Examining the relationship between fish herbivore biomass, coral and macroalgal cover on Singapore’s heavily disturbed reefs

Background. Herbivores play a critical role in structuring benthic communities on tropical coral dominated reefs because they remove macro and microalgae, which might otherwise overgrow adult corals or prevent successful recruitment of juveniles. Reducing herbivory has been implicated in promoting phase shifts from coral dominance to other ecosystem states following acute disturbances. Turbidity and sedimentation are key physical processes that also structure coral reef communities because they limit light penetration and may interfere with biological processes such as heterotrophy. Singapore’s coral reefs have been heavily impacted by human activities for decades and experience very high levels of sedimentation and turbidity. Macroalgae, particularly Sargassum spp. are abundant on the reef flats, while adjacent reef slopes are dominated by diverse coral assemblages, yet nothing is known about the role of herbivory in structuring these heavily disturbed reefs. Methods. To gain a better understanding of the role of herbivore biomass and potential herbivory in structuring these disturbed reefs, we quantified benthic cover of corals and macroalgae, fish biomasses and urchin abundance at eight representative sites among Singapore’s southern islands during two seasons. In addition we filmed herbivory assays on replicate experimental macroalgal stands (4 separate macroalgal species) and natural EAM patches at three of these sites to estimate species-specific rates of herbivory. Using generalised linear mixed models we examine the relationship between herbivore abundances and benthic cover of corals and macroalgae. Results. Average coral cover at 3-4 m depth was surprisingly high (~40%) compared to other Indo-Pacific reefs, considering decades of chronic anthropogenic impacts. In contrast, the average biomass of herbivorous fishes (~4 g m-2) was more typical of degraded and overfished reefs, while urchin abundances were within the range found at other Indo-Pacific sites. Herbivorous fish were not observed feeding on macroalgal assays during 29 h of filming, with all bites counted on EAM carried out primarily by territorial damselfish. There was a significant but weak relationship between coral cover and herbivore biomass but none for macroalgae or between urchin abundance and benthic cover. Discussion. Relatively high coral cover and low macroalgal cover at 3-4 m depth has been maintained on these highly disturbed reefs, demonstrating possible resilience to coralmacroalgal phase shifts despite decades of chronic human disturbances. We suggest that low light levels at 3-4 m depth may limit algal growth rates so that only low levels of herbivory are necessary to prevent seaweeds from outcompeting corals on turbid reefs such as those in the present study. PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.1907v1 | CC-BY 4.0 Open Access | rec: 30 Mar 2016, publ: 30 Mar 2016