Disturbance effects to coral recruitment dynamics on the Great Barrier Reef

Coral recruitment represents a critical phase in the development of coral populations, important to the recovery of coral reefs affected by anthropogenic and natural disturbances. Successful recruitment is significant to the resilience of coral reefs, and degraded reefs often exhibit declining rates of coral recruitment through a poorly understood combination of reduced adult fecundity, decreased settlement, increased competition for space, and high rates of early mortality. The stages of successful recruitment are complex and, whilst evidence indicates that climate change disturbances can adversely reduce coral recruitment, empirical investigations into ecological interactions are limited.

The first section of this thesis used experimental manipulations to investigate how anthropogenic ocean acidification (OA) affects both pre- and post-settlement processes associated with coral recruitment. The first data chapter (Chp. 2) focussed on how OA altered the encrusting benthic community and its subsequent effects on coral settlement. Strikingly, the only preferred settlement substrate in the experimental controls (Titanoderma) was avoided by coral larvae as pCO2 increased, and other substrata were selected. Chapter 3 then used a series of laboratory settlement assays to isolate the effect of OA on the survival and settlement of coral larvae with three ecologically important species of crustose coralline algae (CCA). Here, with all CCA species, the rates of coral settlement declined as pCO2 increased, but the magnitude of this effect was highest with Titanoderma. The last experimental chapter (Chp. 4) tested whether the reduced growth of coral recruits caused by OA would increase their mortality by prolonging their vulnerability to an acute disturbance: fish herbivory on surrounding algal turf. Compared to ambient conditions, recruits needed to double their size at the highest pCO2 to escape incidental grazing mortality. This general trend was observed with three groups of predators (blenny, surgeonfish and parrotfish), and the magnitude of the effect was highest with parrotfish.

The final data chapter (Chp. 5) of this thesis used permanently marked benthic plots and settlement tiles in a disturbed reef flat and reef slope habitat to investigate how life-history and adult stock influenced early recovery dynamics over a three year period. The reef slope was characterised by higher amounts of available settlement substrata, more than twice the rates of coral recruitment, and significantly higher recruit survival compared to the reef flat. In both habitats, as adult coral populations increased, so too did the density of recruits, yielding a significant positive stock-recruitment relationship. This positive stock-recruitment relationship was confined to species that brood their larvae, while no relationship was found for spawning corals. However, the stock-recruitment function did not influence the early recovery in either habitat. Instead, it was the presence of particular fast-growing acroporids on the reef slope that drove the rapid increase in coral cover, demonstrating the importance of life-history traits in assessing coral assemblage recovery. Using population matrices, major recruitment bottlenecks were determined for coral taxa common to Indo-Pacific reefs.

This thesis has captured some of the effects of both natural and anthropogenic disturbances on processes intrinsic to coral recruitment ecology. Of particular importance are the documented interactions among different biological groups – i.e. corals, algae, and herbivores – influenced by different kinds of stressors – e.g. ocean acidification, competition, and herbivory. Benthic communities often respond to perturbations in complex and unpredictable ways, so increasing our understanding of how recruiting corals respond to acute and chronic disturbances assists in the predictive modelling of benthic communities to make better informed management decisions.

Doropoulos C., 2012. Disturbance effects to coral recruitment dynamics on the Great Barrier Reef. PhD thesis, University of Queensland, 166 p. Thesis (restricted access).

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