THE PHYSICAL ENVIRONMENT, MARINE HABITATS, AND CHARACTERISTICS OF THE MARINE FAUNA

The Physical Environment The geography, physiography, climate and oceanography of the region have been summarised by Deegan (1992). These are discussed below in relation to how they affect habitats and fauna. The sea surface temperature range of 20-33°C places the Montebellos within Ekman's (1976) tropical zone, delineated from the subtropical zone by the 20°C minimum isotherm. In terms of biogeographical provinces, determined principally by water temperature, the Montebellos fall within the Dampierian or Northern Australian Tropical Province (Wilson and AlIen, 1987). The quantity and quality of suspended particulate matter is an important environmental parameter for marine organisms. The waters of the Montebellos are little influenced by terrigenous sediments from the mainland or the islands themselves because there is insignificant freshwater runoff in the area. Most turbidity is the result of wave action due to the shallowness of the area and the high tidal range; turbidity conditions of the Montebellos are typical of the mid-shelf and are unlike those of the atolls on the outer shelf edge, such as the Rowley Shoals; which are typically oceanic. The semi-diurnal tidal regime, with a moderately high range at springs (3.5 m), also contributes to turbidity at the Montebellos. An important physical environmental factor influencing the marine fauna in shallow water at the Montebellos is the frequency of occurrence of tropical cyclones. Examination of cyclone tracks provided by the Bureau of Meteorology showed that in the 16 years between 1977/78 and 1991/92 a total of 9 cyclones passed within approximately 1° of the islands, an average of 0.6 per year. The marine biota, particularly of shallow and intertidal areas, is therefore subject to frequent natural perturbation. Communities are consequently likely to be either resilient or transient. An example of the latter is the tabular Acropora on the western barrier reef flat, the distribution and percentage cover of which appears to be particularly dynamic, possibly as a result of cyclones. Probable recent cyclone damage evident in shallow areas of the lagoons during the survey, may have resulted from cyclones "Ian" (Feb /March 1992), "llona" (December 1988) and/or "Orson" (April 1989). In broad morphological terms, the Montebello Islands and associated reefs resemble the shape of an arrowhead, comprising a central "chain" of islands with unusually irregular or convolu.ted coastlines lying on a north-south axis. These islands are in close proximity to one another and are separated by narrow channels, which generally run east-west. The northernmost island in this chain (Northwest Island) forms the apex and from it an almost unbroken barrier reef runs to the south-west and a large elongate island (Trimouille Island) and series of smaller islands runs to the south-east (Figure 1).