Nutrients and plankton spatial distributions induced by a coastal eddy in the Gulf of Lion. Insights from a numerical model

A plankton functional types model forced by a hydrodynamic model is used in the present work to study the impacts of a coastal eddy on the distribution of nutrients and plankton in the western part of Gulf of Lion (NW Mediterranean Sea). This study, based on a realistic simulation of the year 2001, focuses on a long-life anticyclonic eddy detected during summer. The studied anticyclonic eddy has some biogeochemical characteristics of those observed in the open ocean as for example the low productivity at their core and rising-up of nutricline on their edges. However the functioning and consequences of such coastal eddy on nutrients and plankton distributions are complicated by potential interactions with topography, wind-induced upwelling along the Languedoc Coast and nearby Northern Current (NC). Especially the proximity of the southern edge of the eddy with the NC makes possible the exchanges of organic matter (e.g. plankton) at times during the eddy's life. The coastal eddy thus transports organic matter from the coastal zone to the offshore domain. The coupled model also suggests the importance of offshore-to-coastal transport by NC through a seeding process of the eddy in plankton at the beginning of its life.A detailed biogeochemical functioning of the studied eddy all along its lifetime is proposed from the model results. At the beginning the eddy mainly acts as a transporter of the plankton wind-induced bloom occurring along the Languedoc Coast several days before the eddy set up. At the same time the eddy shows upwards vertical velocities on its edges creating upwelling of nutrient-enriched waters. The process of eddy-induced upwelling is notably enhanced on the eddy western side along the Roussillon Coast due to the shelf topography. The around-eddy advection and relative lower temperatures prevent the large development of phytoplankton along the Roussillon Coast and the eddy-induced bloom is predicted to occur near the northwestern side of the eddy. The eddy-induced and wind-induced blooms of phytoplankton then merge on the eastern side by advection and this process of aggregation may explain the persistent occurrence of a filament of high chlorophyll observed on the offshore edge of the eddy throughout the study period. The changes in plankton composition due to a combination of top-down and bottom-up processes during the coast-to-offshore transport may explain the disappearance of the filament south of 43°N at the surface.

► Biogeochemical properties of eddy in Gulf of Lion (Mediterranean Sea). ► Eddy impact on nutrients and plankton spatial distributions. ► Coupled biogeochemical-physical modelling. ► Eddy-related horizontal and vertical processes: phytoplankton development. ► High chlorophyll filament on edge of eddy, confirmed using SeaWiFS images.