Search

Increasing ocean temperatures associated with ongoing climate change have resulted in regional reductions in the cover of live coral and increasing concerns that coral reefs will be overgrown by macroalgae. The likelihood of macroalgal overgrowth will, however, depend on the thermal sensitivities of the macroalgae themselves. We exposed recently settled propagules of the common canopy-forming macroalga Sargassum swartzii and adult thalli of three species of Sargassum (S. swatzii, S. cristaefolium, S. polycystum) to three experimental temperatures: ambient, + 2 °C, and + 3.5 °C, reflective of summer minimum, mean, and maximum temperatures for the region. Growth and survival of Sargassum swartzii propagules were assessed over 48 days, and the growth, physical toughness, elemental composition, and susceptibility to herbivory of adult thalli were assessed after short-term exposure (2-weeks) to experimental temperatures. Growth and survival of S. swartzii propagules were reduced by 43% and 84%, respectively, when cultured at the elevated (+ 3.5 °C) temperature compared to ambient temperature. Similarly, elevated temperature resulted in a 17–49% decline in the growth of adult Sargassum thalli relative to controls. Susceptability of S. swartzii and S. cristaefolium to herbivory (i.e. mass removed by herbivores) was 50% less for thalli cultured at elevated (+ 3.5 °C) compared to ambient temperature, but this pattern was not related to changes in the physical or chemical properties of the thalli as a result of elevated temperature. The negative effects of elevated temperatures on the growth and survival of both Sargassum propagules and adult thalli will likely restrict the capacity of Sargassum, and potentially other macroalgae, to establish in new areas, and may also threaten the persistence of existing macroalgal meadows under future ocean temperatures. The thermal sensitivities of tropical Sargassum, together with those of corals, suggest ongoing ocean warming may lead to novel reef ecosystems that are low in both coral cover and macroalgal cover.

For organisms with discrete life-histories, any mechanism that enhances growth and/or survival at critical life-history transitions may significantly influence adult population size. On coral reefs, structurally complex microhabitats enhance the early post-settlement growth and/or survival of fishes and corals; however, the importance of such microhabitats to the early life stages of tropical macroalgae is largely unknown. Here, we investigate the effect of crevices on the recruitment, growth and survival of propagules of a common coral reef macroalga, Sargassum swartzii. We settled S. swartzii propagules onto terracotta settlement tiles that had a series of 3 mm deep crevices evenly spaced on their top (exposed) surface. Recruitment of S. swartzii was 21% greater, but propagules were 18% shorter, 18-days post-settlement within crevices than on adjacent exposed surfaces. Exposing tiles to local fish assemblages for five days showed that survival of propagules was 90% greater in crevices than on exposed areas of the tiles on the reef crest, but not on the reef flat. Underwater video footage revealed that few fishes fed from within the crevices (18% of all bites) with the majority of feeding being concentrated on the exposed surface of tiles. Interestingly, small-bodied fishes from the family Blenniidae (predominantly Ecsenius spp.) accounted for the majority of the feeding activity on the tiles, and likely contributed to the mortality of propagules. Structurally complex microhabitats, such as crevices, that shelter vulnerable early post-settlement propagules from herbivory may therefore be important for the persistence of macroalgae on coral reefs.

The importance of herbivory in both preventing and reversing shifts to macroalgae dominance on coral reefs has been extensively investigated. However, most studies examining the capacity for herbivores to consume fleshy macroalgae (e.g., Sargassum) have investigated removal of the ‘leafy’ biomass without considering the susceptibility of other components of the macroalga, in particular the holdfast, to herbivory. Here, we investigate the susceptibility of Sargassum components (blades, stipes and holdfasts) to herbivory and investigate the capacity for Sargassum to regrow following damage to the holdfast. We placed entire thalli of Sargassum swartzii on the reef crest at Lizard Island, northern Great Barrier Reef, for 24 d, and used photographs and video recordings to quantify rates of removal over this period. The blades of the S. swartzii were rapidly removed (100% in 2 d), whereas the stipes were less susceptible to herbivores, with 72% of experimental thalli having partial stipes remaining after 24 d. Only one holdfast (out of 54) was removed during the experiment, while all of the remaining holdfasts were largely undamaged. When S. swartzii holdfasts were experimentally damaged, we found no detectable effect on thallus height or holdfast size among regrown thalli after 1 y. There was, however, a 50% reduction in survival for S. swartzii individuals when 75% of the holdfast was removed. This study shows that holdfasts of S. swartzii are extremely resistant to herbivory, and that incidental bites on S. swartzii holdfasts are unlikely to affect their growth or survival unless three-quarters of the holdfast is removed. The capacity of Sargassum to regenerate from damaged holdfasts, coupled with the low rate of herbivory on holdfasts, suggests that sustained browsing (preventing regrowth of the stipe and blades) may be more important in reversing macroalgae dominance than physical removal of holdfasts by herbivorous fishes.

