Standing stocks and production rates of phytoplankton and planktonic copepods in the Inland Sea of Japan

Standing stocks and production rates of phytoplankton and planktonic copepods were investigated at 15 stations in the Inland Sea of Japan during four cruises in October–November 1979, January, April and June 1980. The overall mean of phytoplankton biomass was relatively constant during the study period, ranging from 2.3 mg chl.a m^–3 in April to 3.6 mg chl.a m^–3 in October–November. Primary production was low in January (mean: 90 mg C m^–2 d^–1), but higher than 375 mg C m^–2 d^–1 on the other occasions. Integrated annual primary production was 122 g C m^–2 yr^–1. In terms of carbon weight,Paracalanus parvus was the most important copepod species. The variation of the mean copepod biomass (range: 7.6 mg C m^–3 in April to 20.2 mg C m^–3 in June) was smaller than that of copepod production, which was estimated by the Ikeda-Motoda's physiological method. Copepod producion was low in cold seasons (0.6 and 0.9 mg C m^–3 d^–1 in January and April, respectively), and increased, following the elevation of primary production, to 4.9 mg C m^–3 d^–1 in June. Annual copepod production was 33.7 g C m^–2 yr^–1, of which herbivore (secondary) production was 26.4 g C m^–2 yr^–1 (21.7% of primary production). The ratios of pelagic planktivorous fish catch and total fish catch to the primary production were 0.82 and 1.8%, respectively, indicating very high efficiency in exploiting fishery resources in the Inland Sea of Japan.     

Geographical and seasonal variations in biomass and estimated production rates of net zooplankton in Yatsushiro Bay, Japan

The biomass and production rate of net zooplankton were studied at eight stations in Yatsushiro Bay, Japan, monthly from May 2002 to April 2003. Based on environmental conditions, the bay was divided into three regions, viz. northern (average depth, salinity and chlorophyll a concentration: 11 m, 31.8 and 6.5 μg l⁻¹, respectively), central (30 m, 32.8 and 3.2 μg l⁻¹, respectively) and southern (43 m, 33.4 and 1.9 μg l⁻¹, respectively). Net zooplankton biomass was high in warm months and low in cold ones, with annual averages of 20.2, 38.8 and 16.4 mg C m⁻³ in the northern, central and southern regions, respectively. Copepods were the most important constituent (>ca. 70% of net zooplankton biomass) in all regions. The northern region was characterized by the dominance of Oithona spp. in summer and Acartia spp. in winter-spring. In the central region, Microsetella norvegica was most pronounced in summer-fall. In both central and southern regions, Calanus sinicus and Eucalanus spp. dominated in winter-spring and fall, respectively. The annual average net zooplankton secondary production rate was 4.4, 7.5 and 3.9 mg C m⁻³d⁻¹ in the northern, central and southern region, respectively. Combining the results from the present study with those from other collaborative works on microzooplankton allowed us to determine the trophic interactions in Yatsushiro Bay. If the secondary producers depend entirely on phytoplankton for food, their daily carbon requirement is equivalent to 12.5, 21.6 and 19.1% of the phytoplankton biomass in the respective regions.     

Size and taxonomic plankton community structure and carbon flow at the equator, 175‡E during 1990–1994

Size and taxonomic structure of plankton community carbon biomass for the 0.2–2000 μm equivalent spherical diameter range were determined at the equator at 175°E in September 1990–1993 and April 1994. Total biomass of the plankton community ranged from 1944 to 3448 mg C m⁻². Phytoplankton, zooplankton and bacteria carbon biomasses were 604–1669 mg C m^-2, 300–797 mg C m², and 968–1200 mg C m^-2, and the percentages were 31–54%, 15–26%, and 29–54%, respectively. Biomass of heterotrophic bacteria was always the largest fraction andProchlorococcus biomass was second. Heterotrophic and autotrophic flagellates and dinoflagellates in the nanoplankton size range and copepods (adults and copepodites) in the mesoplankton range were also high. Relatively small biomass was observed in the microplankton size range. The differences in integrated biomass of plankton community for El Nin˜o type oligotrophic conditions of September 1990–1993 and non-El Nifio type mesotrophic conditions of April 1994 were generally small compared with the interannual difference during 1990–1993. However, the percentage ofProchlorococcus in phytoplankton carbon biomass was larger in non-El Nin˜o year. Biomasses of cyanobacteria, diatom, dinoflagellates, nauplii of copepods, and crustaceans other than copepods were larger in the non-El Nin˜o year. Primary production increased significantly from El Nin˜o to non-El Nin˜o years. Carbon flow through the plankton food chain was estimated using the plankton carbon biomass data, primary production measurements, and published empirical relationships.     

