EFFECTS OF CHRONIC HIGH SEDIMENTATION ON FACIES PATTERNS, CORAL ABUNDANCE, DIVERSITY AND MORPHOLOGY – LAS CLAVILLINAS, DOMINICAN REPUBLIC

Spectacular outcrops in the Enriquillo Valley of the western Dominican Republic provide an opportunity to study the character of Holocene reefs and the processes that shaped them. An outcrop near Las Clavillinas reveals a transition from coastal plain to forereef facies over a distance of only 100 meters. A study was conducted to compare facies patterns within this outcrop to those documented at Cañada Honda, 7 km to the west and toward the head of the deeply embayed shoreline. Data on coral diversity, abundance and morphology were collected using 1-m by 1-m quadrats (100 points) placed successively along 8 vertical transects through all major reef facies. The reef's proximity to shore and the steep watershed behind argue for at least periodic heavy sedimentation. However, the number of coral species identified in both outcrops (21 at Las Clavallinas; 28 at Cañada Honda) and the high coral abundance (>50%) rival more open Caribbean reefs and are greater than values reported for many modern, sediment-stressed reefs. The primary reef builders at Las Clavillias include: A) branching corals, dominated by Acropora cervicornis, and B) massive species, most notably Siderastrea spp. At Cañada Honda, coral morphology varies from hemispherical (mostly Colpophyllia natans) to conical (mostly Siderastrea spp.) to columns (Montastrea faveolata) along a demonstrated gradient of increasing sediment stress. At Las Clavallinas, the shape of Siderastrea spp. generally shifts from conical to columnar, presumably a response to sediment stress higher than that at Cañada Honda. In addition, the abundance of the slightly less sediment-tolerant M. favolata is lower than at Cañada Honda (1% vs 2.8%) and colony size is greatly reduced (<10 cm vs. >30-40 cm) – again a response to elevated sediment stress. While the abundance of M. favolata is small at both sites, it contributes measurably to patterns revealed by cluster analysis, and its reduction is felt to be significant. Further studies on sediment type and coral growth rate are underway. This study has added to our understanding of changes in local reef-community structure and coral morphology in response to elevated sedimentation. The higher-than-expected coral abundance and diversity may in turn contrast the effects of natural, chronic sedimentation with acute stress of anthropogenic origin.