Abstract
The removal of macroalgal biomass is critical to the health of coral reef ecosystems. Previous studies on relatively intact reefs with diverse and abundant fish communities have quantified rapid removal of macroalgae by herbivorous fishes, yet how these findings relate to degraded reef systems where fish diversity and abundance are markedly lower and algal biomass substantially higher, is unclear. We surveyed roving herbivorous fish communities and quantified their capacity to remove the dominant macroalga Sargassum ilicifolium on seven reefs in Singapore; a heavily degraded urbanized reef system. The diversity and abundance of herbivorous fishes was extremely low, with eight species and a mean abundance ~1.1 individuals 60 m−2 recorded across reefs. Consumption of S. ilicifolium varied with distance from Singapore’s main port with consumption being 3- to 17-fold higher on reefs furthest from the port (Pulau Satumu: 4.18 g h−1; Kusu Island: 2.38 g h−1) than reefs closer to the port (0.35–0.78 g h−1). Video observations revealed a single species, Siganus virgatus, was almost solely responsible for removing S. ilicifolium biomass, accounting for 83% of the mass-standardized bites. Despite low herbivore diversity and intense urbanization, macroalgal removal by fishes on some Singaporean reefs was directly comparable to rates reported for other inshore Indo-Pacific reefs.
Highlights
The removal of macroalgal biomass is critical to the health of coral reef ecosystems
Macroalgal removal across reefs in Singapore showed a high degree of spatial variation consistent with patterns reported from other Indo-Pacific reef systems[10, 18, 19, 21, 39, 40]
Si. virgaKtu.svaigienSscis. ghobbSacn. rivulatuSsi. javSui.scanaliculaPt.ulsittoDra. lpisrosopotSaie.ncioaralNlin. ufislamentosus assays due to browsers in Singapore was low (0.35–4.18 g h−1) compared to offshore reef studies on the GBR (2–100 g h−1)[41] and Ningaloo (42–53 g h−1)[42], but was directly comparable to rates reported from coastal reefs in the Seychelles (0.28–10 g h−1)[40] and nearshore reefs on the GBR (0.34–4.29 g h−1)[10, 19, 39] using similar methods
Summary
The removal of macroalgal biomass is critical to the health of coral reef ecosystems. Local anthropogenic stressors (e.g. overfishing, coastal development and pollution), coupled with increasing effects of climate change, have caused regional declines in live coral cover (e.g., Caribbean[1] and Great Barrier Reef 2) These external impacts can alter the balance between primary producers and consumers resulting in some reefs being overgrown by fleshy macroalgae (or seaweed)[3,4,5]. The vast majority of studies that have directly quantified macroalgal browsing on coral reefs have been conducted on reef systems in regions with relatively diverse and abundant herbivorous fish populations[12] It remains unclear how these findings relate to heavily degraded, urbanized reef systems where the diversity and abundance of herbivorous fishes are typically lower, and macroalgal biomass substantially higher. The objectives of this study were to: (1) quantify spatial variation in the removal of Sargassum among reefs with differing coral and macroalgal cover spanning the southern islands of Singapore, and (2) determine whether the fish species responsible for removing macroalgal biomass changes significantly over these scales
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