Potential effects of a cyclone on benthic algal production and yield to grazers on coral reefs across the central Great Barrier Reef

Abstract

Rates of production of the epilithic algal community (EAC) and rates of yield to large herbivorous grazers were measured on inshore, mid-shelf and outer-shelf coral reefs in the central Great Barrier Reef (GBR) in February 1985 and February 1986. In February 1985 rates of production and yield were significantly lower inshore (approximately 10 km from the coast) than on the mid-shelf (approximately 50 km from the coast) and outer-shelf (approximately 100 km from the coast). In February 1986 cyclone Winifred passed through the central section of the GBR, causing extensive disturbance of shallow-water sediments inshore and substantially increasing both rainfall and run-off from the major river systems in the area. Inshore rates of both production of the EAC and yield to large grazers increased by a factor of five between February 1985 and February 1986. This resulted in production and yield inshore not significantly different from that on the mid-shelf and outer-shelf in February 1986. Compared to February 1985, the EAC inshore in February 1986 had a significantly lower carbon/nitrogen ratio, a significantly higher standing crop of nitrogen and a significantly higher yield of nitrogen to grazers. It is suggested that reworking of mineralized organic nitrogen from inshore sediments and increased river run-off, both due to the cyclone, increased the availability of nutrients on inshore coral reefs. These nutrients were incorporated into benthic algal production and passed rapidly to the next trophic level. These results are not consistent with the idea that high biomass of algae on inshore reefs is due to higher production, in turn due to higher nutrient input to those reefs. This conclusion has important implications for the interpretation and management of cross-shelf differences in reef benthos.