Factors controlling suspended sediment on inner-shelf coral reefs, Townsville, Australia

Abstract

This study was undertaken to describe the characteristics of suspended sediment concentrations (SSCs) of marine waters near inner-shelf fringing coral reefs and relate these to the prevailing oceanographic and meteorological conditions. Using logging optical backscatter nephelometers, SSCs were measured at fringing reefs at Magnetic Island and on the adjacent inner-shelf, Cleveland Bay, N.E. Australia. Continuous measure-ments were made over a period of 4 months, representing possibly the most comprehensive set of SSC data collected near coral reefs. Wind, current and wave data were also collected. Temporal and spatial variation in near-bed SSCs is high. Periods of strong southeasterly regional winds generate swells, which, within 1 km of the reefs, produce near-bed SSCs of well over 200 mg/l. At the fringing coral reefs at Arthur and Geoffrey Bays, SSCs were less than 5 mg/l for most of the time and rarely exceeded 40 mg/l, but there were a number of periods of over 24 h when near-bed SSCs continuously exceeded 20 mg/l. The height of locally produced, short-period wind-waves is the dominant control on the magnitude of near-bed SSCs at the reef sites, and thus the wind regime heavily influences conditions for coral communities. The magnitude of the tide is of lesser importance. However, it is likely that flushing of these bays by tidal currents is important in preventing a long-term build-up of SSC in the water around the coral reefs.