The role of coral interactions in maintaining unoccupied substrate in the central Great Barrier Reef

Prior to commencement of the main study, samples of "unoccupied" substrate were collected from less than 2 cm and from greater than 10 cm distant from coral colonies at a depth of 10 m on Davies Reef. Samples were examined under x10 magnification and the biota attached to the substrate categorised.Survey plots were established on the fore-reef slopes of Pandora Reef, Rib Reef, Myrmidon Reef and Davies Reef in the central Great Barrier Reef and Flinders Reef in the Coral Sea. To investigate substrate occupancy, 2 X 2 m plots were sampled in high diversity areas at depths of 10 m at Myrmidon Reef and 7.5 m at Davies Reef. Plots were also sampled at 25 m at Davies Reef and in the wave exposed area of Myrmidon Reef at 2 m. Substrate cover values within the interactive reach of coral colonies were recorded from contiguous areas around each coral colony out to their maximum reach. For many species this reach was known. If it was unknown, 1 cm was assigned as this is the minimum for most. The percent cover of bare substrate, corals, soft corals and algae was determined around every colony and totalled for each plot. Total coral perimeter and perimeter undergoing interaction with another species, was also measured.The mean cover values in the plots as a whole was also determined. A tape measure was laid irregularly and randomly through the sample plots so that in all places the tape was in contact with the substrate and followed all reef contours. The substrate at 5 cm intervals along the tape was recorded together with whether that point lay within interactive reach of a coral. From each plot, 200 to 1000 points were recorded.Additional data were obtained from all 5 reefs using the tape method, at 2 or 5 m depth intervals to 35 m deep or to the foot of the reef slope. These data were used to determine the amount of unoccupied substrate at different depths and distances of the reef from shore, as well as the proportion of the total unoccupied reef substrate which lay within interactive reach of corals.In March 1981, 10 permanently marked quadrats with a total area of 11.2 m² were established at Davies Reef at a depth of 10m and examined at three monthly intervals for 1 year. At time 0, interactions in the quadrats were counted and categorised and descriptions and sketches were made to permit them to be relocated later. Photographs were taken of each quadrat to aid relocation, but the resolution was not adequate for taking measurements. During subsequent visits, each interaction was reexamined to determine whether it continued, or the apparent cause of its termination. Newly commenced interactions were also recorded.
 It is often assumed that space on a coral reef slope is limited and that corals compete for it. This study was undertaken to:(1) quantify unoccupied substrate on reef surfaces and determine whether any of it is attributable to coral colonies(2) examine some of the relevant temporal aspects of coral interactions(3) show the broad spatial pattern of unoccupied space across the Great Barrier Reef transect and determine what proportion of it might be accounted for by the interactive mechanisms of corals

Organization: Australian Ocean Data Network