Usually fewer diatoms during mild winters in a pristine Mediterranean coastal site

Studies conducted in a well-preserved Mediterranean coastal area (PhytoCly site, Calvi Bay, Corsica, NW Mediterranean) have shown that winter intensity, through the associated upward flux of nutrients, is a key driver of phytoplankton blooms during the winter-spring period, when the subsurface water is cold (i.e. ≤13.5°C) (a).
Here we use a long-term time series (14 years, 2007-2020) to investigate the relationship between winter intensity and surface phytoplankton characteristics throughout the year at the PhytoCly site. A chemotaxonomic approach based on HPLC analysis of pigments was applied to a series of biweekly samples to determine phytoplankton biomass and identify the major phytoplankton groups, using their pigment signature.
In years categorized as having no winter, mild, moderate, and severe winters, the mean annual time-integrated phytoplankton biomass (Tchl a) displayed a consistent increase with rising winter intensity, ranging from 76 to 290 ng Tchl a L-1 in years with no winter and severe winter conditions, respectively. Cyanobacteria and prymnesiophytes dominated the total phytoplankton biomass, but their biomass fluctuations showed little correlation with winter intensity. In contrast, diatoms and prasinophytes co-varied with Tchl a.
In the only year of the series marked by extreme winter conditions (2008), there was a reduced accumulation of phytoplankton biomass in the Bay, as well as a sharp decrease in the biomass of the 4 dominant groups.
These results exclude the year 2012. In that year, winter intensity was low, while mean annual Tchl a was high (394 ng Tchl a L-1), as were the biomasses of diatoms and prasinophytes. This may be related to the peculiar atmospheric conditions in the Gulf of Lions, which triggered a massive formation of dense water in February 2012 (b) and had a significant impact on the nutrient redistribution in the Ligurian Sea, including the western Corsican coast, where exceptionally high nutrient concentrations were measured.
There was no long-term trend in phytoplankton characteristics between 2007 and 2020, but Tchl a, diatoms and prasinophytes reflected year-to-year changes in local winter intensity, which were sometimes masked by basin-wide processes such as deep water formation in the Gulf of Lions. (a) https://doi.org/10.1016/j.pocean.2015.05.027
(b) https://doi.org/10.1002/grl.50331