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Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flos-aquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll a (Chl a) and N2 fixation. We used regression models relating N2 fixation to Chl a, along with remote-sensing-based estimates of Chl a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment–water exchange, and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl a with empirical models relating N2 fixation rates to Chl a.

A method for automatically counting and measuring sizes and motility behavior of zooplankton and phytoplankton in water samples is presented. Two video cameras are focused on separate optical chambers of different sizes. The chambers are filled and emptied repeatedly by synchronized pumps. Real-time motion analysis is performed by computer on the respective video feeds. Fluorescence from chlorophyll a (Chl a) is imaged at single pixel resolution. Measured parameters for individual organisms include size, swimming velocity, motility patterns, and chlorophyll fluorescence density. The system was tested during a mesocosm experiment where it was mounted on one of several mesocosm columns. The results were validated against Chl a measurements and microscopy counts. A sampling interval of 1 per day revealed detailed dynamics of chlorophyll activity as well as shifts in both the phytoplankton and zooplankton community structure over the course of a month. A helix coefficient, a metric related to organism motility behavior, showed substantial variation over time, consistent with changing plankton communities. Sampling rates as frequent as 1 per hour enables detailed analysis of diurnal vertical migration and similar phenomena at fixed sampling points.

High-throughput sequencing for analysis of environmental microbial diversity has evolved vastly over the last decade. Currently the go-to method for microbial eukaryotes is short-read metabarcoding of variable regions of the 18S rRNA gene with

The concentrations of ambient nutrients and dissolved organic carbon (DOC) in northern coastal ecosystems often show large variations, due to the spatiotemporal differences in terrestrial inputs. How these variations affect the stoichiometry of coastal planktonic organisms is, however, poorly known. Here we assessed the spatiotemporal variability of C, nitrogen (N), and phosphorous (P) concentrations of the seawater on the elemental stoichiometry of seston and dominant mesozooplankton taxa in a coastal area of the northern Baltic Sea. The freshwater inflow peaked in spring following the snowmelt and brought a significant amount of DOC, but not N and P to the coastal system. DOC was the main environmental descriptor for seston C:N stoichiometry. The C:N ratio of seston from 0.7 to 50 μm and mesozooplankton followed the temporal pattern of water C:N ratio, while the temporal trend of bacteria C:N showed an opposite pattern. Our results also indicated that the C:N ratio of seawater controlled both seston and mesozooplankton C:N ratios. Our findings imply that inflows of terrestrial DOC alter the stoichiometry and reduce the nutritional quality of planktonic food webs in northern coastal ecosystems.

Global warming scenarios indicate that in subarctic regions, the precipitation will increase in the future. Coastal bacteria will thus receive increasing organic carbon sources from land runoff. How such changes will affect the function and taxonomic composition of coastal bacteria is poorly known. We performed a 10-day experiment with two isolated bacteria

Marine macroalgae are used worldwide for human consumption, animal feed, cosmetics and agriculture. In addition to beneficial nutrients, macroalgae contain halogenated natural products (HNPs), some of which have toxic properties similar to those of well-known anthropogenic contaminants. Sixteen species of red, green and brown macroalgae were collected in 2017–2018 from coastal waters of the northern Baltic Sea, Sweden Atlantic and Norway Atlantic, and analyzed for bromoanisoles (BAs) and methoxylated bromodiphenyl ethers (MeO-BDEs). Target compounds were quantified by gas chromatography-low resolution mass spectrometry (GC-LRMS), with qualitative confirmation in selected species by GC-high resolution mass spectrometry (GC-HRMS). Quantified compounds were 2,4-diBA, 2,4,6-triBA, 2′-MeO-BDE68, 6-MeO-BDE47, and two tribromo-MeO-BDEs and one tetrabromo-MeO-BDE with unknown bromine substituent positions. Semiquantitative results for pentabromo-MeO-BDEs were also obtained for a few species by GC-HRMS. Three extraction methods were compared; soaking in methanol, soaking in methanol–dichloromethane, and blending with mixed solvents. Extraction yields of BAs did not differ significantly (p > 0.05) with the three methods and the two soaking methods gave equivalent yields of MeO-BDEs. Extraction efficiencies of MeO-BDEs were significantly lower using the blend method (p < 0.05). For reasons of simplicity and efficiency, the soaking methods are preferred. Concentrations varied by orders of magnitude among species: ∑2BAs 57 to 57 700 and ∑5MeO-BDEs < 10 to 476 pg g−1 wet weight (ww). Macroalgae standing out with ∑2BAs >1000 pg g−1 ww were Ascophyllum nodosum, Ceramium tenuicorne, Ceramium virgatum, Fucus radicans, Fucus serratus, Fucus vesiculosus, Saccharina latissima, Laminaria digitata, and Acrosiphonia/Spongomorpha sp. Species A. nodosum, C. tenuicorne, Chara virgata, F. radicans and F. vesiculosus (Sweden Atlantic only) had ∑5MeO-BDEs >100 pg g−1ww. Profiles of individual compounds showed distinct differences among species and locations. EcoChange

