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A cool upwelling filament off Namibia, southwest Africa: preliminary measurements of physical and biological features
Deep-SeaResearch.Vol. 37. No. I I. pp. 1753-1772, 1990. Printed in Great Britain.
0198-0149/90 $3.00 + 0.00 ~) 1990 Pergamon Press plc
A cool upweiling filament off Namibia, southwest Africa: preliminary measurements of physical and biological features F. A. SHILLINGTON,*W. T. PETERSON,?:~L. HlYrCHINGS,']"T. A. PROBYN,* H. N. WALDRON* and J. J. AGENBAGI" (Received 29 August 1989; in revisedform 4 June 1990; accepted 21 June 1990) Ahstrset--Mesoscale physical and biological measurements were made in a cool upwelling filament in the Benguela Current system, off Namibia in February 1988. The filament had been visible on a satellite infra-red image 2 weeks prior to the survey, and was found to be in a decaying state. Temperature, salinity, 0-50 m integrated nitrate, ammonium, oxygen and plankton data have enabled a consistent picture of the structure to be formed, involving four different water bodies. These are (1) typical continental shelf upwelled water inside the shelf break, (2) filament water characterized by salinity of 35.2, high surface integrated nitrate, small phytoplankton cells, and large numbers of salps, (3) warm, blue oligotrophic water to the north, and (4) a warm, high salinity anticyclonic eddy to the south. It is suggested that this southern eddy may have originated from the Agulhas Current near Cape Town. Anomalously low nitrate concentrations, that could not be explained, were found in a 100 m thick layer between 100 and 200 m depth on the southern edge of the structure.
INTRODUCTION
EASTERNboundary currents are well known for the upwelling features that occur near the coast. In these processes, large amounts of cool, deep, nutrient-rich water are transported away from the coast and equatorward. It has been shown recently (e.g. FLAMENTet al., 1985; Kol~so and HtrtER, 1986) that this offshore transport of cool water can occur in narrow bands perpendicular to the coast in so-called "filaments, jets, or squirts." The general physical mechanism for forcing of such structures several hundred kilometres from the coast is not yet fully understood. A possible candidate for the offshore forcing of filaments in the Benguela region would be a detached Agulhas ring or eddy moving up the west coast of southern Africa. LUTJEHAaMSand GORDON(1987) observed the shedding of an Agulhas ring at sea southwest of Cape Town, and O~ON and EVANS(1986) suggested that Agulhas rings may be among the most energetic in the world. They characterized the ring by the 10°C isotherm and found that peak velocities could be as high as 1 m s- l in an anticyclonic sense. Their results indicated a typical ring radius of 130 km. OLSON and EVANS (1986) quote an unpublished paper by MCCARTNEV et al. (in preparation), indicating observations of an Agulhas ring at 23°S, 5°W. This is near the latitude of the filament measurements reported here, although further to the west. *Department of Oceanography, University of Cape Town, Rondebosch 7700. South Africa. ?Sea Fisheries Research Institute, Private Bag X2, Roggebaai 8012, South Africa. ~:Present address: Marine Sciences Research Center, SUNY. Stony Brook. NY 11794. U.S.A. 1753
1754
F.A. SrIILLIN~rO.~et al.
Recent cruise data from an investigation of an Agulhas ring in the South Atlantic have shown that very long filaments from the Benguela region can be connected to passing Agulhas rings (DuUCOMnE RAE et al., 1989). GORDONand HAXaY (1990) have excellent GEOSAT altimetry evidence of a large number of Agulhas rings in the South Atlantic and, in particular, describe CTD measurements of a ring near 28°S, 9°E which is to the west of the study area described below. From the viewpoint of biological oceanography, one obvious question to be addressed is whether or not this localized offshore transport has any effect on productivity in the offshore regions influenced by the filaments. It is also important to know whether filaments can act as a mechanism for the export of carbon from the productive inner shelf region to the deeper ocean. Another freature of at least two eastern boundary current regions is that the zooplankton fauna resident on the poleward end of the current system are quite different from the fauna living nearer the equator (JoH~sor~ and BRINTON, 1963; FLEMXNGER, 1964). The faunal boundary between the ends of the upwelling system is a region of transition between boreal (austral) and subtropical fauna in the northern (southern) hemisphere. In the California Current system, this boundary is located at and just south of Cape Mendocino. In the Benguela Current system, the boundary is near Meob Bay, 24.5"S, to the north of Luderitz (CHAPMANand SHANNON, 1985; SHANNOn and PILLAR,1986; AG~NBA~ and SHAr~NOr~,1988). In both instances, the location of the faunal boundaries is coincident with regions of offshore filaments, suggesting that the filaments may help to act as physical barriers to the equatorward transport of fauna from the poleward side. Curiously, strong faunal boundaries do not seem to exist off the Humboldt Current system, since nearly all the copepod species common off Peru are found off Chile (PETERSONet al., 1988). The main copepod species associated with the South Atlantic are now described. Calanoides caranatus has a dipausal stage V, which enables it to cross upwellings throughout the Indian and Atlantic oceans. In the Benguela Current system, it is normally associated with the cool upwelling shelf waters. Paraclanus p a r v u s (sensulatu) is the most common in neritic upwelled waters on the shelf, with occasional occurrences far offshore, presumably after strong offshore advection. Rhincalanus nasutus is also characteristic of shelf waters in the Benguela region. Calanus australis is characteristic of the Agulhas Bank warm shelf waters. The numbers of these species drop rapidly in offshore waters (DE DECKER, 1984). Cool filaments extending offshore from the Benguela Current system have been seen on early satellite infra-red imagery (e.g. see Fig. 1 taken from V^N FOREESTet al., 1984) and a statistical study of filamentary features in the Benguela has been undertaken by LUTJEr~s_~S and SrocrroN (1986) using data from the geostationary satellite METEOSAT. The purpose of the present study was to locate a suitable filament and make in situ measurements of its physical, biological and plankton properties and distributions. A central hypothesis was to determine whether the filament water was nutrient rich, and thus capable of measurably enhancing the primary and secondary production of what would normally be oligotrophic water. It was anticipated that the question of carbon export away from the more productive shelf region might also be addressed. DATA AND METHODS On the basis of local past experience, it was derided to make the first measurements in an upwelling filament near Luderitz. This site has been identified as one which often
Physical and biological features of a cool upwelling filament
1755
exhibits a strong thermal feature, at least in the surface waters. Due to persistent cloud cover in the region of interest, the most recent satellite infra-red image available at the commencement of the cruise on 17 February 1988, was that of 2 February 1988. Since it was not possible to initially find the main feature at 26.5°S, 13.5°E (visible on the 2 February image) a smaller frontal feature centred at about 27.5°S, 14°E was chosen for sampling (Fig. 2a,b). On 20 February 1988, updated NOAA-9 satellite infra-red temperature information revealed that the filament originally chosen for sampling had moved half a degree to the north of its position as seen on the 2 February image. The ship was directed northwards to this larger filament and a transect was made from south to north measuring the surface temperature, salinity, and vertical temperature with XBTs while underway at 10 kn. From the northern edge, the ship's track turned west for 20 nm. A southward ~,.10
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Sea surface rneasuremems from thermosalinograph for 18-23 February 1988: (a) sea surface temperature, (b) surface salinity in parts per thousand.
1756
F.A. SrnLLIN~rONet al.
