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.

There is a long record (almost 100 years) of dissolved phosphorus (P) and oxygen (O2) concentrations for the waters of the Baltic. The records are 'noisy' due to complex water exchange and deep mixing, but the increasing concentration of dissolved phosphorus over the last 30 years is clear from Fig. 6.30. This increase in nutrient concentration has fuelled primary production and has increased the flux of organic matter to the deep waters. Measurements of dissolved oxygen in deep waters of the Baltic show a steady decline over the last 100 years (Fig. 6.30), consistent with an increase in rates of oxygen consumption due to increasing organic matter inputs —overall, a clear example of eutrophication.

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).

1958 1963 1968 1973 1980

Year

Fig. 6.30 Oxygen and phosphorus concentrations in the Baltic Sea. Dark line through data is a regression line and thin line marks zero O2 concentration. DIP, dissolved inorganic phosphorus. Data plots after Fonselius (1981) and Nehring (1981), with permission from Elsevier Science.

1958 1963 1968 1973 1980

Year

Fig. 6.30 Oxygen and phosphorus concentrations in the Baltic Sea. Dark line through data is a regression line and thin line marks zero O2 concentration. DIP, dissolved inorganic phosphorus. Data plots after Fonselius (1981) and Nehring (1981), with permission from Elsevier Science.

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.

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