Seasonal horizontal and vertical variability in primary production and standing stocks of phytoplankton and zooplankton in the Cretan Sea and the Straits of the Cretan Arc (March 1994–January 1995)

Phytoplankton communities, production rates and chlorophyll levels, together with zooplankton communities and biomass, were studied in relation to the hydrological properties in the euphotic zone (upper 100 m) in the Cretan Sea and the Straits of the Cretan Arc. The data were collected during four seasonal cruises undertaken from March 1994 to January 1995.The area studied is characterised by low nutrient concentrations, low ¹⁴C fixation rates, and impoverished phytoplankton and zooplankton standing stocks. Seasonal fluctuations in phytoplankton densities, chlorophyll standing stock and phytoplankton production are significant; maxima occur in spring and winter and minima in summer and autumn. Zooplankton also shows a clear seasonal pattern, with highest abundances occurring in autumn–winter, and smallest populations in spring–summer. During summer and early autumn, the phytoplankton distribution is determined by the vertical structure of the water column.Concentrations of all nutrients are very low in the surface waters, but increase at the deep chlorophyll maximum (DCM) layer, which ranges in depth from about 75–100 m. Chlorophyll-a concentrations in the DCM vary from 0.22–0.49 mg m⁻³, whilst the surface values range from 0.03–0.06 mg m⁻³. Maxima of phytoplankton, in terms of cell populations, are also encountered at average depths of 50–75 m, and do not always coincide with chlorophyll maxima. Primary production peaks usually occur within the upper layers of the euphotic zone.There is a seasonal succession of phytoplankton and zooplankton species. Diatoms and ‘others’ (comprising mainly cryptophytes and rhodophytes) dominate in winter and spring and are replaced by dinoflagellates in summer and coccolithophores in autumn. Copepods always dominate the mesozooplankton assemblages, contributing approximately 70% of total mesozooplankton abundance, and chaetognaths are the second most abundant group.     

Photosynthate allocation in a temperate sea over an annual cycle: the relationship between protein synthesis and phytoplankton physiological state

The seasonal and vertical variations in the patterns of photosynthate allocation into biomolecules by natural phytoplankton assemblages were determined, together with their species composition, in a coastal station of the central Cantabrian Sea (southern Bay of Biscay). Chlorophyll-a concentration ranged from values below 20 mg m⁻² in winter to values above 80 mg m⁻² during spring and during an upwelling event in summer. Low primary production rates (<300 mgC m⁻² d^-1) were measured during winter and during summer stratification periods. The rate of C fixation during summer upwelling conditions exceeded 3500 mgC m⁻² d⁻¹. In terms of photosynthate partitioning, proteins were the dominant fraction, as they typically accounted for >30% of total photo-assimilated C, with polysaccharides and low molecular weight metabolites showing incorporation percentages around 10–30%. Relative C incorporation into lipids was generally <15%. Recurrent patterns of vertical variability in photosynthate partitioning were observed: the relative synthesis of proteins increased toward the bottom of the euphotic zone, whereas the relative C incorporation into polysaccharides and lipids tended to be higher near the surface. When primary production decreased, the synthesis of proteins was maintained more than that of other molecules. Throughout the year, the relative synthesis of proteins was inversely correlated with phytoplankton biomass, production and growth rate. The conservation of protein synthesis under growth-limiting conditions and the enhancement of lipid and polysaccharide synthesis when irradiance is high seem to constitute general patterns of photosynthate partitioning in marine phytoplankton. In our study, these patterns represented metabolic strategies of phytoplankton in response to changing environmental factors, rather than the effect of variations in the species composition of the community.     

