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.

尽管诸多珊瑚礁已从珊瑚主导转变为藻类主导，但这类群落转型对珊瑚-藻类相互作用的影响及其内在机制仍未得到充分阐释。在微观尺度上，目前尚不清楚珊瑚-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。本研究通过野外观测与流室中的微传感器测量发现，珊瑚（团块状滨珊瑚，massive Porites）与厚层草皮藻（turf algae）、大型藻类（macroalgae）及蓝细菌（cyanobacteria）——这些在野外均为对珊瑚具有较强竞争能力的成功竞争者——形成的界面，在夜间会呈现出较厚的扩散边界层与低氧环境。与之相反，珊瑚与壳状珊瑚藻（crustose coralline algae）、同种珊瑚及薄层草皮藻——这些竞争能力较弱的藻类——形成的界面，仅具有较薄的扩散边界层与夜间轻度低氧环境。此外，草皮藻界面处的扩散边界层厚度与低氧程度随流速升高而降低，但当厚层草皮藻处于上游位置时，这一规律不再适用。本研究结果证实了水介导的物质传输机制在珊瑚-藻类相互作用中的重要性。群落向藻类主导的转变，尤其是高密度藻类群落的形成，可能会在珊瑚边界处形成更厚的扩散边界层、更强的低氧环境，以及更高浓度的有害代谢物与病原体，进而可能促进藻类对活珊瑚的覆生。这类效应可能受到流速与水流取向的调控。