Ocean warming and elevated carbon dioxide: multiple stressor impacts on juvenile mussels from southern Chile

Abstract The combined effect of increased ocean warming and elevated carbon dioxide in seawater is expected to have significant physiological and ecological consequences at many organizational levels of the marine ecosystem. In the present study, juvenile mussels Mytilus chilensis were reared for 80 d in a factorial combination of two temperatures (12 and 16°C) and three pCO2 levels (380, 700, and 1000 µatm). We investigated the combined effects of increasing seawater temperature and pCO2 on the physiological performance (i.e. feeding, metabolism, and growth). Lower clearance rate (CR) occurred at the highest pCO2 concentration (1000 µatm) compared with the control (380 µatm) and with the intermediate concentration of pCO2 (700 µatm). Conversely, CR was significantly higher at 16°C than at 12°C. Significant lower values of oxygen uptake were observed in mussels exposed to 1000 µatm pCO2 level compared with those exposed to 380 µatm pCO2. Scope for growth (SFG) was significantly lower at the highest pCO2 concentration compared with the control. Mussels exposed to 700 µatm pCO2 did not show significantly different SFG from the other two pCO2 treatments. SFG was significantly higher at 16°C than at 12°C. This might be explained because the experimental mussels were exposed to temperatures experienced in their natural environment, which are within the range of thermal tolerance of the species. Our results suggest that the temperature rise within the natural range experienced by M. chilensis generates a positive effect on the processes related with energy gain (i.e. feeding and absorption) to be allocated to growth. In turn, the increase in the pCO2 level of 1000 µatm, independent of temperature, adversely affects this species, with significantly reduced energy allocated to growth (SFG) compared with the control treatment.