Climate change: effects on ecosystems in the ice and at the ice edge
Ecosystems in the Arctic are particularly sensitive to climate change. A circumpolar study of arctic ecosystems concluded that climate change is the single most important of all the factors impacting arctic ecosystems. The significant changes in sea ice in the Arctic over the past decades affect the plant and animal life associated with the ice.
Species that are totally dependent on ice as a habitat are the ones affected first and strongest by the rapid changes we currently see in the sea ice. Such species include ice algae, ice amphipods, ringed seals and polar bears. The indirect effects on species that are only partly dependent on ice are less clear; this group includes polar cod and seabirds that exploit the high productivity at the ice edge seasonally in their life cycles.
Phytoplankton and zooplankton
The ice edge is crucial for the marine ecosystem. Biological productivity at the edge of the ice is particularly great when daylight increases and the ice begins to melt, coinciding with an ample supply of nutrients. Such conditions lead to phytoplankton bloom in the water masses. The bloom propagates up through the food chain to zooplankton, fish, seabirds and marine mammals. Ice algae on the underside of the ice start their bloom as soon as the light is strong enough, as much as two months before the phytoplankton bloom. This lengthens the productive season in the areas with sea ice, and many grazers have adapted accordingly.
A longer growing season, as a result of less sea ice, may yield higher phytoplankton productivity in some areas. Primary production increased by 20% from 1998 to 2009 in the Arctic as a whole, but the regional variations are considerable: in some areas productivity decreased or remained unchanged. The timing and the species composition of the bloom also change. In the long-term perspective, is not clear how these changes will affect species that are directly or indirectly dependent on this primary production. EAltered growing conditions will undoubtedly affect different species in different ways. With the estimated changes in sea ice coverage, ice algae populations will decrease or even disappear completely from the Barents Sea. What effect this would have on the zooplankton that graze on these primary producers is not known. Models and estimates disagree concerning whether the total zooplankton biomass will increase or decrease; either way, the species composition is expected to change.
Seabirds follow their prey, and one probable consequence of changing availability of food (caused by changes in ice coverage) is that the range of some species will shift. A key factor in this context is whether the birds are specialists or can feed on several different prey species. The effects on seabird reproduction are closely linked to the distance between feeding grounds and suitable nesting sites. Seabirds’ reproductive success also relies on the chicks hatching when there is an adequate supply of food; this timing has evolved over thousands of years. At the ice edge, seabirds such as the ivory gull are also common. Seabirds transport nutrients farther up the food chain.
The Brünnich’s guillemot population in nesting areas in Svalbard has decreased steadily (by 5% per year) for the last 15 years (MOSJ-indicator). Altered availability of suitable prey species is believed to be the main cause of this decline.
Researchers believe that the northward retreat of the ice edge will leave the gulls in worse physical condition, decrease their reproductive success, and force them to expand their range to include suboptimal feeding grounds and prey. This is being studied in ICE-Ecosystems.
Ice-dependent marine mammals
Among marine mammals, it is mainly ringed seals and polar bears that require sea ice in their habitat. Both the polar bear and the ringed seal follow the drift ice northward through the summer, returning to coastal areas in the autumn. Changes in sea ice extent will thus have direct impact on these animals.
The ringed seal depends on ice to maintain its way of life. In particular, it requires landfast sea ice in fjords, in front of glaciers and around islands. These seals give birth to their pups in snow caves on the ice; they moult on ice, and they rest on ice. Less snow and less ice means that the seal cannot carve itself a snow cave above a breathing hole, and will be forced to give birth on the bare ice. This leaves the pups unprotected from weather and wind and makes them easier prey for predators such as polar bears and arctic foxes. In addition, the ringed seal needs ice year-round and finds much of its food near the ice. Studies in Canada have shown a declining ringed seal population, and this has been attributed mainly to climate change and changing ice conditions. In Svalbard, it has been noted that the ringed seals on the west coast have not had enough ice for normal reproduction since 2005, and it is reasonable to assume that the population is falling.
The polar bear is the top predator in the Arctic. It is dependent on ice for hunting, because it is in the drift ice that the bear finds its most important prey: ringed seals. The expected reduction of sea ice cover will lead to major changes in the availability of food for polar bears, both because the distribution of prey animals will change, and because some of today’s blubber-rich prey species may disappear. Population decline, worsening physical condition, and changes in polar bear distribution and behaviour are already seen in the Arctic, especially in the southern parts of the bear’s range. Reproduction rate and changes in body condition are monitored in polar bears around Svalbard, and they vary in parallel with large-scale changes in climate. However, other factors also influence the outcome in this area, and the causative links are not yet fully understood. The number of birthing dens on Hopen and Kongsøya shows clearly that few females come to these islands in autumns when the ice forms late. The number of days when there has been sea ice around all five of the most important denning areas has fallen off sharply since 1979. Some years the ice has been late in all of these areas simultaneously, and in such years, females who have spent the summer in the pack ice may have trouble reaching the denning sites. So far, however, there is no evidence that changes in climate have had any obvious effect on the polar bear population around Svalbard.
In Antarctica, the ice-associated parts of the ecosystem are also affected by regional changes in sea ice, particularly west of the Antarctic Peninsula. A possible decrease in krill biomass near the Antarctic Peninsula may be related to reduced sea ice in this area. Early in their development, krill appear to be need ice-related structures to survive. The extent of sea ice around the Peninsula has gone down by 40% over the past 25 years. The retreat of the ice in itself, along with its influence on krill populations, has had a significant negative impact on the ice-dependent Adélie penguin. Other penguin species, such as the gentoo penguin, move south to new habitats as the ice disappears, leaving behind excellent breeding grounds with ample food resources.
- Arctic Biodiversity Assessment Synthesis. The Conservation of Arctic Flora and Fauna (CAFF)
- J, Eamer et.al. 2013. Life Linked to Ice: A guide to sea-ice-associated biodiversity in this time of rapid change. CAFF Assessment Series No. 10. Conservation of Arctic Flora and Fauna, Iceland. ISBN: 978-9935-431-25-7.