F. G. Figueiras

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian Upwelling System

Daily changes in the concentrations of carbon and nitrogen species were monitored during the course of a Lagrangian drifter experiment in a recurrent upwelling filament south of Cape Finisterre (NW Iberian Upwelling System). A drifting buoy released at the southern edge of the upwelling centre generated by the Cape moved 60km southwestwards from 3 to 7 August 1998. Organic matter in the 50m deep study water mass (average 77±2 µM C) consisted of: 57µ MC of dissolved organic matter (DOM) with a C/N molar ratio of 19±2; 6µM C of DOM with a C/N ratio of 9±2, and 14µM C of 50% DOM and 50% suspended organic matter (POM susp) with a C/N ratio of 6.0±0.4. Net conversion of consumed inorganic salts into accumulated TOM=POMsusp+DOM was 40% for nitrogen and 30% for carbon. Since the parcel of water crossed the shelf-edge, these conversion efficiencies are equivalent to net horizontal export-ratio of 0.4 and 0.3 respectively. A second drifter was deployed in the offshore-end of the filament, and was displaced 20km west between 14 and 17 August 1998. Nitrate was exhausted in the surface water and no significant changes were observed in the variables measured during the course of the second experiment. Low C/N ratios (6.5±0.4) and rapid POMsusp/DOM inter-conversion in the 20 µM C excess observed in the study volume points to the persistence of the labile materials formed on the shelf during transport to the ocean. Our data demonstrate

Evidence of in situ diel vertical migration of a red-tide microplankton species in Ria de Vigo (NW Spain)

The migration capacity of red-tide species in the natural environment was studied at a station in the Ría de Vigo (Rías Bajas, NW Spain) over a 24 h period in September 1991. The Ría de Vigo, where red tides are frequent, normally shows a positive estuarine circulation and is subjected to seasonal upwelling and downwelling phenomena. A marked diel pattern was observed for five species that are capable of causing red tides (Ceratium furca, Scrippsiella trochoidea, Dinophysis acuminata, Mesodinium rubrum, and Eutreptiella sp.). Such diel behaviour could be clearly advantageous in a stratified environment where light and nutrients are often in two separate layers. Active movement enables species such as dinoflagellates and some ciliates to exploit high levels of irradiance at the surface during the day and to take up nutrients in deeper layers at night. Patchy distribution of phytoplankton, shear-induced horizontal dispersion and density variations were considered, but none of them accounted for the vertical changes observed. Vertical migration is thought to be one of the mechanisms that could promote blooms in nutrient-depleted surface layers.

Short-term variability of heterotrophic bacterioplankton during upwelling off the NW Iberian margin

Short-term variability of heterotrophic bacterioplankton was studied in a recently upwelled water mass at the NW Iberian margin (August 1998). Bacterioplankton abundance (BA), biomass (BB), production (BP), and specific pro- duction (SBP) were monitored during two Lagrangian drift experiments, one along the shelf-edge, the other off-shelf along an upwelling filament. Other measurements included chlorophyll a (Chla), primary production (PP), suspended particulate organic carbon (POC) and nitrogen (PON), and dissolved organic carbon (DOC) and nitrogen (DON). Although primary production was significantly higher during the shelf-edge drift experiment, bacterial biomass in the euphotic zone (2.68 to 22.20µgC.l 1 ) was not significantly different from that in the offshore filament. In contrast, bacterial production (0.13-3.52µgC.l 1 .d 1 ), estimated using an empirically determined 14 C-leucine to carbon conver- sion factor, and bacterial growth rates (doubling time, DT: 3.9-29.7d), were significantly higher during the shelf-edge drift (BP: 1.50±0.11 versus 0.50±0.02µgC.l 1 .d 1 ; DT: 6.9±0.3 versus 16.2±0.9 d; p0.01). Depth-integrated BB over the euphotic zone comprised 15±1% of phytoplankton biomass during shelf-edge drift and 39±4% under the more oligotrophic conditions in the filament. However, daily BP to net primary production ratios were not significantly different in the two regions (6±1% versus 7±1%). BA, BB, BP and SBP were enhanced in the later part of the shelf- edge drift following a pronounced increase in both PP and gross DOC production, suggesting that phytoplankton was a source of substrates for bacteria in recently upwelled waters. This contrasted with the filament drift in which short- term variability of bacterioplankton was much less pronounced and there was no correlation between BP and PP. In both regions, SBP and DOC in the euphotic zone were significantly correlated (p 0.005) indicating some regulatory effect of DOC over bacterial activity. Bacterial carbon demand in the euphotic zone ranged from 22.5 to 44.0µmolC.m 3 .h 1 , and 10.6 to 11.3µmolC.m 3 .h 1 and represented 62% and 43% of overall gross DOC production during shelf-edge and filament drift experiments. During the filament drift, the relatively high bacterial doubling times coupled with high percentage of non-metabolised DOC (ca. 60%) suggested control of bacteria by either bioavailability of DOM or inorganic nutrients. Between 40 and 60% of gross DOC production was not metabolised by heterotrophic bacteria, thus allowing DOC net accumulation and off-shore advection.  2001 Elsevier Science Ltd. All rights reserved.

