WATER EUTROPHICATION AND ZOOPLANKTON DISTRIBUTION IN THE BALTIC SEA UNDER ABNORMAL WARMING CONDITIONS IN SUMMER OF 2021

Abstract

Data on the structure, abundance and spatial distribution of plankton were obtained for the Russian water area in the south-eastern part, as well as in the central part of the Baltic Sea, during the period of abnormally warm climatic conditions in the summer of 2021. The highest abundance of zooplankton and concentrations of chlorophyll “a” (eutrophic level) were in the coastal area and seaward to a depth of 40 m. Under the conditions of the exclusively open sea, water eutrophication decreased to the mesotrophic level. The lowest values of chlorophyll “a”, the abundance and biomass of zooplankton were observed in the deep-water zone (depths of 100–200 m), located mainly in the Gdansk Deep and Gotland Deep. The excess of the climatic norm in 2021 had a multidirectional effect on plankton. The intensive development of algae, favorable for the development of zooplankton, expand from the coastal zone to the seaward area to depths of 40 m. At the same time, there was a decrease in the abundance of the largest and most valuable zooplankton species (Pseudocalanus elongatus and others) for planktophagous fish. Such changes in plankton have been observed in recent decades and were clearly manifested in the anomalously warm 2021.