The DNA extraction protocol described in this document follows the standard protocol provided with the Zymobiomics DNA miniprep kit (https://files.zymoresearch.com/protocols/_d4300t_d4300_d4304_zymobiomics_dna_miniprep_kit.pdf) with a few modifications. The protocol is part of the project "DNA-metabarcoding of marine phytoplankton" funded by the Swedish Environmental Protection Agency and the Swedish Agency for Marine and Water Management. Phytoplankton play a key role in global biogeochemical cycles and form the basis of marine food webs. Quantitative assessment of the abundance and biodiversity of phytoplankton is therefore a natural component of environmental monitoring programs. Until now such monitoring has largely been based on microscopic counting. This is a time consuming process and requires personnel with high skill for identifying the organisms, and the smallest cells can hardly be identified with this method. There is hence an urge for faster, more precise and reproducible ways of quantifying plankton species in environmental samples. High-throughput sequencing of taxonomic marker genes (metabarcoding) has emerged as a popular alternative for studying plankton diversity. However, also this method has its limitations, and in order to be robustly implemented in monitoring programs, protocols need to be optimised and validated. The main goal of the project is to evaluate high-throughput sequencing as a tool to investigate the diversity of phytoplankon in the seas surrounding Sweden, with special focus on non-indigenous species and harmful algae. Specifically we want to test effects of storage of samples, sample volume, replication and background DNA in the water (“eDNA”), develop and evaluate an improved method for identifying phytoplankton based on sequencing a longer region of the rRNA operon and compare results from metabarcoding with results from microscope-based methods. The tests will be conducted on water samples collected as part of the Swedish National Marine Monitoring programme. This will provide a large data set covering all major sea areas surrounding Sweden and allow us to compare the metabarcoding results with taxonomic identifications obtained using microscopy as part of the monitoring programme. This collaborative project will be conducted by a team of researchers with complementary expertise in plankton ecology, marine monitoring, molecular biology and bioinformatics.

Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flosaquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll-a (Chl-a) and N2 fixation. We used regression models relating N2 fixation to Chl-a, along with remote sensing-based estimates of Chl-a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment-water exchange and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl-a with empirical models relating N2 fixation rates to Chl-a.

In this study, we measured depth-dependent benthic microalgal primary production in a Bothnian Bay estuary to estimate the benthic contribution to total primary production. In addition, we compiled data on benthic microalgal primary production in the entire Baltic Sea. In the estuary, the benthic habitat contributed 17 % to the total annual primary production, and when upscaling our data to the entire Bothnian Bay, the corresponding value was 31 %. This estimated benthic share (31 %) is three times higher compared to past estimates of 10 %. The main reason for this discrepancy is the lack of data regarding benthic primary production in the northern Baltic Sea, but also that past studies overestimated the importance of pelagic primary production by not correcting for system-specific bathymetric variation. Our study thus highlights the importance of benthic communities for the northern Baltic Sea ecosystem in general and for future management strategies and ecosystem studies in particular. Electronic supplementary material The online version of this article (doi:10.1007/s13280-016-0778-5) contains supplementary material, which is available to authorized users.

