Relative dispersion of surface drifters in a barrier reef region

Abstract

The spreading of two clusters of satellite‐tracked surface drifters was quantified by ‘relative dispersion’ and ‘relative diffusivity’ (the time derivative of relative dispersion) in two regions of the southern Great Barrier Reef (GBR): a low‐density reef matrix of complex topography and a shelf lagoon over flat bathymetry. Although based on a small data set, averaged values of these parameters over a 12 day period with concurrent measurements for the two regions indicated that relative dispersion and diffusivity of drifter pairs were 182 and 52 times greater, respectively, along the reef matrix than found in the lagoon. Relative diffusivities in the southern GBR were anisotropic and varied largely in space and time within the spatial scales of 1–100 km and temporal scales of hours to 16–20 days; they were considerably enhanced by high‐frequency tidal processes. Submesoscale processes (<20 km) were important in the southern GBR, particularly in areas with complex topography where secondary circulations around the reefs and regions of steep bathymetry caused abrupt increase in dispersion. Although the dispersion was much higher along the reef matrix, the presence of ring circulations around the islands and wake eddies in the lee of islands can act as trapping mechanisms to keep particles within the reef matrix, retarding the loss to open water areas. This provides an efficient physical mechanism for both enhancing larval connectivity between reefs and, at the same time, trapping larvae when in the vicinity of reefs.