section of five CTD stations was executed across the filament during 21 February 1988, and a total of 17 stations were occupied both in and out of the filament over the next 3 days (Figs 2 and 4b). Figure 4b indicates the sampling network in relationship to the surface temperature field derived from the NOAA-9 satellite imagery for 19 February 1988. Physical measurements
Surface temperature and salinity were monitored continuously at 3 m depth while underway, using a Hewlett-Packard thermosalinograph. Vertical temperature and salinity profiles were made with a Neil Brown CTD, lowered to 300 m, with a sample written to magnetic tape every 2 m. Geopotential anomalies with respect to the 300 dB surface were later calculated using the international equation of state (GILL, 1982). Biological measurements
Ten 4-fitre Nisldn sample bottles were triggered on a rosette sampler at standard intervals 0, 10, 20, 30, 40, 50, 75, 100, 150 and 200 m to obtain samples for nutrients (nitrate, ammonium, nitrite, silicate, phosphate), oxygen and salinity. Nitrate concentration was measured according to the method of STmCKLANDand P~SONS (1972) modified for use with a Technicon Auto Analyser II (MOSTEt~T, 1983). Ammonium concentrations were determined in triplicate immediately on collection following GXASS•OFF (1976) scaled down to 5 ml. Samples for urea determination were stored frozen for about 2 weeks prior to analyses according to GRASSHO~ (1976), but scaled down to 5 ml sample volume. Dissolved oxygen analyses were performed by Wrinkler titration on board using the azide method of MONTC,O~ERYet al. (1964) to prevent interference from nitrite. Water samples for the determination of chlorophyll a concentration were collected from standard depths of 0, 10, 20, 30, 40, 50 and 75 m. Subsamples ranging in volume from 100 to 500 ml were taken from the Niskin bottles, filtered through GF/F filters, immediately homogenized in 90% acetone, allowed to extract for 30 rain in the dark at 18°C, and then read on a Turner Designs fiuorometer (YE~'TSC~ and MEm'ZEL, 1963). An additional subsample was taken from the Niskin bottle triggered at the depth of fluorescence maximum, poured through a 10/~m Nitex screen, and a 100-500 ml subsarnple filtered through a GF/F filter for analysis of chlorophyll a in the less than 10 ~m fraction. Total phytoplankton community nitrogen uptake was measured in 1 litre 200 ~m screened samples spiked with 0.1 mmol N m -3, either I~NH4CI (99.7 at%), NalSNO3 (99.6 at%) or CO(15NH2)2 (99.1 at%). Samples were collected from depths corresponding to the 100, 50, 25, 10 and 1% light levels. Additional 5-litre incubations at the 1% light depth were post-fractionated through a 20/~m plankton mesh and <2/~m Nuclepore filter to identify the predominant size classes involved in the different nutrient species. All experiments were initiated about midday local time and were run for 4 h under simulated in situ light conditions. Experiments were terminated by filtration onto 47 mm Whatman GF/F filters which were stored frozen for later analyses. Samples were prepared according to the Kjeldahl/Rittenberg oxidation procedure (FIEDLERand PROKSCH, 1975) for xsN:14N isotope analysis by emission spectroscopy. Absolute rates of nitrogen uptake were calculated using paniculate nitrogen concentrations measured at the end of the incubations, thereby accounting for any biomass increase that may have occurred during incubation (DU~DALE and WmK~XSON, 1986;
Physicaland biologicalfeaturesof a coolupwellingfilament
1757
COLLOS,1987). Ammonium uptake rates were corrected for isotope dilution using the exponential model of GILBERTet al. (1982). Particulate and aqueous 15N enrichment.in excess of natural abundance were used in all calculations.
Zooplankton measurements Zooplankton was collected with a WP-2 net (200 ~m mesh, 0.57 m diameter mouth) hauled vertically from 50 m to the surface. Samples were preserved with buffered formalin. In the laboratory two or four subsamples (volume of 3.9 ml) were taken with a piston pipette, and all individuals in each subsample were counted (usually about 30 animals). It is estimated that about 5% of the total zooplankton samples were subsampled and that the numbers in the major taxa varied between 200 and 400 animals. Most taxa were identified to species. For measurements of gut fluorescence and egg production, individual zooplankton were first collected by suspending a 0.5 m diameter, 200~tm mesh net fitted with a 2-litre cod end at depths of 10-20 m. The net sampled for 5 rain while the ship drifted, a technique that produces damage-free copepods. Immediately upon being brought aboard the contents of the cod end were poured gently through a 900/~m Nitex mesh (to remove gelatinous zooplankton) into a bucket containing 15 litres of seawater. A subsample of this was quickly poured onto a 120/~m mesh and individuals were picked from the mesh with jeweller's forceps, placed into 10 ml centrifuge tubes, 90% acetone was added, then the capped tubes placed in a freezer overnight where the pigments extracted. After 24 h, pigment concentrations were determined with a Turner Designs Fluorometer. For egg production measurements, females of Centropages brachiatus, Calanus australis and Calanoides carinatus were picked from the sample of living zooplankton with a wide-bore pipette and incubated in 1 litre plastic bottles, 1 female per bottle for Calanus and Calanoides, and 3 per bottle for Centropages. Bottles were previously filled with 60/zm filtered seawater collected from 20 or 30 m (the depth of the Chl a maximum). Females were incubated for 24 h after which the contents were filtered onto a 60 ~tm Nitex screen, rinsed into a sample bottle and preserved with formalin. In the laboratory females and any eggs produced were enumerated. RESULTS
Surface features Figure 2 shows surface temperature and salinity which was sampled continuously from the underway thermosalinograph. There is a 3°C front across the northern filament, but only a 0.1 change in salinity. The surface temperature structure closely matches that from the NOAA-9 satellite image taken on 19 February (see Fig. 4) although this part of the survey was carried out between 20 and 23 February 1988. An objectively computer-contoured map of geopotential anomaly with respect to the 300 dB surface (Fig. 3) shows a dynamic high to the north and south of the study area (corresponding to the warm southern eddy in the satellite image for 2 February, not shown), with a saddle point along the line of Stas 7-12. This can be interpreted as a geostrophic flow to the southwest between Stas 5 and 6 and to the southeast between Stas 2 and 3. There is a sluggish flow towards the north across the line of Stas 13--17.
1758
F . A . SmLLINGION et aL
s
16~
12 ° E
13 °
1
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Fig. 3. Ob~tively computer-contoured geopotential anomaly with respect to 300 dB. Stations are numbered FC 2-17.
Figure 4a shows surface temperature and velocity vectors derived from feature tracking between two NOAA-9 images for 19 and 20 February 1988. The technique used to derive these vectors is described in AGENeAGand SHANNON(1988). From the few vectors derived in the region very close to the survey, it can be seen that the surface flow to the south of the filament is towards the west. The drift vectors on the northern side of the filament show some evidence of an anticyclonic (antidockwise) eddy of warmer water. Most of the vectors derived using this technique are closer to the coast and reflect a tendency for the surface water to be moving offshore to the west, in response to the strong southerly winds (up to a maximum of 20 m s -1) experienced on 18 and 19 February. Vertical structure within the filament is shown in Figs 5--7 for three legs sampled on 21, 22 and 23 February (Stas 2-6, 7-13 and 14-17, respectively). Results from the three different legs are now discussed in detail.
Section 1 (Fig. 5) At the north edge of the filament, Stas 2 and 3 (Fig. 5), there is a slight temperature inversion at about 30 m which is compensated by the salinity maximum. The ot section shows a strong pycnocline between 20 and 30 m which corresponds to a thermocline of some 4--5°C. From Fig. 5 it is clear that only Sta. 5 appears to be in the filament having slightly cooler, and relatively fresher (35.2) water. Both nitrate and ammonium concentrations show a very distinctive signal. In the upper 100 m, the nitrate section shows a high concentration tongue shoaling from the south between depths of 75--40 m, with values decreasing from 16--6 mmol N m -3, while ammonium reaches a maximum value of I mmol N m -3 in the upper 10 m (Stas 3 and 4) and between 50 and 75 m. Below 100 m, the nitrate concentration falls below 0.5 mmol N m - 3 in Stas 4--6. This region of low nitrate concentration has increasingly high values of ammonium (maximum of 2 mmol N m-3). Station 4 shows slight evidence of oxygen depletion (<3 ml 1-1). There is a Subsurface maximum in Chl a concentration at depths of
Physical and biological features of a cool upwelling filament
Fig. 1. NOAA-1 enhanced infra-red (channel 4 VHRR) image of the Benguela Current system along the west coast of southern Africa (15 June 1979) (after VAN F O ~ ' T et al., 1984).
1759
1760
F . A . SHILLINGTONet al.
(a)
Physical and biological features of a cool upwelling filament
1761
30--40 m along this and the second transect line. The maximum is most pronounced between Stas 2 and 4 and is weak at Stas 5, 6 and 7.
Section 2 (Fig. 6) The temperature section shows evidence of cool water uplift at Sta. 10 and from the salinity section it can be seen that Stas 9 and 10 show more uniform salinity values of around 35.2. There are intrusive tongues of high salinity water (35.5-35.6) from both the north and south of the filament at 40-50 m depth. The at section indicates that there is very little flow perpendicular to this line of stations except near the surface between Stas 13 and 11. Once again nitrate and ammonium signals are very distinctive. Nitrate concentration is generally low in the upper 100 m with highest values (10-14 mmol N m-3) at Stas 9 and 10 at 75-100 m depth. Ammonium on the other hand shows patchy high values at 30 m (up to 5 mmol N m -3) at Sta. 10 and at 100 m at Sta. 7. Between 100 and 200 m there is again a region of very low nitrate (<0.5 mmol N m -3) at Stas 7 and 8. The oxygen shows some lower values for Sta. 10. Chlorophyll a is at a maximum between 30 and 40 m, with most pronounced values at Stas 8-11.
Section 3 (Fig. 7) Temperature, salinity and at at these four stations were indicative of coastal inshore conditions, with evidence of slight uplift of the isotherms and the isopycnals towards Sta.
