Primary production and zooplankton
The disappearance of seasonal sea ice will result in increased primary production in the Barents Sea (Øiestad, 1990; Loeng et al. 2005, Ellingsen et al., 2008). The disappearance of seasonal sea ice would eliminate the ice-edge blooms, which would be replaced by blooms resembling those in the more productive Atlantic waters and their timing would be determined by the onset of seasonal stratification.
Loeng et al. (2005) suggested the spring bloom would occur earlier and this would enhance annual primary production by extending the growing season. They also stated that regions where the seabed or the depth of mixing is <40 m are likely to favour diatom blooms, whereas if mixing extended to about 80 m it would likely favour Phaeocystis. Thus, projected stronger winds are likely to result in Phaeocystis becoming more common than at present in the northern and eastern regions of the Barents Sea. If the surface mixed layer extends beyond about 80 m, it is possible that a low-productive community dominated by nanoflagellates would be favoured.
This would imply little transfer of carbon to herbivores and sediments because the grazers would be largely ciliates (Sakshaug and Walsh, 2000).
Ellingsen et al. (2008), using a coupled biological-physical model, found a slight (8%) increase in the mean level of phytoplankton production between 1995 and 2059, due principally to increases in the northern Barents. This is a result of a combination of higher light levels in areas of decreased ice extent and higher nutrient levels from the increased influence in the Atlantic waters. This compares to the 30% increase suggested earlier by Slagstad and Wassmann (1996) between heavy and light ice years.
One effect of climatic changes is changes in run-off from land, due to increased precipitation and melting. Such changes could have a large impact on the phytoplankton abundance, species composition, and production. An increase in the run-off could increase the amount of nutrients added to coastal water, leading to higher phytoplankton activity and changes in the stoichiometric environment (changes in the N:P:Si ratio). An increased run-off could also alter the light regime with more humic substances (DOM) and a stronger light attenuation as well as an increased degree of stratification. The outcome of such changes, on the species composition, could be either an increase in smaller flagellates and dinoflagellates due to low light and strong stratification or species that takes advantages of higher nutrient concentration (e.g. diatoms). It should also be noted that climate warming may act to reduce the supply rate of nutrients, because warming and increased input of sea ice melting can lead to increased stratification of the water column, thus reducing the mixing of nutrient rich deepwater with the layers higher in the water column where primary production occurs (Sakshaug and Slagstad 1992; Wassmann et al. 2006; Loeng and Drinkwater 2007; Tremblay and Gagnon 2008).
Loeng et al. (2005) noted the risk of a mismatch with zooplankton in the event of earlier phytoplankton blooms and the potential of less food supply to fish (Hansen et al., 1996). In such a case, vertically exported production and protozoan biomass are likely to increase. However, a match with phytoplankton blooms could be achieved by arctic copepods, such as C. glacialis, which can adjust its egg production to the development of the phytoplankton bloom, whether early or late in the season. The expected northward extension of warm water inflows would carry with it temperate zooplankton resulting in a northward shift in their distribution (Skjoldal et al., 1987) while ice fauna, such as the large amphipods would suffer massive loss of habitat because of the disappearance of multi-year ice (Loeng et al., 2005). Ellingsen et al. (2008) also predicted that the Atlantic zooplankton production, primarily Calanus finmarchicus, would increase by about 20% and spread farther eastward while the Arctic zooplankton biomass would decrease significantly (by 50%) resulting in an overall decrease in zooplankton production in the Barents Sea. The increased abundance of Atlantic zooplankton is believed to be caused by higher transport into the Barents through inflow of warm Atlantic water (Stenevik and Sundby, 2007) and to faster turnover rates due to the higher temperatures (see Tittensor et al., 2003). Increased amounts of pelagic plankton eating fishes, such as blue whiting and mackerel may also trigger decreases in abundance of Atlantic zooplankton. In addition, it is uncertain how jellyfishes, an important group of predators on zooplankton, may respond to climate changes.
