Identifying drivers controlling the distribution of a keystone species in a changing Arctic

Abstract

Current climate change as a consequence of large-scale global emissions of greenhouse gases results in an unsurpassed warming of the Arctic, leading to melting of sea ice and glaciers. This results in an increased freshening of the ocean that, in combination with warming, affects the unique features of the Arctic environment and biodiversity. While the impact of changes in ice and temperatures on physical and chemical processes is well documented, the effects on the marine biology in the Arctic remain largely understudied. Greenland is the world's largest island stretching from 59°N to 83 °N, en-compassing 12% of the world's coastline. The largely north-south orientated coastlines of Greenland constitute a unique climate gradient from the subarctic to the High Arctic, along which multiple species meet their distribution limits. However, the biology of Greenland's coastal and intertidal systems has received limited attention and remains poorly understood. Therefore, by linking ecology, physiology and genetics of a keystone species, the blue mussel, this thesis aims at increasing knowledge of the Greenland intertidal zone by specifically studying what climatic and physiological factors determine the distribution and polarward limits of intertidal species. Hitherto, it has been commonly accepted that only one blue mussel species (Mytilus edulis) inhabited the Arctic. However, by utilizing genetic tools, we revealed that three blue mussel species inhabit the Arctic, and that the blue mussel M. edulis dominates in Southwest Greenland, while the congener M. trossulus dominates in the North (Paper I). Historically, work on the distribution of blue mussels in West Greenland has only been descriptive, but I quantified the abundance and population dynamics of the genus as far north as 77°N (Paper II). In doing so, I found that sub-zero air temperatures and air exposure time are of central importance for the distribution, and that abundances are controlled at the earliest life stage, not during adulthood. In addition, I performed a series of laboratory experiments to elucidate the importance of the physiology to the distribution. One proposed hypothesis was that polar-ward distribution is controlled by failed gonad maturation and reproduction. However, I showed that blue mussels at their northernmost limit in North Greenland are capable of producing mature gonads and spawn, and that spat settles annually (Paper III). Thus, I conclude suppressed gonadal development is not a key factor in shaping polar-ward distribution limits. Another tested hypothesis is the oxygen- and capacity-limited thermal tolerance hypothesis, which suggests that low temperatures control species distribution through failed aerobic metabolism. I studied this in Paper IV but found no indications of limited aerobic performance in populations from either South or North Greenland (Paper IV). Instead, it seems that blue mussels are capable of adjusting their aerobic performance on both temporal and spatial scales (Paper V). Furthermore, blue mussels at their polar-ward limit are facing prolonged winters with limited pelagic primary production. To investigate how they survive the prolonged winter, I used fatty acids to study their food preferences, both while ice-covered and during summer (Paper VI). I found, that the population feeds extensively on diatoms while covered by intertidal sea ice, but after the ice breaks up, food consists mainly of pelagic dinoflagellates. Thus, this population likely does not face severe starvation during ice cover, as long as ice-algae growth is sustained. Finally, I exposed the blue mussel Mytilus edulis simultaneously to a natural (sub-zero air temperatures) and a chemical stressor (lead, Pb) to investigate the effects of multiple stressors on survival (Paper VII). These two stressors were chosen, because natural Greenlandic M. edulis populations are exposed to both stressors near their polar-ward limit. Chemical stress has been found to decrease thermal tolerance of some ectotherms, thus making them vulnerable to sub-zero air temperatures. I found no interacting effects in the study, but since natural populations are commonly exposed to multiple stressors in their environment, acknowledging the potential importance of such interactions are necessary to understand species ecology and distribution and should be studied further. In conclusion, the drivers of species distribution in the Greenland intertidal zone are complex, and much remains to be done. However, by combining field and laboratory work within ecology, physiology and genetics, this thesis contributes new knowledge of processes and traits determining the distribution and distribution limits of intertidal species in the Arctic. This knowledge is important for our understanding of climate change impacts - now and in the future