Hypoxia and Acidification Have Additive and Synergistic Negative Effects on the Growth, Survival, and Metamorphosis of Early Life Stage Bivalves

Low oxygen zones in coastal and open ocean ecosystems have expanded in recent decades, a trend that will accelerate with climatic warming. There is growing recognition that low oxygen regions of the ocean are also acidified, a condition that will intensify with rising levels of atmospheric CO2. Presently, however, the concurrent effects of low oxygen and acidification on marine organisms are largely unknown, as most prior studies of marine hypoxia have not considered pH levels. We experimentally assessed the consequences of hypoxic and acidified water for early life stage bivalves (bay scallops, Argopecten irradians, and hard clams, Mercenaria mercenaria), marine organisms of significant economic and ecological value and sensitive to climate change. In larval scallops, experimental and naturally-occurring acidification (pH, total scale  = 7.4–7.6) reduced survivorship (by >50%), low oxygen (30–50 µM) inhibited growth and metamorphosis (by >50%), and the two stressors combined produced additively negative outcomes. In early life stage clams, however, hypoxic waters led to 30% higher mortality, while acidified waters significantly reduced growth (by 60%). Later stage clams were resistant to hypoxia or acidification separately but experienced significantly (40%) reduced growth rates when exposed to both conditions simultaneously. Collectively, these findings demonstrate that the consequences of low oxygen and acidification for early life stage bivalves, and likely other marine organisms, are more severe than would be predicted by either individual stressor and thus must be considered together when assessing how ocean animals respond to these conditions both today and under future climate change scenarios.

近数十年来，沿海与开阔大洋生态系统中的低氧区（low oxygen zones）已持续扩张，且这一趋势将随气候变暖进一步加速。学界日益达成共识：海洋低氧区同时伴随酸化（acidification）现象，且这一状况将随大气二氧化碳（CO₂）浓度升高而加剧。然而目前，低氧与酸化对海洋生物的协同效应仍鲜为人知——这是因为此前绝大多数海洋缺氧（hypoxia）研究均未考量pH值水平。本研究通过实验评估了低氧（hypoxic）与酸化水体对双壳类（bivalves）早期生活史阶段个体的影响，实验对象为兼具重要经济与生态价值、且对气候变化敏感的两种生物：海湾扇贝（Argopecten irradians）与硬壳蛤（Mercenaria mercenaria）。对于幼虫阶段的扇贝，实验诱导与自然发生的酸化环境（总标度pH值为7.4~7.6）会使存活率降低50%以上；低氧（浓度为30~50 μM）则会抑制生长与变态发育，抑制幅度超过50%；而两种胁迫因子的联合作用会产生叠加的负面效应。但对于早期生活史阶段的硬壳蛤而言，低氧水体仅会导致死亡率升高30%，而酸化水体则会显著抑制生长，抑制幅度达60%。而后期发育阶段的硬壳蛤则对单一的低氧或酸化胁迫具有耐受性，但当同时暴露于两种胁迫条件时，其生长速率会显著降低40%。综合来看，本研究结果表明：低氧与酸化对双壳类早期生活史阶段个体，以及极有可能对其他海洋生物造成的影响，比单一胁迫因子单独作用时的预测结果更为严重。因此，在评估海洋生物当前及未来气候变化情景下对上述环境条件的响应时，必须将这两种胁迫因子纳入联合考量范畴。