MEPS 556:223-235 (2016)  -  DOI: https://doi.org/10.3354/meps11817

ABSTRACT: 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^-1; moderate = 3-9 mg O₂ l^-1), 2 cycling pH levels (extreme = 6.8-8.1; moderate = 7.2-7.8), and static normoxia (7.5 mg O₂ l^-1) 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^-1) 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.

KEY WORDS: Diel-cycling hypoxia · Diel-cycling pH · Eutrophication · Flounder · Hypoxia · Ocean acidification