Transient hydrographic and chemical conditions affecting microplankton populations in the coastal transition zone of the Iberian upwelling system (NW Spain) in September 1986

The coastal transition zone and adjacent continental shelf of the Iberian upwelling system was studied in September 1986, during the seasonal transition from upwelling-favored to downwellingfavored winds. The most striking features in the coastal transition zone were: (1) a poleward flow of high salinity Eastern North Atlantic Central Water (subtropical) off the Rias Baixas (Galician western coast); and (2) an anticyclonic eddy off Cape Ortega1 (Galician northern coast). Chemical and biological similarities between both structures, clearly different from the surrounding oceanic waters, suggest that the eddy was an isolated and aged parcel of water originating from the poleward flow. The continental shelf was characterized by: (1) outwelling of chlorophyll-rich waters from the four large coastal embayments (the “Rias Baixas”) in the western coast and; (2) an upwelling front off the northern coast. The coexistence of opposite hydrographic structures, as the poleward flow and the upwelling front, was the consequence of transitional wind conditions in September-October, and we hypothesized transitional conditions to be crucial for the development of the eddy. Both the poleward flow and the eddy precluded the shelf-edge exchange of microplankton populations developed over the shelf, leading to massive in situ sedimentation and subsequent nutrient mineralization over the shelf.

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

Abstract Two Lagrangian drift experiments were carried out at the NW Iberian margin. The first tracked a body of nutrient-rich, 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 near-surface 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 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.

The response of phytoplankton production to periodic upwelling and relaxation events at the Iberian shelf break: estimates by the 14C method and by satellite remote sensing

Abstract The primary productivity of the NW Iberian margin has been determined by a combination of 14 C incorporation experiments on natural phytoplankton assemblages and satellite remote sensing with the ocean colour sensor, SeaWiFS. The approach to the field determination of phytoplankton production was Eulerian, involving measurements on a fixed grid of stations covering the region from 41°30′N to 43°00′N and 09°00′W to 10°30′W. Three different laboratories made primary production measurements over a 3-year period. Each group used variations of the 14 C method—in situ incubations and two modelling approaches based on P – E parameter derivation. An intercalibration experiment showed acceptable agreement between the three estimates. Measurements by the three groups on eight cruises have been merged to provide estimates of regional productivity. However, the temporal coverage of the data was too limited to provide robust seasonal estimates since few experiments were done in the autumn and winter months. A second approach to the estimation of primary production utilised surface pigment concentrations derived from satellite remote sensing. Two models were used. Firstly, an empirical relationship was found between depth-integrated primary production and surface chlorophyll concentration, which explained 78% of the variance in the production estimates. The second approach used a semi-analytical model which incorporates irradiance and temperature as well as chlorophyll-derived parameters. Both models have been applied to SeaWiFS-derived, surface chlorophyll concentrations to estimate primary production during 1998, 1999 and 2000 for three regions—the shelf, the slope and the open ocean of the region of study. Using the semi-analytical model, the mean primary production for the 3 years is estimated to be 319 g C m −2 year −1 for the shelf, 280 g C m −2 year −1 for the slope and 217 g C m −2 year −1 for the adjacent open ocean. Interannual variation in production on the shelf ranged from 334 g C m −2 year −1 in 1998 to 301 g C m −2 year −1 in 2000. It is estimated that upwelling increased primary production, relative to that in the open ocean, by ∼50%. Satellite-derived estimates of the ratio of new production to primary production were 0.37, 0.28 and 0.40 for the shelf region during the upwelling seasons of 1998, 1999 and 2000.

