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.

本研究通过实验室实验探究了昼夜循环溶解氧（DO）和pH对当年生夏季比目鱼（Paralichthys dentatus）的影响。实验采用完全交叉的3×3设计，将比目鱼暴露于2种循环DO水平（极端组：1-11 mg O₂/L；中度组：3-9 mg O₂/L）、2种循环pH水平（极端组：6.8-8.1；中度组：7.2-7.8），以及静态常氧（normoxia，7.5 mg O₂/L）和pH（7.5）条件下，持续20天。循环条件模拟了比目鱼育苗栖息地夏季的DO和pH波动，实验分3次部分析因试验开展。结果显示：在所有pH处理组中，暴露于极端昼夜循环DO的鱼类生长显著降低，而其他处理组未观察到一致的生长抑制；平均日低pH（6.87）和高pCO₂（10000 µatm）的循环处理对生长既无独立效应，也无与低氧（hypoxia）的交互效应；比目鱼表现出生长率恢复现象——在1-10天暴露于极端循环低氧和pH后，随后10天置于静态DO（7.5 mg O₂/L）和pH（7.5）条件下，其生长速率提升两倍；在极端昼夜循环DO和pH条件下持续暴露20天，比目鱼未表现出生长率驯化（定义为长期暴露期间生长速率提升）；暴露2-3周后，极端循环条件组的比目鱼死亡率超过90%。这些实验结果表明，极端昼夜循环DO和pH会显著影响夏季比目鱼的生长与存活，且生长速率的降低由DO驱动。