Spatial and temporal variability in sediment denitrification within an agriculturally influenced reservoir

Reservoirs are intrinsically linked to the rivers that feed them, creating a river–reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river–reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 lg N2O g ash free dry mass of sediments (AFDM) 1 h 1 or 0–2.7 lg N2O g dry mass of sediments (DM) 1 h 1 at reservoir sites, vs. 0–12 lg N2O gAFDM 1 h 1 or 0–0.27 lg N2O gDM 1 h 1 at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO3 -N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO3 -N concentration up to a threshold of 0.88 mg NO3 -N l , above which it appeared NO3 -N was no longer limiting. Denitrification assays were amended seasonally with NO3 -N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO3 -N during fall and winter when ambient NO3 -N was low. Abbreviations: AFDM – ash free dry mass; ANOVA – analysis of variance; APHA – American Public Health Association; C – carbon; chl a – chlorophyll a; CV – coefficient of variation; DM – dry mass; DOC – dissolved organic carbon; Figure – figure; g – gram; He – helium; h – hour; km – kilometers; l – liter; LSMeans – least-squared means; LSV – Lake Shelbyville, Illinois; m – meter; mg – milligram; ml – milliliter; mM – millimolar; N – nitrogen; N + C – nitrogen plus carbon; N2 – di-nitrogen gas; N2O – nitrous oxide; NH4 + – ammonium; NO3 – nitrate; O2 – oxygen; SE – standard error; TDN – total dissolved nitrogen; TN – total nitrogen; USEPA – United States Environmental Protection Agency; USGS – United Stated Geologic Survey; lg – microgram