A comparison of inorganic carbon dynamics in two subtropical estuaries during spring floods: Apalachicola Bay, FL, and St. Joseph Bay, FL

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). For this reason, pH, dissolved inorganic carbon (DIC), alkalinity and pCO2 are reported for two adjacent, subtropical, hydrologically distinct estuaries located on the Florida panhandle in the northeast Gulf of Mexico. Apalachicola Bay, FL, is a river-dominated system with an extensive floodplain that relies on spring flooding to deliver nutrients and dissolved and particulate material to drive algal production in March/April. Conversely, St. Joseph Bay receives no influential terrestrial freshwater inflows; precipitation, small creeks, an aquifer and the Gulf County Canal are its major sources. During the annual spring flooding this year, samples collected from Apalachicola River and East Bay sites recorded low pH values, between 5.75 and 6.26, most likely due to high influxes of tannins and fulvic acids washed into the bay from uplands (Livingston, 1984). Dry Bar and West Pass had higher pH values (6.90 and 7.38, respectively), which resulted from the mixing of river water with higher pH Gulf water. The river and Dry Bar (mid-bay) samples had similar DIC (610 µatm/kg and 590 µatm/kg); however, Dry Bar samples had twice the carbonate alkalinity of river samples and therefore much lower pCO2 values. Biological activity could also account for lower pCO2 in the bay. Even though all Apalachicola Bay sites acted as sources of CO2 to the atmosphere, those sites influenced by Gulf water had the lowest pCO2 values, with West Pass at 1,020 µatm, Dry Bar at 2,269 µatm and upriver at 10,826 µatm. St. Joseph Bay is a high salinity lagoon ringed by extensive sea grass beds. The pH at St. Joseph Bay sites were higher and had a smaller range, 8.049 to 8.184, than Apalachicola Bay samples, and their DIC values were two to three times higher than the DIC in Apalachicola Bay. Sites 1 and 2 (at Port St. Joe) had the highest (1918 µm/kg) and lowest (1688 µm/kg) DIC values, with samples from four other sites having DIC near 1800 µm/kg. The lowest alkalinity at a St. Joseph Bay site, (site 2, 1952 µm/kg) was nearly three times higher than the highest Apalachicola Bay alkalinity at West Pass (701 µm/kg). Of the six sites sampled, only station 1 had a pCO2 equal to the present atmospheric concentration of 389 µatm. The pCO2 of the other five sites ranged between 254 and 324 µatm, with St. Joseph Bay acting predominantly as a CO2 sink.