Light profiles and coral communities beneath tabulate Acropora corals, Great Barrier Reef

Light profiles beneath tabulate corals on the seaward reef slopes of Rib and Broadhurst Reefs, central Great Barrier Reef, were measured in November 1980. A total of 14 nearly circular colonies, each supported by a vertical central stem were selected for light measurements: Acropora cytherea (9), Acropora hyacinthus (3), Acropora subulata (1) and Acropora clathrata (1). The corals chosen were located at a depth of approximately 9.5m and separated from other tabulate corals by at least 0.5m.

Irradiance (mW cm-2) was measured with a remote probe, having a horizontally held cosine collector with a flat spectral response corresponding to photosynthetically active radiation (PAR).

Under one table each of Acropora cytherea and Acropora hyacinthus, rules were laid horizontally along the north pointing line of radius and vertically from table to substrate. Light measurements were made at 25 points under the table radius at the intersections of 5 vertical lines spaced equally from stem to perimeter and 5 horizontal lines spaced equally from 5 cm below the undersurface of the table to the substrate. Contours of the light conditions under the radii were produced by interpolation from the readings at the 25 points.

Beneath the remaining 12 Acropora spp. tables, light measurements were taken at 5 horizontally spaced positions along the substrate. Then, with the semicircular half of the table bisected by the radial line of measurement covered with aluminium foil representing completely anastomosed or light-impermeable tables, irradiance measurements were repeated. Irradiance on each table surface was recorded. In all cases unobscured sun persisted during the measurement period. Height, and several radii of each table were measured and a piece taken for identification.

Coral species beneath the semicircular sections of the tables were identified. The distance of each colony from the stem was measured and their distribution categorised into 5 concentric zones. Quadrats of 0.5 x 0.5 m were laid beside 7 other tablulate corals. The colonies in the quadrats were counted and identified, and cover was estimated by eye. The values obtained were compared to those beneath the tables.