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Glycine betaine (GBT) is a nitrogenous osmolyte ubiquitous throughout the marine environment. Despite its widespread occurrence and significance in microbial cycling, knowledge of the seasonality of this compound is lacking. Here, we present a seasonal dataset of GBT concentrations in marine suspended particulate material. Analysing coastal waters in the Western English Channel, GBT peaked in summer and autumn but did not follow the observed maxima in total phytoplankton biomass or chlorophyll a. Instead, we found evidence that GBT concentrations were associated with specific phytoplankton groups or species, particularly in the summer when GBT correlated with dinoflagellate biomass. In contrast, autumn maxima corresponded with a period of rapidly changing salinity and nutrient availability, with potential contributions from some phytoplankton species and Harpacticoid copepods. This suggests distinct environmental drivers for different periods of the GBT seasonality. Building on evidence that GBT and dinoflagellate biomass peak in summer, concomitantly with low nutrients, we propose that GBT positively affects dinoflagellate fitness, allowing them to outcompete other plankton when inorganic nutrients are depleted. By using this assumption, we improved the performance of a marine ecosystem model to reproduce the observed increase in dinoflagellates biomass in the transition from spring to summer. This work sheds light on the interplay between phytoplankton succession, competitive advantage and changing environmental factors relevant to climate change. It paves the way for future multidisciplinary research aiming to understand the importance of dinoflagellates in key coastal ecosystems and their potential significance for methylamine production, compounds relevant for particle growth in atmospheric chemistry.

Heterotrophic protists are essential components of the marine ecosystem, yet they are often excluded from monitoring programmes. With limited resources, monitoring strategies need to be optimised considering both scientific knowledge and available resources. In doing so, it is crucial to understand how sampling frequency affects the value of the data. We analysed 11 years of weekly heterotrophic protist time-series data from Station L4 in the Western English Channel to explore how different sampling intervals impact data quality. In the L4 dataset, comprising 55 protist taxa, the reduction of sampling frequency from weekly to four times a year at specific seasons decreased the number of taxa encountered by 38% for ciliates and 29% for heterotrophic dinoflagellates while the mean annual biomass or its mean variation were not affected. Furthermore, when samples were taken only four times a year, biomass peaks of the ten most important taxa were often missed. The primary motivator for this study was furthering the development of the heterotrophic protist monitoring in temperate and subarctic marine areas, e.g. the Baltic Sea. Based on our findings, we give recommendations on sampling frequency to optimise the value of heterotrophic protist monitoring.

Choline and glycine betaine (GBT) are utilized as osmolytes to counteract osmotic stress, but also constitute important nutrient sources for many marine microbes. Bacterial catabolism of these substrates can then lead to the production of climate active trace gases such as methylamine and methane. Using radiotracers, we investigated prokaryotic choline/GBT uptake and determined biotic and abiotic factors driving these processes in the Western English Channel, UK. Kinetic uptake parameters indicated high affinity (nM range) for both osmolytes and showed a seasonal pattern for choline uptake. Generalized linear modeling of uptake parameters suggested a significant influence of sea surface temperature and salinity on prokaryotic uptake of both osmolytes. The presence of diatoms significantly influenced prokaryotic choline/GBT uptake dynamics. Choline uptake was further related to the occurrence of Phaeocystis spp., which were highly abundant in the phytoplankton community during spring, and dinoflagellates abundance during summer. While Rhodobacteraceae were the most important bacterial drivers for prokaryotic choline uptake, prokaryotic GBT uptake was associated with various groups such as SAR11 (Pelagibacterales) and Gammaproteobacteria, suggesting a wider capacity for GBT catabolism than previously recognized. Furthermore, using a newly developed approach we determined the first available data for dissolved GBT concentrations in seawater and found both osmolytes to be at the sub-nanomolar range. Together, this study improves our understanding of the biogeochemical cycling of these environmentally important osmolytes and highlights how their cycles may be affected by a changing climate.\ud \ud

Size-spectral approaches quantify the efficiency of energy transfer through food webs, but theory and field studies disagree over how changes in temperature, nutrients, and extreme weather impact on this efficiency. We address this at two scales: via 6 years of weekly sampling of the plankton size spectrum at the Plymouth L4 shelf sea site, and via a new, global-scale, meta-analysis of aquatic size spectra. The time series showed that with summertime nutrient starvation, the energy transfer efficiency from picoplankton to macroplankton decreased (i.e., steepening slopes of the size spectra). This reflected increasing dominance by small cells and their microbial consumers. The extreme storms in winter 2013/2014 caused high metazoan mortality, steep size-spectral slopes, and reduced plankton biomass. However, recovery was within months, demonstrating an inbuilt resilience of the system. Both L4 and our meta-analysis showed steep slopes of normalized size spectra (median −1.11). This reflects much lower values, either of trophic transfer efficiency (3.5%) or predator–prey mass ratio (569), compared to commonly quoted values. Results from the meta-analysis further showed that to represent energy transfer faithfully, size spectra are best constructed in units of carbon mass and not biovolume, and span a mass range of > 107. When this range is covered, both the meta-analysis and time series show a dome-shaped relationship between spectral slopes and plankton biomass, with steepening slopes under increasingly oligotrophic and eutrophic conditions. This suggests that ocean warming could decrease the efficiency of energy transfer through pelagic food webs via indirect effects of increasing stratification and nutrient starvation.

Theory and observations suggest that low frequency variation in marine plankton populations, or red noise, may arise through cumulative integration of white noise atmospheric forcing by the ocean and its amplification within food webs. Here, we revisit evidence for the integration of stochastic atmospheric variations by comparing the power spectra of time series of atmospheric and oceanographic conditions to the population dynamics of 150 plankton taxa at Station L4 in the Western English Channel. The power spectra of oceanographic conditions (sea surface temperature, surface nitrate) are redder than those of atmospheric forcing (surface wind stress, net heat fluxes) at Station L4. However, plankton populations have power spectral slopes across trophic levels and body sizes that are redder than atmospheric forcing but whiter than oceanographic conditions. While zooplankton have redder spectral slopes than phytoplankton, there is no significant relationship between power spectral slope and body size or generation length. Using a predator-prey model, we show that the whitening of plankton time series relative to oceanographic conditions arises from noisy plankton bloom dynamics in this strongly seasonal system. The model indicates that, for typical predator-prey interactions, where the predator is on average 10 times longer than the prey, grazing leads to a modest reddening of phytoplankton variability by their larger and longer lived zooplankton consumers. Our findings suggest that, beyond extrinsic forcing by the environment, predator-prey interactions play a role in influencing the power spectra of time series of plankton populations.

