- Email: [email protected]
Seasonal influences on size-fractionated chlorophyll a concentrations and primary production in the north-west Indian Ocean
Deep-Sea Research II 46 (1999) 701—723
Seasonal influences on size-fractionated chlorophyll a concentrations and primary production in the north-west Indian Ocean Graham Savidge*, Linda Gilpin Queen+s University of Belfast, Marine Laboratory, Portaferry, Co Down BT22 1PF, UK Received 3 May 1996; received in revised form 21 March 1997; accepted 1 June 1998
Abstract Chlorophyll a (chl a) concentrations and primary production by the 0.2—2, 2—18 and '18 lm phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8°N 68°E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the C technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chl a and production values were recorded in the coastal upwelling zone with values of 69 mg m\ and 3800 mg C m\ d\, respectively. The maxima, which were distributed patchily in this region, were dominated by the '18 lm size-fraction. Over the remainder of the transect chl a concentrations and production averaged 30 mg m\ and 1500 mg C m\ d\, respectively, with approximately equal contributions by the three size-fractions in the case of chl a at the majority of stations, but in general, with a maximum in production in the 0.2—2 lm fraction. Immediately following cessation of the SW monsoon wind, chl a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chl a concentrations and production across the section were dominated by the 0.2—2 lm size-fraction, with average chl a and production values of the order of 20 mg m\ and 750 mg C m\ d\, respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chl a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon. 1999 Elsevier Science Ltd. All rights reserved.
*Corresponding author. Fax: 0044 12477 28902; e-mail: [email protected]. 0967-0645/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 1 2 4 - 6
702
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
1. Introduction Estimates of primary production, by allowing quantification of the input of carbon to a system, must play a central role in any integrated biogeochemical oceanographic programme. During the two UK ‘‘ARABESQUE’’ cruises in the northwest Indian Ocean in September—October and November—December 1994 (Burkill, 1999), which represented the UK contribution to the JGOFS Indian Ocean Programme, detailed primary production estimates were derived from C uptake rates based on two incubation procedures. For the first, samples were incubated under artificial irradiance conditions to allow estimation of the defining parameters of phytoplankton photosynthesis and subsequent derivation of primary production. These data are presented elsewhere in this volume (Sathyendranath et al., 1999). Parallel determinations of primary production also were made during the two cruises using either in situ or simulated in situ incubations. These estimates, which employed size-fractionation of the samples, form the basis of the present contribution. Although several earlier sets of observations of primary production in the Indian Ocean are available (Ryther et al., 1966; Kabanova, 1968; Krey, 1973; Krey and Babenerd, 1976; Qasim, 1977, 1982; Banse, 1987), in the majority of instances the data were averaged over coarse spatial and temporal scales and are hence of limited value for integration into detailed process studies. More recently, greater insight into annual changes in the control of phytoplankton production has been provided from analyses of CZCS imagery (e.g. Banse and McClain, 1986; Brock et al., 1991, 1993, 1994; Brock and McClain, 1992). However, these data, though of obvious value in assessing and modelling basin-scale changes and differences, are of restricted value for detailed biogeochemical process studies. More recent ship-based studies include those of Owen et al. (1993), Jochem et al. (1993) and Jochem (1995). The present data suite allows assessment of the seasonal changes in the controls of phytoplankton growth in the NW Indian Ocean covering both the major shifts in the controlling environmental parameters and the spatial variability of the parameters across the region. The results presented will be discussed in relation to relevant data currently available, highlighting certain of the major problems apparent in the interpretation of the data sets. Comparisons of estimates of primary production derived from samples incubated under artificial irradiance (Sathyendranath et al., 1999) and from samples incubated under in situ conditions, as described in the present paper, will form the basis of a separate publication.
2. Methods Samples for the estimation of size-fractionated chlorophyll a (chl a) concentrations and primary production were taken during RRS Discovery Cruises D210 and D212 in the northwest Indian Ocean between 25 August—5 October and 16 November— 19 December 1994, respectively. The cruise track was similar for both cruises (Fig. 1) and consisted of a transect aligned approximately southeastward from Masirah Bay on the southern coast of Oman (19°30.3 N 58°09.1 E) to 8°00.0 N 68°00.0 E, followed
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
703
Fig. 1. Map showing location of UK ARABESQUE transect and position of sampling stations, RRS Discovery cruises D210 and D212, August—October and November—December 1994.
by a northward leg to 14°20.0 N 67°00.0 E; and for D210, a return to stations A3 (16°02.2 E) and A1. On cruise D212, RRS Discovery returned directly to Muscat from station A9. On both cruises, selected stations were occupied for extended periods to allow replicate daily sampling. Full details of the strategies of the two cruises are given in Burkill (1999). Water samples to be used for the estimation of chl a concentrations and primary production were collected as near as possible within an hour before dawn from ten depths between the surface and the 1% light depth (LD) corresponding to the 97, 55, 33, 20, 14, 7, 4.5, 3, 2.1 and 1% LDs. The appropriate sample depths for each station were estimated from the downwelling irradiance profile obtained from the nearest CTD cast taken during daylight hours. All samples were collected using clean 10-l Niskin water sample bottles attached to a Rosette sampler integral to a Neil Brown CTD fitted with calibrated downwelling and upwelling irradiance sensors. Subsamples from each depth for estimating primary production were gently run off into 1-l acid-washed polycarbonate bottles using clean silicone tubing, and decanted
704
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
into quadruplicate acid-washed 60 ml polycarbonate incubation bottles. Ten lCi of NaHCO were subsequently added to each incubation bottle. Three bottles were incubated either in situ or in an ‘‘on deck’’ incubator under the appropriate LD irradiance conditions, with the remaining bottle incubated in the dark. All samples were manipulated, incubated and maintained in subdued light prior to their deployment. Samples to be used for the estimation of the size-fractionated chl a were fractionated sequentially on to 18, 2.0 and 0.2 lm polycarbonate membranes, with the 18 lm sample being filtered under gravity and vacuum pressures of approximately 630 and 510 mm Hg being applied to assist filtration through the 2 and 0.2 lm filters, respectively. The filters were extracted overnight in the dark at 4°C in 90% acetone and the fluoroscence subsequently measured using a Turner Designs 10000R Fluorometer. No blank correction was necessary for the membrane filters. The fluorometer was calibrated following each cruise against a range of concentrations of pure chl a (Sigma C 6144) made up from a stock solution, the concentration of which was determined spectrophotometrically. Chl a concentrations of parallel unfractionated samples filtered onto GF/F filters were also estimated fluorometrically on both cruises D210 and D212 (Sathyendranath et al., 1999). The means of the summed fractionated chl a concentrations for the two cruises derived from the present data set were 0.73 and 0.30 mg m\, respectively, with the corresponding means derived by Sathyendranath et al. (1997) being 0.71 and 0.38 mg m\. The correlation coefficients for the cruise means were 0.957 (n"54) and 0.901 (n"49) for cruises D210 and D212, respectively. The difference in the means of the two data sets was considered satisfactory, given the contrasting approaches to sample filtration employed. No corrections were applied to the present data set resulting from the intercomparison. The chl a data obtained fluorometrically in the present study during D210 and D212 were also compared with the corresponding values determined by HPLC analysis using parallel unfractionated samples (see Barlow et al., 1999). Comparisons of the means of fluorometrically derived total chl a concentrations for D210 (0.54 mg m\; n"51) and D212 (0.28 mg m\; n"16) with the corresponding means derived from the HPLC method (0.40 and 0.16 mg m\) obtained by Barlow et al. (1999) indicated significantly higher concentrations derived from the fluorometric analyses. A comparable conclusion in respect to the fluorometrically estimated samples also was recorded by Sathyendranath et al. (1999). Although the reasons for the discrepancy between the fluorometric and HPLC determined values are not clear, two possible reasons are advanced that may have contributed to the differences. Fluorometrically determined values of chl a have been shown to be elevated in the presence of chlorophylls b and c (Bianchi et al., 1995), the latter component being particularly associated with diatoms. High concentrations of diatoms ('72 lg Cl\) were recorded at the inshore stations during D210 (Tarran et al., 1999), the chl c from which may have contributed to the elevation of the fluorometrically determined chl a concentration. On D212, although the biomass of the diatoms was generally at least an order of magnitude lower than recorded on the earlier cruise, their maximal concentration was located in the deeper part of the
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
705
euphotic zone (Tarran et al., 1999), where the discrepancy between the fluorometric: HPLC assays also appeared maximal. The presence of a substantial prochlorophyte population at station A6 and A7 on both cruises was also noted by Barlow et al. (1999). These populations are characterized by relatively high concentrations of divinyl chl a, the presence of which would again lead to the elevation of fluorometrically determined chl a concentrations at these stations. Correction of the HPLC derived chl a data from these stations for the di-vinyl chl a concentration increased the mean total chl a concentration to 0.28 mg m\ compared to a fluorometrically derived value of 0.32 mg m\ for the corresponding samples. Samples to be used for the estimation of primary production were incubated either in situ or under simulated in situ conditions, as dictated by the cruise schedule. The in situ samples were incubated at the depths from which they were collected attached to a line suspended from a toroid buoyed by small flotation packs. The productivity rig was attached by a 25-m line to a marker buoy and drifted free from the ship during deployment. The incubation was commenced within an hour of dawn and continued until dusk when samples were brought inboard and held in dark boxes on the deck of the ship to complete the 24-h incubation. Triplicate 60 ml samples to be incubated under the simulated in situ irradiance conditions were placed in perspex tubes covered with combinations of blue and neutral density filters to simulate the range of LD irradiances. The dark sample bottles were incubated in similar darkened Perspex tubes. The incubation tubes, which were mounted in a frame located approximately 60 cm above the deck of the ship, were cooled by running seawater supplied from a depth of 4 m through the tubes. At dusk, the incubation bottles were removed from the tubes and placed, as for the in situ samples, in dark boxes on the deck of the ship overnight to complete the 24 h incubation. Following incubation, samples were filtered through a filter cascade fitted with 18, 2 and 0.2 lm polycarbonate membranes, with vacuum pressure applied as previously. The filters were placed in ‘‘Ponivial’’ plastic scintillation vials and fumed for approximately 10 min over concentrated HCl before drying overnight in a dessicator. 2.5 ml of Optiphase Hisafe 2 scintillation fluid were then added to each vial prior to counting shipboard in a LKB Rackbeta scintillation counter. Standardisation of the added NaHCO was carried out daily. Column production per day was estimated for each size-fraction from both the in situ and simulated in situ production data by integration over the euphotic zone, as defined by the 1% PAR depth. A comparison of parallel in situ and simulated in situ column production estimates for all three size-fractions obtained for cruises D210 and D212 demonstrated regression relationships respectively of simulated in situ production"1.035 (in situ production)!38.2 mg C m\ d\ (n"17; r"0.838) and simulated in situ production"0.941 (in situ production)#10.3 mg C m\ d\ (n"18; r"0.904). This analysis omitted anomalous data from the '18 lm fraction sample for station A1 obtained during D210, where the simulated in situ estimate was approximately twice that derived from the in situ estimate. Although comparability was recorded between the column in situ and simulated in situ production values, discrepancies were noted between individual in situ and simulated in situ values over the depth profiles. Fig. 2 demonstrates the ratio of the
706
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 2. Average ratios of primary production by the '18, 2—18 and 0.2—2 lm size-fractions estimated from simulated in situ and in situ incubations (OD/IS) at various light depths sampled during (a) Cruise D210 and (b) Cruise D212.
