Effects of diel-cycling hypoxia and acidification on juvenile weakfish Cynoscion regalis growth, survival, and activity

During summertime, dissolved oxygen (DO) and pH/pCO(2) 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 O-2 l(-1); moderate cycling DO, 3-9 mg O-2 l(-1); and constant normoxia, 7.5 mg O-2 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 x 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 O-2 l(-1) and mean daily highs of 7.8-9.5 mg O-2 l(-1) (mean daily low pH 7.03-7.47 and high pCO(2) (1) similar to 16 000-5000 mu atm). However, 100% mortality occurred within similar to 10 d at extreme diel-cycling hypoxia during which DO cycled between mean daily lows of 1.5-2.1 mg O-2 l(-1), mean daily low pH 6.99-7.44, and associated high pCO(2) of similar to 16 500-5500 mu atm. We found no clear independent or interactive effect of pH/pCO(2) on growth or survival despite pH being as low as 6.86 and pCO(2) as high as similar to 20 000 mu atm. 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.