A numerical investigation of phytoplankton and Pseudocalanus elongatus dynamics in the spring bloom time in the Gda sk Gulf

A nutrient–phytoplankton–zooplankton–detritus (1D-NPZD) ‘phytoplankton {Phyt} and Pseudocalanus elongatus {Zoop} dynamics in the spring bloom time in the Gda sk Gulf. The 1D-NPZD model consists of three coupled, partial second-order differential equations of the diffusion type for phytoplankton {Phyt}, zooplankton {Zoop}, nutrients {Nutr} and one ordinary first-order differential equation for benthic detritus pool {Detr}, together with initial and boundary conditions. In this model, the {Zoop} is presented by only one species of copepod (P. elongatus) and {Zoop} is composed of six cohorts of copepods with weights (W_i) and numbers (Z_i); where . The calculations were made for 90 days (March, April, May) for two stations at Gda sk Gulf with a vertical space step of 0.5m and a time step of 900 s. The flow field and water temperature used as the inputs in the biological model 1D-NPZD were reproduced by the prognostic numerical simulation technique using hydrographic climatological data. The results of the numerical investigations described here were compared with the mean observed values of surface chlorophyll-a and depth integrated P. elongatus biomass for 10 years, 1980–1990. The slight differences between the calculated and mean observed values of surface chlorophyll-a and zooplankton biomass are ca. 10–60 mg C m⁻³ and ca. 5–23 mg C m⁻², respectively, depending on the location of the hydrographic station. The 1D-NPZD model with a high-resolution zooplankton module for P. elongatus can be used to describe the temporal patterns for phytoplankton biomass and P. elongatus in the centre of the Gda sk Gulf.     

The gonad morphology and maturation in Arctic Calanus species

Gonad morphology and maturation of the dominant Arctic copepod species Calanus glacialis and C. hyperboreus were studied using a combination of observations on whole animals (“whole-body analysis”) and histological sections to describe basic reproductive processes. The comparison with C. finmarchicus shows that the internal organisation of the gonads is the same in the Calanus species: from the dorsally laying ovary, two anterior diverticula extent into the head region and two posterior diverticula extend into the abdomen. In the diverticula, the oocytes increase in size and development stage in a dorso–ventral direction. There were no physiological differences between the oocytes in the diverticula and oviducts in any of the three species. During gonad development, the morphological modifications of the gonads mainly refer to an increase in number, developmental stage and size of the oocytes both in diverticula and oviducts. During spawning the most ventral layer of oocytes is released, and then the adjacent dorsal layer begins final maturation. Thus, the final maturation processes occur simultaneously as it is essential to produce clutches.     

Pigments, size and distribution of Synechococcus spp. in the Black Sea

Pigments, size and distribution of Phycoerythrin-containing unicellular cyanobacteria Synechococcus spp. within the euphotic zone were studied for the first time in April–May 1994 in the western and southwestern Black Sea by epifluorescence microscopy and flow-cytometry. Synechococcus was present in varying quantities at every station and depth studied. Surface spatial distribution of Synechococcus revealed that cells were much more abundant in offshore waters than near coastal regions under the direct influence of the Danube river. Minimum and maximum cell concentrations ranged between 9×10² and 1.45×10⁵ cells/ml at the surface, between 2×10³ and 1.23×10⁵ cells/ml at the chlorophyll sub-maximum layer, and between 1.3×10² and 3.5×10² at the nitrite maximum layer. Cells at the chlorophyll sub-maximum layer (based on in-situ fluorometer readings) fluoresce brighter and longer than the ones at the surface and lower depths. Spectral properties of chromophore pigment types of total 64 clonal isolates from different depths down to the lower layer of the euphotic zone (60 m) in the southern Black Sea coast revealed that all have type 2 phycoerythrobilin in common, lacking in phycourobilin. In vivo fluorescence emission maxima for the phycoerythrobilin were about the same (578 nm) for all isolates. All isolates examined showed in vivo absorption maxima at between 435 and 442 nm and at about 681 nm due to chlorophyll-a. Based on the flow cytometer mean forward light scatter data for size distribution, it could be concluded that cells at the surface mixed layer (0–10 m) were larger in cell size than the cells at lower depths (20–60 m).     