Disturbances to ecosystems are often considered in terms of their potential to disrupt the feedbacks maintaining a particular ecosystem regime. The likelihood of doing so, however, will be dependent on the nature of the regime and its resilience to disturbance. Within coral reef ecosystems, shifts from coral- to macroalgal dominance are becoming increasingly common and, once established, are difficult to reverse. Most studies to date have focused on the biological removal of macroalgae by herbivores, however, none have considered the resilience of macroalgal beds to physical removal by storms. We simulated the physical removal of macroalgal biomass from Sargassum beds on an inshore coral reef on Australia's Great Barrier Reef, and monitored recovery for 11months. Trimming Sargassum biomass but leaving the holdfast intact had no detectable effect on the density, height or biomass of Sargassum compared to adjacent intact, or control, areas after 5 months. In areas where holdfasts were also removed, Sargassum biomass recovered to 50% of control biomass after 11months. Given the importance of holdfasts to the resilience of Sargassum beds, we also investigated the ability of herbivores to remove and/or damage holdfasts. Exposing pieces of dead coral with attached Sargassum holdfasts to local herbivore assemblages resulted in a 70% decline in the number of holdfasts over 4months compared to those protected from herbivores. Synthesis. Beds of Sargassum are extremely resilient to the physical removal of algal biomass, and as such, storms alone are unlikely to lead to permanent reductions in Sargassum abundance. Persistent reductions in the abundance of macroalgae on degraded coral reefs will not only require the physical or biological removal of algal biomass, but critically, sufficient densities of herbivores capable of removing holdfasts.

Cross-shelf differences in coral reef benthic and fish assemblages are common, yet it is unknown whether these assemblages respond uniformly to environmental disturbances or whether local conditions result in differential responses of assemblages at different shelf positions. Here, we compare changes in the taxonomic and functional composition, and associated traits, of herbivorous reef fish assemblages across a continental shelf, five years before and six months after two severe cyclones and a thermal bleaching event that resulted in substantial and widespread loss of live hard coral cover. Each shelf position maintained a distinct taxonomic assemblage of fishes after disturbances, but the assemblages shared fewer species among shelf positions. There was a substantial loss of species richness following disturbances within each shelf position. Total biomass of the herbivorous fish assemblage increased after disturbances on mid- and outer-shelf reefs, but not on inner-shelf reefs. Using trait-based analyses, we found there was a loss of trait richness at each shelf position, but trait specialisation and originality increased on inner-shelf reefs. This study highlights the pervasiveness of extreme environmental disturbances on ecological assemblages. Whilst distinct cross-shelf assemblages can remain following environmental disturbances, assemblages have reduced richness and are potentially more vulnerable to chronic localised stresses.

Understanding how the impact of different herbivores varies spatially on coral reefs is important in qualifying the resistance of coral reefs to disturbance events and identifying the processes that structure algal communities. We used assays of six common macroalgae (Acanthophora spicifera, Caulerpa taxifolia, Galaxaura rugosa, Laurencia sp. Sargassum sp., and Turbinaria ornata) and remote underwater video cameras to quantify herbivory in two habitats (reef crest and slope) across multiple sites on Orpheus Island, Great Barrier Reef. Rates of herbivory varied among macroalgal taxa, habitats, and sites. Reductions in algal biomass were greatest for Sargassum sp. (36 % 4 h−1), intermediate for A. spicifera, Laurencia sp., C. taxifolia, and T. ornata (17–33 % 4 h−1) and lowest for G. rugosa (6 % 4 h−1). Overall, rates of herbivory were generally greater on the reef crest (30 % 4 h−1) than the reef slope (21 % 4 h−1). This difference in rates of herbivory coincided with a marked shift in the dominant herbivores between habitats. Kyphosus vaigiensis, despite only feeding on three species of macroalgae (Sargassum sp., T. ornata, and A. spicifera), was responsible for 34 % of all bites recorded on the reef crest yet did not take a single bite from algae on the reef slope. In contrast, Siganus doliatus took bites on every species of algae in both habitats, accounting for 40 % of bites on the reef crest and 74 % of all bites recorded on the reef slope. This difference in the number of macroalgal species targeted by herbivores and the habitat/s in which they feed adds another dimension of complexity to our understanding of coral reef herbivore dynamics.

Benthic macroalgae often occur in close association with other macroalgae, yet the implications of such associations on coral reefs are unclear. We selected three pairs of commonly associated macroalgae on inshore reefs of the Great Barrier Reef and exposed them, either independently or paired, to herbivore assemblages. Pairing the palatable alga Acanthophora with the calcified and chemically defended Galaxaura resulted in a 69 % reduction in the consumption of Acanthophora, but had no effect on the consumption of Galaxaura. The reduced consumption of Acanthophora was related to 53–85 % reductions in the feeding rates of two herbivorous fish species, Kyphosus vaigiensis and Siganus doliatus. Neither Acanthophora nor Sargassum were afforded protection when paired with the brown macroalga Turbinaria. Although limited to one of the three species pairings, such associations between algae may allow the ecological persistence of palatable species in the face of intense herbivory, enhancing macroalgal diversity on coral reefs.