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Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (∼105 cells ml-1) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (∼105 cells ml-1) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (∼105 cells ml-1) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (similar to 10(5) cells ml(-1)) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (∼105 cells ml-1) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3-V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 x 10(4) cells ml(-1). This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3-V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 x 10(4) cells ml(-1). This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3-V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 x 10(4) cells ml(-1). This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3-V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 x 10(4) cells ml(-1). This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3-V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 x 10(4) cells ml(-1). This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3–V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 × 104 cells ml–1. This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord. Copyright © 2022 Laber, Pontiller, Bunse, Osbeck, Pérez-Martínez, Di Leo, Lundin, Legrand, Pinhassi and Farnelid., Funding: This study was supported by grants from the Swedish Research Council FORMAS (project no. 2017-00468) and Stiftelsen Anna-Greta and Holger Crafoords Fond (project no. CR2019-0012) to HF. BP, CMGO, and CB acknowledge support from the KVA fund for internationalization and scientific renewal at the Sven Lovén Centre for Marine Sciences. Support was also given through the marine strategic research program EcoChange to JP and CL. CB acknowledges funding by HIFMB, a collaboration between the Alfred-Wegener-Institute, Helmholtz-Center for Polar and Marine Research, and the Carl-von-Ossietzky University Oldenburg, initially funded by the Ministry for Science and Culture of Lower Saxony (MWK) and the Volkswagen Foundation through the “Niedersächsisches Vorab” grant program (grant number ZN3285).

In parallel to phytoplankton community dynamics, transparent exopolymeric particles (TEP) and exopolymeric substances (EPS) were investigated along the salinity gradient of a temperate estuary (Seine estuary, Normandy, France) over the course of a year. The phytoplankton community was mainly dominated by marine diatom species (especially Skeletonema sp., Nitzschia sp., and Paralia sulcata) associated with a spring bloom of pico-eukaryotes and the development of Cryptophyceae in summer. The decreases in species richness and salinity were correlated along the estuary and a significant exponential relationship between species richness and primary production was identified. Concentrations of TEP and EPS (soluble and bound carbohydrates) are highly dynamic in this estuary and can reach respectively 69 mgC L−1, and 33 mgC L−1. TEP distribution was mainly related to physical factors (hydrodynamics, maximum turbidity zone formation and sediment resuspension) probably produced by stressed or dying phytoplankton, while EPS appeared to be excreted during the phytoplankton spring bloom. Soluble and bound EPS appear to be related to Skeletonema sp. and Cryptophyceae occurrences. This paper presents the dynamic pattern of these carbon pools, which play an important role in the trophic network and influence the flocculation processes involved in the fate of both organic and inorganic matter.

In parallel to phytoplankton community dynamics, transparent exopolymeric particles (TEP) and exopolymeric substances (EPS) were investigated along the salinity gradient of a temperate estuary (Seine estuary, Normandy, France) over the course of a year. The phytoplankton community was mainly dominated by marine diatom species (especially Skeletonema sp., Nitzschia sp., and Paralia sulcata) associated with a spring bloom of pico-eukaryotes and the development of Cryptophyceae in summer. The decreases in species richness and salinity were correlated along the estuary and a significant exponential relationship between species richness and primary production was identified. Concentrations of TEP and EPS (soluble and bound carbohydrates) are highly dynamic in this estuary and can reach respectively 69 mgC L−1, and 33 mgC L−1. TEP distribution was mainly related to physical factors (hydrodynamics, maximum turbidity zone formation and sediment resuspension) probably produced by stressed or dying phytoplankton, while EPS appeared to be excreted during the phytoplankton spring bloom. Soluble and bound EPS appear to be related to Skeletonema sp. and Cryptophyceae occurrences. This paper presents the dynamic pattern of these carbon pools, which play an important role in the trophic network and influence the flocculation processes involved in the fate of both organic and inorganic matter.

In parallel to phytoplankton community dynamics, transparent exopolymeric particles (TEP) and exopolymeric substances (EPS) were investigated along the salinity gradient of a temperate estuary (Seine estuary, Normandy, France) over the course of a year. The phytoplankton community was mainly dominated by marine diatom species (especially Skeletonema sp., Nitzschia sp., and Paralia sulcata) associated with a spring bloom of pico-eukaryotes and the development of Cryptophyceae in summer. The decreases in species richness and salinity were correlated along the estuary and a significant exponential relationship between species richness and primary production was identified. Concentrations of TEP and EPS (soluble and bound carbohydrates) are highly dynamic in this estuary and can reach respectively 69 mgC L−1, and 33 mgC L−1. TEP distribution was mainly related to physical factors (hydrodynamics, maximum turbidity zone formation and sediment resuspension) probably produced by stressed or dying phytoplankton, while EPS appeared to be excreted during the phytoplankton spring bloom. Soluble and bound EPS appear to be related to Skeletonema sp. and Cryptophyceae occurrences. This paper presents the dynamic pattern of these carbon pools, which play an important role in the trophic network and influence the flocculation processes involved in the fate of both organic and inorganic matter.

