Understanding the effects of natural processes on coral–algal competition is an important step in identifying the role of macroalgae in perturbed coral reef ecosystems. However, studies investigating coral–algal interactions are often conducted in response to a disturbance, and rarely incorporate seasonal variability. Here, naturally occurring coral–algal interactions were assessed in situ four times a year over 2 years across eight sites spanning diverse benthic communities. In over 6500 recorded coral–algal interactions, cyanobacteria and turf algae were found to be the most damaging regardless of season, resulting in visible damage to coral in greater than 95% of interactions. Macroalgae that primarily compete using chemical mechanisms were found to be more damaging than those that compete using physical mechanisms (e.g. abrasion), with both groups demonstrating decreased competitive ability in summer. While crustose coralline algae were the least damaging to competing coral, during summer, it became three times more competitive. Our results demonstrate that the competitive ability of macroalgae and the outcomes of coral–algal competition can fluctuate in seasonal cycles that may be related to biomass, production of chemical defences and/or physical toughness. The results of this study have important implications for understanding the trajectory and resilience of coral reef ecosystems into the future.

了解自然过程对珊瑚-藻类竞争的影响是确定大型藻类在珊瑚礁生态系统中的作用的重要一步。但是，研究珊瑚与藻类相互作用的研究通常是针对扰动而进行的，很少纳入季节性变化。在这里，对天然存在的珊瑚-藻类相互作用进行了原位评估在过去2年中，每年四次，遍及不同底栖生物群落的八个地点。在超过6500个记录的珊瑚与藻类的相互作用中，无论季节如何，蓝藻和草皮藻类的危害最大，导致超过95％的相互作用对珊瑚的可见破坏。发现主要利用化学机制竞争的大型藻类比利用物理机制竞争的大型藻类（例如磨蚀性）更具破坏性，两组均表明夏季竞争能力下降。尽管甲壳藻类藻类对竞争性珊瑚的危害最小，但在夏季，其竞争性却提高了三倍。我们的结果表明，大型藻类的竞争能力和珊瑚-藻类竞争的结果会在可能与生物量有关的季节性周期中波动，产生化学防御和/或物理韧性。这项研究的结果对于理解未来珊瑚礁生态系统的轨迹和复原力具有重要意义。