Seawater carbonate chemistry and Mytilus edulis biological processes during experiments, 2010

CO2 emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO2 enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO2 in Kiel Fjord is elevated for large parts of the year due to upwelling of CO2 rich waters. Peak pCO2 values of >230 Pa (>2300 µatm) and pHNBS values of <7.5 are encountered during summer and autumn, average pCO2 values are ~70 Pa (~700 µatm). In contrast to previously described naturally CO2 enriched sites that have suggested a progressive displacement of calcifying auto- and heterotrophic species, the macrobenthic community in Kiel Fjord is dominated by calcifying invertebrates. We show that blue mussels from Kiel Fjord can maintain control rates of somatic and shell growth at a pCO2 of 142 Pa (1400 µatm, pHNBS = 7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO2 values of ~100 Pa (~1000 µatm) prevail. Our findings indicate that calcifying keystone species may be able to cope with surface ocean pHNBS values projected for the end of this century when food supply is sufficient. However, owing to non-linear synergistic effects of future acidification and upwelling of corrosive water, peak seawater pCO2 in Kiel Fjord and many other productive estuarine habitats could increase to values >400 Pa (>4000 µatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution.二氧化碳排放正引发海洋酸化。预测海洋群落对酸化的脆弱性颇具挑战，因多数实验研究无法纳入物种适应过程。因此，天然富二氧化碳海域可作为未来群落结构变化的宝贵替代研究位点。本研究描述了一处位于西波罗的海的天然类似位点。基尔峡湾（Kiel Fjord）的海水二氧化碳分压（pCO₂）在一年中的多数时段处于升高状态，这源于富二氧化碳水团的上升流作用。夏秋两季可观测到超过230帕（>2300微大气压）的峰值pCO₂，以及低于7.5的NBS标度pH值（pH_NBS）；该海域年平均pCO₂约为70帕（~700微大气压）。与此前报道的天然富二氧化碳位点研究提出的“钙化自养与异养物种逐渐被替代”的结论不同，基尔峡湾的大型底栖生物群落（macrobenthic community）以钙化无脊椎动物（calcifying invertebrates）为优势类群。本研究显示，采自基尔峡湾的蓝贻贝（blue mussels）可在142帕（1400微大气压，pH_NBS=7.7）的pCO₂环境下，维持体细胞与壳体生长的正常调控速率。幼贻贝种群补充（recruitment）量在夏季达到峰值，而此时海水正处于约100帕（~1000微大气压）的高pCO₂环境中。研究结果表明，在食物供给充足的条件下，作为关键物种（keystone species）的钙化生物或可应对本世纪末海洋表层预计达到的NBS标度pH值水平。然而，由于未来海洋酸化与腐蚀性水团上升流之间存在非线性协同效应，基尔峡湾及其他诸多高生产力河口生境（estuarine habitats）的海水峰值pCO₂可能升至超过400帕（>4000微大气压）。此类变化极有可能影响钙化过程与种群补充，并加剧壳体的外部溶蚀。