UC San Diego

Along upwelling margins, pH and oxygen vary on multiple temporal and spatial scales, and in many places levels are decreasing with climate change. Continuous monitoring in nearshore settings along an upwelling-influenced margin revealed strong, semidiurnal fluctuations, week-long reduction events, and a tight positive correlation between oxygen and pH. Laboratory experiments were conducted to assess implications of pH and oxygen changes for invertebrate gamete and larval performance. At levels reflecting nearshore conditions, there were effects of low pH on fertilization success in echinoids and larval development and size of two Mytilus mussel species, but there was no apparent effect of low oxygen alone or in combination with pH. Fertilization experiments indicated that pH variability present within the habitat of Strongylocentrotus franciscanus could hinder fertilization success when timing of spawning coincides with low pH conditions. The incorporation of semidiurnal pH fluctuations, the dominant scale of observed temporal variability, into laboratory experiments alleviated negative effects of reduced pH in both Mytilus species studied. Furthermore, at lower pH, high variance in echinoid sperm performance and in larval size of Mytilus spp. suggests the raw material exists for evolutionary adaptation to reduced pH. Population variance in combination with temporal and spatial variation in pH may be increasingly important in future, low-pH oceans. Additionally, the observation of species-specific responses to pH among congeneric echinoids and mytilid mussels implies that we cannot assume similar sensitivity to reduced pH based on taxonomic relatedness. Further understanding of responses to ocean acidification may be aided by knowledge of larval pH-exposure history. The development of a larval-based geochemical proxy revealed that U/Ca in larval shells reflected differing pH exposures of mussel larvae. Application to outplanted larvae developing along the San Diego coastline demonstrated that higher U/Ca in larval shells can reflect upwelling and exposure to low pH. Notably, present-day pH conditions are at times low enough to elicit significant effects on fertilization in S. franciscanus, on larval development of Mytilus spp., and on the geochemical composition of larval shells. These effects could influence the sustainability and persistence of these commercially harvested species as ocean acidification intensifies along upwelling margins