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Since the Industrial Revolution, humans have been releasing metals into the environment in damaging quantities. Aquatic ecosystems are particularly sensitive to such pollution since their food chains generally contain more trophic levels than on land and so bioaccumulation is enhanced.

Concentrations of cadmium and mercury have decreased in rivers in the EU since the late 1970s, reflecting the success of measures to eliminate pollution of these two List I substances under the dangerous substances directive (Figure 4.2). However, this information should be treated with some caution as the data are from relatively few stations and may not be representative. The dangerous substances directive also requires the pollution of List II substances to be reduced. List II metals include zinc, copper, nickel, chromium and lead. Data from the Rhine and Elbe indicate that the levels of some of these metals have also been reduced since the late 1980s (Figure 4.3).

In the Rhine, levels of certain heavy metals were reduced by between 50 and 90 % by the end of the 1980s compared with the early 1970s, though the Rhine is still subject to sizeable inputs of pollutant substances. This reduction was achieved by the control and reduction of point sources of these metals, and had a positive impact on aquatic communities in the Rhine.

In the Elbe, there have been considerable reductions in inputs of almost all substances, mainly as a result of the drastic drop in production and of factory closures but also due to the construction and modernisation of sewage treatment plants. However, heavy metal pollution is still high. The reduction in mercury was due primarily to the discontinuation of the amalgam process for chloride production in two factories in the new German Länder and the Czech Republic, and to remediation of existing contaminated sites.