Instream Processes Alter the Bioavailability of Phosphorus Exports From Agricultural Watersheds During High Flow Events

Abstract:

Reoccurring summer harmful algal blooms (HABs) in the Western Basin of Lake Erie have been attributed to an increase in dissolved reactive phosphorus (DRP) loads from watersheds dominated by agriculture. It is well established that instream processes such as sorption/desorption or autotrophic uptake can shape the timing, form and magnitude of P exports to recipient ecosystems during low flow periods. Yet, the majority of this phosphorus (P) runoff which fuels HABs in Lake Erie, and many other systems, is delivered during high flow events, which are predicted to increase in frequency and magnitude in response to climate change. The importance of instream processes during high flow events are rarely examined and generally assumed to be negligible. Here, we hypothesized that P sorption by suspended sediments during transit downstream represents a significant DRP sink, which decreases DRP loading to Lake Erie during high flows and ultimately mediates HABs severity. To evaluate our hypothesis, we measured P sorption kinetics of suspended sediments during thirteen intermediate and high flow events from six Maumee river tributaries between January and June 2019. As predicted, suspended sediments acted as a sink for DRP in 99% of samples indicating the potential for significant DRP sorption during transit. We found that volumetric sorption rates were determined by stream flow and mass of sediment in transit. Mass-specific sorption rates as well as other sorption kinetics (EPC₀, S_max, & NAP) were determined by suspended sediment size and organic matter content as well as by other unidentified site-specific characteristics. Thus, landscape level changes within the watershed have the potential to alter sorption rates via links with stream flow, sediment erosion, nutrient concentrations, and organic matter content. Estimates of total P sorption during transit from our study sites to Lake Erie indicate that in the absence of P sorption, DRP loading to Lake Erie might be significantly higher. Our work shows that streams in the Maumee river watershed are likely providing a major unrecognized ecosystem service to Lake Erie, which should be incorporated into decision making regarding P mitigation practices within the watershed.