Diel and tidal pCO2 x O2 fluctuations provide physiological refuge to early life stages of a coastal forage fish

Coastal ecosystems experience substantial natural fluctuations in pCO(2) and dissolved oxygen (DO) conditions on diel, tidal, seasonal and interannual timescales. Rising carbon dioxide emissions and anthropogenic nutrient input are expected to increase these pCO(2) and DO cycles in severity and duration of acidification and hypoxia. How coastal marine organisms respond to natural pCO(2) x DO variability and future climate change remains largely unknown. Here, we assess the impact of static and cycling pCO(2) x DO conditions of various magnitudes and frequencies on early life survival and growth of an important coastal forage fish, Menidia menidia. Static low DO conditions severely decreased embryo survival, larval survival, time to 50% hatch, size at hatch and post-larval growth rates. Static elevated pCO(2) did not affect most response traits, however, a synergistic negative effect did occur on embryo survival under hypoxic conditions (3.0 mg L-1). Cycling pCO(2) x DO, however, reduced these negative effects of static conditions on all response traits with the magnitude of fluctuations influencing the extent of this reduction. This indicates that fluctuations in pCO(2) and DO may benefit coastal organisms by providing periodic physiological refuge from stressful conditions, which could promote species adaptability to climate change.