Seawater carbonate chemistry and calcification rate in the Bermuda reef community, 2010

Despite the potential impact of ocean acidification on ecosystems such as coral reefs, surprisingly, there is very limited field data on the relationships between calcification and seawater carbonate chemistry. In this study, contemporaneous in situ datasets of seawater carbonate chemistry and calcification rates from the high-latitude coral reef of Bermuda over annual timescales provide a framework for investigating the present and future potential impact of rising carbon dioxide (CO2) levels and ocean acidification on coral reef ecosystems in their natural environment. A strong correlation was found between the in situ rates of calcification for the major framework building coral species Diploria labyrinthiformis and the seasonal variability of [CO32-] and aragonite saturation state omega aragonite, rather than other environmental factors such as light and temperature. These field observations provide sufficient data to hypothesize that there is a seasonal "Carbonate Chemistry Coral Reef Ecosystem Feedback" (CREF hypothesis) between the primary components of the reef ecosystem (i.e., scleractinian hard corals and macroalgae) and seawater carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO32-] and omega aragonite conditions, and rates of coral calcification due to the photosynthetic uptake of CO2. In late summer, rates of coral calcification are suppressed by release of CO2 from reef metabolism during a period of strong net heterotrophy. It is likely that this seasonal CREF mechanism is present in other tropical reefs although attenuated compared to high-latitude reefs such as Bermuda. Due to lower annual mean surface seawater [CO32-] and omega aragonite in Bermuda compared to tropical regions, we anticipate that Bermuda corals will experience seasonal periods of zero net calcification within the next decade at [CO32-] and omega aragonite thresholds of ~184 micro moles kg-1 and 2.65. However, net autotrophy of the reef during winter and spring (as part of the CREF hypothesis) may delay the onset of zero NEC or decalcification going forward by enhancing [CO32-] and omega aragonite. The Bermuda coral reef is one of the first responders to the negative impacts of ocean acidification, and we estimate that calcification rates for D. labyrinthiformis have declined by >50% compared to pre-industrial times.

尽管海洋酸化对珊瑚礁等生态系统具有潜在影响，但令人意外的是，目前关于钙化作用与海水碳酸盐化学之间关联的野外实地数据仍极为匮乏。本研究依托百慕大高纬度珊瑚礁的年际尺度原位（in situ）同步监测数据集，涵盖海水碳酸盐化学与钙化速率两项核心指标，为探究大气二氧化碳（CO₂）浓度上升及海洋酸化对自然环境中珊瑚礁生态系统的当前与未来潜在影响提供了研究框架。研究发现，优势造礁珊瑚物种团块脑珊瑚（Diploria labyrinthiformis）的原位钙化速率，与水体碳酸根离子浓度[CO₃²⁻]及文石饱和状态Ω_文石（omega aragonite）的季节波动呈现显著强相关，而非光照、温度等其他环境因子。基于上述野外观测数据，我们得以提出“碳酸盐化学-珊瑚礁生态系统反馈”（Carbonate Chemistry Coral Reef Ecosystem Feedback, CREF）假说，即礁生态系统核心组分——造礁石珊瑚与大型藻类——与海水碳酸盐化学之间存在此类反馈机制。在初夏时节，礁体底栖组分的强净自养作用通过光合固碳吸收CO₂，提升了水体[CO₃²⁻]与Ω_文石水平，进而促进珊瑚钙化速率。至夏末，礁体代谢释放CO₂，此时系统处于强净异养状态，珊瑚钙化速率因此受到抑制。尽管相较于百慕大这类高纬度礁体，其他热带礁体的该季节性CREF机制强度有所减弱，但这类机制极有可能广泛存在于全球热带珊瑚礁生态系统中。相较于热带海域，百慕大海域的年平均表层海水[CO₃²⁻]与Ω_文石水平更低，我们预测，未来十年内，当水体[CO₃²⁻]与Ω_文石分别降至约184微摩尔每千克（μmol·kg⁻¹）与2.65的阈值时，百慕大珊瑚将进入季节性净钙化归零状态。不过，依据CREF假说，礁体在冬季与春季的净自养作用可通过提升[CO₃²⁻]与Ω_文石水平，延缓净钙化归零或去钙化现象的发生。百慕大珊瑚礁是首批受到海洋酸化负面影响的珊瑚礁生态系统之一，我们估算，团块脑珊瑚的钙化速率较工业化前时期已下降超过50%。