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. This research was undertaken to determine:1. whether corals and filamentous algal turfs were directly competing for space on reefs2. whether the outcome of competition was enhanced by increased availability of nutrients and sediments due to closer proximity to terrestrial sources.

本研究选取3个位点开展藻席（algal turfs）与珊瑚的种间竞争研究，这3个位点分别位于古尔德岛（Goold Island）岸礁的礁坡，以及大棕榈岛（Great Palm Island）上的库利加里湾（Coolgaree Bay）与坎农湾（Cannon Bay）。所选位点旨在代表受赫伯特河（Herbert River）径流影响的跨陆架梯度暴露特征。赫伯特河北侧的古尔德岛直接处于洪水羽流的路径上，此类羽流通常向北移动；而位于赫伯河南侧的大棕榈岛则距岸更远，极少受到河流径流的影响。大棕榈岛上的两个位点存在差异：库利加里湾位于该岛受庇护的近岸一侧，坎农湾则更易受外海水体影响。

1996年10月，我们在每个礁体上布设了12个13×19 cm的样方（quadrat），每个样方均包含藻席与滨珊瑚（Porites lobata）的直接交界区域。先对所有样方进行拍照，随后将样方随机分为3组，每组4个样方，分别接受3种处理：第一种处理为使用钢丝刷与油漆刮刀清除样方内的藻席，但不破坏交界区域的珊瑚组织；第二种处理为刮除珊瑚组织与黏液的表层，但不彻底移除活珊瑚组织；第三种处理为对照组，不进行任何人工干预。7个月后，再次对所有样方进行拍照。

将每个样方的初始与终期照片中的藻-珊瑚交界位置描摹至塑料胶片上，将两张描摹图进行叠加，并进一步描记交界区域发生变化的部分。随后扫描描摹图，利用图像分析软件测算藻席与珊瑚的覆盖面积。

本研究于1998年10月采集了量化水质数据，测定指标包括溶解态无机氮、总氮、总磷、叶绿素a、悬浮颗粒物、浊度与盐度。通过将该批数据与1992年以来在相同位点开展的多轮调查所获的同类数据整合，得到了长期数据集。本研究旨在明确两个科学问题：
1. 珊瑚与丝状藻席（filamentous algal turfs）是否在礁体空间上存在直接竞争关系；
2. 当距离陆地污染源更近，导致营养盐与沉积物可利用性提升时，是否会强化该竞争的结果。