Krill are abundant and ecologically important zooplankton that inhabit dynamic environments characterized by strong natural variability, but global ocean change is shifting the range of conditions that they experience. Laboratory tests reveal that krill are sensitive to ocean acidification despite residing in naturally low pH areas, showing the importance of modulating factors for determining their responses. This study combines laboratory manipulations with field observations across a strong natural water chemistry gradient in Puget Sound, Washington, USA to investigate the effects of pH and oxygen on adult female North Pacific krill, Euphausia pacifica. Enzyme activities of the Electron Transport System (ETS) and aminoacyl-tRNA synthetases (AARS) were used as indices of zooplankton metabolism and growth, respectively, and were paired with traditional incubation methods. Acclimation to pH and oxygen conditions in the laboratory did not reveal effects on respiration rate, ETS, or AARS activity of krill. However, field observations showed that respiratory potential, as estimated by ETS activity, decreased with decreasing oxygen, declining 9% (95% confidence interval 2.5–15%) over the range of conditions we observed (3.9–8.1 mg O₂ L⁻¹). This reduction would depress the metabolic potential of krill in areas of stressful conditions (concurrent low pH), though krill also displayed a high degree of inter-individual variability. Although differences in age structure suggest different patterns of recruitment between E. pacifica populations in areas with stressful conditions and those without, populations persist at stressful sites. Lower temperature of waters with low oxygen and pH, as well as high food concentrations, may contribute to these populations’ success.

磷虾是丰富的且具有重要生态意义的浮游动物，它们栖息在以强烈自然变异为特征的动态环境中，但是全球海洋变化正在改变它们所经历的条件范围。实验室测试表明，尽管磷虾位于自然的低pH值区域，但它对海洋酸化敏感，表明调节因子对于确定其响应的重要性。这项研究将实验室操作与在美国华盛顿州普吉特海湾的强烈自然水化学梯度上的现场观察相结合，以研究pH和氧气对成年雌性北太平洋磷虾（Euphausia pacifica）的影响。电子传输系统（ETS）和氨酰基-tRNA合成酶（AARS）的酶活性分别用作浮游动物代谢和生长的指标，并与传统的孵化方法配对。在实验室中适应pH值和氧气条件并没有显示出对磷虾的呼吸速率，ETS或ARS活性的影响。但是，野外观察表明，根据ETS活性估算，呼吸潜能随着氧气的减少而降低，在我们观察到的条件范围内（3.9–8.1 mg O ₂ L ^-1）下降了9％（95％置信区间2.5–15％）。）。尽管磷虾还表现出高度的个体间变异性，但这种降低会降低磷虾在压力条件下（同时存在的低pH值）区域的代谢潜力。尽管年龄结构的差异表明，在有压力条件的地区和没有压力条件的地区，太平洋大肠杆菌种群之间的募集方式不同，但种群仍然存在于压力较大的地区。氧气和pH值较低的水域温度较低，以及食物浓度较高，可能有助于这些人群的成功。