Impact of sea grass density on carbonate dissolution in Bahamian sediments

Abstract

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 (σCO2) and Ca2+ 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, σCO2, 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.