DISTRIBUTION OF THE GENUS <em>CYCLOTELLA</em> IN RELATION TO pH IN PRECAMBRIAN SHIELD LAKES: IMPLICATIONS FOR DIATOM-INFERRED pH.

Recent research on relationships between diatoms and pH suggests that the genus Cyclotella exhibits a strong relationship with lake acidity, being almost totally absent below pH 5.5. This decline has been used as an indicator of lake acidification in paleolimnological studies. In this study C. stelligera V.H. and C. kützingiana Thwaites were abundant in Precambrian Shield lakes with pH as low as 4.5. Cyclotella comta (Ehr.) Kütz. was found in lakes of pH < 5.5., but maximum abundance was observed in lakes of pH > 5.5. Cyclotella michiganiana Skv. was found in lakes of pH >6.0. These results indicate that the use of C. stelligera, C. kützingiana, and possibly C. comta, in paleolimnological investigations of lake acidification, should be approached with caution. These taxa may exhibit a decline in abundance with decreasing lake-water pH, but this is partially a morphometric effect not necessarily related to anthropogenic acidification. [ABSTRACT FROM AUTHOR]