Period Of Phytoplankton Growth Research Articles

Deconstructing co-occurring marine heatwave and phytoplankton bloom events in the Arkona Sea in 2018

Between May and August 2018, two separate marine heatwaves (MHWs) occurred in the Arkona Sea in the western Baltic Sea. These heatwaves bookended an extended period of phytoplankton growth in the region. Data from the Ocean and Land Colour Instrument (OLCI) on board the European Sentinel-3 satellite revealed an eddy-like structure containing high chlorophyll a (Chl-a) concentrations (ca. 25 mg.m-3) persisting for several days at the end of May in the Arkona Sea. Combining ocean colour observations, a coupled bio-optical ocean model and a particle tracking model, we examined the three dimensional relationship between these co-occurring MHW and phytoplankton bloom events. We find that the onset of the MHW in May provided the optimal conditions for phytoplankton growth, i.e. sufficient light and nutrients. Wind-driven surface eddy circulation, geostrophic eddy stirring and transient submesoscale dynamics along the edges of the eddy provided a transport path for nutrient fluxes and carbon export, and helped to sustain the phytoplankton bloom. The bloom may have indirectly had an enhancing effect on the MHW, through the impact of water constituent-induced heating rates on air-sea energy fluxes. The subsurface signature of the MHW plays a critical role in de-coupling surface and subsurface dynamics and terminating the phytoplankton bloom. Subsurface temperature anomalies of up to 8°C between 15 and 20 m depth are found to persist up to 15 days after the surface signature of the MHW has disappeared. The study reveals how surface and subsurface dynamics of MHWs and phytoplankton blooms are connected under different environmental conditions. It extends our knowledge on surface layer processes obtained from satellite data.

Phytoplankton Phenology in the Coastal Zone of Cyprus, Based on Remote Sensing and In Situ Observations

Alterations in phytoplankton biomass, community structure and timing of their growth (phenology), are directly implicated in the carbon cycle and energy transfer to higher trophic levels of the marine food web. Due to the lack of long-term in situ datasets, there is very little information on phytoplankton seasonal succession in Cyprus (eastern Mediterranean Sea). On the other hand, satellite-derived measurements of ocean colour can only provide long-term time series of chlorophyll (an index of phytoplankton biomass) up to the first optical depth (surface waters). The coupling of both means of observations is essential for understanding phytoplankton dynamics and their response to environmental change. Here, we use 23 years of remotely sensed, regionally tuned ocean-colour observations, along with a unique time series of in situ phytoplankton pigment composition data, collected in coastal waters of Cyprus during 2016. The satellite observations show an initiation of phytoplankton growth period in November, a peak in February and termination in April, with an overall mean duration of ~4 months. An in-depth exploration of in situ total Chl-a concentration and phytoplankton pigments revealed that pico- and nano-plankton cells dominated the phytoplankton community. The growth peak in February was dominated by nanophytoplankton and potentially larger diatoms (pigments of 19’ hexanoyloxyfucoxanthin and fucoxanthin, respectively), in the 0–20 m layer. The highest total Chl-a concentration was recorded at a station off Akrotiri peninsula in the south, where strong coastal upwelling has been reported. Another station in the southern part, located next to a fish farm, showed a higher contribution of picophytoplankton during the most oligotrophic period (summer). Our results highlight the importance of using available in situ data coupled to ocean-colour remote sensing, for monitoring marine ecosystems in areas with limited in situ data availability.

Cell death in individual freshwater phytoplankton species: relationships with population dynamics and environmental factors

