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Abstract There is limited information on how the nutrient and freshwater input affects water column carbonate chemistry in the estuaries along the northern Gulf of Mexico. In this study, we assess the seasonal and spatial variability in carbonate chemistry in the Barataria Basin, a eutrophic estuary adjacent to the mouth of the Mississippi River. Eleven stations were sampled along a salinity gradient during the winter (January), spring (April), summer (July), and fall (October) of 2021. Surface and bottom water samples were collected for the analyses of dissolved inorganic carbon (DIC); total alkalinity (TA); and nitrite plus nitrate (NO₂ + NO₃), phosphate (PO₄), and dissolved silica (SiO₄). Dissolved CO₂(pCO₂) was measured in the surface water. Seasonal surface DIC and TA values ranged from 1553 to 2582 μmol kg⁻¹and 1217 to 2217 μmol kg⁻¹, respectively. DIC and TA varied seasonally and showed an increasing trend from fresh stations to saline stations. The highest DIC and TA values were observed during the fall season, likely due to the increased contribution of DIC and TA from adjacent marshes as a result of enhanced porewater exchange. In contrast to DIC and TA, pCO₂decreased with the increase of salinity. The seasonal and spatial patterns in carbonate chemistry could not be explained solely by physical mixing and reflected complex interactions between biogeochemical processes driven by nutrient supply and temperature as well as tidal flushing and material exchanges with adjacent marshes. 

Abstract The seabed and the water column are tightly coupled in shallow coastal environments. Numerical models of seabed‐water interaction provide an alternative to observational studies that require concurrent measurements in both compartments, which are hard to obtain and rarely available. Here, we present a coupled model that includes water column biogeochemistry, seabed diagenesis, sediment transport and hydrodynamics. Our model includes realistic representations of biogeochemical reactions in both seabed and water column, and fluxes at their interface. The model was built on algorithms for seabed‐water exchange in the Regional Ocean Modeling System and expanded to include carbonate chemistry in seabed. The updated model was tested for two sites where benthic flux and porewater concentration measurements were available in the northern Gulf of Mexico hypoxic zone. The calibrated model reproduced the porewater concentration‐depth profiles and benthic fluxes of O₂, dissolved inorganic carbon (DIC), TAlk, NO₃and NH₄. We used the calibrated model to explore the role of benthic fluxes in acidifying bottom water during fair weather and resuspension periods. Under fair weather conditions, model results indicated that bio‐diffusion in sediment, labile material input and sediment porosity have a large control on the importance of benthic flux to bottom water acidification. During resuspension, the model indicated that bottom water acidification would be enhanced due to the sharp increase of the DIC/TAlk ratio of benthic fluxes. To conclude, our model reproduced the seabed‐water column exchange of biologically important solutes and can be used for quantifying the role of benthic fluxes in driving bottom water acidification over continental shelves. 

Abstract Rivers and wetlands are a major source of terrestrial derived carbon for coastal ocean margins. This results in a net loss of terrestrial carbon into the shelf water and their subsequent transport to interior ocean basin. This study investigates the transport of dissolved inorganic carbon (DIC) in the surface‐mixed layer of Louisiana Shelf in northern Gulf of Mexico (nGOM) adjacent to the Wax Lake Delta (WLD) and Barataria Bay (BB), which represent contrasting net land gain and net land loss areas in this region. DIC samples were collected, in conjunction with short‐lived radium isotopes²²⁴Ra (t_1/2 = 3.66 days) and²²³Ra (t_1/2 = 11.43 days) samples during June and September 2019, to quantify shelf transport of DIC in the surface‐mixed layer during period of high and low river flow, respectively. Radium distribution implied shelf mixing rates of 140–6,759 and 63–2,724 m² s⁻¹for WLD and BB regions, respectively, with more than tenfold decrease in rates between the two seasons. Net shelf transport of DIC was found to be highest for the WLD region in June, highlighting the importance of freshwater discharge in exporting DIC. An upscaling of our study for the entire Louisiana Shelf indicates that 1.54–20.19 × 10⁹ mol C d⁻¹transported in June 2019 and 0.34–8.12 × 10⁹ mol C d⁻¹in the form of DIC was exported across the shallow region of the shelf during high and low river flow seasons, representing an important source of DIC to the NGOM.