Two Lagrangian experiments in the Iberian Upwelling System: tracking an upwelling event and an off-shore filament

Progress in Oceanography 51 (2001) 221–248 www.elsevier.com/locate/pocean

Two Lagrangian experiments in the Iberian Upwelling System: tracking an upwelling event and an off-shore filament Ian Joint a,*, Mark Inall 1,b, Ricardo Torres b, Francisco G. Figueiras c, Xose´ A. ´ lvarez-Salgado c, Andrew P. Rees a, E. Malcolm S. Woodward a A b

a Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey, LL59 5EY, UK c C.S.I.C., Instituto de Investigacio´ns Marin˜as, Eduardo Cabello 6, E-36208 Vigo, Spain

Abstract Two Lagrangian drift experiments were carried out at the NW Iberian margin. The first tracked a body of nutrientrich, upwelled water as it moved south along the shelf break over a 5 day period. The second experiment, of similar duration, followed a water mass as it moved into the deep ocean in an off-shelf filament. This paper describes the background to and aims of each experiment. The overall objective was to quantify chemical and biological processes relating to the additional potential for the ocean at the shelf margins to sequester atmospheric CO2 in upwelling regions. The first experiment began at a time of intense wind-driven upwelling; within 2 days, the wind speed had moderated and the system entered a relaxation period with greatly reduced upwelling. The patch of upwelled water was marked by a single buoy array and it moved south along the shelf break. Transport was initially rapid but slowed with reducing wind speed. The temperature–salinity characteristics were consistent with sampling only a single water mass throughout the experiment. A model of particle trajectories showed slight deviation from the actual movement of the marked water mass, but overall the data support the assumption that the experiment was Lagrangian. During a 5 day experimental period, nutrients were utilised with a N:P ratio of 18.3 and N:Si of 4. Nutrient concentrations first reduced in the nearsurface but depletion deepened in the water column during the experiment. At the beginning of the experiment, the highest chlorophyll concentrations were in the surface 15m but this was replaced by a subsurface chlorophyll maximum at 30m. There was a shift from a small flagellate and dinoflagellate dominated photosynthetic phytoplankton assemblage to a diatom dominated assemblage. A high biomass of heterotrophic dinoflagellates and ciliates was also present. Canonical correlation analysis between environmental variables and microplankton assemblages, as defined by principal component analysis, suggested that a considerable part of DON production resulted from trophic relationships rather than direct release from phytoplankton. The second experiment followed a water mass marked with 5 Argos drifting buoys for 5 days as the water drifted off shelf in an off-shore filament. This water mass was extremely oligotrophic; nitrate concentrations were typically ⬍10nmol l⫺1 in the upper 20m and ammonium concentrations were 20–40nmol l⫺1. Chlorophyll concentrations were

* Corresponding author. Tel.: +44-1752-633100; fax: +44-1752-633101. E-mail address: [email protected] (I. Joint). 1 Present address: Scottish Association for Marine Sciences, Dunstaffnage Marine Laboratory, Oban, Argyll, PA34 4AD, UK. 0079-6611/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 1 ) 0 0 0 6 8 - 4

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very low and the phytoplankton assemblage was dominated by picoplankton. Diatoms were largely absent from the nano- and microplankton fractions, which were dominated by dinoflagellates. The data presented in this paper are a general description of the experiments and form the background to the more detailed descriptions given in the individual papers that make up this Special Issue of Progress in Oceanography.  2001 Elsevier Science Ltd. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

2. Methods . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sampling strategy . . . . . . . . . . . . . . . . . 2.2. Nutrient analyses . . . . . . . . . . . . . . . . . 2.3. Dissolved and particulate carbon and nitrogen 2.4. Chlorophyll . . . . . . . . . . . . . . . . . . . . . 2.5. Microplankton . . . . . . . . . . . . . . . . . . .

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3. Experiment 1 — the upwelling drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Track of the drifting buoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. A Lagrangian experiment? Evidence that the same water mass was sampled . 3.3. Temporal changes in temperature, nutrient salts and suspended organic matter 3.4. Development of phytoplankton-microplankton biomass . . . . . . . . . . . . . . 3.5. Microplankton assemblage composition . . . . . . . . . . . . . . . . . . . . . . . .

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4. Experiment 2 — the off-shore filament . . . . . . . . . . . . . . . . . . . . 4.1. Tracks of the drifting buoys . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Temperature, nutrients and suspended organic matter in the filament 4.3. Chlorophyll and microplankton in the filament . . . . . . . . . . . . .

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

1. Introduction It is generally accepted that the global climate is changing as a consequence of the burning of fossil fuels. However, current estimates of the sinks of anthropogenically produced carbon dioxide, whether on land or in the ocean, have large uncertainties (Houghton et al., 1996). Since biological processes are important mechanisms removing CO2 from the atmosphere, it is of crucial importance for understanding the impact of human activity on the biosphere that we have accurate estimates of global primary production. Field, Behrenfeld, Randersson and Falkowski (1998) suggested that annual biological fixation of carbon is globally about 100 petagrams per year, with photosynthesis in terrestrial and oceans ecosystems accounting for similar proportions. These estimates are based on satellite remote sensing and Field et al. (1998) highlighted the large degree of heterogeneity in marine systems. The oligotrophic regions have very low productivity, but in others, variable physical forcing results in high phytoplankton activity. One of the

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major reasons for enhanced primary production in the oceans is wind-driven upwelling of deep, nutrientrich water into the euphotic zone, which sustains high levels of primary production along a number of continental margins. Wind-driven upwelling events can be very variable and the contribution of upwelling to estimates of global primary production, and hence to the flux of carbon between the ocean and atmosphere, is poorly quantified. The greatest uncertainties relate to exchanges at the open ocean boundary (Gattuso, Frankignoulle, & Wollast, 1998). For that reason, the Ocean Margin Exchange (OMEX) project investigated the Iberian margin with the aim of quantifying atmospheric fluxes of CO2, autotrophic and heterotrophic activity in the water column and sedimentation of organic matter into the benthos of the shelf, slope and deep ocean. The present study was designed to investigate short-time scales and, through Lagrangian drift experiments, to focus on the effect of upwelling on biological production and the carbon cycle. The shelf seas of the northwest Iberian Peninsula are extremely productive and the rias of Galicia support one of the most intensive shellfish farming industries in Europe (Tenore et al., 1995). The region’s high productivity results from intense periods of wind-driven upwelling during the summer months, which bring nutrient-rich water from below the thermocline up into the euphotic zone, hence increasing phytoplankton production. The upwelling results from northerly winds. The rectangular shape of the NW Iberian Peninsula, means that almost any wind with a northerly component can result in upwelling, consequently upwelling events are frequent. The upwelling index of Smyth, Miller, Groom and Lavender (2001) derived from an analysis of remotely sensed sea surface temperatures (SST) indicates the high frequency of upwelling in this region. Upwelling is well characterised by satellite remote sensing through the reductions in SST. The duration of upwelling periods is variable, but generally lasts for about 2 weeks (Moncoiffe, Alvarez-Salgado, Figueiras, & Savidge, 2000). Periods of relaxation alternate between upwelling events. When winds are northerly during upwelling events, the dominant flow is to the south. Any body of nutrient-rich water upwelled from below the thermocline to the surface will move southwards along the narrow shelf of the Iberian peninsula. Within a few days, as the water gains heat, SST rises, at the same time nutrient concentrations decline through the activity of the phytoplankton assemblage. This system offers a good opportunity to study the temporal changes, which occur in plankton populations as a result of the introduction of nutrients. The experiment described here attempted to measure as many relevant parameters as possible within a time scale representative of an upwelling/relaxation period. Another dominant feature of the hydrography of the region is the occurrence of filaments. Satellite images of SST (Smyth et al., 2001) reveal filaments of cooler water extending out into the open ocean from the shelf. These suggest there is significant off-shore transport. Such filaments are often associated with upwelling regions (Haynes, Barton, & Pilling, 1993) but their quantitative significance to off-shelf transport of plankton and organic carbon is unknown. So a second objective of this study was to determine biological activity within a filament and to quantify vertical and horizontal fluxes. Since both the upwelling and filament regions are very dynamic, the best way to understand the response of plankton to the perturbations associated with upwelling, is to attempt to follow a patch of water over a period of time and study the changes taking place in the plankton. Such a Lagrangian approach has been used successfully in recent years to determine the effect of iron fertilization on carbon fixation rates in the equatorial Pacific (Behrenfeld, Bale, Kolber, Aiken, & Falkowski, 1996). In the present study, two Lagrangian experiments were undertaken. In the first experiment, the southerly movement of upwelled water was tracked along the shelf edge by following a single buoy for a period of 5 days. The second experiment in an off-shore filament, five Argos drifting buoys (Barton, Torres-Almarza, Inall, & Sherwin, 2001) were use to track the movements of the water, also over period of five days. After the experiment four of the Argos drifting buoys continued to be tracked for a further 6 months (Barton et al., 2001) to achieve a better understanding of long term transport. As a background to this Special Issue, this paper describes a number of aspects of the two experiments; a basic rationale for the experiments; how closely the ship was able to follow a single water mass in each