Arrigo et al. (2008) discussed the general topic of pelagic versus benthic production with the loss of sea ice. Earlier sea ice melt and the subsequent release of ice algal communities to the water column at a time when surface waters are cold and zooplankton growth rates are low could result in low zooplankton abundance and reduced grazing, thereby increasing the sinking flux of particulate matter from the sea ice to the sediments. However, if advection of increasingly warm surface waters is responsible for the early losses of sea ice, zooplankton growth may not be negatively impacted and carbon export may remain unchanged or even diminish. Furthermore, reduced sea-ice cover has been proposed to favour a pelagic-dominated ecosystem over the more typical coupled sea-ice algae and benthos ecosystem (Piepenburg, 2005). This ecosystem switch could reduce the vertical export of organic carbon and decrease pelagic-benthic coupling, despite overall increases in phytoplankton productivity. Thus what will happen in regards to pelagic-benthic coupling remains unclear.
Changes in fish production and distribution
If warming causes phytoplankton to increase, this is expected to result in an overall increase in fish production. For example, model studies show that higher primary production tends to lead to an increase in cod recruitment in the Barents Sea (Svendsen et al., 2007). Higher temperatures should also lead to improved growth rates of the fish and together with increased recruitment is expected to lead to increased fish yields (Drinkwater, 2005; Stenevik and Sundby, 2007). Increased overall production is expected to produce increased catches of cod, haddock and other species (ACIA, 2005). Cod are expected to spawn farther north and new spawning sites will likely be established (Sundby and Nakken, 2008; Drinkwater 2005).
Possible impacts on the capelin population were explored by Huse and Ellingsen (2008). The movement of the Polar Front farther north and east (Figure 2) will result in a shift in the adult capelin distribution towards the north-eastern Barents Sea, consistent with distributional changes under observed cold and warm years by Gjøsæter (1998). Capelin were also predicted to spawn earlier and to shift their spawning sites eastwards from their present position off northern Norway and establish new spawning locations along Novaya Zemlya (Huse and Ellingsen, 2008). Herring, blue whiting and possibly Atlantic mackerel will spread farther eastward resulting in new species interactions and potentially change the structure and function of the Barents Sea ecosystem (Stenevik and Sundby, 2007). For example, in chapter Current and expected state of the ecosystem - Conclusions about state of the ecosystem of this report it is described how a larger herring stock even on a short term basis may have profound impact on the ecosystem by reducing biomass of zooplankton and severely affect recruitment of capelin. Indirectly, this may affect a number of species, including cod, seabirds and mammals. Salmon abundance likely will increase in Russian waters as previously observed under warmer conditions (Lajus et al. 2005) and also extend to northern Svalbard. The distribution shifts of fish will result in a higher proportion of the fish (such as cod and haddock) into Russian waters although because of expected increases in total production, the total number of fish in both the Norwegian and Russian economic zones should increase (Stenevik and Sundby, 2007). The extent that fish will expand farther east and north will depend not only upon changes in ocean conditions, but also upon the degree of future fishing intensity. Indeed, examining the effect of different management regimes on Norwegian cod fisheries in conjunction with climate change, Eide (2008) concluded that these management schemes will play a more significant role than climate change on the economic performance of the fishing industry in the Barents Sea.
Bioclimatic envelopes are a set of physical and biological conditions that are suitable to a given species and are generally identified from present associations. Cheung et al. (2008) determined the responses of Atlantic cod and capelin to climate change after 30 years using bioclimate envelope models that included sea temperatures, bathymetry, habitat and distance from sea ice. They found that for cod in the Barents Sea there would be an increase in overall abundance with a shift in distribution eastward and northward with a large increase in the Russian zone (Figure 4.6.3) similar to the projections made by Drinkwater (2005) and Stenevik and Sundby (2007). For the polar cod, Cheung et al. (2008) suggested the population would disappear from the Barents Sea after approximately 30 years.
