No abstract

Natural variation and changing climate in coastal oceans subject meroplanktonic organisms to broad ranges of pH and oxygen ([O2 ]) levels. In controlled-laboratory experiments we explored the interactive effects of pH, [O2 ], and semidiurnal pH fluctuations on the survivorship, development, and size of early life stages of two mytilid mussels, Mytilus californianus and M. galloprovincialis. Survivorship of larvae was unaffected by low pH, low [O2 ], or semidiurnal fluctuations for both mytilid species. Low pH (<7.6) resulted in delayed transition from the trochophore to veliger stage, but this effect of low pH was absent when incorporating semidiurnal fluctuations in both species. Also at low pH, larval shells were smaller and had greater variance; this effect was absent when semidiurnal fluctuations of 0.3 units were incorporated at low pH for M. galloprovincialis but not for M. californianus. Low [O2 ] in combination with low pH had no effect on larval development and size, indicating that early life stages of mytilid mussels are largely tolerant to a broad range of [O2 ] reflective of their environment (80-260 μmol kg(-1) ). The role of pH variability should be recognized as an important feature in coastal oceans that has the capacity to modulate the effects of ocean acidification on biological responses.

As the field of ocean acidification has grown, researchers have increasingly turned to laboratory experiments to understand the impacts of increased CO₂ on marine organisms. However, other changes such as ocean warming and deoxygenation are occurring concurrently with the increasing CO₂ concentrations, complicating the understanding of the impacts of anthropogenic changes on organisms. This experimental aquarium design allows for independent regulation of CO₂ concentration, O₂ levels, and temperature in a controlled environment to study the impacts of multiple stressors. The system has the flexibility for a wide range of treatment chemistry, seawater volumes, and study organisms. Control of the seawater chemistry is achieved by equilibration of a chosen gas mixture with seawater using a Liqui-Cel^® membrane contactor. Included as examples, two experiments performed using the system have shown control of CO₂ at values between approximately 500 and 1400 μatm and O₂ at values from 80 to 240 μmol kg⁻¹. Temperature has been maintained to 0.5 °C or better in the range of 10–17 °C. On a weeklong timescale, the system has achieved variability in pH of less than 0.007 pH units and in oxygen concentration of less than 3.5 μmol kg⁻¹. Longer experiments, over a month in duration, have been completed with control to better than 0.08 pH units and 13 μmol kg⁻¹ O₂. The ability to study the impacts of multiple stressors in the laboratory simultaneously, as well as independently, will be an important part of understanding the response of marine organisms to a high-CO₂ world