Phytoplankton and heterotrophic bacteria are key groups at the base of aquatic food webs. In estuaries receiving riverine water with a high content of coloured allochthonous dissolved organic matter (ADOM), phytoplankton primary production may be reduced, while bacterial production is favoured. We tested this hypothesis by performing a field study in a northerly estuary receiving nutrient-poor, ADOM-rich riverine water, and analyzing results using multivariate statistics. Throughout the productive season, and especially during the spring river flush, the production and growth rate of heterotrophic bacteria were stimulated by the riverine inflow of dissolved organic carbon (DOC). In contrast, primary production and photosynthetic efficiency (i.e. phytoplankton growth rate) were negatively affected by DOC. Primary production related positively to phosphorus, which is the limiting nutrient in the area. In the upper estuary where DOC concentrations were the highest, the heterotrophic bacterial production constituted almost 100% of the basal production (sum of primary and bacterial production) during spring, while during summer the primary and bacterial production were approximately equal. Our study shows that riverine DOC had a strong negative influence on coastal phytoplankton production, likely due to light attenuation. On the other hand DOC showed a positive influence on bacterial production since it represents a supplementary food source. Thus, in boreal regions where climate change will cause increased river inflow to coastal waters, the balance between phytoplankton and bacterial production is likely to be changed, favouring bacteria. The pelagic food web structure and overall productivity will in turn be altered. Fil: Andersson, Agneta. Universidad de Umea; Suecia. Umeå Marine Sciences Centre; Suecia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Brugel, S.. Universidad de Umea; Suecia. Umeå Marine Sciences Centre; Suecia Fil: Paczkowska, Joanna Marianna. Universidad de Umea; Suecia. Umeå Marine Sciences Centre; Suecia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rowe, O.F.. Universidad de Umea; Suecia. Umeå Marine Sciences Centre; Suecia. University of Helsinki; Finlandia Fil: Figueroa, D.. Universidad de Umea; Suecia. Umeå Marine Sciences Centre; Suecia Fil: Kratzer, S.. Stockholms Universitet; Suecia Fil: Legrand, C.. Linnaeus University; Suecia

Ongoing marine monitoring programs are seldom designed to detect changes in the environment between different years, mainly due to the high number of samples required for a sufficient statistical precision. We here show that pooling over time (time integration) of seasonal measurements provides an efficient method of reducing variability, thereby improving the precision and power in detecting inter-annual differences. Such data from weekly environmental sensor profiles at 21 stations in the northern Bothnian Sea was used in a cost-precision spatio-temporal allocation model. Time-integrated averages for six different variables over 6 months from a rather heterogeneous area showed low variability between stations (coefficient of variation, CV, range of 0.6-12.4%) compared to variability between stations in a single day (CV range 2.4-88.6%), or variability over time for a single station (CV range 0.4-110.7%). Reduced sampling frequency from weekly to approximately monthly sampling did not change the results markedly, whereas lower frequency differed more from results with weekly sampling. With monthly sampling, high precision and power of estimates could therefore be achieved with a low number of stations. With input of cost factors like ship time, labor, and analyses, the model can predict the cost for a given required precision in the time-integrated average of each variable by optimizing sampling allocation. A following power analysis can provide information on minimum sample size to detect differences between years with a required power. Alternatively, the model can predict the precision of annual means for the included variables when the program has a pre-defined budget. Use of time-integrated results from sampling stations with different areal coverage and environmental heterogeneity can thus be an efficient strategy to detect environmental differences between single years, as well as a long-term temporal trend. Use of the presented allocation model will then help to minimize the cost and effort of a monitoring program.

