Thresholds of plankton community change in a Mediterranean coastal area : results from a long-term (1979-2014) time series

In the Mediterranean Sea, which has been identified as a hotspot for climate change, there is evidence for impacts of climate change on marine organisms. However, the lack of information on environmental drivers and associated thresholds limits our ability to forecast possible changes in biodiversity and ecological interactions.
In this presentation, we use a unique long-term (1979-2014) time series performed in a Mediterranean coastal area unbiased by local anthropogenic pressure (Bay of Calvi, Corsica) to understand how climate variation controls phyto- and zooplankton dynamics and possibly affect artisanal and small-scale fisheries exploiting areas near the coast.
From high-frequency field data, we describe a mechanism that links winter physics, nutrient replenishment of the surface layer and plankton dynamics under the different combinations of meteorological conditions that occurred during the 36 years of observations. We identify threshold values of physical variables below and above which they strongly impact nutrient availability, phyto- and zooplankton bloom characteristics and seasonality succession of plankton functional groups. We provide identification of environmental thresholds beyond which diatoms, which are crucial for sustaining fish populations, collapse. We discuss links between climate-related changes in plankton availability, fish recruitment and the success of small-scale fisheries. We show that the mechanism we identified from the 1979 to 2014 period of observation in the Bay of Calvi is pertinent to other Northwestern Mediterranean areas, stressing the importance of winter conditions in determining the state of Mediterranean pelagic ecosystems.
We highlight that the thresholds obtained from our long-term time series provide key information for improving model scenarios of the impact of climate change on Mediterranean ecosystems.