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2e" Fig. 4. (a) NOAA-9 infra-red image for 19 February 1988, with satelfite-derived surface drift vectors. (b) Positions of the sampfing network in relationship to the surface temperature field. Solid dots are station positions and upright numbers next to them are the station numbers, starting from Stu. 2. The station spacing is nominally 10 nmi (18.5 km). Surface temperatures are simplified from Fig. 4a.
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17, the closest inshore station of the cruise. Between Stas 14 and 15 there was a marked colour front at the surface with associated sharp changes in the biological variables. The vertical distribution of Chl a at Stas 16 and 17 suggest a different water body, and this is corroborated in the zooplankton data discussed below. Nitrate values generally increased with depth in this section. At 200 m there was a gradient of nitrate with the highest values at Sta. 17 (26 mmol N m-3). Ammonium has a tongue of high values at 50 m with a maximum at 75 m at Sta. 16. Within the Chl a maximum layer, all of the Chl a passed through a 10/~m screen at stations in the centre of the first two transect lines (Stas 3-5 and 8-11) (Fig. 8). The data on depth of Chl a maximum and 'the per cent C h l a < 1 0 ~um suggest a tongue of water
1763
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extending from southwest to northeast over the shelf. South and east of this tongue (Stas 6 and 7, and 14-17), a significant proportion of cells were larger than 10~m and there was no well-defined subsurface Chl a maximum. Water masses
A general description of the water masses in this region from historical data has been given by St'IANNON (1985). This is also summarized in Fig. 9 by the solid line which represents data from April 1986, Sta. SNO4, SNEC II (1986). The T-S curve shows a salinity maximum of 35.6 at 50 m depth. Antarctic intermediate water is present at a
1764
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temperature of 5°C, and a salinity of 34.6 at about 600 m depth• Data from Stas 2 to 6 have been plotted for our measurements down to 300 m. Clearly evident is a band of 35.2 salinity water with temperatures in the range 13-20°C. This is assumed to represent warmed, mature upwelled water that has been previously at the surface nearer the coast. Examination of the individual T-S curves for each station shows a background structure similar to SNO4 for Stas 2, 3 and 6. The CTD traces also show evidence of layering which can be interpreted as mixing at the edges of the filament. The distribution of the dominant zooplankton taxa also suggests that several water bodies were present during our study. A comparison of Fig. 10 with Fig. 8 shows that salps in particular were abundant only where the Chi a maximum was most pronounced and where all of the Chl a passed through the 10 pm screen (at Stas 5, 8, 9 and 10). Two copepod species which are numerically dominant in the Benguela upwelling system
Physical and biological features of a cool upwelling filament
1765
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(Paracalanus parvus and C. brachiatus) had their greatest abundance along the southeastern edge of the tongue. They were either uncommon or totally absent at stations along the northern edge of the tongue and only moderately abundant within the salp tongue (Fig. 10). C. carinatus was most abundant within the salp-rich zone, correlating with the enhanced Chl a concentration. Inshore of the tongue, at and inshore of the shelf break, a different water body was encountered, one dominated by euphausiid eggs and larvae and greatly reduced numbers of the three dominant upwelling copepod species. Taken as a whole the plankton data suggest the presence of four water bodies. For the zooplankton, (1) Stas 2, 3, and 13 (northern part of study area), (2) Stas 4, 5, 8, 9 and 10 (core of the salp distribution), (3) 6, 7, 11, 14 and 15 (east of the salp distribution the copepod species were typical of the coastal upwelling zone), and (4) a shelf-break group characterized by high abundances of eupausiid eggs, nauplii and calyptopis. The existence of four water bodies is corroborated in the Chl a data as well: (1) the northern water of relatively lower Chl a concentration, (2) the tongue of water of higher Chl a concentration with all cells passing through a 10~tm Nitex screen, (3) the southern and northern edges of 4.4
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1766
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Physical and biological features of a cool upwelling filament
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the tongue at Stas 5 and 6 and Sta. 13, characterized by a weak subsurface maximum and larger cells, and (4) water inside the shelf break characterized by the highest Chl a concentrations and largest cell size measured. Depth profiles of nitrogen uptake also provide evidence for distinct water bodies in the study area (Fig. 12). At Sta. 3, primary production was supported almost entirely by N!trogen uptake rate ( m real N m "3 h-I ) ,
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F . A . SmI.IJN6"roN et al.
1768
Table 1. Nitrogen uptake integrated to the 1% light level depth. The f-ratio is the proportion of nitrate in the total nitrogen uptake (EeeL£Y and PErE~SON, 1979). Units are/~mol N m -2 h -1 Sta.
3 8 14
Table 2.
NH~
NO~
Urea
f-ratio
138.2 51.5 339.0
3.8 19.5 222.7
2.5 52.9 589.8
0.03 0.16 0.19
Uptake of the three nitrogen species by different microplankton size classesat the 1% light level. Units are~mol N m -3 h -1 200-20/~m
Sta 3 8 14
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0.1 0.0 0.0
0.0 0.0 0.0
2.0 2.5 3.5
0.1 0.0 0.0
0.5 1.0 0.5
7.7 2.1 3.4
0.4 0.0 0.0
1.3 0.0 0.1
Table 3. Egg production data for Centropages brachiatus, Calanus australis and Calanoides carinatus. Units are in eggs female- 1 day- 1 Sta.
Centropages
Calanas
1
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--
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4.1 4.2 2.5 0.8 1.2 1.3 -41.0
8
9 10 11 14
15
Calanoides
0.5 0.0
5.0 4.0
1.9 5.5
ammonium, with a subsurface uptake maximum situated deeper in the water column than in the salp core (Sta. 8) and in the more typically coastal area (Sta. 14). The contribution from nitrate, as evidenced by the f-ratio (Eert~Y and ~ N , 1979), was elevated at Stas 8 and 14 as was the overall level of primary productivity (Table 1). Nitrogen uptake by different size classes also revealed distinct variations between the different water bodies. At Sta. 3 the uptake of ammonium by picoplankton (<3/~m) dominated phytoplankton nitrogen assimilation, whereas at Stas 8 and 14, nanoplankton (20-2 #m) and netplankton (200-20 #m), respectively, were the most active (Table 2). The egg production data (Table 3) shows that the copepods were moderately unstressed only at Stas 2, 6, 14 and 15, stations outside the tongue of water containing salps and small
1769
Physical and biological features of a cool upwelling filament
Table 4. Gut fluorescence data for female Centropages brachiatus, Calanus australis, Calanoides carinatus andfifth copepodites of Rhincalanus nasutus. A blank indicates that no gut data were availablefor the plankton at that station Sta.
Time
1 2 5 6 7 8 9 10 11 12 13 14 15 16 17
1600 0830 1815 2050 O720 0915 1120 1445 1820 2000 2200 0830 1230 1540 1820
Centropages
Calanus
Calanoides
Rhincalanus
0.11
0.07 0.31 0.10 0.19 0.42 1.01
0.75 0.47 0.27 0.09 1.05 0.78 0.43 0.90
0.08 0.12 0.63 O.52
0.58 0.70 0.22 0.86 1.34 1.17 3.16
phytoplankton cells. Within the tongue, egg production by C. brachiatus was about 15 eggs day -1. The higher value (50 eggs day -1) is about 50% of the maximum rate of egg production known for this species. For the other two species (C. australis and C. carinatus) high egg production rates were only observed at Stas 14 and 15 within water characterized by high abundance of these upwelling species and where Chl a concentration was relatively high. Rates observed at these two stations are near the maximum for these species. Within the salp plume egg production rates were about 10% of maximum for C. brachiatus and 5% of maximum for C. australis (based on unpublished data of Peterson). The gut fluorescence data, though sketchy, suggest that relatively good feeding conditions were encountered only in the water at stations outside the salp tongue (Stas 6 and 7 and 11-17. Table 4 lists the data). DISCUSSION
AND CONCLUSIONS
The measurements presented in this paper are the first of their kind in a filament of the Benguela Current system. Taken as a whole, the physical and biological variables measured form a consistent picture of the structure of the filament. This is summarized in the cartoon in Fig 13 where it can be seen that we encountered four significantly different water bodies. These consist of: (1) mature coastal water previously upwelled from about 300 m (temperature 14°(2, salinity 35.2), with dominantly coastal fauna (euphausiid eggs, nauplii and calyptopis larvae). This water was the productive area in terms of nitrogen uptake with a considerably active netphytoplankton community. (2) The filament itself, which was at least 2 weeks old when we started sampling it (judging from the satellite image of 2 February 1988), and hence in a slowly decaying state. This feature contained large numbers of salps. Uptake of ammonium by nanoplankton and picoplankton dominated phytoplankton nitrogen uptake. (3) The warm, high salinity surface water to the south which appears to be an anticyclonic eddy (which may have been spawned off the Agulhas
1770
F . A . SHILUNOTONetal.
