Dataset for: Short-term Arsenic Cycling in a Shallow, Polymictic Lake

Abstract

We observed repeated diel oscillations in arsenic (As) concentrations in the bottom waters of a shallow, temperate lake during a weeklong measurement period. Arsenic concentrations were highest during the morning or midday and lowest in the evening. In this work, we explore four mechanistic hypotheses to explain the diel As cycles based on the physical and biogeochemical processes that were investigated during the study. Despite pH being known to control As cycles in rivers, we determined that this mechanism was inconsistent with As dynamics observed in Lake Killarney. Instead, we found that iron and manganese concentrations oscillated simultaneously with As concentrations and, thus, concluded that redox conditions adjacent to the lakebed controlled the near-bed availability of these three elements. However, based on timescale analysis, we determined that biogeochemical processes at the sediment water interface alone could not have led to the daily oscillations in bottom water concentrations. Rather, turbulence from convective mixing was necessary to transport dissolved species from the lakebed into bottom waters. Notably, we saw that the timing and intensity of peaks in convectively-driven turbulence were consistent with observed diel fluctuations in bottom water As. Our results indicate that physical mixing is key in controlling As transport and concentrations on diel timescales within shallow lakes. The daily cycling of redox-sensitive elements in shallow lakes and the potential physical controls on this phenomenon should be considered when designing sampling methods to assess the environmental health and water quality of contaminated sites.