Diel-cycling hypoxia and pH impacts on juvenile summer flounder growth and survival

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University of Delaware
A series of laboratory experiments was conducted to determine the impact of diel-cycling dissolved oxygen (DO) and pH on young-of-the-year summer flounder (Paralichthys dentatus). Flounder were exposed to two cycling DO levels (extreme = 1-11 mg O2 l-1 ; moderate = 3-9 mg O2 l-1 ), two cycling pH levels (extreme = 6.8-8.1; moderate = 7.2-7.8), and a constant normoxia (7.5 mg O2 l -1 ) & pH (7.5) control treatment in a fully crossed 3X3 experimental design at 25°C. DO and pH levels were chosen to reflect summertime conditions in shallow estuarine nursery habitat. Growth rate, overall change in mass, was significantly reduced in fish exposed to the most extreme diel-cycling hypoxia, across all pH treatments, throughout the course of 20 d experiments. No consistent growth detriment was observed in the other treatments, indicating a) that a moderate DO cycle has little or no impact on growth and b) that pH has neither an independent nor interactive effect with hypoxia on growth rate. In a series of experiments to examine growth rate recovery and acclimation, flounder experienced an initial growth rate detriment when subjected to extreme diel-cycling hypoxia and pH for 10 days, but then exhibited growth rate recovery (growth returned to control levels) when exposed to static normoxia and normal pH conditions the following 10 days. Flounder did not exhibit growth rate acclimation when subjected to extreme diel-cycling hypoxia and pH for an extended period, as growth rate detriment persisted for 20 d of exposure. Flounder exhibited behavioral observations indicative of stress when exposed to diel-cycling hypoxia and eventually exhibited mortality after two to three weeks of exposure to extreme diel-cycling hypoxia and pH. These results demonstrate that prolonged and severe diel-cycling hypoxia and pH degrades summer flounder nursery habitat as flounder exhibited growth rate detriment, and ultimately mortality, under laboratory conditions exhibited in shallow, highly eutrophied estuaries.