Acid-Stress Effects on Stream Biology

Running waters are sensitive and rapid indicators of how whole watersheds become anthropogenically affected by, e.g. acidification. This paper reports and discusses the results of Swedish freshwater acidification research, for the period 1988-1993 and earlier. Changed biotic patterns are exemplified by increased occurrence of those green algae that indicate an increase in nutrients (nitrogen), reduced species richness of invertebrates (especially mayflies, crustaceans, gastropods), a general shift in proportion from invertebrate grazers towards shredders, decreasing populations of fish (salmonids, roach, burbot, minnow). Impact on birds (dipper, grey wagtail) appears less validated. The mechanisms for the changes in individual, population and community levels include elevated hydrogen, aluminum and cadmium concentrations that affect ion balance and respiration in fish and invertebrates, but also various behavior patterns (avoidance reaction, downstream movement, choice 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 neither predatory insects nor flycatchers seem to accumulate Al. It seems less likely that cadmium is a serious threat to invertebrates in "normal" concentrations at low pH. Iron precipitation can affect feeding ability and respiration of mayfly nymphs. That humic substances may mitigate metals still seems uncertain for fish and invertebrates. Generally, most changes in the biotic patterns of streams seem to be related to abiotic impact routes. Fewer changes are due to changed biotic interrelations, but some examples of changed competitive situations are given for invertebrates and fish. In all these cases of sublethal acidification stress, the ultimate effect is that growth, development and reproduction of the organisms are retarded. Relevant and sufficient knowledge seems to be lacking in three research fields of acidification impact on streams; viz increasing occurrence of green algae in acidified streams; role of invertebrates in decomposition of leaves in acid waters; and recovery processes of fish and invertebrates after liming.