A simple approach for identifying and measuring acidification of freshwater

Abstract

ACIDIFICATION of lakes and rivers accompanied by the loss of fish populations is a problem in Scandinavia^1–4, northeastern USA^5,6, and southeastern Ontario, Canada^7,8. These areas have granitic or other siliceous bedrock types, thin and patchy podsolic soils, and extremely soft and poorly buffered surface waters, and receive precipitation which is decidedly acidic (volume-weighted average pH 4.0–4.6). Sulphate, whose principal source is precipitation, is usually the major anion in these acidified waters. In similar areas such as northern Scandinavia and northwestern Ontario, in which precipitation is not acidic (pH>5.0) such oligotrophic pristine softwaters generally have pH levels >5.5, and bicarbonate is the major anion^9–11. Acidification of such waters entails a decrease in the bicarbonate buffer (alkalinity) with only minor decreases in pH, and then after exhaustion of the bicarbonate an increase in acidity to pH levels well below 5.0. Because of the severe damage to fish populations, and other biological consequences, the decreases in pH that characterise the later stages of the acidification process are of major concern. We need, therefore, a simple ‘early warning’ indicator that identifies lakes and rivers which are undergoing the first stage of acidification, the loss of alkalinity, but which have not yet reached the stage of marked pH decreases. Furthermore, we need quantitative estimates of the degree of acidification. We show here that excess sulphate concentrations, or current calcium concentrations and alkalinities can provide such an estimate.