Climate change predictions indicate that coastal and estuarine environments will receive increased terrestrial runoff via increased river discharge. This discharge transports allochthonous material, containing bioavailable nutrients and light attenuating matter. Since light and nutrients are important drivers of basal production, their relative and absolute availability have important consequences for the base of the aquatic food web, with potential ramifications for higher trophic levels. Here, we investigated the effects of shifts in terrestrial organic matter and light availability on basal producers and their grazers. In twelve Baltic Sea mesocosms, we simulated the effects of increased river runoff alone and in combination. We manipulated light (clear/shade) and carbon (added/not added) in a fully factorial design, with three replicates. We assessed microzooplankton grazing preferences in each treatment to assess whether increased terrestrial organic matter input would: (1) decrease the phytoplankton to bacterial biomass ratio, (2) shift microzooplankton diet from phytoplankton to bacteria, and (3) affect microzooplankton biomass. We found that carbon addition, but not reduced light levels per se resulted in lower phytoplankton to bacteria biomass ratios. Microzooplankton generally showed a strong feeding preference for phytoplankton over bacteria, but, in carbon-amended mesocosms which favored bacteria, microzooplankton shifted their diet towards bacteria. Furthermore, low total prey availability corresponded with low microzooplankton biomass and the highest bacteria/phytoplankton ratio. Overall our results suggest that in shallow coastal waters, modified with allochthonous matter from river discharge, light attenuation may be inconsequential for the basal producer balance, whereas increased allochthonous carbon, especially if readily bioavailable, favors bacteria over phytoplankton. We conclude that climate change induced shifts at the base of the food web may alter energy mobilization to and the biomass of microzooplankton grazers. Fil: Meunier, Cédric L.. Universidad de Umea; Suecia Fil: Liess, Antonia. Universidad de Umea; Suecia. Halmstad University; Suecia Fil: Andersson, Agneta. Universidad de Umea; Suecia. Umea Marine Sciences Centre; Suecia Fil: Brugel, Sonia. Universidad de Umea; Suecia. Umea Marine Sciences Centre; Suecia Fil: Paczkowska, Joanna Marianna. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Umea; Suecia. Umea Marine Sciences Centre; Suecia Fil: Rahman, Habib. Universidad de Umea; Suecia Fil: Skoglund, Bjorn. Universidad de Umea; Suecia Fil: Rowe, Owen F.. Universidad de Umea; Suecia. Umea Marine Sciences Centre; Suecia

Uptake of 15 N-labelled nitrate, ammonium, and urea was measured over a quasi-annual cycle in the Cape Bathurst Polynya in the Amundsen Gulf and on the Mackenzie Shelf, during the Canadian Arctic Shelf Exchange Study (CASES) in 2003 and 2004. Before the phytoplankton bloom and in autumn, nitrogen uptake was slow, representing less than 5% of annual consumption. Uptake rates increased exponentially after ice retreat and within 3 wk reached a maximum of 38.6 mmol N m ―2 d ―1 . During spring, NO 3 ― uptake supported new production of 166 mmol N m ―2 and f-ratios rose from 0.1―0.2 to 0.6―0.9. Filter fractionation showed that GF/F filters retained 93.1 ± 1.3% of the 15 N incorporated into particulate matter, suggesting that phytoplankton were responsible for the majority of the N uptake. Although free-living bacteria took up relatively more 15 N in autumn and in the lower part of the euphotic zone than phytoplankton, their assimilation of inorganic N had little effect on water column integrated f-ratios or new production. Urea supplied almost half the N assimilated by phytoplankton annually and about 80 % of the regenerated production during the spring bloom. Total new production, estimated from water column integrated 15 N-nitrogen uptake rates and linear models that interpolated rates over unsampled periods, was 342―415 mmol N m ―2 yr ―1 . Total annual N production for the region was 1.24-1.48 mol N m ―2 yr ―1 .

