The effects of recent environmental changes in Lake Waahi on two forms of the common smelt Retropinna retropinna, and other biota

Abstract

Since 1977 open cast (open pit) coal mining has increased discharge of silt-laden wastes into Lake Waahi, a small shallow lake in the Waikato River system, New Zealand. Phenolic wastes from a coal carbonization factory on the major tributary also have entered the lake in high but variable concentrations since 1974. During and after the summer of 1978–1979 total suspended solids and turbidity were high, the formerly dense macrophyte beds disappeared, the summer phytoplankton community changed from dominance by blue-green algae to diatoms, the eel fishery failed and the black swan population decreased to low levels. The once abundant lake resident form of the common smelt declined drastically between 1975 and 1981 and apparently was extinct by 1983. The migratory form, although abundant until 1981, was scarce by 1983. Low water levels in the outlet stream and the malfunction of a fish pass in the flood control structure located near its mouth may have impeded upstream migration in the spring of 1982. The major cause for the changes in Lake Waahi was the addition of silt; however, the loss of macrophytes may have caused increased toxicity of phenols, particularly to eels, and also promoted continual resuspension of sediments by more effective wind-produced circulation. Remedial measures are considered.