Increased metal concentrations have been associated with freshwater acidification. Continuous-flow acute toxicity tests were conducted in soft water to determine the effect of pH on the toxicity of cadmium, copper, and zinc to small (1–6 g) steelhead trout (Salmo gairdneri). LC50 values were calculated for 96- and 168-h exposure periods in waters of pH 4.7, 5.7, and 7.0. Test fish were significantly more tolerant of the metals at the lowest pH value than at higher pH's. The 96-h LC50 values at pH 4.7, 5.7, and 7.0 were 671, 97, and 66 μg∙L−1 for zinc, 66.0, 4.2, and 2.8 μg∙L−1 for copper, and 28.0, 0.7, and < 0.5 μg∙L−1 for cadmium, respectively. The 168-h results were similar to the 96-h values. These results indicate that for the metals tested, toxicity is ameliorated in depressed pH waters over short exposure periods, such as may occur during snowmelt runoff. The possibility of hydrogen ion interference with metal uptake is postulated.

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Sediment cores from nine lakes in southern Norway (N) and six in northern New England (NE) were dated by 37 Cs, 210Pb and in NE also by pollen, and were analyzed geochemically and for diatoms. Cores from two N and three NE lakes were analyzed for cladocerans. 37 Cs dating is unreliable in these lakes, probably due to mobility of Cs in the sediment. In Holmvatn sediment, an up-core increase in Fe, starting ca. 1900, correlates with geochemical indications of decreasing mechanical erosion of soils. Diatoms indicate a lake acidification starting in the 1920's. We propose that soil Fe was mobilized and runoff acidified by acidic precipitation and/or by soil acidification resulting from vegetational succession following reduced grazing. Even minor land use changes or disturbances in lake watersheds introduce ambiguity to the sedimentary evidence relating to atmospheric influences. Diatom counts from surface sediments in 36 N and 31 NE lakes were regressed against contemporary water pH to obtain coefficients for computing past pH from subsurface counts. Computed decreases of 0.3-0.8 pH units start between 1890 and 1930 in N lakes already acidic (pH 5.0-5.5) before the decrease. These and lesser decreases in other lakes start decades to over a century after the first sedimentary indications of atmospheric heavy metal pollution. It is proposed that the acidification of precipitation accompanied the metal pollution. The delays in lake acidification may be due to buffering by the lakes and watersheds. The magnitude of acidification and heavy metal loading of the lakes parallels air pollution gradients. Shift in cladoceran remains are contemporary with acidification, preceding elimination of fishes.

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