In 2011, Lake Erie experienced the largest harmful algal bloom in its recorded history, with a peak intensity over three times greater than any previously observed bloom. Here we show that long-term trends in agricultural practices are consistent with increasing phosphorus loading to the western basin of the lake, and that these trends, coupled with meteorological conditions in spring 2011, produced record-breaking nutrient loads. An extended period of weak lake circulation then led to abnormally long residence times that incubated the bloom, and warm and quiescent conditions after bloom onset allowed algae to remain near the top of the water column and prevented flushing of nutrients from the system. We further find that all of these factors are consistent with expected future conditions. If a scientifically guided management plan to mitigate these impacts is not implemented, we can therefore expect this bloom to be a harbinger of future blooms in Lake Erie.extreme precipitation events | climate change | aquatic ecology | Microcystis sp. | Anabaena sp.

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We estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for 1974, 1978, 1982, 1987, and 1992. NAPI quantifies all anthropogenic inputs of P (fertilizer use, atmospheric deposition, and detergents) as well as trade of P in food and feed, which can be a net input or output. Fertilizer was the dominant input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed. NAPI increased in relation to area of disturbed land (R 2 = 0.90) and decreased with forested and wetland area (R 2 = 0.90). Export of P by rivers varied with NAPI, especially for the 18 watersheds of LM (R 2 = 0.93), whereas the relationship was more variable among the six LE watersheds (R 2 = 0.59). On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. A comparison of our results with others as well as nitrogen (N) budgets suggests that fractional export of P may vary regionally, as has been reported for N, and the proportion of P inputs exported by rivers appears lower than comparable findings with N.Keywords Phosphorus Á NAPI Á P export Á River Á Watershed Á Nutrient budget Á Nutrient loading Á Lake Michigan Á Lake Erie Abbreviations LELake Erie LM Lake Michigan NANI Net anthropogenic nitrogen input NAPI Net anthropogenic phosphorus input NLCD National Land Cover Database RMSE Root mean squared error

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