Mesozooplankton dynamics in nearshore waters of the Great Barrier Reef

Zooplankton dynamics (community composition, juvenile somatic growth rate, adult egg production, secondary production) were studied in coastal waters of the Great Barrier Reef. Two sectors were compared, one adjacent to a catchment of near-pristine land use patterns, the other to a more intensively farmed catchment. Sampling was conducted in the austral winter (August) and summer (January–March) of two succeeding years. Gradients in zooplankton community composition were weak, with only moderate effects of season and sector. Overall, 37% of zooplankton biomass was in the 73–150 μm size fraction, 26% in the 150–350 μm fraction, and 38% was >350 μm. There was no biomass difference and only small differences in community composition between samples taken during the day and at night; ostracods and large calanoid copepods were occasionally more common at night. Carbon-specific growth rates averaged 0.29 d⁻¹ for cyclopoid copepods and 0.35 d⁻¹ for calanoid copepods, with no difference between sectors. Calanoid copepod growth showed a significant relationship to chlorophyll concentration, but cyclopoid copepods did not. Copepod egg production was low (7.9 ± 5.9 eggs female⁻¹ d⁻¹) and apparently food-limited. Copepod secondary production was lower in August (mean = 2.6, range 1.4–4.0 mg C m⁻² d⁻¹) than in January–March (mean = 8.5, range 2.4–15.5 mg C m⁻² d⁻¹). Secondary production by mesozooplankton in the 73–100 μm size range averaged 0.9% of total phytoplankton production.     

Structure and photosynthetic properties of phytoplankton assemblages in a highly dynamic system, the Northern Adriatic Sea

The photosynthetic properties of phytoplankton populations as related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area whereas small phytoflagellates (mainly 19′-hexanoyloxyfucoxanthin, chlorophyll b, 19′-butanoyloxyfucoxanthin) occurred outside the front. The front was defined by the steep gradient in density in the surface layer separating low-salinity coastal waters from the offshore waters.Physical features of the area strongly influenced phytoplankton biomass distributions, composition and size structure. After high volumes of Po River discharge several gyres and meanders occurred in the area off the river delta in February. Decreasing river discharge and the subsequent disappearance of the gyres and the spreading dilution of the river plume was observed in March. The dynamic circulation of February resulted in high photosynthetic capacity of the abundant phytoplankton population (>3.40 mg m⁻³). In March, the slow circulation and an upper low-salinity water layer, segregated from the deeper layers, resulted in lack of renewal of this water mass. The huge phytoplankton biomass, up to 15.77 mg chl a m⁻³, became nutrient depleted and showed low photosynthetic capacity. In February, an exceptionally high P_max^B, 20.11 mg C (mg chl a)⁻¹ h⁻¹ was recorded in the Po River plume area and average P_max^B was three-fold in February as compared to the March recordings, 10.50 mg C (mg chl a)⁻¹ h⁻¹ and 3.22 mg C (mg chl a)⁻¹ h⁻¹, respectively.The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area will help to predict phytoplankton response and biomass distributions. In the frontal area, despite a considerable variability in environmental conditions, our findings have shown that the phytoplankton assemblages will compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, as a result, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.     

Geographical and seasonal variations in abundance,biomass and estimated production rates of microzooplankton in the Inland Sea of Japan

We measured abundance and biomass of 3 major groups of microzooplankton, i.e. tintinnids, naked ciliates and copepod nauplii, at 21 stations in the Inland Sea of Japan in October 1993, January, April and June 1994. The average abundance of the microzooplankton over the entire Inland Sea of Japan ranged from 2.39×10⁵ indiv. m^–3 in January to 4.00×10⁵ indiv. m^–3 in April. Ciliated protozoans, i.e. tintinnids plus naked ciliates, numerically dominated the microzooplankton. The average biomass of the microzooplankton was exceedingly high in October (8.62 mg C m^–3) compared to that in the other months (2.06, 2.79 and 2.68 mg C m^–3 in January, April and June, respectively). The ciliated protozoans also dominated in terms of biomass except in October, when copepod nauplii were more important. Estimated production rate of the microzooplankton was highest in October (average: 6.02 mg C m^–3d^–1) and followed in order by June, April and January (1.94, 1.14 and 0.54 mg C m^–3d^–1, respectively). Due to higher specific growth rate, the production rate by the ciliated protozoans far exceeded that by the copepod nauplii. The trophic importance of the microzooplankton in the pelagic ecosystem of the Inland Sea of Japan was assessed by estimating carbon flow through the microzooplankton community.     

