Abstract. 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 < 2) resulting from the influence of
terrestrial carbon input and light limitation on photosynthesis. High
NCPO2Ar rates (105.0±59.2 mmol C m−2 d−1, up to 235.4 mmol C m−2 d−1) were observed in the Mississippi and Atchafalaya
plumes at intermediate salinities between 15 and 30 where light and nutrients
were both favorable for phytoplankton production. NCPO2Ar rates
observed in the high-salinity, oligotrophic offshore waters (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.
Spatial distribution of lipid biomarkers, dinoflagellate cysts and pollen in coastal marine surface sediments in the northern Gulf of Mexico
