Controls of phosphorus loading and transport in the Cuyahoga River of northeastern Ohio,USA

An urban stream differs from a natural stream in that it commonly contains anthropogenic nutrients from a variety of diffuse and point sources (e.g., urban runoff, industrial and municipal effluents). The Cuyahoga River as one of such stream systems receives on average 30% of water from over a dozen wastewater treatment plants (WWTPs) and exports over 300 Mg/yr of total phosphorus (TP) into Lake Erie. Municipal effluents account for at least two thirds of the TP loading and 90% of the effluent TP is in the form of soluble reactive phosphorus (SRP), i.e., the highly bioavailable form in aquatic ecosystems. Owing to its dominance of the effluent P, the Cuyahoga River may pose a disproportionately greater ecological risk to Lake Erie. Here we report results of TP and SRP as measured on water samples collected from 12 locations along the Cuyahoga River and its major tributary – Tinkers Creek under three different flow conditions in July 2007 through May 2008. Our results show that the loadings of SRP and TP increased progressively with the effluent inputs increasing from the upper basin downstream. We found the loadings of SRP and TP are not only affected by the amount of effluent P input but also regulated by stream flow regimes. Effluent P is more likely to be transformed during storm runoff events, whereas TP is more likely to be retained under low flow conditions. As a result, most of the TP loading was exported during the storm and intermediate flow conditions, whereas most of the SRP loading was delivered during the low and intermediate flow conditions. These results suggest that stream hydrology has played an important role in the loading and transport of P across the Cuyahoga River as it dictated the amount, form, and timing of P exported to Lake Erie.