Macroalgal herbivory on recovering versus degrading coral reefs

Macroalgal-feeding fishes are considered to be a key functional group on coral reefs due to their role in preventing phase shifts from coral to macroalgal dominance, and potentially reversing the shift should it occur. However, assessments of macroalgal herbivory using bioassay experiments are primarily from systems with relatively high coral cover. This raises the question of whether continued functionality can be ensured in degraded systems. It is clearly important to determine whether the species that remove macroalgae on coral-dominated reefs will still be present and performing significant algal removal on macroalgal-dominated reefs. We compared the identity and effectiveness of macroalgal-feeding fishes on reefs in two conditions post-disturbance—those regenerating with high live coral cover (20–46 %) and those degrading with high macroalgal cover (57–82 %). Using filmed Sargassum bioassays, we found significantly different Sargassum biomass loss between the two conditions; mean assay weight loss due to herbivory was 27.9 ± 4.9 % on coral-dominated reefs and 2.2 ± 1.1 % on reefs with high macroalgal cover. However, once standardised for the availability of macroalgae on the reefs, the rates of removal were similar between the two reef conditions (4.8 ± 4.1 g m−2 h−1 on coral-dominated and 5.3 ± 2.1 g m−2 h−1 on macroalgal-dominated reefs). Interestingly, the Sargassum-assay consumer assemblages differed between reef conditions; nominally grazing herbivores, Siganus puelloides and Chlorurus sordidus, and the browser, Siganus sutor, dominated feeding on high coral cover reefs, whereas browsing herbivores, Naso elegans, Naso unicornis, and Leptoscarus vaigiensis, prevailed on macroalgal-dominated reefs. It appeared that macroalgal density in the surrounding habitat had a strong influence on the species driving the process of macroalgal removal. This suggests that although the function of macroalgal removal may continue, the species responsible may change with context, differing between systems that are regenerating versus degrading