in situ: simulated in situ production values based on the estimates of production per unit volume made during the two cruises. At LDs*20%, the ratio of the production determined from the simulated in situ measurements to the in situ was generally (1 whereas for the samples incubated at LDs (20%, the in situ: simulated in situ ratio was generally '1, except for the two deepest sample means for the D210 data set. The possible causes of the skewness in the in situ: simulated in situ ratios are not clear. Checks of the surface water temperature as recorded from the CTD with the temperature of the water circulating through the simulated in situ incubator indicated temperature differences of (0.5°C. However, it is possible that the higher temperature of the water flowing through the incubator tubes containing the samples from the cooler thermocline or sub-thermocline layers may have contributed to the increased ratios recorded for the deeper samples. The transmission of the light filters used to obtain the range of irradiances (Lee Filters 061 Mist Blue) was originally estimated in the laboratory using a constant light source and LiCor PAR cosine collector. The transmission was rechecked for each incubation tube when set up on deck using a LiCor PAR 4n collector, and although the transmission was generally greater in the tubes than estimated for the same combination of filters in the laboratory, the differences were usually of the order of 10% or less. No systematic marked increases in the water column irradiance attenuation were recorded over the
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
707
course of the experiments. One possibility that may account for the skewed distribution is that the ambient irradiance received by the simulated in situ samples was greater than that received by the in situ samples; this may have resulted from the irradiance directly received by the simulated in situ samples being enhanced by light reflected from the ship’s superstructure and deck fittings. On the assumption that the in situ data are less influenced by light and temperature factors, these results have been reported wherever possible. At those locations where it was only possible to carry out simulated in situ incubations, a factor derived from the simulated in situ: in situ comparison data obtained for each cruise was used to transform the simulated in situ production estimates to an equivalent in situ value. These data are identified in the figures as appropriate.
3. Results 3.1. Distribution of chlorophyll a The distribution of column chl a concentrations for the three size-fractions estimated over the euphotic zone is shown for the outward and return legs of D210 and D212 in Fig. 3. For the outward leg of D210, three regimes based on the relative distribution of the three size-fractions were identified (Fig. 3a). Between stations AS5 and A2, that is to the north of the appropriate mean location of the axis of the Findlater Jet (Brock et al., 1993), considerable variability was apparent in the contribution of the three size-fractions to the column chl a concentration. At three sampling locations, stations AS5, A1 and A2, chl a was predominantly associated with the '18 lm fraction with a maximum concentration in excess of 40 mg m\ being recorded at station A1. At the remainder of the stations in this regime, the concentrations of chl a in the 0.2—2 and 2—18 lm size-fractions were approximately equal, with a mean of the order of 10 mg m\, but with a considerably lower concentration associated with the '18 lm fraction. It is likely that the variability present at the inshore end of the transect reflected the influence of mesoscale processes and structure within the inshore upwelling zone in controlling phytoplankton growth (Elliott and Savidge, 1990; Young and Kindle, 1994). A consistent chl a distribution was recorded at stations A5—A7 and A7—A9 on both the outward and return legs, respectively, of D210 (Fig. 3a and b), with maximal concentrations, averaging 13.2 mg m\, associated with the 0.2—2 lm size-fraction and minimal concentrations of the order of 2.8 mg m\ consistently associated with the '18 lm fraction. Particularly stable chl a conditions were noted at stations A7—A9 on the return leg. The relative distribution of chl a between the size-fractions at these stations was similar to that recorded at those stations on the outward leg between AS5 and A2 not characterised by '18 lm fraction dominance. Station A3 on the outward leg of D210 was located at the transition between the coastwards stations AS5—A2 and the offshore stations A5—A7. The column chl a concentration at this station over the four consecutive days of observation was characterised by an approximately equal contribution from all three size-fractions. The average column
708
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 3. Distribution of size-fractionated chl a concentration along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
709
chl a concentration was 10.8 mg m\, a value comparable to that recorded for the 0.2—2 lm size-fraction at the offshore stations. Conditions similar to those found at A3 on the outward leg were also apparent at the three innermost stations, this is A10—A1, on the return leg of D210 (Fig. 3b). During D212, comparable conditions were observed at all stations on both the outward and return legs. Owing to the limited number of stations sampled on the return transect, the data for both legs are presented in Fig. 3c. The relative contributions of the three size-fractions were similar to those at the offshore stations during D210, with a clear dominance of the 0.2—2 lm chl a fraction at all stations (average chl a concentration 12.4 mg m\ on the outward leg) and minimal concentrations being recorded for the '18 lm fraction. The vertical distribution of size-fractionated chl a concentration across the major part of the outward transect of D210 (Fig. 4a) showed an essentially homogeneous distribution over the euphotic zone, with the dominance of the '18 lm fraction being clearly exemplified inshore of station A3. At the southernmost end of the transect at station A7 there was clear evidence for the presence of a subsurface chlorophyll maximum with the phytoplankton community dominated by the 0.2—2 lm fraction. On the return leg of D210 (Fig. 4b), the chl a associated with the '18 lm fraction was distributed homogeneously throughout the euphotic zone across the transect. However, stratification was apparent in the corresponding vertical distribution of the two smaller size-fractions especially between station A7—A9, with the higher concentrations being recorded in the deeper part of the euphotic zone. The comparable data for the outward and return legs of D212 (Fig. 4c) again demonstrated a relatively homogeneous vertical distribution of chl a concentration for all size-fractions inshore of station A3. Although not immediately apparent in Fig. 4c, a subsurface chlorophyll maximum was also present at stations A6—A8 during cruise D212 as indicated from continuous CTD fluorometric data and also from continuous UOR data collected across the transects (M. Pinkerton, pers. comm.). However, the magnitude of the increased chl a concentration associated with the subsurface maximum at these stations (typically 0.25—0.3 mg m\) was relatively low compared to the average concentrations in the surface mixed layer (0.1—0.15 mg m\), such that the feature may not have been clearly resolved in Fig. 4c. As for the previous cruise, chl a concentrations in the subsurface maximum were dominated by the 0.2—2 lm fraction. The distribution of total chl a concentrations across the transect for both D210 and D212 (Fig. 5) showed surprisingly little variation with the exception of the inshore stations on D210 dominated by the '18 lm size-fraction and the first sampling day at station A7 on the same cruise, which was dominated by the 0.2—2 lm chl a fraction, confined chiefly to the subsurface chlorophyll maximum. The maximum chl a concentration on the outward leg of D210 observed at station A1 was 69.1 mg m\, compared with a similar value of 60.2 mg m\ at the first sampling day at station A7. The average chl a concentration at those stations across the transect where the contribution from the three SFs was approximately similar was 28.8 mg m\. On the return leg of D210, column chl a concentrations increased northwards across the transect (Fig. 5b), with a minimal value of 20.2 mg m\ at station A7 and a maximal
710
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 4. Vertical distribution of size-fractionated chl a concentrations (mg m\) along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Line across transect denotes lower limit of sampling, assumed to be base of euphotic zone.
value of 32.0 mg m\ at station A3. The mean chl a concentration across the transect was 26.9 mg m\. For the outward leg of D212, the corresponding minimum and maximum column chl a values (Fig. 5c) were 12.8 and 21.1 mg m\, respectively, with a mean value across the transect of 17.8 mg m\. No distributional trend was apparent along the transect. Consistently higher values were recorded (mean" 22.7 mg m\) from the three stations on the return leg of D212, suggesting considerable temporal mesoscale variability in the distribution of chl a within the subsurface chlorophyll maximum, the main contributor to column chl a concentrations at the outer stations.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
711
Fig. 4. Continued.
3.2. Distribution of primary production A broad similarity was evident across the transect for both D210 and D212 in the patterns of distribution of size-fractionated primary production (Fig. 6) and the corresponding distributions of column chl a concentration (Fig. 3). On the outward leg of D210, peaks in primary production by the '18 lm size-fraction were recorded at those stations inshore of A3 where the chl a concentrations exhibited a similar fraction dominance. Maximal production was consistently associated with the 0.2—2 lm size-fraction at the other stations in the inshore regime (Fig. 6a). Comparison of these data with the appropriate chl a concentrations suggest higher photosynthesis per unit chl a by the 0.2—2 lm fraction compared to the other fractions. A close overall similarity between the size-fractionated chl a and
712
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 4. Continued.
production values was apparent for the remaining stations sampled during D210 (Fig. 6a and b) with the exception that the chl a peak noted for the 0.2—2 lm size-fraction on the first sampling day at station A7 was not reflected in the associated production value. Although the contribution to production on both the outward and return legs of D212 was dominated by the 0.2—2 lm fraction, as was noted for the comparable chl a data, relatively more variability was observed in the distribution of production (Fig. 6c). In particular, production of the smallest size-fraction was high at the three innermost stations of the transect and increased over the sampling period at station A3, remaining elevated through to station A5.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
713
Fig. 5. Distribution of total ('0.2 lm) chl a concentration along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
714
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 6. Distribution of size-fractionated primary production along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
715
On the outward leg of D210 (Fig. 7a), total column production was highest at the inshore stations AS5 and AS2 and also on the first sampling day at A1, with column production averaging 3130 mg C m\ d\. The sharp decrease in production at A1 after the first sampling day, to values between 810 and 1250 mg C m\ d\ was assumed, as previously, to reflect a change in hydrographic conditions owing to mesoscale variability. A secondary maximum of approximately 1800 mg C m\ d\ was evident at station A2 followed by a general decrease in values seawards with minimal values, averaging 1020 mg C m\ d\, being recorded at the two outermost stations of the transect. On the return leg of D210 (Fig. 7b), minimal production, of the order of 900 mg C m\ d\ was again recorded at the two outermost stations with values increasing to an average of 1500 mg C m\ d\ at the inner stations. During D212, column production was in the range 1000—1300 mg C m\ d\ at the two inshore stations AS5 and AS3 and, as during D210, demonstrated high production only on the first day at A1 (Fig. 7c). Temporal variability was also recorded at station A3 where column production on successive days increased from 300 to '1000 mg C m\ d\. Offshore from A3, the trend towards lower rates of production ((600 mg C m\ d\) at the outermost stations was again apparent. As indicated previously from the data showing the vertical distribution of the chl a size-fractions (Fig. 4), the subsurface chlorophyll maximum was only consistently defined at the outermost station associated with the oligotrophic waters as defined by the surface nutrient concentrations (Woodward et al., 1999). Detail of the distribution of the chl a size-fractions in the subsurface chlorophyll maximum is presented in Fig. 8a for a representative station (A7/49) sampled during D210. The data show a marked maximum chl a concentration of 0.40 mg m\ recorded at 63 m for the smallest size-fraction compared to an average of 0.18 mg m\ in the upper mixed layer. The comparable production data (Fig. 8b) demonstrate a secondary maximum of 9.8 mg C m\ d\ in the profile centred on 60 m. By defining the depth horizons of the subsurface chlorophyll maximum as the depths at which the chl a concentrations were 1.5 times greater than those in the upper 25 m of the water column, the contribution of production in the subsurface chlorophyll maximum to total column production by the three size-fractions was determined (Table 1). Significant contributions to column production by the subsurface chlorophyll maximum were recorded only at station A7, A8 and A9, with only the extreme southern station A7 demonstrating a clear subsurface chlorophyll maximum on both cruises. From consideration of the data for stations A7—A9, it is apparent that the nanophytoplankton (2—18 lm) fraction made the greatest percentage contribution to the subsurface chlorophyll maximum production. However, on the basis of absolute carbon fixation, maximum production was attributable to the picophytoplankton (0.2—2 lm) fraction, this reflecting the higher rates of production by the smallest fraction.
