Strength and duration of diel pH and dissolved oxygen cycles control the survival and performance of early life stage North Atlantic bivalves (Mercenaria mercenaria, Crassostrea virginica, Argopecten irradians and Mytilus edulis)

Many economically important bivalves spawn during the summer months when diel cycles of dissolved oxygen (DO) and pH occur in estuaries. Little is known, however, regarding how cycles of differing durations and magnitudes affect these organisms. Here, larval bivalves ( Mercenaria mercenaria, Crassostrea virginica, Argopecten irradians ) and juvenile mussels ( Mytilus edulis ) were exposed to cycles of low DO and pH of varying duration (4-, 6-, 8-, and 12-h) and strength (moderate: DO range ~ 6 mg L −1 , pH range ~ 0.6 and severe: DO range ~ 10 mg L −1 , pH range ~ 0.9) compared to positive (normoxic and normocapnic) and negative (hypoxic and acidified) static controls. Growth, survival, respiration and clearance rates were measured. During experiments, 12 h of nocturnal hypoxia and acidification coupled with mildly hyperoxic (~11.3 mg L −1 DO) and hypocapnic (~8.13 pH) conditions by day significantly reduced survival in larval C. virginica , M. mercenaria , and A. irradians in all experiments ( p < 0.05), while 12 h of nocturnal hypoxia and acidification without hyperoxic and hypocapnic conditions did so in only half of experiments indicating that hyperoxia and hypocapnia were additional and significant stressors. Six hours of low DO/pH significantly reduced survival in only 16 % of experiments, indicating that larval bivalves are more impacted by longer duration and greater magnitude cycles of DO and pH compared to cycles of shorter duration or lower magnitude. Across species, M. mercenaria larvae were more resilient to nocturnal hypoxia and acidification than A. irradians and C. virginica . The growth and survival of juvenile M. edulis were unaffected by nocturnal hypoxia and acidification but mussels experienced significantly increased clearance and respiration rates under these conditions ( p < 0.01) evidencing physiological mechanisms for coping with these stressors. Collectively, this study demonstrates that the impacts of diel DO and pH cycles on early life stage bivalves are dependent upon cycle duration, cycle intensity, bivalve life stage, and bivalve species. • Cycling from nocturnal hypoxia - acidification to mild hyperoxia- hypocapnia reduced larval bivalve survival in all experiments. • Cycling from nocturnal hypoxia - acidification to normoxia- normocapnia reduced survival of larval bivalves in 50 % of experiments. • Nocturnal hypoxia and acidification caused increased clearance and respiration rates in juvenile mussels. • The impacts of diel DO and pH cycles on early life stage bivalves depend on cycle duration, cycle intensity, life stage, and species.