This deliverable is composed of two reports which propose indice of phytoplankton composition, based respectively on the results of pigment and genetic diversity methods, and the results of microscopy method. Proposals based on the results of pigment and genetic diversity methods. The present work is a continuation of several studies carried out in the past (Siano and Delmas, 2013, 2015) on investigating an indicator of phytoplankton diversity using two methods complementary to optical microscopy: the genetic diversity analyzed with metabarcoding (exhaustive analysis of the diversity and relative abundance of OTUs, i.e. groups of sequences identified by their genetic resemblance) and the pigmentary diversity (analysis of the diversity and abundance of pigments produced by photosynthetic cells). As part of the present study we aim to (i) to deepen our knowledge on the spatio‐temporal structure of phytoplankton assemblages in relation with environmental forcings for three size classes (0.2‐3 μm, 3‐20μm, >20μm, here respectively defined by convention as picophytoplankton, nanophytoplankton and microphytoplankton), and (ii) to verify the hypothesis that certain groups of the phytoplankton could be representative and descriptor of their corresponding size class. This work relies on data collected during four sampling campaigns: three of them at spatial scale (Pelgas 2012, 2013, and Phytec campaigns) and one at temporal scale (Dynapse). During Pelgas 2012 and 2013 campaigns, the sampling was done in the Gironde plume during May, on three coast‐tooffshore transects. The Phytec campaign was carried out in the Iroise sea (Bay of Brest) and the south area of Britain, including the Loire plume, bays of Quiberon, Vilaine, and Concarneau. Finally, the Dynapse campaign was carried out in the bay of Concarneau, between March and July 2012. The statistical analyses performed on metarbarcoding data show a clear separation of the three size classes studied and suggest that picophytop, Ce livrable est composé de deux rapports faisant des propositions pour un indice de composition du phytoplancton, basés respectivement sur les résultats des méthodes de diversité pigmentaire et génétique, et sur les résultats de la méthode microscopie. Propositions basées sur les résultats des méthodes de diversité pigmentaire et génétique (19 pages, pages 13 à 31 du document pdf). Ce travail s’inscrit dans la continuité des études effectuées précédemment (Siano et Delmas, 2013, 2015) sur la recherche d’un indicateur de diversité du phytoplancton en utilisant deux méthodes complémentaires à la microscopie optique : la diversité génétique analysée par approche de metabarcoding (analyse exhaustive de la diversité et abondance relative des OTUs, i.e. groupes des séquences identifiés pour leur ressemblance génétique) et la diversité pigmentaire (analyse de la diversité et de l’abondance des pigments produits par les cellules photosynthétiques). Dans le cadre de cette étude nous avons cherché à : (i) approfondir nos connaissances sur la structure spatiotemporelle des assemblages phytoplanctoniques en rapport aux forçages environnementaux pour trois classes de taille (0,2‐3 μm, 3‐20μm, >20μm, ici définies par convention respectivement picophytoplancton, nanophytoplancton et microphytoplancton) et (ii) vérifier l’hypothèse que certains groupes du phytoplancton pourraient être représentatifs et descripteurs de leur classe de taille correspondante. Cette étude s’est basée sur les données recueillies lors de quatre campagnes d’échantillonnage : trois d’entre elles à l’échelle spatiale (campagnes Pelgas 2012, 2013 et campagne Phytec) et une à l’échelle temporelle (Dynapse). Lors des campagnes Pelgas 2012 et 2013, l’échantillonnage a été réalisé dans le panache de la Gironde au mois de mai, sur trois radiales côte‐large. La campagne Phytec a été effectuée en juin dans la mer d’Iroise (rade de Brest) et la zone Bretagne sud, incluant le panache de la Loire, les Baies de Quiberon

Marine picophytoplankton are responsible for a significant proportion of primary production in the ocean, and they are most abundant in warm and oligotrophic oceans. We compiled 2 datasets to evaluate 2 hypotheses related to the direct effects of temperature on picophytoplankton growth: (1) the optimal growth temperatures of picophytoplankton are higher than those of other larger phytoplankton; and (2) the activation energies of picophytoplankton growth are higher than those of larger phytoplankton. We found that based on the laboratory data, the optimal temperatures for picophytoplankton growth were not significantly different from those of other phytoplankton after controlling the effect of environmental mean temperature; however, the activation energies of picophytoplankton were marginally significantly higher than those of larger phytoplankton. From the field data, the growth rates of Synechococcus and picoeukaryotes in creased with increasing temperature and nitrate and chl a concentrations, whereas the growth rates of Prochlorococcus were not dependent on temperature and decreased with light intensity and nitrate concentrations. When keeping other factors constant, the activation energies of growth rates of Synechococcus and picoeukaryotes were 0.53 +/- 0.07 (mean +/- SE) and 0.62 +/- 0.11 eV higher, respectively, than the 0.36 eV estimated for bulk phytoplankton. Our results suggest that Synechococcus and picoeukaryotes might benefit from warming in mesotrophic waters, and the growth of Prochlorococcus might be retarded by increasing temperature and light levels but might benefit from the increasing oligotrophication in oligotrophic surface oceans.

Studies on phytoplankton community diversity cannot be limited microphytoplankton (cells>20 ìm), it is indeed necessary to take into account also the nano (3-20ìm) picophytoplankton (<3 ìm) components. In the frame of this work aimed at studying temporal phytoplankton community diversity two complementary methodologies of phytoplankton sample analysis, pigment and genetic (metarbarcoding) diversity, have been used as alternative to routine morphological analysis carried out at light microscopy. The REPHY station (047-P-016 Concarneau Large) off-shore the Concarneau Bay has been sampled during the annual highest phytoplankton production (March-July 2012) with a frequent sample interval strategy (twice per week). Results obtained by both pigment and metabarcoding analyses showed a good resolution of all phytoplancton size classes (micro-, nano-, picophytoplankton) diversity, from a groups/class level (pigments) to a species/genetic clades taxonomic resolution (metabarcoding). Size class dynamics of phytoplankton communities is clearly controlled by the high nutrient concentrations measured during the study period, with pico-nano phytoplankton dominating in periods of nutrient depletion. The metabarcoding approach allowed the identification of protistan groups characteristic of all size classes and showed that diversity of each compartments evolve independently. Sequences of piconanophytoplankton together represent in most of the samples at more than the 50% of total identified OTUs (Operational Taxonomic Units), suggesting that pico- and nanophytoplakton communities are dominant compartments of the phytoplankton of the Concarneau Bay., Afin d'appréhender la diversité du phytoplancton dans toutes ses composantes, on ne peut plus se limiter au seul micro-phytoplancton, il est indispensable de considérer le nano et le pico-phytoplancton. Pour l’étude de la biodiversité des communautés phytoplanctoniques, deux types d’approches alternatives et complémentaires à l’approche morphologique classique obtenue par microscope optique ont été utilisés : diversité pigmentaire et diversité génétique par approche de metabarcoding. Le point REPHY (047-P-016 Concarneau Large) situé en baie de Concarneau a été échantillonné pendant la période la plus importante de développement du phytoplancton (mars- juillet 2012) avec une fréquence bihebdomadaire. Les résultats obtenus par les méthodes d’analyses pigmentaires et de metabarcoding montrent une bonne résolution de la biodiversité phytoplanctonique, pouvant représenter au niveau des groupes ou familles (diversité pigmentaires) ou au niveau des espèces ou des clades génétiques (metabarcoding) toutes les classe de taille du phytoplancton y compris celles du pico (<3μm) ou du nano (3-20 μm), qui normalement ne sont pas prise en compte dans les analyses morphologiques de microscopie optique. La structure de taille du phytoplancton est clairement contrôlée par la richesse en nutriments des eaux, les formes pico et nanophytoplanctoniques dominent dès que les sels nutritifs deviennent limitants. L’approche metabarcoding a permis d’identifier des groupes de protistes typiques du micro, nano et picoplancton, et la diversité des OTUs évolue dans le temps de façon différente pour les trois classes de taille. Les séquences du pico et nanophytoplancton ensemble représentent dans la plupart des cas plus du 50% du nombre total d’OTUs identifiées, pouvant être ainsi dominantes dans la communauté phytoplanctonique de la Baie de Concarneau

