Diurnal fluctuations in CO₂and dissolved oxygen concentrations do not provide a refuge from hypoxia and acidification for early-life-stage bivalves

This study assessed the effects of constant and diurnally fluctuating acidification and hypoxia on the survival, growth, and development of larval stages of 3 bivalves indigenous to the east coast of North America: bay scallops Argopecten irradians, hard clams Mercenaria mercenaria, and eastern oysters Crassostrea virginica. Bivalves were exposed to ideal (pH = ∼7.9, dissolved oxygen [DO] = ∼7 mg l^-1), acidified (pH = ∼7.2, DO = ∼7 mg l^-1), hypoxic (pH = ∼7.9, DO = ∼2 mg l^-1), and acidified and hypoxic (pH = ∼7.2, DO = ∼2 mg l^-1) conditions, as well as treatments that fluctuated between ideal conditions by day and acidified, hypoxic, or acidified and hypoxic conditions by night. Continuously acidified conditions reduced survival of larvae of all 3 species, slowed growth of larval bay scallops and eastern oysters, and delayed the development of bay scallop larvae. Continuously hypoxic conditions reduced the survival, growth, and development of larval bay scallops and slowed the development of larval hard clams. Simultaneous exposure to continuously low pH and DO yielded more negative effects than each factor independently. Diurnal exposure to low pH and/or low DO rarely altered, and never fully mitigated, the negative effects of hypoxia and/or acidification despite significantly higher mean pH and DO levels. This suggests that pH and DO fluctuations were too intense, and/or the durations of normoxic and normcapnic conditions were not long enough for bivalve larvae to overcome the physiological stress of hypoxia and acidification. Therefore, the diurnal fluctuations of pH and DO in this study did not provide a temporal refuge from hypoxia and acidification for North Atlantic bivalve larvae, suggesting that such fluctuations in an ecosystem setting can be a significant threat to these larvae.