Control of phytoplankton bloom by winter conditions in a Mediterranean coastal area : results from a long–term study (1979–2011)

The occurrence of a late-winter (February-March) phytoplankton bloom is one of the most unifying features of Mediterranean pelagic ecosystems. However, very little is known about the mechanisms that regulate the interannual variability of this phytoplankton bloom in coastal areas. In the present study, we show how winter physical forcing and environmental drivers control surface phytoplankton bloom dynamics in terms of the onset of the bloom, its duration, and its intensity through the analysis of a long-term time series (1979-2011) at the oligotrophic PhytoCly station (Bay of Calvi, Corsica, northwestern Mediterranean). We define a winter intensity index (WII) that integrates wind forcing and water temperature, and discuss strong correlations between WII and both surface nutrient fertilization and phytoplankton bloom intensity. Our study is consistent with reports that phytoplankton blooms can develop in homogenous water columns, and it does not fit the conventional idea that phytoplankton are blooming in temperate oceans because of warming/stratification of the surface layer.