.....
• -:-
~0
.',
'
- ~
•
Fig. 13. Cartoon showing the four main different water bodies found on this cruise. These are (1) the biologicallypoor blue water to the north of the filament, (2) the warm saline water to the south of the filament (an Agulhas ring or eddy?), (3) the filament water, and (4) the coastal upweiled shelf water. The proposed structure is supported by the physical, biological and plankton measurements. (Depths at the stations sampled ranged from 300 m inshore to greater than 3000 m offshore.)
Current to the south near Cape Town; see CHAPMANet al., 1987; GosOoN and HAXm,, 1990). (4) The warm, high salinity water to the north of the filament which was biologically inactive at the time of the cruise. Here picoplankton made the largest contribution to phytoplankton nitrogen uptake. This proposed structure is coherent in the 0-50 m integrated nitrate, (Fig. 11) small Chl a-bearing cells, salps, surface temperature from the satellite, and dynamic topography. Data from a cruise 2 weeks prior to ours ( B o ~ , personal communication) show a very low nitrate surface layer 80-100 m thick centred at 27°S, 13°18'E, which was about 70 km SSE of our Sta. 6. Due to the monitoring nature of the earlier cruise, their stations are more widely separated (60 km) in the region of the filament, and so only a limited comparison can be made with their data. Low nitrate concentrations in the euphotic zone, warm temperatures (21°C), and high salinity (35.6) are indicative of properties of an Agulhas eddy or ring (C'r~l,~t~,~ et al., 1987). As yet there is no convincing explanation for the 100 m thick depleted nitrate layer between 100 and 200 m depths at Stas 4-8. Dentitrification has been ruled out since the oxygen values are not exceptionally low at these depths. The most plausible explanation is that the low surface nitrates measured by BOYD(personal communication) in the upper 80100 m to the south of the filament, 2 weeks prior to this cruise, have been overlain by nutrient-rich water of coastal origin. Also difficult to explain is the high ammonium associated with this nitrate-poor water. We suggest that the filament is in a decaying state, and hence that there has been biological activity prior to our measurements. It is clear that the measurements reported here indicate that the feature sampled was rather shallow (about 50 m) in the biological variables. This appears to be different from some of the recent results from the Coastal Transition Zone measurements in the Cafifomia Current System (EOS, 1988), where the active filaments appear to be much
Physical and biological features of a cool upwelling filament
1771
d e e p e r . W e c a n o n l y a s s u m e t h a t t h i s d i f f e r e n c e is b e c a u s e w e s a m p l e d a n o l d , d e c a y i n g f i l a m e n t , at t h e e n d of t h e s e a s o n a l u p w e l l i n g c y c l e ( l a t e F e b r u a r y ) .
Acknowledgements--We wish to thank the captain and crew of the R.S. Benguela for assistance in the data collection at sea; Mr Ike Marais of the Hartebeestehoek Satellite receiving station for supplying the NOAA-9 computer compatible tapes in near real time; Mr Pierre Malan for handling the CTD and oxygen titrations on board; Mrs Verheye-Dua for the nutrient analysis, and the Benguela Ecology Programme of the Foundation for Research and Development, CSIR, for funds towards this project. Suggestions from an anonymous referee helped improve the final manuscript.
REFERENCES AGENBAGJ. J. and L. V. SHANNON(1988) A suggested physical explanation for the existence of a biological boundary at 24°30'S in the Benguela system. South African Journal of Marine Research, 6, 119-132. CHAPMAN P. and L. V. SHANNON (1985) The Benguela ecosystem II. Chemistry of related processes. Oceanography and Marine Biology Annual Reviews, 23, 183-251. CHAPMANP., C. M. DUNCOMBeRAE and B. R. ALLANSON(1987) Nutrients, chlorophyll and oxygen relationships in the surface layers at the Agulhas Retroflection. Deep-SeaResearch, 34, 1399-1416. COLLOSY. (1987) Calculations of 15N uptake rates by phytoplankton assimilating one or several nitrogen sources. Applied Radiation and Isotopes, 38, 275-382. DE DeCKEI~A. H. B. (1984) Near surface Copepod distribution in the south-western Indian and south-eastern Atlantic. Annals of the South African'Museum, 93, 303-370. DUCDALER. C. and F. C. WILKE~SON(1986) The use of 15N to measure nitrogen uptake in eutrophic oceans; experimental considerations. Limnology and Oceanography, 31,673-690. DUNCOMSERAE C. M., L. V. ShANnON and F. A. SmLUNOTON(1989) An Agulhas ring in the South Atlantic ocean. South African Journal of Science, 85,747-748. EPPLEY R. W. and B. J. PETERSON(1979) Particulate organic matter flux and planktonic new production in the deep ocean. Nature, 282,677-680. EOS (1988) Abstracts from the Coastal Transition Zone: filaments off California, 69, 1258-1261. F1EDLEi R. and G. PROKSCH(1975) The determination of nitrogen-15 by emission and mass spectrometry in biochemical analysis: a review. Analytica Chirnica Acta, 78, 1-62. FLAMEm"P., L. A ~ I and L. WASHSU~ (1985) The evolving structure of an upwelling filament. Journal of Geophysical Research, 90, 11765-11778. FLEMINOBRA. (1964) Distributional atlas of calanoid copepods in the California Current region, part 1, CaICOFI Atlas, no. 2, Scripps Institute of Oceanography, La Jolla, California. GILL A. E. (1982) Atmosphere-ocean interaction. Academic Press, London, 626 pp. GILaErr P. M., F. L~PSHULTZ,J. J. MCCABTnVand M. M. ALTAB~r(1982) Isotope dilution models of uptake and remineralisation of ammonium by marine plankton. Limnology and Oceanography, 27, 639-650. GORDON A. L. and W. F. HAXeV (1990) Agulhas eddies invade the South Atlantic ocean--evidence from GEOSAT altimeter and shipboard CTD. Journal of Geophysical Research, 95, 3117-3126. GItASSHOFFK. (1976) Methods of sea wateranalysis. Verlag Chemie, Weinheim. JORNSON M. W. and E. BmNTON(1963) Biological species, water masses, and currents. In: The sea, Vol. 2. Chapt. 18, John Wiley, New York, pp. 381-414. Kos~o P. M. and A. HuYEx (1986) CTD and velocity surveys of seaward jets off northern california, July 1981 and 1982. Journal of Geophysical Research, 91, 7680-7690. LtrrJEHA~S J. R. E. and P. L. STOCKTON(1986) Kinematics of the upwelling front off southern Africa. In: The 8enguela and comparable ecosystems, A. I. L. PAYNE, J. A. GULLANOand K. H. BRINK, editors, South African Journal of Marine Science, 5, 35-49. LUTJEt~m~S J. R. E. and A. L. GO~DON(1987) Shedding of an Agulhas ring observed at sea. Nature, 325, 138140. MONTOONE~YH. A. C., M. S. TnOM and A. CocxeuItN (1964) Determination of dissolved oxygen by the Winkler method and the solubility of oxygen in pure water and sea-water. Journal of Applied Chemistry, 14, 280296. MOSTnT S. A. (1983) Procedures used in South Africa for the automatic photometric determination of micro nutrients in sea-water. South African Journal of Marine Science, 1,189-198.
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OLSOND. B. and R. H. EVANS(1986)Rings of the Agulhas Current. Deep-Sea Research, 33, 27-42. PETEnSONW. T., D. F. ARCOS,G. B. McMANUS,H. DAM,D. BELLANTO~i,T. JOHNSONand P. TlSELIUS(1988) The nearshore zone during coastal upwelling: daily variability and coupling between primary and secondary production off central Chile. Progress in Oceanography, 20, 1--40. SHANNONL. V. (1985)The Benguela ecosystem 1. Evolution of the Benguela, physical features and processes. Oceanography and Marine Biology Annual Reviews, 23, 105-182. SHANNONL. V. and S. PILLAR(1986) The Benguela ecosystem II1. Plankton. Oceanography and Marine Biology Annual Reviews, 24, 65-170. SNEC II (1986) Datos informativos. Datos basicos de la Campana Oceanografica "SNEC II" en las costas de Namibia, E. GtrnERREZ,C. MAnRASEand P. Ruaw.s, editors, Vol. 18, 63 pp. STaIeKLANDJ. D . H. and T. R. PARSONS(1972) A practical handbook of sea-water analysis. Fisheries Research Board of Canada Bulletin, 167, 2nd edn, 310 pp. VAN FOREESTD., F. A. SmLLIN~TONand R. LEGECKTS(1984) Large scale, stationary, frontal features in the Benguela Current system. Continental Shelf Research, 3, 465-474. YENTSCtiC. S. and D. W. MENZF.L(1963) A method for the determination of phytoplankton chlorophyll and phaeophytic by fluorescence. Deep-Sea Research, 10, 221-231.
