The carbon dioxide system on the <scp>M</scp>ississippi <scp>R</scp>iver‐dominated continental shelf in the northern <scp>G</scp>ulf of <scp>M</scp>exico: 1. Distribution and air‐sea CO₂ flux

Abstract

River-dominated continental shelf environments are active sites of air-sea CO₂ 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 (<i>p</i>CO₂) was measured, and the air-sea CO₂ flux was calculated. Results show that CO₂ exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO₂ during spring and early summer, and it was neutral or a weak source of CO₂ to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO₂ in low-salinity zones (0≤S<17) to a strong CO₂ 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 <i>p</i>CO₂ values were only observed in narrow regions near freshwater sources, and the distribution of undersaturated <i>p</i>CO₂ generally reflected the influence of freshwater inputs along the shelf. Systematic analyses of <i>p</i>CO₂ variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen-enhanced biological removal, along with mixing processes, dominated <i>p</i>CO₂ variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshore <i>p</i>CO₂ distribution and to affect regional air-sea CO₂ flux estimates. Overall, the study region acted as a net CO₂ sink of 0.96 ± 3.7 mol m^-2 yr^-1 (1.15 ± 4.4 Tg C yr^-1).