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.