Evaluating the effects of diel-cycling hypoxia and pH on growth and survival of juvenile summer flounder Paralichthys dentatus

Effects of diel-cycling dissolved oxygen (DO) and pH on young-of-the-year summer flounder Paralichthys dentatus were examined in laboratory experiments. Flounder were exposed to 2 cycling DO levels (extreme = 1-11 mg/O2 l; moderate = 3-9 mg/O2 l), 2 cycling pH levels (extreme = 6.8-8.1; moderate = 7.2-7.8), and static normoxia (7.5 mg/ O2 l) and pH (7.5) in a fully crossed 3*3 experimental design for 20 d. Cycling conditions reflected summertime DO and pH fluctuations in flounder nursery habitat. Experiments were conducted over 3 partial factorial trials. Growth was significantly reduced in fish exposed to the most extreme diel-cycling DO, across all pH treatments, with no consistent growth reduction in other treatments. Cycling treatments with mean daily low pH (6.87) and high pCO2 (10000 µatm) had neither an independent nor interactive effect, with hypoxia, on growth. Flounder exhibited growth rate recovery. Following initial growth reduction when exposed to extreme diel-cycling hypoxia and pH over Days 1-10, growth increased 2-fold under static DO (7.5 mg/O2 l) and pH (7.5) conditions over the following 10 d. Flounder did not exhibit growth rate acclimation, defined as increased growth during prolonged exposure, under extreme diel-cycling DO and pH for 20 d. Flounder experienced mortality (>90% of individuals) after 2-3 wk exposure to extreme diel-cycling DO and pH. These experimental results demonstrate that extreme diel-cycling DO and pH can significantly impact summer flounder growth and survival and that the growth rate reduction is driven by DO.

本研究通过室内受控实验，探究了昼夜循环溶解氧（diel-cycling dissolved oxygen, DO）与pH对当年生夏季牙鲆幼鱼（Paralichthys dentatus）的影响。实验设置了2组循环溶解氧水平：极端组为1~11 mg·L⁻¹，中等等级组为3~9 mg·L⁻¹；2组循环pH水平：极端组为6.8~8.1，中等等级组为7.2~7.8；同时设置了恒定常氧（normoxia，7.5 mg·L⁻¹）与恒定pH（7.5）的对照组，采用完全交叉3×3实验设计，持续培养20天。该循环条件模拟了牙鲆育苗栖息地夏季的溶解氧与pH波动特征。实验共开展3次部分因子重复试验。在所有pH处理组中，暴露于最极端昼夜循环溶解氧条件的幼鱼生长均显著受抑，其余处理组未出现一致的生长抑制现象。日均最低pH为6.87、二氧化碳分压（pCO₂）达10000 µatm的循环处理组，既未对生长产生独立效应，也未与低氧（hypoxia）产生交互效应。牙鲆表现出生长速率恢复现象：在第1~10天暴露于极端昼夜循环低氧与pH条件后出现初始生长抑制，随后10天在恒定溶解氧（7.5 mg·L⁻¹）与pH（7.5）条件下，其生长速率提升了2倍。在20天的极端昼夜循环溶解氧与pH暴露过程中，牙鲆未表现出生长速率驯化现象——生长速率驯化被定义为长期暴露下生长速率的提升。暴露于极端昼夜循环溶解氧与pH条件2~3周后，牙鲆的死亡率超过90%。本实验结果表明，极端昼夜循环溶解氧与pH可显著影响夏季牙鲆的生长与存活，且生长速率抑制主要由溶解氧变化驱动。