0198-0149/90 $3.00 + 0.00 ~) 1990 Pergamon Press plc
A cool upweiling filament off Namibia, southwest Africa: preliminary measurements of physical and biological features F. A. SHILLINGTON,*W. T. PETERSON,?:~L. HlYrCHINGS,']"T. A. PROBYN,* H. N. WALDRON* and J. J. AGENBAGI" (Received 29 August 1989; in revisedform 4 June 1990; accepted 21 June 1990) Ahstrset--Mesoscale physical and biological measurements were made in a cool upwelling filament in the Benguela Current system, off Namibia in February 1988. The filament had been visible on a satellite infra-red image 2 weeks prior to the survey, and was found to be in a decaying state. Temperature, salinity, 0-50 m integrated nitrate, ammonium, oxygen and plankton data have enabled a consistent picture of the structure to be formed, involving four different water bodies. These are (1) typical continental shelf upwelled water inside the shelf break, (2) filament water characterized by salinity of 35.2, high surface integrated nitrate, small phytoplankton cells, and large numbers of salps, (3) warm, blue oligotrophic water to the north, and (4) a warm, high salinity anticyclonic eddy to the south. It is suggested that this southern eddy may have originated from the Agulhas Current near Cape Town. Anomalously low nitrate concentrations, that could not be explained, were found in a 100 m thick layer between 100 and 200 m depth on the southern edge of the structure.
INTRODUCTION
EASTERNboundary currents are well known for the upwelling features that occur near the coast. In these processes, large amounts of cool, deep, nutrient-rich water are transported away from the coast and equatorward. It has been shown recently (e.g. FLAMENTet al., 1985; Kol~so and HtrtER, 1986) that this offshore transport of cool water can occur in narrow bands perpendicular to the coast in so-called "filaments, jets, or squirts." The general physical mechanism for forcing of such structures several hundred kilometres from the coast is not yet fully understood. A possible candidate for the offshore forcing of filaments in the Benguela region would be a detached Agulhas ring or eddy moving up the west coast of southern Africa. LUTJEHAaMSand GORDON(1987) observed the shedding of an Agulhas ring at sea southwest of Cape Town, and O~ON and EVANS(1986) suggested that Agulhas rings may be among the most energetic in the world. They characterized the ring by the 10°C isotherm and found that peak velocities could be as high as 1 m s- l in an anticyclonic sense. Their results indicated a typical ring radius of 130 km. OLSON and EVANS (1986) quote an unpublished paper by MCCARTNEV et al. (in preparation), indicating observations of an Agulhas ring at 23°S, 5°W. This is near the latitude of the filament measurements reported here, although further to the west. *Department of Oceanography, University of Cape Town, Rondebosch 7700. South Africa. ?Sea Fisheries Research Institute, Private Bag X2, Roggebaai 8012, South Africa. ~:Present address: Marine Sciences Research Center, SUNY. Stony Brook. NY 11794. U.S.A. 1753
1754
F.A. SrIILLIN~rO.~et al.
Recent cruise data from an investigation of an Agulhas ring in the South Atlantic have shown that very long filaments from the Benguela region can be connected to passing Agulhas rings (DuUCOMnE RAE et al., 1989). GORDONand HAXaY (1990) have excellent GEOSAT altimetry evidence of a large number of Agulhas rings in the South Atlantic and, in particular, describe CTD measurements of a ring near 28°S, 9°E which is to the west of the study area described below. From the viewpoint of biological oceanography, one obvious question to be addressed is whether or not this localized offshore transport has any effect on productivity in the offshore regions influenced by the filaments. It is also important to know whether filaments can act as a mechanism for the export of carbon from the productive inner shelf region to the deeper ocean. Another freature of at least two eastern boundary current regions is that the zooplankton fauna resident on the poleward end of the current system are quite different from the fauna living nearer the equator (JoH~sor~ and BRINTON, 1963; FLEMXNGER, 1964). The faunal boundary between the ends of the upwelling system is a region of transition between boreal (austral) and subtropical fauna in the northern (southern) hemisphere. In the California Current system, this boundary is located at and just south of Cape Mendocino. In the Benguela Current system, the boundary is near Meob Bay, 24.5"S, to the north of Luderitz (CHAPMANand SHANNON, 1985; SHANNOn and PILLAR,1986; AG~NBA~ and SHAr~NOr~,1988). In both instances, the location of the faunal boundaries is coincident with regions of offshore filaments, suggesting that the filaments may help to act as physical barriers to the equatorward transport of fauna from the poleward side. Curiously, strong faunal boundaries do not seem to exist off the Humboldt Current system, since nearly all the copepod species common off Peru are found off Chile (PETERSONet al., 1988). The main copepod species associated with the South Atlantic are now described. Calanoides caranatus has a dipausal stage V, which enables it to cross upwellings throughout the Indian and Atlantic oceans. In the Benguela Current system, it is normally associated with the cool upwelling shelf waters. Paraclanus p a r v u s (sensulatu) is the most common in neritic upwelled waters on the shelf, with occasional occurrences far offshore, presumably after strong offshore advection. Rhincalanus nasutus is also characteristic of shelf waters in the Benguela region. Calanus australis is characteristic of the Agulhas Bank warm shelf waters. The numbers of these species drop rapidly in offshore waters (DE DECKER, 1984). Cool filaments extending offshore from the Benguela Current system have been seen on early satellite infra-red imagery (e.g. see Fig. 1 taken from V^N FOREESTet al., 1984) and a statistical study of filamentary features in the Benguela has been undertaken by LUTJEr~s_~S and SrocrroN (1986) using data from the geostationary satellite METEOSAT. The purpose of the present study was to locate a suitable filament and make in situ measurements of its physical, biological and plankton properties and distributions. A central hypothesis was to determine whether the filament water was nutrient rich, and thus capable of measurably enhancing the primary and secondary production of what would normally be oligotrophic water. It was anticipated that the question of carbon export away from the more productive shelf region might also be addressed. DATA AND METHODS On the basis of local past experience, it was derided to make the first measurements in an upwelling filament near Luderitz. This site has been identified as one which often
Physical and biological features of a cool upwelling filament
1755
exhibits a strong thermal feature, at least in the surface waters. Due to persistent cloud cover in the region of interest, the most recent satellite infra-red image available at the commencement of the cruise on 17 February 1988, was that of 2 February 1988. Since it was not possible to initially find the main feature at 26.5°S, 13.5°E (visible on the 2 February image) a smaller frontal feature centred at about 27.5°S, 14°E was chosen for sampling (Fig. 2a,b). On 20 February 1988, updated NOAA-9 satellite infra-red temperature information revealed that the filament originally chosen for sampling had moved half a degree to the north of its position as seen on the 2 February image. The ship was directed northwards to this larger filament and a transect was made from south to north measuring the surface temperature, salinity, and vertical temperature with XBTs while underway at 10 kn. From the northern edge, the ship's track turned west for 20 nm. A southward ~,.10
"
I
I
18
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. N I,
lg E
~ 5 . 3
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,
I
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1¢
!
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o
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15"
Sea surface rneasuremems from thermosalinograph for 18-23 February 1988: (a) sea surface temperature, (b) surface salinity in parts per thousand.
