Experiments on ocean acidification effects on mussel calcification

Ocean acidification (OA) is generally assumed to negatively impact calcification rates of marine organisms. At a local scale however, biological activity of macrophytes may generate pH fluctuations with rates of change that are orders of magnitude larger than the long-term trend predicted for the open ocean. These fluctuations may in turn impact benthic calcifiers in the vicinity. Combining laboratory, mesocosm and field studies, such interactions between OA, the brown alga Fucus vesiculosus, the sea grass Zostera marina and the blue mussel Mytilus edulis were investigated at spatial scales from decimetres to 100s of meters in the western Baltic. Macrophytes increased the overall mean pH of the habitat by up to 0.3 units relative to macrophyte- free, but otherwise similar, habitats and imposed diurnal pH fluctuations with amplitudes ranging from 0.3 to more than 1 pH unit. These amplitudes and their impact on mussel calcification tended to increase with increasing macrophyte biomass to bulk water ratio. At the laboratory and mesocosm scales, biogenic pH fluc- tuations allowed mussels to maintain calcification even under acidified conditions by shifting most of their calcification activity into the daytime when biogenic fluctuations caused by macrophyte activity offered temporal refuge from OA stress. In natural habitats with a low biomass to water body ratio, the impact of biogenic pH fluctuations on mean calcification rates of M. edulis was less pronounced. Thus, in dense algae or seagrass habitats, macrophytes may mitigate OA impact on mussel calcification by raising mean pH and providing temporal refuge from acidification stress.海洋酸化（Ocean Acidification, OA）通常被认为会对海洋生物的钙化速率产生负面影响。然而在局地尺度上，大型水生植物的生物活动可引发pH值波动，其变化速率较开阔海洋的长期预测趋势高出数个数量级。这类波动进而会对周边的底栖钙化生物造成影响。本研究结合实验室、中型实验生态系（mesocosm）与野外调查，在波罗的海西部的分米至数百米空间尺度下，探究了海洋酸化、褐藻墨角藻（Fucus vesiculosus）、海草大叶藻（Zostera marina）以及蓝贻贝（Mytilus edulis）之间的相互作用。相较于无大型水生植物但其余条件一致的生境，大型水生植物可将栖息地的平均pH值提升最高达0.3个单位，并引发幅度介于0.3至1个pH单位以上的昼夜pH波动。这类波动的幅度及其对贻贝钙化的影响，往往随大型水生植物生物量与水体总体积之比的升高而增强。在实验室与中型实验生态系尺度下，生物源性pH波动使得贻贝即便处于酸化环境中也能维持钙化速率——它们会将多数钙化活动转移至日间，此时大型水生植物活动引发的生物源性波动可为其提供躲避海洋酸化胁迫的时间庇护所。在生物量与水体之比偏低的自然生境中，生物源性pH波动对蓝贻贝平均钙化速率的影响相对较弱。因此，在密集的藻类或海草生境中，大型水生植物可通过提升平均pH值、为贻贝提供躲避酸化胁迫的时间庇护所，从而缓解海洋酸化对其钙化过程的负面影响。