Changes in algal, coral and fish assemblages along water quality gradients on the inshore Great Barrier Reef

Macroalgae, hard corals, octocorals, and fish were surveyed on 10 to 13 inshore coral reefs of the Great Barrier Reef, along a water quality gradient in two regions with contrasting agricultural land use. A water quality index was calculated for each reef based on available data of particulate and dissolved nutrients, chlorophyll and suspended solids. Strong gradients in ecological attributes occurred along the water quality gradient. Macroalgae of the divisions Rhodophyta and Chlorophyta increased with increasing nutrients, while Phaeophyta remained similar. Octocoral richness and abundances of many hard coral and octocoral taxa decreased, and none of the hundreds of species increased. At reefs in higher nutrient environments, hard coral and octocoral assemblages were composed of subsets of the many species found in lower nutrient environments, whereas fish and macroalgal assemblages consisted of contrasting suites of species. The study identifies species groups that are likely to increase or decrease in abundance with changing water quality.