Spatial and Temporal Distributions of Carbonate Saturation State on the Mississippi River Dominated Northern Gulf of Mexico

Carbonate saturation state (Ω_Ar) serves as an index of ocean acidification; however, its variation on river-dominated continental shelves remains unclear. Samples of total alkalinity (TA), dissolved inorganic carbon (DIC), and nutrients were taken from nine cruises on the Mississippi and Atchafalaya River dominated continental shelf during 2006 to 2010. The distributions of TA and DIC on sea surface generally follow salinity distributions, i.e. low on the inner shelf and high on the outer shelf. The riverine calcium concentration was considered to calculate Ω_Ar and the result showed high Ω_Ar (4 to 6) along the axis of the river plume trajectory, moderate (~4) on the surface open gulf, and low (< 3) in the deep gulf. Strong seasonal variation of Ω_Ar was observed in the river endmembers, high in spring and low in winter. The Ω_Ar variation was dominated by mixing when salinities < 18. In waters of higher salinities, deficits of DIC from the conservative mixing lines were positively correlated to deficits of NO₃ and the slope of regression line was close to the Redfield ratio after removing data affected by unbalanced Si and N. The fact that Ω_Ar was highly correlated to ΔDIC suggests that Ω_Ar was dominated by biological activity. Finally, model simulations suggest that the effect of increasing riverine TA (increasing Ω_Ar in low salinities zone) and the effect of decreasing riverine nitrate flux (decreasing Ω_Ar in middle-to-high salinities zone) on Ω_Ar might compensate each other in this study area.