Impact of Sea Grass Density on Carbonate Dissolution in Bahamian Sediments

Carbonate dissolution has been widely observed in shallow water tropical sediments. However, sediment budgets have generally not been closed with respect to the amount of acid required to produce the observed carbonate dissolution. Recently it has been suggested that enhanced oxygen transport into sediments through the roots and rhizomes of sea grasses might play a role in resolving this mass balance problem. We conducted studies of sea grass–carbonate sediment interactions around Lee Stocking Island, Exuma Islands, Bahamas to further examine this problem. Our studies showed that alkalinity, total dissolved inorganic carbon (SCO2) and Ca21 increased with depth in the pore waters, while pH and calculated carbonate ion concentration decreased with depth. These observations are consistent with the occurrence of carbonate dissolution in these sediments. The magnitude of pore water alkalinity, SCO2, and Ca21 changes was also related to sea grass density, with the largest gradients seen in the sediments of dense sea grass beds. Calculations suggested that less than ;50% of the O2 needed to drive aerobic respiration (and ultimately carbonate dissolution via CO2 production) could be supplied by transport processes such as diffusion, bioturbation, and physical pore water advection. Furthermore, the O2 needed to balance the carbonate dissolution budget could be provided by the transport of ,15% of the photosynthetically derived O2 to the sediments through sea grass roots and rhizomes without enhancing the removal of carbonate dissolution end products. Thus sea grasses play an important role in controlling the rates of carbonate dissolution in shallow water tropical marine sediments. The dissolution of calcium carbonate in marine sediments is an important component of the oceanic and global carbon cycle on a variety of spatial and temporal scales (e.g., Morse and Mackenzie 1990; Milliman 1993). Carbonate dissolution occurs in aerobic sediments as a result of aqueous CO2 production during oxic remineralization of sediment organic matter (SOM, or CH2O here), expressed here approximately as CH2O 1 O2 → CO2 1 H2O (1) (ignoring any alkalinity contributions associated with N and P remineralization, as well as acid production associated with the oxidation of metal sulfides such as FeS). In sediments overlain by bottom waters supersaturated with respect to calcium carbonate, the metabolic acid produced by this reaction is first neutralized by titration with dissolved carbonate ion (e.g., Emerson and Bender 1981), producing bicarbonate according to, CO2 1 H2O 1 CO → 2HCO 22 2 3 3 (2) 1 Corresponding author ([email protected]).