Temporal variation in macroalgal removal: insights from an impacted equatorial coral reef system

Macroalgal removal is a critical ecosystem function yet few studies have considered its temporal variability, especially on impacted reefs with limited herbivorous fish biodiversity. To address this, we quantified macroalgal removal and mass-standardised bite rates of herbivorous fishes monthly from July 2016 to June 2017 using a series of transplanted Sargassum ilicifolium assays and underwater video cameras on three degraded coral reefs in Singapore: Pulau Satumu, Kusu Island, and Terumbu Pempang Tengah. Our results revealed a distinct temporal pattern in macroalgal herbivory (proportion of biomass removed and mass-standardised bite rates) rates across all sites, increasing from July and decreasing from January, with the highest rates recorded in December (28.10 ± 3.05 g 3.5 h−1; 208.24 ± 29.99 mass-standardised bites 3.5 h−1) and the lowest in May (0.86 ± 0.17 g 3.5 h−1; 9.55 ± 3.19 mass-standardised bites 3.5 h−1). These coincided with the S. ilicifolium growth cycle, confirming previous evidence that herbivory rates are closely linked to macroalgal condition. Video analyses revealed nine species feeding over a year (31,839 bites; 8702.89 mass-standardised bites), with Siganus virgatus responsible for ∼ 80% of the total mass-standardised bites. Siganus virgatus took the largest proportion of bites monthly, except between April and June, when Scarus rivulatus was dominant, suggesting temporal constraints in functional roles

Accreting coral reefs in a highly urbanized environment

Globally, many coral reefs have fallen into negative carbonate budget states, where biological erosion exceeds carbonate production. The compounding effects of urbanization and climate change have caused reductions in coral cover and shifts in community composition that may limit the ability of reefs to maintain rates of vertical accretion in line with rising sea levels. Here we report on coral reef carbonate budget surveys across seven coral reefs in Singapore, which persist under chronic turbidity and in highly disturbed environmental conditions, with less than 20% light penetration to 2 m depth. Results show that mean net carbonate budgets across Singapore’s reefs were relatively low, at 0.63 ± 0.27 kg CaCO3 m−2 yr−1 (mean ± 1 SE) with a range from − 1.56 to 1.97, compared with the mean carbonate budgets across the Indo-Pacific of 1.4 ± 0.15 kg CaCO3 m−2 yr−1, and isolated Indian Ocean reefs pre-2016 bleaching (~ 3.7 kg CaCO3 m−2 yr−1). Of the seven reefs surveyed, only one reef had a net negative, or erosional budget, due to near total loss of coral cover (\u3c 5% remaining coral). Mean gross carbonate production on Singapore’s reefs was dominated by stress-tolerant and generalist species, with low-profile morphologies, and was ~ 3 kg m−2 yr−1 lower than on reefs with equivalent coral cover elsewhere in the Indo-Pacific. While overall these reefs are maintaining and adding carbonate structure, their mean vertical accretion potential is below both current rates of sea level rise (1993–2010), and future predictions under RCP 4.5 and RCP 8.5 scenarios. This is likely to result in an increase of 0.2–0.6 m of water above Singapore’s reefs in the next 80 yr, further narrowing the depth range over which these reefs can persist

Growth and carbonate production of crustose coralline algae on a degraded turbid reef system

Crustose coralline algae (CCA) and other encrusting calcifiers drive carbonate production on coral reefs. However, little is known about the rates of growth and calcification of these organisms within degraded turbid reef systems. Here we deployed settlement cards (N = 764) across seven reefs in Singapore for two years to examine spatio-temporal variation in encrusting community composition and CCA carbonate production. Our results showed that CCA was the dominant encrusting taxa (63.7% ± 18.3SD) across reefs. CCA carbonate production rates (0.009–0.052 g cm−2 yr−1) were less than half of those reported for most Indo-Pacific reefs, but similar to other turbid reef systems. Highest CCA carbonate production rates were observed furthest from Singapore\u27s main shipping port, due to a relative increase in CCA cover on the offshore reefs. Our results suggest that proximity to areas of high industrialisation and ship traffic may reduce the cover of encrusting calcifying organisms and CCA production rates which may have negative, long-term implications for the stabilisation of nearshore reefs in urbanised settings

Fear effects associated with predator presence and habitat structure interact to alter herbivory on coral reefs

Foraging decisions made by consumers are driven by a range of factors, including non-consumptive predation effects. These effects are often mediated by both the presence or absence of predators, and the structure of the surrounding habitat that may visually occlude prey, thus increasing the predation risk. Under such circumstances, it is likely that prey will be warier, and this will be reflected in their rates of browsing. We used models of the predatory coral reef fish Plectropomus leopardus and experimentally manipulated the density of the macroalga Sargassum ilicifolium to investigate how these factors interact on a coral reef in Singapore. We found that the interaction between predator- and habitat associated fear effects influence the rate of herbivory, with declining rates with increasing macroalgal density, likely due to visual occlusion by macroalgae making it more difficult to detect predators, and thus increasing wariness in browsers. The predator model appeared to have an impact on browsing, but only at low-densities of Sargassum. Our results suggest that when fishes' knowledge of their surroundings is less certain, they will respond with a heightened wariness, regardless of acute predation cues

Temporal variation in macroalgal removal: insights from an impacted equatorial coral reef system

10.1007/s00227-020-03806-7Marine Biology1681

Chemically Mediated Interactions with Macroalgae Negatively Affect Coral Health but Induce Limited Changes in Coral Microbiomes

Allelopathic chemicals facilitated by the direct contact of macroalgae with corals are potentially an important mechanism mediating coral–macroalgal interactions, but only a few studies have explored their impacts on coral health and microbiomes and the coral’s ability to recover. We conducted a field experiment on an equatorial urbanized reef to assess the allelopathic effects of four macroalgal species (Bryopsis sp., Endosiphonia horrida, Hypnea pannosa and Lobophora challengeriae) on the health and microbiomes of three coral species (Merulina ampliata, Montipora stellata and Pocillopora acuta). Following 24 h of exposure, crude extracts of all four macroalgal species caused significant coral tissue bleaching and reduction in effective quantum yield. The corals were able to recover within 72 h of the removal of extracts, except those that were exposed to L. challengeriae. While some macroalgal extracts caused an increase in the alpha diversity of coral microbiomes, there were no significant differences in the composition and variability of coral microbiomes between controls and macroalgal extracts at each sampling time point. Nevertheless, DESeq2 differential abundance analyses showed species-specific responses of coral microbiomes. Overall, our findings provide insights on the limited effect of chemically mediated interactions with macroalgae on coral microbiomes and the capacity of corals to recover quickly from the macroalgal chemicals