Marine picophytoplankton are responsible for a significant proportion of primary production in the ocean, and they are most abundant in warm and oligotrophic oceans. We compiled 2 datasets to evaluate 2 hypotheses related to the direct effects of temperature on picophytoplankton growth: (1) the optimal growth temperatures of picophytoplankton are higher than those of other larger phytoplankton; and (2) the activation energies of picophytoplankton growth are higher than those of larger phytoplankton. We found that based on the laboratory data, the optimal temperatures for picophytoplankton growth were not significantly different from those of other phytoplankton after controlling the effect of environmental mean temperature; however, the activation energies of picophytoplankton were marginally significantly higher than those of larger phytoplankton. From the field data, the growth rates of Synechococcus and picoeukaryotes in creased with increasing temperature and nitrate and chl a concentrations, whereas the growth rates of Prochlorococcus were not dependent on temperature and decreased with light intensity and nitrate concentrations. When keeping other factors constant, the activation energies of growth rates of Synechococcus and picoeukaryotes were 0.53 +/- 0.07 (mean +/- SE) and 0.62 +/- 0.11 eV higher, respectively, than the 0.36 eV estimated for bulk phytoplankton. Our results suggest that Synechococcus and picoeukaryotes might benefit from warming in mesotrophic waters, and the growth of Prochlorococcus might be retarded by increasing temperature and light levels but might benefit from the increasing oligotrophication in oligotrophic surface oceans.

Marine picophytoplankton are responsible for a significant proportion of primary production in the ocean, and they are most abundant in warm and oligotrophic oceans. We compiled 2 datasets to evaluate 2 hypotheses related to the direct effects of temperature on picophytoplankton growth: (1) the optimal growth temperatures of picophytoplankton are higher than those of other larger phytoplankton; and (2) the activation energies of picophytoplankton growth are higher than those of larger phytoplankton. We found that based on the laboratory data, the optimal temperatures for picophytoplankton growth were not significantly different from those of other phytoplankton after controlling the effect of environmental mean temperature; however, the activation energies of picophytoplankton were marginally significantly higher than those of larger phytoplankton. From the field data, the growth rates of Synechococcus and picoeukaryotes in creased with increasing temperature and nitrate and chl a concentrations, whereas the growth rates of Prochlorococcus were not dependent on temperature and decreased with light intensity and nitrate concentrations. When keeping other factors constant, the activation energies of growth rates of Synechococcus and picoeukaryotes were 0.53 +/- 0.07 (mean +/- SE) and 0.62 +/- 0.11 eV higher, respectively, than the 0.36 eV estimated for bulk phytoplankton. Our results suggest that Synechococcus and picoeukaryotes might benefit from warming in mesotrophic waters, and the growth of Prochlorococcus might be retarded by increasing temperature and light levels but might benefit from the increasing oligotrophication in oligotrophic surface oceans.

By using a global data set (NOMAD), improved algorithms were developed for deducing from ocean color remote sensing data the concentrations of phytoplankton pigments, total chlorophyll alpha (TChl_alpha), zeaxanthin (Zea) and fucoxanthin (Fuco), and the associated primary phytoplankton groups, namely, Prochlorococcus, Synechococcus, haptophytes and diatoms, in the oceans. A modified classification for the waters of the world's ocean based on the dominant phytoplankton group was then devised. By using a local data set from the northern South China Sea (NSCS), algorithms were also developed for assessing the cell abundances of picophytoplankton, Prochlorococcus, Synechococcus and pico-eukaryotes, in this region. These algorithms were then applied to the remotely sensed data from the MODerate-resolution Imaging Spectrometer at Aqua sensor (MODIS-Aqua) to study the abundances and the distributional patterns of picophytoplankton in the entire NSCS. The results agreed well with field observations, which were available only from selected locations in the study area. On a basin scale, the results were consistent with expected variations in phytoplankton pigments, phytoplankton community composition, and picophytoplankton biomass, in response to the major known environmental phenomena in the NSCS, such as the progressive increase in biological productivity towards the coast, the runoff from the Pearl River, higher biological productivity in the wintertime, upwelling around the Taiwan Bank, winter upwelling northwest of the Luzon Island, and the transformation and dissipation of internal waves off the Dongsha Atoll. This work represents the first effort in estimating the cell abundances of the principal picophytoplankton groups in the oceans from space. The general application of this approach to the global ocean needs to be further validated and calibrated with a more extensive data base from field observations in other parts of the oceans. (C) 2012 Elsevier Inc. All rights