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experiment, i.e. how closely these experiments approximated to Lagrangian; a description of temporal changes in temperature, chlorophyll and nutrient concentrations in the water masses sampled. Finally, the changes in the major phytoplankton taxa, which occurred during each experiment, are discussed.

2. Methods 2.1. Sampling strategy The measurements took place during cruise CD114 of the RRS Charles Darwin between 29 July and 24 August 1998. The first experiment investigated an upwelling and relaxation event (Table 1). A drifting buoy was used to mark a water mass and measurements were made of the chemistry and biology in the surface mixed layer. The position for deployment was chosen on the basis of satellite remote sensing (both chlorophyll concentration from SeaWiFS and sea surface temperature from AVHRR) transmitted to the ship in near real time (⬍6h after satellite overpass). A region of cold water with elevated chlorophyll concentrations was chosen and CTD stations were sampled along a transect from the coast to the shelf break. On the basis of these data a position was chosen for deployment where there was evidence of high nutrient concentrations and increasing phytoplankton biomass in the surface mixed layer. The drifting buoy array comprised a small surface float, with radar reflector and radio beacon, which was designed to offer minimum windage. A semi-submerged toroid buoy, 1.2m in diameter was attached Table 1 Lagrangian experiment in an upwelling event showing positions of sampling stations and mean velocity of drifting buoy between each CTD station. Drift rate is the mean rate of movement between CTD stations and mean wind speeds and direction are calculated for 30 minutes before and after the start of the CTD profile Date

Time (GMT) CTD Number Latitude (°N)

Longitude (°W)

3 August

03:29 09:37 13.47 00:27 02:45 08:29 14:16 23:59 01:27 02:42 09:35 13:58 18:44 01:12 02:40 08:36 14:44 17:12 17:57 23:21 02:45 08:20 12:46

09°23.89’ 09°21.80’ 09°22.92’ 09°23.00’ 09°20.41’ 09°19.50’ 09°20.82’ 09°24.20’ 09°25.04’ 09°24.37’ 09°23.50’ 09°23.78’ 09°24.23’ 09°24.24’ 09°24.66’ 09°25.90’ 09°25.11’ 09°25.89’ 09°26.00’ 09°26.43’ 09°25.88’ 09°26.70’ 09°26.51’

4 August

5 August

6 August

7 August

9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

42°36.95’ 42°34.40’ 42°31.04’ 42°24.09’ 42°22.71’ 42°19.70’ 42°18.30’ 42°14.94’ 42°15.00’ 42°13.67’ 42°12.20’ 42°11.26’ 42°09.74’ 42°08.72’ 42°08.08’ 42°07.50’ 42°06.95’ 42°05.84’ 42°05.87’ 42°05.30’ 42°05.20’ 42°03.80’ 42°04.13’

Drift rate (m Mean wind Mean wind speed (m s–1) direction (°) s–1)

0.25 0.43 0.34 0.53 0.28 0.15 0.39 0.22 0.58 0.12 0.11 0.17 0.08 0.25 0.09 0.07 0.26 0.06 0.06 0.06 0.14 0.04

6.1 9.1 11.0 8.4 7.1 8.1 10.8 9.1 9.1 8.8 5.4 5.7 5.5 7.1 6.9 4.5 4.7 5.6 5.6 4.6 1.1 2.0 3.7

9 356 350 14 10 358 352 359 18 17 19 339 335 25 35 52 353 353 352 339 295 165 247

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to this marker buoy with a 50m line. Suspended from this toroid were 3 sediment traps (Olli, Wexels Riser, Wassmann, Ratkova, & Arashkevich, 2001) at 30m, 40m, 50m and bottle racks at 8 depths to 40m for in situ primary production incubations (Joint & Pomroy, 1983). The sediment traps were the largest structures on the array and these acted as drogues at 30m, 40m, 50m depth. Contact between the ship and buoy was maintained by radar and by the DF radio beacon. The drifting buoy was deployed at 42°37’N: 09°24’W at 0329 GMT on 3 August 1998 and the water mass adjacent to the buoy was sampled repeatedly until 1246 GMT on 7 August at 42°04’N: 09°27’W. A total of 23 CTD profiles were completed over this period. In the second experiment (Table 2), a patch of water was marked with five Argos drifting buoys, drogued at 20m. The position for their deployment was again based on satellite remote sensing and CTD survey. A position was chosen, which was seaward of the shelf break where ADCP surveys indicated there was an offshore current; full details are given in Barton et al. (2001). Four buoys were deployed at the corners of an 11km diagonal square around 41°56.4’N: 9°50.9’W where a central Argos buoy incorporating a thermistor chain was deployed, which acted as the primary marker for sampling of the water patch. At each sampling time, the ship was positioned about 200m downwind of the marker buoy. A SeaBird CTD system with transmissometer and fluorometer was deployed and water samples were taken with 10 l Niskin bottles with external spring closing mechanisms, on a 24 bottle rosette. A large number of measurements were made on the water samples most of which are described in other papers in this volume. 2.2. Nutrient analyses Nutrient concentrations were determined with a five channel Technicon AAII, segmented flow autoanalyser. The chemical methodologies used were: nitrate — Brewer & Riley (1965); nitrite — Grasshoff (1976); phosphate — Kirkwood (1989); silicate — Kirkwood (1989), and ammonia — Mantoura and Woodward (1983). Nitrate and nitrite at low nanomolar concentrations were measured using the Table 2 Lagrangian experiment in an offshore filament: positions of sampling stations and mean velocity of drifting buoy between each CTD station. Drift rate is the mean rate of movement between CTD stations and mean wind speeds and directions are calculated for 30 minutes before and after the start of the CTD profile Date

Time (GMT) CTD Number Latitude (°N)

Longitude (°W)

14 August

02:26 13:52 01:26 02:44 13:48 01:35 02:58 13:41 01:38 02:45 13:45 00:11 02:36 13:40 02:59 04:01

09°50.03’ 09°51.02’ 09°53.45’ 09°54.16’ 09°56.17’ 09°58.33’ 09°58.84’ 09°58.75’ 09°59.89’ 10°00.66’ 10°04.11’ 10°06.20’ 10°07.40’ 10°07.15’ 10°06.05’ 10°06.07’

15 August

16 August

17 August

18 August

19 August

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

41°56.63’ 41°55.80’ 41°54.06’ 41°54.13’ 41°53.12’ 41°52.26’ 41°51.99’ 41°51.98’ 41°52.50’ 41°52.14’ 41°50.09’ 41°48.04’ 41°47.97’ 41°47.07’ 41°45.60’ 41°45.77’

Drift rate (m Mean wind Speed (m s⫺1) s⫺1)

