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

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 168C) and three pCO₂ levels (380, 700, and 1000 matm). 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 matm) compared with the control (380 matm) and with the intermediate concentration of pCO₂ (700 matm). Conversely, CR was significantly higher at 168C than at 128C. Significant lower values of oxygen uptake were observed in mussels exposed to 1000 matm pCO₂ level compared with those exposed to 380 matm pCO₂. Scope for growth (SFG) was significantly lower at the highest pCO₂ concentration compared with the control. Mussels exposed to 700 matm pCO₂ did not show significantly different SFG from the other two pCO₂ treatments. SFG was significantly higher at 168C than at 128C. 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 matm, independent of temperature, adversely affects this species, with significantly reduced energy allocated to growth (SFG) compared with the control treatment.