Temporal changes of phytoplankton biomass in the western Black Sea shelf waters: Evaluation by satellite data (1998-2018)

This study investigated temporal changes of phytoplankton biomass with particular focus on blooms and their underlying mechanisms within different shelf regions in the western part of the Black Sea for the poorly studied post-eutrophication period. We combined satellite chlorophyll with carbon-to-Chl a ratios to obtain a better proxy of phytoplankton biomass, and applied an algorithm to partition these data into a base seasonal variation and blooms. The base phytoplankton biomass was characterized by a seasonal maximum in October-November, highest values in the Romanian inner shelf region that receives inputs directly from the Danube River, and downward time trends in all shelf regions, coinciding with reduced nitrate inputs from the Danube. The majority of phytoplankton blooms were short-lived events (lasting up to 5 days), which accounted for over 60% of total bloom events. The initiation of spring blooms coincided with the peak riverine input for the Romanian inner shelf and with the formation of the thermocline in other regions. Autumn blooms were observed with the breakdown of the thermocline in October-November. These were most likely sustained by nutrient accumulation in bottom waters originating from riverine inputs in spring and regenerated nutrients from sedimenting organic matter and sediments. Large diatoms may contribute to autumn blooms in the outer shelf regions through harvesting new nitrate from deeper waters and hosting endosymbiotic cyanobacteria to extract new nitrogen from the atmosphere. Finally, high intensity of upwelling processes could support winter blooms in the Bulgarian outer shelf region.

Automated summary

This study investigated the temporal changes of phytoplankton biomass and blooms in different shelf regions in the western part of the Black Sea. The results showed that the base phytoplankton biomass had a seasonal peak in October-November, with the highest values in the Romanian inner shelf region. The majority of blooms were short-lived events, with spring blooms coinciding with peak riverine input and the formation of thermocline in other regions. Autumn blooms were likely sustained by nutrient accumulation and regenerated nutrients from sedimenting organic matter and sediments. Large diatoms may contribute to autumn blooms in outer shelf regions through harvesting new nitrate and hosting endosymbiotic cyanobacteria.