CHARACTERIZING NUTRIENT FLUXES AT THE SEDIMENT-WATER INTERFACE IN A EUTROPHIC MIDWESTERN RESERVOIR

Carlyle Lake, an impoundment of the Kaskaskia River in southwestern Illinois, drains a predominately agricultural watershed (90% of the land cover) and is subject to periodic algal blooms. The spatial distribution of nutrients in the water column (SRP, NO_x-N, NH₃-N) and sediments (total P, C, and N) were characterized to understand patterns of nutrient loading and storage. Additionally, 15 intact sediment cores were collected and manipulated in the laboratory to examine mechanisms of nutrient flux across the sediment-water interface (SWI) that may trigger algal blooms. Core treatment conditions included simulated advection (shaking to mimic potential nutrient fluxes during wind events) and diffusion (using site water or nutrient-free DI water). Spatially, water column nutrients sampled during a drier period were highest near the dam wall. The inlet to the lake was sampled during flooding and had overall higher nutrient concentrations and lower chlorophyll levels compared to samples taken when the lake level was lower. Mean sediment total P was 936 mg/kg, but ranged from <800 mg/kg near inlet tributaries up to 1,661 mg/kg at the dam. Total P was significantly correlated (p < 0.001) with sedimentary organic matter and total C. Intact core experiments of advective flux along the SWI showed SRP decreased from 0.117 ppm to below detection in 6 hr, but NO_x-N and NH₃-N increased linearly over the first 5 d of the experiment (0.015 ppm/hr and 0.0013 ppm/hr, respectively). For the diffusive treatments, site water cores showed no significant change in water column nutrient levels over the experiment, whereas DI water treatments resulted in increasing dissolved P and N-species concentrations over 22 d (SRP = 0.0016 ppm/hr, NO_x-N = 0.0007 ppm/hr, NH₃-N = 0.0023 ppm/hr). Manipulated core sediments had lower extractable NO_x-N compared to initial/untreated cores (2.56 - 2.86 mg/kg less; p < 0.001), but there was no significant difference in extractable NH₃-N and total P. Findings indicate both advective and diffusive conditions at the SWI may trigger the flux of nutrients important for algal growth from lake sediments: diffusion contributes both N and P to depleted water columns, while advection increases water column N, but decreases dissolved P.