Residence times of neutrally-buoyant matter such as larvae, sewage or nutrients on coral reefs

Abstract

Coral reef flushing times at an individual reef scale are specified and a general formula to determine these times is developed. The formula is confirmed by comparison with residence times predicted by numerical small-scale reef models, including those from a 4 month unsteady current simulation of John Brewer Reef on Australia's Great Barrier Reef. The method proves to be a satisfactory alternative to the numerical modelling. When neutrally-buoyant material around a reef is removed by the currents, the concentrations decay exponentially. The decay rate depends primarily on free stream current and reef dimensions. Secondary factors are the tidal excursion, shelf depth, lagoon size and residual current in the lee of the reef. These factors, when combined into a decay coefficient, specify the rate of loss of neutrally-buoyant material (e.g. some larvae, pollutants and sewage) from a coral reef and its surrounds. The analytical formula can be used to predict the flushing rates or the percentage of material still remaining on a reef after a selected time interval. We demonstrate that material can remain on or near typical reefs in common weather conditions for several weeks.