Short-Term Arsenic Cycling in a Shallow, Polymictic Lake

Arsenic (As), a harmful contaminant present in many urban lakes, can negatively impact lake ecosystem health when aqueous concentrations are elevated. We observed repeated diel oscillations in As concentrations in the bottom waters of a shallow, temperate lake during a weeklong period. 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 diel 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 that redox conditions adjacent to the lakebed likely 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. 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. Understanding the daily cycling of As in shallow lakes is essential for predicting the degree to which lake biota are exposed to this contaminant. Diel oscillations in As concentrations should be considered when designing sampling methods to assess the water quality of contaminated sites. Arsenic is a cancer-causing toxin found in many urban lakes. The processes that control arsenic concentrations in lakes, especially on short timescales, are not fully understood. In this study, we examined daily patterns in arsenic concentrations in a shallow, arsenic-contaminated lake near Seattle, WA, USA. We observed that concentrations of arsenic, as well as concentrations of iron and manganese, increased and decreased on a 24-hr period in the water near the lake bottom. We discuss several mechanisms that could potentially create these daily cycles. Dissolved oxygen concentrations also exhibited daily cycles. We hypothesize that the oxygen cycles may affect how much arsenic is released from the lake sediment into the water throughout the day. However, we also show that a physical mixing process was necessary for the vertical transport of arsenic. From measurements of water velocities, we found that thermal convection, or the mixing of water due to heat differences, could explain the daily cycles in arsenic concentrations we observed. Because arsenic can be taken up into the lake food web, it is important to understand how arsenic concentrations change throughout the day and how changing concentrations may affect the plants and animals in the lake. Arsenic concentrations exhibit diel oscillations in the bottom water of a shallow, polymictic lake during the summertime Diel oscillations in bottom water dissolved oxygen may control rates of arsenic transport into lake water at short timescales Convective mixing from nighttime lake surface cooling is necessary to create the diel cycles in bottom water arsenic concentrations