The spread of the invasive cladoceran Cercopagis pengoi has been well documented in southern areas of the Baltic Sea, however, little research on this invasive species (nor the zooplankton community) has focused on the Gulf of Bothnia (Bothnian Sea and Bay). We analysed data collected over a 12–13 year period at two main stations, one coastal and one offshore, to examine the occurrence of C. pengoi, invasion dynamics, effects on natural zooplankton communities and associated environmental factors. Nine other stations in the Gulf of Bothnia were also examined and the contribution to three-spined stickleback (Gasterosteus aculeatus) diet was quantified. The zooplankton community response apparently differed between coastal and offshore stations with Bosmina, Eurytemora, and Acartia populations being influenced during peak abundances of C. pengoi. It appears that the native zooplankton community has some resilience, returning to its prior structure outside of peak invasion periods. C. pengoi, where present, contributed significantly to stickleback diet. We explored possible barriers for C. pengoi in the Bothnian Bay, suggesting that the low productive Bothnian Bay ecosystem may be incapable of supporting such a predator. This highlights the need for further studies, especially in the light of global climate change.

The phytoplankton community of the Mackenzie shelf and the Amundsen Gulf (south- eastern Beaufort Sea) was characterized (e.g. chlorophyll a biomass, primary production and taxon- omy) during autumn 2002 (23 September to 14 October) and 2003 (30 September to 14 November). Spatial differences were evident, particularly in early autumn. Total phytoplankton biomass and the contribution of large cells (>5 µm) to biomass were higher in the Amundsen Gulf than on the Mackenzie shelf. The community of autotrophic cells (>10 µm) was numerically dominated by diatoms in the Amundsen Gulf and by dinoflagellates on the Mackenzie shelf. The abundance of chlorophytes revealed the influence of the Mackenzie River on the Mackenzie shelf. Contrary to 2002, when all measurements were from early October, the phytoplankton community of the Amund- sen Gulf in 2003 presented the characteristics of a late bloom, which presumably peaked in late Sep- tember. In early autumn, however, primary production rates were similar for both years, averaging 75 mg C m -2 d -1 . High primary production-to-biomass ratios and overall dominance of small cells (

The distribution of picophytoplankton (0.2–2 µm) and nanophytoplankton (2–20 µm) in the Beaufort Sea–Mackenzie Shelf and Amundsen Gulf regions during autumn, 2002 is examined relative to their ambient water mass properties (salinity, temperature and nutrients: nitrate + nitrite, phosphate, and silicate) and to the ratio of variable to maximum fluorescence, Fv/Fm. Total phytoplankton and cell abundances ( a = 0.9 mg m − 3 ). No significant differences in Fv/Fm were evident in the different water masses, indicating that rate limiting conditions for photosynthesis and growth were uniform across the whole system, which was in a pre-winter stage, and was probably already experiencing light limitation as a result of shortening day lengths.

Depth profiles of heterotrophic bacteria abundance were measured weekly over a 6-month period from December to May in Franklin Bay, a 230 m-deep coastal Arctic Ocean site of the southeastern Beaufort Sea. Total bacteria, low nucleic acid (LNA) and high nucleic acid (HNA) bacteria abundances were measured using flow cytometry after SYBR Green I staining. The HNA bacteria abundance in surface waters started to increase 5–6 weeks after phytoplankton growth resumed in spring, increasing from 1 × 10 5 to 3 × 10 5 cells mL − 1 over an 8-week period, with a net growth rate of 0.018 d − 1 . LNA bacteria response was delayed by more than two months relative to the beginning of the phytoplankton biomass accumulation and had a lower net growth rate of 0.013 d − 1 . The marked increase in bacterial abundance occurred before any significant increase in organic matter input from river discharge (as indicated by the unchanged surface water salinity and DOC concentrations), and in the absence of water temperature increase. The abundance of bacteria below the halocline was relatively high until January (up to 5 × 10 5 cells mL − 1 ) but then decreased to values close to 2 × 10 5 cells mL − 1 . The three-fold bacterial abundance increase observed in surface waters in spring was mostly due to HNA bacteria, supporting the idea that these cells are the most active.

