Competition between corals and algal turf along a water quality gradient in the nearshore central Great Barrier Reef

The three sites chosen to study competition between algal turfs and coral were located on the reef slope of the fringing reefs at Goold Island, and Coolgaree Bay and Cannon Bay on Great Palm Island. The sites were chosen to represent a cross-shelf gradient in exposure to runoff from the Herbert River. Goold Island, north of the Herbert River, lies directly in the path of flood plumes, which generally move northward. Palm Island, south of the Herbert River is further offshore and rarely exposed to river runoff. The two sites on Great Palm Island differed in that Coolgaree Bay is on the sheltered, inshore side of the island and Cannon Bay is more exposed to offshore waters.

In October 1996, twelve 13 x 19 cm quadrats, selected to include a direct boundary between turf algae and coral (Porites lobata) were marked out at each reef. The quadrats were photographed and then four quadrats were randomly assigned to each of three treatments. In the first treatment algae was removed from the quadrat using a wire brush and paint scraper without damaging the coral tissue along the boundary. The second treatment involved scraping the top layer of the tissue and mucus of the coral, but not completely removing the living coral tissue. The third treatment was the control and no manipulation occurred. Quadrats were photographed again after 7 months.

The position of the boundary between algae and coral were traced from the initial and final photograph of each quadrat onto plastic film. The tracings were overlapped and a further tracing made of areas along the boundary that had changed. The tracings were scanned and the area of algae and coral was measured using image analysis software.

Quantitative water quality data was collected for this study in October 1998. Dissolved inorganic and total nitrogen and phosphorus, chlorophyll a, suspended particulate matter, turbidity and salinity were measured. A longer term data set was achieved by combining these data with similar data collected from the same sites during various surveys since 1992.