WAMSI 2 - Dredging Node - Project 5.4 - Recovery mechanisms

The objective of Project 5.4 was to determine whether recovery of seagrass in the Pilbara following disturbance is by sexual (recruitment from seeds) or asexual (vegetative regrowth from rhizome extension) means, and the relative importance of each, thereby determining the capacity, timeframes and mechanisms of recovery from light and sediment deposition effects. This was achieved through an experiment in which plots cleared of seagrass were enclosed by a barrier or left open, and changes in cover were compared to unmanipulated control (and procedural control) plots. The experiment was done at two sites, a 2-m deep (the 'shallow' site) and a 6-m deep (the 'deep' site). The experiment was intended to run for 6 months, but after 5 months disturbance caused by Tropical Cyclone (TC) Olwyn reduced seagrass cover at both sites, and removed the experimental apparatus at the shallow site. Nevertheless, results yielded by surveys prior to TC Olwyn unambiguously showed strong evidence for recovery through vegetative regrowth (full recovery in cleared plots with no barriers, plus a pattern of increasing cover from the edges of the plots) and no evidence for recovery through recruitment from seeds (no seagrass was ever recorded in any cleared plot with a barrier). The main species of seagrass present was Halophila ovalis, which is widespread throughout northwestern Australia. The transferability of inferences from this study to other places in the Pilbara is hampered by the substantial variation in abundance and species composition from place to place - different species might have different mechanisms of recovery. The nature of the experimental disturbance (complete removal of all seagrass, including roots and rhizomes) is a reasonable facsimile of a severe dredging-induced disturbance, but the spatial extent of the experimental clearances (~0.5 m2) is orders of magnitude smaller than the spatial extents of dredging-induced mortality induced elsewhere: it is plausible that recovery of patches within meadows is more easily achieved through rhizome extension than recovery that encompasses spatial extents of hectares.

项目5.4的研究目标为明确皮尔巴拉（Pilbara）海域受扰动后的海草恢复途径究竟是有性繁殖（通过种子萌发定植）还是无性繁殖（通过根茎延伸实现营养再生），并量化二者的相对重要性，进而明确海草在光照与沉积物沉积胁迫下的恢复能力、恢复时限与恢复机制。本研究通过受控实验开展相关研究：将已清除海草的样地分别设置封闭屏障或保持开放状态，并将其盖度变化与未受干扰的对照样地（含程序对照样地）进行对比。实验设置了两个站位：水深2米的“浅水区站位”与水深6米的“深水区站位”。实验原计划持续6个月，但在推进至第5个月时，热带气旋（Tropical Cyclone, TC）“奥尔温”引发的扰动导致两个站位的海草盖度均出现下降，且浅水区站位的实验装置完全损毁。尽管如此，热带气旋来临前的调查结果已提供确凿证据证实：海草恢复主要通过营养再生途径实现——无屏障的清除样地实现了完全恢复，且样地边缘区域的盖度随时间逐渐提升；未发现任何通过种子萌发定植实现恢复的证据（所有设置屏障的清除样地均未监测到海草生长）。本研究涉及的优势海草物种为喜盐草（Halophila ovalis），该物种广泛分布于澳大利亚西北部海域。由于不同区域的海草丰度与物种组成存在显著差异，本研究结论对皮尔巴拉其他海域的推广适用性受到限制——不同海草物种可能具备不同的恢复机制。此外，本实验的扰动方式为彻底清除所有海草（包括根系与根茎），该方式可近似模拟严重疏浚引发的扰动，但实验清除样地的空间尺度（约0.5平方米）远小于其他地区疏浚导致的海草死亡范围（相差数个数量级）。由此可推测：海草草场内局部斑块的恢复通过根茎延伸即可高效实现，而涉及公顷级空间尺度的整体恢复则难度更高。