Using automated imaging technologies, it is now possible to generate previously unprecedented volumes of plankton image data which can be used to study the composition of plankton assemblages. However, the current need to manually classify individual images introduces a bottleneck into processing chains. Although Machine Learning techniques have been used to try and address this issue, past efforts have suffered from accuracy limitations, especially in minority classes. Here we use state-of-the-art methods in Deep Learning to investigate suitable architectures for training an automated plankton classification system which achieves high efficacy for both abundant and rare taxa. We collected live plankton from Station L4 in the Western English Channel and imaged 11,371 particles covering 104 taxonomic groups using the automated plankton imaging system FlowCam. The image set contained a severe class imbalance, with some taxa represented by > 600 images while other, rarer taxa were represented by just 14. We demonstrate that by allowing multiple Deep Learning models to collaborate in a single classification system, classification accuracy improves for minority classes when compared with the best individual model. The top collaborative model achieved a 6 % improvement in F1 accuracy over the best individual model, while overall accuracy improved by 3.2 %. This resulted in a 97.4 % overall accuracy score and a 96.2 % F1 macro score on a separate holdout test set containing 104 taxonomic groups. Based on a survey of similar studies in the literature, we believe collaborative deep learning models can significantly improve the accuracy of existing automated plankton classification systems.

Ecological communities are shaped by a complex interplay between abiotic forcing, biotic regulation and demographic stochasticity. However, community dynamics modelers tend to focus on abiotic forcing overlooking biotic interactions, due to notorious challenges involved in modeling and quantifying inter-specific interactions, particularly for species-rich systems such as planktonic assemblages. Nevertheless, inclusive models with regard to the full range of plausible drivers are essential to characterizing and predicting community response to environmental changes. Here we develop a Bayesian model for identifying, from in-situ time series, the biotic, abiotic and stochastic factors underlying the dynamics of species-rich communities, focusing on the joint biomass dynamics of biologically meaningful groups. We parameterize a multivariate model of population co-variation with an explicit account for demographic stochasticity, density-dependent feedbacks, pairwise interactions, and abiotic stress mediated by changing environmental conditions and resource availability, and work out explicit formulae for partitioning the temporal variance of each group in its biotic, abiotic and stochastic components. We illustrate the methodology by analyzing the joint biomass dynamics of four major phytoplankton functional types namely, diatoms, dinoflagellates, coccolithophores and phytoflagellates at Station L4 in the Western English Channel using weekly biomass records and coincident measurements of environmental covariates describing water conditions and potentially limiting resources. Abiotic and biotic factors explain comparable amounts of temporal variance in log-biomass growth across functional types. Our results demonstrate that effective modelling of resource limitation and inter-specific interactions is critical for quantifying the relative importance of abiotic and biotic factors.

Continental margins are disproportionally important for global primary production, fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has been an ongoing summertime decline of key biota-large diatoms, dinoflagellates and copepods-that traditionally fuel higher tropic levels such as fish, sea birds and marine mammals. Here, we combine multiple time series with in situ process studies to link these declines to summer nutrient stress and increasing proportions of picophytoplankton that can comprise up to 90% of the combined pico- and nanophytoplankton biomass in coastal areas. Among the pico-fraction, it is the cyanobacterium Synechococcus that flourishes when iron and nitrogen resupply to surface waters are diminished. Our field data show how traits beyond small size give Synechococcus a competitive edge over pico- and nanoeukaryotes. Key is their ability to grow at low irradiances near the nutricline, which is aided by their superior light-harvesting system and high affinity to iron. However, minute size and lack of essential biomolecules (e.g. omega-3 polyunsaturated fatty acids and sterols) render Synechococcus poor primary producers to sustain shelf sea food webs efficiently. The combination of earlier spring blooms and lower summer food quantity and quality creates an increasing period of suboptimal feeding conditions for zooplankton at a time of year when their metabolic demand is highest. We suggest that this nutrition-related mismatch has contributed to the widespread, ~50% decline in summer copepod abundance we observe over the last 60 years. With Synechococcus clades being prominent from the tropics to the Arctic and their abundances increasing worldwide, our study informs projections of future food web dynamics in coastal and shelf areas where droughts and stratification lead to increasing nutrient starvation of surface waters.

When designing models for predicting phytoplankton biomass or characterizing traits, it is useful to aggregate the myriad of species into a few biologically meaningful groups and focus on group-level attributes, the common practice being to combine phytoplankton species by functional types. However, biogeochemists and plankton ecologists debate the most applicable grouping for describing phytoplankton biomass patterns and predicting future community structure. Although trait-based approaches are increasingly being advocated, methods are missing for the generation of trait-based taxa as alternatives to functional types. Here we introduce such a method and demonstrate the usefulness of the resulting clustering with field data. We parameterize a Bayesian model of biomass dynamics and analyze long-term phytoplankton data collected at Station L4 in the Western English Channel between April 2003 and December 2009. We examine the tradeoffs encountered regarding trait characterization and biomass prediction when aggregating biomass by (1) functional types, (2) the trait-based clusters generated by our method, and (3) total biomass. The model conveniently extracted trait values under the trait-based clustering, but required well-constrained priors under the functional type categorization. It also more accurately predicted total biomass under the trait-based clustering and the total biomass aggregation with comparable root mean squared prediction errors, which were roughly five-fold lower than under the functional type grouping. Although the total biomass grouping ignores taxonomic differences in phytoplankton traits, it predicts total biomass change as well as the trait-based clustering. Our results corroborate the value of trait-based approaches in investigating the mechanisms underlying phytoplankton biomass dynamics and predicting the community response to environmental changes.

