Biogeochemical cycling of carbon dioxide in estuaries and the continental shelf of the southeastern United States
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In the first half of the dissertation, partial pressure of carbon dioxide (pCO2), dissolved inorganic carbon (DIC), and total alkalinity (TA) were measured at both high tide and low tide in the surface water of three Georgia estuaries from September 2002 to May 2004. Of the three estuaries, Sapelo and Doboy Sounds are marine-dominated estuaries, while Altamaha Sound is a river-dominated estuary. During all sampling months, the three estuaries were supersaturated in CO2 with respect to the atmosphere (390 - 3380 μatm). The calculated annual air-water CO2 flux in Altamaha Sound (69.3 mmol m-2 d-1) is 2.4 times that of Sapelo and Doboy Sounds (28.7-29.4 mmol m-2 d-1). The higher CO2 degassing in the river-dominated estuary is largely fueled by CO2 loading from the river. Due to the substantial differences between river- and marine-dominated estuaries, current estimates of air-water CO2 fluxes in global estuaries (which are based almost entirely on river-dominated estuaries) could be overestimated. In the second half of the dissertation, surface water pCO2, as well as vertical distributions of DIC, TA, dissolved oxygen (DO), dissolved organic carbon (DOC), pH, nitrate (NO3-) plus nitrite (NO2-), phosphate (PO43-), silicic acid (H4SiO4), δ13C and Δ14C of DIC, and δD and δ18O of H2O, was measured over the entire continental shelf from North Carolina to Florida (also known as South Atlantic Bight, or SAB) during six months from January 2005 to May 2006. Results indicate that the SAB is a net sink of atmospheric CO2 on an annual and whole shelf basis (-0.48±0.21 mol m-2 yr-1). The inner shelf is a source of +1.20±0.24 mol m-2 yr-1, while the middle and outer shelves are sinks of -1.23±0.19 and -1.37±0.21 mol m-2 yr-1, respectively. After removing pCO2 variations due to the annual temperature cycle and air-sea gas exchange, residual pCO2 is calculated. Residual pCO2, along with salinity, DIC, DOC, is then used to evaluate mechanisms controlling pCO2 in this region. DIC results show that SAB is strongly controlled landside by carbon inputs and freshwater dilution from terrestrial sources, and seaside by biological activity on the middle and outer shelf.