40. The impact of high pCO2, both static and fluctuating, on whole-organism thermal tolerance

Robert P. Ellis (1)*, Mauricio A. Urbina (1,2), Cameron Hird (1), Ceri Lewis (1)

1 Department of Biosciences, College of Life and Environmental Sciences, University of Exeter, Exeter, UK
2 Departamento de Zoología, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Casilla 160-C, Concepción, Chile

Background
Organisms’ thermal tolerance has traditionally been used as a predictor of the effect of climate change on species distributions. However, CO2-driven climate change is occurring alongside a parallel increase in seawater pCO2 which is driving ocean acidification. In many coastal regions these changes in pCO2 are not occurring as stable linear decreases over time, but manifest as highly dynamic, fluctuating conditions over a range of temporal and spatial scales. It is not yet fully understood how increases in pCO2, and hence OA, influence an organisms thermal tolerance, and therefore accurately projecting shifts in species distribution due to climate change remains challenging. Consequently, understanding how any complex fluctuations in pCO2 influence thermal tolerance of marine organisms is key for accurately predicting the impact of climate change on future species distributions.

Methods
We exposed the harbour ragworm, Hediste diversicolor, to one of four pCO2 regimes for 14 days prior to monitoring metabolic rate and upper and lower thermal tolerance (CTmax and CTmin). Treatments were chosen to represent ambient (400 µatm static), 2100 RCP 8.5 (950 µatm static) and 2300 A2 (1900 µatm static) emissions scenarios. Furthermore a fluctuating treatment (cycling between 400 µatm and 1900 µatm representing a semidiurnal tidal cycle) was included.

Findings
These experiments show that both metabolic rate and organismal thermal tolerance were significantly affected by seawater pCO2, and moreover the pCO2 regime (static or fluctuating) affected the magnitude of worms’ responses to high CO2 perturbations.

Conclusions
Understanding the impact of pCO2 on thermal tolerance is a key component for accurately projecting/modelling shifts in species distribution in a changing ocean. Moreover for an accurate representation of this effect we have shown that incorporating measures of fluctuating pCO2 regimes over different temporal or spatial scales is vital.