The Coral Sea

This article provides a brief review of the Coral Sea, and lists the main body of literature written about this important geographic region. The Coral Sea hosts a high diversity of geomorphic and oceanographic features, giving rise to numerous habitats and ecosystems, from abyssal seafloor over 4,000 m deep to vegetated coral cays above the ocean’s surface. This area hosts important habitats and migration corridors that sustain unique assemblages of organisms. Many of the Coral Sea’s ecosystems and ecological processes remain to be explored and described through scientific research.

The Coral Sea is the body of water east of the Great Barrier Reef, bounded by Papua New Guinea to the north, the Tasman Front to the south, and the west Pacific island chains of the Solomons, Vanuatu and New Caledonia to the east. The Australian portion of the Coral Sea has the Exclusive Economic Zone boundary as its eastern border.

The seabed off eastern Australia was shaped by seafloor spreading and the subsidence of major carbonate platforms along the continental shelf (Davies et al. 1989). The geological structure of the Coral Sea is characterised by abyssal plain to the northeast, a series of plateaux and slopes etched by undersea canyons and separated by deep ocean trenches, and the northern end of a volcanic seamount chain to the south (Keene et al. 2008). In shallower waters, 18 coral reef systems emerge from structural high points on the plateaux or from the tops of seamounts, many with multiple small reefs forming their perimeter. Approximately 49 vegetated and unvegetated cays provide the Coral Sea's only terrestrial habitats. The Coral Sea spans five eco-physical sub-regions, each with unique geomorphic features, ocean currents, drivers of productivity and ecological communities (Brewer et al. 2007).

Three major surface currents dominate the Coral Sea region: The South Equatorial Current, the Hiri Current and the East Australian Current. The westward-flowing South Equatorial Current enters the Coral Sea as a series of jets between the Solomon Islands, Vanuatu and New Caledonia. East of the Great Barrier Reef, the South Equatorial Current bifurcates to form the northern Hiri Current, a slow clockwise gyre in the Gulf of Papua, and the southerly East Australian Current (Ganachaud et al. 2007). The bifurcation point moves seasonally north and south, sending west Pacific water across the north of the continent to feed the Indonesian Throughflow in the Timor Sea in winter (Wyrtki 1960), and strengthening the East Australian Current in summer (Scully-Power 1973). The East Australian Current brings warm, low-nutrient water and tropical species southwards as a series of eddies, and is then pushed eastwards by the Tasman Front off northern New South Wales (Zann 2000). Very little is known about how small-scale circulation patterns affect the Coral Sea’s ecology, but slow eddies have been discovered over some of the plateaux that may retain fauna and promote endemism (Middleton et al. 1994; Brewer et al. 2007).

Only three of the Coral Sea’s 18 reef systems – Coringa-Herald, Lihou Reef and Osprey Reef – have been subject to scientific research. One of the most striking observations has been the high variability in cover or density of live coral and fish between reefs, with very low coral cover and fish density at Coringa-Herald (Ceccarelli et al. 2008), and considerably greater abundance of life at Lihou Reef (Ceccarelli et al. 2009) and Osprey Reef (Andrews et al. 2008; Ayling and Choat 2008). Reef size, the degree of sheltered habitat available, and the isolation of each reef from other reef systems is thought to be the most important driver of the abundance and composition of organisms on these oceanic reefs. Low coral cover seems to be a common feature of small, isolated reefs with a general lack of sheltered habitat. Highly exposed reef crests are cemented with a ridge of crustose coralline algae, and wave-swept reef flats are dominated by low-lying algal turf (Byron et al. 2001; Neil and Jell 2001). More sheltered areas are higher in live coral cover, providing greater habitat complexity and hosting larger abundances of invertebrates and fish (Ceccarelli et al. 2009). Coral, fish and invertebrate populations show key differences from the Great Barrier Reef (Planes et al. 2001; Oxley et al. 2003; Sinclair et al. 2007) and some affinities with the western Pacific and reefs of the Arafura and Timor Seas (Endean 1957; Benzie and Williams 1992; Hooper et al. 1999). Some reefs support high densities of sharks and other predators, and protection in no-take Commonwealth Marine Reserves has led to healthy populations of otherwise exploited species (Ceccarelli et al. 2009).

