Effects of sewage pollution on coral-reef communities

Sewage pollution is an increasing problem in tropical marine environments. In this revlew we synthesize present knowledge of the effects of sewage pollution on coral-reef communities, and suggest directions for future research. A wide range of sewage impacts on coral-reef communities has been reported. Little or no impact has been observed on some reefs in well-flushed waters that receive small quantities of effluent, whereas large d~scharges of effluent into poorly-flushed lagoons and bays have caused major changes in species composition and abundance. The 3 components of sewage effluent most detrimental to coral communities are nutrients, sediments, and toxic substances. Nutrient enrichment by sewage effluent may enhance benthic algal biomass and primary product~on in the water column. Increased primary production in the water column favors benthic filter-feeding invertebrates which, with the benthic algae, may out-compete corals and other reef-building organisms. Anthropogenic inputs of dissolved nutrients and organic particulate matter may also depress oxygen levels. While heavy sediment loads on corals may be lethal, lesser quantities may inhibit growth, cause changes in the growth forms of colonies, decrease coral cover, alter species composition of reef-building organisms, and inhibit coral recruitment. Toxic substances may induce metabolic changes in corals, decrease rates of growth and reproduction, or reduce viability of corals. Although further research is needed on all three major components of sewage effluent, the most critical need is for comprehensive, long-term studies of sewage impacts. The combined effects of particulate and toxic substances are especially poorly documented at this time.