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 w ere collected with a suction apparatus across an exposure gradient from (1) the reef crest at highly exposed outer barrier reefs, (2) the reef crest o f moderately exposed midshelf reefs, and (3) the reef slope of sheltered mi dshelf reefs. The biomass of algae and detritus decreased from sheltered mi dshelf reefs to moderately exposed midshelf reefs to highly exposed outer b arrier reefs. This decrease was significant only for detritus (P < 0.005). Wave energies were calculated across the exposure gradient with the wave mo del WAMGBR. Detrital mass was inversely correlated with predicted wave ener gies and fitted a polynomial relationship (P < 0.001) and explained 52.8% o f the variation. A similar relationship was also found between algal mass a nd wave energy (P < 0.001) but only explained 30.0% of the variation. The n utritional value of samples in protein amino acids and starch was assessed. The amino acid composition of detritus and algae was similar and not consi dered nutritionally different, whereas the concentration of protein amino a cids 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(-1)). These results demonstrate that detritus is a potentially val uable food source to grazing fishes on coral reefs.