Rising air temperatures will increase intertidal mussel abundance in the Arctic

Knowledge about the consequences of Arctic warming for marine biogeography remains limited. Mytilus mussels play a key ecological role in the littoral zone, and they are expected to be sensitive to climate change. Here we used a space-for-time approach as a first attempt to infer the coupling between climate warming and population structure and abundance of intertidal mussels along a latitudinal climate gradient in west Greenland. We analysed trends in air temperatures, collected seasonal temperature data from intertidal microhabitats, and quantified abundance, growth and age structure at 73 sites in 5 fjords. We also compared tolerance to sub-zero temperatures between seasons and latitudes. Since 1958, atmospheric temperatures have increased significantly in all fjords. Consequently, the number of days with temperatures below the lower lethal temperature, LT50 (−13°C), has decreased by up to 57%, suggesting that constraint by low temperature is weakening. Abundance declined >95% with increasing latitude from an average of 23.67 to 0.71 ind. 0.0625 m−2 in mid-intertidal habitats. However, no latitudinal differences in growth, average size (6.3−9 cm), age (1.5−2.5 yr) and mortality (55.9−62.5 yr−1) were found. The abundance of 1 yr old recruits declined with latitude, indicating that geographical patterns are controlled at the earliest life stages. Air temperatures and exposure time were important drivers for latitudinal patterns in abundance, with adults being increasingly limited to favourable microhabitats in the lower intertidal to the North. Combined, the data suggest that increased warming will facilitate an increase in intertidal mussel abundance in Greenland, and potentially, across the Arctic.