Dissolved inorganic carbon dynamics in two subtropical estuaries: Apalachicola Bay, FL, and St. Joseph Bay, FL

Background/Question/Methods

Currently, the world’s marginal seas pump CO₂ into the ocean and act as carbon sinks (Tsunogai et al. 1999). However, recent estimates suggest that CO₂ release by estuaries nearly balances the continental shelf sink (scaled globally), but data from estuaries is sparse (Frankignoulle et al.1998; Tsunogai et al., 1999).Except for work in the South Atlantic Bight (Cai and Wang, 1998; Cai et al. 2000), there is a paucity of data describing carbon fluxes in subtropical and tropical systems. For this study, pH,dissolved organic carbon (DIC), alkalinity and pCO₂were measured in two adjacent but hydrologically distinct estuaries located in the northeast Gulf of Mexico. Apalalachicola Bay, FL, is a dynamic, river-dominated system that relies on spring floods to deliver nutrients and dissolved and particulate material to drive algal production, and is subject to drought in autumn. Conversely, St. Joseph Bay, a high salinity lagoon ringed by sea grass beds, receives no influential terrestrial freshwater inflows.

Results/Conclusions

In this study, we sampled six sites in each bay during spring and fall seasons. Following the annual spring flooding, Apalachicola River and East Bay recorded low pH values between 5.75 and 6.26 due to a high influx of tannin, lignin and fulvic acid complexes washed into the bay from upland swamps. In addition, low dissolved inorganic carbon (DIC) values, ranging from 125 µm/kg and 734 µm/kg, caused Apalachicola Bay to act as a source of CO₂ to the atmosphere, with pCO₂ values ranging from 1,020 µatm (West Pass) to 10,826 µatm (upriver site). St. Joseph Bay acted mainly as a CO₂ sink. St. Joseph Bay pH ranged from 8.049 to 8.184, and its DIC values were two to three times higher than the DIC in Apalachicola Bay, ranging from 1688 to 1918 µm/kg. Of the six sites sampled, only one station’s pCO₂ value was equal to the present atmospheric concentration of 389 µatm. The pCO₂ of the other five sites was between 254 and 324 µatm. The influx of low pH, low alkalinity river water influenced by upland inputs caused Apalachicola Bay to be a source of CO₂. St. Joseph’s Bay, having no significant freshwater inputs, had higher pH, DIC and alkalinity associated with Gulf of Mexico water, and was therefore a CO₂ sink.