The carbon dioxide system on the Mississippi River-dominated continental shelf in the northern Gulf of Mexico: 1. Distribution and air-sea CO flux.

River-dominated continental shelf environments are active sites of air-sea CO exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide (CO) was measured, and the air-sea CO flux was calculated. Results show that CO exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO during spring and early summer, and it was neutral or a weak source of CO to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO in low-salinity zones (0≤S<17) to a strong CO sink in the middle-to-high-salinity zones (17≤S<33), and finally was a near-neutral state in the high-salinity areas (33≤S<35) and in the open gulf (S≥35). High CO values were only observed in narrow regions near freshwater sources, and the distribution of undersaturated CO generally reflected the influence of freshwater inputs along the shelf. Systematic analyses of CO variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen-enhanced biological removal, along with mixing processes, dominated CO variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshore CO distribution and to affect regional air-sea CO flux estimates. Overall, the study region acted as a net CO sink of 0.96 ± 3.7 mol m yr (1.15 ± 4.4 Tg C yr).