NUTRIENT CYCLING IN THE LOWER MISSISSIPPI RIVER FLOODPLAIN: ROLE OF BACKWATER ENVIRONMENTS IN NITROGEN AND PHOSPHORUS UPTAKE

Abstract

The Mississippi River watershed encompasses more than a third of the nation’s land area, and is home to much of the nation’s agricultural production. As a result of generous fertilizer application, the watershed has incredibly high nitrate and phosphate concentrations. This study investigates the role of backwater environments (backwater lakes, flooded forests, and secondary channels) in nitrogen and phosphorus uptake in the Lower Mississippi River over a two month period in the summer of 2014, using biweekly sampling and analysis for chlorophyll, anion and cation concentrations, membrane inlet mass spectrometry, and stable isotope analysis of nitrate. It asks which environments serve as the largest sink for nitrogen and phosphorus and by which mechanisms the uptake occurs. Our findings indicate that backwater lakes and flooded forests have the highest uptake of nitrate via denitrification, but that over the study time, they did not take up phosphorus. We also present evidence to support that the main channel and secondary channels are sites of denitrification, but not over the short distances measured in this study.