Human effects on regional seas 1 the Baltic

The Baltic Sea (Fig. 6.30) is a large regional sea, receiving drainage from much of northern and central Europe. The hydrography of the Baltic is complex, consisting of a number of deep basins separated by shallow sills. As a result, the waters of the deep basins can be isolated from exchange with one another—and from the atmosphere—on timescales of years. 

The isolated deep waters of the Baltic have probably always had low oxygen concentrations. However, the declining trend over recent years means that, in some areas, oxygen concentrations have fallen to zero (anoxic). Under anoxic conditions, respiration of organic matter by microbial sulphate (SO4-) reduction has produced hydrogen sulphides (HS ) (plotted as negative oxygen in Fig. 6.30). 

The Baltic contrasts with the nearby North Sea, where oxygen concentrations rarely fall to low levels, despite large inputs of nutrients. This is because the North Sea is shallow and its waters exchange freely with those of the North Atlantic providing a constant supply of oxygen-rich surface water to the North Sea deeper waters. 

---