Seawater carbonate chemistry and coral-coral competition

As carbon dioxide (CO2) levels increase, coral reefs and other marine systems will be affected by the joint stressors of ocean acidification (OA) and warming. The effects of these two stressors on coral physiology are relatively well studied, but their impact on biotic interactions between corals are poorly understood. While coral-coral interactions are less common on modern reefs, it is important to document the nature of these interactions to better inform restoration strategies in the face of climate change. Using a mesocosm study, we evaluated whether the combined effects of ocean acidification and warming alter the competitive interactions between the common coral Porites astreoides and two other mounding corals (Montastraea cavernosa or Orbicella faveolata) common in the Caribbean. After 7 days of direct contact, P. astreoides suppressed the photosynthetic potential of M. cavernosa by 100% in areas of contact under both present (28.5°C and 400 μatm pCO2) and predicted future (30.0°C and 1000 μatm pCO2) conditions. In contrast, under present conditions M. cavernosa reduced the photosynthetic potential of P. astreoides by only 38% in areas of contact, while under future conditions reduction was 100%. A similar pattern occurred between P. astreoides and O. faveolata at day 7 post contact, but by day 14, each coral had reduced the photosynthetic potential of the other by 100% at the point of contact, and O. faveolata was generating larger lesions on P. astreoides than the reverse. In the absence of competition, OA and warming did not affect the photosynthetic potential of any coral. These results suggest that OA and warming can alter the severity of initial coral-coral interactions, with potential cascading effects due to corals serving as foundation species on coral reefs.

随着二氧化碳（CO₂）浓度持续升高，珊瑚礁与其他海洋生态系统将面临海洋酸化（OA）与海洋增温的双重胁迫。目前学界对这两种胁迫因子对珊瑚生理的影响已有较为深入的研究，但二者对珊瑚间生物相互作用的调控效应仍有待阐明。尽管现代珊瑚礁上珊瑚种间竞争的发生频率相对较低，但为应对气候变化优化珊瑚修复策略，厘清此类种间互作的本质具有重要的科学与应用价值。本研究依托中型实验生态系统（mesocosm）实验，探究了海洋酸化与增温的联合效应是否会改变加勒比海海域常见的三种团块状珊瑚间的竞争互作，即星状孔珊瑚（Porites astreoides）与大杯脑珊瑚（Montastraea cavernosa）、瘤状合叶珊瑚（Orbicella faveolata）之间的竞争关系。实验结果显示，经过7天的直接接触培养，在当前环境条件（28.5℃、400 μatm pCO₂）与未来预测气候情景（30.0℃、1000 μatm pCO₂）下，星状孔珊瑚均可使接触区域内大杯脑珊瑚的光合潜力完全受抑（抑制率达100%）。与之形成对比的是，在当前环境条件下，大杯脑珊瑚仅能使接触区域内星状孔珊瑚的光合潜力降低38%；而在未来气候情景下，其对星状孔珊瑚光合潜力的抑制幅度可达100%。星状孔珊瑚与瘤状合叶珊瑚在接触培养7天后也呈现出相似的竞争模式，但至接触培养第14天时，二者在接触区域内的光合潜力均被对方完全抑制，且瘤状合叶珊瑚对星状孔珊瑚造成的病灶面积显著大于后者对其造成的损伤。在无种间竞争的对照组中，海洋酸化与增温并未对任意一种珊瑚的光合潜力产生显著影响。本研究结果表明，海洋酸化与增温能够改变珊瑚种间初始竞争的强度；由于珊瑚是珊瑚礁生态系统的建群物种（foundation species），此类胁迫介导的种间互作变化可能引发潜在的级联生态效应。