4. Discussion The observations described in this report are considered to be of particular significance since they spanned both the end of the SW monsoon season in the
716
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 7. Distribution of total ('0.2 lm) primary production along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
717
Fig. 8. Vertical distribution of (a) size-fractionated chl a concentrations and (b) primary production at station A7/49, Cruise D210. Table 1 Contribution to column primary production (mg C m\ d\) by subsurface chlorophyll maximum for the three defined size-fractions and by total phytoplankton community, RRS Discovery cruises 210 and 212 SCM (%)
2—18 lm column production
SCM (%)
'18 lm column production
SCM (%)
Total column production
SCM (%)
D210 outward A5 180 A7 47
3.6 11.0
383 159
0.2 59.3
966 764
0 57.4
1530 970
0.9 55.4
D210 return A8 45 A9 42
15.0 3.3
134 221
52.4 71.6
760 966
50.4 30.4
939 1229
49.0 36.9
D212 outward and return A3 35 7.8 A7 39 31.3 A8 82 3.7
42 61 59
31.8 34.8 18.7
383 445 433
0 22.7 0
561 545 575
3.5 24.7 2.4
Station
0.2—2 lm column production
northwest Indian Ocean and the initiation of the subsequent relaxation phase, together with a period well within the relaxation phase (Burkill, 1999). The demonstration of the presence of phytoplankton communities with contrasting size-fraction characteristics to the NW and SE of station A3 on the initial leg of D210 corresponded
718
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
with comparable data obtained from phytoplankton taxonomic studies (Tarran et al., 1999). To the NW of station A3, the phytoplankton assemblage was shown to be dominated by diatoms at those locations exhibiting marked dominance by the '18 lm size-fraction, with the cyanobacteria contributing maximally to the smaller size-fractions. To the SE of station A3 through to station A6, the phytoplankton biomass was dominated by coccolithophorids, particularly at depths greater than 20 m, and also, again, by cyanobacteria. Numerically, both dinoflagellates and autotrophic pico-eucaryotes also exhibited maximal abundances at these stations (Tarran et al., 1999). A marked decrease in all principal phytoplankton taxa was evident at the outermost station A7, these being replaced by a maximum in the prochlorophytes. A local minimum in the biomass of the majority of the taxa was evident at station A3, which marked the transition between the two size-fraction-defined communities. The demonstration of a boundary in the distribution of the phytoplankton communities in the vicinity of station A3 may be related tentatively to the wind distribution and hydrography observed during D210. Burkill (1999) presents hourly averages of wind stress and direction recorded during both D210 and D212. During D210 the wind was confirmed as blowing consistently from a southwesterly direction until approximately 20 September during occupation of station A7 when a marked shift to a northerly quarter and reduced speeds were noted. This rapid change in wind velocity and the timing is characteristic of the end of the SW monsoon in the northwestern Indian Ocean (Savidge et al., 1990). In the earlier part of D210, a maximum in wind speed of the order of 23 m s\ was recorded between 4 and 5 September, immediately after RRS Discovery arrived in the coastal waters of southern Oman. During the subsequent detailed survey of the coastal area through until 9 September, the wind speed gradually decreased to an average of 13 m s\. Savidge et al. (1990) previously have shown the speed of the SW monsoon wind to vary temporally in the southern Oman coastal region during the monsoon season. However, following commencement of the main transect, a secondary maximum in wind strength of approximately 18 m s\ was observed at station A2. South-eastwards of A2 to the furthest station A7, the mean wind strength consistently decreased with a minimum of 5 m s\ recorded at A7 on 19 September. The progressive decrease in the wind speed was accompanied by an increasing westerly component in the wind direction, such that at station A7 on 19 September, the wind was effectively from a westwards as opposed to a south-westerly direction. The mean position of the main axis of the SW monsoon, that is the Findlater Jet (Findlater, 1969), coincides approximately with the location of station A3 (Brock et al., 1993). The Jet is usually assumed to mark the boundary between the coastal and immediate offshore upwelling regions defined by the positive wind stress curl and the offsore downwelling region associated with the negative wind stress curl (Brock et al., 1991). Some local variation in the position of the Jet may be expected over the course of the SW monsoon season and it is hence assumed that the position of the Jet during D210 was represented by the secondary wind speed maximum recorded adjacent to station A2. This interpretation is consistent with the decreasing wind speed and westwards shift in wind direction observed to the south of the Jet, representative of a negative wind stress curl. From the data available, it is not clear whether the wind
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
719
speed maximum recorded adjacent to the coast was a transient feature or was associated with a secondary circulation related to the proximity of the coast. Hydrographic data presented by Woodward et al. (1999) indicate the presence of a regime boundary in the vicinity of station A2. The marked surfaceward gradient of isopycnals and nutrient isolines to the north of A2 is assumed to reflect active upwelling. Although upwelling also was suggested at A3 by the NO distribution data, this was not confirmed by the corresponding density distribution. Between A2 and A5, a progressive deepening of the isopycnals was recorded, consistent with downwelling conditions, as may be typically found to the south of the Jet. The relatively high surface nutrient concentrations also recorded between A2 and A5 suggest that the downwelling water spread southwards from its area of origin adjacent to the Jet. To the south of A5, the data of Woodward et al. (1999) indicated strongly oligotrophic conditions. The occurrence of similar size-fractionated chl a concentrations at stations A10—A1 on the return leg of D210 to those recorded at station A3 on the outward leg of D210 may also reflect transition conditions. Whereas, however, on the outward leg of D210 the community reflected a geographically defined transition zone, the transition conditions at A10—A1 on the return leg of D210 reflected a temporal change in the wind conditions. On the outward leg, strong southwesterly winds prevailed at the northernmost stations while on the return leg some three weeks later relatively calm conditions prevailed with only light northerly winds (Burkill, 1999) representative of the transition between the SW monsoon season and the initiation of the relaxation phase. Clearly, with the onset of calm conditions, the upwelling and associated surface nutrient enrichment processes would cease resulting in a change in the structure of the prevailing phytoplankton community as observed. A broad correspondence was apparent between the surface chl a concentrations and column primary production values recorded in the present study and the large scale monthly distributions of these parameters as presented by Krey and Babenerd (1976). It is more instructive, however, to compare the present data with previous more locally restricted data sets relevant to the particular timings of the present surveys. The most appropriate comparison can be made with data obtained by Ryther et al. (1966) between late September and early November 1963 from the upwelling zone off Oman. For stations north of 18°N, which may arbitrarily be assumed to be in the Omani coastal upwelling zone, Ryther et al. (1966) recorded a mean production of 2230 mg C m\ d\ (n"18) compared to a mean obtained at stations AS5 and AS3 in the present study of 3080 mg C m\ d\. Considerably variability was evident in the values of Ryther et al. (1966) from this region (range: 840—6580 mg C m\ d\), consistent with the presence of marked mesoscale variability. Between 16 and 18°N, the zone dominated by the positive curl upwelling, the data from Ryther et al. (1966) show a decrease in the mean production to 1550 mg C m\ d\ (n"3) similar to the decrease recorded at stations A1 and A2 (mean"2050 mg C m\ d\, n"5). In the latter case, the mean was skewed by a high initial value at station A1 (Fig. 7a), which may have resulted from a transient influence from the immediately adjacent coastal upwelling zone. South of the mean position of the Jet axis between approximately 9°—16°N, the average production at stations A5—A7 was 910 mg C m\ d\ (n"6) compared to a corresponding value of 1060 mg C m\ d\ (n"11) derived from the
720
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
data of Ryther et al. (1966). From this comparison it is apparent that there was considerable correspondence between the present data and that obtained at a comparable time of year in 1963. Further comparison may be made between the mean chl a concentrations in the photic layer and column production values recorded from the present cruises and those obtained by Owens et al. (1993) along a similar transect carried out during September—October 1986 (Table 2). It is clear that there was a considerable degree of correspondence between the chl a concentrations, but in general production values for 1994 were considerably higher than those recorded for 1986. This effect was particularly marked at the two furthest offshore stations in strongly oligotrophic waters where production was approximately three times higher in 1994. The factors responsible for this difference are not obvious, but may possibly be related to differences in the intensity of the SW monsoon between contrasting year (Brock and McClain, 1992) or the location of the position of maximum windstress (Haake et al., 1993). Such influences may have altered the phytoplankton community composition with a consequent effect on the phytoplankton production: biomass ratio. Further insight into the problems encountered in the interpretation of chl a data from the NW Indian Ocean may be obtained from comparison of the 1994 data with observations and predictions derived from CZCS satellite imagery of the region. CZCS data obtained by Brock and McClain (1992) were used to emphasize the difference in the phytoplankton biomass between the Omani coastal upwelling area and the offshore upwelling area influenced by the positive wind stress curl to the north of the Findlater Jet. The data, which referred to the late phase of the SW monsoon and which were obtained over the four years 1979—1982, indicated typical surface chl a concentrations of the order of 5 mg m\ to the south in the positive curl zone. These values are very considerably higher than the mean photic layer values obtained both in the present investigation and by Owens et al. (1993) (Table 2: stations 16 and 14). In addition, the data in Table 2 do not indicate a significant difference in the surface photic layer chl a concentrations between the two zones. The interannual consistency within the separate shipboard and CZCS imagery data sets suggests the possibility of a calibration effect in the satellite data, perhaps reflecting the difficulties of obtaining good calibrations in a variable oceanographic environment. In relation to the Table 2 Mean mixed layer chl a concentrations (mg m\) and column production values (mg C m\ day\) obtained at comparable stations in NW Indian Ocean in 1986 and 1994. 1986 data from Owens et al. (1993) Station no.