By using a global data set (NOMAD), improved algorithms were developed for deducing from ocean color remote sensing data the concentrations of phytoplankton pigments, total chlorophyll alpha (TChl_alpha), zeaxanthin (Zea) and fucoxanthin (Fuco), and the associated primary phytoplankton groups, namely, Prochlorococcus, Synechococcus, haptophytes and diatoms, in the oceans. A modified classification for the waters of the world's ocean based on the dominant phytoplankton group was then devised. By using a local data set from the northern South China Sea (NSCS), algorithms were also developed for assessing the cell abundances of picophytoplankton, Prochlorococcus, Synechococcus and pico-eukaryotes, in this region. These algorithms were then applied to the remotely sensed data from the MODerate-resolution Imaging Spectrometer at Aqua sensor (MODIS-Aqua) to study the abundances and the distributional patterns of picophytoplankton in the entire NSCS. The results agreed well with field observations, which were available only from selected locations in the study area. On a basin scale, the results were consistent with expected variations in phytoplankton pigments, phytoplankton community composition, and picophytoplankton biomass, in response to the major known environmental phenomena in the NSCS, such as the progressive increase in biological productivity towards the coast, the runoff from the Pearl River, higher biological productivity in the wintertime, upwelling around the Taiwan Bank, winter upwelling northwest of the Luzon Island, and the transformation and dissipation of internal waves off the Dongsha Atoll. This work represents the first effort in estimating the cell abundances of the principal picophytoplankton groups in the oceans from space. The general application of this approach to the global ocean needs to be further validated and calibrated with a more extensive data base from field observations in other parts of the oceans. (C) 2012 Elsevier Inc. All rights

By using a global data set (NOMAD), improved algorithms were developed for deducing from ocean color remote sensing data the concentrations of phytoplankton pigments, total chlorophyll alpha (TChl_alpha), zeaxanthin (Zea) and fucoxanthin (Fuco), and the associated primary phytoplankton groups, namely, Prochlorococcus, Synechococcus, haptophytes and diatoms, in the oceans. A modified classification for the waters of the world's ocean based on the dominant phytoplankton group was then devised. By using a local data set from the northern South China Sea (NSCS), algorithms were also developed for assessing the cell abundances of picophytoplankton, Prochlorococcus, Synechococcus and pico-eukaryotes, in this region. These algorithms were then applied to the remotely sensed data from the MODerate-resolution Imaging Spectrometer at Aqua sensor (MODIS-Aqua) to study the abundances and the distributional patterns of picophytoplankton in the entire NSCS. The results agreed well with field observations, which were available only from selected locations in the study area. On a basin scale, the results were consistent with expected variations in phytoplankton pigments, phytoplankton community composition, and picophytoplankton biomass, in response to the major known environmental phenomena in the NSCS, such as the progressive increase in biological productivity towards the coast, the runoff from the Pearl River, higher biological productivity in the wintertime, upwelling around the Taiwan Bank, winter upwelling northwest of the Luzon Island, and the transformation and dissipation of internal waves off the Dongsha Atoll. This work represents the first effort in estimating the cell abundances of the principal picophytoplankton groups in the oceans from space. The general application of this approach to the global ocean needs to be further validated and calibrated with a more extensive data base from field observations in other parts of the oceans. (C) 2012 Elsevier Inc. All rights

There is a large body of evidence supporting a major role of heterotrophic bacteria in dimethylsulphoniopropionate (DMSP) utilisation as a source of reduced sulphur. However, a role for phototrophic microorganisms has been only recently described and little is known about their contribution to DMSP consumption and the potential modulating effects of sunlight. In an attempt to ascertain the relative quantitative roles of heterotrophic bacteria and picophytoplankton in the osmoheterotrophic uptake of DMSP-sulphur upon exposure to natural sunlight conditions, we incubated northwestern Mediterranean waters under various optical filters and used an array of bulk and single-cell activity methods to trace the fate of added 35S-DMSP. Flow cytometry cell sorting confirmed dark 35S uptake by Prochlorococcus, Synechococcus and heterotrophic bacteria, the latter being the most efficient in terms of uptake on a cell volume basis. Under exposure to full sunlight, however, the relative contribution of Synechococcus was significantly enhanced, mainly because of the inhibition of heterotrophic bacteria. Microautoradiography showed a strong increase in the proportion of Synechococcus cells actively taking up 35S-DMSP, which, after full sunlight exposure, made up to 15% of total active Bacteria. Parallel incubations with 3H-leucine generally showed no clear responses to light. Finally, size-fractionated assimilation experiments showed greater relative cyanobacterial assimilation during the day than at night compared with that of heterotrophic bacteria. Our results show for the first time a major influence of sunlight in regulating the competition among autotrophic and heterotrophic picoplankton for DMSP uptake at both the daily and seasonal time scales.

The abundance, size, and fluorescence of picophytoplankton cells were investigated during the summer (July-August of 2009) and winter (January of 2010) extending from near-shore coastal waters to oligotrophic open waters in northern South China Sea, under the influence of contrasting seasonal monsoons. We found that the median abundance of Prochlorococcus averaged over top 150 m decreased nearly 10 times in the winter compared to the summer in the whole survey area, while median abundance of Synechococcus and picoeukaryotes increased 2.6 and 2.4 folds, respectively. Vertical abundance profiles of picoeukaryotes usually formed a subsurface maximum during the summer with the depth of maximal abundances tracking the depth of nutricline, whereas their vertical distributions were more uniform during the winter. Size and cellular fluorescence of Prochlorococcus and Synechococcus usually increased with depth in the summer, while the size of picoeukaryotes was smallest at the depth of maximal abundances. Size, cellular fluorescence, and chlorophyll-to-carbon ratio of Prochlorococcus and Synechococcus in surface waters were generally higher in the winter than in the summer and onshore than offshore, probably resulting from different temperature, nutrient, and light environments as well as different ecotype compositions. Prochlorococcus cells were most abundant in warm and oligotrophic environments, while the abundance of Synechococcus and picoeukaryotes was the highest in waters with intermediate chlorophyll and nutrient concentrations. The distributional patterns of picophytoplankton groups are consistent with their specific physiology documented in previous studies and can be possibly predicted by environmental physical and chemical variables. (C) 2011 Elsevier Ltd. All rights reserved.