Vertical profiles of particulate organic matter and its relationship with chlorophyll-a in the upper layer of the NE Mediterranean Sea

Particulate organic matter (POM), nutrients, chlorophyll-a (CHL) and primary production measurements were performed in the upper layer of three different regions (cyclonic, anticyclonic and frontal+peripherial) of the NE Mediterranean Sea in 1991–1994. Depth profiles of bulk POM exhibited a subsurface maximum, coinciding with the deep chlorophyll maximum (DCM) established near the base of the euphotic zone of the Rhodes cyclone and its periphery, where the nutricline was situated just below the euphotic zone for most of the year. Moreover, the POM peaks were broader and situated at shallower depths in late winter–early spring as compared to its position in the summer–autumn period. Under prolonged winter conditions, as experienced in March 1992, the characteristic POM feature disappeared in the center of the Rhodes cyclone, where the upper layer was entirely occupied by nutrient-rich Levantine deep water. Deep convective processes in the cyclonic gyre led to the formation of vertically uniform POM profiles with low concentrations of particulate organic carbon (POC) (2.1 μM), nitrogen (0.21 μM), total particulate phosphorus (PP) (0.02 μM) and chlorophyll-a (0.5 μg/L) in the euphotic zone. Though the Levantine deep waters ascended up to the surface layer with the nitrate/phosphate molar ratios (28–29) in March 1992, the N/P molar ratio of bulk POM in the upper layer was low as 10–12, indicating luxury consumption of phosphate during algal production. Depth-integrated primary production in the euphotic zone ranged from 38.5 for oligotrophic autumn to 457 mg C m⁻² day⁻¹ for moderately mesotrophic cool winter conditions.     

Distribution, reproduction and development of Calanus species in the Northeast water in relation to environmental conditions

The vertical and horizontal distribution of mesozooplankton biomass and its composition, together with the reproduction and development of the three dominant herbivorous copepods Calanus finmarchicus, C. glacialis and C. hyperboreus, were studied in the Northeast Water between the end of May and end of July, 1993. Biomass varied between 0.15 and 10.6 g m⁻² and was lower on the shallow banks. Highest biomass was found on the shelf slope and in the entrances to Belgica and Westwind Troughs. There was no seasonal trend during the study period. Among the zooplankton taxa, copepods were dominant, with 84% biomass of all other taxa, followed by chaetognaths with 14%. The large Calanus species made up 91% of copepod biomass. The most abundant species, C. glacialis, inhabited areas of low current speeds on Belgica and Ob Bank, C. hyperboreus dominated shelf slope and trough stations, while C. finmarchicus was most abundant in the Return Atlantic Current along the shelf slope and also eastern Belgica Trough. C. glacialis was the only Calanus species spawning during this period, but young copepodites of the other species were also abundant. Egg production of C. glacialis was at a maximum by our arrival and continued to at least mid August. According to the results from starvation experiments, its egg production was fuelled by food uptake, but was decoupled from phytoplankton chlorophyll until July, indicating ice-algae and microzooplankton as an alternative food source. Only when the polynya approached its maximum extent was a close relationship to phytoplankton established. Due to both spatial and temporal heterogeneity of the occurrence of young copepodite stages it was impossible to follow the growth of cohorts of developmental stages. Herbivorous carbon requirements estimated from egg production rates were mostly less than one third of the phytoplankton stock. From egg production and the distribution of young stages, the outer Westwing Trough seems to be the centre of biological activities. This may be related to the formation of young ice in winter in this area, which was found to carry a large mass of under-ice algae. High biomass but low production in Belgica Trough indicate this as an advective and expatriated community of C. finmarchicus and C. hyperboreus, where grazing is negligible, at least during early summer.     

Microscale variability in the distributions of large fluorescent particles observed in situ with a planar laser imaging fluorometer

It has been known for decades that particle-size and biomass spectra show regular patterns in the ocean, and that these patterns often show systematic variations with other properties such as total biomass, nutrient concentration, season, and distance (both vertical and horizontal). The recent finding of the ubiquitous nature of layers of phytoplankton < 1 m thick prompted us to explore the fine- and microscale vertical variations of size- and fluorescence-abundance spectra in the ocean. Using a two-dimensional planar laser imaging system mounted on a free-falling platform, we quantified the properties of large fluorescent particles ( 20 μm–2 cm) through the water column, obtaining images every 10–30 cm. These images showed systematic relationships of the spectral properties to total chlorophyll: increased proportions of the smallest particles at high chlorophyll concentrations, and a lengthening of the spectral size range at high total chlorophyll concentrations (more large particles at high chlorophyll concentrations). Further, we observed significant variations of the spectral properties over scales of 1 m and less, and recorded the frequent occurrence of unusual layers of large particles. Our new instrument, which is sensitive to thin layers of enhanced phytoplankton biomass, shows the planktonic community to be highly structured vertically on scales of 1–2 m, particularly within the DCM.     

