Numerical simulation of larval dispersal and recruitment within the Great Barrier Reef and its application to the Acanthaster phenomenon and coral reef resource management

Dight, Ian John (1991) Numerical simulation of larval dispersal and recruitment within the Great Barrier Reef and its application to the Acanthaster phenomenon and coral reef resource management. PhD thesis, James Cook University of North Queensland.

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Abstract

This research was motivated by debate surrounding the causes of population fluctuations of the crown-of-thorns starfish Acanthaster planci within the Great Barrier Reef (GBR), Australia. The patterns of Acanthaster population spread have led to the widespread belief that the propagation of populations is by means of larval dispersal, and that population fluctuations are strongly influenced by larval recruitment. The spatial and temporal scales over which larvae disperse, and of the Acanthaster phenomenon itself, pointed to the need for a more complete picture of the large-scale dynamics of water motion and larval dispersal. The research aimed to investigate the relationship between reefs (reef connectivity) which results from pelagic larval dispersal. Modelling has provided the only feasible approach to this problem.

An understanding of the observed patterns of Acanthaster population spread throughout the GBR has necessitated the development of numerical models which are capable of simulating the hydrodynamics associated with large assemblages of reefs over the time scale of larval dispersal. The model is modular in design and has incorporated the principal processes forcing water circulation within the GBR. Its development has required a new modelling approach which pays attention to the geometric representation of reef structure and the large-scale dynamics of water motion, while incorporating the fine-scale features of water circulation over and around individual reefs. The particle tracking component of the model uses a Lagrangian 'marker and cell' technique which runs under time-dependent tidal flow and a randomly varying wind field based upon an historical record. The entrainment of larvae from the mainstream water circulation into the near-field circulation of reefs is incorporated into the dispersal model through a parameterization scheme based on schematized reefs. Attention has been paid to the accurate representation of the dynamics of water motion.

This study identifies hydrodynamic processes which result in distinctive patterns of larval dispersal and recruitment at three spatial scales: (1) regional scale (100s kilometres); (2) shelf scale (10s kilometres); and (3) inter-reef scale (kilometres). Regional scale variation may result in differences in the level and pattern of recruitment and, therefore, in population density and age-structure. Shelf scale variation can result in passive mechanisms which both concentrate and separate larval pools, which in turn can account for some of the cross-shelf variation in species abundance and community structure that has been documented. Inter-reef scale variation results in differences in potential recruitment of larvae to coral reefs as a consequence of a combination of both stochastic and deterministic factors, including reef size and shape, the location of trajectory intercept with respect to the reef edge, and the state of the tide.

Model simulations of the large-scale patterns of larval dispersal and reef connectivity are compared with the distribution of adults and the observed pattern of Acanthaster population spread throughout the central GBR. Results provide a physical explanation for many of the observed features of Acanthaster population spread throughout the region. These include: (1) identification of a source region from which Acanthaster populations are proposed to have spread; (2) the apparent southward spread of populations from this region; (3) the high incidence of outbreaks on mid-shelf reefs south from Green Island; (4) the susceptibility of particular reefs to repeated recruitment, notably Green Island and many others which border the main lagoon; and (5) the cessation of outbreaks causing high coral mortality throughout the mid-shelf reefs within the Central Section of the Marine Park. The high incidence of outbreaks on inner matrix reefs south from Green Island can be explained as a purely physical phenomenon, without the need to invoke human interference in the ecology of these more accessible reefs.

Model results are also applied to other issues, including cross-shelf and regional variation in community structure, life-history theory and the management of tropical marine resources. Variation in larval transport and reef connectivity is believed to be reflected in regional and cross-shelf differences in community structure and population abundance of many species of invertebrate and fish within the central GBR. Larval dispersal and recruitment to coral reefs is shown to be a highly unpredictable and variable process. Inappropriate scales of dispersal and predation by reef-associated species are identified as potentially significant sources of larval mortality during the dispersal phase of many tropical marine species. There appears to be a decided disadvantage to extending the pre-competent period of larval development beyond a relatively short period of 4 days if predation within the pelagic environment or from reef-associated species is intense. Consideration of how physical oceanographic processes might interact with life-history characteristics, and thereby influence species distributions and dynamics, is identified as a potentially rewarding area for future research.

Larval dispersal models are well suited to management issues and applications. Most coral reef invertebrates and fish have a pelagic larval phase to their life-cycle, during which they have the potential to disperse quite widely. Recruitment from this larval pool is likely to structure communities and influence the population abundance of many species. On this basis, models which are capable of predicting reef connectivity can provide a stronger, and more rational, biological basis for the management of tropical marine resources.

Item ID: 33766
Item Type: Thesis (PhD)
Keywords: Acanthaster populations; COTS; crown-of-thorns starfish; Acanthaster planci; larvae; larval distribution; mathematical models; hydrodynamics; regional variation; oceanography; Great Barrier Reef; GBR
Date Deposited: 29 May 2015 05:04
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050103 Invasive Species Ecology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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