0.05 0.11 0.21 0.08 0.08 0.17 ⬍0.01 0.04 0.31 0.15 0.12 0.19 0.04 0.07 0.08

Mean wind direction (°)

8.9 6.9

299 141

9.5 2.7

40 351

3.7 0.9

82 150

9.4 11.5

141 309

11.4 8.7

311 343

11.4

353

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methodology of Garside (1982), and low nanomolar ammonia concentrations by the method of Jones (1991). Water samples from the Rosette system were sub-sampled into clean bottles and the analyses of nutrient concentrations were in every case completed within 4 hours of sampling. Clean handling procedures were used to avoid contamination of the samples. 2.3. Dissolved and particulate carbon and nitrogen The methods for the analysis of dissolved and particulate organic carbon and nitrogen are described ´ lvarez-Salgado et al. (2001). Briefly, DOM was measured on samples filtered through precomfully by A busted Glass Fibre filters (GF/F) filters and acidified with H3PO4 to pH ⬍2. These were then sealed in glass ampoules and stored at 4°C until analysed in the Instituto de Investigacio´ ns Marin˜ as. Seawater for POC and PON analysis was subsampled into 2 litre polycarbonate flasks and immediately filtered with a vacuum filtration system (filtration pressure ⬍0.3kg cm⫺2) to collect the particulate material on to GF/F filters. The filters were dried on silica gel and frozen to ⫺20°C until analysed in the laboratory. Measurements were carried out using a “Perkin Elmer 2400 CHN” analyser. 2.4. Chlorophyll Chlorophyll concentrations were measured by the fluorometric method of Holm-Hansen, Lorenzen, Holmes and Strickland (1965). Aliquots of either 100 or 200 mls water collected during the experiments were filtered through polycarbonate filters with pore sizes of 5µm, 2µm and 0.2µm, which were stored frozen until return to the laboratory. Pigments were extracted by adding 90% (v/v) acetone to the filters, which were stored in the dark at 4°C for ca 12h prior to analysis. 2.5. Microplankton Microplankton samples were collected at several depths in the photic layer. The organisms were identified to species level whenever possible and enumerated using an inverted microscope and composite sedimentation chambers. The dimensions of each microplankton species or group identified were measured and cell volumes were estimated by approximation to the nearest geometric shape (Edler, 1979). These biovolumes were then used to estimate the cell carbon content with conversion factors published by Strathmann (1967) for diatoms and dinoflagellates, Verity et al. (1992) for flagellates, and Laws et al. (1984) for ciliates. Principal Component Analysis (PCA) based on correlation matrices of abundance was used to determine the main microplankton assemblages. Species abundances were transformed to log (X +1) where X is the number of individual per 100ml of seawater. Bias resulting from double zeros in the correlation matrix was reduced by considering species that were present in ⬎70% of the samples. A Canonical Correlation Analysis (CCA) was performed on the physico–chemical variables (salinity, temperature, chlorophyll, particulate organic nitrogen, dissolved organic nitrogen and nitrate plus nitrite concentrations) and the phytoplankton components extracted by PCA, to identify objective relationships between environmental variables and the phytoplankton assemblages. CCA was only carried out on the data from the upwelling drift experiment, because only one component with ecological meaning was extracted by the PCA of the microplankton data from the filament experiment. All data are archived at the British Oceanographic Data Centre, as part of the OMEX Project. 3. Experiment 1 — the upwelling drift The objectives of the first experiment in the upwelling region were to quantify changes in chemistry and biology of a body of water, which had been recently upwelled into the surface mixed layer. These aims were achieved by frequent sampling of the surface 150m over the five day period.

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3.1. Track of the drifting buoy Figure 1 shows the track of the drifting buoy as it moved south along the shelf break. The position of each dawn CTD profile is shown in relation to the bottom topography of the region. Table 1 lists the time and distance covered by the drifter between each sampling event and the wind speed at the time of sampling. Rate of movement was faster during the first two days when the wind was strongest; during this time, the buoy drifted at speeds varying between 苲0.25 and 苲0.53 m s⫺1. Mean wind speeds were 8–10 m s⫺1. Towards the end of the experiment, wind speeds had fallen significantly lower and the drift rate of the buoy had slowed to ⬍0.1 ms⫺1. From the middle of 5 August to the end of the experiment, the winds were light enough for this period to be a relaxation event, when upwelling was either considerably reduced or suspended. The effect of this relaxation was clearly visible in the satellite images (Smyth et al., 2001) as a shrinking in the area of cool SSTs. 3.2. A Lagrangian experiment? Evidence that the same water mass was sampled Figure 2 shows the hydrographic structure determined from the 23 CTD casts. The main features are the downward slope of the isotherms over the first 苲24 hours, variation in the high salinity core at 苲50m and warming of the upper few tens of metres towards the end of the drift experiment. The surface warming was almost certainly a local response to the relaxation of northerly winds. The T/S characteristics on the 26.9 isopycnal surface (苲80m) are plotted in Fig. 3 to determine if significant changes had occurred in properties of the water masses sampled. Points on the T/S diagram cluster into 2 groups; during the first 24 hours the water was cooler and fresher. This is consistent with the start position of the drift being in an area of upwelled water to the north and closer to the coast. After the first 24 hours, some scatter in the data indicates a slight weakening of the high salinity core. Overall, however, variations of water properties viewed in T/S space show no more variability than one might expect of subtropical Eastern North Atlantic Central Water (ENACWT) at these positions. We conclude that no dramatic change in water mass characteristics occurred during the period of sampling as the buoy drifted south along the shelf edge. Further evidence of any deviation from Lagrangian behaviour was sought in the velocity structure. The

Fig. 1. Tracks of the sampling track during 2 Lagrangian experiments. The first experiment tracked a body of upwelled water from 3 to 7 August 1998. The second experiment was in an off-shore filament which was sampled for 5 days from 14 August 1998. The positions of the CTD sampling station at dawn each day are indicated.

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Fig. 2. Sections of salinity, temperature and density (sigma θ) taken from CTD casts made during the drift experiment in the upwelling region.

Fig. 3. TS diagram on sigma θ=26.9 surface. T and S values taken directly from CTD data. 䊏 3 August, 쐌 4 August, 䉬 5 August, + 6 August.

current structure measured with the hull-mounted ADCP (Fig. 4) indicates three characteristic regimes. During the first 苲24 hours, a wind driven surface jet with strong southward and weaker eastward flow was apparent. During the next two days (4 and 5 August) the currents were characterised by a return to tidally dominated variations (M2 tide, frequency 12.4 hours), with some baroclinic (depth varying) structure evident. From 6 August, the tidal velocities weakened, which was commensurate with drift out into deeper water and a weakening of the tidal currents as a result of the increase in depth. At the same time there was an increase and shoaling of the northward component, so there was an increase in northward transport of water throughout the entire water column measured. Vertical shear, a change in horizontal currents with depth, is of key interest when discussing a Lagrangian

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Fig. 4. Absolute currents as measured by the hull-mounted ADCP.

type experiment. One deficiency of hull-mounted ADCP data is that its shallowest velocity measurement is at 苲20m. The shear in the upper 20m, that was probably considerable during the period of strong winds at the beginning of the experiment, was therefore unknown. It is possible to estimate whether or not the buoy was affected by wind driven currents in the upper 20m (U20) by comparing the position of the drifting buoy with the trajectory of a particle at different depths, as determined by the ADCP measurements. Figure 5 shows the position of the buoy and the U20 trajectory. These did not diverge greatly during 4 August when the wind was strongest (Table 1). However Fig. 6a shows that, over the same period, the trajectory for the upper 20m moved further to the south (8km) and to the west (3km) than the trajectory at 60m (U60). The greatest difference between buoy and U20 trajectories occurred during 5 August, which accounted for half of the total (苲16km) divergence between buoy position and U20 trajectory shown in Fig. 5. During 5 August, U20 moved further south than U60, and U60 moved further south than U100. The southward acceleration of the buoy during 5 August can only be accounted for by strong southward currents in the upper 20m dragging the whole mooring 苲8km to the south relative to the U20, and 苲11km further south than U60. After 5 August, there was much less divergence between all the trajectories. Calculations for the subsequent days indicate the relative trajectories were near circular (Fig. 6b), illustrating the advective influence of the internal (baroclinic) tide. In summary, although the buoy ended up 苲16km further south than the U20 trajectory and 苲30km further south than the U60 trajectory, there is no evidence to suggest that different water masses were sampled during either experiment. So the following data on the response of plankton to the upwelling event are presented with the assumption that the experiment was truely Lagrangian.

