Late Quaternary diatom and chemical profiles from a meromictic lake in Quebec, Canada

Abstract

An investigation of the chemistry and diatoms in the sediments of a small hardwater meromictic lake in Quebec indicated remains of both freshwater and brackish (halophilic) water diatoms in the organic sediments laid down after the lake was isolated by isostatic rebound from the Champlain Sea, ∼11,000 yr. ago. Water in deeper strata of the lake evidently was not completely fresh, giving rise to ectogenic meromixis. The loss of marine salts from the lake is believed to have taken 3000 yr. or so and that as this happened, salts of biological origin gradually accumulated in the monimolimnion so that maintenance of meromixis became biogenic. The sediment chemistry reflects primary successional changes in the vegetation of the area rather than a decline of ectogenic meromixis. As forest cover developed and soils stabilised on the rebounding land, erosional processes gave way to leaching processes as the source of nutrients to the lake. The appearance of eutrophic indicator diatoms, and an increased flux of nutrients to the sediments, suggest that a cycle of eutrophication occurred in the lake following the decline of hemlock, ∼4800 yr. ago. The recent sediments show an increase in concentrations of Cu, Zn and Pb as a result of anthropogenic activities.