Response of phytoplankton to climate-driven changes in a Mediterranean coastal area : results from 4 decades of observations (1979 - 2018)

In the Mediterranean coastal areas, most of the long-term studies of plankton dynamics concern highly urbanized areas, where long-term variability reflects the combined effects of climate and anthropogenic forcing. Here we use a unique long-term time series (1979-2018) performed at a fixed station in a well-preserved NW Mediterranean coastal area (Bay of Calvi, Corsica) to understand how climate variation drives changes in phytoplankton biomass and composition.
From a high-frequency data set, which combines environmental parameters and HPLC pigments, we highlight the importance of winter conditions to determine the state of phytoplankton community structure in surface waters. We show high variability in total biomass and phytoplankton composition resulting from changes in winter intensity associated with both interannual and decadal fluctuations.
We identify three distinct periods. The 1980s (1979-1988), the 1990s (1989-1998) and the 2000s (1999-2018) are characterized by moderate, mild and highly variable winters, respectively. The last years (2014-2018) show very mild winters and increasing winter water temperature, associated with decreasing trends in rainfall, total phytoplankton biomass and diatom abundance. Moderate and severe winters are favorable for diatoms, which do not develop during mild winters, when nutrient replenishment is not sufficient to support their growth.
In situ measurements and satellite-derived temperature and Tchl a in the Western Corsican Current are consistent with in situ data from the Bay of Calvi. This suggests that changes observed in the Bay of Calvi reflect phenomena occurring at a larger scale, and that our data set can contribute to predict changes of phytoplankton dynamics under different climatic scenarios.