Centre for Sustainable Aquatic Ecosystems, Harry Butler Institute, Murdoch University, Murdoch, Western Australia 6150, Australia. Electronic address: sorcha.cronin-o'
Centre for Sustainable Aquatic Ecosystems, Harry Butler Institute, Murdoch University, Murdoch, Western Australia 6150, Australia.
Sci Total Environ. 2024 Dec 1;954:176356. doi: 10.1016/j.scitotenv.2024.176356. Epub 2024 Sep 17.
Due to human activity, ecosystems are exceeding their ecological thresholds and shifting into undesired alternative stable states with new ecological configurations. Despite their purported ubiquity, it is uncertain whether estuaries can exist in multiple stable states. We use data from a 3.5-year study of invertebrate communities in an Australian estuary that is usually closed to the ocean to test for their existence. Sampling spanned a 1.5-year period of hypersalinity (>40 ppt) during a prolonged estuary closure, where salinity reached 122 ppt, and for two years during and after the estuary opened to the ocean when salinities were mesohaline (5-19 ppt). Two distinct community states occurred before and after the sandbar breached, with an intermediary period of invertebrate community impoverishment due to sediment scouring. During the closure, the community was simple (average of one taxa 100 cm) and dominated by larvae of terrestrial insects, most notably the halotolerant, non-biting midge Tanytarsus barbitarsis. After opening, the richness and abundance of invertebrates increased (average of four taxa and 84 individuals 100 cm) as polychaetes, molluscs and crustaceans colonised the estuary, although recovery was incomplete according to previous species records. Duration of estuary closure and salinity were the strongest drivers of composition. This study, together with evidence from the literature, suggests a salinity threshold of 60-65 ppt between states. These empirical data meet key criteria of alternative states, i.e. a clear transition between two distinct self-sustaining communities, indicating a regime shift triggered by an exogenous event. Our findings suggest that temporarily open and closed estuaries can exist in alternative stable states, with prolonged closures, hypersalinity, and sandbar breaching being key determinants of the switch between states. This situation may apply to other low-inflow estuarine systems, particularly in arid, semi-arid, or seasonally arid climates, and may become more frequent with human-induced climate change.
由于人类活动，生态系统正超过其生态阈值，转变为具有新生态结构的非理想稳定状态。尽管据称它们无处不在，但仍不确定河口是否可以存在于多个稳定状态中。我们使用了澳大利亚一个通常与海洋隔绝的河口的 3.5 年无脊椎动物群落数据来检验它们的存在。采样跨越了一个 1.5 年的时间，期间该河口经历了长时间的关闭，盐度达到了 122ppt，随后在河口向海洋开放的两年期间和之后，盐度为中盐度（5-19ppt）。在沙洲决堤之前和之后出现了两种截然不同的群落状态，中间有一个因泥沙冲刷而导致无脊椎动物群落贫瘠的时期。在关闭期间，群落简单（平均每 100 厘米有一个分类群），主要由陆地昆虫的幼虫组成，尤其是耐盐、不吸血的摇蚊 Tanytarsus barbitarsis。开放后，随着多毛类、软体动物和甲壳类动物在河口的殖民化，无脊椎动物的丰富度和丰度增加（平均每 100 厘米有四个分类群和 84 个个体），尽管根据以前的物种记录，恢复并不完整。河口关闭的持续时间和盐度是影响组成的最强驱动因素。这项研究与文献中的证据一起表明，状态之间存在 60-65ppt 的盐度阈值。这些经验数据符合替代状态的关键标准，即两个明显不同的自我维持群落之间的清晰过渡，表明外生事件引发了制度转变。我们的研究结果表明，暂时开放和关闭的河口可以存在于替代的稳定状态中，长时间的关闭、高盐度和沙洲决堤是状态之间转换的关键决定因素。这种情况可能适用于其他低流入河口系统，特别是在干旱、半干旱或季节性干旱气候下，并且随着人为气候变化的发生可能会变得更加频繁。
