Fine sediment budget on an inner shelf island of the Great Barrier Reef

The deposition and removal budget of fine sediments on the windward and leeward sides of an inner-shelf coral-fringed island (High Island) of the central Great Barrier Reef was examined.Oceanographic instruments were deployed at 3, 7, and 12 m depths during 3-15 January 2005, along two transects (windward=seaward, leeward=landward side). At each of the 6 stations, 2 sediment traps and an Analite nephelometer were mounted 0.8 m above the reef substratum. A current meter was deployed at the two 7 m depths (windward and leeward sides), with the sensors located 0.5 m above the substratum. The current meters measured 1 min-averaged currents and tides every 10 min, as well as a 20-min long burst of wave height data every hour. The nephelometers recorded 10 s averaged data (sampled every 0.5 s) every 5 min and had wipers to prevent biofouling. The sediment traps were replaced daily 3-7 January, then left for one week during a storm.The amount of sediment resuspended was measured using an Analite nephelometer using the upper 90 percentile of the nephelometer readings fromeach 3 min run. The resuspender was run at 3 replicate patches of bare substratum near each mooring site in the week before the storm. To determine how much riverine sediment is deposited on the windward and leeward sides reef and whether this sediment is flushed out during resuspension events. The data show that total sedimentation rates were about 2000 mg cm-2 yr-1 at all depths, with 30% to 60% imported from the reef-surrounding waters during calm periods, and the remaining material locally resuspended during storms. Storms resuspended fine sediment at depths less than ~ 5.5 m on the leeward reef side and ~12 m on the windward side. In these shallow waters there appeared to be a net annual sediment balance between import and export by resuspension events. Below these depths, there was no resuspension during storms; further there was a ten-fold increase of the sedimentation rate during storms and most of this additional mud originated from resuspended material from shallower waters on the island slopes. The mud had accumulated on these deeper coral reefs to levels of about 9000 mg DW cm-2, equivalent to ~ 4 years of sediment deposition that is presumably flushed out only during tropical cyclones. The data show that increased rates of advection of fine suspended sediments from soil erosion on land can result in increased sediment accumulation on inshore coral reefs below the depth of storm resuspension.

本研究针对大堡礁中部内陆架珊瑚礁环绕岛屿（海高岛，High Island）的向风侧与背风侧细颗粒沉积物的沉积-移除收支情况展开了分析。2005年1月3日至15日期间，研究团队沿两条断面布设海洋观测设备，其中向风侧对应向海侧，背风侧对应向陆侧，布设水深分别为3 m、7 m及12 m。在6个站位的每一处，均于珊瑚礁基底上方0.8 m处安装了2台沉积物捕集器（sediment traps）与1台Analite型浊度计（Analite nephelometer）。两台设于7 m水深的海流计（current meter）分别布设于向风侧与背风侧，其传感器设于基底上方0.5 m处。海流计每10分钟记录一组1分钟平均的海流与潮汐数据，并每小时采集一段时长20分钟的波浪高度连续采样数据。浊度计每5分钟记录一组10秒平均的观测数据，其采样频率为每0.5秒1次，且配备了刮拭装置以防止生物附着（biofouling）。沉积物捕集器于1月3日至7日期间每日更换，随后在一场风暴过程中连续静置一周未进行更换。沉积物再悬浮量通过Analite型浊度计测得，具体为每3分钟观测周期内浊度计读数的上90百分位数。在风暴来临前的一周，研究人员在每个锚定站点附近的3块裸露基底重复样区中开展了沉积物再悬浮模拟实验。本研究旨在明确两大科学问题：一是有多少河流来源沉积物沉积在向风侧与背风侧的珊瑚礁区域；二是这些沉积物是否会在再悬浮事件中被冲刷移除。观测数据显示，所有水深的总沉积速率约为2000 mg·cm⁻²·yr⁻¹，其中30%至60%的沉积物在平静时段由礁周水体输入，剩余物质则在风暴期间由本地沉积物再悬浮产生。风暴可在背风侧礁区约5.5 m以浅海域、向风侧礁区约12 m以浅海域引发细颗粒沉积物再悬浮。在上述浅水区，经再悬浮事件实现的沉积物输入与输出之间，年度净收支基本处于平衡状态。在上述深度以下的海域，风暴期间未发生沉积物再悬浮；且风暴期间沉积速率提升了十倍，其中新增的大部分泥质沉积物来源于岛坡浅水区的再悬浮物质。这些较深水珊瑚礁区域的泥质沉积物累积量约为9000 mg 干重（DW）·cm⁻²，相当于约4年的沉积物沉积总量，据推测这些沉积物仅会在热带气旋期间被冲刷移除。数据表明，陆地土壤侵蚀产生的细颗粒悬浮沉积物平流输运速率提升，会导致风暴再悬浮深度以下的近岸珊瑚礁区域的沉积物累积量增加。