Seawater carbonate chemistry, calcification and shell size of hard clam Mercenaria mercenaria during experiments, 2011@en

Increasing atmospheric carbon dioxide threatens to decrease pH in the world's oceans. Coastal and estuarine calcifying organisms of significant ecological and economical importance are at risk; however, several biogeochemical processes drive pH in these habitats. In particular, coastal and estuarine sediments are frequently undersaturated with respect to calcium carbonate due to high rates of organic matter remineralization, even when overlying waters are saturated. As a result, the post-larval stages of infaunal marine bivalves must be able to deposit new shell material in conditions that are corrosive to shell. We measured calcification rates on the hard clam, Mercenaria spp.,in 5 post-larval size classes (0.39, 0.56, 0.78, 0.98, and 2.90 mm shell height) using the alkalinity anomaly method. Acidity of experimental water was controlled by bubbling with air-CO2 blends to obtain pH values of 8.02, 7.64, and 7.41, corresponding to pCO2 values of 424, 1120, and 1950 µatm. These pH values are typical of those found in many near-shore terrigenous marine sediments. Our results show that calcification rate decreased with lower pH in all 5 size classes measured. We also found a significant effect of size on calcification rate, with the smaller post-larval sizes unable to overcome dissolution pressure. Increased calcification rate with size allowed the larger sizes to overcome dissolution pressure and deposit new shell material under corrosive conditions. Size dependency of pH effects on calcification is likely due to organogenesis and developmental shifts in shell mineralogy occurring through the post-larval stage. Furthermore, we found significantly different calcification rates between the 2 sources of hard clams we used for these experiments, most likely due to genotypic differences. Our findings confirm the susceptibility of the early life stages of this important bivalve to decreasing pH and reveal mechanisms behind the increased mortality in post-larval juvenile hard clams related to dissolution pressure, that has been found in previous studies.大气二氧化碳浓度升高正威胁全球海洋pH值下降。兼具重要生态与经济价值的沿海及河口钙化生物正面临生存风险，但诸多生物地球化学过程会调控此类生境的pH水平。具体而言，即便上覆水体处于碳酸钙饱和状态，沿海与河口沉积物因有机质再矿化速率极高，往往呈现碳酸钙不饱和状态。因此，底栖海洋双壳类的幼虫后期阶段，必须能够在对贝壳具有腐蚀性的环境中沉积新的贝壳基质。本研究采用碱度异常法（alkalinity anomaly method），针对5个幼虫后期体型等级（壳高分别为0.39、0.56、0.78、0.98和2.90 mm）的硬壳蛤属（Mercenaria spp.）开展钙化速率测定。通过通入空气-二氧化碳混合气调控实验水体酸度，设置8.02、7.64、7.41三个pH梯度，对应的二氧化碳分压（pCO2）分别为424、1120和1950 µatm。上述pH值常见于多数近岸陆源海洋沉积物生境。实验结果显示，在所测定的5个体型等级中，钙化速率均随pH值降低而下降。同时，体型大小对钙化速率存在显著影响：体型更小的幼虫后期个体无法抵消溶解压力，而体型更大的个体可通过更高的钙化速率克服溶解压力，实现腐蚀性环境下的贝壳基质沉积。pH对钙化作用的影响存在体型依赖性，这可能与幼虫后期的器官发生及贝壳矿物学的发育转变相关。此外，本研究使用的两个不同来源的硬壳蛤种群间钙化速率存在显著差异，这大概率源于基因型差异。本研究证实了该重要双壳类早期生活史阶段对pH值下降的易感性，并揭示了此前研究中观测到的幼虫后期稚贝因溶解压力导致死亡率升高的内在机制。