Vikebø et al. (2007) examined the potential impact of a reduction in the thermohaline circulation (THC) in the North Atlantic on the larval drift of the North-east Arctic cod. This circulation pattern brings warm water north which cools, sinks and returns as a deep water current. Using a Regional Ocean Modelling Systems (ROMS), they imposed a 3 times present river discharge to the Nordic Seas and the Arctic Ocean greatly reduces the strength of the THC by 35%. This is near the projected reduction of around 25% in the THC predicted by the end of the 21st century in the IPCC (2007) report. Vikebø et al (2007) found that this reduction results in a south and westward drift of cod year classes from the Barents onto the Norwegian and Svalbard shelves, a reduction in the numbers of pelagic juveniles that survive, and an increase in the proportion of larvae and juveniles advected along West Svalbard and possibly into the Arctic Ocean. These latter would not be expected to survive, however.
The results of long-term simulations by STOCOBAR model show that a temperature increase of 1-4C° in the Barents Sea will lead to acceleration of cod growth and maturation rates. This will positively affect the general production of the cod stock but on the other hand, cannibalism will also increase, which will have a negative effect on cod recruitment and the total cod abundance.
The summarized consequences of a temperature increase in the Barents Sea for the cod stock and catches are presented in Figure 4.6.4. The harvest control rule for cod in the simulations correspondents to the present management strategy, which is based at the precautionary approach. The cod yield for the all temperature scenarios were calculated using existing values of the biological references points for the cod stock.
Benthos, cephalopods and shellfish
With increasing temperatures, temperate benthic species are expected to become more frequent and the species composition of the benthos will change. A shift in the benthic communities towards boreal species at the expense of Arctic species is expected, as observed in the early 20th Century warming (Blacker, 1957; Nesis, 1960). Such changes will affect benthic production (i.e. food for demersal fishes and other vertebrates) and may therefore have considerable management implications. In addition, the marginal ice zone is an area of high benthic productivity because of large amounts of ice algae from melting ice sinking to the bottom. If sea ice is lost or greatly reduced, this production pulse will be greatly reduced or disappear. Also, much of the production in these areas may shift from benthic to the pelagic species, resulting in dominance of pelagic species. However, there is considerable uncertainty associated with this (see section on primary production and zooplankton above).
Future fluctuations in zoobenthic communities will be related to the temperature tolerance of the animals and the future temperature of the seawater. Whereas a majority of the boreal forms have planktonic larvae that need a fairly long period to develop into maturity, arctic species do not (Thorson, 1950). Consequently, boreal species should be quick to spread with warm currents in periods with warming, whereas the more stenothermal arctic species will perish quickly. During periods of cooling, the arctic species, with their absence of pelagic stages, should slowly follow the receding warm waters. Boreal species that can survive in near-freezing water could continue to live in the cooler areas.
Polar bears, ringed seals, bearded seals, harp seals and hooded seals are all dependent on sea ice. It is the primary foraging habitat for polar bears, and a resting and breeding habitat for all of these seals. Additionally, some of the seals feed on ice-associated prey. As a result of climate warming and the associated loss of sea ice, distribution and abundance of these species are expected to decrease in the Barents Sea. Some observations supporting this expectation have already been made. In the recent warm years in the Barents Sea, reproduction has been low in ringed seals and harp seals. Pup mortality of harp seals has also been high in the White Sea. No effects of declining sea ice have been detected for polar bears in the Barents Sea, but in areas of Canadian Arctic, reduced body condition and lowered rates of reproduction have been observed as a consequence of a longer ice-free season.
Three species of whales, beluga whales, narwhal and bowhead whales are associated with sea ice. However, the linkage between these species and sea ice is less well understood than for the other ice-associated marine mammals. Sea ice is thought to provide a predation shield, and may also serve to reduce competition for food. But, because of our lack of detailed ecological data on these species in the Barents Sea region it is hard to predict what will happen to these species in a warmer climate.
Several species of marine mammals are found only in the ice-free season and ice-free regions of the Barents Sea. Climate warmingis expected to result in these species spending longer periods of time in the Barents Sea and expanding their distribution north and eastwards. Observations supporting these predictions have already been made. An increasing number of fin whales have been observed to the north of Svalbard, and boreal species such as sei whales and harbour porpoises have been observed at very high latitudes in recent years. Killer whales also appear to be arriving in at high latitudes very early in the spring.
