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.
This study was undertaken to examine light profiles and coral species composition beneath tabulate corals. These observations were used to assess the competitive value of shading.

1980年11月，研究人员于大堡礁中部的里布礁（Rib Reef）与布罗德赫斯特礁（Broadhurst Reef）的向海礁坡上，对板状珊瑚下方的光照剖面展开测量。本次研究共选取14个以垂直中央立柱支撑的近乎圆形珊瑚群落开展光照测量，分别为杯形轴孔珊瑚（Acropora cytherea）9个、紫扇轴孔珊瑚（Acropora hyacinthus）3个、尖轴孔珊瑚（Acropora subulata）1个以及网格轴孔珊瑚（Acropora clathrata）1个。所选珊瑚的生长水深约为9.5米，且与其他板状珊瑚的间距不小于0.5米。
光照强度（单位：毫瓦每平方厘米，mW cm⁻²）采用搭载远程探头的设备进行测量，该探头配备水平安装的余弦采光器，其光谱响应曲线平坦，适配光合有效辐射（Photosynthetically Active Radiation, PAR）的测量需求。在杯形轴孔珊瑚（Acropora cytherea）与紫扇轴孔珊瑚（Acropora hyacinthus）各一个珊瑚盘下方，沿指向北的半径方向水平布设标尺，并从珊瑚盘垂直向下延伸至基底。珊瑚盘下方半径方向的光照测量点共设25个，由5条从立柱到珊瑚盘边缘均匀分布的垂直线，与5条从珊瑚盘下表面下方5厘米处至基底均匀分布的水平线相交构成。通过对25个测量点的读数进行插值计算，绘制出半径方向下方的光照条件等值线图。
针对其余12个轴孔珊瑚属（Acropora spp.）的珊瑚盘，研究人员在基底上沿水平方向设置5个等间距测点开展光照测量。随后，将珊瑚盘以测量半径线平分的半圆形区域用铝箔覆盖，以模拟完全融合且不透光的珊瑚盘，并重做光照强度测量。同时记录每个珊瑚盘表面的光照强度。所有测量时段均为晴朗无云的晴天。
研究人员测量了每个珊瑚盘的高度与若干半径参数，并采集样本用于物种鉴定。对珊瑚盘半圆形区域下方的珊瑚物种进行了分类鉴定，测量了每个群落与立柱的距离，并将其分布划分为5个同心环带。在另外7个板状珊瑚旁布设0.5×0.5米的样方（quadrats），对样方内的珊瑚群落进行计数与物种鉴定，并通过目视估算珊瑚盖度。将上述样方数据与珊瑚盘下方的测量结果进行对比分析。
本研究旨在探究板状珊瑚下方的光照剖面特征与珊瑚物种组成，以此评估遮阴作用的竞争价值。