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SynopsisAcidification in geologically-sensitive areas subject to appreciable acidic deposition has affected the activities of microbes, plants and animals in poorly-buffered freshwater ecosystems in Europe and N. America. Biota can be influenced directly by changes in water quality during both short acidic episodes and longer-term sustained periods of acidification: they are affected indirectly by alterations to the balance of acid-sensitive and acid-tolerant species at different trophic levels sometimes leading to a lessening of grazing pressures. Together the chemical and biological alterations result in adverse effects on some biogeochemical processes including the increased mobilisation and leaching of biologically active elements such as mercury, copper and zinc. Many field observations have now been corroborated by field (whole-lake) and laboratory experiments.Decomposition of organic matter, attributable to microbes, has been found to be affected adversely at some locations. Conspicuously, acidification favours the prolific growth of filamentous algae particularly species of Mougeolia: it also shifts the balance of diatoms, minimising the occurrence of circumneutral species and favouring that of acidophilous and acidobiontic types. Because of their ‘resistant’ siliceous skeletons, it has been possible to trace the historical progression of acidification by examining the remains of diatoms in sediment accumulations. While their species diversity is significantly decreased, the biomass and productivity of dinoflagellates and chrysophytes are only slightly affected.Like that of phytoplankton (dinoflagellates, chrysophytes and some diatoms), the species diversity of zooplankton is significantly decreased by acidification which has variable effects on the biomass and productivity of the group as a whole: the loss of daphnids is particularly conspicuous. Of the benthic macroinvertebrates, snails, crayfish, clams and freshwater shrimps (amphipods) have been found to be acid-sensitive whereas dragonflies and water boatmen (corixids) increase, particularly where populations of predetory fish have decreased.Acidification has been responsible for the loss of fish from significant parts of Norway and Sweden: losses, partial or complete, have also been documented in geologically-sensitive areas of the U.K. and other parts of Europe, also Canada and the U.S.A. Eels seem to be the least sensitive to acidic conditions, followed by pike, trout, minnow and roach — the latter being the most sensitive. Prior to being eliminated, populations of fish become unbalanced because of recruitment failure and the mean age, and sometimes size, of the survivors increases. Accumulations of manganese and mercury have been detected in fish taken from acidified freshwaters. Short-term events (episodes) of extreme acidity and/or large concentrations of water-soluble aluminium have led to major fish kills.The development of embryonic and larval stages of amphibians, including the common frog, toad and natterjack toad, is acid-sensitive. The distribution of fish-eating birds may be influenced by the effects of acidification on the performance of their prey. That of the dipper, a riverine bird, as well as its breeding success, has been restricted where acidification has decreased the availability of its macroinvertebrate prey. Deleterious effects on fish-eating mammals of reduced supplies of prey, sometimes with accumulations of mercury and cadmium and attributable directly and indirectly to acidification, have not, as yet, been confirmed.

Extensive studies of precipitation chemistry during the last 20 yr have clearly shown that highly polluted precipitation falls over large areas of Scandinavia, and that this pollution is increasing in severity and geographical extent. Precipitation in southern Norway, Sweden, and Finland contains large amounts of H+,SO~, and NO 3 ions, along with heavy metals such as Cu, Zn, Cd, and Pb, that originate as air pollutants in the highly industrialized areas of Great Britain and central Europe and are transported over long distances to Scandinavia, where they are deposited in precipitation and dry-fallout.In Norway the acidification of fresh waters and accompanying decline and disappearance of fish populations were first reported in the 1920s, and since then in Sorlandet (southernmost Norway) the salmon have been eliminated from several rivers and hundreds of lakes have lost their fisheries.Justifiably, acid precipitation has become Norway's number-one environmental problem, and in 1972 the government launched a major research project entitled 'Acid precipitation -effects on forest and fish', (the SNSF-project). Studies of freshwater ecosystems conducted by the SNSF-project include intensive research at 10 gauged watersheds and lake basins in critical acid-areas of southern Norway, extensive surveys of the geographical extent and severity of the problem over all of Norway, and field and laboratory experiments on the effect of acid waters on the growth and physiology of a variety of organisms.Large areas of western, southern, and eastern Norway have been adversely affected by acid precipitation. The pH of many lakes is below 5.0, and sulfate, rather than bicarbonate, is the major anion. Lakes in these areas are particularly vulnerable to acid precipitation because their watersheds are underlain by highly resistant bedrock with low Ca and Mg contents.Apart from the well-documented decline in fish populations, relatively little is known about the effects of acid precipitation on the biology of these aquatic ecosystems. Biological surveys indicate that low pHvalues inhibit the decomposition of allochthonous organic matter, decrease the species number of phytoand zooplankton and benthic invertebrates, and promote the growth of benthic mosses.Acid precipitation is affecting larger and larger areas of Norway. The source of the pollutants is industrial Europe, and the prognosis is a continued increase in fossil-fuel consumption. The short-term effects of the increasing acidity of freshwater ecosystems involve interference at every trophic level. The long-term impact may be quite drastic indeed.Water, A Jr, and Soil Pollution 6 (1976) 483-499. A ll Rights Reserved