Validation of the 3D biogeochemical model MIRO&CO with field nutrient and phytoplankton data and MERIS-derived surface chlorophyll a images

This paper presents results obtained with MIRO&CO-3D, a biogeochemical model dedicated to the study of eutrophication and applied to the Channel and Southern Bight of the North Sea (48.5°N–52.5°N). The model results from coupling of the COHERENS-3D hydrodynamic model and the biogeochemical model MIRO, which was previously calibrated in a multi-box implementation. MIRO&CO-3D is run to simulate the annual cycle of inorganic and organic carbon and nutrients (nitrogen, phosphorus and silica), phytoplankton (diatoms, nanoflagellates and Phaeocystis), bacteria and zooplankton (microzooplankton and copepods) with realistic forcing (meteorological conditions and river loads) for the period 1991–2003. Model validation is first shown by comparing time series of model concentrations of nutrients, chlorophyll a, diatom and Phaeocystis with in situ data from station 330 (51°26.00′N, 2°48.50′E) located in the centre of the Belgian coastal zone. This comparison shows the model's ability to represent the seasonal dynamics of nutrients and phytoplankton in Belgian waters. However the model fails to simulate correctly the dissolved silica cycle, especially during the beginning of spring, due to the late onset (in the model) of the early spring diatom bloom. As a general trend the chlorophyll a spring maximum is underestimated in simulations. A comparison between the seasonal average of surface winter nutrients and spring chlorophyll a concentrations simulated with in situ data for different stations is used to assess the accuracy of the simulated spatial distribution. At a seasonal scale, the spatial distribution of surface winter nutrients is in general well reproduced by the model with nevertheless a small overestimation for a few stations close to the Rhine/Meuse mouth and a tendency to underestimation in the coastal zone from Belgium to France. PO₄ was simulated best; silica was simulated with less success. Spring chlorophyll a concentration is in general underestimated by the model. The accuracy of the simulated phytoplankton spatial distribution is further evaluated by comparing simulated surface chlorophyll a with that derived from the satellite sensor MERIS for the year 2003. Reasonable agreement is found between simulated and satellite-derived regions of high chlorophyll a with nevertheless discrepancies close to the boundaries.     

Inorganic carbon fluxes through the boundaries of the Greenland Sea Basin based on in situ observations and water transport estimates

A carbon budget for the exchange of total dissolved inorganic carbon C_T between the Greenland Sea and the surrounding seas has been constructed for winter and summer situations. An extensive data set of C_T collected over the years 1994–1997 within the European Sub-polar Ocean Programmes (ESOP1 and ESOP2) are used for the budget calculation. Based on these data, mean values of C_T in eight different boxes representing the inflow and outflow of water through the boundaries of the Greenland Sea Basin are estimated. The obtained values are then combined with simulated water transports taken from the ESOP2 version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The fluxes of inorganic carbon are presented for three layers; a surface mixed layer, an intermediate layer and a deep layer, and the imbalance in the fluxes are attributed to air–sea exchange, biological fixation of inorganic carbon, and sedimentation. The main influx of carbon is found in the surface and the deep layers in the Fram Strait, and in the surface waters of direct Atlantic origin, whereas the main outflux is found in the surface layer over the Jan Mayen Fracture Zone and the Knipovich Ridge, transporting carbon into the Atlantic Ocean via the Denmark Strait and towards the Arctic Ocean via the Norwegian Sea, respectively. The flux calculation indicates that there is a net transport of carbon out of the Greenland Sea during wintertime. In the absence of biological activity, this imbalance is attributed to air sea exchange, and requires an oceanic uptake of CO₂ of 0.024±0.006 Gt C yr⁻¹. The flux calculations from the summer period are complicated by biological fixation of inorganic carbon, and show that data on organic carbon is required in order to estimate the air–sea exchange in the area.     

