Freshwater, density gradients and biological processes in cold, brackish seas : aspects of the biogeophysical fluid dynamics characterising the Baltic Sea

Abstract: The dominant component in the ocean water is freshwater. In this thesis its mixing and the freshwater-induced dynamics are discussed, with cold, brackish basins such as the Baltic Sea especially in mind. A mathematical framework is derived to allow for a consistent way of adding freshwater effects to the governing equations. The conservation equations in the presence of mixing by small-scale, irreversible molecular processes or large-scale stirring patterns are derived and the resulting differences are discussed. The lateral advection of freshwater from coastal sources is demonstrated on the basis of observations from the Gulf of Riga and Gulf of Finland. The advection is found to take place as a well-defined coastal current, in particular during spring when a minimum in the climatological wind stress is found. The baroclinic stability of a coastal current of this type is studied analytically and numerically, and a functional dependence between the cross-shore eddy transports and the bottom slope is derived. The role of edge waves in the stability problem and their sensitivity to bottom boundary characteristics is emphasized, and their biological relevance is discussed.Spatial temperature anomalies are shown to be dynamically passive at low temperatures in the Baltic. Observations and a numerical simulation confirm that the density gradients in off-shore areas of the Northern Baltic result from lateral gradients of freshwater. Under these conditions, the phytoplankton spring bloom results from freshwater-induced stratification.As opposed to non-reactive freshwater, many effluents that are carried to the sea by coastal freshwater discharge are transported exactly like freshwater, but being biologically reactive they have a limited range of transport. The biological reaction rate, in turn, is modified by the physical processes dealt with in this thesis, as shown from the observations and numerical results. The range of influence and eventual fate of such effluents is of relevance for the coastal societies. A requisite theoretical biogeophysical basis for socioeconomic inferences concerning coastal eutrophication is thus provided by the freshwater dynamics discussed this thesis.