Coral Sea reefs are subject to a high degree of disturbance, with exposure to a high frequency of tropical cyclones and periods of hot, still weather that can lead to coral bleaching events. One of the marine surveys of Lihou Reef documented a major bleaching event, where 65% of the hard coral cover was bleached (Oxley et al. 2005). There are also records of this event from Osprey Reef, where bleaching was higher in deeper water, possibly because deep-water corals were less adapted to high temperature and UV irradiation (Salih et al. 2006). Recovery of impacted reefs appears variable; the Lihou reef coral community was exhibiting clear signs of early recovery in 2008 (Ceccarelli et al. 2009), while Coringa-Herald, last surveyed in 2007, did not (Ceccarelli et al. 2008). Coral species that are naturally more resistant to bleaching dominated tracts of the recovering reefs. Recently discovered low-light (mesophotic) coral communities at depths of 30-150m on the Osprey Reef slope may provide larvae for the recovery of isolated reefs after major disturbance (Bongaerts et al. 2011).

The islets and cays support terrestrial ecosystems, from simple invertebrate food chains based on carrion and detritus on unvegetated cays (Heatwole 1971), to well-developed communities that include 15% of Australia’s Pisonia grandis forests (Batianoff et al. 2009). The cays provide habitat for nesting and roosting seabirds, many of which are of conservation significance, and nesting endangered green turtles (Harvey et al. 2005; Baker et al. 2008). The Coringa-Herald cays are the best-studied, and host at least 30 species of plants that make up 17 distinct vegetation communities. Approximately 70% of plant species are of wider Melanesian provenance, and are essentially a subset of species found in mainland habitats of similar type and latitude. The number of plant species present at any given time on the Coringa-Herald cays is dependent on immigration, survival and extinction rates. It is likely that most species were dispersed along prevailing currents, such as the east-to-west South Equatorial Current, surface currents driven by the south-easterly trade winds and seasonal monsoon winds, and migratory seabirds (Batianoff et al. 2008).

Keystone species are potentially more important in this harsh environment in their role of promoting or protecting biodiversity. On coral cays, the keystone species of plants are those that are abundant and resilient enough to provide consistent habitat for other species of plants and animals (Batianoff et al. 2010). Among terrestrial invertebrates the Guinea ant, Tetramorium bicarinatum, was found to be a key driver of these communities, acting as a keystone species and indicator of active, above-ground invertebrate populations (Greenslade 2008). The native vegetation of Coral Sea cays has evolved without fire regimes, grazing or common insect predation. As a result, some keystone species such as Pisonia grandis have not developed resistance to mainland pests such as scale insects, and have been subject to dramatic impacts from these pests in the last decade. The infestation of scale insects also led to a shift in community structure in the cays' invertebrate fauna (Smith and Papacek 2004; Batianoff et al. 2008; Greenslade 2008).

Deeper Coral Sea ecosystems are still being explored, but have so far revealed a great diversity of geological formations. In the deep ocean, geomorphic features and substrate composition are the main drivers of ecological communities (Beaman 2010), and recent discoveries include coralline sponges considered ‘living fossils’ (Woerheide and Reitner 1996), diverse cold-water coral communities (Beaman et al. 2009), and high abundances of predatory fish and sharks in the deeper reaches of coral reefs (Sarano and Pichon 1988). Geomorphic features such as canyons, shelf breaks, seamounts and reefs can create patches of high productivity in the waters above them, and therefore attract aggregations of pelagic species. Deep-sea benthic communities may provide important foraging areas for deep-diving cetaceans and fish (Brewer et al. 2007). The Coral Sea’s pelagic realm is frequented by numerous threatened and migratory cetaceans, turtles and sharks, as well as tunas and billfishes important to fisheries and conservation. The pelagic realm of the southern Coral Sea has been the site of in-depth trophic studies, revealing three tiers of consumers that are controlled by large densities of mid-trophic fish and squid (Revill et al. 2009; Griffiths et al. 2010). In the nutrient-poor Coral Sea, aggregations of productivity are highly significant and are likely to affect behaviour, reproduction and migration patterns of species that exploit them (AIMS 2011; Young et al. 2011).

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