Ocean Acidification

Global warming is causing the oceans to simultaneously warm, decrease in pH and increase in pCO₂, stressors that will have deleterious impacts on physiological function in marine biota. To address questions on future vulnerabilities in a conspicuous component of the shallow water biota of temperate eastern Australia, the impact of projected (2100 and beyond) temperature and pH/pCO₂ change on respiration in the brittle star Ophionereis schayeri was investigated. Survivorship at elevated temperature was also investigated. Temperature and pH conditions experienced by this species in the field were determined to place experiments in the context of present and near future scenarios and to assess current tolerance levels with regard to tidal variation in temperature and pH. The acute metabolic response (M_O2) of O. schayeri to pH/pCO₂ was measured with animals acclimated to 19 °C, low summer sea surface temperature (SST) and 25 °C (+ 1 °C max summer SST). Temperature had the predicted effect on oxygen uptake, with the Q₁₀ being 2.6 at normal pH 8.2. It was predicted that hypercapnia/low pH (7.4–7.8) would be stressful to O. schayeri and that this would be reflected by a decrease in respiration rate. Hypercapnia (pCO₂ ≥ 1000 μatm) had a narcotic effect causing lethargy in O. schayeri. In the pH 7.8 treatments narcotic effects may have contributed to the decrease in oxygen uptake observed. This pH level is near the lower range of pH experienced by this species in the field and it is likely that short term metabolic depression may be a routine response strategy for O. schayeri. However, at pH 7.6 and 7.4 O. schayeri exhibited a significant increase in oxygen uptake associated with copious secretion of mucus, a stress response. During five week acclimation experiments at control (22 °C) and elevated (25 °C) temperatures, mortalities were 1.3% and 15%, respectively. The brittle stars did not cope with higher temperatures (≥ 26 °C). It appears that 25 °C approximates the thermotolerance threshold for O. schayeri indicating that this species is living near its upper limit. Local populations of this brittle star may not cope with a more permanent increase in environmental pCO₂ and temperature due to climate change.

Christensen A. B., Nguyen H. D., & Byrne M., in press. Thermotolerance and the effects of hypercapnia on the metabolic rate of the ophiuroid Ophionereis schayeri: Inferences for survivorship in a changing ocean. Journal of Experimental Marine Biology and Ecology doi:10.1016/j.jembe.2011.04.002. Article (subscription required).