Influence of basin-scale and mesoscale physical processes on biological productivity in the Bay of Bengal during the summer monsoon

Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data collected during the peak summer monsoon in July–August, 2003. Three different and spatially varying physical processes were identified in the upper 300 m: (I) anticyclonic warm gyre offshore in the southern Bay; (II) a cyclonic eddy in the northern Bay; and (III) an upwelling region adjacent to the southern coast. In the warm gyre (>28.8 °C), the low salinity (33.5) surface waters contained low concentrations of nutrients. These warm surface waters extended below the euphotic zone, which resulted in an oligotrophic environment with low surface chlorophyll a (0.12 mg m⁻³), low surface primary production (2.55 mg C m⁻³ day⁻¹) and low zooplankton biovolume (0.14 ml m⁻³). In the cyclonic eddy, the elevated isopycnals raised the nutricline upto the surface (NO₃–N > 8.2 μM, PO₄–P > 0.8 μM, SiO₄–Si > 3.5 μM). Despite the system being highly eutrophic, response in the biological activity was low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high (Chl a – 0.25 mg m⁻³, PP – 9.23 mg C m⁻³ day⁻¹), and mesozooplankton biovolume (1.12 ml m⁻³) was rich. Normally in oligotrophic, open ocean ecosystems, primary production is based on ‘regenerated’ nutrients, but during episodic events like eddies the ‘production’ switches over to ‘new production’. The switching over from ‘regenerated production’ to ‘new production’ in the open ocean (cyclonic eddy) and establishment of a new phytoplankton community will take longer than in the coastal system (upwelling). Despite the functioning of a cyclonic eddy and upwelling being divergent (transporting of nutrients from deeper waters to surface), the utilization of nutrients leading to enhanced biological production and its transfer to upper trophic levels in the upwelling region imply that the energy transfer from primary production to secondary production (mesozooplankton) is more efficient than in the cyclonic eddy of the open ocean. The results suggest that basin-scale and mesoscale processes influence the abundance and spatial heterogeneity of plankton populations across a wide spatial scale in the BoB. The multifaceted effects of these physical processes on primary productivity thus play a prominent role in structuring of zooplankton communities and could consecutively affect the recruitment of pelagic fisheries.     

Nutrient enrichment and phytoplankton response in an Adriatic karstic estuary

Nutrients, chlorophyll a, primary production (¹⁴C), and standard oceanographic parameters were measured seasonally from 1983 to 1988 along the axis of a karstic estuary of the central Adriatic Sea (the Krka River estuary). Because of anthropogenic phosphorus discharges, the surface-layer orthophosphate concentrations (up to 1.7 mmol m⁻³), phytoplankton biomass (chlorophyll a up to 23 mg m⁻³) and primary production (up to 108 mg C m⁻³ h⁻¹) were significantly higher in ibenik Bay (lower estuary) than in the other estuarine subregions, and the coastal sea in particular. In contrast, nitrate and orthosilicate (up to 59 and 65 mmol m⁻³, respectively) distributions during autumn and winter were ascribed to dilution of Krka River nutrients along the estuary. As a consequence, the surface-layer inorganic N/P ratio was extremely high in the upper estuary (averages up to 180), but this ratio was reduced up to three times in ibenik Bay and the coastal sea. In spring and summer, nitrate and orthosilicate, but not orthophosphate, were almost exhausted from the water because of biological utilization. In the saline layer below the halocline (depth 2–5 m) oxygen saturation varied over a large range, particularly in the upper estuary (16–176%), and nutrient concentration ratios differed from those in the surface layer. A nutrient regeneration stoichiometric model was derived, based on a linear regression analysis: AOU:Si:N:P = 276:16:6:0.4. Anthropogenic nutrient inputs should be urgently reduced to re-establish a natural nutrient environment.     

