Dissolved carbon export and CO2 outgassing from the lower Mississippi River - Implications of future river carbon fluxes

Air temperatures, precipitation, and river discharge have been steadily increasing in many river basins over the last century. This is especially the case with North America's largest river basin, the 3.2-million-km(2) Mississippi River Basin, which comprises over 41% of the total land surface area in the contiguous USA. Yet we lack an understanding of how these changes will influence the export of terrigenous carbon through river systems into coastal waters and the atmosphere. This study investigated concentrations and mass fluxes of DOC, DIC, and CO2 during 2015-2018 in the Lower Mississippi River. We found an annual riverine export of 3.95 Tg C and 12.25 Tg C for DOC and DIC into the Gulf of Mexico, with 75 and 67% of their respective loads discharged during the wet season from January through July. The riverine mass exports represent a flux rate of 1.4 g DOC-C m(-2) and 4.2 g DIC-C-2 from the Mississippi River Basin, which are considerably higher than those reported for other river basins. Surface water of the Lower Mississippi River was constantly supersaturated by dissolved CO2, with its partial pressure (pCO(2)) ranging from 526 to 3961 mu atm and an average outgassing rate of 654 g C m(-2) yr(-1). Export of DOC and pCO(2) measurements were substantially greater than estimates reported over ten years prior, accompanied by significant increases in river discharge and air temperatures. These findings strongly suggest that DOC and CO2 fluxes from the Mississippi River Basin into coastal waters and the atmosphere will very likely increase in the future due to increasing biological productivity and subsequent carbon export to rivers under increasing discharge and air temperatures.