Under‐Ice Ecological and Biogeochemical Dynamics at the Onset of Spring Thaw in Four Arsenic‐Contaminated Subarctic Lakes

Interactions between aquatic organisms and contaminants in under‐ice lake environments are poorly characterized relative to ice‐free seasons. Here, we compared under‐ice arsenic biogeochemical processes and plankton community composition in four subarctic lakes spanning an arsenic contamination gradient. We focused on a short (7–10 days) late winter transitional period where lakes began to thaw but were still ice‐covered; a potentially significant but understudied period of rapid biological and chemical change. We observed decreases in conductivity over the late winter transitional period, due to snowmelt influx. We also observed decreases in ice thickness and increases in photosynthetically active radiation, dissolved oxygen, and water temperatures, which corresponded with a proliferation of under‐ice phytoplankton. The two study lakes with the highest under‐ice arsenic concentrations experienced a dilution in the surface waters following the influx of snowmelt, while meltwater was likely a source of arsenic enrichment to the study lake with the lowest under‐ice arsenic concentration. The introduction of oxygen into surface waters also shifted the arsenic pool to a higher relative fraction of arsenate compared to arsenite for lakes experiencing under‐ice anoxia. The organic arsenic pool increased at spring thaw onset in 2022, as would be expected based on the increase in biological activity, a result not replicated in 2023. Arsenite was a statistically significant driver of late winter variation in phytoplankton assemblages, but not for zooplankton or rotifers. Overall, this study generates new insights into under‐ice arsenic cycling and ecotoxicity in subarctic lakes, which remain ice‐covered for much of year.