Mesozooplankton biomass and grazing responses to Cyclone Opal, a subtropical mesoscale eddy

As part of E-Flux III cruise studies in March 2005, plankton net collections were made to assess the effects of a cyclonic cold-core eddy (Cyclone Opal) on the biomass and grazing of mesozooplankton. Mesozooplankton biomass in the central region of Cyclone Opal, an area of uplifted nutricline and a subsurface diatom bloom, averaged 0.80±0.24 and 1.51±0.59 g DW m⁻², for day and night tows, respectively. These biomass estimates were about 80% higher than control (OUT) stations, with increases more or less proportionately distributed among size classes from 0.2 to >5 mm. Though elevated relative to surrounding waters south of the Hawaiian Islands (Hawai’i lee), total biomass and size distribution in Cyclone Opal were almost exactly the same as contemporary measurements made at Stn. ALOHA, 100 km north of the islands, by the HOT (Hawaii Ocean Time-series) Program. Mesozooplankton biomass and community composition at the OUT stations were also similar to ALOHA values from 1994 to 1996, preceding a recent decadal increase. These comparisons may therefore provide insight into production characteristics or biomass gradients associated with decadal changes at Stn. ALOHA. Gut fluorescence estimates were higher in Opal than in ambient waters, translating to grazing impacts of 0.11±0.02 d⁻¹ (IN) versus 0.03±0.01 d⁻¹ (OUT). Over the depth-integrated euphotic zone, mesozooplankton accounted for 30% of the combined grazing losses of phytoplankton to micro- and meso-herbivores in Opal, as compared to 13% at control stations. Estimates of active export flux by migrating zooplankton averaged 0.81 mmol C m⁻² d⁻¹ in Cyclone Opal and 0.37 mmol C m⁻² d⁻¹ at OUT stations, 53% and 24%, respectively, of the carbon export measured by passive sediment traps. Migrants also exported 0.18 mmol N m⁻² d⁻¹ (117% of trap N flux) in Cyclone Opal compared to 0.08 mmol N m⁻² d⁻¹ (51% of trap flux) at control stations. Overall, the food-web importance of mesozooplankton increased in Cyclone Opal both in absolute and relative terms. Diel migrants provided evidence for enhanced export flux in the eddy that was missed by sediment trap and ²³⁴Th techniques, and migrant-mediated flux was the major export term in the observed bloom-perturbation response and N mass balance of the eddy.     

Primary production and nutrient assimilation in the Iberian upwelling in August 1998

