Abstract
Abstract. Stratification plays an essential role in the marine ecosystem, with a shallow mixed layer being one of the preconditions for enhanced primary production in the ocean. In the Baltic Sea, the general understanding is that the upper mixed layer (UML) is well below the euphotic zone in winter. In this study, we demonstrate that wintertime shallow stratification is common in the Gulf of Finland and it forms at a depth comparable to the euphotic zone in January–February. The onset of restratification is likely associated with the annual cycle of westerly winds, which ease off in late January–early February. Stratification is first invoked along the northern coast by the westward advection of riverine water forced by easterly winds and is expanded to the south when the prevailing wind direction changes from an easterly to a westerly direction. Haline stratification emerges approximately 1 month later in the southern part of the gulf. Winter restratification can occur in the entire gulf and also in the absence of ice; thus, it is a regular seasonal feature in the area. Interannual variations in the wintertime UML correspond with variations in the North Atlantic Oscillation. Chlorophyll a concentrations in winter can be comparable to mid-summer; the limiting factor for phytoplankton bloom in winter is likely insufficient solar radiation.
Highlights
Upper-layer stratification is an important characteristic in the dynamics of the pelagic ecosystem
The water column becomes stable in spring and the mixed layer is shallower than the euphotic zone, so that the spring bloom is triggered when solar radiation is sufficiently strong (Fleming and Kaitala, 2006; Jaanus et al, 2006; Lips et al, 2014; Wasmund et al, 1998)
Lateral chlorophyll a (Chl a) extent was closely linked to the salinity structure, with higher Chl a concentration associated with lower salinity and vice versa
Summary
Upper-layer stratification is an important characteristic in the dynamics of the pelagic ecosystem. The upper mixed layer (UML), with a typical depth of 10–20 m, forms in spring and is separated from the rest of the water column by a seasonal thermocline. The thermocline is eroded by thermal convection, wind stirring, and current shear-induced mixing, and the mixed layer deepens down to the sea bottom or the halocline at 40–80 m depth in autumn–winter (e.g., Lass et al, 2003; Liblik and Lips, 2017; Väli et al, 2013). The water column becomes stable in spring and the mixed layer is shallower than the euphotic zone, so that the spring bloom is triggered when solar radiation is sufficiently strong (Fleming and Kaitala, 2006; Jaanus et al, 2006; Lips et al, 2014; Wasmund et al, 1998).
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