Sediment transport and the distribution of shallow-water gorgonians

Abstract

The objective of this study was to provide evidence supporting the contention that sediment transport (bedload) is a major environmental factor controlling the distribution of shallow-water gorgonian species. Gorgonians were surveyed annually from 2003-2008 in 3 replicated sets of transects at 2 shelf edge, 3 midshelf and 3 nearshore reefs on the southwest coast of Puerto Rico. Except for the shelf edge reefs, the transects were located in the fore reef terrace, break and base zones. A Reciprocal Averaging (RA) ordination analysis was used to identify the environmental factor underlying the distribution of gorgonian species. Substratum slope was used as a proxy of bedload levels to assess the overall RA results among and within reefs. Within reef RA scores decreased monotonically from the reef terrace to the slope to the base in all applicable cases. This pattern is consistent with sediment transport because increases in substratum slope from the terrace to the base of reefs should result in decreasing levels of bedload within reefs. RA scores were highly related to substratum slope (r 2 =0.79, p<0.01) supporting the premise that sediment transport controls the distribution of gorgonian species.