Short-time scale development of a Gymnodinium catenatum population in the Ria de Vigo (NW Spain)

Wind direction and fresh water runoff determine the circulation pattern of the Ría de Vigo (NW Spain), which in turn influence the selection and distribution of its phytoplankton populations. Coastal winds with a south–southwesterly component reverse the positive estuarine circulation in the Ría, causing an off‐shore to in‐shore flow of surface waters and, consequently, the outflow of inner waters via deeper layers. We found that this reversal imposed a selective force on the phytoplankton population: diatoms, which could not counteract the sinking movement of the surface waters, were diminished, while dinoflagellates remained in the water column. From the end of September to the beginning of October 1993, an accumulation of Gymnodimium catenatum Graham was observed coinciding with an intrusion of coastal water induced by westerly winds which provoked a reversal in the circulation of the Ría. The slow reestablishment of the positive estuarine circulation pattern, which was due to a weak coastal upwelling and considerable fresh water runoff, allowed the population of G. catenatum to flourish.

Photosynthetic parameters and primary production in the Bransfield Strait: relationships with mesoscale hydrographic structures

Abstract During January 1994, the photosynthetic response (P–E curves) of phytoplankton in the eastern part of the Bransfield Strait (Antarctica) was studied in relation to the mesoscale hydrographic structures in the area. The most important hydrographic features found in the study area were: (i) the Bransfield Strait front which separates surface Bellingshausen waters from surface Weddell Sea water in the northern part of the Strait; (ii) further to the North, the Weddell–Scotia Confluence north of Elephant Island; (iii) to the South, the ice-edge and the associated lenses of melting waters at the south-eastern part of the sampling area. These three structures were associated with zones of shallower mixing depth ( 1 mg Chl m−3). The photosynthetic response of the phytoplankton was also affected by the hydrographic structures. The phytoplankton of the well-mixed Weddell waters showed a lower light saturation parameter (Ek 100 μmol m−2 s−1). There were no differences in the light saturation parameters of phytoplankton samples from the different water bodies found in the region as demonstrated by a t-test for paired comparisons (0.88>P>0.46). The average Ek was 87±26 μmol m−2 s−1 and not significantly different (t-test NS, P=0.83) from the mean irradiance in the upper mixed layer (Zuml) without the Zuml≥150 m stations. The slope of the PmB vs. α relationship was 59±5 μmol m−2 s−1 (r2=0.66, P 1.5 g C m−2 d−1 between King George Island and Elephant Island.

Photosynthesis, primary production and phytoplankton growth rates in Gerlache and Bransfield Straits during Austral summer: cruise FRUELA 95

Bio-opticalparameters, primary production, and phytopl ankton growth rates were determined in the southern part of the Bellingshausen Sea, the western part of the Bransfield Strait, and the Gerlache Strait, during December 1995 and early January 1996. Three water bodies were present at the surface (o100 m); Bellingshausen warm waters (BWW), Bellingshausen cold waters (BCW), and transitional Weddell waters (TWW), which were separated by a thermal front. High chlorophyll concentrations were found in the upper mixed layer (UML) of the BCW waters (5.5774.04 mg m � 3 ), with intermediate values in BWW (2.8571.24 mg m � 3 ) and the lowest values in the TWW (1.5370.94 mg m � 3 ). The broadband photosynthetic parameters ðP B ; a B and EKPARÞ were highest in BWW and lowest in TWW, except for EKPAR; which did not show significant differences between water bodies. Spectralbio-opticalparameters ðaph; f max and EKPURÞ showed a similar distribution to chlorophyll concentrations except EKPUR; which was highest in BWW. A comparison between light-saturation parameters and water-column irradiance suggests that photosynthesis was not light-limited in the mixed layer. However, transitory situations exist where photosynthesis could be close to limitation at the bottom of the photic layer when high chlorophyll concentrations were present in the surface layers. The high maximum quantum yields (0.07370.032 molC (mol photons) � 1 ) also suggest that iron limitation of photosynthesis should not be occurring in the region during the cruise. The mean primary production rates were 1.1170.68 g Cm � 2 d � 1 in the TWW, 2.2870.98 g C m � 2 d � 1 in the BCW, and 2.6870.94 g C � 2 d � 1 in the BWW. The Gerlache Strait and frontal zones were the most productive, with values analogous to those of upwelling areas. Carbon-specific growth rates in UML (0.4370.16 d � 1 ) were similar to those of temperate seas, neither nutrient- nor light-limited. Growth rates in the UML of BCW (0.5470.20 d � 1 ) were higher than those of TWW (0.3770.08 d � 1 ) and BWW (0.3570.09 d � 1 ). The results indicate that factors as microzooplankton grazing or sinking should control the standing