Control of plankton phenology by climate variation in a Mediterranean coastal area : results from a long-term study (1979-2011)

Physical processes are known to play major roles in marine plankton succession. However, few studies have addressed the mechanisms that regulate phyto- and zooplankton phenology in a context of changing climate. Here we used a unique long-term (1979-2011) time series performed in a Mediterranean coastal area unbiased by local anthropogenic pressure (PHYTOCLY station, Bay of Calvi, Corsica) to understand how environmental forcing affects the timing, duration and magnitude of the winter-spring phyto- and zooplankton blooms. We showed that phyto- and zooplankton blooms were bottom-up controlled by the establishment of favourable abiotic conditions, i.e. nutrient replenishment by vertical mixing under specific water temperature and wind conditions, for which thresholds were defined. According to the intensity of winter characteristics, there were strong differences in both the abundance and composition of phyto- and zooplankton during the winter-spring period. Our study is consistent with the recent reports that, when occurring, diatoms peaks were added to the initial phytoplankton groups instead of replacing them. In contrast, zooplankton groups followed a replacement sequence. Based on the results provided by our time series, we show that plankton phenology in the Bay of Calvi is highly controlled by climate variation and exhibits contrasted patterns in response to different scenarios of environmental forcing.