Seasons Of Change: The Influence Of Seasonality On Arsenic Biogeochemical Cycling And Ecotoxicity To Plankton Communities In Mining-Impacted Lakes Near Yellowknife (Northwest Territories, Canada)

Abstract

Relatively little is known about the biogeochemical cycling and bio-uptake of arsenic by freshwater organisms outside of the summer season, despite concerns related to its toxicity and persistence in contaminated aquatic systems. This is particularly true for subarctic lakes that remain ice-covered for the majority of the year. Recent research in shallow, subarctic lakes near Yellowknife (Northwest Territories, Canada) has found that arsenic concentrations undergo significant increases during the winter and changes in chemical form (i.e., speciation), due to cryoconcentration (solute exclusion during lake-ice formation) and sediment efflux under anoxic conditions. This has important implications for its toxicity to under-ice plankton communities, as well as the bio-uptake and transfer of arsenic through lake food webs. This dissertation integrated a field-based study of arsenic biogeochemistry and plankton ecology in five Yellowknife-area lakes that spanned a gradient of legacy arsenic contamination (1.1 – 455 g L-1), incorporating seasonal dynamics. Over a period of 2.5 years (April 2022 – June 2024), the lakes were sampled multiple times during both the open-water and under-ice seasons for water chemistry, plankton community data, and bulk plankton tissue samples for the analysis of arsenic bioaccumulation in phytoplankton and zooplankton. Two key, novel findings emerged from this research: (1) The winter and seasonal transitional periods may be more relevant for arsenic bioavailability and ecotoxicity; and (2) Total arsenic concentrations in lake water did not strongly correlate with arsenic bioaccumulation in plankton, which suggests that other lake-specific factors (particularly dissolved organic carbon (DOC)) influenced the overall bioavailability of arsenic across the five study lakes. As well, this dissertation provides new data on associations between arsenic and individual zooplankton and phytoplankton taxa, including data on specific forms of arsenic (e.g., As(V), As(III)) that are not routinely measured in field-based arsenic toxicity studies. Overall, this dissertation highlights the importance of sampling across limnological gradients and seasons when assessing arsenic impacts on freshwater ecosystems, particularly in subarctic lakes which remain ice-covered for the majority of the year.