Recurring bleaching events disrupt the spatial properties of coral reef benthic communities across scales

Marine heatwaves are causing recurring coral bleaching events on tropical reefs that are driving ecosystem change. Yet little is known about how bleaching and subsequent coral mortality impacts the spatial properties of tropical seascapes, such as patterns of organism spatial clustering and heterogeneity across scales. Changes in these spatial properties can offer insight into ecosystem recovery potential following disturbance. Here we repeatedly quantified coral reef benthic spatial properties around the circumference of an uninhabited tropical island in the central Pacific over a 9-year period that included a minor and severe marine heatwave. Benthic communities showed increased biotic homogenisation following both minor and mass bleaching, becoming more taxonomically similar with less diverse intra-island community composition. Hard coral cover, which was highly spatially clustered around the island prior to bleaching, became less spatially clustered following minor bleaching and was indiscernible from a random distribution across all scales (100–2000 m) following mass bleaching. Interestingly, the reduced degree of hard coral cover spatial clustering was already evident by the onset of mass bleaching and before any dramatic wholesale loss in island-mean coral cover occurred. Reductions in hard coral spatial clustering may therefore offer an early indication of the ecosystem becoming degraded prior to mass coral mortality. In contrast, the spatial clustering of competitive fleshy macroalgae remained unchanged through both bleaching events, while crustose coralline algae and fleshy turf algae became more spatially clustered at larger scales (200–700 m) following mass bleaching. Overall, benthic community spatial patterning became less predictable following bleaching and was no longer reflective of gradients in long-term environmental drivers that typically structure these remote reefs. Our findings provide novel insights into how climate-driven marine heatwaves can impact the spatial properties of coral reef communities over multiple scales.海洋热浪正引发热带珊瑚礁反复出现珊瑚白化事件，进而推动生态系统发生转变。然而，目前学界对珊瑚白化及后续珊瑚死亡如何影响热带海洋景观（seascapes）的空间属性——例如不同尺度下的生物空间聚集（spatial clustering）与异质性（heterogeneity）模式——仍缺乏足够认知。上述空间属性的变化，可为理解扰动后生态系统的恢复潜力提供重要依据。本研究于9年周期内，对中太平洋一座无人热带岛屿周边海域的珊瑚礁底栖（benthic）空间属性开展了重复量化，研究期间经历了一次轻度与一次极端海洋热浪事件。无论轻度还是大规模白化事件后，底栖群落均出现生物均质化（biotic homogenisation）程度加剧的现象：群落分类学相似度提升，岛内群落组成的多样性显著降低。白化发生前，造礁珊瑚覆盖率在岛屿周边海域呈现高度空间聚集特征；轻度白化后，其空间聚集程度有所下降；大规模白化后，其在所有尺度（100~2000米）下均与随机分布无显著差异。值得注意的是，造礁珊瑚覆盖率的空间聚集程度下降，在大规模白化发生时便已显现，甚至早于岛屿平均珊瑚覆盖率出现大幅整体损失之前。因此，造礁珊瑚空间聚集程度的下降，或可成为大规模珊瑚死亡前生态系统退化的早期预警信号。与之形成鲜明对比的是，两类具有竞争优势的肉质大型藻类（fleshy macroalgae）的空间聚集程度在两次白化事件中均未发生改变；而大规模白化后，壳状珊瑚藻（crustose coralline algae）与肉质 turf藻（fleshy turf algae）在较大尺度（200~700米）下的空间聚集程度显著提升。总体而言，白化事件后底栖群落的空间格局可预测性下降，且不再反映通常塑造这些偏远珊瑚礁的长期环境驱动因子梯度。本研究结果为气候驱动的海洋热浪如何多尺度影响珊瑚礁群落的空间属性提供了全新的科学见解。