Biogeochemical processing along the continuum from the lower Atchafalaya River to the northern Gulf of Mexico

The largest hypoxic area in the US (and second largest in the world) forms each summer in the northern Gulf of Mexico (GOM) downstream of the Mississippi (MR) and Atchafalaya Rivers (AR), the primary sources of freshwater, sediment, nitrogen, and phosphorus delivery to the region.  The AR is regulated to be ~30% of the combined flow of the Red River and diverted MR water.  While 100% of the AR discharge is delivered to the hypoxic region of the shelf, only about half of the MR flow goes westward toward the region. In June 2008, we initiated a monthly sampling program, consisting of 20 stations, to characterize net changes in nutrient and organic matter concentrations as water moves below USGS monitoring stations on the lower AR, through Atchafalaya Bay and into the northern GOM; greatly improving current estimates (that rely on concentrations from upstream locations in MR and AR or from MR plume) of the fluxes of organic matter and nutrients to offshore regions of the GOM experiencing seasonal coastal hypoxia.  Specifically, we will present monthly patterns of nitrate, ammonium, dissolved organic nitrogen, phosphate, silicate, dissolved organic carbon, particulate organic carbon and nitrogen, total nitrogen, phosphorus, and carbon concentrations and fluxes.