Methyl Mercury Levels in a Polluted Prairie River–Lake System: Seasonal and Site-Specific Variations, and the Dominant Influence of Trophic Conditions

Abstract

The Moose Jaw River – Qu'Appelle River system of southern Saskatchewan (Canada) and its chain of hypertrophic lakes are polluted with mercury (Hg) and sewage from urban point-sources. Analysis of water, suspended particles, and bottom sediments sampled at different localities at different times of year (from April to November) revealed that methyl mercury (CH₃Hg⁺) levels are not controlled by the total abundance of inorganic Hg but are highly dependent on the abundance of organic matter and planktonic algae. CH₃Hg⁺ levels were anomalously high in the presence of sewage effluents and associated algal blooms and at times of reduced river discharge, when the flushing and dilution of nutrients were minimal. At elevated organic carbon, nitrogen, and chlorophyll concentrations, CH₃Hg⁺ levels were high even if inorganic Hg levels were low; but at low organic carbon, nitrogen, and chlorophyll concentrations, CH₃Hg⁺ levels were low even if inorganic Hg levels were high. The results are ascribed principally to stimulation of the activities of Hg-methylating microorganisms by organic nutrients from algae and sewage. The possibility of amerliorating the Hg problem by reducing nutrient pollution is discussed.