Chl a concentration
Column production
1986
1994
1986
1994
1986
1994
16 14 5 3
AS5, AS2 A2 A5 A7
1.66 1.63 0.38 0.18
1.63 1.30 0.38 0.26
2668 1391 466 300
3283 2042 1530 1026
Mean of two values.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
721
differences in the phytoplankton biomass status of the coastal and offshore upwelling zones, it is at present not clear to what extent these arbitrarily defined regions may be considered as separate entities (Brock and McClain, 1992) or whether the biomass in the offshore area is influenced by export from the coastal upwelling zone as mediated by mesoscale eddy and offshore filament activity (Young and Kindle, 1994). Possibly the most valuable insight into the phytoplankton dynamics of the NW Indian Ocean has been provided by Brock et al. (1993, 1994). Both studies modelled the interactions of the main physical controls on phytoplankton growth over the four main seasons in the area: the SW and NE monsoon seasons and the two intermonsoon seasons. More valuable comparisons can be made between the present data and the earlier study in which production over the whole water column was modelled as opposed to production in the surface mixed layer. One of the major predictions of the 1993 study, which considered conditions at a station in the offshore positive curl upwelling zone, was of a sharp and major decrease in total column production from approximately 2000 to 500 mg C m\ d\ following the cessation of the SW monsoon wind in late September. In the present study the average column production at stations A1 and A2 in the offshore upwelling zone on the outward leg of D210 when the SW monsoon wind prevailed was 1670 mg C m\ d\, a value comparable to that given by Brock et al. (1993). Although the SW monsoon had relaxed on the return leg of D210, the column production at station A1 remained at 1440 mg C m\ d\, indicating no substantial decrease in the two week period following the relaxation of the SW wind. At this time, the '18 lm size-fraction still represented a substantial contribution to production at this location. During D212, some seven weeks into the intermonsoon season, the mean column production at station A1 and A2 had decreased to 350 mg C m\ d\, with a clear dominance in production shown by the 0.2—2 lm fraction. Thus there is a considerable degree of agreement between the production values predicted by the model of Brock et al. (1993) for their station in the positive curl region and those observed in the present study, although it is clear that a finite time is required for the system to adjust to the lower production level following relaxation of the SW monsoon wind. Although Brock et al. (1993) predicted that 60% of the total production in the positive curl region would be contributed by the subsurface chlorophyll maximum, the present observations indicated that, with the exception of station A3 on D221, significant contributions to column production by the subsurface chlorophyll maximum were only made at the southernmost stations of the transect in the oligotrophic waters. UOR data obtained by M. Pinkerton (pers. comm.) confirmed that the subsurface chlorophyll maximum was restricted to the furthest oceanic stations on both ARABESQUE cruises. In the present investigation, the major contribution to subsurface chlorophyll maximum production at these sites was by the picoplankton (0.2—2 lm size-fraction), in agreement with the observations made by Jochem et al. (1993) at a similar location in May 1987. One of the more equivocal questions in relation to the NW Indian Ocean concerns the level of production in the regions away from the upwelling zones, particularly those areas to the south of the axis of the main SW monsoon wind jet. Based on the very low concentrations of new nutrients typically observed in the surface mixed layer
722
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
(Wyrtki, 1971), it was assumed in early studies of the Indian Ocean that this region would exhibit low production similar to that of other sub-tropical oligotrophic oceanic areas. This view was supported by the early data of Ryther et al. (1966) and Kabanova (1968), in which production levels generally (200 mg C m\ d\ were recorded, and later by that of Owens et al. (1993) for their station 3 in September 1987 where production averaged 300 mg C m\ d\. However, this conclusion concerning low production is not clear-cut. Coarsely resolved temporal production data presented by Qasim (1982) indicated high production levels of the order of 900 mg C m\ d\ in the NW Indian Ocean in both the SW and NE monsoon seasons, that is similar to the values recorded in the present study. The more localised data set obtained by Jochem et al. (1993) during a Lagrangian experiment at a station in the oligotrophic region during the spring intermonsoon season in May 1987 demonstrated a high average production of 730 mg C m\ d\. In this latter study maximal production, as in the present investigation at the comparable oligotrophic stations, was associated with the picophytoplankton. Clearly, the data presented in the present investigation support the concept of relatively high production in the offshore oligotrophic region of the NW Indian Ocean, although the temporal and spatial resolution of the data sets currently available do not allow firm conclusions to be drawn on this subject. It may be noted, however, that the hydrographic data presented for both cruises by Woodward et al. (1999) for the strongly oligotrophic stations indicate substantive NO concentrations either in or slightly above the thermocline. The vertical transfer of nutrients, which may result from the influence of mixing, is likely to be reflected in increased production in the subsurface chlorophyll maximum and thus assist in maintaining increased column production values. This phenomenon clearly has implications for the modelling of production in the Indian Ocean owing to the spatial extent of the strongly oligotrophic region. It is hoped, however, that the data obtained during the intensive Indian Ocean JGOFS Programme will assist in resolving the issue. Acknowledgements We wish to thank the Principal Scientists of RRS Discovery cruises 210 and 212, Professor R.F.C. Mantoura and Dr. P.H. Burkill respectively, for their efforts in enabling the UK ARABESQUE Indian Ocean Programme and for leading the cruises. Our thanks go also to the Captains and crews of RRS Discovery and also to Alan Pomroy and Ian Joint of the Plymouth Marine Laboratory without whose considerable assistance this work would not have been possible. The generous help of Amersham International plc towards the provision of C radiotracer is gratefully acknowledged. References Banse, K., 1987. Seasonality of phytoplankton chlorophyll in the central and northern Arabian Sea. Deep-Sea Research 34, 713—723. Banse, K., McClain, C.R., 1986. Winter blooms of phytoplankton in the Arabian Sea as observed by the Coastal Zone Color Scanner. Marine Ecology Progress Series 34, 201—211.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
723
Barlow, R.G., Mantoura, R.F.C., Cummings, D.C., 1999. Monsoonal influence on the dstribution of phytoplankton pigments in the Arabian Sea. Deep-Sea Research II 46, 677—699. Bianchi, T.S., Lambert, C., Biggs, D.C.,1995. Distribution of chlorophyll a and phaeopigments in the northwestern Gulf of Mexico: a comparison between fluorometric and high-performance liquid chromatography measurements. Bulletin of Marine Science 56, 25—32. Brock, J.C., McClain, C.R., 1992. International variability in phytoplankton blooms observed in the northwestern Arabian Sea during the southwest Monsoon. Journal of Geophysical Research 97, 733—750. Brock, J.C., McClain, C.R., Luther, M.E., Hay, W.W., 1991. The phytoplankton bloom in the northwestern Arabian Sea during the southwest Monsoon of 1979. Journal of Geophysical Research 96, 20 623—20 642. Brock, J.C., Sathyendranath, S., Platt, T., 1993. Modelling the seasonality of subsurface light and primary production in the Arabian Sea. Marine Ecology Progress Series 101, 209—221. Brock, J.C., Sathyendranath, S., Platt, T., 1994. A model study of seasonal mixed-layer primary production in the Arabian Sea. Proceedings of the Indian Academy of Scieneces (Earth and Planetary Sciences) 103, 65—78. Burkill, R.H., 1999, ARABESQUE: An overview. Deep-Sea Research II 46, 529—547. Elliot, A.J., Savidge, G., 1990. Some features of upwelling off Oman. Journal of Marine Research 48, 319—333. Findlater, J., 1969. A major low-level air current near the Indian Ocean during the northern summer. Quarterly Journal of the Royal Meteorological Society 95, 362—380. Haake, B., Ittekkot, V., Rixen, T., Ramaswamy, V., Nair, R.R., Curry, W.B., 1993. Seasonality and interannual variability of particle fluxes to the deep Arabian Sea. Deep-Sea Research I 40, 1323—1344. Jochem, F.J., 1995. Phototrophic picoplankton community structure in three different pelagic regimes in the Arabian Sea. Marine Ecology Progress Series 117, 307—314. Jochem, F.J., Pollehne, F., Zeitschel, B., 1993. Productivity regime and phytoplankton size structure in the Arabian Sea. Deep-Sea Research II 40, 711—735. Kabanova, Y.G., 1968. Primary production of the northern part of the Indian Ocean. Oceanology 8, 214—225. Krey, J., 1973. Primary productivity in the Indian Ocean, I. In: Zeitschel, B. (Ed.), The Biology of the Indian Ocean, Ecological Studies, Analysis and Synthesis, vol. 3. Springer, Berlin, pp. 115—130. Krey, J., Babenerd, B., 1976. Phytoplankton production. Atlas of the International Indian Ocean Expedition. Universitat Kiel, Institut fur Meereskunde, 70 pp. Owens, N.J.P., Burkill, P.H., Mantoura, R.F.C., Woodward, E.M.S., Bellan, I.E., Aiken, J., Howland, R.J.M., Llewellyn, C.A., 1993. Size-fractionated primary production and nitrogen assimilation in the northwestern Indian Ocean. Deep-Sea Research II 40, 697—709. Qasim, S.Z., 1977. Biological productivity of the Indian Ocean. Indian Journal of Marine Sciences 6, 122—137. Qasim, S.Z., 1982. Oceanography of the northern Arabian Sea. Deep-Sea Research 29, 1041—1068. Ryther, J.H., Hall, J.R., Pease, A.K., Bakun, A., Jones, M.M., 1966. Primary organic production in relation to the chemistry and hydrography of the western Indian Ocean. Limnology and Oceanography 11, 371—380. Sathyendranath, S., Stuart, V., Irwin, B.D., Maass, H., Savidge, G., Gilpin, L., Platt, T., 1999. Seasonal variations in bio-optical properties of phytoplankton in the Arabian Sea. Deep-Sea Research II 46, 633—653. Savidge, G., Lennon, J., Matthews, A.J., 1990. A shore-based survey of upwelling along the coast of Dhofar region, southen Oman. Continental Shelf Research 10, 259—275. Tarran, G.A., Burkill, P.H., Edwards, E.S., Woodward, E.M.S., 1999. Phytoplankton community structure in the Arabian Sea during and after the SW Monsoon, 1994. Deep-Sea Research II 46, 655—676. Woodward, E.M.S., Rees, A.P., Stephens, J.A., 1999. Influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea. Deep-Sea Research II 46, 571—591. Wyrtki, K., 1971. Oceanographic Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, 531 pp. Young, D.K., Kindle, J.C., 1994. Physical processes affecting availability of dissolved silicate for diatom production in the Arabian Sea. Journal of Geophysical Research 99, 22 619—22 632.