This work evaluates the sensitivity of CO 2 air–sea gas exchange in a coastal site to four different model system configurations of the 1D coupled hydrodynamic–ecosystem model GOTM–ERSEM, towards identifying critical dynamics of relevance when specifically addressing quantification of air–sea CO 2 exchange. The European Sea Regional Ecosystem Model (ERSEM) is a biomass and functional group-based biogeochemical model that includes a comprehensive carbonate system and explicitly simulates the production of dissolved organic carbon, dissolved inorganic carbon and organic matter. The model was implemented at the coastal station L4 (4 nm south of Plymouth, 50°15.00’N, 4°13.02’W, depth of 51 m). The model performance was evaluated using more than 1500 hydrological and biochemical observations routinely collected at L4 through the Western Coastal Observatory activities of 2008—-2009. In addition to a reference simulation (A), we ran three distinct experiments to investigate the sensitivity of the carbonate system and modeled air–sea fluxes to (B) the sea-surface temperature (SST) diurnal cycle and thus also the near-surface vertical gradients, (C) biological suppression of gas exchange and (D) data assimilation using satellite Earth observation data. The reference simulation captures well the physical environment (simulated SST has a correlation with observations equal to 0.94 with a p> 0.95). Overall, the model captures the seasonal signal in most biogeochemical variables including the air–sea flux of CO 2 and primary production and can capture some of the intra-seasonal variability and short-lived blooms. The model correctly reproduces the seasonality of nutrients (correlation > 0.80 for silicate, nitrate and phosphate), surface chlorophyll-a (correlation > 0.43) and total biomass (correlation > 0.7) in a two year run for 2008–2009. The model simulates well the concentration of DIC, pH and in-water partial pressure of CO 2 (pCO 2 ) with correlations between 0.4–0.5. The model result suggest that L4 is a weak net source of CO 2 (0.3–1.8 molCm − 2 year − 1 ). The results of the three sensitivity experiments indicate that both resolving the temperature profile near the surface and assimilation of surface chlorophyll-a significantly impact the skill of simulating the biogeochemistry at L4 and all of the carbonate chemistry related variables. These results indicate that our forecasting ability of CO 2 air–sea flux in shelf seas environments and their impact in climate modeling should consider both model refinements as means of reducing uncertainties and errors in any future climate projections.

It is difficult to make skillful predictions about the future dynamics of marine phytoplankton populations. Here, we use a 22‐year time series of monthly average abundances for 198 phytoplankton taxa from Station L4 in the Western English Channel (1992–2014) to test whether and how aggregating phytoplankton into multi‐species assemblages can improve predictability of their temporal dynamics. Using a non‐parametric framework to assess predictability, we demonstrate that the prediction skill is significantly affected by how species data are grouped into assemblages, the presence of noise, and stochastic behavior within species. Overall, we find that predictability one month into the future increases when species are aggregated together into assemblages with more species, compared with the predictability of individual taxa. However, predictability within dinoflagellates and larger phytoplankton (>12 μm cell radius) is low overall and does not increase by aggregating similar species together. High variability in the data, due to observational error (noise) or stochasticity in population growth rates, reduces the predictability of individual species more than the predictability of assemblages. These findings show that there is greater potential for univariate prediction of species assemblages or whole‐community metrics, such as total chlorophyll or biomass, than for the individual dynamics of phytoplankton species., Marine phytoplankton population dynamics are inherently noisy and difficult to predict. Here, using observations of phytoplankton abundance from the English Channel, we found that the predictability of phytoplankton assemblages typically exceeds predictability for individual species and that predictability increases with assemblage size. A companion numerical model reveals that aggregation across species reduces the effect of measurement error and stochasticity in phytoplankton growth rates on predictability.

WOS:000450233300040; The Plymouth L4 time plankton series in the Western English Channel is a textbook example of a shallow, stratifying shelf sea system. Over its 30 yr of weekly sampling, this site has provided a diverse and contrasting suite of numerical and conceptual models of plankton bloom formation, phenology, and seasonal succession. The most recent of these papers, Kenitz et al. (2017) has initiated this comment, partly because we feel that it has presented a slightly misleading picture of the plankton composition at this site, and of a robust, recurring seasonal succession. We address this by illustrating the extent of inter-annual variability in phenology that occurs at the site, and which needs to be captured better within models. However our main aim is to foster a much better integration of the variety of top-down and bottom-up processes that have all been suggested to be key in driving seasonal succession. Some of these, particularly the multiple grazing and growth controls contributing to the so-called "loophole hypothesis" may be complementary, but others, such as the role of copepod feeding traits in driving species succession (Kenitz et al. 2017) offer testable competing hypotheses. The basic assumptions and outputs of all these models need to be validated more critically, both against time series data and process studies that include the finding of unselective feeding. We suggest that the variability in plankton phenology (and not just mean timing and amplitude) could be used to diagnose the performance of alternative models of plankton succession.

The pelagic copepods Oithona similis and Calanus helgolandicus have overlapping geographic ranges, yet contrast in feeding mode, reproductive strategy, and body size. We investigate how these contrasting traits influence the seasonality of copepod abundance and reproductive output under environmental variation, using time series data collected over 25 years at the Western Channel Observatory station L4. The proportional change in Egg Production Rate (EPR, eggs female-1 d-1) over the annual cycle was ~10-fold and similar for both species, although EPR of O. similis was only ~ 11% that of C. helgolandicus. The timing of EPR maxima for O. similis coincided with increased Sea Surface Temperature (SST) in summer, likely due to a temperature-dependent brooding period. Conversely, EPR of broadcast spawning C. helgolandicus was more strongly related to Net Heat Flux (NHF) and diatom biomass, both parameters associated with the spring phytoplankton bloom. In both species, female body mass negatively correlated with SST, with a 7.5% reduction in body mass per °C in C. helgolandicus compared to just 2.3% in O. similis. Finally, seasonality of EPR and adult and copepodite abundance was strongly decoupled in both species, suggesting that optimum conditions for reproduction and abundance occur at different times of the year.

Observations made within a cold filament in the Mauritanian upwelling system demonstrate that intense submesoscale circulations at the peripheral edges of the filament are likely responsible for anomalously high levels of observed primary productivity by resupplying nutrients to the euphotic zone. Measurements made on the shelf within the recently upwelled water reveal that primary production (PP) of 8.2 gC/m−2 day−1 was supported by nitrate concentrations (NC) of 8 mmol m−3. Towards the front that defined the edge of the filament containing the upwelled water as it was transported offshore, PP dropped to 1.6 gC m−2 day−1 whilst NC dropped to 5.5 mmol m−3. Thus, whilst the observed nutrients on the shelf accounted for 90% of new production, this value dropped to ∼60% near the filament’s front after accounting for vertical turbulent fluxes and Ekman pumping. We demonstrate that the N15 was likely to have been supplied at the front by submesoscale circulations that were directly measured as intense vertical velocities ⩾100 m day−1 by a drifting acoustic Doppler current profiler that crossed a submesoscale surface temperature front. At the same time, a recently released tracer was subducted out of the mixed layer within 24 h of release, providing direct evidence that the frontal circulations were capable of accessing the reservoir of nutrients beneath the pycnocline. The susceptibility of the filament edge to submesoscale instabilities was demonstrated by O(1) Rossby numbers at horizontal scales of 1–10 km. The frontal circulations are consistent with instabilities arising from a wind-driven nonlinear Ekman buoyancy flux generated by the persistent northerly wind stress that has a down-front component at the northern edge of the inshore section of the filament. The prevalence of submesoscale instabilities and their associated vertical circulations are proposed to be a key mechanism operating at sub-grid scales and sustaining new production throughout the upwelling system.

