TERRESTRIAL SEDIMENT ACCUMULATION IN A CORAL REEF ENVIRONMENT, LA PARGUERA, PUERTO RICO, U.S.A

Sediment runoff from coastal development is widely considered a major threat to the viability of coral reefs. To evaluate this concern, the delivery of terrestrial and marine-derived sediment into a coral reef environment adjacent to La Parguera, PR was examined on a cross-shelf transect over a 100-year time-scale via high-resolution radionuclide and sedimentological techniques. La Parguera is a small coastal village on Puerto Rico's southwestern coast that has experienced considerable land use change over the past century. With its increasing popularity as a tourist destination, La Parguera has undergone significant mangrove and seagrass bed removal for the construction of marinas, hotels, and residences. This development is anticipated to have enhanced erosive processes and the delivery of terrestrial sediment to the reef system. Shore cores (<2.0m) were collected from inter-reef sediments along a transect in a protected bay, backreef, and forereef environment. Cores were analyzed to determine mass/sediment accumulation rates (MAR/SAR) and sediment type (siliciclastic vs. carbonate). MAR and SAR values vary considerably (0.28-1.3 g/cm²/y and 0.21-1.3 cm/y) and show a general correlation with core localities. MAR and SAR values generally decrease with increasing distance from the land, with greatest rates found in backreef areas where the seabed is more sheltered from wave attack. Carbonate percentages exhibit the reverse relationship, an increase with increasing distance from land. Collectively, these suggest a notable (up to 2X) increase in the amount of terrestrial sediment accumulating in discrete areas (e.g., backreef), although mixing effects cannot be ruled out. La Parguera's reefs are relatively healthy compared to other areas in the Caribbean, and the physiological and ecological impacts of these changes are unknown. However, data suggest backreef lagoonal areas may be at greatest risk to stresses (e.g., reduced light levels) associated with increases in the accumulation of terrestrial sediments.