ABSTRACT Understanding and predicting changes in phytoplankton populations requires knowledge of losses due not only to sedimentation and grazing, but also to intrinsic processes (here, collectively termed ‘cell death’). Cell death is poorly understood, especially in freshwater phytoplankton, but experiments in culture often suggest involvement of abiotic factors (e.g. temperature, light, nutrients). The occurrence of cell death was examined in a simple, natural environment: a small, well-mixed, temperate, urban pond during a period of phytoplankton growth, from mid-July to mid-November. Abundances of 18 phytoplankton taxa were measured weekly and fluorescence microscopy and staining was used to detect dead cells (using SYTOX which measures loss of membrane integrity) and cells undergoing cell death (using Annexin-V, which measures lipid inversions of membranes, an early signal of cell death). Dead and dying cells occurred in most phytoplankton taxa, but incidence and timing varied considerably, e.g. species like the chlorophyte Ankistrodesmus spiralis showed 20–30% of cells staining with SYTOX and Annexin in late autumn when the population was decreasing, while the dinoflagellate Peridinium sp. showed staining of up to 50% of cells with STYOX throughout the period, and the cyanobacterium Microcystis aeruginosa occasionally showed staining of 100% of cells with SYTOX. Overall, there was some association between cell death staining and growth phase with 10–15% of the total community showing SYTOX and Annexin staining in late autumn, when most populations were declining. Cell death could not be correlated with thresholds or rapid changes in abiotic conditions (e.g. temperature, irradiance) or with indicators of nutrient limitation (e.g. N:P ratios). While abiotic factors have been clearly implicated in cell death within unialgal culture experiments, in natural freshwater ecosystems interactions between biotic factors, such as pathogens or allelopathy, may play greater roles in losses related to cell death and be distinct for different taxa.

Spatial Distribution and Seasonal Dynamics of the Chlorophyll a Concentration in the Sea of Azov Based on MERIS Images

The chlorophyll a concentration (Cchl a) in the Sea of Azov is estimated by the two-band NIR-red algorithm [34] from MERIS images for 2002–2012. The sea-truth spectrophotometric measurements and MERIS remote estimates of Cchl a are compared. The monthly average Cchl a values are mapped from MERIS data for its lifetime for the first time. The features of the spatiotemporal distribution of Cchl a are ascertained. Differences between the seasonal dynamics of Cchl a in the Sea of Azov according to the literature data and the dynamics derived from MERIS data are found, namely: the summer–autumn phytoplankton growth period is longer than the spring period and is characterized by higher Cchl a values throughout the water area.

Enzyme‐level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean

AbstractIn the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO3−) have been used to investigate NO3− assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the 15N/14N (δ15N) and 18O/16O (δ18O) of NO3− and nitrite (NO2−) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ15N and δ18O of NO3− + NO2− increase roughly equally, as expected for NO3− assimilation; however, the δ15N of NO3−‐only (measured after NO2− removal) increases more than does NO3−‐only δ18O. Differencing indicates that NO2− has an extremely low δ15N, often < −70‰ versus air. These observations are consistent with the expression of an equilibrium N isotope effect between NO3− and NO2−, likely due to enzymatic NO3−‐NO2− interconversion. Specifically, we propose reversibility of the nitrite oxidoreductase (NXR) enzyme of nitrite oxidizers that, having been entrained from the subsurface during late summer mixed layer deepening, are inhibited by light. Our interpretation suggests a role for NO3−‐NO2− interconversion where nitrifiers are transported into environments that discourage NO2− oxidation. This may apply to surface regions with upwelling, such as the summertime Antarctic. It may also apply to oxygen‐deficient zones, where NXR‐catalyzed interconversion may explain previously reported evidence of NO2− oxidation.

Phytoplankton phenology indices in coral reef ecosystems: Application to ocean-color observations in the Red Sea

Phytoplankton, at the base of the marine food web, represent a fundamental food source in coral reef ecosystems. The timing (phenology) and magnitude of the phytoplankton biomass are major determinants of trophic interactions. The Red Sea is one of the warmest and most saline basins in the world, characterized by an arid tropical climate regulated by the monsoon. These extreme conditions are particularly challenging for marine life. Phytoplankton phenological indices provide objective and quantitative metrics to characterize phytoplankton seasonality. The indices i.e. timings of initiation, peak, termination and duration are estimated here using 15years (1997–2012) of remote sensing ocean-color data from the European Space Agency (ESA) Climate Change Initiative project (OC-CCI) in the entire Red Sea basin. The OC-CCI product, comprising merged and bias-corrected observations from three independent ocean-color sensors (SeaWiFS, MODIS and MERIS), and processed using the POLYMER algorithm (MERIS period), shows a significant increase in chlorophyll data coverage, especially in the southern Red Sea during the months of summer NW monsoon. In open and reef-bound coastal waters, the performance of OC-CCI chlorophyll data is shown to be comparable with the performance of other standard chlorophyll products for the global oceans. These features have permitted us to investigate phytoplankton phenology in the entire Red Sea basin, and during both winter SE monsoon and summer NW monsoon periods. The phenological indices are estimated in the four open water provinces of the basin, and further examined at six coral reef complexes of particular socio-economic importance in the Red Sea, including Siyal Islands, Sharm El Sheikh, Al Wajh bank, Thuwal reefs, Al Lith reefs and Farasan Islands. Most of the open and deeper waters of the basin show an apparent higher chlorophyll concentration and longer duration of phytoplankton growth during the winter period (relative to the summer phytoplankton growth period). In contrast, most of the reef-bound coastal waters display equal or higher peak chlorophyll concentrations and equal or longer duration of phytoplankton growth during the summer period (relative to the winter phytoplankton growth period). The ecological and biological significance of the phytoplankton seasonal characteristics are discussed in context of ecosystem state assessment, and particularly to support further understanding of the structure and functioning of coral reef ecosystems in the Red Sea.

