Seawater carbonate chemistry and variation in community development@en

Ocean acidification represents a pervasive environmental change that is predicted to affect a wide range of species, yet our understanding of the emergent ecosystem impacts is very limited. Many studies report detrimental effects of acidification on single species in lab studies, especially those with calcareous shells or skeletons. Observational studies using naturally acidified ecosystems have shown profound shifts away from such calcareous species, and there has been an assumption that direct impacts of acidification on sensitive species drive most ecosystem responses. We tested an alternative hypothesis that species interactions attenuate or amplify the direct effects of acidification on individual species. Here, we show that altered competitive dynamics between calcareous species and fleshy seaweeds drive significant ecosystem shifts in acidified conditions. Although calcareous species recruited and grew at similar rates in ambient and low pH conditions during early successional stages, they were rapidly overgrown by fleshy seaweeds later in succession in low pH conditions. The altered competitive dynamics between calcareous species and fleshy seaweeds is probably the combined result of decreased growth rates of calcareous species, increased growth rates of fleshy seaweeds, and/or altered grazing rates. Phase shifts towards ecosystems dominated by fleshy seaweed are common in many marine ecosystems, and our results suggest that changes in the competitive balance between these groups represent a key leverage point through which the physiological responses of individual species to acidification could indirectly lead to profound ecosystem changes in an acidified ocean.海洋酸化 (Ocean Acidification) 是一种普遍存在的环境变化，据预测会影响众多物种种群，但目前学界对其涌现性生态系统影响的认知仍十分有限。诸多研究在实验室条件下报道了酸化对单一物种种群的不利影响，尤其是那些具有钙质外壳或骨骼 (calcareous shells or skeletons) 的类群。依托自然酸化生态系统开展的观测研究显示，此类钙质物种种群在群落中的占比出现显著下降；学界此前普遍假设，酸化对敏感物种的直接影响是驱动多数生态系统响应的核心因素。我们检验了一项替代假说：物种间的相互作用会减弱或放大酸化对单个物种的直接影响。本研究发现，钙质物种与肉质海藻之间的竞争动态发生改变，会在酸化环境下引发显著的生态系统偏移。尽管在早期演替阶段，钙质物种在正常pH与低pH环境中的种群补充与生长速率并无显著差异，但在低pH环境的后续演替阶段中，它们会迅速被肉质海藻覆盖并挤占生存空间。钙质物种与肉质海藻之间竞争动态的改变，可能是钙质物种生长速率下降、肉质海藻生长速率提升，以及/或牧食速率改变共同作用的结果。在全球诸多海洋生态系统中，向肉质海藻占主导的生态系统发生群落相移的现象十分普遍；本研究结果表明，这两类群之间的竞争平衡改变是关键调控节点——单个物种对酸化的生理响应可通过该节点，间接引发酸化海洋中显著的生态系统变化。