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.

本研究选取的三处样地用于探究丝状藻垫（algal turfs）与珊瑚的竞争关系，均坐落于岸礁的礁坡之上，分别为古尔德岛（Goold Island）样地，以及大棕榈岛（Great Palm Island）上的库尔加里湾（Coolgaree Bay）与坎农湾（Cannon Bay）。所选样地旨在代表受赫伯特河（Herbert River）径流影响的跨陆架暴露梯度：赫伯特河以北的古尔德岛，直接处于通常向北扩散的洪水羽流路径之中；而位于赫伯特河以南的大棕榈岛则离岸更远，极少受到河流径流的影响。大棕榈岛上的两处样地存在显著差异：库尔加里湾位于该岛遮蔽的近岸一侧，坎农湾则更邻近外海水域。

1996年10月，我们在每处礁坡上布设了12个13×19 cm的样方，所有样方均选取包含丝状藻垫与团块滨珊瑚（Porites lobata）直接边界的区域。对所有样方完成拍照后，将12个样方随机分配至三种处理组，每组各4个样方：第一组采用钢丝刷与油漆刮刀去除样方内的藻类，且不破坏边界处的珊瑚组织；第二组刮除珊瑚组织与黏液的表层，但未完全移除活珊瑚组织；第三组为对照组，不进行任何人为干预。7个月后，再次对所有样方进行拍照。

将每个样方的初始与最终照片中的藻-珊瑚边界描摹至塑料胶片上，将两份描摹图叠加后，进一步描摹出边界处发生变化的区域。将描摹图扫描后，采用图像分析软件测量藻类与珊瑚的覆盖面积。

本研究于1998年10月采集了定量水质数据，测定指标涵盖溶解态无机氮、总氮、总磷、叶绿素a、悬浮颗粒物、浊度及盐度。通过将1992年以来针对同一样地开展的多轮调查所获取的同类水质数据与之合并，最终得到长期水质数据集。

本研究旨在探究两个核心科学问题：其一，珊瑚与丝状藻垫是否在礁体空间上存在直接竞争关系；其二，当样地距离陆地源更近，导致营养盐与沉积物可利用性提升时，竞争结果是否会因此发生改变。