Distribution of phytoplankton in the southern Black Sea in summer 1996, spring and autumn 1998

The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June–July 1996 and March–April and September 1998. A total of 150 species were identified, 50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×10³ cells l⁻¹ in spring to 110×10³ cells l⁻¹ in autumn and biomass from 250 μg l⁻¹ in summer to 1370 μg l⁻¹ in spring. Based on the extensive sampling grid from June–July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.     

The trophic role of the tunicate Salpa thompsoni in the Antarctic marine ecosystem

During a repeat grid survey and drogue study carried out in austral summer 1994/95, the abundance and feeding activity of salps were estimated in the Lazarev Sea region from net tows and in situ measurements of gut fluorescence. Throughout the survey area, Salpa thompsoni accounted for >95% of the total salp stock while Ihlea racovitzai was consistently represented in very low abundances. Maximum densities of S. thompsoni, with ≈4000 ind. 1000 m⁻³, were recorded in the Marginal Ice Zone (MIZ) in December when chlorophyll-a concentrations were well below 1 mg m⁻³. A dramatic decrease in salp stock was observed at the beginning of January, when S. thompsoni virtually disappeared from the most productive area of the MIZ where chlorophyll-a concentrations had by then reached bloom levels of 1.5–3 mg (Chl-a) m⁻³. In situ grazing measurements showed that throughout the cruise S. thompsoni exhibited the highest ingestion rates per individual of any of the most abundant components of the grazing pelagic community, with maxima of ≈160 μg (pigm) ind. ⁻¹ d⁻¹. These feeding rates are 3 to 5 times higher than those previously obtained using in vitro incubations. The total daily consumption of the population of S. thompsoni varied from 0.3 to 108% of daily primary production. We suggest that competitive removal of food by S. thompsoni, rather than direct predation, is responsible for the low krill abundances generally associated with salp swarms.     

Seasonal and interannual variation of physical and biological processes during 1994–2001 in the Sea of Japan/East Sea: A three-dimensional physical–biogeochemical modeling study

A Pacific basin-wide physical–biogeochemical model has been used to investigate the seasonal and interannual variation of physical and biological fields with analyses focusing on the Sea of Japan/East Sea (JES). The physical model is based on the Regional Ocean Model System (ROMS), and the biogeochemical model is based on the Carbon, Si(OH)₄, Nitrogen Ecosystem (CoSiNE) model. The coupled ROMS–CoSiNE model is forced with the daily air–sea fluxes derived from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis for the period of 1994 to 2001, and the model results are used to evaluate climate impact on nutrient transport in Mixed Layer Depth (MLD) and phytoplankton spring bloom dynamics in the JES.The model reproduces several key features of sea surface temperature (SST) and surface currents, which are consistent with the previous modeling and observational results in the JES. The calculated volume transports through the three major straits show that the Korea Strait (KS) dominates the inflow to the JES with 2.46 Sv annually, and the Tsugaru Strait (TS) and the Soya Strait (SS) are major outflows with 1.85 Sv and 0.64 Sv, respectively. Domain-averaged phytoplankton biomass in the JES reaches its spring peak 1.8 mmol N m^− 3 in May and shows a relatively weak autumn increase in November. Strong summer stratification and intense consumption of nitrate by phytoplankton during the spring result in very low nitrate concentration at the upper layer, which limits phytoplankton growth in the JES during the summer. On the other hand, the higher grazer abundance likely contributes to the strong suppression of phytoplankton biomass after the spring bloom in the JES. The model results show strong interannual variability of SST, nutrients, and phytoplankton biomass with sudden changes in 1998, which correspond to large-scale changes of the Pacific Decadal Oscillation (PDO). Regional comparisons of interannual variations in springtime were made for the southern and northern JES. Variations of nutrients and phytoplankton biomass related to the PDO warm/cold phase changes were detected in both the southern and northern JES, and there were regional differences with respect to the mechanisms and timing. During the warm PDO, the nutrients integrated in the MLD increased in the south and decreased in the north in winter. Conversely, during the cold PDO, the nutrients integrated in the MLD decreased in the south and increased in the north. Wind divergence/convergence likely drives the differences in the southern and northern regions when northerly and northwesterly monsoon dominates in winter in the JES. Subjected to the nutrient change, the growth of phytoplankton biomass appears to be limited neither by nutrient nor by light consistently both in the southern and northern regions. Namely, the JES is at the transition zone of the lower trophic-level ecosystem between light-limited and nutrient-limited zones.     

