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Capricorn Eddy A Driver Prominent in the Ecology and Future of the Southern Great Barrier Reef (GBR)

The study reported in this paper focuses on the Capricorn Eddy, a mesoscale eddy feature, which typically forms within an indentation of a continental shelf in the southern Great Barrier Reef (GBR). In this study relevant-scale sea surface dynamics were studied at mesoscale and sub-mesoscale by the use of satellite data at moderate resolution (1 km). In situ measurements and model data that are available were used to validate satellite observations as well as to specify the nature of the processes that are occurring within the water column. The characteristic features are identified and physical theory is employed to develop an understanding of processes that are associated. In particular, it is shown that the effect of the eddy is to raise cooler oceanic subsurface water, that is nutrient enriched, and transport it to the reef zone, and eventually into the lagoon. It is demonstrated in this study that to determine the biological responses on the scale of reef communities, linkages between large scale oceanography and the mesoscale and submesoscale patterns are crucial, and may be key to understanding climate change impacts at the relevant spatial scales.

Rapid warming and ocean acidification has occurred in tropical oceans on a scale that has not occurred for at least 720,000 years, and possibly as long as 20 million years (Hoegh-Guldberg at al., 2007). It appears that the ability of biological systems to keep up with the changes has been exceeded as a result of the rapidity of these environmental changes that have occurred in distribution and genetic structure. Widespread extinction of many species and collapse of ecosystems are predicted by many analysts if the current rates of change of atmospheric carbon dioxide continue at their present rates (Myers & Knoll, 2001; Vernon, 2008; Wake and Vrendenberg, 2008).

The finer temporal and special scales have remained uncharted, which represents a challenge for the research and ecosystem management community, though broad characteristics of these changes have been documented. For example, as waters have warmed there has been an increased frequency and intensity of coral bleaching events, and expectations are that sea temperatures will soon exceed the threshold at which mass bleaching and mortality on a yearly basis (Hoegh-Guldberg at al., 1999; Sheppard, 2003; Hoegh-Guldberg at al., 2007). However, it is apparent that no 2 bleaching events are identical, in terms of their temporal and spatial scales, and their overall intensity (Oliver et al., 2008). Variability of timing and stress (Weeks et al., 2008), the physical conditions predominating in a particular location, as well as the communities and their resilience (Berkelmans & Willis, 1999; Brown et al., 2002; McClanahan et al., 2007), will drive significant differences from site to site in the ecological impacts and the responses that are required from managers of natural resources.

According to Weeks et al. at present, the underlying reasons for this variability have been poorly described and are much less well understood. However, it can be assumed that variation on a small scale may ultimately result from patterns of causative physical factors and mechanisms. It is certain that cooling influences, such as cooler waters being upwelled from greater depths, are important to understanding how other factors; salinity changes, solar radiation, sedimentation, nutrients, may influence the outcome of primary stresses such as temperature. The resilience of reefs to rapid environmental changes that are expected in an enhanced greenhouse world may ultimately be determined by these coinciding factors. In this respect, an understanding of the dynamics of ocean processes, on scales from global to local, will become imperative within the goal of understanding and predicting the impacts of global change. The design of investigations that are process-oriented of key issues, as well as the projection of local effects of climate change based on projections of climate change on the larger scale ocean and atmospheric dynamics, would be guided by such predictability. These local projections and understandings are critical to any effective natural ecosystem management response.

The Capricorn Eddy

It was first noted in 1970 in a surface drifter study (Woodhead, 1970) that there was an eddy in the region adjacent to the mouth of the Capricorn Channel. It was postulated later that the establishment of a stable cyclonic eddy in the lee of the shelf bathymetry was contributing to north-westward flow in the Capricorn Channel region (Griffin et al., 1987). It has been shown by subsequent oceanographic deployments and studies of sea surface temperatures by satellite that these cyclonic features trigger upwelling along the continental shelf (Kleypas & Burrage, 1994; Middleton et al., 1994; Burrage et al., 1996).


The circulation of the East Australian Current (EAC) and consequent eddy dynamics are primary mechanisms of forcing in the southern Great Barrier Reef (GBR), having a direct impact on the ecology and the future of the ecosystem. An understanding of the patterns and processes relevant at spatial scales is key understanding the impacts of climate change. In regard to the future and anticipated responses from managers of coral reef ecosystems this has huge importance. In regards to this, the regional and local impacts of recent mass coral bleaching event on the Great Barrier Reef have been distinct, though the reasons for these patterns have not been clear. In this paper Weeks et al. have explored the critical links between oceanography at the large scale and the mesoscale and submesoscale patterns of physical and biological conditions that are crucial to the determination of biological responses on the scale of reef communities and sections of the reef. An understanding of which reefs are more naturally adapted to, and are more environmentally resilient, and more able to withstand the impacts of a warming ocean, and will produce more accurate tools for predicting patterns of mass coral bleaching, will be provided by incorporation of oceanographic variability. The design of process-oriented investigations of key issues, as well as the local effects of climate change based on projections of climate change on the larger scale ocean and dynamics of the atmosphere, which are critical to any effective management response, would be guided by such predictability.

Sources & Further reading

  1. Weeks, S. J., A. Bakun, C. R. Steinberg, R. Brinkman and O. Hoegh-Guldberg (2010). "The Capricorn Eddy: a prominent driver of the ecology and future of the southern Great Barrier Reef." Coral Reefs 29(4): 975-985.


Author: M. H. Monroe
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