MEPS 564:163-174 (2017)  -  DOI: https://doi.org/10.3354/meps11966

ABSTRACT: During summertime, dissolved oxygen (DO) and pH/pCO₂ cycle daily in shallow estuarine waters used by young fishes as nursery habitat. Laboratory experiments on juvenile weakfish Cynoscion regalis examined individual and interactive impacts of present-day levels of diel-cycling hypoxia and acidification on growth and activity. Fish were exposed to 3 DO treatments (extreme cycling DO, 1-11 mg O₂ l^-1; moderate cycling DO, 3-9 mg O₂ l^-1; and constant normoxia, 7.5 mg O₂ l^-1) and 3 pH treatments (extreme cycling pH, 6.8-8.1; moderate cycling pH, 7.2-7.8; and constant pH, 7.5) for 20 d in a 3 × 3 factorial design. Growth was not impacted by moderate diel cycles of DO and acidification with mean daily lows of 3.0-3.3 mg O₂ l^-1 and mean daily highs of 7.8-9.5 mg O₂ l^-1 (mean daily low pH 7.03-7.47 and high pCO₂ ~16000-5000 µatm). However, 100% mortality occurred within ~10 d at extreme diel-cycling hypoxia during which DO cycled between mean daily lows of 1.5-2.1 mg O₂ l^-1, mean daily low pH 6.99-7.44, and associated high pCO₂ of ~16500-5500 µatm. We found no clear independent or interactive effect of pH/pCO₂ on growth or survival despite pH being as low as 6.86 and pCO₂ as high as ~20000 µatm. Furthermore, fish that survived as much as 10 d exposure to extreme and moderate diel cycles of DO and acidification experienced no residual negative impact on growth following return to normoxia and static pH. Our results suggest that juvenile weakfish have substantial tolerance of diel cycles of oxygenation and acidification encountered in shallow estuarine nursery habitat.

KEY WORDS: Cynoscion regalis · Weakfish · Growth · Hypoxia · pH · pCO₂ · Ocean acidification · Diel-cycling · Estuary