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 a transition for coral–algal interactions and their underlying mechanisms remain 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 coral in the field, are characterized 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.

尽管多数珊瑚礁已从珊瑚主导态转变为藻类主导态，但此类转变对珊瑚-藻类相互作用及其潜在作用机制的影响仍有待深入解析。在微观尺度下，学界尚不清楚珊瑚-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及其竞争能力发生变化。本研究借助野外观测与流动腔室内的微传感器测量，证实块状滨珊瑚（Porites）与三类在野外可有效竞争珊瑚的藻类——厚层垫状藻、大型藻类以及蓝细菌——形成的界面，具有夜间扩散边界层增厚且伴随低氧的特征；与之相对，当珊瑚与竞争能力较弱的壳状珊瑚藻、同种个体以及薄层垫状藻形成界面时，其扩散边界层较薄且夜间低氧程度较低。进一步研究发现，界面处的扩散边界层厚度与低氧程度随流速升高而降低，但当厚层垫状藻处于上游位置时，该规律不再成立。本研究结果佐证了水介导的物质转运机制在珊瑚-藻类相互作用中的重要性。向藻类主导态的群落转变，尤其是密集藻类群落的形成，可能会在珊瑚边界处形成更厚的扩散边界层、更高程度的低氧，以及更高浓度的有害代谢物与病原体，进而促进藻类对活珊瑚的过度侵占。此类效应可能受流速与水流朝向的调控。