Seasonal Mixing and Biological Controls of the Carbonate System in a River-Dominated Continental Shelf Subject to Eutrophication and Hypoxia in the Northern Gulf of Mexico

Large rivers export a large amount of dissolved inorganic carbon (DIC) and nutrients to continental shelves; and subsequent river-to-sea mixing, eutrophication, and seasonal hypoxia (dissolved oxygen < 2 mg?L?1) can further modify DIC and nutrient distributions and fluxes. However, quantitative studies of seasonal carbonate variations on shelves are still insufficient. We collected total alkalinity (TA), DIC, and NO3? data from nine cruises conducted between 2006 and 2010 on the northern Gulf of Mexico continental shelf, an area strongly influenced by the Mississippi and Atchafalaya Rivers. We applied a three-end-member model (based on salinity and potential alkalinity) to our data to remove the contribution of physical mixing to DIC and nitrate distribution patterns and to derive the net in situ removal of DIC and nitrate (ADIC and ANO3?, respectively). Systematic analyses demonstrated that the seasonal net DIC removal in the near surface water was strong during summer and weak in winter. The peak in net DIC production in the near-bottom, subsurface waters of the inner and middle sections of the shelf occurred between July and September; it was coupled, but with a time lag, to the peak in the net DIC removal that occurred in the near-surface waters in June. A similar 2 month delay (i.e., January vs. November) could also be observed between their minima. A detailed examination of the relationship between ADIC and ANO3? demonstrates that net biological activity was the dominant factor of DIC removal and addition. Other effects, such as air?sea CO2 gas exchange, wetland exports, CaCO3 precipitation, and a regional variation of the Redfield ratio, were relatively minor. We suggest that the delayed coupling between eutrophic surface and hypoxic bottom waters reported here may also be seen in the carbon and nutrient cycles of other nutrient-rich, river-dominated ocean margins worldwide. Superscript/Subscript Available

Automated summary

The study found that strong seasonal net DIC removal occurred in the near-surface water during summer on the northern Gulf of Mexico continental shelf, while the net DIC production peak in the near-bottom, subsurface waters was observed between July and September. The delayed coupling between eutrophic surface and hypoxic bottom waters suggests a similar pattern may be seen in other nutrient-rich, river-dominated ocean margins worldwide.