Community structure of mesopelagic copepods (>500 μm) in the Ligurian Sea (Western Mediterranean)

The distribution of mesopelagic copepods between 250 and 1000 m depth was studied along a transect off the coast of Villefranche-sur-Mer in June 1991 by means of a BIONESS multiple-net sampler. Among the most abundant species at all stations were Pleuromamma gracilis, Paraeuchaeta acuta and overwintering Calanus helgolandicus CV copepodites, each species inhabiting different depth layers with maximum abundances in the 250–350 m, 450–550 m and the 700–850 m layer, respectively. Cluster analysis of the vertical distribution of all species caught revealed three distinct strata (‘TOP', ‘INTERMEDIATE', ‘BOTTOM') consisting of characteristic species that occurred in the same stratum at all stations. Differences in total abundances of the species assemblage occupying each layer are discussed with regard to predation pressure by mesopelagic macroplanktonic crustaceans and fish. We show that each stratum is characterized by distinct trophic interactions and life strategies, which play an important role in the structuring of mesopelagic zooplankton communities.     

Convection and the seeding of the North Atlantic bloom

Observations of vertical velocities in deep wintertime mixed layers using neutrally buoyant floats show that the convectively driven vertical velocities, roughly 1000 m per day, greatly exceed the sinking velocities of phytoplankton, 10 m or less per day. These velocities mix plankton effectively and uniformly across the convective layer and are therefore capable of returning those that have sunk to depth back into the euphotic zone. This mechanism cycles cells through the surface layer during the winter and provides a seed population for the spring bloom. A simple model of this mechanism applied to immortal phytoplankton in the subpolar Labrador Sea predicts that the seed population in early spring will be a few percent of the fall concentration if the plankton sink more slowly than the mean rate at which the surface well-mixed layer grows over the winter. Plankton that sink faster than this will mostly sink into the abyss with only a minute fraction remaining by spring. The shallower mixed layers of mid-latitudes are predicted to be much less effective at maintaining a seed population over the winter, limiting the ability of rapidly sinking cells to survive the winter.     

Size-fractionated phytoplankton biomass and species composition in the Indian sector of the Southern Ocean during austral summer

During the late austral summer of 1994, Antarctic waters were characterized by low phytoplankton biomass. Along the 62°E meridian transect, between 49°S and 67°S, chlorophyll (Chl.) a concentration in the upper 150 m was on average 0.2 mg m⁻³. However, in the Seasonal Ice Zone (SIZ) chlorophyll a concentrations were higher, with a characteristic deep chlorophyll maximum. The highest value (0.6 mg Chl. a m⁻³) was measured at the Antarctic Divergence, 64°S, corresponding to the depth of the temperature minimum (100 m). This deep biomass maximum decreased from South to North, disappeared in the Permanently Open Ocean Zone (POOZ) and reappeared with less vigour in the vicinity of the Polar Front Zone (PFZ). In the SIZ, the upper mixed layer was shallow, biomass was higher and the >10 μm fraction was predominant. In this zone the >10 μm, 2–10 μm and <2 μm size fractions represented on the average 46%, 25.1% and 28.9% of the total integrated Chl. a stock in the upper 100 m, respectively. The phytoplankton assemblage was diverse, mainly composed of large diatoms and dinoflagellate cells which contributed 42.7% and 33.1% of the autotrophic carbon biomass, respectively. Moving northwards, in parallel with the decrease in biomass, the biomass of autotrophic pico- and nanoflagellates (mainly Cryptophytes) increased steadily. In the POOZ, the picoplanktonic size fraction contributed 47.4% of the total integrated Chl. a stock. A phytoplankton community structure with low biomass and picoplankton-dominated assemblage in the POOZ contrasted with the relatively rich, diverse and diatom-dominated assemblage in the SIZ. These differences reflect the spatial and temporal variations prevailing in the Southern Ocean pelagic ecosystem.     

