Elevated arsenic concentrations in Crowley Lake derive from upstream geothermal inputs. We examined the water column of Crowley Lake under stratified and unstratified conditions, seeking evidence for algal uptake and transformation of arsenic and its deposition to and release from the sediments. Vertical profiles of other elements, which might either influence or track the cycling of arsenic, were also examined. Manganese and phosphorus concentrations increased with depth below the oxycline under stratified conditions, consistent with a sediment source of these elements. However, these elements did not accumulate in the hypolimnion during the period of stratification. This can be explained by accounting for the dynamics of reservoir operation in which water withdrawn from the hypolimnion is replaced from the epilimnion or surface inflows. Depletion of phosphorus in the surface water was incomplete during stratification, suggesting that phosphorus is not a limiting nutrient. Vertical profiles of total arsenic during stratification did not provide evidence for release of arsenic from the sediment; concentrations were either uniform with depth or showed a mid-depth minimum at the oxycline attributable to internal recycling within the water column. There was neither depletion of arsenic nor evidence for methylated arsenic species in the productive surface water. Arsenic was present as arsenate in the epilimnion and as a mixture of arsenate and arsenite in the hypolimnion. In the absence of an efficient mechanism to transport arsenic from the water column to the sediments, the substantial mass flux of arsenic through Crowley Lake results in only a moderate accumulation of arsenic in the sediments. Arsenic in the sediments appears to be sequestered by sulfide phases, thus release of arsenic from Crowley Lake sediments should not constitute a threat to the water quality of the Los Angeles Aqueduct. | Elevated arsenic concentrations in Crowley Lake derive from upstream geothermal inputs. We examined the water column of Crowley Lake under stratified and unstratified conditions, seeking evidence for algal uptake and transformation of arsenic and its deposition to and release from the sediments. Vertical profiles of other elements, which might either influence or track the cycling of arsenic, were also examined. Manganese and phosphorus concentrations increased with depth below the oxycline under stratified conditions, consistent with a sediment source of these elements. However, these elements did not accumulate in the hypolimnion during the period of stratification. This can be explained by accounting for the dynamics of reservoir operation in which water withdrawn from the hypolimnion is replaced from the epilimnion or surface inflows. Depletion of phosphorus in the surface water was incomplete during stratification, suggesting that phosphorus is not a limiting nutrient. Vertical profiles of total arsenic during stratification did not provide evidence for release of arsenic from the sediment; concentrations were either uniform with depth or showed a mid-depth minimum at the oxycline attributable to internal recycling within the water column. There was neither depletion of arsenic nor evidence for methylated arsenic species in the productive surface water. Arsenic was present as arsenate in the epilimnion and as a mixture of arsenate and arsenite in the hypolimnion. In the absence of an efficient mechanism to transport arsenic from the water column to the sediments, the substantial mass flux of arsenic through Crowley Lake results in only a moderate accumulation of arsenic in the sediments. Arsenic in the sediments appears to be sequestered by sulfide phases, thus release of arsenic from Crowley Lake sediments should not constitute a threat to the water quality of the Los Angeles Aqueduct.