Assessing Phosphorus Loading Targets for Lake Erie Algal Blooms and Hypoxia under Climate Variability

Abstract A decade ago, the United States and Canada revised Lake Erie’s phosphorus loading targets to address increasing severity of western basin harmful algal blooms (HABs) and central basin hypoxia. To mitigate HABs, targets were set for spring (March–July) loads of total phosphorus (860 metric tonnes [MT]) and dissolved reactive phosphorus (186 MT) from the Maumee River. To raise the average central basin summer hypolimnetic dissolved oxygen concentration above 2 mg/L, annual total phosphorus (TP) load target of 6000 MT/year was established. We update, apply, and improve HAB and hypoxia models to re-evaluate load–response curves, estimate temporal lags, and quantify the influence of climate. Both models are developed within Bayesian frameworks to provide uncertainty quantification. Under the current climate, the HAB and hypoxia targets require a 21% reduction in the 2020–2024 average spring total bioavailable phosphorus (TBP) load, and an 18% reduction in the annual TP load. Once loading goals are achieved, we project an average lag of 7.0 and 9.0 years for hypoxia and HAB. Under moderate climate change, midcentury targets require a 38% reduction from today’s spring TBP loads and a 45% reduction of annual TP load. Larger loading reductions will be required during wet years.