How will this change?

The largest concern facing the polar environment at present is undoubtedly climate change. There are many studies on this subject that agree that it is likely that the polar ice caps will diminish in size, but disagree on the extent and timescale over which this will occur. For example, Comiso (2003) predicted that the extent of new sea ice would reduce by 3.1% per decade, and perennial sea ice would reduce by 8.5% per decade. While the reduction of sea ice cover to any extent will have a significant impact on this environment, it is more likely that this environment will be changed than eliminated. The extent of the effects, their timescale and how the intricate links between the ecosystem and it’s habitat can only be guessed at at this point. However, some basic hypotheses can still be drawn.

While it can be inferred that rising sea temperatures are already reducing the extent of sea ice’s coverage of the ocean, it is highly unlikely that the ice will be completely eradicated. Instead, it is more likely that the ice will be severely diminished during the spring and summer months, and relatively diminished as a whole. If this is the case, this could hold serious consequences for krill, as the reduction in thawing ice during the spring and summer months would lead to a reduction in diatoms and other organisms from the ice made available to the krill, as well as a reduction in algal blooms caused by release of nutrients. However, these effects would be counteracted somewhat by the fact that there would be a greater amount of primary production due to phytoplankton having more access to light. The distribution of the krill species associated with sea ice would be changed,  however, from associating with a hard substrate to free swimming in the water column. As well as this, krill would have to change their feeding behavior from grazing hard substrates to consuming free-floating phytoplankton, and there would be no guarantee that these organisms would provide the same dietary supplement that those associated with sea ice would.

All in all, it is likely that the species of krill that were associated with sea ice will suffer a reduction in abundance due to a reduction in food supply, as well as a removal of shelter for larvae and adults, leading to increased capture by predators.

It is also possible that this shift in ecosystem structure that could prove detrimental to krill populations could also prove beneficial to other types of organisms. A key example of this is that of salps, a highly abundant family of tunicate zooplanktonic organisms. These worms have been known to increase in abundance in response to declines in krill populations, most likely due to competition for the common resource of phytoplankton, as well as the fact that salps aren’t reliant on sea ice for survival. If salps replace krill as the most abundant type of zooplanktonic organism, a significant source of food for many organisms will be removed, leading the restructuring of the ecosystem around zooplanktonic organisms rather than higher predators (Legendre et al, 1992)

These potential changes, whilst being mere hypotheses, illustrate the severe consequences posed by the disruption of the link between krill and sea ice.

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