Characterize variables in biogeochemical conditions and spatial distribution of bacterial communities across the English Channel in two main productive seasons

The English Channel features a wide range of ecological habitats that hosts numerous biodiversity resources and are submitted to natural and/or anthropogenic pressures. During the EcoPel oceanographic campaigns (spring and summer 2018) in French waters of the English Channel and North Sea, a variety of coastal pelagic habitats were sampled for analyzing biogeochemical variables and bacterial communities. Results of PCA suggest that main environmental variables were SPM, PIM, and NO2-/NO3+ in spring and salinity, SPM, Si, and Chl-a in summer. The Shannon index suggested summer alpha diversity had higher richness and equitability compared to spring. A clear seasonality in the bacterial community structure was also revealed by hierarchical cluster analysis. Most of spring communities had lower proportion of Bacteroidetes while most of summer communities had higher proportion of Proteobacteria, Actinomycetota, Verrucomicrobia, and Planctomycetes. Based on distance correlations and statistical significance, the spring taxonomic composition were strongly correlated with Chl-a, PO4, POM, SPM, and PIM, while the summer composition was strongly correlated with salinity. According to hierarchical cluster analyses, both biogeochemical variables and bacterial communities seem to be clustered in parallel with the coast, evidencing the main influence of coastal-offshore gradients and implying possible links with river inputs and phytoplankton/algae dynamics.