Remote sensing of particulate absorption coefficients and their biogeochemical interpretation: A case study in the Irish Sea

We propose a method for (i) obtaining particulate absorption coefficients from optical remote sensing reflectance signals and (ii) partitioning these coefficients into separate components for phytoplankton cells and mineral particles. This method makes the initial assumption that absorption by coloured dissolved organic matter (CDOM) in the region of interest is either constant, or so low that its variability can be neglected. It uses a slightly modified version of the Quasi-Analytical Algorithm (Lee et al., 2009) to recover the non-water absorption (anw) and particulate backscattering (bbp) coefficients, and then obtains an estimate of absorption by CDOM, and the backscattering to absorption ratios for minerals and phytoplankton, by fitting upper and lower boundaries to the resulting data cluster. These ratios are employed in linear un-mixing equations to resolve the contributions of phytoplankton and minerals to the particulate absorption coefficient. The performance of the method was evaluated using reflectance spectra generated by an optical model of the Irish Sea which incorporated realistic specific inherent optical properties and distributions of constituent concentrations. It was then used to recover absorption coefficients for phytoplankton and suspended minerals in the 488nm waveband from MODIS remote sensing data for the central Irish Sea for the period May 2005–November 2012. The partitioned coefficients showed strong seasonality, with suspended mineral absorption peaking in winter and phytoplankton absorption becoming dominant in spring and summer. By employing locally determined specific inherent optical properties, it was possible to estimate the average annual cycles in the concentrations of the two constituents. These cycles proved to be consistent with previously published models of particle dynamics in the region in which mineral concentrations are elevated by winter mixing and increasing water column stability in spring is followed by a period of phytoplankton growth. We conclude that it is possible to resolve the separate contributions of phytoplankton and minerals to particulate absorption in waters where these constituents are the dominant determinants of optical variability, and that the partitioned absorption coefficients can provide valuable insights into the dynamics of biogenic and minerogenic particles in shelf sea ecosystems.

Linking atmospheric dimethyl sulfide and the Arctic Ocean spring bloom

AbstractWe measured atmospheric dimethyl sulfide (DMS) mixing ratios at approximately hourly intervals over a 1 year period (April 2010 to March 2011) in the Atlantic sector of the Arctic Ocean (Svalbard, Norway; 78.5°N, 11.8°E). The mixing ratios varied by several orders of magnitude over time scales of less than several days, and occasionally reached 200−300 parts per trillion by volume during the major phytoplankton growth period (May to September), whereas during the winter months (October to April) the mixing ratios were on the order of a few parts per trillion by volume. Our results, based on analyses using multiple data products (atmospheric DMS mixing ratios, satellite‐derived ocean colors, and meteorological datasets), indicated that weekly variability in the DMS mixing ratios at Svalbard was highly correlated with variability in the chl‐a concentration in waters in the vicinity of Svalbard (r = 0.89). Hourly‐to‐daily variability in the DMS mixing ratios were satisfactorily explained by changes in the trajectory, altitude, and speed of air masses passing the DMS sources prior to reaching Svalbard. The observed coupling between DMS mixing ratios and chl‐a concentration is surprising, and indicates that the variability in chl‐a concentrations in the study area represents the change in the abundance of phytoplankton capable of producing DMS. The intensive monitoring of DMS levels at Svalbard enabled us to identify in situ production and the flux of oceanic DMS over the Arctic region. It thus constitutes a useful analytical tool for detecting changes in DMS production associated with variations in phytoplankton productivity resulting from changes in sea ice extent as a consequence of Arctic seasonality and warming.

