Recent investigations of surface water and precipitation chemistry have aroused considerable concern about the potential effects of acidic precipitation on aquatic ecosystems in the north-central United States. The mean weighted pH of precipitation at two monitoring stations in north-central Wisconsin during 1980 was 4.45 and 4.63 (7), or about 10-14 times more acidic than 'normal' rainfall, which has a theoretical pH of 5.6 (8). Many watersheds in northern Wisconsin have acid soils with low buffering capacity (9). Consequently, the water of most lakes in these watersheds is very soft and slightly acidic. Analyses of 275 northern Wisconsin lakes by Eilers et al. (10) during 1979 indicated that 36% of the lakes were susceptible and an additional 50% were potentially susceptible to acidification from acid precipitation.