Seabirds that are dependent on sea ice may be affected directly by climate changes. ivory gulls, for example, which feed in the marginal ice zone or in openings in the ice, distribution and abundance is expected to decrease or the species may disappear totally from the Barents Sea. Similar responses can be expected for other species that are dependent on sea ice. Direct effects like reduced breeding success due to heavy rain- and snowfall early in the breeding season, might be an increasing problem if weather gets worse.
For other seabird species, climate change effects are more likely to occur indirectly through changes in distribution and abundance of prey species. This means that some species may be negatively affected. For example, little auks Alle alle feed on large energy rich Arctic zooplankton, which will be replaced by smaller and less energy rich Atlantic species in a warmer climate. This will probably cause abundance of little auks to decrease substantially throughout their current range in the Barents Sea.
Other species may be affected positively by climate warming if their food sources are positively affected. For example, if capelin shift to a more northern and easterly distribution, seabirds that are dependent on capelin may expand in these parts of the Barents Sea, and increase in the southern parts.
In general, climate warming will tend to cause infectious organisms to acquire a more northerly distribution. The response for each pathogen species will vary from no response to possible large responses. Thus, new species of infectious organisms will be established in the Barents Sea, but it is difficult to predict which ones.
Because infectious organisms may have profound effects on the dynamics of host populations and structure of ecosystem, this may affect the overall dynamics of the ecosystem in Barents Sea.
Overall impact on the ecosystem
To understand the overall effect of climate change on the ecosystem in the Barents Sea, it is necessary to look across different groups of organisms and take into account how species interact and influence each other in the ecosystem. As described above, there is considerable uncertainty associated with how individual groups of organisms will respond. For example, based on our current understanding, we cannot predict whether abundance of a central group like zooplankton will increase or decrease in a warmer Barents Sea. This uncertainty does not get smaller when we attempt to put together a broader picture and take into account the many complex ways species may interact in the ecosystem. Therefore, it is impossible to predict in detail how the ecosystem will respond to climate warming.
A more useful approach can therefore be to analyse different types of changes. It is possible to distinguish between two types of ecosystem responses to climate warming. One type is ecosystem shifts, in which basic parts of the structure of the ecosystem is changed irreversibly. Such changes can have large effects on biodiversity and productivity of the system. An example is the changes that have occurred in the Northwest Atlantic, where crustaceans have taken over as dominating group after collapses of cod and other large predatory fishes. The other main type of change can be termed smaller changes. Here, the main groups in the ecosystem will remain the same, although species composition may change.
As described above, we can predict with fair certainty that a number of such smaller changes will occur as a result of climate warming. In particular it can be predicted with fair certainty that southern (boreal) species will become more northerly distributed and enter the Barents Sea. Ice dependent species will likely decline or disappear. Thus, species composition will change considerably. In addition, several of the species that are present in the Barents Sea today may shift to a more north-easterly distribution and/or change in abundance.
Such small changes may spread to other species in the ecosystem. For example, Norwegian spring spawning herring tends to produce strong year classes in warm years. Strong year classes of herring can have strong negative effects on capelin and have caused the capelin stock to collapse three times since the mid 1980s. As described in chapter General background description of the ecosystem - Ecosystem interactions - Overall picture, this has had large effects on other species in the ecosystem, including zooplankton, cod, seabirds and marine mammals. If a warmer climate causes herring to produce strong year classes more often, negative effects on capelin may become more persistent, with potentially considerable consequences for the ecosystem (this is discussed in more detail in chapter Current and expected state of the ecosystem - Conclusions about state of the ecosystem).
It is possible to draw up a scenario where such changes and other smaller changes get so numerous that they can no longer be absorbed by the existing structure in the ecosystem. If so, the ecosystem may shift irreversibly to another state. Climate change has contributed to such large shifts in other marine ecosystems, but most often as a factor in combination with other types of impact, such as fisheries and pollution. It is therefore reasonable to assume that the probability of climate induced regime shifts in the Barents Sea will increase with increasing pressure from other factors such as fisheries, which has a large impact on the ecosystem today. Another important factor to consider is acidification. As described below, acidification can cause considerable changes in the ecosystem and thus potentially amplify the effects of climate change on the ecosystem.