The abundance, size, and fluorescence of picophytoplankton cells were investigated during the summer (July-August of 2009) and winter (January of 2010) extending from near-shore coastal waters to oligotrophic open waters in northern South China Sea, under the influence of contrasting seasonal monsoons. We found that the median abundance of Prochlorococcus averaged over top 150 m decreased nearly 10 times in the winter compared to the summer in the whole survey area, while median abundance of Synechococcus and picoeukaryotes increased 2.6 and 2.4 folds, respectively. Vertical abundance profiles of picoeukaryotes usually formed a subsurface maximum during the summer with the depth of maximal abundances tracking the depth of nutricline, whereas their vertical distributions were more uniform during the winter. Size and cellular fluorescence of Prochlorococcus and Synechococcus usually increased with depth in the summer, while the size of picoeukaryotes was smallest at the depth of maximal abundances. Size, cellular fluorescence, and chlorophyll-to-carbon ratio of Prochlorococcus and Synechococcus in surface waters were generally higher in the winter than in the summer and onshore than offshore, probably resulting from different temperature, nutrient, and light environments as well as different ecotype compositions. Prochlorococcus cells were most abundant in warm and oligotrophic environments, while the abundance of Synechococcus and picoeukaryotes was the highest in waters with intermediate chlorophyll and nutrient concentrations. The distributional patterns of picophytoplankton groups are consistent with their specific physiology documented in previous studies and can be possibly predicted by environmental physical and chemical variables. (C) 2011 Elsevier Ltd. All rights reserved.

The abundance, size, and fluorescence of picophytoplankton cells were investigated during the summer (July-August of 2009) and winter (January of 2010) extending from near-shore coastal waters to oligotrophic open waters in northern South China Sea, under the influence of contrasting seasonal monsoons. We found that the median abundance of Prochlorococcus averaged over top 150 m decreased nearly 10 times in the winter compared to the summer in the whole survey area, while median abundance of Synechococcus and picoeukaryotes increased 2.6 and 2.4 folds, respectively. Vertical abundance profiles of picoeukaryotes usually formed a subsurface maximum during the summer with the depth of maximal abundances tracking the depth of nutricline, whereas their vertical distributions were more uniform during the winter. Size and cellular fluorescence of Prochlorococcus and Synechococcus usually increased with depth in the summer, while the size of picoeukaryotes was smallest at the depth of maximal abundances. Size, cellular fluorescence, and chlorophyll-to-carbon ratio of Prochlorococcus and Synechococcus in surface waters were generally higher in the winter than in the summer and onshore than offshore, probably resulting from different temperature, nutrient, and light environments as well as different ecotype compositions. Prochlorococcus cells were most abundant in warm and oligotrophic environments, while the abundance of Synechococcus and picoeukaryotes was the highest in waters with intermediate chlorophyll and nutrient concentrations. The distributional patterns of picophytoplankton groups are consistent with their specific physiology documented in previous studies and can be possibly predicted by environmental physical and chemical variables. (C) 2011 Elsevier Ltd. All rights reserved.

There is a large body of evidence supporting a major role of heterotrophic bacteria in dimethylsulphoniopropionate (DMSP) utilisation as a source of reduced sulphur. However, a role for phototrophic microorganisms has been only recently described and little is known about their contribution to DMSP consumption and the potential modulating effects of sunlight. In an attempt to ascertain the relative quantitative roles of heterotrophic bacteria and picophytoplankton in the osmoheterotrophic uptake of DMSP-sulphur upon exposure to natural sunlight conditions, we incubated northwestern Mediterranean waters under various optical filters and used an array of bulk and single-cell activity methods to trace the fate of added 35S-DMSP. Flow cytometry cell sorting confirmed dark 35S uptake by Prochlorococcus, Synechococcus and heterotrophic bacteria, the latter being the most efficient in terms of uptake on a cell volume basis. Under exposure to full sunlight, however, the relative contribution of Synechococcus was significantly enhanced, mainly because of the inhibition of heterotrophic bacteria. Microautoradiography showed a strong increase in the proportion of Synechococcus cells actively taking up 35S-DMSP, which, after full sunlight exposure, made up to 15% of total active Bacteria. Parallel incubations with 3H-leucine generally showed no clear responses to light. Finally, size-fractionated assimilation experiments showed greater relative cyanobacterial assimilation during the day than at night compared with that of heterotrophic bacteria. Our results show for the first time a major influence of sunlight in regulating the competition among autotrophic and heterotrophic picoplankton for DMSP uptake at both the daily and seasonal time scales.

Spatial and temporal scales of variability in phyto- and bacterioplankton abundance were investigated in a deep atoll lagoon (Ahe, French Polynesia), Ahe Lagoon showed a classical phytoplankton composition, but its picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes) concentrations are higher than the mean for Polynesian atoll lagoons. Phyto- and bacterioplankton descriptors were highly variable, with mean CVs of >20% in most cases: day night, day-to-day, vertical, lagoonal and seasonal. Phytoplankton biomass, which was estimated by in vivo fluorescence, was higher in the deep layer than in the upper layer. This pattern was detected on a day night scale, showing transitory vertical gradients, and through long-term events that were recorded during day-to-day measurements, especially during low wind conditions. The main factors potentially driving this vertical structure of the phytoplankton appeared to be photoinhibition in upper layers and a bottom effect providing more favourable nutrient conditions in deeper layers. Day-to-day fluctuation was the major source of temporal variation and appeared to be highly driven by vertical patterns and mixing events showing phytoplankton supply from deep layers. At the lagoon scale, the picophytoplankton exhibited a patchy structure that is closely linked to the geomorphology of the rim, lagoon depth and proximity of the pass. Stations close to the pass were characterized by consistently lower Synechococcus and picoeukaryote concentrations. Lastly, warm periods showed higher Synechococcus, nanophytoplankton and bacterial concentrations. Prochlorococcus and picoeukaryotes showed no seasonal trend, but had an annual variability that could be related to variations in exchanges between the lagoon and the ocean.