Primary production was measured during two Lagrangian experiments in the Iberian upwelling. The first experiment, in a body of upwelled water, measured day-to-day changes in phytoplankton activity as the water mass moved south along the shelf break. Nutrient concentrations decreased over a five day period, with concomitant increases in phytoplankton biomass. Initially the maximum phytoplankton biomass was in the upper 10m but after four days, a sub-surface chlorophyll maximum was present at 30m. Depth-integrated primary production at the beginning of the experiment was 70mmolC.m⁻².d⁻¹ (838mgC.m⁻².d⁻¹) and reached a maximum of 88mmolC.m⁻².d⁻¹ (1053mgC.m⁻².d⁻¹) on day 3. On day 1, the picoplankton fraction (<2μm) was slightly more productive than larger (>5μm) phytoplankton, but the increase in overall production during the drift experiment was by these larger cells. Nitrate was the dominant nitrogen source. As nutrient concentrations declined, ammonium became increasingly more important as a nitrogen source and the f-ratio decreased from 0.7 to 0.5. Picoplankton cells (<2μm) were responsible for most (65–80%) of the ammonium uptake. The C:N:P uptake ratios were very close to the Redfield ratio for the first four days but as nutrients became depleted high C:N uptake ratios (11 to 43) were measured. Over the period of the experiment, nitrate concentration within the upper 40m decreased by 47.91mmolN.m⁻². In vitro estimates, based on ¹⁵N nitrate uptake, accounted for 56% of the decrease in nitrate concentration observed in the drifting water mass. Ammonium uptake over the same four day period was 16.28mmolN.m⁻², giving a total nitrogen uptake of 43.18mmolN.m⁻².In the second experiment, an offshore filament was the focus and a water mass was sampled as it moved offshore. Nutrient concentrations were very low (nitrate was <10nmol l⁻¹ and ammonium was 20–40nmol l⁻¹). Primary production rate varied between 36mmolC.m⁻².d⁻¹ (436mgC.m⁻².d⁻¹) and 21mmolC.m⁻².d⁻¹ (249mgC.m⁻².d⁻¹). Picophytoplankton was the most productive fraction and was responsible for a constant proportion (ca 0.65) of the total carbon fixation. Uptake rates of both nitrate and ammonium were between 10 and 20% of those measured in the upwelling region. Urea could be a very significant nitrogen source in these waters with much higher uptake rates than nitrate or ammonium; urea turnover times were ca. one day but the source of the urea remains unknown. Urea uptake had a profound effect on calculated f ratios. If only nitrate and ammonium uptake was considered, f ratios were calculated to be 0.42–0.46 but inclusion of urea uptake reduced the f ratio to <0.1. The primary production of this oligotrophic off-shore filament was driven by regenerated nitrogen.     

The estimation of annual production by phytoplankton in Akkeshi Bay,Japan

The annual net and gross primary production by phytoplankton in Akkeshi Bay Hokkaido, are estimated to be 146±25 g C m^–2y^–1 and 416±53 g C m^–2y^–1, respectively. The annual means of the net and gross efficiency between primary production and solar radiation are estimated to be 0.26 % and 0.79 %, respectively.     

Biomass and estimated production rates of metazoan zooplankton community in a tropical coral reef of Malaysia

Quantitative research on composition, biomass and production rates of zooplankton community is crucial to understand the trophic structure in coral reef pelagic ecosystems. In the present study, micro‐ (35–100 μm) and net‐ (>100 μm) metazooplankton were investigated in a fringing coral reef at Tioman Island of Malaysia. Sampling was done during the day and night in August and October 2004, and February and June 2005. The mean biomass of total metazooplankton (i.e. micro + net) was 3.42 ± 0.64 mg C·m^?3, ranging from 2.32 ± 0.75 mg C·m^?3 in October to 3.26 ± 1.77 mg C·m^?3 in August. The net‐zooplankton biomass exhibited a nocturnal increase from daytime at 131–264% due to the addition of both pelagic and reef‐associated zooplankton into the water column. The estimated daily production rates of the total metazooplankton community were on average 1.80 ± 0.57 mg C·m^?3·day^?1, but this increased to 2.51 ± 1.06 mg C·m^?3·day^?1 if house production of larvaceans was taken into account. Of the total production rate, the secondary and tertiary production rates were 2.20 ± 1.03 and 0.30 ± 0.06 mg C·m^?3·day^?1, respectively. We estimated the food requirements of zooplankton in order to examine the trophic structure of the pelagic ecosystem. The secondary production may not be satisfied by phytoplankton alone in the study area and the shortfall may be supplied by other organic sources such as detritus.     

Spatial variability of the species composition,abundance, and productivity of the phytoplankton in the white sea in the late summer period

The species composition, cell concentration (N), and biomass (B) of the phytoplankton, as well as the chlorophyll a (Chl a) concentration, primary production (PP), and the concentrations of the dissolved inorganic micronutrients (phosphorus, silica, nitrogen as nitrite), were estimated for Kandalaksha Bay (KB), Dvina Bay (DB), and the basin (Bas) of the White Sea in August of 2004. The micronutrient concentrations were lower compared to the average long-term values for the summer period. The Chl a concentration varies from 0.9 to 2.0 mg/m³ for most of the studied areas, reaching up to 7.5 mg/m³ in the Northern Dvina River estuary. The surface water layer of the DB was the most productive area, where the PP reached up to 270–375 mg C/(m³ day). The phytoplankton biomass varied from 11 to 205 mg C/m³ with the highest values observed in the Bas and DB. Three groups of stations were defined during the analysis of the phytoplankton’s species composition similarity. The dinoflagellates Dinophysis norvegica and Ceratium fusus were particular to the phytoplankton assemblages in the KB; the diatom Ditylum brightwellii was particular to the upper and central parts of the DB. These three phytoplankton species were less abundant in the Bas.     

