Abstract. “Legacy phosphorus” is the historical accumulation of phosphorus (P) in soils and sediments due to past human inputs. River networks represent a potential sink and/or source of legacy P, with many in-channel processes potentially governing the storage and mobilization of P over time. The objective of this study was to evaluate the potential contribution of the in-channel release of legacy P to bioavailable P transport in streams during summer low-flow conditions across a land use gradient in Minnesota, USA. We addressed this objective through the synthesis of (1) water quality and streamflow (Q) data collected for 143 gauged watersheds across the state of Minnesota between 2007–2021 (22 750 total samples); (2) water quality data from 33 additional ditch, stream, and river sites in Minnesota sampled under low-flow conditions in the summer of 2014; and (3) water quality data collected from tile drainage outlets for 10 monitored farm fields between 2011–2021. We used geospatial data and a random forest modeling approach to identify possible drivers of bioavailable P concentrations during summer low flows for gauged watersheds. During low flows in late summer, between one-third to one-half of the gauged watersheds we studied exhibited soluble reactive phosphorus (SRP) concentrations that were above previously identified thresholds for eutrophication of 0.02–0.04 mg L−1. For many of these watersheds, stream SRP concentrations in late summer were above those observed in tile drainage outlets. Elevated SRP concentrations during late-summer low flows weakened concentration–discharge relationships that would otherwise appear to indicate more strongly mobilizing SRP–Q responses across other seasons and flow conditions. While wastewater discharge likely contributed to elevated P concentrations for watersheds with high densities of treatment plants, many watersheds did not have substantial wastewater impacts. The most important variables for predicting bioavailable P concentrations during late-summer low-flow conditions in a random forest model were land use in riparian areas (particularly crop cover); soil characteristics including soil erodibility, soil permeability, and soil clay content; agricultural intensity (reflected via higher pesticide use, higher phosphorus uptake by crops, and higher fertilizer application rates); watershed precipitation; and stream temperature. These findings suggest that, for stream and river sites heavily impacted by past and current P inputs associated with agriculture and urbanization, biogeochemical processes mediated by climate and geology can result in the release of legacy P from in-channel stores during late-summer low-flow conditions. As summers become hotter and, at times, drier – which are predicted changes in this region – conditions for the release of legacy P stored in stream and river channels will likely become more prolonged and/or more acute, increasing eutrophication risk.