1756
F.A. SrnLLIN~rONet al.
section of five CTD stations was executed across the filament during 21 February 1988, and a total of 17 stations were occupied both in and out of the filament over the next 3 days (Figs 2 and 4b). Figure 4b indicates the sampling network in relationship to the surface temperature field derived from the NOAA-9 satellite imagery for 19 February 1988. Physical measurements
Surface temperature and salinity were monitored continuously at 3 m depth while underway, using a Hewlett-Packard thermosalinograph. Vertical temperature and salinity profiles were made with a Neil Brown CTD, lowered to 300 m, with a sample written to magnetic tape every 2 m. Geopotential anomalies with respect to the 300 dB surface were later calculated using the international equation of state (GILL, 1982). Biological measurements
Ten 4-fitre Nisldn sample bottles were triggered on a rosette sampler at standard intervals 0, 10, 20, 30, 40, 50, 75, 100, 150 and 200 m to obtain samples for nutrients (nitrate, ammonium, nitrite, silicate, phosphate), oxygen and salinity. Nitrate concentration was measured according to the method of STmCKLANDand P~SONS (1972) modified for use with a Technicon Auto Analyser II (MOSTEt~T, 1983). Ammonium concentrations were determined in triplicate immediately on collection following GXASS•OFF (1976) scaled down to 5 ml. Samples for urea determination were stored frozen for about 2 weeks prior to analyses according to GRASSHO~ (1976), but scaled down to 5 ml sample volume. Dissolved oxygen analyses were performed by Wrinkler titration on board using the azide method of MONTC,O~ERYet al. (1964) to prevent interference from nitrite. Water samples for the determination of chlorophyll a concentration were collected from standard depths of 0, 10, 20, 30, 40, 50 and 75 m. Subsamples ranging in volume from 100 to 500 ml were taken from the Niskin bottles, filtered through GF/F filters, immediately homogenized in 90% acetone, allowed to extract for 30 rain in the dark at 18°C, and then read on a Turner Designs fiuorometer (YE~'TSC~ and MEm'ZEL, 1963). An additional subsample was taken from the Niskin bottle triggered at the depth of fluorescence maximum, poured through a 10/~m Nitex screen, and a 100-500 ml subsarnple filtered through a GF/F filter for analysis of chlorophyll a in the less than 10 ~m fraction. Total phytoplankton community nitrogen uptake was measured in 1 litre 200 ~m screened samples spiked with 0.1 mmol N m -3, either I~NH4CI (99.7 at%), NalSNO3 (99.6 at%) or CO(15NH2)2 (99.1 at%). Samples were collected from depths corresponding to the 100, 50, 25, 10 and 1% light levels. Additional 5-litre incubations at the 1% light depth were post-fractionated through a 20/~m plankton mesh and <2/~m Nuclepore filter to identify the predominant size classes involved in the different nutrient species. All experiments were initiated about midday local time and were run for 4 h under simulated in situ light conditions. Experiments were terminated by filtration onto 47 mm Whatman GF/F filters which were stored frozen for later analyses. Samples were prepared according to the Kjeldahl/Rittenberg oxidation procedure (FIEDLERand PROKSCH, 1975) for xsN:14N isotope analysis by emission spectroscopy. Absolute rates of nitrogen uptake were calculated using paniculate nitrogen concentrations measured at the end of the incubations, thereby accounting for any biomass increase that may have occurred during incubation (DU~DALE and WmK~XSON, 1986;
Physicaland biologicalfeaturesof a coolupwellingfilament
1757
COLLOS,1987). Ammonium uptake rates were corrected for isotope dilution using the exponential model of GILBERTet al. (1982). Particulate and aqueous 15N enrichment.in excess of natural abundance were used in all calculations.
Zooplankton measurements Zooplankton was collected with a WP-2 net (200 ~m mesh, 0.57 m diameter mouth) hauled vertically from 50 m to the surface. Samples were preserved with buffered formalin. In the laboratory two or four subsamples (volume of 3.9 ml) were taken with a piston pipette, and all individuals in each subsample were counted (usually about 30 animals). It is estimated that about 5% of the total zooplankton samples were subsampled and that the numbers in the major taxa varied between 200 and 400 animals. Most taxa were identified to species. For measurements of gut fluorescence and egg production, individual zooplankton were first collected by suspending a 0.5 m diameter, 200~tm mesh net fitted with a 2-litre cod end at depths of 10-20 m. The net sampled for 5 rain while the ship drifted, a technique that produces damage-free copepods. Immediately upon being brought aboard the contents of the cod end were poured gently through a 900/~m Nitex mesh (to remove gelatinous zooplankton) into a bucket containing 15 litres of seawater. A subsample of this was quickly poured onto a 120/~m mesh and individuals were picked from the mesh with jeweller's forceps, placed into 10 ml centrifuge tubes, 90% acetone was added, then the capped tubes placed in a freezer overnight where the pigments extracted. After 24 h, pigment concentrations were determined with a Turner Designs Fluorometer. For egg production measurements, females of Centropages brachiatus, Calanus australis and Calanoides carinatus were picked from the sample of living zooplankton with a wide-bore pipette and incubated in 1 litre plastic bottles, 1 female per bottle for Calanus and Calanoides, and 3 per bottle for Centropages. Bottles were previously filled with 60/zm filtered seawater collected from 20 or 30 m (the depth of the Chl a maximum). Females were incubated for 24 h after which the contents were filtered onto a 60 ~tm Nitex screen, rinsed into a sample bottle and preserved with formalin. In the laboratory females and any eggs produced were enumerated. RESULTS
Surface features Figure 2 shows surface temperature and salinity which was sampled continuously from the underway thermosalinograph. There is a 3°C front across the northern filament, but only a 0.1 change in salinity. The surface temperature structure closely matches that from the NOAA-9 satellite image taken on 19 February (see Fig. 4) although this part of the survey was carried out between 20 and 23 February 1988. An objectively computer-contoured map of geopotential anomaly with respect to the 300 dB surface (Fig. 3) shows a dynamic high to the north and south of the study area (corresponding to the warm southern eddy in the satellite image for 2 February, not shown), with a saddle point along the line of Stas 7-12. This can be interpreted as a geostrophic flow to the southwest between Stas 5 and 6 and to the southeast between Stas 2 and 3. There is a sluggish flow towards the north across the line of Stas 13--17.
1758
F . A . SmLLINGION et aL
s
16~
12 ° E
13 °
1
14 °
Fig. 3. Ob~tively computer-contoured geopotential anomaly with respect to 300 dB. Stations are numbered FC 2-17.
Figure 4a shows surface temperature and velocity vectors derived from feature tracking between two NOAA-9 images for 19 and 20 February 1988. The technique used to derive these vectors is described in AGENeAGand SHANNON(1988). From the few vectors derived in the region very close to the survey, it can be seen that the surface flow to the south of the filament is towards the west. The drift vectors on the northern side of the filament show some evidence of an anticyclonic (antidockwise) eddy of warmer water. Most of the vectors derived using this technique are closer to the coast and reflect a tendency for the surface water to be moving offshore to the west, in response to the strong southerly winds (up to a maximum of 20 m s -1) experienced on 18 and 19 February. Vertical structure within the filament is shown in Figs 5--7 for three legs sampled on 21, 22 and 23 February (Stas 2-6, 7-13 and 14-17, respectively). Results from the three different legs are now discussed in detail.
Section 1 (Fig. 5) At the north edge of the filament, Stas 2 and 3 (Fig. 5), there is a slight temperature inversion at about 30 m which is compensated by the salinity maximum. The ot section shows a strong pycnocline between 20 and 30 m which corresponds to a thermocline of some 4--5°C. From Fig. 5 it is clear that only Sta. 5 appears to be in the filament having slightly cooler, and relatively fresher (35.2) water. Both nitrate and ammonium concentrations show a very distinctive signal. In the upper 100 m, the nitrate section shows a high concentration tongue shoaling from the south between depths of 75--40 m, with values decreasing from 16--6 mmol N m -3, while ammonium reaches a maximum value of I mmol N m -3 in the upper 10 m (Stas 3 and 4) and between 50 and 75 m. Below 100 m, the nitrate concentration falls below 0.5 mmol N m - 3 in Stas 4--6. This region of low nitrate concentration has increasingly high values of ammonium (maximum of 2 mmol N m-3). Station 4 shows slight evidence of oxygen depletion (<3 ml 1-1). There is a Subsurface maximum in Chl a concentration at depths of
Physical and biological features of a cool upwelling filament
Fig. 1. NOAA-1 enhanced infra-red (channel 4 VHRR) image of the Benguela Current system along the west coast of southern Africa (15 June 1979) (after VAN F O ~ ' T et al., 1984).
1759
1760
F . A . SHILLINGTONet al.
(a)
Physical and biological features of a cool upwelling filament
1761
30--40 m along this and the second transect line. The maximum is most pronounced between Stas 2 and 4 and is weak at Stas 5, 6 and 7.
Section 2 (Fig. 6) The temperature section shows evidence of cool water uplift at Sta. 10 and from the salinity section it can be seen that Stas 9 and 10 show more uniform salinity values of around 35.2. There are intrusive tongues of high salinity water (35.5-35.6) from both the north and south of the filament at 40-50 m depth. The at section indicates that there is very little flow perpendicular to this line of stations except near the surface between Stas 13 and 11. Once again nitrate and ammonium signals are very distinctive. Nitrate concentration is generally low in the upper 100 m with highest values (10-14 mmol N m-3) at Stas 9 and 10 at 75-100 m depth. Ammonium on the other hand shows patchy high values at 30 m (up to 5 mmol N m -3) at Sta. 10 and at 100 m at Sta. 7. Between 100 and 200 m there is again a region of very low nitrate (<0.5 mmol N m -3) at Stas 7 and 8. The oxygen shows some lower values for Sta. 10. Chlorophyll a is at a maximum between 30 and 40 m, with most pronounced values at Stas 8-11.
