DECADAL VARIABILITY OF ZOOPLANKTON COMMUNITIES OF THE NW MEDITERRANEAN SEA (WEEKLY SAMPLED FROM 1995 TO 2006) IN RELATION WITH CLIMATIC FORCING.

Abstract : The inter-annual variability of the pelagic ecosystems of the Ligurian Sea is investigated combining original datasets (from 1995 to 2006 collected weekly) of zooplankton abundances, hydrology and local weather conditions obtained in the bay of Villefranche-sur-mer. Two main patterns of zooplankton dynamics were observed with a shift between 1999 and 2000. The first period was characterized by high precipitation and mild air temperature during the winter. This induced lower salinity and higher seawater temperature and low density of surface seawater during the winter. These waters were characterize by low loads of nutrients. During these years, zooplankton total biovolume was also lower as shown by the strong negative anomalies in the time series. Starting in 2000, the climate changed toward drier and colder winters with denser surface water and more intense convections as suggested by higher nutrients concentrations. An increase of the abundances of all zooplankton categories was observed with a doubling of the total zooplankton average annual means and a change in the zooplankton phenology with a spring development happening 2.5 weeks earlier during these years. These results could be explained by a strong bottom-up control on the pelagic ecosystem of the Ligurian Sea at the inter-annual scale. Whereas the summer thermal stratification increase was often suggested to drive long-term dynamic in Ligurian Sea zooplankton, our results highlight the strong influence of the Winter convection properties as the main factor governing inter-annual changes in zooplankton abundance. The effect of global climate cycles will be discussed as being possible factors driving the pelagic ecosystem in the NW Mediterranean Sea.