Stable nitrogen isotope dynamics of a mesoscale iron enrichment experiment in the NE Subarctic Pacific

We report the response in the natural abundance of the stable isotopes of nitrogen (δ15N) during the Fe-enrichment experiment SERIES (Subarctic Ecosystem Response to Iron Enrichment Study) in the NE Pacific. Samples were collected for isotope analysis of nitrate, particulate material (including size fractionated samples), and particles trapped in the water column from beneath the Fe-enriched patch. In all sample types, δ15N changed in response to increased phytoplankton productivity after the Fe enrichment. The nitrate concentration and δ15N of nitrate were inversely related, the result of the opposing effects of isotope fractionation during nitrate assimilation and the addition of new nitrate by periodic mixing of water from outside the Fe patch. During the growth period a decrease in the difference of the δ15N of particulate nitrogen and nitrate occurred that was attributed to physical mixing, shifts in growth from regenerated nitrogen sources to nitrate, and the change in the community assemblage from<5-μm phytoplankton cells to a larger assemblage dominated by diatoms. The surface-tethered sediment trap δ15N samples indicate that the nitrate isotope fractionation signal in surface waters was not transported below the permanent mixed layer until the end of the phytoplankton growth period, and therefore only the highest values associated with the isotope fractionation process were recorded in the sinking material from the patch. An important conclusion from this study is that mesoscale physical mixing effects and nitrogen remineralization can reduce the expression of isotope fractionation during phytoplankton growth, explaining why the high fractionation values measured in laboratory studies are not commonly observed in the natural environment.

Optical characteristics of two contrasting Case 2 waters and their influence on remote sensing algorithms

This paper describes the results of measurements of inherent and apparent optical properties of two contrasting Case 2 waters (Southern Baltic and off the west coast of Ireland). The experiments were carried out over two seasons, both before the peak of the phytoplankton growth period, and during the bloom events. The first study was made when the concentration of chlorophyll (phytoplankton pigment) was similar in both waters and in the range of 0.6–3.23 mg m −3. The second study was made when concentrations of chlorophyll were very high, reaching 14 mg m −3 in the west coast of Ireland and 70 mg m −3 in Southern Baltic. Optical measurements of surface reflectance were made with a profiling spectroradiometer. The spectral shapes of the particulate absorption spectra and the diffuse attenuation coefficient in both regions and seasons are compared. The contribution of detritus and coloured dissolved organic matter absorption to the apparent optical properties of water have impact on the accuracy of remote sensing retrieval algorithm for chlorophyll a. Differences have been found between algorithms based on conventional spectral bands (e.g. SeaWiFS) and proposed new spectral channels. The most accurate ratio for chlorophyll retrieval (490/550) was for the western Irish shelf ( R 2=76%, and the standard error of the estimate ranged from 30% to 37%), with poor results for this ratio in the Baltic. The R rs (550/590) ratio gave better results for the Baltic ( R 2=75%, and the standard error of the estimate ranged from 20% to 55%), with poor results for the Irish shelf. The results show that a unique combination of spectral bands needs to be applied in Baltic waters to achieve an acceptable accuracy of the in-water remote sensing algorithm whereas the more commonly accepted band ratios were acceptable for the Irish Shelf. This suggests that for accurate determination of chlorophyll from satellite-borne sensors in different coastal waters a wider choice of spectral bands is needed.

Reduction of the Trophic Status of a Water Body with the Use of Large-Size Zooplankton

It is shown that large-size zooplankton can be used as a natural filter in flow-through plants for reducing the trophic status of water bodies and improve water transparency. Experiments in Chistye Prudy pond in the period of blue-green algae blooming show large-size zooplankton to reduce phytoplankton biomass in the pond throughout the period of phytoplankton growth. Daphnia magna specimens (with a biomass of 92 to 3450 mg/l) in lothic ecosystems are found to reduce the biomass of dominating Anabaena spirodes blue-green algae when this biomass attains its maximum.

Changes in the development of the winter-spring phytoplankton bloom in the Bay of Calvi (NW Mediterranean) over the last two decades: a response to changing climate?

