Three-dimensional model of atoll hydrodynamics Eric L. Peterson¹ *, Maria Beger² and Silvia Pinca³ ¹ Centre for Marine Studies at the University of Queensland, Brisbane 4072 Australia ² The Ecology Centre at the University of Queensland, Brisbane 4072 Australia ³ Marine Science Program at the College of the Marshall Islands, Majuro, Marshall Islands * Corresponding author: E L Peterson Email: e.peterson@uq.edu.au Abstract The objectives of this paper were three-fold: (a) Develop a three-dimensional method of modelling hydrodynamics in and around atolls. (b) Establish physical processes of benthic shear velocity and shear stress as significant for habitat suitability of coral reefs. (c) Enrich morphological modelling detail in case of Rongelap atoll, for example. The last objective is still a work-in progress, but the present paper has provided a good review of atoll modelling methods and established our benthic shear velocity as a habitat suitability indicator, measured by a new tool: the “under water weather balloon”, which can be used by resource assessment surveyors to quantify the hydrodynamic climate at dive sites. Sound modelling methodology is a powerful tool that can answer many questions, such as what drives larval settlement in and around an isolated atoll and what would be the likely trajectories of sand, larvae, and anthropogenic pollutants. Hydrodynamic modelling may be useful to classify regions of an atoll’s sub-units on the basis of bathymetry and hydrodynamics to establish to direction and magnitude of bottom shear velocity, and to specifically empower resource managers to estimate the fate of effluent from aquaculture cages, for example. Keywords atoll, habitat suitability, hydrodynamics Introduction Why are hydrodynamics important for coral atolls? The life history of most coral reef organisms involve a sedentary and a pelagic phase. The pelagic phase is important to dispersal of animals on small, regional and global scales. It is widely recognized that ocean currents transport pelagic larvae from their native populations to new settlement reefs. While currents between atolls facilitate the exchange of propagules between atolls, it is likely that a large proportion of recruitment is derived from native populations (Cowen, et al. 2000, Jones, et al. 1999, Swearer, et al. 2002). Shear stress at the substrate- water interface differentially influences the ability of inorganic and organic matter to settle (Peterson 1999b), and may be a substantial factor in the patchy precedence of events in the reef-building process. What is an atoll? The word originated from “atolu” used in the Maldives. Our working definition is an isolated ring of shoals, enclosing a central lagoon, surrounded by deep ocean. Atoll morphology is believed to be the remnant of subsiding volcanic peaks, where coral reefs sustain accretion of substrate just below rising sea-level while an exposed peak may persist for some time, surrounded by expanding lagoon (Darwin 1842). Rising sea level or subsiding basement, with respect to isolated volcanic cones, appears to have formed many atolls. Darwin’s theory was challenged in the 20th century by “glacial control theory”, evidenced by the stand of sea level 90 meters below present sea level during the last ice age. This partially explains why coral deposits are heightened on atoll rims, effected by relatively sudden sea level rise from melting of the glaciers (Daly 1915). The depths to the lagoon floors of most atolls are less than 70 m below present sea-level (Vecsei 2003), supporting the view that short term fluctuating sea levels have considerable influence on atoll morphology. Within the rim, atoll lagoon bathymetry is generally relatively flat, except where punctuated by near-surface pinnacles (Purdy, Winterer 2001). So we should model lagoons as relatively flat bottomed, between two alternative recent sea levels (warm level above and ice age level below). Atoll reef structures have recently been drilled in the Indian Ocean (Braithwaite, et al. 2000) to suggest other modifications to the Darwinian theory are required to suit the geological and hydrodynamic history of the equatorial Indian Ocean. Indeed, the Darwinian model of atoll morphology was founded in the Pacific. The hydrodynamic model we have developed is suitable for north and south Pacific locations in trade-winds with steep walled reefs which reflect surf. Research at Bikini found the morphology of grooves in the seaward face of the reef coupled with surge channels in the reef flat are perfectly tuned to dissipate the prevailing wave periods (Munk, Sargent 1948). Because we are interested in the transport and settlement of larvae, nutrients, and sediment, the present study concerns net flow through passes and over reef flats, rather than the oscillatory dynamics within the narrow fringing surf line at the windward face of an atoll. So we need a model to have a water level just inside the surf zone raised above the general sea level, so that water always 1434 Proceedings of 10th International Coral Reef Symposium, 1434-1439 (2006)