Seasonal influences on size-fractionated chlorophyll a concentrations and primary production in the north-west Indian Ocean Graham Savidge*, Linda Gilpin Queen+s University of Belfast, Marine Laboratory, Portaferry, Co Down BT22 1PF, UK Received 3 May 1996; received in revised form 21 March 1997; accepted 1 June 1998
Abstract Chlorophyll a (chl a) concentrations and primary production by the 0.2—2, 2—18 and '18 lm phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8°N 68°E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the C technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chl a and production values were recorded in the coastal upwelling zone with values of 69 mg m\ and 3800 mg C m\ d\, respectively. The maxima, which were distributed patchily in this region, were dominated by the '18 lm size-fraction. Over the remainder of the transect chl a concentrations and production averaged 30 mg m\ and 1500 mg C m\ d\, respectively, with approximately equal contributions by the three size-fractions in the case of chl a at the majority of stations, but in general, with a maximum in production in the 0.2—2 lm fraction. Immediately following cessation of the SW monsoon wind, chl a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chl a concentrations and production across the section were dominated by the 0.2—2 lm size-fraction, with average chl a and production values of the order of 20 mg m\ and 750 mg C m\ d\, respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chl a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon. 1999 Elsevier Science Ltd. All rights reserved.
*Corresponding author. Fax: 0044 12477 28902; e-mail: [email protected]. 0967-0645/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 1 2 4 - 6
702
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
1. Introduction Estimates of primary production, by allowing quantification of the input of carbon to a system, must play a central role in any integrated biogeochemical oceanographic programme. During the two UK ‘‘ARABESQUE’’ cruises in the northwest Indian Ocean in September—October and November—December 1994 (Burkill, 1999), which represented the UK contribution to the JGOFS Indian Ocean Programme, detailed primary production estimates were derived from C uptake rates based on two incubation procedures. For the first, samples were incubated under artificial irradiance conditions to allow estimation of the defining parameters of phytoplankton photosynthesis and subsequent derivation of primary production. These data are presented elsewhere in this volume (Sathyendranath et al., 1999). Parallel determinations of primary production also were made during the two cruises using either in situ or simulated in situ incubations. These estimates, which employed size-fractionation of the samples, form the basis of the present contribution. Although several earlier sets of observations of primary production in the Indian Ocean are available (Ryther et al., 1966; Kabanova, 1968; Krey, 1973; Krey and Babenerd, 1976; Qasim, 1977, 1982; Banse, 1987), in the majority of instances the data were averaged over coarse spatial and temporal scales and are hence of limited value for integration into detailed process studies. More recently, greater insight into annual changes in the control of phytoplankton production has been provided from analyses of CZCS imagery (e.g. Banse and McClain, 1986; Brock et al., 1991, 1993, 1994; Brock and McClain, 1992). However, these data, though of obvious value in assessing and modelling basin-scale changes and differences, are of restricted value for detailed biogeochemical process studies. More recent ship-based studies include those of Owen et al. (1993), Jochem et al. (1993) and Jochem (1995). The present data suite allows assessment of the seasonal changes in the controls of phytoplankton growth in the NW Indian Ocean covering both the major shifts in the controlling environmental parameters and the spatial variability of the parameters across the region. The results presented will be discussed in relation to relevant data currently available, highlighting certain of the major problems apparent in the interpretation of the data sets. Comparisons of estimates of primary production derived from samples incubated under artificial irradiance (Sathyendranath et al., 1999) and from samples incubated under in situ conditions, as described in the present paper, will form the basis of a separate publication.
2. Methods Samples for the estimation of size-fractionated chlorophyll a (chl a) concentrations and primary production were taken during RRS Discovery Cruises D210 and D212 in the northwest Indian Ocean between 25 August—5 October and 16 November— 19 December 1994, respectively. The cruise track was similar for both cruises (Fig. 1) and consisted of a transect aligned approximately southeastward from Masirah Bay on the southern coast of Oman (19°30.3 N 58°09.1 E) to 8°00.0 N 68°00.0 E, followed
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
703
Fig. 1. Map showing location of UK ARABESQUE transect and position of sampling stations, RRS Discovery cruises D210 and D212, August—October and November—December 1994.
by a northward leg to 14°20.0 N 67°00.0 E; and for D210, a return to stations A3 (16°02.2 E) and A1. On cruise D212, RRS Discovery returned directly to Muscat from station A9. On both cruises, selected stations were occupied for extended periods to allow replicate daily sampling. Full details of the strategies of the two cruises are given in Burkill (1999). Water samples to be used for the estimation of chl a concentrations and primary production were collected as near as possible within an hour before dawn from ten depths between the surface and the 1% light depth (LD) corresponding to the 97, 55, 33, 20, 14, 7, 4.5, 3, 2.1 and 1% LDs. The appropriate sample depths for each station were estimated from the downwelling irradiance profile obtained from the nearest CTD cast taken during daylight hours. All samples were collected using clean 10-l Niskin water sample bottles attached to a Rosette sampler integral to a Neil Brown CTD fitted with calibrated downwelling and upwelling irradiance sensors. Subsamples from each depth for estimating primary production were gently run off into 1-l acid-washed polycarbonate bottles using clean silicone tubing, and decanted
704
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
into quadruplicate acid-washed 60 ml polycarbonate incubation bottles. Ten lCi of NaHCO were subsequently added to each incubation bottle. Three bottles were incubated either in situ or in an ‘‘on deck’’ incubator under the appropriate LD irradiance conditions, with the remaining bottle incubated in the dark. All samples were manipulated, incubated and maintained in subdued light prior to their deployment. Samples to be used for the estimation of the size-fractionated chl a were fractionated sequentially on to 18, 2.0 and 0.2 lm polycarbonate membranes, with the 18 lm sample being filtered under gravity and vacuum pressures of approximately 630 and 510 mm Hg being applied to assist filtration through the 2 and 0.2 lm filters, respectively. The filters were extracted overnight in the dark at 4°C in 90% acetone and the fluoroscence subsequently measured using a Turner Designs 10000R Fluorometer. No blank correction was necessary for the membrane filters. The fluorometer was calibrated following each cruise against a range of concentrations of pure chl a (Sigma C 6144) made up from a stock solution, the concentration of which was determined spectrophotometrically. Chl a concentrations of parallel unfractionated samples filtered onto GF/F filters were also estimated fluorometrically on both cruises D210 and D212 (Sathyendranath et al., 1999). The means of the summed fractionated chl a concentrations for the two cruises derived from the present data set were 0.73 and 0.30 mg m\, respectively, with the corresponding means derived by Sathyendranath et al. (1997) being 0.71 and 0.38 mg m\. The correlation coefficients for the cruise means were 0.957 (n"54) and 0.901 (n"49) for cruises D210 and D212, respectively. The difference in the means of the two data sets was considered satisfactory, given the contrasting approaches to sample filtration employed. No corrections were applied to the present data set resulting from the intercomparison. The chl a data obtained fluorometrically in the present study during D210 and D212 were also compared with the corresponding values determined by HPLC analysis using parallel unfractionated samples (see Barlow et al., 1999). Comparisons of the means of fluorometrically derived total chl a concentrations for D210 (0.54 mg m\; n"51) and D212 (0.28 mg m\; n"16) with the corresponding means derived from the HPLC method (0.40 and 0.16 mg m\) obtained by Barlow et al. (1999) indicated significantly higher concentrations derived from the fluorometric analyses. A comparable conclusion in respect to the fluorometrically estimated samples also was recorded by Sathyendranath et al. (1999). Although the reasons for the discrepancy between the fluorometric and HPLC determined values are not clear, two possible reasons are advanced that may have contributed to the differences. Fluorometrically determined values of chl a have been shown to be elevated in the presence of chlorophylls b and c (Bianchi et al., 1995), the latter component being particularly associated with diatoms. High concentrations of diatoms ('72 lg Cl\) were recorded at the inshore stations during D210 (Tarran et al., 1999), the chl c from which may have contributed to the elevation of the fluorometrically determined chl a concentration. On D212, although the biomass of the diatoms was generally at least an order of magnitude lower than recorded on the earlier cruise, their maximal concentration was located in the deeper part of the
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
705
euphotic zone (Tarran et al., 1999), where the discrepancy between the fluorometric: HPLC assays also appeared maximal. The presence of a substantial prochlorophyte population at station A6 and A7 on both cruises was also noted by Barlow et al. (1999). These populations are characterized by relatively high concentrations of divinyl chl a, the presence of which would again lead to the elevation of fluorometrically determined chl a concentrations at these stations. Correction of the HPLC derived chl a data from these stations for the di-vinyl chl a concentration increased the mean total chl a concentration to 0.28 mg m\ compared to a fluorometrically derived value of 0.32 mg m\ for the corresponding samples. Samples to be used for the estimation of primary production were incubated either in situ or under simulated in situ conditions, as dictated by the cruise schedule. The in situ samples were incubated at the depths from which they were collected attached to a line suspended from a toroid buoyed by small flotation packs. The productivity rig was attached by a 25-m line to a marker buoy and drifted free from the ship during deployment. The incubation was commenced within an hour of dawn and continued until dusk when samples were brought inboard and held in dark boxes on the deck of the ship to complete the 24-h incubation. Triplicate 60 ml samples to be incubated under the simulated in situ irradiance conditions were placed in perspex tubes covered with combinations of blue and neutral density filters to simulate the range of LD irradiances. The dark sample bottles were incubated in similar darkened Perspex tubes. The incubation tubes, which were mounted in a frame located approximately 60 cm above the deck of the ship, were cooled by running seawater supplied from a depth of 4 m through the tubes. At dusk, the incubation bottles were removed from the tubes and placed, as for the in situ samples, in dark boxes on the deck of the ship overnight to complete the 24 h incubation. Following incubation, samples were filtered through a filter cascade fitted with 18, 2 and 0.2 lm polycarbonate membranes, with vacuum pressure applied as previously. The filters were placed in ‘‘Ponivial’’ plastic scintillation vials and fumed for approximately 10 min over concentrated HCl before drying overnight in a dessicator. 2.5 ml of Optiphase Hisafe 2 scintillation fluid were then added to each vial prior to counting shipboard in a LKB Rackbeta scintillation counter. Standardisation of the added NaHCO was carried out daily. Column production per day was estimated for each size-fraction from both the in situ and simulated in situ production data by integration over the euphotic zone, as defined by the 1% PAR depth. A comparison of parallel in situ and simulated in situ column production estimates for all three size-fractions obtained for cruises D210 and D212 demonstrated regression relationships respectively of simulated in situ production"1.035 (in situ production)!38.2 mg C m\ d\ (n"17; r"0.838) and simulated in situ production"0.941 (in situ production)#10.3 mg C m\ d\ (n"18; r"0.904). This analysis omitted anomalous data from the '18 lm fraction sample for station A1 obtained during D210, where the simulated in situ estimate was approximately twice that derived from the in situ estimate. Although comparability was recorded between the column in situ and simulated in situ production values, discrepancies were noted between individual in situ and simulated in situ values over the depth profiles. Fig. 2 demonstrates the ratio of the
706
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 2. Average ratios of primary production by the '18, 2—18 and 0.2—2 lm size-fractions estimated from simulated in situ and in situ incubations (OD/IS) at various light depths sampled during (a) Cruise D210 and (b) Cruise D212.
