ALBERT

Description: Net community production (NCP) in the surface water of the northern Gulf of Mexico (nGOM) and its coupling with the CO2 system were examined during the productive spring season. NCP was estimated using multiple approaches: (1) underway O2 and Ar ratio, (2) oxygen changes during light/dark bottle oxygen incubations, and (3) non-conservative changes in dissolved inorganic carbon or nutrients. These methods all showed high spatial variability of NCP and displayed similar patterns along the river–ocean mixing gradient, showing high production rates in plume regions. NCPO2Ar estimated from high-resolution O2 and Ar underway measurement indicated heterotrophic conditions at the high-nutrient and high-turbidity Mississippi River end (-51.3±11.9 mmol C m−2 d−1 when salinity  35.5) were close to zero due to nutrient limitation. Air–sea CO2 fluxes generally showed corresponding changes, from being a strong CO2 source in the river channel (55.5±7.6 mmol C m−2 d−1), to a CO2 sink in the plume (-13.4±5.5 mmol C m−2 d−1), and to being nearly in equilibrium with the atmosphere in offshore waters. Overall, the surface water of the nGOM was net autotrophic during spring 2017, with an area-weighted mean NCPO2Ar of 21.2 mmol C m−2 d−1, and was a CO2 sink of −6.7 mmol C m−2 d−1. A temporal mismatch between in situ biological production and gas exchange of O2 and CO2 was shown through a box model to result in decoupling between NCPO2Ar and CO2 flux (e.g., autotrophic water as a CO2 source outside the Mississippi River mouth and heterotopic water as a CO2 sink in the Atchafalaya coastal water). This decoupling was a result of in situ biological production superimposed on the lingering background pCO2 from the source water because of the slow air–sea CO2 exchange rate and the buffering effect of the carbonate system.