Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes

Despite the widely accepted importance of fish herbivory on coral reefs, few studies have considered the temporal variability in the nature of algal-herbivore interactions. We therefore quantified monthly feeding intensity on Sargassum sp. bioassays for 12 mo with remote underwater video cameras deployed to identify the herbivores responsible for macroalgal removal on an inshore island of the Great Barrier Reef, Australia. Significantly higher removal rates were observed during the summer months whereas winter months were characterized by 4 times lower removal rates. However, rather than being simply changes in the feeding activity of a single species, this temporal pattern in herbivory also incorporated changes in the species responsible for the removal of Sargassum. Video analyses revealed that, of the 43 herbivore species recorded from the bay, only 3 played a significant role in Sargassum removal: Kyphosus vaigiensis, Naso unicornis and Scarus rivulatus. K. vaigiensis, a rudderfish, was primarily responsible for the removal of Sargassum during the summer months (83% of the total recorded bites; 85 553 bites). There was almost no feeding activity on Sargassum by K. vaigiensis during the winter months (82 bites). However, there was a reciprocal increase in feeding intensity on Sargassum by parrotfishes in the winter months, particularly S. rivulatus (71 bites during summer versus 2884 bites in winter). This temporal variability in herbivore functional roles suggests that functional redundancy on reefs may be less than previously assumed in that the feeding activities of fishes may be both spatially and temporally constrained.