Temporal variability of phytoplankton biomass and algae blooms in the open Black Sea

Abstract

This study investigated long-term changes and seasonality of phytoplankton carbon biomass and algae blooms in the open Black Sea surface layer within the post-eutrophication period (1998–2022). Phytoplankton carbon biomass was calculated from satellite-derived chlorophyll a concentration using carbon-to-chlorophyll a ratio from in situ measurements. This allowed us to identify the months experiencing algal blooms within this period (August – November). We show that phytoplankton biomass and bloom frequency were negatively coupled to the vertical density gradient, suggesting that these features were driven possibly by large diatoms harvesting nutrients from deeper depths. Our results also suggest that photoinhibition, together with high vertical density gradients, could reduce phytoplankton biomass and blooms in the surface layer during summer, with the first signs of alleviation in August. Our study also discusses the large differences in drivers of the phytoplankton community between three distinct periods: the pre-eutrophication (before the early 1970 s), intense phase of eutrophication (mid-1980 s – early 1990 s) and post-eutrophication, and conclude that decadal changes in climate and nutrient inputs caused substantial changes in phytoplankton biomass and algae blooms.