Long-term trends in the phytoplankton community along the French coast of the English Channel and southern Bight of the North Sea were studied and related to physico-chemical factors and large-climatic indices. Phytoplankton and hydrological data were acquired through three transects. Sampling took place between 1992 and 2011 as part of the French Phytoplankton Monitoring Network (REPHY) and the Regional Nutrients Monitoring Network (SRN). Trends in time-series were identified with dynamic linear models tailored to environmental monitoring data characteristics (e.g. irregular sampling frequency, missing data). Temporal and spatial patterns in the phytoplankton community were explained with a partial triadic analysis. Relationships between the phytoplankton community composition, environmental factors, and climatic indices were assessed using a redundancy analysis (RDA). The analyses revealed long-term changes in the community composition, characterized by a temporal structure that remained common to all transects. The abundance of some groups of taxa such as the one composed by Gymnodinium and Gyrodinium, as well as the group of Pseudo-nitzschia increased during the study period, whereas the abundance of other taxa as for example Guinardia and the group of Coscinodiscus and Stellarima globally decreased. More generally, the proportion of dinoflagellates relative to diatoms increased. Trends in environmental variables were also observed in most sites and related to decreases in nutrient concentrations and an increasing trend in salinity. The RDA indicated that the Atlantic Multidecadal Oscillation index and salinity were the main factors defining the temporal structure of the phytoplankton community. This suggests that variations observed in the phytoplankton community are linked to hydro-climatic changes in the coastal environment.

The decrease in water nutrients confirms oligotrophication in the period 1998-2016. ► The decrease in phytoplankton biomass was associated with a decrease in diatom abundance. ► The dominant taxa shifted from Skeletonema-Chaetoceros to Chaetoceros-Pseudo-nitzschia. ► The median proportion of dinoflagellates relative to diatoms increased significantly. ► The total dinoflagellate abundance did not change significantly.

Keywords: phytoplankton environment niche position tolerance kernel density outlying mean index a b s t r a c t Relating environmental factors to species occurrence is a fundamental step in a better understanding of the community structure, the spatial and temporal dynamics of populations and how species may respond to environmental changes. In this paper, we used phytoplankton occurrence data, from a French national phytoplankton monitoring network, and environmental variables with the aim of characterizing the realized ecological niches of phytoplankton groups. We selected 35 phytoplankton taxa representing the most important taxa in terms of occurrence frequency and abundance along the French coast of the eastern English Channel, the Southern Bight of the North Sea and the Atlantic Ocean. We show that environmental variables such as nutrient concentration, water temperature, irradiance and turbidity can be considered key factors controlling phytoplankton dynamics and influencing the community structure. By using a statistical framework based on an ordination technique the community structure was analyzed and interpreted in terms of niche overlap, marginality and tolerance. The most marginal taxon was Dactyliosolen and the most tolerant was Skeletonema. Non-marginal taxonomic units could be generalist and specialist, while marginal taxa were rather specialist. Specialist and marginal taxa globally showed lower values of overlap.

In France, REPHY (Observation and Surveillance Network for Phytoplankton and Hydrology in coastal waters) and REPHYTOX (Monitoring Network for Phycotoxins in marine organisms) have been contributing to long-term time series on ocean health for more than 30 years. The aim of this paper is to describe these networks and to highlight their key results. Over the last 20 years, phytoplankton flora analysis on French coasts from the Channel to Mediterranean has shown that the five "emblematic" taxa are Chaetoceros, Skeletonema, Cryptophyceae, Leptocylindrus and Pseudo-nitzschia. The latter, together with the taxa of interest Dinophysis + Phalacroma, Alexandrium, and Karenia, have been consistently recorded along the entire French coastline. However, when taking into account frequency of occurrence some taxa exhibit more distinct geographical distributions. In particular, the occurrence of Phaeocystis appeared to be strongly specific to the northern coasts of the Channel. French coasts have been regularly affected since the 1980s by the presence of toxins in bivalve molluscs, leading to bans on fishing and sale of shellfish during periods of varying duration. Three categories of toxins were involved. PST and AST were absent from the French coasts, respectively before 1988 and 2000. DST (Diarrheic Shellfish Toxins) have affected many areas along the whole coast every year since 1987. For PST (Paralytic Shellfish Toxins), only a few areas have been affected, sometimes sporadically, since 1988 in the Channel, 1993 in the Atlantic, and 1998 in the Mediterranean. Many areas have been impacted since 2000 by AST (Amnesic Shellfish Toxins) episodes, mainly affecting scallops in the Channel and on Atlantic coasts. The patterns of change of shellfish toxicity episodes showed no real trend in any province over the entire period 1987-2018. Highlights► REPHY and REPHYTOX have been contributing to ocean survey for more than 30 years. ► Five taxa are "emblematic" of phytoplankton flora from the Channel to Mediterranean. ► Four harmful taxa have been consistently recorded along the entire French coastline. ► Episodes of toxicity in shellfish mainly started in the 1980s and 90s. ► Patterns of change of toxicity episodes have not shown a real trend for 30 years.

Abstract. The evolution of eutrophication parameters (i.e., nutrients and phytoplankton biomass) during recent decades was examined in coastal waters of the Vilaine Bay (VB, France) in relation to changes in the Loire and Vilaine rivers. Dynamic linear models were used to study long-term trends and seasonality of dissolved inorganic nutrient and chlorophyll a concentrations (Chl a) in rivers and coastal waters. For the period 1997–2013, the reduction in dissolved riverine inorganic phosphorus (DIP) concentrations led to the decrease in their Chl a levels. However, while dissolved inorganic nitrogen (DIN) concentrations decreased only slightly in the Vilaine, they increased in the Loire, specifically in summer. Simultaneously, phytoplankton in the VB underwent profound changes with increase in biomass and change in the timing of the annual peak from spring to summer. The increase in phytoplankton biomass in the VB, manifested particularly by increased summer diatom abundances, was due to enhanced summer DIN loads from the Loire, sustained by internal regeneration of DIP and dissolved silicate (DSi) from sediments. The long-term trajectories of this case study evidence that significant reduction of P inputs without simultaneous N abatement was not yet sufficient to control eutrophication all along the Loire–Vilaine–VB continuum. Upstream rivers reveal indices of recoveries following the significant diminution of P, while eutrophication continues to increase downstream, especially when N is the limiting factor. More N input reduction, paying particular attention to diffuse N sources, is required to control eutrophication in receiving VB coastal waters. Internal benthic DIP and DSi recycling appears to have contributed to the worsening of summer VB water quality, augmenting the effects of anthropogenic DIN inputs. For this coastal ecosystem, nutrient management strategies should consider the role played by internal nutrient loads to tackle eutrophication processes.