Role of the Atchafalaya River Basin in regulating export fluxes of dissolved organic carbon, nutrients, and trace elements to the Louisiana Shelf

The abundance and fluxes of selected dissolved trace elements (TE), nutrients, and dissolved organic carbon (DOC) were quantified in the Atchafalaya River Basin to better understand its influence on the chemical regime of the Louisiana Shelf, a region where bottom water hypoxia occurs annually during late spring to early fall. Water samples were collected from throughout the entire Atchafalaya River Basin including from the Mississippi and Red Rivers as well as basin swamp waters during April and November 2010, and June 2011, which represent "typical" high and low, and "unusual" high river discharges, respectively. Within the total dissolved (<0.45 mu m) pool, most of the TEs were mainly partitioned to the <0.02 Jim dissolved phase with the exception of Cr, Cs, Fe, Pb and Zn which were dominantly in the colloidal (0.02-0.45 Jim) phase. In the Atchafalaya River, seasonal concentration variations in nutrients, DOC and most TEs were similar to those in the Mississippi River, reflecting a major contribution of water from the Mississippi River. Contributions of the Red River to the Atchafalaya River's DOC and nutrients were estimated to be 1-35%, similar to previous estimates for this system. The Red River contribution to the fluxes of Co, Cr, Cs, Fe, Mn, Pb, Rb, and Zn was generally disproportionally high (>20%), exceeding its hydrological contribution (similar to 10%) to the Atchafalaya River, due to greatly enriched concentrations of these elements in the Red River. The Atchafalaya River Basin wetland/floodplain contribution to chemical constituents was noticeable, accounting for more than a 20% decrease of some elements (Cd, Cs, Cr, Cu, Fe, Mo, P, U, and Zn). Mn was the only element with persistent addition during all sampling campaigns. However, the contributions from the Red River were greater than the wetland/floodplain contributions for DOC, nutrients and most of TEs (except Mn), suggesting that the Red River plays a disproportionate role in regulating water quality in the Atchafalaya River. Overall, the Atchafalaya River with its chemical modification of Mississippi River water via contributions from the Red River and interactions with floodplain wetlands, plays a critical role in chemical distributions on the Louisiana Shelf in the Northern Gulf of Mexico. Thus, the Atchafalaya River contribution should be adequately accounted for in biogeochemical studies and models of trace elements and nutrients on the Louisiana Shelf, particularly during periods of bottom water hypoxia. (C) 2019 The Authors. Published by Elsevier B.V.