Variability in plankton community structure, metabolism, and vertical carbon fluxes along an upwelling filament (Cape Juby, NW Africa)

The variability in dissolved and particulate organic matter, plankton biomass, community structure and metabolism, and vertical carbon fluxes were studied at four stations (D₁–D₄), placed along a coastal-offshore gradient of an upwelling filament developed near Cape Juby (NW Africa). The filament was revealed as a complex and variable system in terms of its hydrological structure and distribution of biological properties. An offshore shift from large to small phytoplankton cells, as well as from higher to lower autotrophic biomass, was not paralleled by a similar gradient in particulate (POC) or dissolved (DOC) organic carbon. Rather, stations in the central part of the filament (D₂ and D₃) presented the highest organic matter concentrations. Autotrophic carbon (POC_Chl) accounted for 53% (onshore station, D₁) to 27% (offshore station, D₄) of total POC (assuming a carbon to chlorophyll ratio of 50), from which nano- and pico-phytoplankton biomasses (POC_{A < 10 μm}) represented 14% (D₁) to 79% (D₄) of POC_Chl. The biomass of small hetrotrophs (POC_{H < 10 μm}) was equivalent to POC_{A < 10 μm}, except at D₁, where small autotrophs were less abundant. Dark community respiration (R_d) in the euphotic zone was in general high, almost equivalent to gross production (P_g), but decreasing offshore (D₁–D₄, from 108 to 41 mmol C m⁻² d⁻¹). POC sedimentation rates (POC_sed) below the euphotic zone ranged from 17 to 6 mmol C m⁻² d⁻¹. Only at D₄ was a positive carbon balance observed: P_g−(R_d + POC_sed) = 42 mmol C m⁻² d⁻¹. Compared to other filament studies from the NE Atlantic coast, the Cape Juby filament presented lower sedimentation rates and higher respiration rates with respect to gross production. We suggest that this is caused by the recirculation of the filament water, induced by the presence of an associated cyclonic eddy, acting as a trapping mechanism for organic matter. The export capacity of the Cape Juby filament therefore would be constrained to the frequency of the interactions of the filament with island-induced eddies.     

Seasonal phytoplankton composition, productivity and biomass in the Neuse River estuary, North Carolina

Phytoplankton community composition, productivity and biomass characteristics of the mesohaline lower Neuse River estuary were assessed monthly from May 1988 to February 1990. An incubation method which considered water-column mixing and variable light exposure was used to determine phytoplankton primary productivity. The summer productivity peaks in this shallow estuary were stimulated by increases in irradiance and temperature. However, dissolved inorganic nitrogen loading was the major factor controlling ultimate yearly production. Dynamic, unpredictable rainfall events determined magnitudes of seasonal production pulses through nitrogen loading, and helped determine phytoplankton species composition. Dinoflagellates occasionally bloomed but were otherwise present in moderate numbers; rainfall events produced large pulses of cryptomonads, and dry seasons and subsequent higher salinity led to dominance by small centric diatoms. Daily production was strongly correlated (r = 0·82) with nitrate concentration and inversely correlated (r = −0·73) with salinity, while nitrate and salinity were inversely correlated (r = −0·71), emphasizing the importance of freshwater input as a nutrient-loading source to the lower estuary. During 1989 mean daily areal phytoplankton production was 938 mgC m⁻², mean chlorophyll a was 11·8 mg m⁻³, and mean phytoplankton density was 1·56 × 10³ cells ml⁻¹. Estimated 1989 annual areal phytoplankton production for the lower estuary was 343 gC m⁻².     