Spatial and temporal scales of variability in phyto- and bacterioplankton abundance were investigated in a deep atoll lagoon (Ahe, French Polynesia), Ahe Lagoon showed a classical phytoplankton composition, but its picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes) concentrations are higher than the mean for Polynesian atoll lagoons. Phyto- and bacterioplankton descriptors were highly variable, with mean CVs of >20% in most cases: day night, day-to-day, vertical, lagoonal and seasonal. Phytoplankton biomass, which was estimated by in vivo fluorescence, was higher in the deep layer than in the upper layer. This pattern was detected on a day night scale, showing transitory vertical gradients, and through long-term events that were recorded during day-to-day measurements, especially during low wind conditions. The main factors potentially driving this vertical structure of the phytoplankton appeared to be photoinhibition in upper layers and a bottom effect providing more favourable nutrient conditions in deeper layers. Day-to-day fluctuation was the major source of temporal variation and appeared to be highly driven by vertical patterns and mixing events showing phytoplankton supply from deep layers. At the lagoon scale, the picophytoplankton exhibited a patchy structure that is closely linked to the geomorphology of the rim, lagoon depth and proximity of the pass. Stations close to the pass were characterized by consistently lower Synechococcus and picoeukaryote concentrations. Lastly, warm periods showed higher Synechococcus, nanophytoplankton and bacterial concentrations. Prochlorococcus and picoeukaryotes showed no seasonal trend, but had an annual variability that could be related to variations in exchanges between the lagoon and the ocean.

Spatial and temporal scales of variability in phyto- and bacterioplankton abundance were investigated in a deep atoll lagoon (Ahe, French Polynesia), Ahe Lagoon showed a classical phytoplankton composition, but its picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes) concentrations are higher than the mean for Polynesian atoll lagoons. Phyto- and bacterioplankton descriptors were highly variable, with mean CVs of >20% in most cases: day night, day-to-day, vertical, lagoonal and seasonal. Phytoplankton biomass, which was estimated by in vivo fluorescence, was higher in the deep layer than in the upper layer. This pattern was detected on a day night scale, showing transitory vertical gradients, and through long-term events that were recorded during day-to-day measurements, especially during low wind conditions. The main factors potentially driving this vertical structure of the phytoplankton appeared to be photoinhibition in upper layers and a bottom effect providing more favourable nutrient conditions in deeper layers. Day-to-day fluctuation was the major source of temporal variation and appeared to be highly driven by vertical patterns and mixing events showing phytoplankton supply from deep layers. At the lagoon scale, the picophytoplankton exhibited a patchy structure that is closely linked to the geomorphology of the rim, lagoon depth and proximity of the pass. Stations close to the pass were characterized by consistently lower Synechococcus and picoeukaryote concentrations. Lastly, warm periods showed higher Synechococcus, nanophytoplankton and bacterial concentrations. Prochlorococcus and picoeukaryotes showed no seasonal trend, but had an annual variability that could be related to variations in exchanges between the lagoon and the ocean.

The major source of reduced sulfur in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and released through food web interactions. Relevant fluxes and concentrations of DMS, its phytoplankton-produced precursor, dimethylsulfoniopropionate (DMSP) and related parameters were measured during an intensive Lagrangian field study in two mesoscale eddies in the Sargasso Sea during July–August 2004, a period characterized by high mixed-layer DMS and low chlorophyll—the so-called ‘DMS summer paradox’. We used a 1-D vertically variable DMS production model forced with output from a 1-D vertical mixing model to evaluate the extent to which the simulated vertical structure in DMS and DMSP was consistent with changes expected from field-determined rate measurements of individual processes, such as photolysis, microbial DMS and dissolved DMSP turnover, and air–sea gas exchange. Model numerical experiments and related parametric sensitivity analyses suggested that the vertical structure of the DMS profile in the upper 60 m was determined mainly by the interplay of the two depth variable processes—vertical mixing and photolysis—and less by biological consumption of DMS. A key finding from the model calibration was the need to increase the DMS(P) algal exudation rate constant, which includes the effects of cell rupture due to grazing and cell lysis, to significantly higher values than previously used in other regions. This was consistent with the small algal cell size and therefore high surface area-to-volume ratio of the dominant DMSP-producing group—the picoeukaryotes.

– The Rhône River (France) is an important river that flows into relatively nutrient-poor Mediterranean waters resulting in the formation of chemical and physical gradients. The distribution of picophytoplankton and bacterioplankton was studied along the salinity gradient in the Rhône River plume in November 1998, March 1999 and April 1999. Abundances of three groups of picophytoplankton (Prochlorococcus [PRO], Synechoccocus [SYN], small eukaryotic cells [PEUK]) and two clusters of bacteria differing by their nucleic acid content were determined by flow cytometry. PRO were abundant in November in the river plume (7-23 × 103 cells.ml-1) but were not observed in March and April. Abundances of SYN (3-20 × 103 cells.ml-1) and PEUK (0.5-4 × 103 cells.ml-1) were similar in November and March. In April, the concentrations of chlorophyll a (Chl a) peaked (> 15 μg.L-1) at intermediate salinities (30 to 35). At this time, the distribution of PEUK followed the same pattern as Chl a concentrations, while SYN abundances were negatively correlated to Chl a concentrations. Bacterial abundance and production were both well correlated to Chl a concentration in April, whereas only bacterial production was correlated with Chl a concentrations in March. In April, the abundance of bacteria with a high nucleic acid content (HNA) was more closely correlated to bacterial production and Chl a concentrations than those of bacteria with a low nucleic acid content (LNA). This suggests that HNA bacteria have a higher contribution to bacterial production than LNA bacteria during this productive period. - RÉSUMÉ. – Le Rhône est un fleuve important qui se déverse dans les eaux pauvres en nutriments de la Méditerranée induisant la formation de gradients chimiques et physiques. La distribution du picophytoplancton et du bactérioplancton dans le panache du Rhône a été étudiée en novembre 1998, et en mars et avril 1999. Les abondances de trois groupes de picophytoplancton (Prochlorococcus [P, info:eu-repo/semantics/published

