Study area
The Baltic Sea is a shallow (mean depth 54 m), strongly stratified brackish water sea with a topography consisting of several sub-basins, and is connected to the North Sea only by narrow straits allowing minimal water exchange with the fully marine North Atlantic Ocean (Leppäranta & Myrberg 2009). It is characterized by substantial riverine input and steep geographical and seasonal gradients in temperature, salinity and nutrient concentrations. The salinity in the Baltic Sea is in some areas low, and ranges from 2 to 25 (Leppäranta & Myrberg 2009).
The Gulf of Riga is located at temperate latitudes with mean summer air temperatures close to 20°C, and in winter around −5°C. The gulf can be ice-covered up to 80 days (Kotta et al. 2008), but often winters can be ice-free too. Winter conditions are tightly coupled to the Northern Atlantic Oscillation (NAO) and years with positive NAO have stronger winds and a substantial increase in rainfall (Kotta et al. 2004). Moreover, cold winters alter the seasonal freshwater inflow, producing a spring freshet (Hänninen et al. 2000). In the Gulf, river discharges mix with brackish Baltic Sea water, yielding average surface salinity between 5.2 and 6.4, whereas bottom layer salinity is >7. However, during spring, surface salinities <2 can be observed in the river plumes.