Latitudinal distribution of microbial plankton abundance,production, and respiration in the Equatorial Atlantic in autumn 2000

Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated ¹⁴C primary production ranged from ∼200 mg C m⁻² d⁻¹ in the TTS region to ∼1300 mg C m⁻² d⁻¹ in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m⁻², respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m⁻² d⁻¹) three-fold larger than net production measured in the TTS region (370 mg C m⁻² d⁻¹). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.     

Grazing by meso- and microzooplankton on phytoplankton in the upper reaches of the Schelde estuary (Belgium/The Netherlands)

In contrast with the marine reaches of estuaries, few studies have dealt with zooplankton grazing on phytoplankton in the upper estuarine reaches, where freshwater zooplankton species tend to dominate the zooplankton community. In spring and early summer 2003, grazing by micro- and mesozooplankton on phytoplankton was investigated at three sites in the upper Schelde estuary. Grazing by mesozooplankton was evaluated by monitoring growth of phytoplankton in 200 μm filtered water in the presence or absence of mesozooplankton. In different experiments, the grazing impact was tested of the calanoïd copepod Eurytemora affinis, the cyclopoid copepods Acanthocyclops robustus and Cyclops vicinus and the cladocera Chydorus sphaericus, Moina affinis and Daphnia magna/pulex. No significant grazing impact of mesozooplankton in any experiment was found despite the fact that mesozooplankton densities used in the experiments (20 or 40 ind. l⁻¹) were higher than densities in the field (0.1–6.9 ind. l⁻¹). Grazing by microzooplankton was evaluated by comparing growth of phytoplankton in 30 and 200 μm filtered water. Microzooplankton in the 30–200 μm size range included mainly rotifers of the genera Brachionus, Trichocerca and Synchaeta, which were present from 191 to 1777 ind. l⁻¹. Microzooplankton had a significant grazing impact in five out of six experiments. They had a community grazing rate of 0.41–1.83 day⁻¹ and grazed up to 84% of initial phytoplankton standing stock per day. Rotifer clearance rates estimated from microzooplankton community grazing rates and rotifer abundances varied from 8.3 to 41.7 μl ind.⁻¹ h⁻¹. CHEMTAX analysis of accessory pigment data revealed a similar phytoplankton community composition after incubation with and without microzooplankton, indicating non-selective feeding by rotifers on phytoplankton.     

Influence of upwelling and river runoff interaction on phytoplankton assemblages in a Middle Galician Ria and Comparison with northern and southern rias (NW Iberian Peninsula)

The first oceanographic research (hydrography, nutrient salts, chlorophyll, primary production and phytoplankton assemblages) in a Middle Galician Ria was carried out in Corme-Laxe during 2001, just a year before the Prestige oil spill, being the only reference to evaluate eventual changes in the phytoplankton community. Due to the small size of this ria (6.5 km²), oceanographic processes were driven by the continental water supplied by Anllons River during the wet season (20–30 m³ s⁻¹ in winter), and the strong oceanic influence from the nearby shelf during the dry season. The annual cycle showed a spring bloom with high levels of chlorophyll (up to 14 μg Chl-a L⁻¹) and primary production (3 g C m⁻² d⁻¹) and a summer upwelling bloom (up to 8 μg Chl-a L⁻¹ and 10 g C m⁻² d⁻¹) where the proximity of the Galician upwelling core (<13.5 °C at sea surface) favors the input of upwelled seawater (up to 9 μM of nitrate and silicate) to the bottom ria layer, even during summer stratification events (primary production around 2 g C m⁻² d⁻¹). Thus, phytoplankton assemblages form a “continuum” from spring to autumn with a predominance of diatoms and overlapping species between consecutive periods; only in autumn dinoflagellates and flagellates characterized the phytoplankton community. In the Middle Rias as Corme-Laxe, the nutrient values, Chl-a, primary production and phytoplankton abundance for productive periods were higher than those reported for the Northern (Ria of A Coruña) and Southern Rias (Ria of Arousa) for year 2001; this suggests the importance of the hydrographic events occurring in the zone of maximum upwelling intensity of the Western Iberian Shelf, where a lack of annual cycles studies exists.