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Fig. 5. Ship position and trajectory at 20m derived from the ADCP data. Triangles and open circles indicate 12 hour intervals for the ship position and 20 m trajectory, respectively, starting at the origin and beginning at 0030 on 3 August 1998.

3.3. Temporal changes in temperature, nutrient salts and suspended organic matter Figure 7 shows the time evolution of temperature (T), nitrate (NO3⫺), nitrite (NO2⫺) and particulate organic nitrogen (PON) in the upper 150m during the drift experiment. Only the 0200 GMT profiles are presented. Marked temperature changes are observed in the near surface as the water mass drifted south along the shelf break. At the beginning of the experiment on 3 August there was a sharp thermocline at 15m and a shallow gradient in temperature below 20m. The temperature in the surface 5m was 15.5°C, and at 50m it was 13.4°C. One day later, the dominant thermocline had deepened to 45m, the temperature in the surface 5m was 15.44°C, whereas at 50m it had risen to 14.23°C. Near surface temperatures continued to increase until 7 August when the surface temperature at 3m depth was 16.98°C, and at 50m it was 14.07°C. During the observation period, the surface 5m warmed by almost 1.5°C over a period of 96 hours. This warming is clearly apparent in the satellite images of Smyth et al. (2001), with the near surface water

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Fig. 6. Relative particle trajectories calculated using the hull-mounted ADCP data, for (a) 20m relative to 60m, and (b) 60m relative to 100m, starting at the origin and beginning at 0030 on 3 August 1998.

Fig. 7. Time evolution of the temperature, nitrate (NO3⫺), nitrite (NO2⫺) and suspended organic nitrogen (PON) profiles during the upwelling drift experiment at dawn from 3 to 7 August 1998. Shaded areas indicate the water volume understudy, with 1% photosynthetic available radiation (PAR) defining the lower limit of depth.

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heating up as it drifted southwards along the shelf break following the upwelling event and during the relaxation period. ´ lvarez-Salgado et al. (2001) show that the contribution of continental runoff to the water mass compoA sition of the surface layer is negligible. More than 99.5% of the shaded area in Fig. 7, with the 1% photosynthetic available radiation (PAR) defining the lower limit of depth, is composed of upwelled ENACW modified only by heating. ENACW extended down from the shallow salinity maximum at 70– 80m during the drift experiment (not shown), to a deep salinity minimum at 450–500m (Pe´ rez, Mourin˜ o, Fraga, & Rı´os, 1993). It should be noted that only the ENACW branches at depths shallower than 150– ´ lvarez-Salgado, 200m (ENACWT) upwell over the western Iberian shelf and reach the photic layer (A Roso´ n, Pe´ rez, & Pazos, 1993). Nitrate concentrations in the 70–150m depth range were 6–8 µmol N l⫺1, as were to be expected for ENACW in the NW Iberian margin during the upwelling season (Castro, Pe´ rez, ´ lvarez-Salgado, & Fraga, 2000). These nitrate concentrations are less than half those of the far “older” A subsurface waters that are upwelled off California, Peru and SW Africa (Castro et al., 2000). These low nutrient concentrations result from the subthermocline waters along the Iberian margin being within the North Atlantic ventilated thermocline region (McCartney & Talley, 1982). The nitrate profile measured in the near surface waters on 3 August suggests that there had already been significant phytoplankton activity within the surface mixed layer before the experiment began. Nutrient concentrations continued to decline in the photic layer during the experiment: between 3 and 5 August nitrate concentrations at 10m reduced from 0.4 to 0.2 µmol N l⫺1; thereafter the concentrations were below the limit of detection of the autoanalyser. At 30m, the nitrate concentration was 3.6 µmol N l⫺1 on 3 August and had declined to ⬍0.1 µmol N l⫺1 by the end of the drift experiment. The profile observed on 4 August is not consistent with there having been a smooth transition with time, since the nitrate concentration at 40m was much lower than on the following day. The anomalous nature of this station is also indicated by the temperature profile; it is possible that sampling coincided with an internal wave, or this profile was not part of a sequence. Relatively high nitrite (⬎0.1µmol N l⫺1) were observed throughout the photic layer at the beginning of the experiment, evolving to a marked subsurface maximum at the base of the pycnocline by 7 August. Ammonium (not shown) was low (⬍0.15 µmol N l⫺1) and remained quite constant throughout the experiment. High nitrate concentrations in the euphotic layer are often accompanied by relatively high nitrite, as observed during 3 August. The nitrite seems to have originated from partial reduction of nitrate by phytoplankton (Wada & Hattori, 1991). As the nitrate becomes progressively reduced in concentration in the euphotic layer, the phytoplankton utilises the nitrite in the near surface water, producing a subsurface nitrite maximum as observed by 7 August. Since the same water mass, ENACW, was sampled throughout the upper 150m, the linear regressions (model II; Sokal & Rolhf, 1995) of nitrate with phosphate and silicate (Fig. 8a and b) give an indication of the stoichiometry of the net consumption of nutrients during the coastal upwelling event. The whole data set has been used in these statistical analyses, i.e. samples taken at 0200, 0900 and 1200 GMT. The correlation coefficient was very good in both cases (r= +0.88) with a N:P slope of 18.3±0.3 mol N:mol P⫺1 and a N:Si slope of 4.0±0.3 mol N:mol Si⫺1. The best fit of the nitrate:silicate data was quadratic rather than linear (dashed line in Fig. 8b). Since nitrate increased with depth, the observed behaviour is consistent with enhanced silica dissolution compared with organic matter mineralisation as one goes deeper in the ENACW domain. The linear silicate:nitrate equation in Fig. 8b (solid line), was obtained using only samples with nitrate ⬍5 µmol l⫺1. The N:P slope is 14% is greater than expected from the average composition of the products of synthesis and early degradation of marine phytoplankton (C106H175O42N16P; Anderson, 1995), suggesting the utilisation of phosphate was more rapid than that of nitrate during this period of nutrient exploitation. The N:Si slope is 300% greater than the average N:Si composition ratio of marine diatoms of 苲1.0 reported by Brzezinski (1985). This is probably a result of the diatoms making a minor contribution to the microplankton assemblages during the drift experiment, despite the upwelling conditions

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Fig. 8. X–Y plots of phosphate versus nitrate (a), nitrate versus silicate (b) and PON versus POC (c) for all waters samples collected in the upper 150m during the upwelling drift experiment. Samples were taken each day at 0200, 0900 and 1200 GMT. The solid line is a linear regression and the dashed line is a quadratic regression for nitrate versus silicate.

