Data from: Evidence for water-mediated mechanisms in coral-algal interactions

Although many coral reefs have shifted from coral to algal dominance, the consequence of such transition for coral-algal interactions and their underlying mechanisms remains poorly understood. At the microscale, it is unclear how diffusive boundary layers (DBLs) and surface oxygen concentrations at the coral-algal interface vary with algal competitors and competitiveness. Using field observations and microsensor measurements in a flow chamber, we show that coral (massive Porites) interfaces with thick turf algae, macroalgae and cyanobacteria, which are successful competitors against the coral in the field, are characterised by a thick DBL and hypoxia at night. In contrast, coral interfaces with crustose coralline algae, conspecifics and thin turf algae, which are poorer competitors, have a thin DBL and low hypoxia at night. Furthermore, DBL thickness and hypoxia at the interface with turf decreased with increasing flow speed, but not when thick turf was upstream. Our results support the importance of water-mediated transport mechanisms in coral-algal interactions. Shifts towards algal dominance, particularly dense assemblages, may lead to thicker DBLs, higher hypoxia and higher concentrations of harmful metabolites and pathogens along coral borders, which in turn may facilitate algal overgrowth of live corals. These effects may be mediated by flow speed and orientation.

尽管全球多数珊瑚礁已从珊瑚主导转变为藻类主导的群落，但此类转变对珊瑚-藻类相互作用的影响及其潜在调控机制仍未得到充分解析。在微观尺度上，学界尚未明确珊瑚-藻类界面处的扩散边界层（diffusive boundary layers, DBLs）与表面氧浓度，如何随藻类竞争者的类型及其竞争力水平发生变化。本研究借助野外原位观测与流动腔室中的微传感器测量，发现野外中可有效竞争珊瑚资源的厚层草皮藻、大型藻类与蓝细菌，与块状滨珊瑚（Porites）形成的界面在夜间会呈现较厚的扩散边界层与严重缺氧环境。与之相对，竞争力较弱的壳状珊瑚藻、同种珊瑚个体与薄层草皮藻与珊瑚形成的界面，仅具有较薄的扩散边界层，且夜间缺氧程度较低。此外，随着水流流速升高，草皮藻与珊瑚界面处的扩散边界层厚度与夜间缺氧程度均有所降低，但当厚层草皮藻位于水流上游时，该变化规律不再成立。本研究结果证实了水介导输运机制在珊瑚-藻类相互作用中的关键作用。向藻类主导群落的转变（尤其是高密度藻类群落），可能会在珊瑚周边形成更厚的扩散边界层、更高程度的缺氧环境，同时伴随有害代谢物与病原体浓度升高，进而促进藻类对活珊瑚的过度生长。此类效应可能受水流流速与水流朝向的调控。