Seawater carbonate chemistry and net community production and net ecosystem calcification in tide pools

Predicting the impacts of ocean acidification in coastal habitats is complicated by bio-physical feedbacks between organisms and carbonate chemistry. Daily changes in pH and other carbonate parameters in coastal ecosystems, associated with processes such as photosynthesis and respiration, often greatly exceed global mean predicted changes over the next century. We assessed the strength of these feedbacks under projected elevated CO2 levels by conducting a field experiment in 10 macrophyte-dominated tide pools on the coast of California, USA. We evaluated changes in carbonate parameters over time and found that under ambient conditions, daytime changes in pH, pCO2, net ecosystem calcification (NEC), and O2 concentrations were strongly related to rates of net community production (NCP). CO2 was added to pools during daytime low tides, which should have reduced pH and enhanced pCO2. However, photosynthesis rapidly reduced pCO2 and increased pH, so effects of CO2 addition were not apparent unless we accounted for seaweed and surfgrass abundances. In the absence of macrophytes, CO2 addition caused pH to decline by ∼0.6 units and pCO2 to increase by ∼487 µatm over 6 hr during the daytime low tide. As macrophyte abundances increased, the impacts of CO2 addition declined because more CO2 was absorbed due to photosynthesis. Effects of CO2addition were, therefore, modified by feedbacks between NCP, pH, pCO2, and NEC. Our results underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification.预测沿海生境海洋酸化的影响时，生物与碳酸盐化学之间的生物物理反馈会使研究变得复杂。沿海生态系统中pH值与其他碳酸盐参数的日变化与光合作用、呼吸作用等过程密切相关，其幅度往往远超未来百年全球平均预测的变化量。我们通过在美国加利福尼亚沿岸10个以大型植物为主的潮池开展野外实验，评估了未来CO₂升高情景下这些反馈的强度。我们对碳酸盐参数的时序变化进行了评估，发现在自然环境条件下，日间pH、二氧化碳分压（pCO₂）、净生态系统钙化（net ecosystem calcification, NEC）以及氧气浓度的变化，与净群落生产（net community production, NCP）速率显著相关。我们在日间低潮时向潮池添加CO₂，这本应降低pH值并提升pCO₂水平。然而，光合作用会快速降低pCO₂并提升pH值，因此除非考虑海藻与鳗草的丰度，否则CO₂添加的影响并不明显。在无大型植物的情况下，日间低潮时添加CO₂会在6小时内使pH值下降约0.6个单位，pCO₂升高约487微大气压（µatm）。随着大型植物丰度提升，CO₂添加的影响逐渐减弱，因为光合作用会吸收更多的CO₂。因此，NCP、pH、pCO₂与NEC之间的反馈作用会改变CO₂添加的影响。本研究结果凸显了沿海大型植物在缓解海洋酸化影响方面的潜在重要性。