Multiple environmental variables influence diatom assemblages across an arsenic gradient in 33 subarctic lakes near abandoned gold mines

Abstract

We examined surface sediment diatom assemblages from 33 subarctic lakes around the historic gold mines in Yellowknife, Northwest Territories (Canada), where lake-water As concentrations ([As]) still range between 1.5 and 2,780 µg/l, even though the roasting of the arsenopyrite-bearing gold ores ceased in 1999. Water chemistry variables related to gold mining pollution (arsenic, antimony, and sulfate) declined with increasing distance from the mines. The diatom assemblages varied substantially in species composition across our study lakes with the planktonic Discostella stelligera/pseudostelligera complex dominating in deep lakes where [As] were less than 5 µg/l. However, in lakes that exceeded the guideline for the protection of aquatic life ([As] > 5 µg/l), opportunistic benthic diatoms (small fragilarioid taxa and Achnanthidium minutissimum) were generally present in higher abundances. The subtle differences in diatom species richness and diversity among lakes with varying [As] suggest these indices may not be affected strongly by long-term arsenic pollution. Furthermore, canonical correspondence analysis of the diatom assemblage and environmental data indicated that nutrient and ionic compounds explained most of the variation in the assemblages, while mining-related variables played a limited role. Our results indicate that multiple limnological variables (nutrients, ions, and contaminants) and subarctic climatic conditions are important factors shaping the diatom assemblage composition in lakes impacted by mining activities and land-use changes near Yellowknife.