Photosynthetic pigments and stable isotopes from suspended particulate organic matter (POM) and surface sediment of the southeast Beaufort Sea, including the Mackenzie shelf and the Amundsen Gulf, were studied during fall 2003 and summer 2004. This multiple-biomarker approach led to an increased understanding of spatial and seasonal variation in pelagic-benthic coupling, as these 2 biomarkers reflect inherent differences in the time scales over which they integrate. Sedi- mentary pigments highlighted the importance of local water-column production as a source of phy- todetrital inputs to the sea floor. In the summer, the dominance of diatoms in the water column was reflected in the sediment by the abundance of fucoxanthin, a pigment broadly found in diatoms. In the fall, a more variable suite of sedimentary pigments reflected inputs from smaller cells such as haptophytes and prasinophytes. While stable isotope composition of the POM showed seasonal vari- ations, i.e. a more marine signature in the summer and a more terrestrial signature in the fall, sedi- mentary stable isotopes revealed geographical differences. Sediment on the Mackenzie shelf sug- gested a terrestrial source of organic matter, while in the Amundsen Gulf, sources of organic matter had a more marine origin. Finally, benthic community compositions and activity (sediment carbon demand) seemed affected by both spatial and seasonal variations in organic matter inputs to the ben- thos. This study stresses the importance of both physical factors (water depth and riverine inputs) and biological production (primary productivity and secondary production) in the determination of organic matter inputs to the benthos.

Pelagic–benthic coupling is relatively well studied in the marginal seas of the Arctic Ocean. Responses of meiofauna with regard to seasonal pulses of particulate organic matter are, however, rarely investigated. We examined the dynamics of metazoan meiofauna and assessed the strength of pelagic–benthic coupling in the Southeastern Beaufort Sea, during autumn 2003 and spring–summer 2004. Meiofauna abundance varied largely (range: 2.3 × 105 to 5 × 106 ind m−2), both spatially and temporally, and decreased with increasing depth (range: 24–549 m). Total meiofauna biomass exhibited similar temporal as well as spatial patterns as abundance and varied from 25 to 914 mg C m−2. Significant relationships between sediment photopigments and various representatives of meiofauna in summer and autumn likely indicate the use of sediment phytodetritus as food source for meiofauna. A carbon-based grazing model provided estimates of potential daily ingestion rates ranging from 32 to 723 mg C m−2. Estimated potential ingestion rates showed that meiofauna consumed from 11 to 477% of the sediment phytodetritus and that meiofauna were likely not food-restricted during spring and autumn. These results show that factors governing the distribution and abundance of metazoan meiofauna need to be better elucidated if we are to estimate the benthic carbon fluxes in marginal seas of the Arctic Ocean.

Over a one-year cycle, several shifts occur in the phytoplankton community of the Bay of Banyuls-sur-Mer, a French northwestern Mediterranean shallow bay. In order to determine what drives these shifts, seasonal changes in littoral phytoplankton are considered together with environmental parameters. Our methodology combined a survey of the physical structure and nutrient contents of the water column with the complete characterization of the phytoplankton community structure at 3-m and 24-m depths. The sampling did provide a detailed record of the seasonal changes and successions that occur in northwestern Mediterranean littoral waters. Intrusions of low salinity water clearly had a significant impact on phytoplankton community structure. Such water freshening events both freshened the surface water and introduced substantial amounts of nutrient. The ratio between nitrogen and phosphorus changed on these occasions and proved to be a reliable clue for the characterization of phytoplankton community structure. Picophytoplankton predominated when the N:P ratio was below 10:1 under summer warm oligotrophic situations ( Synechococcus and Prochlorococcus ) and at the start of water nutrient enrichment (picoeukaryotes). As the N:P ratio increased in late winter–early spring period, eukaryotic phytoplankton was promoted at the expense of Synechococcus and Prochlorococcus . Successions in pico-, nano- and microphytoplankton communities appeared mainly to be driven by differences in the ability to acquire nutrients and by differences in species-specific growth rates. Further characterization of picophytoplankton from samples collected during summer when the N:P ratio reached 5:1 suggested the existence of two cyanobacteria sub-populations. This study clearly shows that northwestern littoral Mediterranean waters are not always well mixed and that nutrient availability in the Bay of Banyuls-sur-Mer depends primarily on low salinity water intrusions.