The development of the winter-spring phytoplankton bloom was investigated in the Bay of Calvi (Corsica, Ligurian Sea, northwestern Mediterranean) in 1979, 1986, 1988, 1997 and 1998. A drastic reduction of phytoplankton biomass was evidenced over the last 2 decades, in relation to long- term changes in climatic and environmental conditions. Between 1979 and 1998, the monthly aver- aged chlorophyll a concentrations at 1 m decreased by about 80% during February, March and April. Simultaneously, major changes to hydrodynamic conditions include warmer water, overall decrease of salinity at 10 m depth, longer periods of bright sunshine and lower wind stress. The changes in environmental conditions were large enough to affect the vertical stability of the water column dur- ing the winter-spring period and to reduce nutrient replenishment of the surface layer prior to the usual period of phytoplankton growth. Until 1986, the main factor driving nutrient replenishment was the winter upward mixing of nutrient-rich deep waters, while the progressive reduction of mixing from 1988 induced nutrient limitation of surface waters in the last decade. The following hypotheses on changes in the development of the winter-spring phytoplankton bloom are made: (1) Until 1986, phytoplankton peaks took place in relatively high-nutrient waters and were diatom-dominated. (2) Between 1986 and 1988, decreasing Si availability led to Si limitation which caused a reduction in diatom abundance. This resulted in the disappearance of the diatom-dominated pulses and in lower phytoplankton biomass and was accompanied by a shift toward non-siliceous phytoplankton. (3) In 1988, 1997 and 1998, decreasing nitrate availability led to nitrate limitation, thus explaining the pro- gressive reduction in non-siliceous phytoplankton biomass. Other, associated changes in benthos assemblages and ichthyofauna are documented. The conclusions from the Bay of Calvi are extended to the whole western Corsican coast. This confirms that the Mediterranean reacts rapidly to external perturbations, which are driven by climate change in that particular area.

The seasonal planktonic cycle in coastal waters of the Canary Islands

The short-term temporal variation in the phytoplankton and mesozooplankton cycles was studied in a coastal area off east Gran Canaria Island. A small phytoplankton bloom, split into two peaks, appeared during late winter (end of February and March), coinciding with the lowest temperatures in the water column. A clear inverse relationship was observed between the biomasses in mesozooplankton and phytoplankton during the bloom period. The peaks in primary production and phytoplankton biomass were uncoupled in time, suggesting that biomass could depend on consumer control (grazing), and primary production on resource control (nutrients). Mesozooplankton grazing represented less than 20% of the primary production, an indication that small zooplankton and protozoans controlled the phytoplankton populations, dominated by picoplanktonic cells (> 60% of the primary production). The ratio between depth-integrated primary production and community respiration (P/R) covaried with primary production (P), showing that changes in P control the trophic status of the system. At P > 400 mgC m -2 day -1 the P/R ratio is > 1, switching the system from heterotrophy to autotrophy, a situation that takes place during the phytoplankton growth period.

Aggregation, sedimentation and biodegradability of phytoplankton-derived material during spring in the Ross Sea, Antarctica

Abstract The sedimentation rates, aggregation and biodegradability of phytoplankton-derived material were investigated in the Ross Sea Polynya during austral spring, 1997. Planktonic organisms and aggregates were collected using a large volume (100 l) sampler. The phytoplankton assemblages were dominated by diatoms and/or Phaeocystis antarctica, a colonial haptophyte. Total community sinking rates of phytoplankton or derived aggregates were low, ranging between 0 and 1 m d−1, except towards mid-December when the sinking rates increased to 3 m d−1. At this time, the co-occurrence of Phaeocystis colonies and diatoms resulted in the enhanced formation of aggregates. Taxon-specific differences in sinking behaviour were observed, in that diatoms and dinoflagellates had higher sinking rates than P. antarctica. The biodegradability of phytoplankton-derived material was estimated from the enzymatic (ectoprotease, ecto-β-glucosidase) activities of free-living vs. particle-attached bacteria. Bacterial production rates were very low until late November, and from mid-December the bacterial activities increased slowly. At the same time, the proportion of attached bacteria increased; nevertheless, their contribution to the total bacterial activity remained low (less than 19% for ectoprotease, 24% for ecto-β-glucosidase and 18% for bacterial production). In contrast, specific (per cell) activities were higher for particle-attached bacteria than for free-living bacteria. Calculations suggest that 1.4–38.5% of the primary production potentially could be removed from the euphotic zone via sinking during spring, and 0.5–11.6% could be remineralized by bacterial metabolism, with a minor contribution of particle-attached bacteria (less than 2% of the primary production). Thus, during the most active period of phytoplankton growth when grazing losses are nearly non-existent, most of the removal of phytoplankton-derived particulate material is by passive sinking, and only a small amount of the particulate organic carbon is heterotrophically oxidized.

