Thermal and trophic stability of deeper Maine lakes in granite watersheds impacted by acid deposition

Abstract

Acid deposition can lead to lake and watershed acidification, increases in lake transparency, and reductions in thermal stability and hypolimnetic oxygen deficits. On the basis of lake surveys during August‐September 1985, we determined to what extent the deeper (maximum depth zm > 17 m) Maine lakes in acid‐sensitive granitic watersheds have registered changes in temperature and oxygen stratification, as compared to 1938–1942, when G. P. Cooper performed the earliest scientific surveys of the state's lakes. After correcting for small but geographically consistent interannual differences in summer hypolimnetic temperatures related to spring turnover, and weather‐dependent differences in mixed layer depth, there has been no significant change in thermal stratification in these Maine lakes over approximately 43 years. On the basis of specific historical contrasts in the late summer metalimnetic, hypolimnetic, and bathylimnetic oxygen concentrations there has been no significant change in lake trophic state or transparency.