Abstract
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 (ΔDIC and ΔNO3–, 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 ΔDIC and ΔNO3– 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.
Highlights
Continental shelves receive large and highly variable amounts of freshwater, dissolved inorganic carbon (DIC), and nutrients
We adopted a river end-member a few weeks earlier than the sampling cruise to perform the calculation in the section “Net Dissolved Inorganic Carbon Removal and Nitrate Removal Over the Water Column.”
We considered the horizontal dispersal (Figures 6, 7) and vertical stratification (Figure 7) and organized the monthly DIC on specified conditions on the inner-to-middle shelf to quantify the spatial–temporal DIC variations. (Note that DIC was from the calculation in the section “Net Dissolved Inorganic Carbon Removal ( DIC) and Nitrate Removal ( NO3) Over the Water Column” by the three-end-member mixing.) In the nearsurface water, the average of DIC values increased from 14.3 μmol·kg−1 in January to 126.6 μmol·kg−1 in June and subsequently decreased to nearly zero (–2.9 μmol·kg−1)
Summary
Continental shelves receive large and highly variable amounts of freshwater, dissolved inorganic carbon (DIC), and nutrients. As such, they may experience seasonal coastal enhanced primary productivity and bottom-water hypoxic [dissolved oxygen (DO) less than 2 mg·L−1] events (Borges and Gypens, 2010; Rabalais et al, 2010; Bauer et al, 2013; Breitburg et al, 2018; Oschlies et al, 2018). Excessive amounts of riverine nutrients can induce eutrophication, which fuels primary production, removing DIC from the water, increasing the flux of organic matter, and subsequently leading to hypoxic events whereby DIC is released back to the water. They found that significant shifts could occur as a result of changes in freshwater discharge and wind direction, where the latter was persistent and favored upwelling
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