Alkenone sea surface temperature in the Okhotsk Sea for the last 15 kyr.

A sediment core collected from the Sea of Okhotsk, a marginal sea of the western North Pacific, has been analyzed for alkenones to reconstruct changes in the sea surface temperatures (SST) over the last 15 kyrs. A comparison of the core-top U 37′ value with the modern surface temperatures indicates that alkenones are produced at present in summer-fall season, a production seasonality probably identical to the one prevailing in the past, as suggested by evidences of enhanced sea ice cover and subsequent limitation of the phytoplankton growth period to summer‐fall at the glacial/deglacial transition. The alkenone downcore profile indicated that summer SST at the early deglaciation period (15 kyrs BP) were around 5 °C lower than today (ca. 11°C). A rapid SST increase (more than 3°C) was found to occur from 13.7 to 11.6 kyrs BP, following the melting water pulse event (MWP-1A). This warming was assumed to be the consequence of a radical change in the atmospheric/oceanic circulations which induced an enhanced heat transport from the sub-tropical to the northern North Pacific. A significant surface water cooling was evidenced from 11 to 8 kyrs BP, possibly caused by the supply of cold Siberian permafrost melt-water to the Okhotsk Sea through the Amur River.

Evaluation of coastal upwelling effects on phytoplankton growth by simulated culture experiments

Upwelling effects of subsurface water on phytoplankton growth were evaluated by 9 simulated culture experiments of coastal upwelling. Particular attention was paid to the effects of nutrient enrichment on the surface phytoplankton by the upwelling of nutrient rich subsurface water and of the exposure of the subsurface phytoplankters to surface radiation. The following are the results obtained: the lag period of phytoplankton growth was inversely related to water temperature; the maximum yield of phytoplankton was proportional to the amounts of available initial nutrients; the specific growth rates of phytoplankton were a function of both the initial nutrient concentrations and water temperature; and the maximum specific growth rate was simply proportional to water temperature. According to the relations found, a simple equation is presented for the estimation of phytoplankton growth in a given upwelling. Succession of species in the phytoplankton assemblage in upwelled water mass was also taken into consideration.

Light absorption in the waters of the Ems-Dollard estuary and its consequences for the growth of phytoplankton and microphytobenthos

Light absorption and concentrations of suspended matter were measured in the Ems-Dollard estuary (The Netherlands) from 1976 to 1980. The mean coefficients of quantum irradiance attenuation ranged from 1.6 in the outer Ems estuary to 6.7 m−1 in the inner part of the estuary, whilst the mean concentrations of suspended matter ranged from 30 in the outer part to 193 mg · m−1 in the inner part. Both factors closely correlate.The consequences of this gradient on the growth of phytoplankton and microphytobenthos are discussed. The mean column irradiance levels in the channels and above the submersed tidal flats were calculated. It is demonstrated that the total irradiance available for phytoplankton depends to a small extent on the irradiance received during the submersion of the tidal flats. This contribution may accelerate spring growth of phytoplankton by 2 weeks. Both the onset and the period of phytoplankton growth calculated for the outer part of the estuary corresponded with actual observations on the phytoplankton biomass in the estuary. The column irradiance for phytoplankton growth constrast with that of microphytobenthos growth which reaches a saturation level throughout most of the year.

Hydrography, micronutrients, and phytoplankton at four stations in Hjeltefjorden, western Norway

Abstract The study covers the period February to August 1978. The hydrography differed little from that of the coastal water in general, possibly with the exception of a more stable surface layer. Salinities at the surface ranged from 27.7‰ to 32.9‰ in the bottom water (250 m) from 34.4 to 34.9‰. The highest temperature was 14.5° C at 0 m in August and the lowest was 4° C at 0 m at the end of March. At 250 m depth the temperature decreased from 7.7° C in February to 6.7° C in July. Oxygen saturation was above 100% during the phytoplankton growth period down to a depth of la-3D m. In the deep water oxygen saturation was 86–88%. Two blooms were recorded, one in March-April and Onein June, both dominated by Skeletonema costatum. The highest cell numbers, 105–1071-1, the highest chlorophyll a concentrations, 2.6–3.3 mg m-3, and the highest carbon assimilation rates, 10–20 mg C m-3h-1 (100 μE m-2sec-1 )-1, were all round during the blooms, At other times cell numbers were low, and chlorophyll a concentrations ...