Transport of particulate organic carbon by the Mississippi River and its fate in the gulf of Mexico

Trefry, J.H., S. Metz, T.A. Nelsen, R.P. Trocine, and B.J. Eadie

ABSTRACT

This study was designed to determine the amount of particulate organic carbon (POC) introduced to the gulf of Mexico by the Mississippi River and assess the influence of POC inputs on the development of hypoxia and burial of organic carbon on the Louisiana continental shelf. Samples of suspended sediment and supporting hydrographic data were collected from the river and >50 sites on the adjacent shelf. Suspended particles collected in the river averaged 1.8 ( 0.3% organic carbon. Because of this uniformity, POC values (in umol l-1) correlated well with concentrations of total suspended organic matter. Net transport of total organic carbon by the Mississippi-Atchafalaya River system averaged 0.48 X 10^12 moles y-1 with 66% of the total organic carbon carried as POC. Concentrations of POC decreased from as high as 600 umol l-1 in the river to < 0.8 umol l-1 in offshore waters. In contrast, the organic carbon fraction of the suspended matter increased from < 2% of the total mass in the river to > 35% along the shelf at >10 km from the river mouth. River flow was a dominant factor in controlling particle and POC distribution; however, time-series data showed that tides and weather fronts can influence particle movement and POC concentrations. Values for apparent oxygen utilization (AOU) increased from 60 umol l-1 to > 200 umol l-1 along the shelf on approach to the region of chronic hypoxia. Short-term increases in AOU were related to transport of more particle-rich waters. Sediments buried on the shelf contained less organic carbon than incoming river particles. Organic carbon and 13dC values for shelf sediments indicated that large amounts of both terrigenous and marine organic carbon are being decomposed in shelf waters and sediments to fuel observed hypoxia.