Detritus as food for grazing fishes on coral reefs

Algal turf assemblages of the northern Great Barrier Reef, Australia, were sampled to determine the nutritional value of detritus and algae. Samples were collected with a suction apparatus across an exposure gradient from (1) the reef crest at highly exposed outer barrier reefs, (2) the reef crest of moderately exposed midshelf reefs, and (3) the reef slope of sheltered midshelf reefs. The biomass of algae and detritus decreased from sheltered midshelf reefs to moderately exposed midshelf reefs to highly exposed outer barrier reefs. This decrease was significant only for detritus (P < 0.005). Wave energies were calculated across the exposure gradient with the wave model WAMGBR. Detrital mass was inversely correlated with predicted wave energies and fitted a polynomial relationship (P < 0.001) and explained 52.8% of the variation. A similar relationship was also found between algal mass and wave energy (P < 0.001) but only explained 30.0% of the variation. The nutritional value of samples in protein amino acids and starch was assessed. The amino acid composition of detritus and algae was similar and not considered nutritionally different, whereas the concentration of protein amino acids was significantly (P < 0.001) higher in detritus (21.2 ± 2.0 mg g -1 ) than in algae (11.8 ± 1.0 mg g -1 ). Starch content was significantly (P < 0.05) higher in algae (7.7 ± 0.9 mg g -1 ) than in detritus (6.0 ± 1.0 mg g -t ). These results demonstrate that detritus is a potentially valuable food source to grazing fishes on coral reefs.