The challenge of using phytoplankton composition for Mediterranean ecosystem health assessment

In recent times, a wide variety of marine ecosystem health indicators has been developed. The first aim of these indicators is to distinguish between healthy from degraded ecosystems with sufficient precision to identify measures or actions that need to be taken in order to maintain or retrieve good environmental status. In Mediterranean coastal waters, only a few phytoplankton composition indicators have been developed in compliance with the EU environmental legislation. This can be in part attributed to the difficulties to describe the causal chain from driving forces (e.g. human activities) through pressures (e.g. nutrient discharges) to impacts on phytoplankton composition.
In this work, we present a new phytoplankton composition index (PPCI, Pressure Pigments Composition Index), which uses phytopigments for identifying and quantifying phytoplankton functional groups. PPCI is a multimetric index that integrates group-specific chemotaxonomic indicators (carotenoids) responding to pressures. The index was developed in the Corsican coastal waters (NW Mediterranean) where well known reference conditions deliver a baseline against which other phytoplankton variables can be anchored. Being very sensitive to human pressures, PPCI detects the effects of anthropogenic disturbances on both quantitative and qualitative phytoplankton communities structure over different spatial and temporal scales. PPCI is transferable over a broad range of coastal zones, and is therefore a useful tool for assessing the marine ecosystem health.
Synergies between in situ pigment data and satellite-derived phytoplankton functional types, obtained using a regionalized version of PHYSAT (downscaling), are presented. In the framework of the MSFD obligations, the next objective will be to identify patterns of homogeneous phytoplankton composition in the French Mediterranean subregion.