Trait values are usually extracted from laboratory studies of single phytoplankton species, which presents challenges for understanding the immense diversity of phytoplankton species and the wide range of dynamic ocean environments. Here we use a Bayesian approach and a trait-based model to extract trait values for four functional types and ten diatom species from field data collected at Station L4 in the Western Channel Observatory. We find differences in maximum net growth rate, temperature optimum and sensitivity, half-saturation constants for light and nitrogen, and density-dependent loss terms across the functional types. We find evidence of very high linear loss rates, suggesting that grazing may be even more important than commonly assumed and differences in density-dependent loss rates across functional types, indicating the presence of strong niche differentiation among functional types. Low half-saturation constants for nitrogen at the functional type level may indicate widespread mixotrophy. At the species level, we find a wide range of density-dependent effects, which may be a signal of diversity in grazing susceptibility or biotic interactions. This approach may be a way to obtain more realistic and better-constrained trait-values for functional types to be used in ecosystem modeling.

The processes and patterns seen in coastal benthic communities can be strongly influenced by the overlying pelagic environmental conditions. Integrating long-term biological and environmental data (both benthic and pelagic) can give insight into the specific relationships between key benthic functional groups and natural temporal changes in the marine environment. The identity and abundance of amphipod species found at Station L4 (Western English Channel) were tracked for 7 years (2008–2014), whilst simultaneously, annual changes in phytoplankton biomass, water temperature, salinity and chlorophyll a concentration were also characterized. The main species were persistent and showed little variability along the study period. Overall, however, there were significant changes in the structure of the whole community between sampling times, highlighting the importance of less numerically-dominant species in driving temporal variability. Surprisingly, the current study did not detect a significant influence of the phytoplankton biomass on benthic amphipod dynamics. On the other hand, there was a clear and constant correlation between bottom water temperatures and amphipod abundance. This pattern is different from that observed in other detritivorous species at L4, highlighting the complexity of benthic-pelagic coupling and the high variability of the response to pelagic conditions among different groups. As a result of the biogeographic position of the Western English Channel, the key role of amphipods in benthic communities, the influence of the temperature in their populations dynamics, as well as the solid baseline provided here and in previous studies, the monitoring of long-term amphipod dynamics in the English Channel could be a valuable tool to evaluate the biological effect of climate change over marine benthic communities.

A 21-year time series of phytoplankton community structure was analysed in relation to Phaeocystis spp. to elucidate its contribution to the annual carbon budget at station L4 in the western English Channel (WEC). Between 1993-2014 Phaeocystis spp. contributed ∼4.6% of the annual phytoplankton carbon and during the March - May spring bloom, the mean Phaeocystis spp. biomass constituted 17% with a maximal contribution of 47% in 2001. Upper maximal weekly values above the time series mean ranged from 63 to 82% of the total phytoplankton carbon (∼42-137mg carbon (C)m-3) with significant inter-annual variability in Phaeocystis spp. Maximal biomass usually occurred by the end of April, although in some cases as early as mid-April (2007) and as late as late May (2013). The effects of elevated pCO2 on the Phaeocystis spp. spring bloom were investigated during a fifteen-day semi-continuous microcosm experiment. The phytoplankton community biomass was estimated at ∼160mgCm-3 and was dominated by nanophytoplankton (40%, excluding Phaeocystis spp.), Phaeocystis spp. (30%) and cryptophytes (12%). The smaller fraction of the community biomass comprised picophytoplankton (9%), coccolithophores (3%), Synechococcus (3%), dinoflagellates (1.5%), ciliates (1%) and diatoms (0.5%). Over the experimental period, total biomass increased significantly by 90% to ∼305mgCm-3 in the high CO2 treatment while the ambient pCO2 control showed no net gains. Phaeocystis spp. exhibited the greatest response to the high CO2 treatment, increasing by 330%, from ∼50mgCm-3 to over 200mgCm-3 and contributing ∼70% of the total biomass. Taken together, the results of our microcosm experiment and analysis of the time series suggest that a future high CO2 scenario may favour dominance of Phaeocystis spp. during the spring bloom. This has significant implications for the formation of hypoxic zones and the alteration of food web structure including inhibitory feeding effects and lowered fecundity in many copepod species.

Autumn phytoplankton blooms represent key periods of production in temperate and high-latitude seas. Biogenic silica (bSiO2) production, dissolution, and standing stocks were determined in the Celtic Sea (United Kingdom) during November 2014. Dissolution rates were in excess of bSiO2 production, indicating a net loss of bSiO2. Estimated diatom bSiO2 contributed ≤10% to total bSiO2, with detrital bSiO2 supporting rapid Si cycling. Based on the average biomass-specific dissolution rate (0.2 day−1), 3 weeks would be needed to dissolve 99% of the bSiO2 present. Negative net bSiO2 production was associated with low-light conditions (−2·day−1). Our observations imply that dissolution dominates Si cycling during autumn, with low-light conditions also likely to influence Si cycling during winter and early spring.

The combined effects of elevated pCO2 and temperature were investigated during an experimentally induced autumn phytoplankton bloom in vitro sampled from the western English Channel (WEC). A full factorial 36-day microcosm experiment was conducted under year 2100 predicted temperature (+4.5 ∘C) and pCO2 levels (800 µatm). Over the experimental period total phytoplankton biomass was significantly influenced by elevated pCO2. At the end of the experiment, biomass increased 6.5-fold under elevated pCO2 and 4.6-fold under elevated temperature relative to the ambient control. By contrast, the combined influence of elevated pCO2 and temperature had little effect on biomass relative to the control. Throughout the experiment in all treatments and in the control, the phytoplankton community structure shifted from dinoflagellates to nanophytoplankton . At the end of the experiment, under elevated pCO2 nanophytoplankton contributed 90 % of community biomass and was dominated by Phaeocystis spp. Under elevated temperature, nanophytoplankton comprised 85 % of the community biomass and was dominated by smaller nanoflagellates. In the control, larger nanoflagellates dominated whilst the smallest nanophytoplankton contribution was observed under combined elevated pCO2 and temperature (∼ 40 %). Under elevated pCO2, temperature and in the control there was a significant decrease in dinoflagellate biomass. Under the combined effects of elevated pCO2 and temperature, dinoflagellate biomass increased and was dominated by the harmful algal bloom (HAB) species, Prorocentrum cordatum. At the end of the experiment, chlorophyll a (Chl a) normalised maximum photosynthetic rates (PmB) increased > 6-fold under elevated pCO2 and > 3-fold under elevated temperature while no effect on PmB was observed when pCO2 and temperature were elevated simultaneously. The results suggest that future increases in temperature and pCO2 simultaneously do not appear to influence coastal phytoplankton productivity but significantly influence community composition during autumn in the WEC.