in situ: simulated in situ production values based on the estimates of production per unit volume made during the two cruises. At LDs*20%, the ratio of the production determined from the simulated in situ measurements to the in situ was generally (1 whereas for the samples incubated at LDs (20%, the in situ: simulated in situ ratio was generally '1, except for the two deepest sample means for the D210 data set. The possible causes of the skewness in the in situ: simulated in situ ratios are not clear. Checks of the surface water temperature as recorded from the CTD with the temperature of the water circulating through the simulated in situ incubator indicated temperature differences of (0.5°C. However, it is possible that the higher temperature of the water flowing through the incubator tubes containing the samples from the cooler thermocline or sub-thermocline layers may have contributed to the increased ratios recorded for the deeper samples. The transmission of the light filters used to obtain the range of irradiances (Lee Filters 061 Mist Blue) was originally estimated in the laboratory using a constant light source and LiCor PAR cosine collector. The transmission was rechecked for each incubation tube when set up on deck using a LiCor PAR 4n collector, and although the transmission was generally greater in the tubes than estimated for the same combination of filters in the laboratory, the differences were usually of the order of 10% or less. No systematic marked increases in the water column irradiance attenuation were recorded over the
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
707
course of the experiments. One possibility that may account for the skewed distribution is that the ambient irradiance received by the simulated in situ samples was greater than that received by the in situ samples; this may have resulted from the irradiance directly received by the simulated in situ samples being enhanced by light reflected from the ship’s superstructure and deck fittings. On the assumption that the in situ data are less influenced by light and temperature factors, these results have been reported wherever possible. At those locations where it was only possible to carry out simulated in situ incubations, a factor derived from the simulated in situ: in situ comparison data obtained for each cruise was used to transform the simulated in situ production estimates to an equivalent in situ value. These data are identified in the figures as appropriate.
3. Results 3.1. Distribution of chlorophyll a The distribution of column chl a concentrations for the three size-fractions estimated over the euphotic zone is shown for the outward and return legs of D210 and D212 in Fig. 3. For the outward leg of D210, three regimes based on the relative distribution of the three size-fractions were identified (Fig. 3a). Between stations AS5 and A2, that is to the north of the appropriate mean location of the axis of the Findlater Jet (Brock et al., 1993), considerable variability was apparent in the contribution of the three size-fractions to the column chl a concentration. At three sampling locations, stations AS5, A1 and A2, chl a was predominantly associated with the '18 lm fraction with a maximum concentration in excess of 40 mg m\ being recorded at station A1. At the remainder of the stations in this regime, the concentrations of chl a in the 0.2—2 and 2—18 lm size-fractions were approximately equal, with a mean of the order of 10 mg m\, but with a considerably lower concentration associated with the '18 lm fraction. It is likely that the variability present at the inshore end of the transect reflected the influence of mesoscale processes and structure within the inshore upwelling zone in controlling phytoplankton growth (Elliott and Savidge, 1990; Young and Kindle, 1994). A consistent chl a distribution was recorded at stations A5—A7 and A7—A9 on both the outward and return legs, respectively, of D210 (Fig. 3a and b), with maximal concentrations, averaging 13.2 mg m\, associated with the 0.2—2 lm size-fraction and minimal concentrations of the order of 2.8 mg m\ consistently associated with the '18 lm fraction. Particularly stable chl a conditions were noted at stations A7—A9 on the return leg. The relative distribution of chl a between the size-fractions at these stations was similar to that recorded at those stations on the outward leg between AS5 and A2 not characterised by '18 lm fraction dominance. Station A3 on the outward leg of D210 was located at the transition between the coastwards stations AS5—A2 and the offshore stations A5—A7. The column chl a concentration at this station over the four consecutive days of observation was characterised by an approximately equal contribution from all three size-fractions. The average column
708
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 3. Distribution of size-fractionated chl a concentration along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
709
chl a concentration was 10.8 mg m\, a value comparable to that recorded for the 0.2—2 lm size-fraction at the offshore stations. Conditions similar to those found at A3 on the outward leg were also apparent at the three innermost stations, this is A10—A1, on the return leg of D210 (Fig. 3b). During D212, comparable conditions were observed at all stations on both the outward and return legs. Owing to the limited number of stations sampled on the return transect, the data for both legs are presented in Fig. 3c. The relative contributions of the three size-fractions were similar to those at the offshore stations during D210, with a clear dominance of the 0.2—2 lm chl a fraction at all stations (average chl a concentration 12.4 mg m\ on the outward leg) and minimal concentrations being recorded for the '18 lm fraction. The vertical distribution of size-fractionated chl a concentration across the major part of the outward transect of D210 (Fig. 4a) showed an essentially homogeneous distribution over the euphotic zone, with the dominance of the '18 lm fraction being clearly exemplified inshore of station A3. At the southernmost end of the transect at station A7 there was clear evidence for the presence of a subsurface chlorophyll maximum with the phytoplankton community dominated by the 0.2—2 lm fraction. On the return leg of D210 (Fig. 4b), the chl a associated with the '18 lm fraction was distributed homogeneously throughout the euphotic zone across the transect. However, stratification was apparent in the corresponding vertical distribution of the two smaller size-fractions especially between station A7—A9, with the higher concentrations being recorded in the deeper part of the euphotic zone. The comparable data for the outward and return legs of D212 (Fig. 4c) again demonstrated a relatively homogeneous vertical distribution of chl a concentration for all size-fractions inshore of station A3. Although not immediately apparent in Fig. 4c, a subsurface chlorophyll maximum was also present at stations A6—A8 during cruise D212 as indicated from continuous CTD fluorometric data and also from continuous UOR data collected across the transects (M. Pinkerton, pers. comm.). However, the magnitude of the increased chl a concentration associated with the subsurface maximum at these stations (typically 0.25—0.3 mg m\) was relatively low compared to the average concentrations in the surface mixed layer (0.1—0.15 mg m\), such that the feature may not have been clearly resolved in Fig. 4c. As for the previous cruise, chl a concentrations in the subsurface maximum were dominated by the 0.2—2 lm fraction. The distribution of total chl a concentrations across the transect for both D210 and D212 (Fig. 5) showed surprisingly little variation with the exception of the inshore stations on D210 dominated by the '18 lm size-fraction and the first sampling day at station A7 on the same cruise, which was dominated by the 0.2—2 lm chl a fraction, confined chiefly to the subsurface chlorophyll maximum. The maximum chl a concentration on the outward leg of D210 observed at station A1 was 69.1 mg m\, compared with a similar value of 60.2 mg m\ at the first sampling day at station A7. The average chl a concentration at those stations across the transect where the contribution from the three SFs was approximately similar was 28.8 mg m\. On the return leg of D210, column chl a concentrations increased northwards across the transect (Fig. 5b), with a minimal value of 20.2 mg m\ at station A7 and a maximal
710
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 4. Vertical distribution of size-fractionated chl a concentrations (mg m\) along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Line across transect denotes lower limit of sampling, assumed to be base of euphotic zone.
value of 32.0 mg m\ at station A3. The mean chl a concentration across the transect was 26.9 mg m\. For the outward leg of D212, the corresponding minimum and maximum column chl a values (Fig. 5c) were 12.8 and 21.1 mg m\, respectively, with a mean value across the transect of 17.8 mg m\. No distributional trend was apparent along the transect. Consistently higher values were recorded (mean" 22.7 mg m\) from the three stations on the return leg of D212, suggesting considerable temporal mesoscale variability in the distribution of chl a within the subsurface chlorophyll maximum, the main contributor to column chl a concentrations at the outer stations.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
711
Fig. 4. Continued.
3.2. Distribution of primary production A broad similarity was evident across the transect for both D210 and D212 in the patterns of distribution of size-fractionated primary production (Fig. 6) and the corresponding distributions of column chl a concentration (Fig. 3). On the outward leg of D210, peaks in primary production by the '18 lm size-fraction were recorded at those stations inshore of A3 where the chl a concentrations exhibited a similar fraction dominance. Maximal production was consistently associated with the 0.2—2 lm size-fraction at the other stations in the inshore regime (Fig. 6a). Comparison of these data with the appropriate chl a concentrations suggest higher photosynthesis per unit chl a by the 0.2—2 lm fraction compared to the other fractions. A close overall similarity between the size-fractionated chl a and
712
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 4. Continued.
production values was apparent for the remaining stations sampled during D210 (Fig. 6a and b) with the exception that the chl a peak noted for the 0.2—2 lm size-fraction on the first sampling day at station A7 was not reflected in the associated production value. Although the contribution to production on both the outward and return legs of D212 was dominated by the 0.2—2 lm fraction, as was noted for the comparable chl a data, relatively more variability was observed in the distribution of production (Fig. 6c). In particular, production of the smallest size-fraction was high at the three innermost stations of the transect and increased over the sampling period at station A3, remaining elevated through to station A5.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
713
Fig. 5. Distribution of total ('0.2 lm) chl a concentration along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
714
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 6. Distribution of size-fractionated primary production along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
715
On the outward leg of D210 (Fig. 7a), total column production was highest at the inshore stations AS5 and AS2 and also on the first sampling day at A1, with column production averaging 3130 mg C m\ d\. The sharp decrease in production at A1 after the first sampling day, to values between 810 and 1250 mg C m\ d\ was assumed, as previously, to reflect a change in hydrographic conditions owing to mesoscale variability. A secondary maximum of approximately 1800 mg C m\ d\ was evident at station A2 followed by a general decrease in values seawards with minimal values, averaging 1020 mg C m\ d\, being recorded at the two outermost stations of the transect. On the return leg of D210 (Fig. 7b), minimal production, of the order of 900 mg C m\ d\ was again recorded at the two outermost stations with values increasing to an average of 1500 mg C m\ d\ at the inner stations. During D212, column production was in the range 1000—1300 mg C m\ d\ at the two inshore stations AS5 and AS3 and, as during D210, demonstrated high production only on the first day at A1 (Fig. 7c). Temporal variability was also recorded at station A3 where column production on successive days increased from 300 to '1000 mg C m\ d\. Offshore from A3, the trend towards lower rates of production ((600 mg C m\ d\) at the outermost stations was again apparent. As indicated previously from the data showing the vertical distribution of the chl a size-fractions (Fig. 4), the subsurface chlorophyll maximum was only consistently defined at the outermost station associated with the oligotrophic waters as defined by the surface nutrient concentrations (Woodward et al., 1999). Detail of the distribution of the chl a size-fractions in the subsurface chlorophyll maximum is presented in Fig. 8a for a representative station (A7/49) sampled during D210. The data show a marked maximum chl a concentration of 0.40 mg m\ recorded at 63 m for the smallest size-fraction compared to an average of 0.18 mg m\ in the upper mixed layer. The comparable production data (Fig. 8b) demonstrate a secondary maximum of 9.8 mg C m\ d\ in the profile centred on 60 m. By defining the depth horizons of the subsurface chlorophyll maximum as the depths at which the chl a concentrations were 1.5 times greater than those in the upper 25 m of the water column, the contribution of production in the subsurface chlorophyll maximum to total column production by the three size-fractions was determined (Table 1). Significant contributions to column production by the subsurface chlorophyll maximum were recorded only at station A7, A8 and A9, with only the extreme southern station A7 demonstrating a clear subsurface chlorophyll maximum on both cruises. From consideration of the data for stations A7—A9, it is apparent that the nanophytoplankton (2—18 lm) fraction made the greatest percentage contribution to the subsurface chlorophyll maximum production. However, on the basis of absolute carbon fixation, maximum production was attributable to the picophytoplankton (0.2—2 lm) fraction, this reflecting the higher rates of production by the smallest fraction.
