Phosphorus transport in a hotter and drier climate: in-channel release of legacy phosphorus during summer low flow conditions

Abstract. The release of bioavailable phosphorus (P) during hot, dry summer periods when conditions are optimal for algal growth in lakes and rivers drives increased eutrophication risk. In addition to external P inputs, water quality is impacted by “legacy P”, i.e., the historical accumulation of P in soils and sediments due to past inputs. River networks represent a potential sink and/or source of legacy P, with many dynamic 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 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 hypothesized that in-stream release of legacy P contributes to elevated concentrations of bioavailable P (i.e., soluble reactive P, SRP) during summer in streams with strong agricultural and/or urban influence, in addition to concurrent contributions from tile drainage systems and point source discharges. We addressed this hypothesis through synthesis of three water quality datasets: 1) water quality and stream flow (Q) data collected for 143 gaged 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 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 machine learning (random forest) approach to identify possible drivers of bioavailable P concentrations during summer low flows for gaged watersheds. Our analysis indicates that between one third to one half of the gaged watersheds we studied exhibited SRP concentrations during low flows in late summer above previously identified thresholds for eutrophication of 0.02–0.04 mg/L. 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. We found that while wastewater discharge contributed to elevated P concentrations for watersheds with high densities of treatment plants, many 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), as well as 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 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 – 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.