Marine EnvironmentMarine Environment

Marine Environment

North Keeling Island is entirely surrounded by a broken, irregular fringing reef, except at the northwest corner. The reef is narrower on the sheltered sides of the island (north and west), and broader on the exposed sides (south and east). On the east coast it is continuous across the mouth of the lagoon, and forms a wide bar which effectively blocks the entrance. Much of the reef is partly exposed at low tide (Gibson-Hill 1948). Along the western side of the island is a wide coral terrace which drops into deep water (Ecology Lab 1994). It is believed there is no seagrass fringing North Keeling (Murray 2003, pers comm.).

North Keeling Island’s lagoon has only a single, narrow entrance on the south-eastern side of the atoll. The opening has no channel through the reef, but is a shallow conduit which drains almost totally at low tide (Woodroffe and McLean 1994). The lagoon is shallow, reported as being nowhere deeper than 8 feet (2.4 m) by Guppy (1889). Its surface sediments are muddy sands, except for two sandy spits which trail in through the entrance. Some of the lagoon is covered with seagrass, and is an important fish nursery area. Periodically, the lagoon entrance becomes blocked, and there is no water movement into or out of the lagoon. When this occurs the water quality in the lagoon deteriorates, and fish and seagrasses die.

In broad terms the Park comprises three major marine habitat types: (1) outer reef slope (subtidal); (2) reef flats including sandy and rocky shores (predominantly intertidal); and (3) lagoon (predominantly subtidal) (Berry 1989).

The Cocos (Keeling) atolls represent the western limit for many species of the Western Pacific biogeographic province. Biogeographic and ecological interest in the marine biota also stems from the extreme isolation and relatively small size of the atolls, from which there are some unusual absences including benthic skates and rays.

According to Berry (1989), the extreme isolation of the islands influences their faunal composition. Those species established at the Cocos (Keeling) Islands must be pelagic as adults, or have long-lived pelagic larval stages. In general, the fauna is relatively depauperate compared with other atolls (Berry 1989 ), but there is evidence of hybridisation which may ultimately result in the evolution of a new species (Choat 2003 pers. comm.). In recent times periodical large-scale natural disturbances including cyclones (Bunce 1988), deoxygenation of lagoon waters (perhaps linked to El Niño events) and outbreaks of crown of thorns starfish, Acanthaster planci, have also reduced the abundance of corals (Berry 1989).

Specific information on marine fauna surrounding North Keeling Island is limited. Taxonomic surveys of the southern atoll were undertaken by the Western Australian Museum in 1989 and the Ecology Laboratory of the Institute of Marine Ecology, University of Sydney, in 1992, with less attention given to the waters surrounding North Keeling Island due to the difficulty of access. Ongoing surveys by Parks Australia staff and researchers from James Cook University is progressively adding to knowledge on marine fauna.