Modelling the silica pump in the Permanently Open Ocean Zone of the Southern Ocean

A coupled 1D physical–biogeochemical model has been built to simulate the cycles of silicon and of nitrogen in the Indian sector of the Permanently Open Ocean Zone of the Southern Ocean. Based on a simplified trophic network, that includes two size classes of phytoplankton and of zooplankton, and a microbial loop, it has been calibrated by reference to surface physical, chemical and biological data sets collected at the KERFIX time-series station (50°40′S–68°25′E). The model correctly reproduces the high nutrient low chlorophyll features typical of the studied area. In a region where the spring–summer mixed layer depth is usually deeper than 60 m, the maximum of chlorophyll never exceeds 1.5 mg m⁻³, and the annual primary production is only 68 g C m⁻² year⁻¹. In the surface layer nitrate is never exhausted (range 27–23.5 mmoles m⁻³) while silicic acid shows strong seasonal variations (range 5–20 mmoles m⁻³). On an annual basis 71% of the primary production sustained by nanophytoplankton is grazed by microzooplankton. Compared to North Atlantic, siliceous microphytoplankton is mainly prevented from blooming because of an unfavourable spring–summer light-mixing regime. Silicic acid limitation (high half saturation constant for Si uptake: 8 mmoles m⁻³) also plays a major role on diatom growth. Mesozooplankton grazing pressure excerpts its influence especially in late spring. The model illustrates the efficiency of the silica pump in the Southern Ocean: up to 63% of the biogenic silica that has been synthetized in the photic layer is exported towards the deep ocean, while only 11% of the particulate organic nitrogen escapes recycling in the surface layer.     

Egg production of the copepod Acartia hongi in Kyeonggi Bay, Korea

Many studies of copepod egg production have shown that food availability and temperature are major factors that influence copepod growth. However, coastal environments are complicated ecosystem and the relationships between growth of copepods and influencing factors are not always clear in nature. We conducted a study along an inner–middle–outer bay transect where variations in environmental parameters would be expected to affect the biomass and egg production rate of A. hongi from February 2001 to December 2001. In this study, we investigated the abundance and biomass with developmental stages and egg production rates of A. hongi in relation to various environmental factors. The copepod A. hongi occurred continuously throughout the year, with a peak abundance in May. In general, the variation in egg production rates showed a similar tendency with the variations in chlorophyll-a throughout the study period. This suggests that phytoplankton biomass is an important factor that affects the egg production of A. hongi. In addition, during the warm season, the egg production of A. hongi was also influenced by the ciliates abundance in the middle and outer bay. Consequently, the egg production of A. hongi is generally affected by food availability in Kyeonggi Bay.     

Production of Acartia omorii (Copepoda: Calanoida) in Ilkwang Bay, southeastern coast of Korea

Production of the marine calanoid copepod Acartia omorii was measured from 2 October 1991 to 8 October 1992 at a station in Ilkwang Bay on the southeastern coast of Korea. A. omorii (nauplii + copepodites + adults) were present in the plankton throughout the year, with seasonal variation in abundance. Biomass of A. omorii was averaged at 0.44 mgC m^− 3, with peaks in February and July, and relatively low biomass in late summer and fall. Egg production rate ranged from 2.4 to 151.9 μgC m^− 3 day^− 1, which was equivalent to 95–6075 eggs m^− 3 day^− 1. Fecundity of an adult female was averaged at 38 eggs female^− 1 day^− 1. Instantaneous growth rates of copepodites were higher than those of nauplii stages. Annual production of A. omorii ranged from 33.5 mgC m^− 3 year^− 1 to 221 mgC m^− 2 year^− 1, showing a seasonal variation of daily production rate with peaks in February and July. The daily production rate of A. omorii was significantly correlated with chlorophyll a concentration. These results suggest that standing stocks and/or productivity of phytoplankton are the major influencing factors, rather than water temperature for the seasonal variation of production of A. omorii in Ilkwang Bay.     