Macroalgae drive the largest CO2 flux fixed globally by marine macrophytes. Most of the resulting biomass is exported through the coastal ocean as detritus and yet almost no field measurements have verified its potential net sequestration in marine sediments. This gap limits the scope for the inclusion of macroalgae within blue carbon schemes that support ocean carbon sequestration globally, and the understanding of the role their carbon plays within distal food webs. Here, we pursued three lines of evidence (eDNA sequencing, Bayesian Stable Isotope Mixing Modeling, and benthic-pelagic process measurements) to generate needed, novel data addressing this gap. To this end, a 13-month study was undertaken at a deep coastal sedimentary site in the English Channel, and the surrounding shoreline of Plymouth, UK. The eDNA sequencing indicated that detritus from most macroalgae in surrounding shores occurs within deep, coastal sediments, with detritus supply reflecting the seasonal ecology of individual species. Bayesian stable isotope mixing modeling [C and N] highlighted its vital role in supporting the deep coastal benthic food web (22–36% of diets), especially when other resources are seasonally low. The magnitude of detritus uptake within the food web and sediments varies seasonally, with an average net sedimentary organic macroalgal carbon sequestration of 8.75 g C·m−2·yr−1. The average net sequestration of particulate organic carbon in sediments is 58.74 g C·m−2·yr−1, the two rates corresponding to 4–5% and 26–37% of those associated with mangroves, salt marshes, and seagrass beds, systems more readily identified as blue carbon habitats. These novel data provide important first estimates that help to contextualize the importance of macroalgal-sedimentary connectivity for deep coastal food webs, and measured fluxes help constrain its role within global blue carbon that can support policy development. At a time when climate change mitigation is at the foreground of environmental policy development, embracing the full potential of the ocean in supporting climate regulation via CO2 sequestration is a necessity.

International audience; In a few recent studies, the action of a bacterial dioxygenase (10S-DOX) on palmitoleic acid was observed within some polar and estuarine settings. To add further mechanistic information regarding the action of this enzyme in marine settings, we measured a range of lipids (sterols, fatty acids and the chlorophyll phytyl side chain) and their biotic and abiotic degradation products in water samples collected in 2018 from two depths (5 m and 25 m) at the temperate oceanographic time series site L4, located in the western English Channel. Lipid distributions indicated a dominance of diatoms and copepods during the spring bloom, while a peak in dinoflagellate activity was evident in samples collected from late summer/autumn, both outcomes being consistent with taxonomic data reported previously for the same sampling site and interval. Monitoring of lipid oxidation products characteristic of different degradation pathways showed a relatively weak effect of photo- and autoxidation processes, with these acting mainly on the more reactive lipids (i.e. chlorophyll and polyunsaturated fatty acids). In contrast, monitoring of biotic degradation processes revealed significant quantities of 10S-hydroxyhexadec-8(E)-enoic acid in samples collected at the end of April (reaching 40% of the residual parent palmitoleic acid), attributed to the involvement of bacterial 10-dioxygenase (10S-DOX) activity during the spring bloom. We propose that this enzyme could be utilised by bacteria to detoxify free fatty acids released by wounded diatoms in the presence of copepods

The lagrangian progression of biogeochemical processes was followed in a filament of the Mauritanian upwelling system, North West Africa, during offshore advection. Inert duel tracers sulphur hexafluoride and helium-3 labelled a freshly upwelled patch of water that was mapped for 8 days. Changes in biological, physical and chemical characteristics were measured including phytoplankton productivity, nitrogen assimilation and regeneration. Freshly upwelled water contained high nutrient concentrations (NO3− = 9.0 ± 0.1 μmol L−1; PO43− = 0.7 ± 0.1 μmol L−1; Si = 2.7 ± 0.1 μmol L−1) but was depleted in N compared to Redfield stoichiometry (N:P = 13.9:1). A maximum primary productivity rate of 0.7 mol C m−2 d−1 was measured on the continental shelf, associated with N-assimilation rates of 43.8 nmol L−1 h−1 for NO3−, 32.8 nmol L−1 h−1 for NH4+ and a phytoplankton community dominated by diatoms and flagellates. Indicators of phytoplankton abundance and activity decreased as the labelled water mass transited the continental shelf slope into deeper water, possibly linked to the mixed layer depth exceeding the light penetration depth. By the end of the study, primary productivity rates of 0.1 mol C m−2 d−1 were measured, associated with N-assimilation rates of 3.9 nmol L−1 h−1 for NO3−, 6.1 nmol L−1 h−1 for NH4+ and lower nutrient concentrations (NO3− = 4.6 ± 0.3 μmol L−1; PO43− = 0.4 ± 0.1 μmol L−1; Si = 0.9 ± 0.1 μmol L−1). Nitrogen regeneration and assimilation took place simultaneously; NH4+ was regenerated at 9.4–85.0 nmol L−1 h−1; NH4+ was oxidised at 0.30–8.75 nmol L−1 h−1; NO2− was oxidised at 25.55–81.11 nmol L−1 h−1. Results highlight the importance of regenerated NH4+ in sustaining phytoplankton productivity and indicate that the upwelled NO3− pool contained an increasing fraction of regenerated NO3− as it advected offshore. By calculating this fraction and incorporating it into an f ratio formulation we estimated that of the 12.38 Tg C of annual regional production, 4.73 Tg C was exportable.

1.There are tens of thousands of species of phytoplankton found throughout the tree of life. Despite this diversity, phytoplankton are often aggregated into a few functional groups according to metabolic traits or biogeochemical role. We investigate the extent to which phytoplankton species dynamics are neutral within functional groups. 2.Seasonal dynamics in many regions of the ocean are known to affect phytoplankton at the functional group level leading to largely predictable patterns of seasonal succession. It is much more difficult to make general statements about the dynamics of individual species. 3.We use a 7 year time-series at station L4 in the Western English Channel with 57 diatom and 17 dinoflagellate species enumerated weekly to test if the abundance of diatom and dinoflagellate species vary randomly within their functional group envelope or if each species is driven uniquely by external factors. 4.We show that the total biomass of the diatom and dinoflagellate functional groups is well predicted by irradiance and temperature and quantify trait values governing the growth rate of both functional groups. The biomass dynamics of the functional groups are not neutral and each has their own distinct responses to environmental forcing. Compared to dinoflagellates, diatoms have faster growth rates, and grow faster under lower irradiance, cooler temperatures, and higher nutrient conditions. 5.The biomass of most species vary randomly within their functional group biomass envelope, most of the time. As a consequence, modelers will find it difficult to predict the biomass of most individual species. Our analysis supports the approach of using a single set of traits for a functional group and suggests that it should be possible to determine these traits from natural communities.

