An expedition organized at the end of February by the Atlantic Branch of Shirshov Institute of Oceanology of the Russian Academy of Sciences made it possible to obtain new data on the development of plankton communities in the Baltic Sea during the winter period.
The research was carried out from the small vessel BRIG F450 in the coastal region of the Baltic Sea and the western part of the Kaliningrad Gulf, connected to the sea by the Baltic Strait, and included hydrophysical sounding, collecting samples to study hydrochemical conditions and water eutrophication in winter.
In the Kaliningrad Gulf, for the first time, studies were conducted under conditions of melting ice – the water temperature at the surface was only 0.7°C, rising to 3°C at the bottom. There was no ice cover in the Baltic Sea, and the temperature in the water column (20–25 m) was about 3°C.
For the first time in the history of observations in the estuarine system of the Kaliningrad Bay, scientists have noted the mass development of phytoplankton at the end of winter. The content of chlorophyll "a" in the surface layer reached 32 μg/dm3 at a water salinity of 4.7 pes, which characterizes the intensive development of cold-water plankton species, which decreased almost an order of magnitude near the bottom. Previous studies conducted in the winter period of open water (late February - early March) demonstrated values 2-3 times lower, and a similar level of phytoplankton abundance was achieved much later - during the spring "bloom" of diatoms and summer cyanobacteria.
Intensive development of phytoplankton (10–12 μg/dm3) was also observed in the surface layer in the coastal region of the Baltic Sea at a salinity of about 7 pes, decreasing many times deeper. The values recorded by scientists in sea waters are close to the maximum values observed in the South-Eastern Baltic during the year, including exceeding the summer values, when cyanobacteria "bloom" is observed in the Baltic Sea. Bioproduction processes influenced hydrochemical conditions, in particular, a twofold decrease in the concentrations of biogenic elements (phosphates) at the surface.
The obtained results will help to clarify the understanding of the functioning of different ecosystems of the Baltic Sea under increased eutrophication of waters and changing climatic conditions. The noted productivity of waters in winter may be a previously little-known feature of the seasonal dynamics of plankton in estuarine and coastal waters in these areas of the Baltic. This phenomenon may have been influenced by climate change, when warmer winters, increased warming of waters and early clearing of water bodies from ice are observed in the region in the modern period. This contributes to earlier development of plankton and may lead to changes in trophic conditions in these ecosystems, including improving conditions for zooplankton as food for planktivorous fish.