(see Section 3.4). Assuming that diatoms had a N:Si molar ratio of 1.0, they could have been responsible for only 25% of the observed nitrate consumption. The intercepts of the regressions in Fig. 8a and b, give key additional information about nutrient cycles along the Iberian margin. Phosphate was still measurable at 0.03±0.01µmol P l⫺1, when nitrate

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concentrations had fallen below the limits of detection of the autoanalyser system; this suggests that nitrate may be the factor limiting primary production. A phosphate excess at zero nitrate had been previously ´ lvarez-Salgado et al., 1993; Castro et al., 2000) and the adjacent rias of Galicia documented in the shelf (A (Nogueira, Pe´ rez, & Rı´os, 1997). A silicate excess (0.6±0.1 µmol Si l⫺1) was also observed at zero nitrate concentrations. This value is within the wide range (0.3–2.0 µmol Si l⫺1) of threshold silicate concentrations necessary for the inherently high growth rate of diatoms at non-limiting silicate concentrations (Egge & Aksens, 1992) and so could be the reason for the reduced contribution of diatoms to the biomass. PON is the initial end-product of nitrate consumption, which is subsequently transformed into DON and ´ lvarez-Salgado et al. (2001) sinking particulate nitrogen in the photic layer. The companion papers of A and Olli et al. (2001) deal with the dissolved and sinking organic matter pools, respectively. The PON profiles are shown in Fig. 7 and in general, they were the inverse of those of nitrate. A PON excess was observed in the photic layer (average ± SD was 1.27±0.35µmol N l⫺1) compared with the constant concentration in the ENACW below (average ± SD at 150m was 0.11±0.01µmol N l⫺1). At the beginning of the drift experiment, PON concentration in the upper 15m was 1.4µmol N l⫺1 and decreased below this depth; the average PON over the photic layer reduced to 0.7µmol N l⫺1. Maximum PON concentrations during the drift experiment were recorded on 4 August at 20m (2.5µmol N l⫺1), with a mean photic layer concentration of 1.8µmol N l⫺1, i.e. more than double that of the previous day. This dramatic PON accumulation in the photic layer accounted for about half of the observed nitrate decrease over the same period. From 5 to 7 August, PON in the photic layer remained at 1.29±0.04 µmol N l⫺1. PON was correlated with POC throughout the upper 150m (Fig. 8c), with a C:N slope of 5.9±0.2 molC molN⫺1. This was 10% lower than the expected value of 6.6 from the average composition of marine phytoplankton (Redfield, Ketchum, & Richards, 1963; Anderson, 1995), suggesting that the nitrogenous fraction of POM was highly labile. This fraction represents about 85% of the POM in the upper 150m; the remaining 15% was composed of N-free POC, as indicated by the intercept of the POC:PON regression of 0.9±0.3µmol C l⫺1 (Fig. 8c). Average C:N ratios for the photic zone were 6.9±0.7molC molN⫺1 in the photic layer and 11.0±1.8 molC molN⫺1 in the 80–150m depth range. This marked increase of the C:N ´ lvarez-Salgado, Doval, & ratio of POC with depth has been previously reported in the Iberian margin by A Pe´ rez (1999) and it is a trend commonly observed in other marine provinces as a result of the more intense recycling of nitrogenous compared to carbonaceous compounds (Copin-Montegut & Copin-Montegut, 1983). PON (Fig. 7) and chlorophyll concentration (Fig. 9) in the photic layer covaried during the drift experiment (r= +0.72, p⬍0.01, n=32). The slope of the POC:chlorophyll plot was equivalent to 57±10gC gChl⫺1,

Fig. 9.

Chlorophyll concentration at dawn each day in the upper 100m of the water column.

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which is very close to the average ratio of 50 characteristic of healthy phytoplankton populations (Longhurst, 1995). The intercept was very large (36±12 µg C l⫺1) indicating that a substantial part of the POC that had accumulated in the photic layer was composed of non-chlorophyll materials, presumably microheterotrophs and detritus. Phytoplankton represented 65% and non-phytoplankton 35% of suspended POM in the photic layer, and the resultant average ± SD direct C/Chl ratio was 91±22gC gChl⫺1. Therefore, during this drift experiment, the system was essentially autotrophic. 3.4. Development of phytoplankton-microplankton biomass As was expected in an upwelling area, the phytoplankton biomass increased during the experiment. Chlorophyll concentration was 1.2µg l⫺1 at 10m at the beginning of the experiment, which is consistent with the observed reduction of nitrate concentration in the surface 10m. During the five day period of the drift experiment, chlorophyll concentrations in the upper 10m declined and a sub-surface chlorophyll maximum developed. Figure 9 shows that the depth of the chlorophyll maximum increased from the upper 20m to 苲35m after 5 days; the maximum chlorophyll concentration measured was 2.6µg l⫺1 at 35m at midday on 6 August. The decline in chlorophyll concentrations in the surface 10m coincided with the reduction in nitrate concentration (Fig. 7). The phytoplankton biomass changed during the drift south from a maximum biomass in the upper 20 m to a chlorophyll maximum at 苲30 m. Microplankton abundance during the 5 day period (Fig. 10a) was dominated by small cells, mainly small flagellates and Cryptophyceae, representing 苲70% of the total cell abundance. Diatoms were scarce (4%) at the beginning of the experiment but had increased in importance by the end (25%), when they contributed to the formation of the subsurface chlorophyll maximum at 苲30 m (see Table 3). The most abundant species of diatoms belonged to the genus Pseudo-nitzschia (Pseudo-nitzschia cf. delicatissima and P. cf. seriata). Dinoflagellates were present (12%) during the whole period but were more abundant during the first three days of observation, when diatoms abundance was lower. Small autotrophic dinoflagellates like Heterocapsa niei, Gymnodinium simplex and G. nanum, as well as small heterotrophic species (Amphidinium flagellans and Gymnodinium agiliforme) coexisted with large heterotrophic dinoflagellates (Gyrodiniumn fusiforme and G. spirale). The abundance of ciliates was low (1%) with the small autotrophic species Mesodinium pulex and the small heterotrophic species Strombidium minutum forming the bulk of the population. However, individuals of large heterotrophic species (Strombidium cornucopiae, S. cornutum, S. cf. testaceum and Metastrombidium cf. sonnifer) were conspicuous during the last two days of the experiment. This microplankton composition, characterised by mixed populations of autotrophic and heterotrophic species, is typical for the upwelling period in the Galicia coast (Figueiras & Niell, 1987) when the increase in phytoplankton numbers in the upwelling provokes an immediate response by microzooplankton grazers. The difference in size of the numerically dominant microplankton species means that the contribution of the four groups to the total biomass differed significantly from the total cell abundance (Fig. 10b). Small flagellates and Cryptophyceae, in conjunction with other small forms like Leucocryptos sp. and Solenicola setigera, contributed only 18% to the total microplankton carbon content. Whereas, dinoflagellates represented on average 67% of the total biomass. This was largely because of the large heterotrophic species Gyrodinium spirale, which alone on average contributed 58% of the total dinoflagellate biomass. Dinoflagellates showed a steady decline in their contribution to total biomass from 80% on day 3 to 46% on day 5, while there was a corresponding increase by diatoms and ciliates. Diatoms represented 2% of total biomass on day 3 and 24% on day 5, and the contributions by ciliates were 1% and 10% respectively. Mean biomass of diatoms and ciliates within the water column was positively correlated (r= +0.96) whereas diatoms and G. spirale showed a negative correlation (r= ⫺0.54). G. spirale was positively correlated (r = +0.78) with the biomass of small forms (labelled “others” in Fig. 10). This suggests that trophic relationships within microplankton evolved from large dinoflagellate/small flagellate interactions at the beginning

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Fig. 10. Mean microplankton cell concentration (a) and carbon biomass (b) at dawn each day in the water column during the coastal upwelling drift experiment.

of the experiment towards a large ciliate/diatom interaction at the end. The body size ratio between the more abundant diatoms (Pseudo-nitzschia cf. seriata, P. cf. delicatissima, Leptocylindrus danicus) and the largest ciliates is well below 0.125 and, therefore, grazing might be possible (Hansen, Bjørnsen, & Hansen, 1994). In addition, bacterial abundance was the highest during the last two days of observations (Barbosa et al., 2001). Table 3 lists the main microplankton species present in water sampled at dawn each day during the upwelling drift. These data refer only to ⬎5µm cells, which can be readily visualised in the light microscope, and so exclude all picoplankton. Picoplankton activity was significant throughout the experiment and Joint, Rees and Woodward (2001) found that primary production rates of picoplankton ⬍2µm were of the same magnitude as the ⬎5µm phytoplankton. The estimated biomass of dinoflagellates was significantly greater than that of diatoms, with the nonpigmented species having the larger biomass (Table 3). The biomass of diatoms increased from 1.63µgC l⫺1

