## ABSTRACT

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

The Great Barrier Reef (GBR) (Figure 1) is characterised by a juxtaposition of regions of low reef density (where the reefs block only 10% of the length along the shelf) and high reef density (where the reefs block about 90% of the length; Pickard et al., 1977). Each of these regions is a few hundred kilometres in length. A large spring-neap tide cycle exists on the GBR. Wolanski (1994) coined the term “sticky water” to explain why regions of high reef density may be less permeable to lowfrequency currents at spring tides than at neap tides due to purely physical reasons. Wolanski and Spagnol (2000) further investigated this effect numerically. They used the two-dimensional model of King and Wolanski (1996) for a model barrier reef. In this idealised bathymetry the reefs were assumed to be rectangular. Also, the prevailing tidal and mean currents were parallel to each other. The prevailing currents were oriented perpendicular to the longest sides of the rectangles. To illustrate the blocking effect, passive tracers were seeded upstream of the matrix of reefs. Only half as much tracers filter through an ideal model reef matrix at spring tides than at neap tides; the rest was deflected sideways. This deflection was due to energy dissipation by bottom friction and island wakes. Further investigation into this effect for a realistic bathymetry and realistic currents could not be carried out due to lack of high resolution bathymetry data for the study region.