Spatial and temporal patterns in reef sediment accumulation and composition, southwestern insular shelf of Puerto Rico

Effects of terrigenous sedimentation are considered a serious threat to Puerto Rico's coral reefs. This study assesses: 1) the composition of sediments accumulating at reef sites on the southwestern shelf of Puerto Rico; 2) the spatial extent to which terrigenous materials are reaching these reefs; and 3) the spatial and temporal variability of sediment composition, grain size and trap collection rates. Sediment traps were deployed at 9 sites from inner shelf to shelf edge. Analyses of total sediment weight (April 2006 – April 2007) show a similar temporal pattern at all sites with June and August having the highest accumulation rates. In general, there is a decrease in trap accumulation rate with depth. Carbon composition analyses indicate that samples consist primarily of calcium carbonate with lesser amounts of terrigenous and organic material, and a slightly higher percentage of terrigenous material in the <63 µm fraction. At a given site, the percentage of terrigenous material is fairly constant regardless of trap collection rate, suggesting that observed changes in accumulation rates are due to resuspension of existing material rather than an influx of new terrigenous material. These results, illustrate that in some reef systems, resuspension of existing bottom sediments is as important as the influx of new terrigenous material.