Identifying the spatial pattern and the drivers of the decline in the eastern English Channel chlorophyll-a surface concentration over the last two decades.

It has been established from previous studies that chlorophyll-a surface concentration has been declining in the eastern English Channel. This decline has been attributed to a decrease in nutrient concentrations in the rivers. However, the decrease in river discharge could also be a cause. In our study, rivers outflows and in-situ data have been compared to time series of satellite-derived chlorophyll-a concentrations. Dynamic Linear Model has been used to extract the dynamic and seasonally adjusted trends of several environmental variables. The results showed that, for the 1998-2019 period, chlorophyll-a levels stayed significantly lower than average and satellite images revealed a coast to offshore gradient. Chlorophyll-a concentration of coastal stations appeared to be related to the declining fluxes of phosphate while offshore stations were more related to nitrate-nitrite. Therefore, we can exclude that the climate variability, through river flows alone, has a dominant effect on the decline of chlorophyll-a concentration.

Trajectories of nutrients concentrations and ratios in the French coastal ecosystems: 20 years of changes in relation with large-scale and local drivers.

Along with their important diversity, coastal ecosystems receive various amounts of nutrients, principally arising from the continent and from the related human activities (mainly industrial and agricultural activities). During the 20th century, nutrients loads have increased following the increase of both the global population and need of services. Alongside, climate change including temperature increase or atmospheric circulation change has occurred. These processes, Ecosystem state changes are hard to monitor and predict. To study the long-term changes of nutrients concentrations in coastal ecosystems, eleven French coastal ecosystems were studied over 20 years as they encompass large climatic and land pressures, representative of temperate ecosystems, over a rather small geographical area. Both univariate (time series decomposition) and multivariate (relationships between ecosystems and drivers) statistical analyses were used to determine ecosystem trajectories as well as typologies of ecosystem trajectories. It appeared that most of the French coastal ecosystems exhibited trajectories towards a decrease in nutrients concentrations. Differences in trajectories mainly depended on continental and human influences, as well as on climatic regimes. One single ecosystem exhibited very different trajectories, the Arcachon Bay with an increase in nutrients concentrations. Ecosystem trajectories based on ordination techniques were proven to be useful tools to monitor ecosystem changes. This study highlighted the importance of local environments and the need to couple uni- and multi-ecosystem studies. Although the studied ecosystems were influenced by both local and large-scale climate, by anthropogenic activities loads, and that their trajectories were mostly similar based on their continental influence, non-negligible variations resulted from their internal functioning.

Three decades of data on phytoplankton and phycotoxins on the French coast: Lessons from REPHY and REPHYTOX.

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.

Harmful algae niche responses to environmental and community variation along the French coast.

Distribution, frequency and intensity of harmful phytoplanktonic species are impacted by changes in environmental conditions. In the Bay of Brest, Alexandrium minutum has been responsible for several harmful algal blooms (HABs) associated with toxin production causing paralytic shellfish poisoning (PSP). Additionally, Lepidodinium chlorophorum causes green water and hypoxia locally in the Bay of Biscay. Previous studies revealed that L. chlorophorum's success was related to possible competitive exclusion. Therefore, the phytoplankton composition and the environmental conditions should be taken into account. This study aims to assess the combined effect of changes in habitat conditions and community structure with the occurrence of HAB species, on a spatial-temporal scale. For the investigation we first used the Hutchinson's niche concept by means of the Outlying Mean Index (OMI) analysis. The OMI analysis enable us to observe the environmental variables defining the ecological niche of the harmful species among the community. Secondly, we used the subniche theory to highlight the environmental variables defining the subniches in cases of high and low abundance of HABs with an estimation of the biological constraint restricting the species' subniche. This was undertaken using the Within Outlying Mean indexes (WitOMI) calculated under environmental conditions promoting high (H) and low (L) abundance bloom. Thirdly, we used the Indicator Species Concept from the Indicator Species Analysis (ISA) to link the biological restriction with potential competing or indicator species. We combined a data set from the French National Phytoplankton and Phycotoxin Monitoring Network (REPHY), the Velyger network (oyster monitoring program) and satellite imagery. A total of 44 stations, over the period of 1998-2017 using 50 taxonomic units. 36 taxa had significant niche and were mostly distributed along nutrient and salinity gradients. The two species of interest L. chlorophorum and A. minutum seemed to have similar affinity for summer-like environmental conditions and both used a marginal habitat compared to the rest of the community. A. minutum had a larger niche due to a greater affinity to the estuarine-like conditions. The subniche of the two species had a similar response to the environmental variation; their respective abundance was partly caused by greater environmental restrains. Their success in abundance appeared to be linked to local hydrodynamics which increases or reduces resources. On the other hand, the biotic pressure exerted upon A. minutum and L. chlorophorum were antagonistic. A possible competitor assemblage was exposed but the analysis was inconclusive. The methodological limitations were discussed as well as a perspective for future similar studies.

Contribution of benthic microalgae to the temporal variation in phytoplankton assemblages in a macrotidal system.

Suspended marine benthic microalgae in the water column reflect the close relationship between the benthic and pelagic components of coastal ecosystems. In this study, a 12-year phytoplankton time-series was used to investigate the contribution of benthic microalgae to the pelagic system at a site along the French-Atlantic coast. Furthermore, all taxa identified were allocated into different growth forms in order to study their seasonal patterns. The highest contribution of benthic microalgae was observed during the winter period, reaching up to 60% of the carbon biomass in the water column. The haptobenthic growth form showed the highest contribution in terms of biomass, dominant in the fall-winter period when the turbidity and the river flow were high. The epipelic growth form did not follow any seasonal pattern. The epiphytic diatom Licmophora was most commonly found during summer. As benthic microalgae were found in the water column throughout the year, the temporal variation detected in the structure of pelagic assemblages in a macrotidal ecosystem was partly derived from the differentiated contribution of several benthic growth forms.