Effect of ocean acidification and pH fluctuations on the growth and development of coralline algal recruits, and an associated benthic algal assemblage@en

Coralline algae are susceptible to the changes in the seawater carbonate system associated with ocean acidification (OA). However, the coastal environments in which corallines grow are subject to large daily pH fluctuations which may affect their responses to OA. Here, we followed the growth and development of the juvenile coralline alga Arthrocardia corymbosa, which had recruited into experimental conditions during a prior experiment, using a novel OA laboratory culture system to simulate the pH fluctuations observed within a kelp forest. Microscopic life history stages are considered more susceptible to environmental stress than adult stages; we compared the responses of newly recruited A. corymbosa to static and fluctuating seawater pH with those of their field-collected parents. Recruits were cultivated for 16 weeks under static pH 8.05 and 7.65, representing ambient and 4*preindustrial pCO2 concentrations, respectively, and two fluctuating pH treatments of daily (daytime pH = 8.45, night-time pH = 7.65) and daily (daytime pH = 8.05, night-time pH = 7.25). Positive growth rates of new recruits were recorded in all treatments, and were highest under static pH 8.05 and lowest under fluctuating pH 7.65. This pattern was similar to the adults' response, except that adults had zero growth under fluctuating pH 7.65. The % dry weight of MgCO3 in calcite of the juveniles was reduced from 10% at pH 8.05 to 8% at pH 7.65, but there was no effect of pH fluctuation. A wide range of fleshy macroalgae and at least 6 species of benthic diatoms recruited across all experimental treatments, from cryptic spores associated with the adult A. corymbosa. There was no effect of experimental treatment on the growth of the benthic diatoms. On the community level, pH-sensitive species may survive lower pH in the presence of diatoms and fleshy macroalgae, whose high metabolic activity may raise the pH of the local microhabitat.珊瑚藻（Coralline algae）易受与海洋酸化（OA）相关的海水碳酸盐系统变化影响。然而，珊瑚藻栖息的近岸环境存在大幅每日pH波动，这可能会改变其对海洋酸化的响应。本研究以先前实验中已在实验条件下定植的幼年珊瑚藻Arthrocardia corymbosa为研究对象，采用新型实验室酸化培养系统，模拟海带林内观测到的pH波动，追踪其生长与发育过程。微观生活史阶段被认为相较于成体阶段对环境胁迫更为敏感，因此我们将新定植的A. corymbosa与野外采集的成体个体对恒定与波动海水pH的响应进行了对比。定植幼体分别在恒定pH 8.05与7.65的条件下培养16周，前者对应当前环境海水pH水平，后者对应4倍工业化前大气二氧化碳分压（pCO₂）浓度，同时设置两组波动pH处理：其一为日间pH 8.45、夜间pH 7.65的每日循环模式，其二为日间pH 8.05、夜间pH 7.25的每日循环模式。所有处理组的新定植幼体均呈现正生长速率，其中以恒定pH 8.05组生长速率最高，波动pH 7.65组最低。该生长模式与成体的响应趋势一致，但成体在波动pH 7.65处理组中生长速率为零。幼体方解石中碳酸镁（MgCO₃）的干重占比从pH 8.05条件下的10%降至pH 7.65条件下的8%，但pH波动对此无显著影响。所有实验处理组中均有多种大型肉质藻类以及至少6种底栖硅藻定植，其来源为与成体A. corymbosa伴生的隐蔽孢子。实验处理对底栖硅藻的生长无显著影响。从群落层面来看，pH敏感物种在底栖硅藻与大型肉质藻类存在的条件下可在更低pH环境中存活，因为这些生物的高代谢活动可提升局部微生境的pH水平。