During recent decades, with continued economic growth, population increase, urbanization and technical development, more and more waste materials have been discharged into bodies of water. Toxic substances and heavy organic loads usually have a severe local impact. This is especially true of smaller bodies of water such as streams, where the composition of invertebrate communities may be used to derive practical indices of pollution severity.
A particularly insidious effect of pollution occurs when toxic materials accumulate within the tissues of a species used as food. Such materials include PCBs and mercury. Fish in the Lower Rhine, for example, accumulate sufficient quantities of phenol for this to be detectable by taste. Heavy metal (especially mercury) accumulation in fish is causing concern about its fitness for human consumption in some areas.
Thermal pollution due to discharges from the cooling systems of power plants affects the aquatic ecosystem to various degrees. Heat influences all biological activity, ranging from feeding habits and reproduction rates of fish via metabolism to changes in nutrient levels, photosynthesis, eutrophication and the degradation rate of organic material.
The possible environmental impact of the long-range transport of sulphur and nitrogen oxides deposited as strong mineral acids in precipitation has emerged as a new problem. Sulphur oxides and nitrogen oxides derived from fossil fuel combustion both contribute appreciably to the acidity of precipitation. Some lakes have shown a decline in pH values of as much as 1.8 units since the 1930s - that is, acidity as measured by hydrogen ion concentration has increased almost a hundred fold. There is evidence that acidification reduces the diversity of plant plankton and affects a number of other organisms in the aquatic food web. The growth of rooted plants is reduced, and the abundance of bog moss (Sphagnum), growing as benthic carpets, increased. The rate of decomposition of organic matter declines and fungal felts may develop over sediments, mineralizing nitrogen and phosphorous much more slowly than the normal bacteria. Productivity of the lake ecosystem is reduced.
Inland water bodies have suffered in many areas from industrial pollution and poor land management. In Scandinavia, for example, hundreds of lakes, particularly small ones, still suffer from acidification and it will take a long time for water quality to return to normal (EEA 1997). All the major rivers in the European part of the former Soviet Union and in Siberia have been diverted into chains of artificial lakes. In most, lake bed sediments are highly polluted, and high inputs of phosphorus and other nutrients have often led to eutrophication. The Aral Sea - which lost one-third of its area, two-thirds of its water and almost all its native organisms as a result of the diversion of its input waters for irrigation (UNEP 1994b) - will probably never recover.