Spatial and Temporal Variability of Phytoplankton Communities in a Tidal Front Ecosystem: the Iroise Sea (N-E Atlantic).

Coastal areas are very productive systems in which phytoplankton is responsible for the major part of the total primary production. The efficiency of the carbon pump depends on the quality of primary producers. Community composition is thus directly driven by environmental factors, which act by selecting species through adaptation or acclimation processes. In this context, mechanisms that promote coexistence are of center of interest in environmental research.

 At a regional scale, the Iroise sea is characterized by an intense tidal front (“Ushant” front) that undergoes a very pronounced seasonal cycle, with a marked signal during the summer. It separates offshore thermally stratified waters from tidally well-mixed areas over the continental shelf. The frontal zone is therefore a highly dynamic structure that sustains a high-intensity phytoplanktonic bloom (>2mg Chla.m^-3).

 Using a regional ecosystem model, this study aims to evaluate the impact of hydrodynamic conditions variability on the onset of various ecological niches which promote specific phytoplankton assemblages. We run a realistic hydrodynamic simulation (ROMS-AGRIF, 1,5 km resolution) coupled with an innovative and model of plankton diversity which allow communities to self-assemble through competition amongst large numbers of phytoplankton types (the MIT-Follows (“DARWIN”) model).

 Our results show a seasonal succession as well as a bioregionalization of phytoplankton community composition, diversity and traits during the summer. In addition to the seasonal variability, tidal cycle (neap/spring tides alternation) is also shown to influence the relative contributions of each size class to the total phytoplankton biomass.

Particular attention will be given to the productivity/diversity relationship and the importance of 'bottom-up' vs. 'top-down' mechanisms in controlling the simulated diversity.