Increased exposure of plankton to arsenic in contaminated weakly-stratified lakes

Abstract

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.

Contrasting arsenic cycling in strongly and weakly stratified contaminated lakes: Evidence for temperature control on sediment–water arsenic fluxes

“…Thus, arsenic concentrations are typically low in oxic waters because of this particle reactivity (Smedley and Kinniburgh ). However, some shallow well‐mixed lakes in our study region have also been shown to maintain high arsenic concentrations during summer months (Barrett et al ). In unstratified or weakly stratified lakes, arsenic tends to be mixed throughout the oxic water column rather than being isolated in the hypolimnion (Martin and Pedersen ; Couture et al ; Barrett et al ).…”

“…S1) with arsenic‐contaminated sediments from legacy pollution due to aerial emissions from the former ASARCO copper smelter in Ruston, WA, now designated the Commencement Bay/Nearshore Tideflats Superfund Site (Gawel et al ). Angle Lake and Lake Killarney have similar concentrations of arsenic in lakebed sediments, but different mixing behaviors and levels of productivity (Table ) (Barrett et al ). Angle Lake is a deep, oligotrophic lake that experiences strong thermal seasonal stratification resulting in anoxic bottom waters, while Lake Killarney is a shallow eutrophic lake that generally maintains a well‐mixed, oxic water column throughout the year (Supporting Information Fig.…”

“…Arsenic settled out downwind of the smelter across the landscape, contaminating surface soils and also lakebed sediments, which contain large amounts of legacy arsenic pollution (Gawel et al ). Initial work on lakes in the impacted region has shown that this sedimentary arsenic may be mobilized into the overlying water column and enter the aquatic food web (Gawel et al ; Barrett et al ).…”

“…Classically, elevated levels of arsenic are associated with anaerobic hypolimnetic waters that form during summer stratification (Aggett and O'Brien ; Azcue and Nriagu ; Senn et al ; Barringer et al ; Wei et al ). In seasonally stratified lakes, arsenic generally remains trapped within the hypolimnion and is typically present in the reduced form as arsenite (Aurilio et al ; Martin and Pedersen ; Durant et al ; Barringer et al , ; Chen et al ; Barrett et al ). Of the lakes impacted by legacy arsenic contamination in the Puget Sound region, those that exhibited seasonal hypolimnetic anoxia had elevated dissolved arsenic in bottom waters at levels correlated to sedimentary arsenic concentrations (Gawel et al ).…”

“…A key outcome of the flux of arsenic in oxic lake water columns is increased potential for biological exposure to aqueous arsenic (Barrett et al ). In oxygenated conditions, arsenic tends to be speciated as arsenate, a phosphate analog, which is mistakenly taken up by phytoplankton during phosphate transport (Rahman et al ).…”