Modification of Coral Reef Zonation by Terrigenous Sediment Stress

Abstract

Four coral reefs nearPonce, Puerto Rico were examined for the effects of terrigenous influx into a reef environment. The four coral reefs are successively farther from a point source of frequently occurring, westward-moving terrigenous sediment plumes which are generated by resuspension of fine-grained sediments. The coral cover was measured from linear photographic transects parallel to each 5 meter depth. Living coral was marked on each photograph, the species identified, and area of cover was measured with a jandel digitizer pad and SigmaScan program. Statistics were compiled for percent cover by species, total cover, and number of colonies. Total coral cover was reduced near the source of terrigenous sediment influx. Coral cover and diversity increased with distance from the source and amounts of sediment trapped on the reefs decreased, suggesting that the plume influx was an important factor contributing to the deterioration of these reefs. Sediment stress has drastically reduced the coral cover and number of species. The reefs with high sediment inputs showed decreased coral species diversity and percent cover. Sediment-resistant coral species tolerated this adverse environment and their percent of cover remained relatively constant. The effects from sediment influx include partial or total burial of coral colonies, bleaching, and colonization of the coral surface by filamentous bluegreen algae and sponges. The reduced light levels resulted in domination of the community by deeper fore-reef coral. INTRODUCTION