Section 3 (Fig. 7) Temperature, salinity and at at these four stations were indicative of coastal inshore conditions, with evidence of slight uplift of the isotherms and the isopycnals towards Sta.
(b)
12"E
14"
24"S
26"
2e" Fig. 4. (a) NOAA-9 infra-red image for 19 February 1988, with satelfite-derived surface drift vectors. (b) Positions of the sampfing network in relationship to the surface temperature field. Solid dots are station positions and upright numbers next to them are the station numbers, starting from Stu. 2. The station spacing is nominally 10 nmi (18.5 km). Surface temperatures are simplified from Fig. 4a.
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2 3 4 s e 5. Vertical sections for leg 1 (Stas 02--06) from north to south. Parametersare temperature, salinity, at, ammonium, nitrate, oxygen, Chi a. Nutrient concentrations are in mmol N m-3, oxygen in ml1-1, andC'hla in mg m-3. Fig.
17, the closest inshore station of the cruise. Between Stas 14 and 15 there was a marked colour front at the surface with associated sharp changes in the biological variables. The vertical distribution of Chl a at Stas 16 and 17 suggest a different water body, and this is corroborated in the zooplankton data discussed below. Nitrate values generally increased with depth in this section. At 200 m there was a gradient of nitrate with the highest values at Sta. 17 (26 mmol N m-3). Ammonium has a tongue of high values at 50 m with a maximum at 75 m at Sta. 16. Within the Chl a maximum layer, all of the Chl a passed through a 10/~m screen at stations in the centre of the first two transect lines (Stas 3-5 and 8-11) (Fig. 8). The data on depth of Chl a maximum and 'the per cent C h l a < 1 0 ~um suggest a tongue of water
1763
Physical and biological features of a cool upwelling filament
0 m
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Vertical-section for leg 2 (Stas 13-7). Note that station 13 causes a dog leg in the line so that it was occupied in the warmer water. Only T,S measurements were available for Sta. 12. Units as in Fig. 5. Fig. 6.
extending from southwest to northeast over the shelf. South and east of this tongue (Stas 6 and 7, and 14-17), a significant proportion of cells were larger than 10~m and there was no well-defined subsurface Chl a maximum. Water masses
A general description of the water masses in this region from historical data has been given by St'IANNON (1985). This is also summarized in Fig. 9 by the solid line which represents data from April 1986, Sta. SNO4, SNEC II (1986). The T-S curve shows a salinity maximum of 35.6 at 50 m depth. Antarctic intermediate water is present at a
1764
F . A . SHILLING'IONet al.
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temperature of 5°C, and a salinity of 34.6 at about 600 m depth• Data from Stas 2 to 6 have been plotted for our measurements down to 300 m. Clearly evident is a band of 35.2 salinity water with temperatures in the range 13-20°C. This is assumed to represent warmed, mature upwelled water that has been previously at the surface nearer the coast. Examination of the individual T-S curves for each station shows a background structure similar to SNO4 for Stas 2, 3 and 6. The CTD traces also show evidence of layering which can be interpreted as mixing at the edges of the filament. The distribution of the dominant zooplankton taxa also suggests that several water bodies were present during our study. A comparison of Fig. 10 with Fig. 8 shows that salps in particular were abundant only where the Chi a maximum was most pronounced and where all of the Chl a passed through the 10 pm screen (at Stas 5, 8, 9 and 10). Two copepod species which are numerically dominant in the Benguela upwelling system
Physical and biological features of a cool upwelling filament
1765
25.5*O 7tO o
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Percentage of Chl a <10/~m at the depth of the chlorophyll maximum.
(Paracalanus parvus and C. brachiatus) had their greatest abundance along the southeastern edge of the tongue. They were either uncommon or totally absent at stations along the northern edge of the tongue and only moderately abundant within the salp tongue (Fig. 10). C. carinatus was most abundant within the salp-rich zone, correlating with the enhanced Chl a concentration. Inshore of the tongue, at and inshore of the shelf break, a different water body was encountered, one dominated by euphausiid eggs and larvae and greatly reduced numbers of the three dominant upwelling copepod species. Taken as a whole the plankton data suggest the presence of four water bodies. For the zooplankton, (1) Stas 2, 3, and 13 (northern part of study area), (2) Stas 4, 5, 8, 9 and 10 (core of the salp distribution), (3) 6, 7, 11, 14 and 15 (east of the salp distribution the copepod species were typical of the coastal upwelling zone), and (4) a shelf-break group characterized by high abundances of eupausiid eggs, nauplii and calyptopis. The existence of four water bodies is corroborated in the Chl a data as well: (1) the northern water of relatively lower Chl a concentration, (2) the tongue of water of higher Chl a concentration with all cells passing through a 10~tm Nitex screen, (3) the southern and northern edges of 4.4
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Temperature-salinity curve for Stas 2--6. Solid line represents data from SNEC (1986) for Sta. SN4 which was very close to the position of the measurements displayed here.
1766
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1767
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the tongue at Stas 5 and 6 and Sta. 13, characterized by a weak subsurface maximum and larger cells, and (4) water inside the shelf break characterized by the highest Chl a concentrations and largest cell size measured. Depth profiles of nitrogen uptake also provide evidence for distinct water bodies in the study area (Fig. 12). At Sta. 3, primary production was supported almost entirely by N!trogen uptake rate ( m real N m "3 h-I ) ,
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F . A . SmI.IJN6"roN et al.
1768
Table 1. Nitrogen uptake integrated to the 1% light level depth. The f-ratio is the proportion of nitrate in the total nitrogen uptake (EeeL£Y and PErE~SON, 1979). Units are/~mol N m -2 h -1 Sta.
3 8 14
Table 2.
NH~
NO~
Urea
f-ratio
138.2 51.5 339.0
3.8 19.5 222.7
2.5 52.9 589.8
0.03 0.16 0.19
Uptake of the three nitrogen species by different microplankton size classesat the 1% light level. Units are~mol N m -3 h -1 200-20/~m
Sta 3 8 14
20-2/~m
<2 #m
NH~"
NOi
Urea
NH~
NO~"
Urea
NI-I~
NO~
Urea
0.0 0.5 6.5
0.1 0.0 0.0
0.0 0.0 0.0
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0.1 0.0 0.0
0.5 1.0 0.5
7.7 2.1 3.4
0.4 0.0 0.0
1.3 0.0 0.1
Table 3. Egg production data for Centropages brachiatus, Calanus australis and Calanoides carinatus. Units are in eggs female- 1 day- 1 Sta.
Centropages
Calanas
1
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--
2 6
-53.7 12.8 20.0 12.2 10.0 60.7 59.3
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8
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15
Calanoides
0.5 0.0
5.0 4.0
1.9 5.5
ammonium, with a subsurface uptake maximum situated deeper in the water column than in the salp core (Sta. 8) and in the more typically coastal area (Sta. 14). The contribution from nitrate, as evidenced by the f-ratio (Eert~Y and ~ N , 1979), was elevated at Stas 8 and 14 as was the overall level of primary productivity (Table 1). Nitrogen uptake by different size classes also revealed distinct variations between the different water bodies. At Sta. 3 the uptake of ammonium by picoplankton (<3/~m) dominated phytoplankton nitrogen assimilation, whereas at Stas 8 and 14, nanoplankton (20-2 #m) and netplankton (200-20 #m), respectively, were the most active (Table 2). The egg production data (Table 3) shows that the copepods were moderately unstressed only at Stas 2, 6, 14 and 15, stations outside the tongue of water containing salps and small
1769
Physical and biological features of a cool upwelling filament
Table 4. Gut fluorescence data for female Centropages brachiatus, Calanus australis, Calanoides carinatus andfifth copepodites of Rhincalanus nasutus. A blank indicates that no gut data were availablefor the plankton at that station Sta.
