The English–Wabigoon River System: III. Selenium in Lake Enclosures: Its Geochemistry, Bioaccumulation, and Ability to Reduce Mercury Bioaccumulation

Abstract

Aquatic biota have been shown previously to bioaccumulate selenium rapidly with a concomitant reduction in accumulation of mercury when exposed to 100 μg Se/L. Using radioisotope techniques this experiment extends these observations by examining the effects of single additions of sodium selenite at 1, 10, and 100 μg Se/L to experimental ecosystems (130 m³) located in mercury-contaminated Clay Lake, northwestern Ontario (50°03′N, 90°30′W). The major sink for added selenium and mercury was the sediment. Movement of ⁷⁵Se into the sediments in control enclosures (< 0.2 μg Se/L) appeared to stabilize within 6 wk; however, movement did not stabilize in the selenium enclosures where selenium remaining in water was largely ionic. At first, selenium accumulated rapidly in net plankton but then declined in correspondence with water concentrations. In Anadonta sp. and Orconectes virilis, selenium increased with time and in proportion to ambient selenium. Stable concentrations of selenium were not achieved in fish after 2 mo, but levels always varied according to the following sequence: Semotilus margarita < Perca flavescens < Catostomus commersoni, ranging from 0.2 to 0.4 μg Se/g (wet wt) at 1 μg Se/L. Accumulation of mercury was reduced in several biota in a concentration-dependent manner, with greatest reductions observed at 100 μg Se/L. These reductions appeared to depend on the position of the organism in the food web. In fish, the reduction was proportional to the amount of selenium accumulated. If not present in excessive amounts, selenium might be of benefit in the treatment of aquatic ecosystems with either atmospheric or point-source mercury pollution problems.