Understanding controls of P movement through watersheds are essential for improved landscape management in intensively managed regions. Here, we analyze observational data from 104 gaged river sites and 176 nongaged river sites within agriculturally dominated watersheds of Minnesota, USA, to understand the role of landscape features, land use practices, climate variability, and biogeochemical processes in total, dissolved and particulate P dynamics at daily to annual scales. Our analyses demonstrate that factors mediating P concentration-discharge relationships varied greatly across watersheds and included near-channel sediment sources, lake and wetland interception, assimilation by algal P, and artificial land drainage. The majority of gaged sites exhibited mobilizing behavior for all forms of P at event (i.e., daily) timescales and chemostatic behavior at annual timescales. The large majority of watershed P export (>70%, on average) occurred during high flow conditions, suggesting that more frequent large storm events arising from climate change will drive increased P losses from agricultural watersheds without substantial management changes. We found that P export could be dominated by dissolved P, particulate P, or an even mix of the two forms, depending on watershed attributes. Implementation of management practices to control P losses must be guided by understanding of how local landscapes interact with current and future climate conditions. Managing for both dissolved and particulate P is required to reduce overall P load in many agricultural watersheds.
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Data supporting the conclusions in this paper can be found in the tables, references and supplemental text. Raw water chemistry from field study sites is available at Springe Link ( http://doi.org/10.1007/s10750‐016‐2911‐7 ). Water chemistry and flow data from gaged sites is available at Department of Natural Resources website ( https://www.dnr.state.mn.us/waters/csg/index.html ) and Minnesota Pollution Control Agency website ( https://www.pca.state.mn.us/wplmn/data‐viewer ). This research was supported by the National Science Foundation under grant 1209402 Water, Sustainability and Climate (WSC)—Category 2, Collaborative: Climate and human dynamics as amplifiers of natural change: a framework for vulnerability assessment and mitigation planning. C.L. Dolph received additional support from the US Environmental Protection Agency under grant. R836166 Valuing Water Quality Improvements in Midwestern Ecosystems: Spatial Variability, Validity and Extent of the Market for Total Value. C.L. Dolph, E. Boardman, A.C. Baker and B. Dalzell also received support from the Minnesota Department of Agriculture under a Clean Water Fund Grant: Measuring and Modeling Watershed Phosphorus Loss and Transport For Improved Management of Agricultural Landscapes. We thank two anonymous reviewers and the journal editors for highly constructive comments that improved this manuscript.