Time
1 2 5 6 7 8 9 10 11 12 13 14 15 16 17
1600 0830 1815 2050 O720 0915 1120 1445 1820 2000 2200 0830 1230 1540 1820
Centropages
Calanus
Calanoides
Rhincalanus
0.11
0.07 0.31 0.10 0.19 0.42 1.01
0.75 0.47 0.27 0.09 1.05 0.78 0.43 0.90
0.08 0.12 0.63 O.52
0.58 0.70 0.22 0.86 1.34 1.17 3.16
phytoplankton cells. Within the tongue, egg production by C. brachiatus was about 15 eggs day -1. The higher value (50 eggs day -1) is about 50% of the maximum rate of egg production known for this species. For the other two species (C. australis and C. carinatus) high egg production rates were only observed at Stas 14 and 15 within water characterized by high abundance of these upwelling species and where Chl a concentration was relatively high. Rates observed at these two stations are near the maximum for these species. Within the salp plume egg production rates were about 10% of maximum for C. brachiatus and 5% of maximum for C. australis (based on unpublished data of Peterson). The gut fluorescence data, though sketchy, suggest that relatively good feeding conditions were encountered only in the water at stations outside the salp tongue (Stas 6 and 7 and 11-17. Table 4 lists the data). DISCUSSION
AND CONCLUSIONS
The measurements presented in this paper are the first of their kind in a filament of the Benguela Current system. Taken as a whole, the physical and biological variables measured form a consistent picture of the structure of the filament. This is summarized in the cartoon in Fig 13 where it can be seen that we encountered four significantly different water bodies. These consist of: (1) mature coastal water previously upwelled from about 300 m (temperature 14°(2, salinity 35.2), with dominantly coastal fauna (euphausiid eggs, nauplii and calyptopis larvae). This water was the productive area in terms of nitrogen uptake with a considerably active netphytoplankton community. (2) The filament itself, which was at least 2 weeks old when we started sampling it (judging from the satellite image of 2 February 1988), and hence in a slowly decaying state. This feature contained large numbers of salps. Uptake of ammonium by nanoplankton and picoplankton dominated phytoplankton nitrogen uptake. (3) The warm, high salinity surface water to the south which appears to be an anticyclonic eddy (which may have been spawned off the Agulhas
1770
F . A . SHILUNOTONetal.
.....
• -:-
~0
.',
'
- ~
•
Fig. 13. Cartoon showing the four main different water bodies found on this cruise. These are (1) the biologicallypoor blue water to the north of the filament, (2) the warm saline water to the south of the filament (an Agulhas ring or eddy?), (3) the filament water, and (4) the coastal upweiled shelf water. The proposed structure is supported by the physical, biological and plankton measurements. (Depths at the stations sampled ranged from 300 m inshore to greater than 3000 m offshore.)
Current to the south near Cape Town; see CHAPMANet al., 1987; GosOoN and HAXm,, 1990). (4) The warm, high salinity water to the north of the filament which was biologically inactive at the time of the cruise. Here picoplankton made the largest contribution to phytoplankton nitrogen uptake. This proposed structure is coherent in the 0-50 m integrated nitrate, (Fig. 11) small Chl a-bearing cells, salps, surface temperature from the satellite, and dynamic topography. Data from a cruise 2 weeks prior to ours ( B o ~ , personal communication) show a very low nitrate surface layer 80-100 m thick centred at 27°S, 13°18'E, which was about 70 km SSE of our Sta. 6. Due to the monitoring nature of the earlier cruise, their stations are more widely separated (60 km) in the region of the filament, and so only a limited comparison can be made with their data. Low nitrate concentrations in the euphotic zone, warm temperatures (21°C), and high salinity (35.6) are indicative of properties of an Agulhas eddy or ring (C'r~l,~t~,~ et al., 1987). As yet there is no convincing explanation for the 100 m thick depleted nitrate layer between 100 and 200 m depths at Stas 4-8. Dentitrification has been ruled out since the oxygen values are not exceptionally low at these depths. The most plausible explanation is that the low surface nitrates measured by BOYD(personal communication) in the upper 80100 m to the south of the filament, 2 weeks prior to this cruise, have been overlain by nutrient-rich water of coastal origin. Also difficult to explain is the high ammonium associated with this nitrate-poor water. We suggest that the filament is in a decaying state, and hence that there has been biological activity prior to our measurements. It is clear that the measurements reported here indicate that the feature sampled was rather shallow (about 50 m) in the biological variables. This appears to be different from some of the recent results from the Coastal Transition Zone measurements in the Cafifomia Current System (EOS, 1988), where the active filaments appear to be much
Physical and biological features of a cool upwelling filament
1771
d e e p e r . W e c a n o n l y a s s u m e t h a t t h i s d i f f e r e n c e is b e c a u s e w e s a m p l e d a n o l d , d e c a y i n g f i l a m e n t , at t h e e n d of t h e s e a s o n a l u p w e l l i n g c y c l e ( l a t e F e b r u a r y ) .
Acknowledgements--We wish to thank the captain and crew of the R.S. Benguela for assistance in the data collection at sea; Mr Ike Marais of the Hartebeestehoek Satellite receiving station for supplying the NOAA-9 computer compatible tapes in near real time; Mr Pierre Malan for handling the CTD and oxygen titrations on board; Mrs Verheye-Dua for the nutrient analysis, and the Benguela Ecology Programme of the Foundation for Research and Development, CSIR, for funds towards this project. Suggestions from an anonymous referee helped improve the final manuscript.
REFERENCES AGENBAGJ. J. and L. V. SHANNON(1988) A suggested physical explanation for the existence of a biological boundary at 24°30'S in the Benguela system. South African Journal of Marine Research, 6, 119-132. CHAPMAN P. and L. V. SHANNON (1985) The Benguela ecosystem II. Chemistry of related processes. Oceanography and Marine Biology Annual Reviews, 23, 183-251. CHAPMANP., C. M. DUNCOMBeRAE and B. R. ALLANSON(1987) Nutrients, chlorophyll and oxygen relationships in the surface layers at the Agulhas Retroflection. Deep-SeaResearch, 34, 1399-1416. COLLOSY. (1987) Calculations of 15N uptake rates by phytoplankton assimilating one or several nitrogen sources. Applied Radiation and Isotopes, 38, 275-382. DE DeCKEI~A. H. B. (1984) Near surface Copepod distribution in the south-western Indian and south-eastern Atlantic. Annals of the South African'Museum, 93, 303-370. DUCDALER. C. and F. C. WILKE~SON(1986) The use of 15N to measure nitrogen uptake in eutrophic oceans; experimental considerations. Limnology and Oceanography, 31,673-690. DUNCOMSERAE C. M., L. V. ShANnON and F. A. SmLUNOTON(1989) An Agulhas ring in the South Atlantic ocean. South African Journal of Science, 85,747-748. EPPLEY R. W. and B. J. PETERSON(1979) Particulate organic matter flux and planktonic new production in the deep ocean. Nature, 282,677-680. EOS (1988) Abstracts from the Coastal Transition Zone: filaments off California, 69, 1258-1261. F1EDLEi R. and G. PROKSCH(1975) The determination of nitrogen-15 by emission and mass spectrometry in biochemical analysis: a review. Analytica Chirnica Acta, 78, 1-62. FLAMEm"P., L. A ~ I and L. WASHSU~ (1985) The evolving structure of an upwelling filament. Journal of Geophysical Research, 90, 11765-11778. FLEMINOBRA. (1964) Distributional atlas of calanoid copepods in the California Current region, part 1, CaICOFI Atlas, no. 2, Scripps Institute of Oceanography, La Jolla, California. GILL A. E. (1982) Atmosphere-ocean interaction. Academic Press, London, 626 pp. GILaErr P. M., F. L~PSHULTZ,J. J. MCCABTnVand M. M. ALTAB~r(1982) Isotope dilution models of uptake and remineralisation of ammonium by marine plankton. Limnology and Oceanography, 27, 639-650. GORDON A. L. and W. F. HAXeV (1990) Agulhas eddies invade the South Atlantic ocean--evidence from GEOSAT altimeter and shipboard CTD. Journal of Geophysical Research, 95, 3117-3126. GItASSHOFFK. (1976) Methods of sea wateranalysis. Verlag Chemie, Weinheim. JORNSON M. W. and E. BmNTON(1963) Biological species, water masses, and currents. In: The sea, Vol. 2. Chapt. 18, John Wiley, New York, pp. 381-414. Kos~o P. M. and A. HuYEx (1986) CTD and velocity surveys of seaward jets off northern california, July 1981 and 1982. Journal of Geophysical Research, 91, 7680-7690. LtrrJEHA~S J. R. E. and P. L. STOCKTON(1986) Kinematics of the upwelling front off southern Africa. In: The 8enguela and comparable ecosystems, A. I. L. PAYNE, J. A. GULLANOand K. H. BRINK, editors, South African Journal of Marine Science, 5, 35-49. LUTJEt~m~S J. R. E. and A. L. GO~DON(1987) Shedding of an Agulhas ring observed at sea. Nature, 325, 138140. MONTOONE~YH. A. C., M. S. TnOM and A. CocxeuItN (1964) Determination of dissolved oxygen by the Winkler method and the solubility of oxygen in pure water and sea-water. Journal of Applied Chemistry, 14, 280296. MOSTnT S. A. (1983) Procedures used in South Africa for the automatic photometric determination of micro nutrients in sea-water. South African Journal of Marine Science, 1,189-198.
1772
F . A . SmLLIN~ronet al.
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