Canopy facilitates seaweed recruitment on subtidal temperate reefs

1. Facilitation through physical stress amelioration has been largely overlooked in subtidal marine ecosystems, where abiotic gradients are perceived to be benign. However, seemingly subtle changes in marine environmental conditions can alter community structure across vast areas, and therefore, the type of interactions and any refuge provided by marine foundation species. This could have substantial implications for community organization. 2. We measured net recruitment rates of subtidal seaweeds under experimentally modified canopy densities, across 1000 km of coastline, to examine the nature of interactions between seaweed canopies and recruits. 3. Contrary to expectations, facilitation, as evidenced by higher recruitment under canopies, was observed throughout all conditions for Scytothalia dorycarpa and under partial canopies at three of four locations for Sargassum spp., whilst competitive interactions remained prevalent for Sargassum under closed canopies. 4. Supply side dynamics were also of major importance to recruitment success for Scytothalia. For Sargassum, the interaction strength between recruits and the canopy became increasingly positive towards lower latitudes, suggesting the canopy environment was mitigating stress across the latitudinal climate gradient. 5. Synthesis. Positive interactions and stress amelioration play an important and previously unrecognized role in determining the recruitment success and viability of seaweeds in subtidal marine ecosystems. These results challenge long held paradigms about the general importance of canopy competition and force a rethink of how seaweed interactions affect habitat resilience to disturbances in subtidal ecosystems.

1. 物理胁迫缓解介导的正相互作用，在潮下带海洋生态系统（subtidal marine ecosystems）中长期被忽视——这类生态系统中非生物梯度（abiotic gradients）曾被认为是温和无害的。然而，海洋环境条件看似细微的变化，可在广阔海域范围内改变群落结构，进而影响物种间相互作用类型与海洋建群种（marine foundation species）所能提供的庇护场所，这对群落组织格局可能产生深远影响。
2. 本研究在长达1000千米的海岸带范围内，通过实验调控海藻冠层密度（canopy densities），测定潮下带海藻的净补充率（net recruitment rates），以探讨海藻冠层与幼体（recruits）之间的相互作用本质。
3. 与预期相悖的是，冠层下补充率更高的现象证实了促进作用：斯氏隐丝藻（Scytothalia dorycarpa）在所有实验条件下均表现出该促进效应；而马尾藻属（Sargassum spp.）物种仅在4个样点中的3个样点的半冠层条件下呈现促进效应，在全冠层条件下其与幼体间仍以竞争相互作用为主。
4. 补给动态（supply side dynamics）对斯氏隐丝藻的补充成功同样具有关键影响。对于马尾藻属物种而言，随着纬度降低，幼体与冠层间的相互作用强度逐渐向正向转变，这表明冠层环境可在纬度气候梯度上缓解胁迫压力。
5. 【综合结论】正相互作用与胁迫缓解在潮下带海洋生态系统的海藻补充成功与生存力维持中发挥着此前未被认知的重要作用。本研究结果挑战了长期以来认为冠层竞争普遍重要的经典范式，并促使我们重新思考：海藻间的相互作用如何影响潮下带生态系统的生境抗干扰恢复力。