ABUNDANCE OF THE EXCAVATING SPONGE CLIONA DELITRIX IN RELATION TO SEWAGE DISCHARGE AT SAN ANDRÉS ISLAND, SW CARIBBEAN, COLOMBIA

Abstract

It is known that the encrusting and excavating Caribbean sponge Cliona delitrix may increase its abundance near sources of sewage. To ascertain whether its current conspicuousness in leeward reefs of San Andrés Island (SW Caribbean, Colombia) is related to organic pollution from local raw sewage discharges, quantitative data on density and cover of this sponge and other benthic components was obtained from belt and line transects at seven stations along the shallow (5-10 m deep) terrace. Coral mucus was sampled to quantify Escherichia coli bacteria, as an approximate indicator of sewage plume influence on benthic biota. A negative multiplicative regression between amount of E. coli in coral mucus and distance from the main raw sewage outlet demonstrated the domestic-wastes origin of the bacteria. Whereas significant E. coli counts occurred only up to 1-2 km from sewage sources, overall sewage influence may extend further as moderate C. delitrix abundances occurred throughout the West shallow terrace of San Andrés, apparently associated to the overall nutrient enrichment from sewage. C. delitrix abundances were lower in the Southwest, farthest from sewage influence, and generally increased towards sewage sources, but decreased near the main sewage outlet. Close to sewage sources, any positive effect on the sponge brought about by the increase in suspended organic matter is probably outweighed by the negative effect that excessive sedimentation has on the sponge itself, and on the quantity and quality of substratum available for settlement.