We present results obtained with SWAMCO-4, a complex model of the marine planktonic system calculating C, N, P, Si, Fe cycling within the upper ocean, the export production and the exchange of CO2 between the ocean and atmosphere. The model, constrained by physical, chemical and biological (grazing, lysis) controls, explicitly details the dynamics of four relevant phytoplankton functional groups with respect to C, N, P, Si, Fe cycling and climate change. Those are diatoms, pico/nano phytoplankton, coccolithophorids, and Phaeocystis spp. whose growth regulation by light, temperature and nutrients has been obtained based on a comprehensive analysis of literature reviews on these taxonomic groups. The performance of SWAMCO-4 is first evaluated in a 1D physical frame throughout its cross application in provinces with contrasted key species dominance, export production, CO2 air-sea fluxes and where biogeochemical time-series data are available for model initialisation and comparison of results. These are: (i) the ice-free Southern Ocean Time Series station KERFIX (50°40S, 68°E) for the period 1993-1994 (diatom-dominated); (ii) the sea-ice associated Ross Sea domain (Station S; 76°S, 180°W) of the Antarctic Environment and Southern Ocean Process Study AESOPS in 1996-1997 (Phaeocystis-dominated); and (iii) the North Atlantic Bloom Experiment NABE (60°N, 20°W) in 1991 (coccolithophorids). We then explore and compare the ocean response to increased atmospheric CO2 by running SWAMCO-4 at the different locations over the last decade. Results show that at all tested latitudes the prescribed increase of atmospheric CO2 enhances the carbon uptake by the ocean. However, the amplitude of the predicted atmospheric CO2 sinks displays large regional and interannual variations due to the actual meteorological forcing that drives the local hydrodynamics. This is particularly true in the marginal ice zone of the Ross Sea (AESOPS) where the magnitude of the predicted annual CO2 sink is po, info:eu-repo/semantics/published

We present results obtained with SWAMCO-4, a complex model of the marine planktonic system calculating C, N, P, Si, Fe cycling within the upper ocean, the export production and the exchange of CO2 between the ocean and atmosphere. The model, constrained by physical, chemical and biological (grazing, lysis) controls, explicitly details the dynamics of four relevant phytoplankton functional groups with respect to C, N, P, Si, Fe cycling and climate change. Those are diatoms, pico/nano phytoplankton, coccolithophorids, and Phaeocystis spp. whose growth regulation by light, temperature and nutrients has been obtained based on a comprehensive analysis of literature reviews on these taxonomic groups. The performance of SWAMCO-4 is first evaluated in a 1D physical frame throughout its cross application in provinces with contrasted key species dominance, export production, CO2 air-sea fluxes and where biogeochemical time-series data are available for model initialisation and comparison of results. These are: (i) the ice-free Southern Ocean Time Series station KERFIX (50°40S, 68°E) for the period 1993-1994 (diatom-dominated); (ii) the sea-ice associated Ross Sea domain (Station S; 76°S, 180°W) of the Antarctic Environment and Southern Ocean Process Study AESOPS in 1996-1997 (Phaeocystis-dominated); and (iii) the North Atlantic Bloom Experiment NABE (60°N, 20°W) in 1991 (coccolithophorids). We then explore and compare the ocean response to increased atmospheric CO2 by running SWAMCO-4 at the different locations over the last decade. Results show that at all tested latitudes the prescribed increase of atmospheric CO2 enhances the carbon uptake by the ocean. However, the amplitude of the predicted atmospheric CO2 sinks displays large regional and interannual variations due to the actual meteorological forcing that drives the local hydrodynamics. This is particularly true in the marginal ice zone of the Ross Sea (AESOPS) where the magnitude of the predicted annual CO2 sink is po, info:eu-repo/semantics/published

1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems. 2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N. 3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton. 4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes. 5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.

Picophytoplankton were inferior competitors for inorganic phosphorus compared to heterotrophic bacteria. This may be due to the source of energy available for the heterotrophs, while cell-size was of minor importance. However, picophytoplankton were superior to large phytoplankton in the competition for nutrients at low concentrations. Biomass of picophytoplankton was low in brownwater lakes and high in clearwater lakes, compared to the biomass of heterotrophic bacteria. The results suggest that picophytoplankton are inferior to heterotrophic bacteria in the competition for inorganic nutrients in brownwater lakes, where the production of heterotrophic bacteria is subsidized by humic dissolved organic carbon (DOC) Relative to large phytoplankton, picophytoplankton were most important in lakes with intermediate water colour, despite the fact that the lowest nutrient concentrations were found in the clearwater lakes. Large phytoplankton in the clearwater lakes may be able to overcome nutrient competition with picophytoplankton by vertical migration. In conclusion, changes in nutrient content, light availability and concentrations of DOC affect the interactions of heterotrophic bacteria, picophytoplankton and large phytoplankton and are therefore important factors for the structure of the food web in the pelagic zones of lakes. Picophytoplankton (planktonic algae and cyanobacteria, < 2 µm) constitute an important component of pelagic food webs. They are linked to larger phytoplankton and heterotrophic bacteria through complex interactions including competition, commensalism and predation. In this thesis, field and laboratory studies on the competitive ability of picophytoplankton are reported.

1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers. 2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket. 3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton.

Picophytoplankton were inferior competitors for inorganic phosphorus compared to heterotrophic bacteria. This may be due to the source of energy available for the heterotrophs, while cell-size was of minor importance. However, picophytoplankton were superior to large phytoplankton in the competition for nutrients at low concentrations. Biomass of picophytoplankton was low in brownwater lakes and high in clearwater lakes, compared to the biomass of heterotrophic bacteria. The results suggest that picophytoplankton are inferior to heterotrophic bacteria in the competition for inorganic nutrients in brownwater lakes, where the production of heterotrophic bacteria is subsidized by humic dissolved organic carbon (DOC) Relative to large phytoplankton, picophytoplankton were most important in lakes with intermediate water colour, despite the fact that the lowest nutrient concentrations were found in the clearwater lakes. Large phytoplankton in the clearwater lakes may be able to overcome nutrient competition with picophytoplankton by vertical migration. In conclusion, changes in nutrient content, light availability and concentrations of DOC affect the interactions of heterotrophic bacteria, picophytoplankton and large phytoplankton and are therefore important factors for the structure of the food web in the pelagic zones of lakes. Picophytoplankton (planktonic algae and cyanobacteria, < 2 µm) constitute an important component of pelagic food webs. They are linked to larger phytoplankton and heterotrophic bacteria through complex interactions including competition, commensalism and predation. In this thesis, field and laboratory studies on the competitive ability of picophytoplankton are reported.

