Effects of water turbidity and sedimentation on the community structure of puerto rican corals

The community structure of hermatypic corals off the west coast of Puerto Rico is quantitatively described in terms of species composition and diversity patterns. Observations were made along a series of line transects from 8-20-m depths. An underwater laboratory provided opportunity for extended field work by saturation diving. The area studied is heavily stressed by alluvial sediments originating from two rivers that discharge into the shelf area close to the reef. The coral community structure on a steep slope (upper East Reef. 11-17-m depth) is compared to a flat reef (West Reef, 20-m depth). Water turbidity and sedimentation seem to be the major factors Thai dictate the distribution of corals in the different reef zones. While coral diversity and living cover at the upper East Reef were high (H'x = 2.196, cover = 79%) and the average water turbidity and sedimentation were low (1.5 Formazin Turbidity Units and 3.0 mg/cm2/day, respectively), coral diversity and cover at the West Reef were low (H'n = 1.830, cover = 30%) and the average water turbidity and sedimentation were appreciably higher (5.5 FTU and 15 mg/enr/day, respectively). It is suggested that the differences in community structure between upper East Reef and West Reef are attributable to turbidity and sedimentation. Montastrea cavernosa was the most abundant species and the major frame-builder of the reefs. This species reflects most of the morphological features typical of corals having greater efficiency in sediment rejection. Other species that seemed to succeed in the heavy regime of sedimentation at the West Reef included Siderastrea radians, S. sidcrca, Diploria strigosa, and Meandrina meandrites.