Abstract
Coral reefs are under increasing pressure from anthropogenic and climate-induced stressors. The ability of reefs to reassemble and regenerate after disturbances (i.e., resilience) is largely dependent on the capacity of herbivores to prevent macroalgal expansion, and the replenishment of coral populations through larval recruitment. Currently there is a paucity of this information for higher latitude, subtropical reefs. To assess the potential resilience of the benthic reef assemblages of Lord Howe Island (31°32′S, 159°04′E), the worlds' southernmost coral reef, we quantified the benthic composition, densities of juvenile corals (as a proxy for coral recruitment), and herbivorous fish communities. Despite some variation among habitats and sites, benthic communities were dominated by live scleractinian corals (mean cover 37.4%) and fleshy macroalgae (20.9%). Live coral cover was higher than in most other subtropical reefs and directly comparable to lower latitude tropical reefs. Juvenile coral densities (0.8 ind.m−2), however, were 5–200 times lower than those reported for tropical reefs. Overall, macroalgal cover was negatively related to the cover of live coral and the density of juvenile corals, but displayed no relationship with herbivorous fish biomass. The biomass of herbivorous fishes was relatively low (204 kg.ha−1), and in marked contrast to tropical reefs was dominated by macroalgal browsing species (84.1%) with relatively few grazing species. Despite their extremely low biomass, grazing fishes were positively related to both the density of juvenile corals and the cover of bare substrata, suggesting that they may enhance the recruitment of corals through the provision of suitable settlement sites. Although Lord Howe Islands' reefs are currently coral-dominated, the high macroalgal cover, coupled with limited coral recruitment and low coral growth rates suggest these reefs may be extremely susceptible to future disturbances.
Highlights
IntroductionReef-building corals, are subject to a diversity of disturbances, ranging from very localised events (e.g. discrete predation events) that kill or injure individual coral polyps, to ocean-scale bleaching events associated with climate change [1,2]
Coral reefs, and reef-building corals, are subject to a diversity of disturbances, ranging from very localised events that kill or injure individual coral polyps, to ocean-scale bleaching events associated with climate change [1,2]
This study revealed that the coral cover at Lord Howe Island, the worlds’ southernmost coral reef, was higher than most other subtropical reefs, and directly comparable to lower latitude reefs of the Great Barrier Reef (GBR), where mean coral cover typically ranges from 18.3–27.0% on inshore reefs to 30.7–33.6% on offshore reefs [22,53]
Summary
Reef-building corals, are subject to a diversity of disturbances, ranging from very localised events (e.g. discrete predation events) that kill or injure individual coral polyps, to ocean-scale bleaching events associated with climate change [1,2]. The diversity, frequency and severity of disturbances affecting reef corals are increasing, especially those disturbances associated with climate change [3,4]. Given the potential importance of macroalgae in the functioning and resilience of coral reef ecosystems, it is not surprising that considerable research effort has focused on determining the factors that influence macroalgal distributions. These studies have identified a number of physical and biological mechanisms that may shape coral reef algal communities; including herbivory, eutrophication, hydrodynamics and sedimentation (e.g., [13,14]). Herbivory is widely accepted as a key determinant of benthic community succession and algal community structure on coral reefs (reviewed by [15])
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