Effects of Acid Mine Wastes on Phytoplankton Communities of Two Northern Ontario Lakes

Abstract

Differences in chemical composition in 1965–67 between two lakes contaminated by uranium-milling wastes and one unaffected lake were related directly to the nature of processes in uranium extraction and subsequent waste treatment. Low pH and high concentrations of SO₄⁻², NO₃⁻¹, and Ca⁺² occurred in the contaminated lakes. Low concentrations of inorganic carbon, which apparently limited production in the contaminated waters, resulted from reduced solubility, loss, and possibly inefficient regeneration of CO₂. Other major nutrients did not appear to be limiting, since nitrates were in greater supply in the contaminated lakes than in the unaffected lake and phosphorus and silica occurred in similar concentrations in all three lakes.Lower phytoplankton populations and lower indices of diversity were found in the contaminated lakes than in the unaffected one. Many species of Bacillariophyceae, Chrysophyceae, and Myxophyceae developed in the unaffected lake but were absent or occurred in extremely low numbers in the contaminated lakes.Average primary productivities were approximately 70 and 30 mg C m⁻² day⁻¹ in the two contaminated lakes and 130 mg C m⁻² day⁻¹ in the unaffected lake. In situ areal and volumetric measurements in laboratory and field bioassays confirmed the importance of inorganic carbon in limiting primary productivity. A potential compensatory mechanism in the contaminated lakes was a deepening of the euphotic zone. Although greater concentrations of inorganic carbon occurred and were assimilated in hypolimnetic than in epilimnetic waters, the mechanism was not sufficient to overcome the effects of reduction in both species diversity and abundance of phytoplankton on the areal primary productivity.