Trichodesmium, a colonial cyanobacterium typically associated with tropical waters, was observed between January and April 2014 in the western English Channel. Sequencing of the heterocyst differentiation (hetR) and 16S rRNA genes placed this community within the Clade IV Trichodesmium, an understudied clade previously found only in low numbers in warmer waters. Nitrogen fixation was not detected although measurable rates of nitrate uptake and carbon fixation were observed. Trichodesmium RuBisCO transcript abundance relative to gene abundance suggests the potential for viable and potentially active Trichodesmium carbon fixation. Observations of Trichodesmium when coupled with a numerical advection model indicate that Trichodesmium communities can remain viable for >3.5 months at temperatures lower than previously expected. The results suggest that Clade IV Trichodesmium occupies a different niche to other Trichodesmium species, and is a cold- or low-light-adapted variant.

Motivation The BioTIME database contains raw data on species identities and abundances in ecological assemblages through time. These data enable users to calculate temporal trends in biodiversity within and amongst assemblages using a broad range of metrics. BioTIME is being developed as a community-led open-source database of biodiversity time series. Our goal is to accelerate and facilitate quantitative analysis of temporal patterns of biodiversity in the Anthropocene. Main types of variables included The database contains 8,777,413 species abundance records, from assemblages consistently sampled for a minimum of 2 years, which need not necessarily be consecutive. In addition, the database contains metadata relating to sampling methodology and contextual information about each record. Spatial location and grain BioTIME is a global database of 547,161 unique sampling locations spanning the marine, freshwater and terrestrial realms. Grain size varies across datasets from 0.0000000158 km2 (158 cm2) to 100 km2 (1,000,000,000,000 cm2). Time period and grain BioTIME records span from 1874 to 2016. The minimal temporal grain across all datasets in BioTIME is a year. Major taxa and level of measurement BioTIME includes data from 44,440 species across the plant and animal kingdoms, ranging from plants, plankton and terrestrial invertebrates to small and large vertebrates. Software format .csv and .SQL., Global Ecology and Biogeography, 27 (7), ISSN:1466-822X, ISSN:1466-8238

WOS:000441489600006; International audience; There has been an upsurge of interest in trait-based approaches to zooplankton, modelling the seasonal changes in the feeding modes of zooplankton in relation to phytoplankton traits such as size or motility. We examined this link at two English Channel plankton monitoring sites south of Plymouth (L4 and El). At L4 there was a general transition from diatoms in spring to motile microplankton in summer and autumn, but this was not mirrored in the succession of copepod feeding traits; for example the ambushing Oithona similis dominated during the spring diatom bloom. At nearby El we measured seasonality of food and grazers, finding strong variation between 2014 and 2015 but overall low mesozooplankton biomass (median 4.5 mg C m(-3)). We also made a seasonal grazing study of five copepods with contrasting feeding modes (Calculus helgolandicus, Centropages typicus, Acartia clausi, Pseudocalanus elongatus and Oithona similis), counting the larger prey items from the natural seston. All species of copepod fed on all food types and differences between their diets were only subtle; the overriding driver of diet was the composition of the prey field. Even the smaller copepods fed on copepod nauplii at significant rates, supporting previous suggestions of the importance of intra-guild predation. All copepods, including O. similis, were capable of tackling extremely long ( \textgreater 500 mu m) diatom chains at clearance rates comparable to those on ciliates. Maximum observed prey:predator length ratios ranged from 0.12 (C. helgolandicus) up to 0.52 (O. similis). Unselective feeding behaviour and the ability to remove highly elongated cells have implications for how copepod feeding is represented in ecological and biogeochemical models.

Lipids are key constituents of marine phytoplankton, and some fatty acids (key constituents of lipids) are essential dietary components for secondary producers. However, in natural marine ecosystems the interactions of factors affecting seasonal phytoplankton lipid composition are still poorly understood. The aim of this study was to assess the roles of seasonal succession in phytoplankton community composition and nutrient concentrations, on the lipid composition of the phytoplankton community. Fatty acid and polar lipid composition in seston was measured in surface waters at the time series station L4, an inshore station in the Western English Channel, from January to December 2013. Redundancy analyses (RDA) were used to identify factors (abiotic and biotic) that explained the seasonal variability in phytoplankton lipids. RDA demonstrated that nutrients (namely nitrogen) explained the majority of variation in phytoplankton lipid composition, as well as a smaller explanatory contribution from changes in phytoplankton community composition. The physiological adaptations of the phytoplankton community to nutrient deplete conditions during the summer season when the water column was stratified, was further supported by changes in the polar lipid to phytoplankton biomass ratios (also modelled with RDA) and increases in the lipid to chlorophyll a ratios, which are both indicative of nutrient stress. However, the association of key fatty acid markers with phytoplankton groups e.g. 22:6 n-3 and dinoflagellate biomass (predominant in summer), meant there were no clear seasonal differences in the overall degree of fatty acid saturation, as might have been expected from typical nutrient stress on phytoplankton. Based on the use of polyunsaturated fatty acids (PUFA) as markers of ‘food quality’ for grazers, our results suggest that in this environment high food quality is maintained throughout summer, due to seasonal succession towards flagellated phytoplankton species able to maintain PUFA synthesis under surface layer nutrient depletion.

Naturally occurring red tides and harmful algal blooms (HABs) are of increasing importance in the coastal environment and can have dramatic effects on coastal benthic and epipelagic communities worldwide. Such blooms are often unpredictable, irregular or of short duration, and thus determining the underlying driving factors is problematic. The dinoflagellate Karenia mikimotoi is an HAB, commonly found in the western English Channel and thought to be responsible for occasional mass finfish and benthic mortalities. We analysed a 19-year coastal time series of phytoplankton biomass to examine the seasonality and interannual variability of K. mikimotoi in the western English Channel and determine both the primary environmental drivers of these blooms as well as the effects on phytoplankton productivity and oxygen conditions. We observed high variability in timing and magnitude of K. mikimotoi blooms, with abundances reaching >1000 cells mL −1 at 10 m depth, inducing up to a 12-fold increase in the phytoplankton carbon content of the water column. No long-term trends in the timing or magnitude of K. mikimotoi abundance were evident from the data. Key driving factors were identified as persistent summertime rainfall and the resultant input of low-salinity high-nutrient river water. The largest bloom in 2009 was associated with highest annual primary production and led to considerable oxygen depletion at depth, most likely as a result of enhanced biological breakdown of bloom material; however, this oxygen depletion may not affect zooplankton. Our data suggests that K. mikimotoi blooms are not only a key and consistent feature of western English Channel productivity, but importantly can potentially be predicted from knowledge of rainfall or river discharge.

