ACID-STRESS EFFECTS ON STREAM BIOLOGY

Running waters are sensitive and rapid indicators of how whole watersh eds become anthropogenically affected by, e.g. acidification. This pap er reports and discusses the results of Swedish freshwater acidificati on research, for the period 1988-1993 and earlier. Changed biotic patt erns are exemplified by increased occurrence of those green algae that indicate an increase in nutrients (nitrogen), reduced species richnes s of invertebrates (especially mayflies, crustaceans, gastropods), a g eneral shift in proportion from invertebrate grazers towards shredders , decreasing populations of fish (salmonids, roach, burbot, minnow). I mpact on birds (dipper, grey wagtail) appears less validated. The mech anisms for the changes in individual, population and community levels include elevated hydrogen, aluminum and cadmium concentrations that af fect ion balance and respiration in fish and invertebrates, but also v arious behavior patterns (avoidance reaction, downstream movement, cho ice of spawning site), and developmental stages (molt and emergence of insects, hatching and growth of early fish stages). Al can ameliorate low pH temporarily but does not biomagnify along food chains, and nei ther predatory insects nor flycatchers seem to accumulate Al. It seems less likely that cadmium is a serious threat to invertebrates in ''no rmal'' concentrations at low pH. Iron precipitation can affect feeding ability and respiration of mayfly nymphs. That humic substances may m itigate metals still seems uncertain for fish and invertebrates. Gener ally, most changes in the biotic patterns of streams seem to be relate d to abiotic impact routes. Fewer changes are due to changed biotic in terrelations, but some examples of changed competitive situations are given for invertebrates and fish. In all these cases of sublethal acid ification stress, the ultimate effect is that growth, development and reproduction of the organisms are retarded. Relevant and sufficient kn owledge seems to be lacking in three research fields of acidification impact on streams; viz. increasing occurrence of green algae in acidif ied streams; role of invertebrates in decomposition of leaves in acid waters; and recovery processes of fish and invertebrates after liming.