Long-term fluctuations (1979 – 2008) of the phytoplankton dynamics in the Bay of Calvi (Corsica, NW Mediterranean): relations with wind stress variability and NAO index

The development of the winter-spring phytoplankton bloom is investigated in the oligotrophic Bay of Calvi (Corsica, Ligurian Sea, Northwestern Mediterranean) since 1979.

Time-series results enlighten that changes in environmental conditions in the Corsican sector have altered the coastal phytoplankton dynamics and induced a drastic reduction of the winter-spring phytoplankton biomass over the last three decades.

At the local scale, variations in winter wind stress control the development of the winter-spring phytoplankton bloom and the structure of the phytoplankton assemblages. Results emphasize that the total phytoplankton biomass (chl a) is higher during high winter wind stress years. From a specific point of view, flagellates are always the dominant component of the phytoplankton bloom communities. However, diatoms tend to be more abundant during high winter wind stress years.

At a wider scale, a link between the intensity of the phytoplankton bloom and the winter NAO index is established.

Lastly, the decrease in phytoplankton abundance coincides with changes in zooplankton populations. As a consequence, the entire pelagic food web dynamics is affected by climate variations and the ecosystem resilience is threatened.