4. Discussion The observations described in this report are considered to be of particular significance since they spanned both the end of the SW monsoon season in the
716
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
Fig. 7. Distribution of total ('0.2 lm) primary production along ARABESQUE transect on (a) D210 outward leg, (b) D210 return leg and (c) D212 outward and return legs. Replicate data sets obtained at selected stations shown.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
717
Fig. 8. Vertical distribution of (a) size-fractionated chl a concentrations and (b) primary production at station A7/49, Cruise D210. Table 1 Contribution to column primary production (mg C m\ d\) by subsurface chlorophyll maximum for the three defined size-fractions and by total phytoplankton community, RRS Discovery cruises 210 and 212 SCM (%)
2—18 lm column production
SCM (%)
'18 lm column production
SCM (%)
Total column production
SCM (%)
D210 outward A5 180 A7 47
3.6 11.0
383 159
0.2 59.3
966 764
0 57.4
1530 970
0.9 55.4
D210 return A8 45 A9 42
15.0 3.3
134 221
52.4 71.6
760 966
50.4 30.4
939 1229
49.0 36.9
D212 outward and return A3 35 7.8 A7 39 31.3 A8 82 3.7
42 61 59
31.8 34.8 18.7
383 445 433
0 22.7 0
561 545 575
3.5 24.7 2.4
Station
0.2—2 lm column production
northwest Indian Ocean and the initiation of the subsequent relaxation phase, together with a period well within the relaxation phase (Burkill, 1999). The demonstration of the presence of phytoplankton communities with contrasting size-fraction characteristics to the NW and SE of station A3 on the initial leg of D210 corresponded
718
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
with comparable data obtained from phytoplankton taxonomic studies (Tarran et al., 1999). To the NW of station A3, the phytoplankton assemblage was shown to be dominated by diatoms at those locations exhibiting marked dominance by the '18 lm size-fraction, with the cyanobacteria contributing maximally to the smaller size-fractions. To the SE of station A3 through to station A6, the phytoplankton biomass was dominated by coccolithophorids, particularly at depths greater than 20 m, and also, again, by cyanobacteria. Numerically, both dinoflagellates and autotrophic pico-eucaryotes also exhibited maximal abundances at these stations (Tarran et al., 1999). A marked decrease in all principal phytoplankton taxa was evident at the outermost station A7, these being replaced by a maximum in the prochlorophytes. A local minimum in the biomass of the majority of the taxa was evident at station A3, which marked the transition between the two size-fraction-defined communities. The demonstration of a boundary in the distribution of the phytoplankton communities in the vicinity of station A3 may be related tentatively to the wind distribution and hydrography observed during D210. Burkill (1999) presents hourly averages of wind stress and direction recorded during both D210 and D212. During D210 the wind was confirmed as blowing consistently from a southwesterly direction until approximately 20 September during occupation of station A7 when a marked shift to a northerly quarter and reduced speeds were noted. This rapid change in wind velocity and the timing is characteristic of the end of the SW monsoon in the northwestern Indian Ocean (Savidge et al., 1990). In the earlier part of D210, a maximum in wind speed of the order of 23 m s\ was recorded between 4 and 5 September, immediately after RRS Discovery arrived in the coastal waters of southern Oman. During the subsequent detailed survey of the coastal area through until 9 September, the wind speed gradually decreased to an average of 13 m s\. Savidge et al. (1990) previously have shown the speed of the SW monsoon wind to vary temporally in the southern Oman coastal region during the monsoon season. However, following commencement of the main transect, a secondary maximum in wind strength of approximately 18 m s\ was observed at station A2. South-eastwards of A2 to the furthest station A7, the mean wind strength consistently decreased with a minimum of 5 m s\ recorded at A7 on 19 September. The progressive decrease in the wind speed was accompanied by an increasing westerly component in the wind direction, such that at station A7 on 19 September, the wind was effectively from a westwards as opposed to a south-westerly direction. The mean position of the main axis of the SW monsoon, that is the Findlater Jet (Findlater, 1969), coincides approximately with the location of station A3 (Brock et al., 1993). The Jet is usually assumed to mark the boundary between the coastal and immediate offshore upwelling regions defined by the positive wind stress curl and the offsore downwelling region associated with the negative wind stress curl (Brock et al., 1991). Some local variation in the position of the Jet may be expected over the course of the SW monsoon season and it is hence assumed that the position of the Jet during D210 was represented by the secondary wind speed maximum recorded adjacent to station A2. This interpretation is consistent with the decreasing wind speed and westwards shift in wind direction observed to the south of the Jet, representative of a negative wind stress curl. From the data available, it is not clear whether the wind
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
719
speed maximum recorded adjacent to the coast was a transient feature or was associated with a secondary circulation related to the proximity of the coast. Hydrographic data presented by Woodward et al. (1999) indicate the presence of a regime boundary in the vicinity of station A2. The marked surfaceward gradient of isopycnals and nutrient isolines to the north of A2 is assumed to reflect active upwelling. Although upwelling also was suggested at A3 by the NO distribution data, this was not confirmed by the corresponding density distribution. Between A2 and A5, a progressive deepening of the isopycnals was recorded, consistent with downwelling conditions, as may be typically found to the south of the Jet. The relatively high surface nutrient concentrations also recorded between A2 and A5 suggest that the downwelling water spread southwards from its area of origin adjacent to the Jet. To the south of A5, the data of Woodward et al. (1999) indicated strongly oligotrophic conditions. The occurrence of similar size-fractionated chl a concentrations at stations A10—A1 on the return leg of D210 to those recorded at station A3 on the outward leg of D210 may also reflect transition conditions. Whereas, however, on the outward leg of D210 the community reflected a geographically defined transition zone, the transition conditions at A10—A1 on the return leg of D210 reflected a temporal change in the wind conditions. On the outward leg, strong southwesterly winds prevailed at the northernmost stations while on the return leg some three weeks later relatively calm conditions prevailed with only light northerly winds (Burkill, 1999) representative of the transition between the SW monsoon season and the initiation of the relaxation phase. Clearly, with the onset of calm conditions, the upwelling and associated surface nutrient enrichment processes would cease resulting in a change in the structure of the prevailing phytoplankton community as observed. A broad correspondence was apparent between the surface chl a concentrations and column primary production values recorded in the present study and the large scale monthly distributions of these parameters as presented by Krey and Babenerd (1976). It is more instructive, however, to compare the present data with previous more locally restricted data sets relevant to the particular timings of the present surveys. The most appropriate comparison can be made with data obtained by Ryther et al. (1966) between late September and early November 1963 from the upwelling zone off Oman. For stations north of 18°N, which may arbitrarily be assumed to be in the Omani coastal upwelling zone, Ryther et al. (1966) recorded a mean production of 2230 mg C m\ d\ (n"18) compared to a mean obtained at stations AS5 and AS3 in the present study of 3080 mg C m\ d\. Considerably variability was evident in the values of Ryther et al. (1966) from this region (range: 840—6580 mg C m\ d\), consistent with the presence of marked mesoscale variability. Between 16 and 18°N, the zone dominated by the positive curl upwelling, the data from Ryther et al. (1966) show a decrease in the mean production to 1550 mg C m\ d\ (n"3) similar to the decrease recorded at stations A1 and A2 (mean"2050 mg C m\ d\, n"5). In the latter case, the mean was skewed by a high initial value at station A1 (Fig. 7a), which may have resulted from a transient influence from the immediately adjacent coastal upwelling zone. South of the mean position of the Jet axis between approximately 9°—16°N, the average production at stations A5—A7 was 910 mg C m\ d\ (n"6) compared to a corresponding value of 1060 mg C m\ d\ (n"11) derived from the
720
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
data of Ryther et al. (1966). From this comparison it is apparent that there was considerable correspondence between the present data and that obtained at a comparable time of year in 1963. Further comparison may be made between the mean chl a concentrations in the photic layer and column production values recorded from the present cruises and those obtained by Owens et al. (1993) along a similar transect carried out during September—October 1986 (Table 2). It is clear that there was a considerable degree of correspondence between the chl a concentrations, but in general production values for 1994 were considerably higher than those recorded for 1986. This effect was particularly marked at the two furthest offshore stations in strongly oligotrophic waters where production was approximately three times higher in 1994. The factors responsible for this difference are not obvious, but may possibly be related to differences in the intensity of the SW monsoon between contrasting year (Brock and McClain, 1992) or the location of the position of maximum windstress (Haake et al., 1993). Such influences may have altered the phytoplankton community composition with a consequent effect on the phytoplankton production: biomass ratio. Further insight into the problems encountered in the interpretation of chl a data from the NW Indian Ocean may be obtained from comparison of the 1994 data with observations and predictions derived from CZCS satellite imagery of the region. CZCS data obtained by Brock and McClain (1992) were used to emphasize the difference in the phytoplankton biomass between the Omani coastal upwelling area and the offshore upwelling area influenced by the positive wind stress curl to the north of the Findlater Jet. The data, which referred to the late phase of the SW monsoon and which were obtained over the four years 1979—1982, indicated typical surface chl a concentrations of the order of 5 mg m\ to the south in the positive curl zone. These values are very considerably higher than the mean photic layer values obtained both in the present investigation and by Owens et al. (1993) (Table 2: stations 16 and 14). In addition, the data in Table 2 do not indicate a significant difference in the surface photic layer chl a concentrations between the two zones. The interannual consistency within the separate shipboard and CZCS imagery data sets suggests the possibility of a calibration effect in the satellite data, perhaps reflecting the difficulties of obtaining good calibrations in a variable oceanographic environment. In relation to the Table 2 Mean mixed layer chl a concentrations (mg m\) and column production values (mg C m\ day\) obtained at comparable stations in NW Indian Ocean in 1986 and 1994. 1986 data from Owens et al. (1993) Station no.
