Transport into a region by itself does not predict that a species can become established in that region and persist, potentially permanently displacing endemic species. The expatriate species may be able to survive in the short term but, because their life histories and physiology are not adapted to the environmental conditions (e.g., temperature, phenology of production, light cycles), they may not reproduce. For example, it has been hypothesized that Alaskan salmon cannot reproduce along the north coast of Alaska (Carothers et al., 2013) and that Bering Sea pollock will not experience a northward shift in distribution because of persistence of very cold water (<0 °C) at depth in the northern Bering Sea (the “cold pool”) and further north (Sigler et al., 2010).
If, on the other hand, subarctic species can adapt to and successfully reproduce in Arctic conditions, then their biogeographic ranges can expand. In the future, with warmer temperatures and earlier and potentially higher primary production with a longer productive season, temperate organisms transported into the Arctic may be able to persist—that is, to reproduce and maintain populations in the Arctic. It also has been suggested that temperate species may have better resistance to ocean acidification (AWI, 2013). Changes in persistence of expatriate species can result in changes in community composition, displacement of endemic Arctic species, changes in pelagic-benthic coupling, changes in the size composition of planktonic and benthic organisms, and thus the availability of prey for forage fish and seabirds and, ultimately, marine mammals.
Recognizing colonization by expatriate marine species is difficult because few long-term records exist (Wassmann et al., 2011). The situation is better for terrestrial ecosystems, for which there are some long-term records (e.g., Jeffries et al., 2012; Post et al., 2013). Lack of understanding of physiological tolerances, temperature-dependent rate processes, and species phenologies also hampers our ability to predict northward expansion of marine and terrestrial organisms. Studies focusing on the potential for expatriate species to survive and persist, including modeling, observations, and experimentation to determine species-specific responses and vital rates under varying environmental conditions, are necessary to gain this predictive capability.
A by-product of many types of phytoplankton is dimethyl sulfide (DMS), which serves as effective condensation nuclei for the formation of clouds. As the Arctic Ocean transitions to a seasonally ice-free state, the resulting shifts in distributions and abundance of phytoplankton are likely to influence DMS production. Large uncertainty surrounds the magnitude of this change on cloud production within and beyond the Arctic.