Diatoms Chaetoceros convolutus Hemiaulus hauckii Leptocylindrus danicus Pleurosigma elongatum Pseudo-nitzschia cf. delicatissima Pseudo-nitzschia cf. seriata Detonula pimula Total Diatom biomass Photosynthetic Dinoflagellates Heterocapsa niei Gymnodinium simplex Total Photosynthetic Dinoflagellate biomass Non-pigmented Dinoflagellates Gyrodinium fusiforme Gyrodinium spirale Non-pigmented Dinoflagellate Biomass Other phytoplankton Cryptophyceae Small flagellates Total Biomass of other phytoplankton Total Ciliate biomass

Date 3 Depth

0 0 0.02 0 0.07 0.14 0 0.34 0.08 0.08 0.39

1.62 35.44 40.50 0.71 5.99 6.70 0.14

1.08 1.45 5.86

4.04 51.16 64.36 3.49 6.87 13.52 0.68

30m

0.01 0 0.44 0.16 0.41 0 0.08 1.63

August 3 15m

4.41 9.13 16.43 1.12

2.43 27.52 35.2

0.75 1.67 2.99

0 0 0.32 0.55 0.39 0.73 0.76 3.31

August 4 15m

1.79 5.05 7.12 1.19

5.66 66.89 77.45

0.42 0.43 1.11

0.02 0 0.28 0.75 0.27 0.39 0.08 2.59

30m

5.49 8.07 15.43 3.04

4.04 86.54 99.06

1.08 2.01 5.78

0.12 1.90 0.02 0 0.50 0.22 0.53 5.60

August 5 15m

0.87 4.26 6.03 0.26

1.62 15.72 23.54

0 0.33 0.53

0.01 0.73 0.05 0 0.29 0.07 0.23 2.38

30m

0.37 2.10 2.80 2.62

1.62 7.86 10.92

0.08 0.21 1.09

0.23 0 0.09 0.62 0.29 1.65 0.99 5.83

August 6 15m

0.55 2.13 2.46 1.20

0.81 23.59 27.24

0.17 0.19 0.96

0 0 0.23 0.36 0.19 0.63 0.53 5.49

30m

1.25 6.87 10.95 7.36

2.43 27.52 31.69

0.25 0.29 0.84

0 0.11 1.38 0.57 0.74 2.50 1.37 7.21

August 7 15m

1.5 6.49 9.75 1.59

1.62 0 6.02

0.25 0.21 1.35

6.35 0.22 1.64 0.40 0.96 10.1 1.68 23.01

30m

Table 3 Microplankton species making major contribution to biomass (µg C l⫺1) during the upwelling drift experiment: water samples taken from 15m and 30m depth at dawn on each day

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at 15m depth on the first day to 7.21µgC l⫺1 at the same depth on 7 August. At 30m, the depth at which the subsurface chlorophyll maximum developed (Fig. 9), the total diatom assemblage increased from 0.34µgC l⫺1 to 23.01µgC l⫺1 over the five day period. Most of this increase in diatom biomass was the result of the growth of Pseudo-nitzschia cf. seriata, although there was also a large increase in Chaetoceros convolutus at 30m on the final day. Small flagellates also made a major contribution to the phytoplankton biomass. Dinoflagellates were a major component of the microplankton throughout the cruise. The pigmented dinoflagellates, which are assumed to be autotrophic although some species may be capable of mixotrophy, showed a steady decline (Table 3) from an initial biomass of 5.86µgC l⫺1 at 15m on 3 August to 0.84µgC l⫺1 on 7 August. The biomass of the autotrophic dinoflagellates was substantially less than cells with no pigment (Table 3), whose biomass was as high as 99µgC l⫺1 at 15m on the third morning of the experiment. Ciliate biomass increased by an order of magnitude during the course of the experiment (Table 3). There were major changes in the estimated biomass of some of the species, which were not consistent with population growth. For example, the major component of the biomass at 30m on 7 August was Pseudonitzschia cf. seriata, with an estimated biomass of 10.1µgC l⫺1. Whereas on the previous day, the biomass at 30m had been 0.63µgC l⫺1. To explain such an increase in biomass, very rapid growth, with a generation time of 6 hours, would be required. This seems unlikely, so it is probable that there was greater lateral and/or horizontal patchiness in the species distributions than would be predicted from the physical characteristics of the water mass, possibly as a result of the influence of outflows from the Rias Baixas (Tilstone, Figueiras, & Fraga, 1994). 3.5. Microplankton assemblage composition The first three components extracted by the PCA accounted for 46% of the total variance. The first principal component (PC 1) explained 25%, the second (PC 2) 14% and the third (PC 3) 7%. Four microplankton species (Cochlodinium brandti, Navicula sp., Cylindrotheca closterium and Cochlodinium helix) showed negative loadings and several diatoms had their highest positive loadings with PC 1 (Table 4). The vertical and temporal distributions of PC1 (Fig. 11) followed that of chlorophyll (Fig. 9) indicating that this component is related to the variability associated with the development of autotrophic microplankton biomass. In fact, PC 1 had a positive correlation with chlorophyll concentration (r= +0.59), diatom carbon biomass (r= +0.68) and small flagellate carbon biomass (r= +0.40). PC 2 reflected differences between two assemblages dominated by large diatoms with positive loads (Proboscia alata, Guinardia delicatula, Dactyliosolen fragilissimus, Pleurosigma elongatum) and small autotrophic, flagellated forms, with negative loads (Gymnodinium simplex, Cryptophyceae and small flagellates); but Gyrodinium spirale and G. fusiforme also had negative loads with PC 2 (Table 4). PC 2 was positively correlated with carbon biomass of small flagellates (r= +0.61) and with dinoflagellates biomass (r= +0.40). Distributions of PC 2 (Fig. 11) indicate that diatoms were confined to deep waters (positive values) during the first three days, whereas small forms and large dinoflagellates were located in the surface waters. During the last two days of sampling, diatoms developed in subsurface waters where a chlorophyll maximum had formed. The chlorophyll maximum at 20m comprised small autotrophic flagellated species (Gymnodinium simplex, Cryptophyceae and small flagellates) as is indicated by the negative values of PC 2 at this depth (Fig. 11). The presence of large diatoms below the chlorophyll maximum has been previously observed in this area at times of upwelling relaxation when subsurface chlorophyll maxima develop (Figueiras & Pazos, 1991). Within and below this maximum, autotrophic species were generally found. Above it, in the surface waters, microplankton assemblages were dominated by large heterotrophic forms. The third component (PC 3) differentiated an assemblage, basically dominated by dinoflagellates (positive values), two diatoms (Nitzschia longuissima and Pleurosigma elongatum) and the autotrophic

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Table 4 Correlation coefficients of the species selected for Principal component Analysis (PCA) with the first 3 principal components. Species are ordered according to PC1. The higher positive and negative loads for PC2 and PC3 are given in bold Taxon Leptocylindrus danicus Detonula pimula Pleurosigma elongatum Pseudo-nitzschia cf. delicatissima Pseudo-nitzschia cf. seriata Nitzschia longuissima Proboscia alata Leptocylindrus minimus Guinardia delicatula Thalassiothrix nitzschioides Strombidium cornucopiae Leucocryptos sp. Heterocapsa niei Gymnodinium simplex Cryptophyceae Dactyliosolen fragilissimus Eutreptiella sp. Strombidium minutum Mesodinium pulex Chaetoceros convolutus Small flagellates Amphidinium flagellans Lohmaniella spiralis Laboea strobila Thalassiosira pseudonana Gymnodinium nanum Navicula ostrearia Torodinium robustum Gyrodinium fusiforme Gyrodinium spirale Rhizosolenia setigera Rhizosolenia styliformis Hemiaulus hauckii Gymnodinium agiliforme Tetraselmis sp. Cochlodinium helix Cylindrotheca closterium Navicula sp. Cochlodinium brandti

