Seasonal rainfall and runoff promote coral disease on Magnetic Island fringing reefs, Great Barrier Reef

Sites used for this study of the coral disease atramentous necrosis (AN) were located in two adjacent bays, Nelly Bay and Geoffrey Bay, on the south-eastern side of Magnetic Island, which is situated within the inner shelf region of the Great Barrier Reef. Both bays have fringing coral reefs and are similar in shape, physical structure, and hydrodynamic setting. The study was conducted between December 2007 and December 2009. Sampling was conducted every 2 weeks in the austral summer (Nov-Apr) and once a month in the winter (May-Oct). Increased sampling frequency in summer was based on the hypothesis that AN increases with warm water temperatures. Disease dynamics were assessed in three permanent 5x5 m quadrats at 3-5 m depth at each site. Coral colonies demonstrating signs of AN were tagged with numbered plastic tags attached to cable ties. Visual surveys were able to clearly distinguish the four stages in the development of AN lesions: Stage 1 lesions are small (1-2 cm diameter) areas of bleached but intact tissue; Stage 2 lesions are white skeleton devoid of tissue; Stage 3 lesions are covered with a white bacterial film; and in Stage 4, a black, sulphurous deposit accumulates under the white film likely the result of opportunistic secondary microbial community. The last two stages were combined in this study because they generally occur simultaneously and an additional stage (S) was used when disease progression stopped.The diseased corals were all colonies of Montipora aequituberculata, which was the most prevalent species of Montipora in the quadrats. New disease cases (disease incidence) were counted and tagged in each plot during each survey. New AN infections, in addition to both lesion progression and cessation, were monitored on individual colonies to elucidate spatio-temporal patterns in disease dynamics. Due to logistical constraints, Geoffrey Bay was not sampled in February 2008.On each sampling occasion, two replicate water samples were collected in 1-L plastic bottles 1 m above the coral and on opposite sides of the quadrats for the analysis of concentrations of: dissolved organic carbon (DOC), chlorophyll a (chl-a), particulate organic carbon (POC), particulate nitrogen (PN), particulate phosphorus (PP) and suspended solids (SS). Due to logistical reasons, only two replicate water samples were used. Salinity was measured at each sampling occasion with a handheld refractometer (r2 Mini, Reichert GmbH, Germany). Temperature was measured using a temperature logger (ODYSSEY data recording systems, Christchurch, New Zealand) attached underneath a sediment trap in both Nelly Bay and Geoffrey Bay. It was retrieved and downloaded approximately every 2 months. Temperature data from sensors were combined with data collected by the Australian Institute of Marine Science (AIMS) sea surface temperature monitoring program in the same two bays (data available at http://www.aims.gov.au). Maximum temperatures were calculated for the periods of 7 and 14 days up to and including the sampling date.Two sediment traps (40 cm high with a diameter of 10 cm) were deployed 10 m apart close to the permanent 5x5 m quadrats at each site. Traps were collected at every second sampling occasion in the winter and on each occasion in the summer. The ash-free dry weight (AFDW) of the sediment was used as a coarse measure of the organic content of the sediment.Rainfall data for Townsville were obtained from the Australian Bureau of Meteorology web site (http://www.bom.gov.au). This study was undertaken to investigate a possible connection between the seasonal dynamics of a coral disease and parameters associated with water quality on the GBR. The aims of the study were to:(i) document seasonal dynamics of atramentous necrosis (AN) and seasonally varying environmental parameters(ii) analyse relationships between disease prevalence and these parameters to identify potential environmental drivers of AN within populations of the coral Montipora aequituberculata on an inshore GBR reef.

本研究针对珊瑚黑色坏死病（atramentous necrosis, AN）的调查样地，位于大堡礁内陆架区磁岛（Magnetic Island）东南侧的两个相邻海湾——内利湾（Nelly Bay）与杰弗里湾（Geoffrey Bay）。两处海湾均发育裙礁，在形态、物理结构与水动力环境上高度相似。研究周期为2007年12月至2009年12月。采样频率设置为：南半球夏季（11月至次年4月）每2周采样1次，冬季（5月至10月）每月采样1次——夏季提升采样频次的依据为“AN发病率随水温升高而上升”的假说。

在每个样地的3-5米水深处，设置3个固定的5×5米样方，用于监测病害动态。表现出AN症状的珊瑚群体，使用绑定扎带的编号塑料标签进行标记。通过目视调查可清晰区分AN病灶发育的四个阶段：阶段1病灶为直径1-2厘米的白化但组织完整的区域；阶段2病灶为裸露无组织的白色骨骼；阶段3病灶表面覆盖白色细菌膜；阶段4则在白色膜下积累黑色硫沉积物，该现象大概率由机会性次生微生物群落活动所致。本研究将后两个阶段合并分析，因其通常同步发生；当病害进程停止时，额外标记为阶段（S）。

所有受感染的珊瑚群体均为结节微孔珊瑚（Montipora aequituberculata），该物种为样方内分布最广泛的微孔珊瑚属物种。每次调查均统计并标记各区域内的新增发病个体（发病率），同时监测单个珊瑚群体的新增AN感染、病灶进展与病害停止情况，以阐明病害动态的时空分布模式。受后勤条件限制，杰弗里湾在2008年2月未开展采样。

每次采样时，在珊瑚上方1米处、样方两侧的位置，使用1升塑料瓶采集两份平行水样，用于分析溶解有机碳（dissolved organic carbon, DOC）、叶绿素a（chlorophyll a, chl-a）、颗粒有机碳（particulate organic carbon, POC）、颗粒氮（particulate nitrogen, PN）、颗粒磷（particulate phosphorus, PP）与悬浮颗粒物（suspended solids, SS）的浓度。受后勤条件限制，本研究仅设置两份平行水样。每次采样均使用手持折射仪（r2 Mini，德国赖希特有限公司（Reichert GmbH, Germany））测定盐度；温度则通过部署于内利湾与杰弗里湾沉积物捕集器下方的温度记录仪（ODYSSEY数据记录系统，新西兰克赖斯特彻奇）进行监测，约每2个月取回并下载数据。将该传感器获取的温度数据，与澳大利亚海洋科学研究所（Australian Institute of Marine Science, AIMS）在上述两海湾开展的海表温度监测项目数据进行整合（数据可从http://www.aims.gov.au获取）。本研究计算了采样日期前7天与14天（含采样当日）的最高水温。

在每个样地的固定5×5米样方附近，间距10米处部署两个沉积物捕集器（高40厘米，直径10厘米）。冬季每两次采样采集一次捕集器样品，夏季则每次采样均采集。以沉积物的灰分干重（ash-free dry weight, AFDW）作为沉积物有机质含量的粗略评估指标。

汤斯维尔市的降雨数据来自澳大利亚气象局官网（http://www.bom.gov.au）。

本研究旨在探究珊瑚病害的季节动态与大堡礁（Great Barrier Reef, GBR）近岸海域水质相关参数之间的潜在关联，具体研究目标为：(i) 记录黑色坏死病（AN）的季节动态以及随季节变化的环境参数；(ii) 分析病害流行率与上述环境参数之间的关联，以识别大堡礁近岸礁区结节微孔珊瑚（Montipora aequituberculata）种群中AN发生的潜在环境驱动因子。