The impact of the seasonal deposition of phytoplankton and phytodetritus on surface sediment bacterial abundance and community composition was investigated at the Western English Channel site L4. Sediment and water samples were collected from January to September in 2012, increasing in frequency during periods of high water column phytoplankton abundance. Compared to the past two decades, the spring bloom in 2012 was both unusually long in duration and contained higher than average biomass. Within spring months, the phytoplankton bloom was well mixed through the water column and showed accumulations near the sea bed, as evidenced by flow cytometry measurements of nanoeukaryotes, water column chlorophyll a and the appearance of pelagic phytoplankton at the sediment. Measurements of chlorophyll and chlorophyll degradation products indicated phytoplankton material was heavily degraded after it reached the sediment surface: the nature of the chlorophyll degradation products (predominantly pheophorbide, pyropheophorbide and hydroxychlorophyllone) was indicative of grazing activity. The abundance of bacterial 16S rRNA genes g−1 sediment (used as a proxy for bacterial biomass) increased markedly with the onset of the phytoplankton bloom, and correlated with measurements of chlorophyll at the surface sediment. Together, this suggests that bacteria may have responded to nutrients released via grazing activity. In depth sequencing of the 16S rRNA genes indicated that the composition of the bacterial community shifted rapidly through-out the prolonged spring bloom period. This was primarily due to an increase in the relative sequence abundance of Flavobacteria.

Calanus helgolandicus is a key copepod of the NE Atlantic and fringing shelves, with a distribution that is expanding northwards with oceanic warming. The Plymouth L4 site has warmed over the past 25-years, and experiences large variations in the timing and availability of food for C. helgolandicus. Here we examine the degree to which these changes translate into variation in reproductive output and subsequently C. helgolandicus population size. Egg production rates (eggs female−1 day−1) were maximal in the spring to early-summer period of diatom blooms and high ciliate abundance, rather than during the equally large autumn blooms of autotrophic dinoflagellates. Egg hatch success was lower in spring however, with a greater proportion of naupliar deformities then also. Both the timing and the mean summer abundance of C. helgolandicus (CI–CVI) reflected those of spring total reproductive output. However this relationship was driven by inter-annual variability in female abundance and not that of egg production per female, which ranged only two-fold. Winter abundance of C. helgolandicus at L4 was much more variable than abundance in other seasons, and reflected conditions from the previous growing season. However, these low winter abundances had no clear carry-over signal to the following season’s population size. Overall, the C. helgolandicus population appears to be surprisingly resilient at this dynamic, inshore site, showing no long-term phenology shift and only a four-fold variation in mean abundance between years. This dampening effect may reflect a series of mortality sources, associated with the timing of stratification in the early part of the season, likely affecting egg sinking and loss, plus intense, density-dependent mortality of early stages in mid-summer likely through predation.

The aim of this paper is to investigate the role of phytoplankton nutritional status in the formation of the spring bloom regularly observed at the station L4 in the Western English Channel. Using a modelling approach, we tested the hypothesis that the increase in light from winter to spring induces a decrease in diatom nutritional status (i.e., an increase in the C:N and C:P ratios), thereby reducing their palatability and allowing them to bloom. To this end, a formulation describing the Stoichiometric Modulation of Predation (SMP) has been implemented in a simplified version of the European Regional Seas Ecosystem Model (ERSEM). The model was coupled with the General Ocean Turbulence Model (GOTM), implemented at the station L4 and run for 10 years (2000–2009). Simulated carbon to nutrient ratios in diatoms were analysed in relation to microzooplankton biomass, grazing and assimilation efficiency. The model reproduced in situ data evolutions and showed the importance of microzooplankton grazing in controlling the early onset of the bloom. Simulation results supported our hypothesis and provided a conceptual model explaining the formation of the diatom spring bloom in the investigated area. However, additional data describing the microzooplankton grazing impact and the variation of carbon to nutrient ratios inside phytoplanktonic cells are required to further validate the proposed mechanisms.

In a warming climate, differential shifts in the seasonal timing of predators and prey have been suggested to lead to trophic “mismatches” that decouple primary, secondary and tertiary production. We tested this hypothesis using a 25-year time-series of weekly sampling at the Plymouth L4 site, comparing 57 plankton taxa spanning 4 trophic levels. During warm years, there was a weak tendency for earlier timings of spring taxa and later timings of autumn taxa. While this is in line with many previous findings, numerous exceptions existed and only a few taxa (e.g. Gyrodinium spp., Pseudocalanus elongatus, and Acartia clausi) showed consistent, strong evidence for temperature-related timing shifts, revealed by all 4 of the timing indices that we used. Also, the calculated offsets in timing (i.e. “mismatches”) between predator and prey were no greater in extreme warm or cold years than during more average years. Further, the magnitude of these offsets had no effect on the “success” of the predator, in terms of their annual mean abundance or egg production rates. Instead numerous other factors override, including: inter-annual variability in food quantity, high food baseline levels, turnover rates and prolonged seasonal availability, allowing extended periods of production. Furthermore many taxa, notably meroplankton, increased well before the spring bloom. While theoretically a chronic mismatch, this likely reflects trade-offs for example in predation avoidance. Various gelatinous taxa (Phaeocystis, Noctiluca, ctenophores, appendicularians, medusae) may have reduced these predation constraints, with variable, explosive population outbursts likely responding to improved conditions. The match–mismatch hypothesis may apply for highly seasonal, pulsed systems or specialist feeders, but we suggest that the concept is being over-extended to other marine systems where multiple factors compensate.