PC1 0.847 0.798 0.763 0.745 0.742 0.696 0.688 0.681 0.667 0.664 0.612 0.593 0.567 0.565 0.563 0.560 0.549 0.486 0.470 0.456 0.450 0.404 0.398 0.365 0.329 0.321 0.302 0.273 0.263 0.235 0.221 0.220 0.188 0.186 0.035 ⫺0.003 ⫺0.113 ⫺0.319 ⫺0.335

PC2 0.121 ⫺0.024 0.411 0.085 0.406 0.429 0.514 0.332 0.470 ⫺0.211 0.183 ⴚ0.499 ⴚ0.428 ⴚ0.629 ⴚ0.608 0.433 ⫺0.118 ⫺0.102 ⫺0.044 0.047 ⴚ0.579 ⴚ0.503 0.154 0.003 ⫺0.260 ⴚ0.501 ⴚ0.523 0.007 ⴚ0.368 ⴚ0.384 0.311 0.213 ⴚ0.541 ⴚ0.544 ⫺0.135 0.258 ⴚ0.554 ⴚ0.585 ⫺0.048

PC3 ⫺0.061 ⴚ0.331 0.269 ⫺0.192 ⫺0.175 0.402 ⫺0.089 0.115 0.023 ⫺0.086 ⫺0.130 ⫺0.026 ⫺0.038 0.186 0.209 ⫺0.226 ⫺0.201 ⫺0.017 0.397 ⴚ0.325 ⫺0.032 0.259 ⫺0.185 ⫺0.219 ⫺0.063 0.233 ⫺0.019 0.237 0.264 0.392 ⴚ0.388 ⴚ0.335 ⴚ0.548 0.292 0.248 0.170 ⴚ0.611 ⴚ0.532 0.063

ciliate Mesodinium pulex, from a diatom assemblage with negative values (Table 4). This component is related to the relative contribution of each microplankton group to the total biomass as indicated by the positive correlation with the percentage of dinoflagellates (r= +0.48) and the negative correlations with diatoms (r= ⫺0.48) and small flagellates (r= ⫺0.30). Its distribution (Fig. 11) shows the clear dominance of dinoflagellates during the first two days of observation and on the last day in the surface layer, above the diatom dominated, subsurface chlorophyll maximum. The diatom community defined by this third component was relatively important on days 5 and 6, during the transition to relaxation, and at the 30m depth subsurface chlorophyll maximum on day 7 (Fig. 11).

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Fig. 11. Time evolution of the three principal components (PC1 – PC 3) profiles extracted by the Principal Component Analysis of microplankton at dawn each day during the upwelling drift experiment.

Separation between large heterotrophic microplankton in the surface waters and large diatoms in and below the chorophyll maximum has been shown for other areas (Cullen, Reid, & Stewart, 1982; Estrada, 1985). We found a similar pattern but in addition, small autotrophic flagellated forms were important during the first stages of the development of the subsurface chlorophyll maximum, which occurred during the upwelling-relaxation event. Table 5 shows the resulting canonical structure of the CCA made with the state variables and the three principal components of microplankton. The first canonical variable relates PC 1 positively with chlorophyll, temperature, PON and, to a lesser extent, with DON. This supports the conclusion that PC 1 defined a microplankton assemblage related with the development of phytoplankton biomass, and is confirmed by Table 5 Canonical correlations of pairs of canonical variables (V1 – V3) and correlations (loads; LV1 – LV3) between original and canonical variables (P ⬍0.01). PC 1 – PC 3 principal components of microplankton, PON particulate organic nitrogen, DON dissolved organic nitrogen Canonical variables Canonical correlation

V1 0.84 LV1

V2 0.62 LV2

V3 0.45 LV3

PC 1 PC 2 PC 3 Salinity Temperature Chlorophyll PON DON Nitrates + Nitrites

0.927 ⫺0.084 ⫺0.340 ⫺0.335 0.735 0.786 0.548 0.243 ⫺0.868

⫺0.136 ⫺0.996 0.021 ⫺0.385 0.360 0.153 0.698 0.721 ⫺0.308

⫺0.349 0.022 ⫺0.940 0.741 0.073 0.101 ⫺0.261 0.472 0.130

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the highest load with chlorophyll. The positive relationship with temperature indicates that this chlorophyll development encompassed upwelling/relaxation and, therefore, water column stratification. This development of the diatom assemblage responsible for the chlorophyll enhancement is not followed by a DON increase. The DON and PON increases are related to the importance of small flagellated forms (negative loads with PC 2) as is indicated by the relationship between negative values of PC 2 and positive values of PON and DON with the second canonical variable (Table 5). Since Gyrodinium spirale and G. fusiforme, the two dinoflagellates responsible for the high biomass values, have important negative loads with PC 2 (see Table 4), it can be deduced that high concentrations of DON and PON were the result of the large dinoflagellates/small forms assemblage in which presumably trophic interactions might be important. The third canonical variable related high salinity values, and also relatively high DON concentrations, with the diatom community defined by PC 3 (negative values on PC 3) and relative low values of biomass (negative values on PC 1). The most striking result of the CCA analysis is that the highest DON concentrations were not associated with high autotrophic biomass (first canonical variable). On the contrary, they were related to high biomass values of heterotrophic/autotrophic microplankton assemblages (second canonical variable), which suggests that a considerable part of DON production may be a result of trophic relationships rather than direct release from phytoplankton (Strom, Benner, Ziegler, & Dagg, 1997).

4. Experiment 2 — the off-shore filament The second experiment had a very similar rationale to the first but took place in an off-shore filament. The position for initial deployment of the drifting buoys was selected on the basis of satellite SST images transmitted to the ship. In this experiment, the water mass was marked with five Argos drifting buoys (Barton et al., 2001). The same sediment trap and production array as was used in the first experiment was deployed again but in this case, it was not used as the primary indicator of water movement. Rather, this was taken from the movement of the central Argos buoy, which was drogued at 20m. The sediment traps were still deployed as a free-floating array, but this time at 8 depths at intervals between 10 and 200m. Even with each sediment trap tending to act as a drogue, the daily movement of this array was still very similar to that of the central Argos buoy drogued at 20m. 4.1. Tracks of the drifting buoys The track followed by the central Argos drifting buoy drogued at 20m is shown in Fig. 1 and the station positions, wind speed at the time of sampling and drift rate between stations are given in Table 2. The drift was consistently off-shelf, even when the wind had a NE component. The rate of drift was slow and the distance covered each day by the marker buoy was much less than during the first experiment. The physics of the filament drift is the subject of another paper (Barton et al., 2001). The conclusion is that this experiment in the off-shelf filament could also be considered to be Lagrangian and that it is appropriate to consider that the day-to-day variations in chemical and biological parameters were not being affected by mixing of different water masses. 4.2. Temperature, nutrients and suspended organic matter in the filament Temperature profiles during the filament drift (Fig. 12) contrasted with those recorded during the first experiment (Fig. 7). A marked seasonal thermocline isolated the warm (⬎17°C) upper mixed layer from the ENACW below. The base of the thermocline was at 40–45m depth, which roughly coincided with the 1% PAR depth through the experiment, except on 15 August when it was at about 30m. Nitrate concentrations were below the detection limits of the autoanalyser in the photic layer, but the

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Fig. 12. Time evolution of the temperature, nitrate (NO3⫺), nitrite (NO2⫺) and suspended organic nitrogen (PON) profiles during the filament drift experiment at dawn from 14 to 17 August 1998. Shaded areas indicate the study water volume, with the 1% photosynthetic available radiation (PAR) defining the lower limit of depth.

application of the more sensitive analytical techniques of Garside (1982) and Jones (1991) allowed us to quantify the nitrate and ammonium concentrations. Figure 13 shows a vertical profile of nitrate and ammonium on 15 August. Nitrate concentrations were ⬍10 nmol l⫺1 in the surface 20m and increased at the thermocline. Ammonium concentrations were also very low (苲20 nmol l⫺1) throughout the upper 100m, but there were occasional samples with increased concentrations (such as at 15m in Fig. 13). Generally,

Fig. 13.

