Proceedings of the 2020 Mississippi Water Resources Conference

Year: 2020 Authors: Nifong R.L., Taylor J.M.

Oxbow lakes are an important but understudied component of river floodplain ecosystems and may serve as nitrogen (N) sinks in agricultural watersheds by creating conditions that promote hotspots and hot moments of denitrification. Denitrification can be an important pathway of reactive nitrogen removal, particularly in waterbodies receiving agricultural runoff. In order to examine seasonal and habitat specific patterns of denitrification and sediment oxygen demand, we collected dissolved gas and nutrient samples during 14 different flow-through intact core incubations over the course of a year within Beasley Lake, an oxbow located off the Sunflower River in the Lower Mississippi River Basin (LMRB). Concurrently, we collected field water samples to examine patterns in ambient water quality. We found that dissolved nutrient and nitrogen gas (N_2-N) fluxes varied in space and time while sediment oxygen demand occurred throughout the year. N_2-N fluxes were positive in at least one habitat during every sampling event. Dissolved nitrate (NO₃^-_-N) fluxes from cores were highest from the early summer through the fall, but became minimal during the winter and early spring. In addition to seasonal patterns of NO₃^-_-N fluxes, we observed positive fluxes of ammonium (NH₄⁺_-N) throughout the year and of phosphate (PO₄^-3-P) during late summer through the winter from sediments. Field water quality measurements were uniformly low with the exception of the planting season when total suspended sediments (TSS), NO₃^-_-N, soluble reactive phosphorus (SRP), and total organic carbon (TOC) peaked. Together, these results provide insight into spatial and temporal patterns of nutrient input and removal within agroecosystems in the LMRB.