Chl a concentration
Column production
1986
1994
1986
1994
1986
1994
16 14 5 3
AS5, AS2 A2 A5 A7
1.66 1.63 0.38 0.18
1.63 1.30 0.38 0.26
2668 1391 466 300
3283 2042 1530 1026
Mean of two values.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
721
differences in the phytoplankton biomass status of the coastal and offshore upwelling zones, it is at present not clear to what extent these arbitrarily defined regions may be considered as separate entities (Brock and McClain, 1992) or whether the biomass in the offshore area is influenced by export from the coastal upwelling zone as mediated by mesoscale eddy and offshore filament activity (Young and Kindle, 1994). Possibly the most valuable insight into the phytoplankton dynamics of the NW Indian Ocean has been provided by Brock et al. (1993, 1994). Both studies modelled the interactions of the main physical controls on phytoplankton growth over the four main seasons in the area: the SW and NE monsoon seasons and the two intermonsoon seasons. More valuable comparisons can be made between the present data and the earlier study in which production over the whole water column was modelled as opposed to production in the surface mixed layer. One of the major predictions of the 1993 study, which considered conditions at a station in the offshore positive curl upwelling zone, was of a sharp and major decrease in total column production from approximately 2000 to 500 mg C m\ d\ following the cessation of the SW monsoon wind in late September. In the present study the average column production at stations A1 and A2 in the offshore upwelling zone on the outward leg of D210 when the SW monsoon wind prevailed was 1670 mg C m\ d\, a value comparable to that given by Brock et al. (1993). Although the SW monsoon had relaxed on the return leg of D210, the column production at station A1 remained at 1440 mg C m\ d\, indicating no substantial decrease in the two week period following the relaxation of the SW wind. At this time, the '18 lm size-fraction still represented a substantial contribution to production at this location. During D212, some seven weeks into the intermonsoon season, the mean column production at station A1 and A2 had decreased to 350 mg C m\ d\, with a clear dominance in production shown by the 0.2—2 lm fraction. Thus there is a considerable degree of agreement between the production values predicted by the model of Brock et al. (1993) for their station in the positive curl region and those observed in the present study, although it is clear that a finite time is required for the system to adjust to the lower production level following relaxation of the SW monsoon wind. Although Brock et al. (1993) predicted that 60% of the total production in the positive curl region would be contributed by the subsurface chlorophyll maximum, the present observations indicated that, with the exception of station A3 on D221, significant contributions to column production by the subsurface chlorophyll maximum were only made at the southernmost stations of the transect in the oligotrophic waters. UOR data obtained by M. Pinkerton (pers. comm.) confirmed that the subsurface chlorophyll maximum was restricted to the furthest oceanic stations on both ARABESQUE cruises. In the present investigation, the major contribution to subsurface chlorophyll maximum production at these sites was by the picoplankton (0.2—2 lm size-fraction), in agreement with the observations made by Jochem et al. (1993) at a similar location in May 1987. One of the more equivocal questions in relation to the NW Indian Ocean concerns the level of production in the regions away from the upwelling zones, particularly those areas to the south of the axis of the main SW monsoon wind jet. Based on the very low concentrations of new nutrients typically observed in the surface mixed layer
722
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
(Wyrtki, 1971), it was assumed in early studies of the Indian Ocean that this region would exhibit low production similar to that of other sub-tropical oligotrophic oceanic areas. This view was supported by the early data of Ryther et al. (1966) and Kabanova (1968), in which production levels generally (200 mg C m\ d\ were recorded, and later by that of Owens et al. (1993) for their station 3 in September 1987 where production averaged 300 mg C m\ d\. However, this conclusion concerning low production is not clear-cut. Coarsely resolved temporal production data presented by Qasim (1982) indicated high production levels of the order of 900 mg C m\ d\ in the NW Indian Ocean in both the SW and NE monsoon seasons, that is similar to the values recorded in the present study. The more localised data set obtained by Jochem et al. (1993) during a Lagrangian experiment at a station in the oligotrophic region during the spring intermonsoon season in May 1987 demonstrated a high average production of 730 mg C m\ d\. In this latter study maximal production, as in the present investigation at the comparable oligotrophic stations, was associated with the picophytoplankton. Clearly, the data presented in the present investigation support the concept of relatively high production in the offshore oligotrophic region of the NW Indian Ocean, although the temporal and spatial resolution of the data sets currently available do not allow firm conclusions to be drawn on this subject. It may be noted, however, that the hydrographic data presented for both cruises by Woodward et al. (1999) for the strongly oligotrophic stations indicate substantive NO concentrations either in or slightly above the thermocline. The vertical transfer of nutrients, which may result from the influence of mixing, is likely to be reflected in increased production in the subsurface chlorophyll maximum and thus assist in maintaining increased column production values. This phenomenon clearly has implications for the modelling of production in the Indian Ocean owing to the spatial extent of the strongly oligotrophic region. It is hoped, however, that the data obtained during the intensive Indian Ocean JGOFS Programme will assist in resolving the issue. Acknowledgements We wish to thank the Principal Scientists of RRS Discovery cruises 210 and 212, Professor R.F.C. Mantoura and Dr. P.H. Burkill respectively, for their efforts in enabling the UK ARABESQUE Indian Ocean Programme and for leading the cruises. Our thanks go also to the Captains and crews of RRS Discovery and also to Alan Pomroy and Ian Joint of the Plymouth Marine Laboratory without whose considerable assistance this work would not have been possible. The generous help of Amersham International plc towards the provision of C radiotracer is gratefully acknowledged. References Banse, K., 1987. Seasonality of phytoplankton chlorophyll in the central and northern Arabian Sea. Deep-Sea Research 34, 713—723. Banse, K., McClain, C.R., 1986. Winter blooms of phytoplankton in the Arabian Sea as observed by the Coastal Zone Color Scanner. Marine Ecology Progress Series 34, 201—211.
G. Savidge, L. Gilpin / Deep-Sea Research II 46 (1999) 701—723
723
Barlow, R.G., Mantoura, R.F.C., Cummings, D.C., 1999. Monsoonal influence on the dstribution of phytoplankton pigments in the Arabian Sea. Deep-Sea Research II 46, 677—699. Bianchi, T.S., Lambert, C., Biggs, D.C.,1995. Distribution of chlorophyll a and phaeopigments in the northwestern Gulf of Mexico: a comparison between fluorometric and high-performance liquid chromatography measurements. Bulletin of Marine Science 56, 25—32. Brock, J.C., McClain, C.R., 1992. International variability in phytoplankton blooms observed in the northwestern Arabian Sea during the southwest Monsoon. Journal of Geophysical Research 97, 733—750. Brock, J.C., McClain, C.R., Luther, M.E., Hay, W.W., 1991. The phytoplankton bloom in the northwestern Arabian Sea during the southwest Monsoon of 1979. Journal of Geophysical Research 96, 20 623—20 642. Brock, J.C., Sathyendranath, S., Platt, T., 1993. Modelling the seasonality of subsurface light and primary production in the Arabian Sea. Marine Ecology Progress Series 101, 209—221. Brock, J.C., Sathyendranath, S., Platt, T., 1994. A model study of seasonal mixed-layer primary production in the Arabian Sea. Proceedings of the Indian Academy of Scieneces (Earth and Planetary Sciences) 103, 65—78. Burkill, R.H., 1999, ARABESQUE: An overview. Deep-Sea Research II 46, 529—547. Elliot, A.J., Savidge, G., 1990. Some features of upwelling off Oman. Journal of Marine Research 48, 319—333. Findlater, J., 1969. A major low-level air current near the Indian Ocean during the northern summer. Quarterly Journal of the Royal Meteorological Society 95, 362—380. Haake, B., Ittekkot, V., Rixen, T., Ramaswamy, V., Nair, R.R., Curry, W.B., 1993. Seasonality and interannual variability of particle fluxes to the deep Arabian Sea. Deep-Sea Research I 40, 1323—1344. Jochem, F.J., 1995. Phototrophic picoplankton community structure in three different pelagic regimes in the Arabian Sea. Marine Ecology Progress Series 117, 307—314. Jochem, F.J., Pollehne, F., Zeitschel, B., 1993. Productivity regime and phytoplankton size structure in the Arabian Sea. Deep-Sea Research II 40, 711—735. Kabanova, Y.G., 1968. Primary production of the northern part of the Indian Ocean. Oceanology 8, 214—225. Krey, J., 1973. Primary productivity in the Indian Ocean, I. In: Zeitschel, B. (Ed.), The Biology of the Indian Ocean, Ecological Studies, Analysis and Synthesis, vol. 3. Springer, Berlin, pp. 115—130. Krey, J., Babenerd, B., 1976. Phytoplankton production. Atlas of the International Indian Ocean Expedition. Universitat Kiel, Institut fur Meereskunde, 70 pp. Owens, N.J.P., Burkill, P.H., Mantoura, R.F.C., Woodward, E.M.S., Bellan, I.E., Aiken, J., Howland, R.J.M., Llewellyn, C.A., 1993. Size-fractionated primary production and nitrogen assimilation in the northwestern Indian Ocean. Deep-Sea Research II 40, 697—709. Qasim, S.Z., 1977. Biological productivity of the Indian Ocean. Indian Journal of Marine Sciences 6, 122—137. Qasim, S.Z., 1982. Oceanography of the northern Arabian Sea. Deep-Sea Research 29, 1041—1068. Ryther, J.H., Hall, J.R., Pease, A.K., Bakun, A., Jones, M.M., 1966. Primary organic production in relation to the chemistry and hydrography of the western Indian Ocean. Limnology and Oceanography 11, 371—380. Sathyendranath, S., Stuart, V., Irwin, B.D., Maass, H., Savidge, G., Gilpin, L., Platt, T., 1999. Seasonal variations in bio-optical properties of phytoplankton in the Arabian Sea. Deep-Sea Research II 46, 633—653. Savidge, G., Lennon, J., Matthews, A.J., 1990. A shore-based survey of upwelling along the coast of Dhofar region, southen Oman. Continental Shelf Research 10, 259—275. Tarran, G.A., Burkill, P.H., Edwards, E.S., Woodward, E.M.S., 1999. Phytoplankton community structure in the Arabian Sea during and after the SW Monsoon, 1994. Deep-Sea Research II 46, 655—676. Woodward, E.M.S., Rees, A.P., Stephens, J.A., 1999. Influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea. Deep-Sea Research II 46, 571—591. Wyrtki, K., 1971. Oceanographic Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, 531 pp. Young, D.K., Kindle, J.C., 1994. Physical processes affecting availability of dissolved silicate for diatom production in the Arabian Sea. Journal of Geophysical Research 99, 22 619—22 632.