BED SEDIMENT MODULATES WATER QUALITY IN AN AGRICULTURAL RESERVOIR

Excess inputs of nutrients (like N and P) from chemical fertilizers and manures used in agriculture can enter aquatic environments, harming human and ecosystem health (e.g., via NO₃^- toxicity or algal blooms). Successful management of nutrient pollution requires understanding N and P dynamics in fluvial systems, which transport excess nutrients to the ocean but can also sequester or transform them in river water columns, bed sediments, or floodplains. River impoundments may be especially important in determining the fate of nutrients in intensively managed landscapes because they slow water transport, extending the interaction time between the water and sediment. We thus investigated the role of reservoir sediment in modulating water quality by assessing nutrient dynamics at the sediment-water interface for Lake Decatur, an impoundment of the agriculturally-impacted Sangamon River in Illinois, United States. We collected sediment samples at 12 sites across the reservoir twice over 21 days to evaluate how nutrient content varied in space and time. We also obtained 12 intact sediment cores from a single site that we manipulated ex situ to simulate nutrient fluxes across the sediment-water interface during periods of quiescence (via diffusion; n = 4) and sediment resuspension due to wind events (via advection; n = 4). Nutrient chemistry in untreated cores (n = 4) was assessed as well. We compared our high spatial and temporal resolution datasets with long-term (1993-2018) water quality data for the reservoir. Bulk sediment N, P, C, and organic matter concentrations did not significantly (p > 0.05) differ between the sampling dates and generally increased towards the dam. Our ex situ experiment established that diffusive and advective sediment conditions similarly reduced water column N concentrations by 52-58 %, likely from denitrification. In contrast, water column P concentrations increased by 106 % via diffusion and 147 % via advection, possibly due to organic matter decomposition or P desorption. Our long-term nutrient mass balance showed that the reservoir consistently reduced incoming N loads by 25 % but enhanced P loads by 117 %, which is analogous to our core experiment results. Reservoir sediment in agricultural watersheds can therefore act as a sink for N but a source for P over both short and long timescales.