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3处用于研究藻席（algal turfs）与珊瑚竞争关系的样地，均位于古尔岛（Goold Island）岸礁（fringing reefs）的礁坡，以及大棕榈岛（Great Palm Island）的库勒加里湾（Coolgaree Bay）与坎农湾（Cannon Bay）。上述样地的选取旨在表征受赫伯特河（Herbert River）径流影响的跨陆架暴露梯度特征。赫伯特河以北的古尔岛，直接处于通常向北移动的洪水羽流（flood plumes）路径上；而位于赫伯特河以南的大棕榈岛则离岸更远，极少受到河流径流影响。大棕榈岛上的两处样地存在差异：库勒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，我们在每处礁区划定了12个13×19 cm的样方（quadrats），每个样方均选取包含藻席与叶状微孔珊瑚（Porites lobata）的直接交界区域，并对样方进行拍照。随后将4个样方随机分配至3种处理组中：第一组处理为藻类清除组，使用钢丝刷与油漆刮刀清除样方内的藻类，且不损伤交界区域的珊瑚组织；第二组处理为珊瑚刮擦组，刮除珊瑚组织表层与粘液，但未完全移除活珊瑚组织；第三组为对照组，不施加任何人工干预。7个月后，再次对所有样方进行拍照。

将每个样方初始与最终照片中的藻-珊瑚交界区域描记至塑料胶片上，将两份描记图重叠后，进一步描记交界区域发生变化的部分。随后扫描描记图，通过图像分析软件（image analysis software）测算藻席与珊瑚的覆盖面积。

本研究于1998年10月采集了定量水质数据，检测指标涵盖溶解态无机氮磷（dissolved inorganic nitrogen and phosphorus）、总氮总磷（total nitrogen and phosphorus）、叶绿素a（chlorophyll a）、悬浮颗粒物（suspended particulate matter）、浊度（turbidity）与盐度（salinity）。通过将本次采集的数据与1992年以来针对同一样地开展的多轮调查获取的同类数据整合，最终构建得到长期监测数据集。

本研究旨在解答两个科学问题：1. 珊瑚与丝状藻席（filamentous algal turfs）是否在礁区空间上存在直接竞争关系；2. 相较于距离陆源（terrestrial sources）更近的区域，营养盐与沉积物可利用性的提升是否会加剧这类竞争的结局。