In 2012, the Western English Channel experienced an unusually large and long-lived phytoplankton spring bloom. When compared with data from the past 20 years, average phytoplankton biomass at Station L4 (part of the Western Channel Observatory) was approximately 3× greater and lasted 50% longer than any previous year. Regular (mostly weekly) box core samples were collected from this site before, during and after the bloom to determine its impact on macrofaunal abundance, diversity, biomass, community structure and function. The spring bloom of 2012 was shown to support a large and rapid response in the majority of benthic taxa and functional groups. However, key differences in the precise nature of this response, as well as in its timing, was observed between different macrofauna feeding groups. Deposit feeders responded almost instantly at the start of the bloom, primarily thorough an increase in abundance. Suspension feeders and opportunistic/predatory/carnivorous taxa responded slightly more slowly and primarily with an increase in biomass. At the end of the bloom a rapid decline in macrobenthic abundance, diversity and biomass closely followed the decline in phytoplankton biomass. With suspension feeders showing evidence of this decline a few weeks before deposit feeders, it was concluded that this collapse in benthic communities was driven primarily by food availability and competition. However, it is possible that environmental hypoxia and the presence of toxic benthic cyanobacteria could also have contributed to this decline. This study shows evidence for strong benthic–pelagic coupling at L4; a shallow (50 m), coastal, fine-sand habitat. It also demonstrates that in such habitats, it is not just planktonic organisms that demonstrate clear community phenology. Different functional groups within the benthic assemblage will respond to the spring bloom in specific manner, with implications for key ecosystem functions and processes, such as secondary production and bioturbation. Only by taking integrated benthic and pelagic observations over such fine temporal scales (weekly) was the current study able to identify the intimate structure of the benthic response. Similar studies from other habitats and under different bloom conditions are urgently needed to fully appreciate the strength of benthic–pelagic coupling in shallow coastal environments.

© 2015 Published by Elsevier Ltd. Primary productivity and subsequent carbon cycling in the coastal zone have a significant impact on the global carbon budget. It is currently unclear how anthropogenic activity could alter these budgets but long term coastal time series of hydrological, biogeochemical and biological measurements represent a key means to better understand past drivers, and hence to predicting future seasonal and inter-annual variability in carbon fixation in coastal ecosystems. An 8-year time series of primary production from 2003 to 2010, estimated using a recently developed absorption-based algorithm, was used to determine the nature and extent of change in primary production at a coastal station (L4) in the Western English Channel (WEC). Analysis of the seasonal and inter-annual variability in production demonstrated that on average, nano- and pico-phytoplankton account for 48% of the total carbon fixation and micro-phytoplankton for 52%. A recent decline in the primary production of nano- and pico-phytoplankton from 2005 to 2010 was observed, corresponding with a decrease in winter nutrient concentrations and a decrease in the biomass of Phaeocystis sp. Micro-phytoplankton primary production (PPM) remained relatively constant over the time series and was enhanced in summer during periods of high precipitation. Increases in sea surface temperature, and decreases in wind speeds and salinity were associated with later spring maxima in PPM. Together these trends indicate that predicted increases in temperature and decrease in wind speeds in future would drive later spring production whilst predicted increases in precipitation would also continue these blooms throughout the summer at this site.

The combined effects of elevated pCO2 and temperature were investigated during an autumn phytoplankton bloom in the Western English Channel (WEC). A full factorial 36-day microcosm experiment was conducted under year 2100 predicted temperature (+4.5 °C) and pCO2 levels (800 μatm). The starting phytoplankton community biomass was 110.2 (±5.7 sd) mg carbon (C) m−3 and was dominated by dinoflagellates (~ 50 %) with smaller contributions from nanophytoplankton (~ 13 %), cryptophytes (~ 11 %)and diatoms (~ 9 %). Over the experimental period total biomass was significantly increased by elevated pCO2 (20-fold increase) and elevated temperature (15-fold increase). In contrast, the combined influence of these two factors had little effect on biomass relative to the ambient control. The phytoplankton community structure shifted from dinoflagellates to nanophytoplankton at the end of the experiment in all treatments. Under elevated pCO2 nanophytoplankton contributed 90% of community biomass and was dominated by Phaeocystis spp., while under elevated temperature nanophytoplankton contributed 85 % of the community biomass and was dominated by smaller nano-flagellates. Under ambient conditions larger nano-flagellates dominated while the smallest nanophytoplankton contribution was observed under combined elevated pCO2 and temperature (~ 40 %). Dinoflagellate biomass declined significantly under the individual influences of elevated pCO2, temperature and ambient conditions. Under the combined effects of elevated pCO2 and temperature, dinoflagellate biomass almost doubled from the starting biomass and there was a 30-fold increase in the harmful algal bloom (HAB) species, Prorocentrum cordatum. Chlorophyll a normalised maximum photosynthetic rates (PBm) increased > 6-fold under elevated pCO2 and > 3-fold under elevated temperature while no effect on PBm was observed when pCO2 and temperature were elevated simultaneously. The results suggest that future increases in temperature and pCO2 do not appear to influence coastal phytoplankton productivity during autumn in the WEC which would have a negative feedback on atmospheric CO2.

A 21-year time series of phytoplankton community structure was analysed in relation to Phaeocystis spp. to elucidate its contribution to the annual carbon budget at station L4 in the western English Channel (WEC). Between 1993-2014 Phaeocystis spp. contributed ∼4.6% of the annual phytoplankton carbon and during the March - May spring bloom, the mean Phaeocystis spp. biomass constituted 17% with a maximal contribution of 47% in 2001. Upper maximal weekly values above the time series mean ranged from 63 to 82% of the total phytoplankton carbon (∼42-137mg carbon (C)m

We used a numerical model to investigate if and to what extent cellular photoprotective capacity accounts for succession and vertical distribution of marine phytoplankton species/groups. A model describing xanthophyll photoprotective activity in phytoplankton has been implemented in the European Regional Sea Ecosystem Model and applied at the station L4 in the Western English Channel. Primary producers were subdivided into three phytoplankton functional types defined in terms of their capacity to acclimate to different light-specific environments: low light (LL-type), high light (HL-type) and variable light (VL-type) adapted species. The LL-type is assumed to have low cellular level of xanthophyll-cycling pigments (PX) relative to the modelled photosynthetically active pigments (chlorophyll and fucoxanthin (FUCO) = PSP). The HL-type has high PX content relative to PSP while VL-type presents an intermediate PX to PSP ratio. Furthermore, the VL-type is capable of reversibly converting FUCO to PX and synthesizing new PX under high-light stress. In order to reproduce phytoplankton community succession with each of the three groups being dominant in different periods of the year, we had also to assume reduced grazing pressure on HL-adapted species. Model simulations realistically reproduce the observed seasonal patterns of pigments and nutrients highlighting the reasonability of the underpinning assumptions. Our model suggests that pigment-mediated photophysiology plays a primary role in determining the evolution of marine phytoplankton communities in the winter-spring period corresponding to the shoaling of the mixed layer and the increase of light intensity. Grazing selectivity however contributes to the phytoplankton community composition in summer.