Elevated levels of arsenic have been reported in surface waters of many urban lakes around the Puget Sound as a result of the widespread heavy metal contamination from the late ASARCO smelter in Ruston, Washington. Arsenic is a neurotoxin and carcinogen and a priority Superfund contaminant. However, the mobility and toxicity of arsenic is not fully understood. Physical and biogeochemical processes that lead to elevated arsenic concentrations in the water column and aquatic organisms have been studied well in thermally stratified lakes, but not in periodically mixed (polymictic) oxygenated lakes. This project examines the mobility, bioaccumulation and toxicity of arsenic in four urban lakes in south King County that range from seasonally stratified and oxygen depleted (anoxic) to polymictic and oxygenated (oxic). Specifically, we aim to discover why one of the lakes has elevated levels of arsenic (up to 40 ppb) in surface waters, yet regularly mixes and thus remains oxygenated. Typically, arsenic is only mobilized from sediments when a lake becomes anoxic during stratification. Monthly water quality parameters (temperature, pH, dissolved oxygen, and specific conductivity) were measured and water samples (alkalinity, chlorophyll, sulfide, iron, arsenic, and nutrients) were collected at multiple depths throughout the water column. Plankton samples were also collected using vertical net tows. Water and plankton samples were analyzed for dissolved and total arsenic (after digestion) by ICP-MS. Phytoplankton and zooplankton in polymictic oxic lakes accumulated more arsenic than plankton in thermally stratified lakes, even though aqueous arsenic concentrations were similar between the lakes. This data suggests that arsenic in polymictic urban lakes is not only bioavailable, but has the potential to travel up the food chain. This project aims to create a model for predicting arsenic bioavailability based on physical lake characteristics to better predict possible toxicity and, therefore, has important implications for lake management.