Vertical profiles of nitrate and ammonium measured at low nanomolar concentrations on 15 August 1998.

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measured ammonium concentrations were very low in the upper water column throughout this experiment. These represent extremely oligotrophic conditions. The nitrite profiles showing a small subsurface maximum (⬍0.15 µmol l⫺1) at about 50m depth just below the thermocline, were also typical of oligothrophic waters. Wada and Hattori (1991) have referred to this as the primary nitrite maximum. Such a maximum is a frequent characteristic of the stratified and N-exhausted oceanic waters off NW Spain (Castro et al., 2000). Wada and Hattori (1991) suggested that light inhibition of nitrifying bacteria, which oxidise nitrite to nitrate, was the reason for the deep nitrite maximum in oligothrophic waters. Phosphate and silicate were well correlated with nitrate throughout the upper 150m (r ⬎ +0.96; Fig. 14a and b). The intercept and slope of the corresponding linear regressions did not differ essentially from those observed in the coastal upwelling drift experiment. As in the upwelling drift experiment, it is clear that the silicate and nitrate fitted a quadratic rather than a linear relationship. Again, a linear regression was obtained for samples with nitrate ⬍5µmol l⫺1, highlighting that, despite nitrate exhaustion in the photic layer, measurable concentrations of phosphate (0.03±0.01 µmol P l⫺1) and silicate (0.7±0.1 µmol Si l⫺1) were still observed. PON levels were lower than in the first experiment in the upwelling region (Fig. 12), as would be expected in oligothrophic conditions. The average (± SD) concentration in the photic layer was 0.98 (±0.15)µmol N l⫺1 and decreased in the ENACW domain to 0.23 (±0.01)µmol N l⫺1 below 100m. It is interesting that, whereas the PON profiles were quasi-homogeneous throughout the photic layer, the chlorophyll profiles were characterised by a deep chlorophyll maximum (DCM) at the base of the thermocline (Fig. 15). This results in very high direct carbon:chlorophyll (C:Chl) ratios in the upper 20m (average ± SD was 380±90gC gChl⫺1). Assuming the same phytoplankton C:Chl ratio as in the first experiment would suggest that only 15% of the POC in the upper 20m was contributed by phytoplankton. Considering the entire photic layer, the C:Chl-a ratio was 295±130gC gChl⫺1 and phytoplankton would make up about 20% of POC. This number is in clear contrast with the 65% obtained in the upwelling experiment. In spite of phytoplankton being a relatively minor fraction of POC, the direct correlation (model II) of POC and PON was very good (Fig. 14c) and the C:N slope of 6.3±0.3 indicates that the material was still labile. The intercept, equivalent to 18±5µg C l⫺1, represented 22% of the average POC in the upper 150m and is higher than the 15% obtained in the first experiment. This suggests that in this N-limited system, the metabolic balance of the phytoplankton might have resulted in excess production of carbohydrates and other cell constituents, which do not contain nitrogen. 4.3. Chlorophyll and microplankton in the filament Vertical profiles of chlorophyll concentration are shown in Fig. 15. Concentrations in the upper 30m were very low (typically ⬍0.3µg l⫺1) but at most stations there was a slight chlorophyll maximum towards the base of the thermocline. Chlorophyll concentrations at these depths were still low; the maximum concentration measured being 1.8µgC l⫺1 at 47m on 14 August during the final CTD cast of the experiment. The phytoplankton assemblage was dominated by picophytoplankton (⬍2µm) which accounted for 苲60% of the total primary production (Joint et al., 2001). Of the larger phytoplankton, almost no diatoms were detected in water samples and the assemblages contained both pigmented and non-pigmented dinoflagellates (Table 6). Most of the biomass of the pigmented dinoflagellates resulted from the presence Gymnodinium simplex and from some large Ceratium species. As in the coastal upwelling experiment, small flagellates were the most abundant microplankton and dinoflagellates accounted for the largest fraction of the biomass (Fig. 16). Total microplankton biomass was well correlated with dinoflagellate (r=0.99) and small flagellate biomass (r=0.80). Gyrodinium spirale accounted for 67% of the total dinoflagellate biomass. Dinoflagellate composition was very similar to that of the upwelling drift experiment except for the presence of large Ceratium species, which are characteristic of open ocean and oligotrophic waters in the region (Figueiras & Pazos, 1991).

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Fig. 14. X–Y plots of (a) phosphate versus nitrate, (b) nitrate versus silicate and (c) PON versus POC for all water samples collected in the upper 150m during the filament drift experiment. Samples were taken at 0200 and 0900 GMT each day. The solid line is a linear regression and the dashed line is a quadratic regression for nitrate versus silicate.

Principal Component Analysis extracted only a single component, which explained 30% of the variance. It related biomass variations among days, as is indicated by its correlation with total biomass (r=0.75), dinoflagellate biomass (r=0.74) and biomass of small flagellates (r=0.63).

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Fig. 15.

245

Chlorophyll concentrations measured at dawn during the drift experiment in the filament.

Table 6 Microplankton species making major contribution to biomass (µgC l⫺1) during the off-shelf filament drift experiment: water samples taken from 10m and 50m depth at dawn on each day Date Depth Autotrophic Dinoflagellates Heterocapsa niei Ceratium cf. concilians Ceratium furca Ceratium horridum Ceratium symmetricum Gymnodinium simplex Oxytoxum cf. crassum Total pigmented Dinoflagellates Non-pigmented Dinoflagellates Amphidinium flagellans Gymnodinium agiliforme Gyrodinium fusiforme Gyrodinium spirale Total non-pigmented Dinoflagellates Small Flagellates Total ciliates

14 August 10m

50m

0.58

0.08

16 August 10m

0.17 0.17

0.08

50m

0.08 0.08

0.66 0.34 0.24

0.17

0.14 1.67

0.10

0.06

1.84

0.16

0.46

2.04

0.22 0.92

0.45 0.92

3.93 5.19 2.63 0

3.93 5.47 2.17 0

0.22 1.23 0.01 15.72 19.13 1.45 1.48

0.62 0.40 1.94 2.46 2.78

17 August 10m

0.92 19.66 20.9 6.16 0

50m

0.25 0.16 1.68 0.60 0.48 4.06 3.36 4.93 2.43 31.51 45.72 7.26 3.10

5. Conclusions This paper has described the general hydrography, nutrient chemistry and the structure of the microplankton assemblage during two experiments, which were designed to be Lagrangian. It is clear that, although there was some deviation between actual and modelled positions of the water masses sampled over the two five day periods, there was no significant evidence of dispersion. Therefore, the changes in biology and chemistry observed during the courses of these drift experiments are assumed to be a consequence of

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Fig. 16. (a) Mean microplankton cell concentration and (b) carbon biomass at dawn in the water column during the drift experiment in the filament.

the activity of the biota within those water masses; i.e. the experiments did not differ significantly from true Lagrangian. This paper provides the context within which to relate the observational data and interpretation of the other papers in this Special Issue.

Acknowledgements We are very grateful to the captain and the crew of RRS Charles Darwin for their assistance during the sampling programme. We thank Denise Cummings for operating the autoanalyser during sampling of

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the upwelling region. This work was supported by the European Union in the framework of the Mast 3 Programme, contract number MAS3-CT97-0076. (OMEX II project), by the DYME Project of the CCMS Plymouth Marine Laboratory, a component of the Natural Environment Research Council and by the Spanish Comisio´ n Interministerial de Ciencia y Tecnologı´a grant No MAR97-1729-CE and MAR1997-2068-CE.

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