Winter stratification phenomena and its consequences in the Gulf of Finland, Baltic Sea

Abstract. Stratification plays an essential role in the marine system. The shallow mixed layer is one of the preconditions for the enhanced primary production in the ocean. The general understanding is that the mixed layer is well deeper than the euphotic zone in the Baltic Sea during winter. In this work, we demonstrate the wintertime stratification is a common phenomenon in the Gulf of Finland. Shallow haline stratification at the depth comparable to the euphotic zone depth forms in late January–early February. Stratification is evoked by the positive buoyancy flux created by the westward advection of riverine water along the northern coast of the gulf. Fresher water and haline stratification appeared approximately one month later in the southern part of the gulf. The phenomena can occur in the whole gulf and also without ice. Chl a concentration and phytoplankton biomass in winter can be comparable to mid-summer. The limiting factor for phytoplankton bloom in winter is likely insufficient solar radiation.