Influence of the heavy fuel spill from the Prestige tanker wreckage in the overlying seawater column levels of copper, nickel and vanadium (NE Atlantic ocean)

The water column above the Prestige wreckage was sampled during two consecutive campaigns: Prestinaut (December 2002) two weeks after the tanker sunk and HidroPrestige0303 (March 2003) one month after the sealing of the main fuel leaks. Samples of the original cargo fuel and the emulsified fuel in the surface of the ocean were also collected. Analysis of the fuel indicated the release of 135 kg of Cu, 1700 kg of Ni and 5300 kg of V from the original fuel to the water column, remaining 35 kg of Cu, 3100 kg of Ni and 13,800 kg of V in the emulsified fuel. The metal partitioning between the water column and the emulsioned floating fuel, Cu > Ni ~ V, are in accordance with the stability index for the metal–nitrogen bond in metalloporphyrins. This release had an impact on dissolved trace metal concentrations in the water column. An increase on dissolved copper (2.8–4.7 nM) and nickel (2.2–8.0 nM) with respect to natural values (1–3 nM for Cu and 1.6–5 nM for Ni) was observed. Values for vanadium (28–35 nM) were in the range of pristine North Atlantic waters (30–36 nM). This contamination was especially observed in the upper water column (0–50 m), associated with the mixing of seawater with the fuel moving upwards, and in deep waters, where the residence time of fuel is higher. Future research in this field should focus on the environmental variables and the processes that control the release of contaminants from fuels for a better assessment of the contamination in oil-spill events.     

Seasonality of picophytoplankton chlorophyll a and biomass in the central Cantabrian Sea, southern Bay of Biscay

Seasonal changes in the abundance and biomass of cyanobacteria (Synechococcus and Prochlorococcus) and picoeukaryotes were studied by flow cytometry in the upper layers of the central Cantabrian Sea continental shelf, from April 2002 to April 2006. The study area displayed the typical hydrographic conditions of temperate coastal zones. A marked seasonality of the relative contribution of prokaryotes and eukaryotes was found. While cyanobacteria were generally more abundant for most of the year (up to 2.4 10⁵ cells mL^− 1), picoeukaryotes dominated the community (up to 10⁴ cells mL^− 1) from February to May. The disappearance of Prochlorococcus from spring through summer is likely related to shifts in the prevailing current regime. The maximum total abundance of picophytoplankton was consistently found in late summer–early autumn. Mean photic-layer picoplanktonic chlorophyll a ranged from 0.06 to 0.53 µg L^− 1 with a relatively high mean contribution to total values (33 ± 2% SE), showing maxima around autumn and minima in spring. Biomass (range 0.58–40.16 mg C m^− 3) was generally dominated by picoeukaryotes (mean ± SE, 4.28 ± 0.27 mg C m^− 3) with an average contribution of cyanobacteria of 30 ± 2%. Different seasonality of pigment and biomass values resulted in a clear temporal pattern of picophytoplanktonic carbon to chlorophyll a ratio, which ranged from 10 (winter) to 140 (summer). This study highlights the important contribution of picoplanktonic chlorophyll a and carbon biomass in this coastal ecosystem.     

A box model approach for a long-term assessment of estuarine eutrophication, Szczecin Lagoon, southern Baltic

We develop a layered “box model” to evaluate the major effects of estuarine eutrophication of the Szczecin lagoon which can be compared with integrating measures (chlorophyll a (Chl a), sediment burial, sediment oxygen consumption (SOC), input and output of total nutrient loads) and use it to hindcast the period 1950–1996 (the years when major increase in nutrient discharges by the Oder River took place). The following state variables are used to describe the cycling of the limiting nutrients (nitrogen and phosphorus): phytoplankton (Phy), labile and refractory detritus (D_N, D_Nref, D_P, D_Pref), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and oxygen (O₂). The three layers of the model include two water layers and one sediment layer. Decrease of the carrying capacity with respect to the increased supply of organic matter of the system with advancing eutrophication over the period studied is parameterized by an exponential decrease of the sediment nitrogen fluxes with increasing burial, simulating changing properties from moderate to high accumulating sediments. The seasonal variation as well as the order of magnitude of nutrient concentrations and phytoplankton stocks in the water column remains in agreement with recent observations. Calculated annual mean values of nutrient burial of 193 mmol N m⁻² a⁻¹ and 23 mmol P m⁻² a⁻¹ are supported by observed values from geological sediment records. Estimated DIN remineralization in the sediments between 100 and 550 mmol N m⁻² a⁻¹ corresponds to SOC measurements. Simulated DIP release up to 60 mmol P m⁻² a⁻¹ corresponds to recent measurements. The conceptual framework presented here can be used for a sequential box model approach connecting small estuaries like the Szczecin lagoon and the open sea, and might also be connected with river box models.