Our MIO colleague Thibaut Wagener (CEM) took part in this study.
Phytoplankton is the generic term for the microscopic algae that carry out photosynthesis in the ocean, just as vegetation does on land. These micro-algae are the focus of a great deal of research because they are a key element in the biological carbon pump and the first link in the marine food chain. But monitoring this 'marine vegetation' is much more difficult than on land, since it is subject to surface currents and vertical movements of water masses, and our observation capabilities are more limited at sea. Our current understanding of the seasonal cycle, or phenology, of phytoplankton is therefore very partial, which hampers our ability to understand both the functioning of marine ecosystems and their evolution in the context of climate change.
Phytoplankton phenology in the Mediterranean Sea has been studied mainly by satellite observations, known as 'ocean colour', which provide a good indicator of the quantity of phytoplankton present in surface waters. Two main categories of phenology have been identified: the first, observed over the vast majority of the basin, is characterised by slight seasonal variations; the second, localised to very specific regions, shows strong seasonal variations marked in particular by the presence of a spring peak.
As part of the MISTRALS project, the DEWEX and PERLE actions have made it possible to extend these results in two of these areas of marked spring bloom: the Gulf of Lion in the north-western Mediterranean and the south of the island of Rhodes in the eastern Mediterranean. Several oceanographic campaigns and a massive deployment of autonomous platforms (underwater gliders and profiling floats) have made it possible to gather unprecedented data. They confirm that the phytoplankton cycle is driven by seasonal changes in the depth of the mixed layer, i.e. the layer of surface water subject to winds and variations in atmospheric temperature. It is the dynamics of this layer that determine the growth of phytoplankton: when it deepens in winter, under the combined effect of increased wind and lower temperatures, it sporadically enriches the surface with nutrient-rich deep water. This phenomenon creates favourable conditions for a bloom the following spring, when the temperature and light levels rise. This bloom then continues until the nutrients are exhausted.
The data also highlighted a specific feature of the Mediterranean with regard to this phenomenon. In the two areas studied, the deep reservoir of nutrients is located relatively close to the surface, which means that nutrient salts can be supplied from the bottom, even in the presence of relatively shallow winter mixing layers. As a result, the intensity of the phytoplankton bloom in spring is not directly correlated with the depth of the winter mixing layer. Incorporating this information into numerical models will improve our ability to predict the evolution of marine ecosystems in the Mediterranean over the coming decades.
Authors
Fabrizio D'Ortenzio1, Vincent Taillandier1, Laurent Coppola2, Thibaut Wagener3