Response of plankton communities of the Bay of Calvi (northwestern Mediterranean) to climate variation over the past three decades

Long-term changes of surface plankton dynamics are studied from three decades at a permanent station in the oligotrophic Bay of Calvi (Corsica, northwestern Mediterranean) where water quality is unbiased by terrestrial inputs. As a distinctive feature of the Bay, a plurimodal plankton bloom occurs generally between January and April, with very large interannual variability reaching one order of magnitude from one year to another. Later in the season, the plankton biomass remains very low from May to December.
Contrarily to the 1979 - 1998 period where a continuous decrease of phyto- and zooplankton biomasses was observed, observations from the last decade (1999 - 2011) emphasize abrupt changes in plankton biomass and bloom duration from one year to another in response to succession of mild and rigorous winters.
Studies conducted to understand mechanisms controlling plankton variability emphasize a close relationship between winter wind stress intensity, water temperature, nutrient inputs and phytoplankton biomass. Total phytoplankton biomass is maximum during years characterized by high winter wind stress and low winter water temperature. From a specific point of view, flagellates are always the dominant component of the winter - spring phytoplankton communities. However, diatoms tend to be more abundant during rigorous winter years. Shifts in phytoplankton community structure contribute to control the dynamics of zooplankton that rely on phytoplankton as food and influence the temporal succession of zooplankton assemblages.
Time-series results indicate the high sensitivity of the plankton ecosystem to climate change and confirm that the Bay of Calvi reacts rapidly to external perturbations, which are driven by climate variation.