Changes in the fish community of a limed lake near Sudbury, Ontario: effects of chemical neutralization or reduced atmospheric deposition of acids?

In the early 1970's, the fish population in Nelson lake (a moderately acid, metal-polluted lake some 28 km from the metal smelters at Sudbury) were seriously degraded; lake trout (Salvelinus namaycush) were scarce, smallmouth bass (Micropterus dolomieui) extinct, and the littoral zone was dominated by acid-tolerant yellow perch (Perca flavescens). The lake was limed in 1975-76, increasing the pH of the water to 6.4 and reducing the metal content. In the following years, the numbers of yellow perch declined rapidly, lake trout became more abundant, and re-introduced smallmouth bass became established. However, there was evidence showing that these improvements were not the result of the neutralization, but of reduced emissions from the smelters, and that the water quality was already improving before 1975. There might be abrupt thresholds for fish population level responses, and neutralization of lakes with initial chemical conditions above these thresholds might not produce distinguishable effects.