Picophytoplankton were inferior competitors for inorganic phosphorus compared to heterotrophic bacteria. This may be due to the source of energy available for the heterotrophs, while cell-size was of minor importance. However, picophytoplankton were superior to large phytoplankton in the competition for nutrients at low concentrations. Biomass of picophytoplankton was low in brownwater lakes and high in clearwater lakes, compared to the biomass of heterotrophic bacteria. The results suggest that picophytoplankton are inferior to heterotrophic bacteria in the competition for inorganic nutrients in brownwater lakes, where the production of heterotrophic bacteria is subsidized by humic dissolved organic carbon (DOC) Relative to large phytoplankton, picophytoplankton were most important in lakes with intermediate water colour, despite the fact that the lowest nutrient concentrations were found in the clearwater lakes. Large phytoplankton in the clearwater lakes may be able to overcome nutrient competition with picophytoplankton by vertical migration. In conclusion, changes in nutrient content, light availability and concentrations of DOC affect the interactions of heterotrophic bacteria, picophytoplankton and large phytoplankton and are therefore important factors for the structure of the food web in the pelagic zones of lakes. Picophytoplankton (planktonic algae and cyanobacteria, < 2 µm) constitute an important component of pelagic food webs. They are linked to larger phytoplankton and heterotrophic bacteria through complex interactions including competition, commensalism and predation. In this thesis, field and laboratory studies on the competitive ability of picophytoplankton are reported.

Picophytoplankton were inferior competitors for inorganic phosphorus compared to heterotrophic bacteria. This may be due to the source of energy available for the heterotrophs, while cell-size was of minor importance. However, picophytoplankton were superior to large phytoplankton in the competition for nutrients at low concentrations. Biomass of picophytoplankton was low in brownwater lakes and high in clearwater lakes, compared to the biomass of heterotrophic bacteria. The results suggest that picophytoplankton are inferior to heterotrophic bacteria in the competition for inorganic nutrients in brownwater lakes, where the production of heterotrophic bacteria is subsidized by humic dissolved organic carbon (DOC) Relative to large phytoplankton, picophytoplankton were most important in lakes with intermediate water colour, despite the fact that the lowest nutrient concentrations were found in the clearwater lakes. Large phytoplankton in the clearwater lakes may be able to overcome nutrient competition with picophytoplankton by vertical migration. In conclusion, changes in nutrient content, light availability and concentrations of DOC affect the interactions of heterotrophic bacteria, picophytoplankton and large phytoplankton and are therefore important factors for the structure of the food web in the pelagic zones of lakes. Picophytoplankton (planktonic algae and cyanobacteria, < 2 µm) constitute an important component of pelagic food webs. They are linked to larger phytoplankton and heterotrophic bacteria through complex interactions including competition, commensalism and predation. In this thesis, field and laboratory studies on the competitive ability of picophytoplankton are reported.

The Mediterranean Thau Lagoon is an important oyster farming area in Europe. Oyster growth rates are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytoplankton, nano-microphytoplankton, dinoflagellates and loricate ciliates such as tintinnids are abundant. However, heterotrophic flagellates and aloricate ciliates have not been investigated. The aim of this study was to assess picophytoplankton, protist and zooplankton abundances in the Lagoon and to investigate the particular structure of the microbial food web, which may explain such paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic picoplankter, with an abundance maximum in summer. On 17 August 1998, following a rainfall event, pico- and nanophytoplankton abundances were not as high as expected and we observed a high abundance of large diatoms. At this time, the available carbon resources were produced by microphytoplankton (84.5%), and picoplank-tonic cells represented only 1.27 % in terms of carbon. Heterotrophic cells were low in abundance and constituted <14% of carbon resources. In order to evaluate the importance of the 'protozoan trophic link' for energy transfer from the microbial food web to large benthic suspension feeders, the oyster Crassostrea gigas was offered a planktonic community as potential prey. In the grazing experiment, all >5 mum flagellates, microphytoplankton, dinoflagellates, ciliates and large zooplankton were retained by the oyster gills. Only flagellates <5 pm and O. tauri were not very well retained (45 and 2% respectively). The high clearance rates of C. gigas found in this experiment can be explained by a low concentration of suspended particulate matter (0.65 mg l(-1)). The oysters adapted their retention mechanism when they Lived in oligotrophic waters. These results indicate that, under the given experimental conditions, pic

The Mediterranean Thau Lagoon is an important oyster farming area in Europe. Oyster growth rates are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytoplankton, nano-microphytoplankton, dinoflagellates and loricate ciliates such as tintinnids are abundant. However, heterotrophic flagellates and aloricate ciliates have not been investigated. The aim of this study was to assess picophytoplankton, protist and zooplankton abundances in the Lagoon and to investigate the particular structure of the microbial food web, which may explain such paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic picoplankter, with an abundance maximum in summer. On 17 August 1998, following a rainfall event, pico- and nanophytoplankton abundances were not as high as expected and we observed a high abundance of large diatoms. At this time, the available carbon resources were produced by microphytoplankton (84.5%), and picoplank-tonic cells represented only 1.27 % in terms of carbon. Heterotrophic cells were low in abundance and constituted <14% of carbon resources. In order to evaluate the importance of the 'protozoan trophic link' for energy transfer from the microbial food web to large benthic suspension feeders, the oyster Crassostrea gigas was offered a planktonic community as potential prey. In the grazing experiment, all >5 mum flagellates, microphytoplankton, dinoflagellates, ciliates and large zooplankton were retained by the oyster gills. Only flagellates <5 pm and O. tauri were not very well retained (45 and 2% respectively). The high clearance rates of C. gigas found in this experiment can be explained by a low concentration of suspended particulate matter (0.65 mg l(-1)). The oysters adapted their retention mechanism when they Lived in oligotrophic waters. These results indicate that, under the given experimental conditions, pic