Chemical Composition and Phytoplankton of Lakes in Southern Ontario

Abstract

Water chemistry in southern Ontario lakes is determined primarily by surface and bedrock geology and the impact of cultivation. In areas of uniform bedrock, elevation and lake size are shown to be important. Total dissolved solids give a poor indication of the electrolyte content of lakes, because of the varied geology. Four main lake types are recognised: (1) base-poor lakes, frequently dystrophic, found on the Precambrian shield; (2) moderately base-rich lakes, in the Bancroft–Rideau lakes region, related to the frequent occurrence of more easily weathered rocks; (3) base-rich lakes found in the limestone region south of the shield, and receiving abundant drainage from cultivated land; and (4) base-rich lakes with high levels of magnesium draining from Silurian dolomitic limestone bedrock and dolomite-rich glacial tills.Phytoplankton populations were found to be related in part to the chemical composition of lake waters. Certain species such as Tabellaria flocculosa and to a lesser extent Asterionella formosa and Dinobryon divergens were associated mainly with base-poor lakes; a far larger number of species, including Synedra acus, Pediastrum duplex, Scenedesmus quadricauda, Cyclotella meneghiniana, Synedra ulna, and Cyclotella kutzingiana, were significantly associated with base-rich conditions.An association diagram for all the species found in the lakes showed several groupings, mostly of species that prefer base-rich conditions. Few species confined to base-poor lakes were found to form associations. Apparent associations between oligotrophic species and species known to prefer eutrophic lakes can be explained by the fact that nutrients were depleted at the time of the survey.