EPIPHYTES ON THALASSIA TESTUDINUM – A MAJOR SOURCE OF CARBONATE SEDIMENT? A CASE STUDY FROM GRAND CAYMAN, BRITISH WEST INDIES

Sea grasses have significantly influenced sedimentation rates in coastal regions since their appearance in the late Cretaceous. The plants are integral to shallow water ecosystems and help to stabilize coastlines worldwide. Thalassia testudinum, which is the most common sea grass in the Caribbean Sea, contributes to carbonate sedimentation by: (1) causing deposition of suspended sediment through reduction of current flow, (2) stabilizing substrates with its complex root system, and (3) providing substrates suitable for epiphytes that, upon death, become part of the sediment. There is, however, little information on the composition of the epiphytic biota on Thalassia and estimates for the amount of sediment generated by epiphytes ranges from nothing to all of the calcareous mud found in a lagoon. Through identification of the epiphytes living on Thalassia, the contribution they make to sedimentation can be better understood.

Extensive Thalassia banks are found in the shallower, near shore areas of the lagoons around Grand Cayman. Diverse assemblages of epiphytes live on leaves of Thalassia within this study area. Coralline red algae, which are the most common epiphytes, colonize Thalassia leaves starting at the tip and gradually extend toward the base of the plant. Other epiphytes include various species of foraminifera, gastropods, coccoliths, ostracods, and sponges. All of these organisms have calcareous skeletons that become part of the sediment once they have died, either separating from the Thalassia leaves upon death, or when the leaves themselves die. Various species of diatoms, which are commonly not mentioned in literature to date, are also found on the sea grass in the lagoons and upon their death will contribute silica to the system. Preliminary data indicate that there is an order of colonization of the leaves in all but one of the lagoons in Grand Cayman. The nutrient levels within a lagoon influence the presence of the most common epiphyte (coralline algae) making it impossible to accurately quantify the overall impact these epiphytes have on sedimentation.