Influence of photoacclimation on the phytoplankton seasonal cycle in the Mediterranean Sea as seen by satellite

Photoacclimation is a cellular process that allows phytoplankton to change the intracellular chlorophyll-a concentration (Chl) in relation to environmental factors such as light and nutrients. This process is currently overlooked by standard operational ocean colour algorithms used to retrieve information about both the phytoplankton standing stock and production. Here, we describe the photoacclimation effect on the phytoplankton seasonal cycle through the chlorophyll to carbon ratio (Chl:C) over the Mediterranean Sea, using SeaWiFS (1998-2007) monthly data. Chlorophyll was calculated through a regionalized empirical algorithm and Carbon was estimated from the particle backscattering, after subtraction of the non-algal contribution (bbpNAP). Historically, bbpNAP has been neglected or assumed constant, and only recently its space-time variability started to be taken into account. Here, bootstrapping simulations are used to compute the monthly bbpNAPs over a former bio-regionalization map of the Mediterranean Sea, all highlighting significant space-time variability. Resulting Chl:C varies within a factor of three, demonstrating the important role that photoacclimation plays at seasonal and basin scales. Lowest Chl:C values (~ 0.0013) are observed during summer; this is due to the combined effect of high irradiation, shallow mixing and low nutrient concentration. Maximum values (~ 0.031) are observed in winter and spring dominated by both low solar radiation, deeper mixing and high nutrient availability. It emerges that a better view of the phytoplankton space and time variability can be achieved through the joint use of Chl and C.