Arsenic contamination of lakebed sediments is widespread due to a range of human activities, including herbicide application, waste disposal, mining, and smelter operations. The threat to aquatic ecosystems and human health is dependent on the degree of mobilization from sediments into overlying water columns and exposure of aquatic organisms. We undertook a mechanistic investigation of arsenic cycling in two impacted lakes within the Puget Sound region, a shallow weakly stratified lake and a deep seasonally stratified lake, with similar levels of lakebed arsenic contamination. We found that the processes that cycle arsenic between sediments and the water column differed greatly in shallow and deep lakes. In the shallow lake, seasonal temperature increases at the lakebed surface resulted in high porewater arsenic concentrations that drove larger diffusive fluxes of arsenic across the sediment-water interface compared to the deep, stratified lake where the lakebed remained~10 C cooler. Plankton in the shallow lake accumulated up to an order of magnitude more arsenic than plankton in the deep lake due to elevated aqueous arsenic concentrations in oxygenated waters and low phosphate : arsenate ratios in the shallow lake. As a result, strong arsenic mobilization from sediments in the shallow lake was countered by large arsenic sedimentation rates out of the water column driven by plankton settling.

Arsenic, a priority Superfund contaminant and carcinogen, is a legacy pollutant impacting aquatic ecosystems in urban lakes downwind of the former ASARCO copper smelter in Ruston, WA, now a Superfund site. We examined the mobility of arsenic from contaminated sediments and arsenic bioaccumulation in phytoplankton and zooplankton in lakes with varying mixing regimes. In lakes with strong seasonal thermal stratification, high aqueous arsenic concentrations were limited to anoxic bottom waters that formed during summer stratification, and arsenic concentrations were low in oxic surface waters. However, in weakly-stratified lakes, the entire water column, including the fully oxic surface waters, had elevated concentrations of arsenic (up to 30μgL) during the summer. We found enhanced trophic transfer of arsenic through the base of the aquatic food web in weakly-stratified lakes; plankton in these lakes accumulated up to an order of magnitude more arsenic on multiple sampling days than plankton in stratified lakes with similar levels of contamination. We posit that greater bioaccumulation in weakly-stratified lakes was due to elevated arsenic in oxic waters. Aquatic life primarily inhabits oxic waters and in the oxic water column of weakly-stratified lakes arsenic was speciated as arsenate, which is readily taken up by phytoplankton because of its structural similarities to phosphate. Our study indicates that mobilization of arsenic from lake sediments into overlying oxic water columns in weakly-stratified lakes leads to increased arsenic exposure and uptake at the base of the aquatic food web.

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Arsenic, a carcinogenic contaminant, has accumulated in the bottom sediments of lakes around the world due to a range of anthropogenic activities, such as mining, smelting, and the application of arsenic-containing pesticides (Gawel et al., 2014;Rice et al., 2002;Smedley & Kinniburgh, 2002). When arsenic is mobilized from sediments into overlying lake water, it can be taken up by primary producers (Barrett et al., 2018;Caumette et al., 2011) and move up the aquatic food web, including into organisms consumed by humans (Hull et al., 2021;Rahman et al., 2012). Both physical and biogeochemical lake processes control arsenic mobilization, transport, and concentration in lake water and thus control ecosystem and human exposure to arsenic. Recent findings show that conditions within shallow lakes, as opposed to deep lakes, facilitate rapid cycling of arsenic between the sediment and overlying lake water (Barrett et al., 2019), and enhance spatial overlap between oxygen-requiring biota and mobilized arsenic (Barrett et al., 2018;Hull et al., 2021).According to lake mixing classifications, most shallow lakes are polymictic, meaning they circulate perennially and only stratify for short periods (Hutchinson & Löffler, 1956;Lewis Jr., 1983). Based on this reasoning, the

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