Freshening increases the susceptibility to heat stress in intertidal mussels (Mytilus edulis) from the Arctic

1. Temperatures in the Arctic are increasing at a faster pace than at lower latitudes resulting in range expansion of boreal species. In Greenland, the warming also drives accelerating melt of the Greenland Ice Sheet resulting in more meltwater entering Greenland fjords in summer. 2. Our aim was to determine if increasing summer temperatures combined with lower salinity can induce expression of stress‐related proteins, e.g. heat shock protein, in boreal intertidal mussels in Greenland, and whether low salinity reduces the upper thermal limit at which mortality occurs. 3. We conducted a mortality experiment, using twelve different combinations of salinity and air temperature treatments during a simulated tidal regime and quantified the change in mRNA levels of five stress‐related genes (hsp24, hsp70, hsp90, sod, p38) in surviving mussels to discern the level of sub‐lethal stress. 4. Heat induced mortality occurred in mussels exposed to an air temperature of 30°C and mortality was higher in treatments with lowered salinity (5 and 15‰), which confirms that low habitat salinity decreases the upper thermal limit of Mytilus edulis. The gene expression analysis supported the mortality results, with the highest gene expression found at combinations of high temperature and low salinity. 5. Combined with seasonal measurements of intertidal temperatures in Greenland, we suggest heat stress occurs in low salinity intertidal area and that further lowered salinity in coastal water due to increased run‐off can make intertidal bivalves more susceptible to summer heat stress. This study thus provides an example of how different impacts of climate warming can work synergistically to stress marine organisms.