THE ROLE OF SEDIMENT IN DRIVING PHOSPHORUS LOADING AND DYNAMICS IN THE HIGHLY ALTERED AGRICULTURAL LE SUEUR RIVER BASIN

The Minnesota River Basin is undergoing rapid hydrologic and geomorphic change as a result of anthropogenic pressures including climate change and intensive agricultural land use, leading to sediment and nutrient loading downstream. The Le Sueur River, a tributary to the Minnesota River, is one of the largest producers of excess sediment and phosphorus (P). To improve our ability to target mitigation for reduction of P loading to the Le Sueur, we build upon a fine-sediment budget to generate a mass-balance for sediment-derived P. Sediments were collected from erosional source areas including alluvial streambanks, glacial till bluffs, ravines, agricultural fields and agricultural ditch-banks; and were analyzed for P content and sorptive properties. These data were used to determine the extent to which sediment serves as a source of P to the Le Sueur River, and to explore the role of in-stream equilibrium processes between sediment and dissolved P in driving watershed scale P dynamics.

Mass balance reveals that, despite the very high sediment loads and high background P concentration of sediment, only 24% of total P load measured at the outlet can be attributed directly to source sediment, with 23% in particulate form and < 1% in dissolved form. Dissolved orthophosphate comprises 37% of the measured average annual total P load at the Le Sueur outlet. Incorporation of data describing sorptive properties of sediment into this budget suggests that 2-24% of the total P exiting the basin may be particulate P that formed via sorption by sediment in the water column, and that the true dissolved P load may be masked by sorption of dissolved P to sediment, potentially reducing the fraction of total P observed as dissolved load by as much as 39%.

While it is frequently assumed that reducing erosion will control P loss in agricultural landscapes, these findings indicate that even if we were able to control all sediment inputs, we would only reduce P loads by less than one quarter of the total exported. Furthermore, conversion of P from dissolved to particulate form via in-stream sorption processes may mute the signal of dissolved P inputs to the channel network. Thus, management practices which address dissolved P source will be equally as important as those targeting erosion in reducing P loading to this system.