View of the unstructured grid at the Whitsunday Islands and zoomed in on Lindeman andShawIslands.

During the past year, two separate projects with external collaborators have enabled AIMS oceanographers to contribute to better understanding of water circulation in theGreat Barrier Reefand its implications for biological and chemical processes.In the first project, in collaboration with modellers fromJamesCookUniversity, a shelf-scale 3-D circulation model of the Great Barrier Reef was used to simulate the fate of materials discharged at the coast and the residence times of water-borne materials within theGreat Barrier Reefshelf sea system. The model results indicate that most materials discharged near the coast travel long distances north and south before leaving the reef system rather than mixing the shorter distance across the continental shelf and into theCoral Sea. A significant proportion of coastal water tracers released in the model remained within the reef system for periods ranging from months to over one year, indicating that biological processes in reef waters and sediments have the opportunity to recycle land-sourced nutrients and other materials many times before they leave the reef.The second project looked at balancing the contrasting demands of a system as large and spatially complex as theGreat Barrier Reefwith the details of circulation and mixing at small spatial scales which are important factors in many biological and chemical processes. A new 2-D non-structured grid model of circulation for theGreat Barrier Reefhas been implemented in collaboration with Belgian modellers from Université Catholique de Louvain. This model, with approximately 1 million grid points for computations, balances detail and effort by having close but irregularly spaced grid points (to 100 m) in reef areas with complicated bathymetry and more widely spaced grid elements (to 2 km) in areas of open water. The new model resolves complex flows around reefs, eddies and flow stagnation points, while capturing large-scale flows in an efficient manner. As this model is further developed, it is expected to bring further realism to computer simulations of fundamental problems such as larval connectivity among reef populations.