Sediment addition drives declines in algal turf yield to herbivorous coral reef fishes: implications for reefs and reef fisheries

Coral reefs around the world are changing rapidly, with overfishing of herbivorous fishes and increased sediment inputs being two of the major local-scale stressors. We therefore assessed the effects of sediment loads and overfishing on the nutritional quality and yield to grazing fishes of algal turfs, within the epilithic algal matrix, on a coral reef at Lizard Island, Australia. Low, ambient and high sediment loads were maintained on turf-covered coral tiles, with and without grazer exclusion cages, for 1 month. Subsequently, algal turfs were removed and analysed for organic carbon and nitrogen content. Under grazer exclusion, sediment additions decreased algal turf biomass by approximately 63%, while algal turf biomass was the highest on tiles with sediments removed. In the presence of grazing fishes, algal turfs in all treatments were cropped by grazers to similar low biomass levels. Nitrogen content of algal turfs followed a similar trend. Effectively, added sediments decreased the potential yield of algal turf biomass and nitrogen to grazing fishes by an average of 2000 and 3300%, respectively. Sediments profoundly affect algal turf yield to grazing herbivorous fishes and, therefore, the productivity of algal turf-based food chains, potentially diminishing reef-based fisheries.