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Declining Sea Ice: Impacts on Arctic Cetaceans

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By Rachael Ragen, SRC intern

Climate change has had a major impact on Arctic waters especially since it is reducing and thinning sea ice. Anthropogenic greenhouse gas emissions have caused the temperature to increase by about 0.2 ºC and almost all of this heat is absorbed by the ocean (Hoegh-Guldberg and Bruno 2010). This negatively impacts the sea ice, which can be problematic for marine mammals since many behaviors are tied to seasonal ice conditions. In March of 1979 there was 16.5 million km2 of Arctic sea ice, but this number decreased to 15.25 million km2 by March of 2009 (Hoegh-Guldberg and Bruno 2010). There are many other effects due to the warming of the oceans. Thermal expansion occurs due to the lowered density of the warmer water causing sea levels to rise. Currents are based upon changes in density due to different temperatures of the water. These may change due to increased warming. The ocean also absorbs excess carbon dioxide from the atmosphere causing ocean acidification, which can have major effects on phytoplankton and zooplankton. This causes problems throughout trophic levels since these organisms make up the basis of many food webs.

Since sea ice is an important factor in the Arctic marine habitat, many marine mammals will experience changes in all aspects of their lives. Some of the most susceptible to these problems are endemic Arctic species such as the narwhal, as they are highly specialized and have trouble altering their habitat. Many other species are thought to shift northward as the temperature continues to increase (Wassmann et al. 2010). The metabolic rates of species also change with temperature and move out of their ideal range (Hoegh-Guldberg and Bruno 2010). The prey of Arctic cetaceans will also be affected by these changes causing a decrease in food and shifts in the food web. The major factor in all of this is sea ice considering the seasonal changes of ice structures the habitat of the marine environment and influences the organisms as well as photosynthetic processes, which have a major impact on the prey of the bowhead whale.

Figure 1: Bowhead whale, (Source: https://upload.wikimedia.org/wikipedia/commons/8/87/A_bowhead_whale_breaches_off_the_coast_of_western_Sea_of_Okhotsk_by_Olga_Shpak%2C_Marine_Mammal_Council%2C_IEE_RAS.jpg)

The bowhead whale is extremely adapted to thick sea ice and can move through nearly solid sea ice cover (Laidre et al. 2008). They rely on copepods and euphasiids but also eat zooplankton as well as pelagic and epibenthic crustaceans (Laidre et al. 2008). Phytoplankton have a specifically timed bloom when the sea ice begins to melt. Zooplankton then feed on these phytoplankton, but if sea ice decreases the water column will be warmed earlier causing the phytoplankton may bloom earlier. This will alter the interaction between zooplankton and phytoplankton possibly having very detrimental effects on the bowhead whale’s major food sources (Laidre et al. 2008).

Figure 2: Beluga (Source: https://c1.staticflickr.com/3/2598/3676156476_e01305bc09_b.jpg)

Belugas are connected with to pack ice and live in waters with a combination of open water, loose ice, and heavy pack ice. (Laidre et al. 2008) As species have a northward shift in their distribution, more predators such as the killer whale could move into the beluga’s habitat. Killer whales prey on narwhals and bowhead whales as well, but it is believed that belugas move into deep, ice-covered waters in order to avoid killer whales. (Laidre et al. 2008) If this ice disappears belugas could lose this protection and become much more susceptible to killer whales.

Figure 3: Narwhal, https://upload.wikimedia.org/wikipedia/commons/4/4e/Pod_Monodon_monoceros.jpg

Narwhals are thought to be the most susceptible of the Arctic cetaceans to changes in sea ice since they are endemic to the Arctic whereas belugas and bowhead whales have a circumpolar distribution (Wassmann et al. 2010). They are highly adapted to pack ice and most of their feeding occurs during winter months in waters with dense pack ice and limited open water. They mostly feed on benthic organisms (Laidre et al. 2008). Decreases or thinning in sea ice could alter their feeding habitats and be detrimental to their prey.

In the end changes in sea ice has many detrimental effects on Arctic cetaceans. As waters warm species are expected to shift northward because they are no longer in their ideal metabolic ranges and their habitats may no longer meet ecological needs (Laidre et al. 2008). Many species such as the humpback whale, minke whale, gray whale, blue whale, pilot whale, killer whale, and harbor porpoises may have altered migration patterns and arrive further north much earlier (Laidre et al. 2008). This will put these species in direct competition with narwhals, belugas, and bowhead whales. Predatory species such as the killer whale may also put more stress on these species due to increased predation. As habitat is lost or altered, the body condition of species will decline. This has a major impact both on cetaceans and prey species. Lowered body condition also makes organisms more susceptible to diseases and epizootics (Laidre et al. 2008). While the decrease in sea ice may initially benefit species like bowhead whales that feed on photosynthetic plankton, it will have unknown effects on the food web. The benefits will likely be short lived and become more detrimental to the habitat (Laidre et al. 2008).

References

Hoegh-Guldberg O, Bruno JF (2010) The impact of climate change on the world’s marine ecosystems. Science 328:1523-1528

Laidre KL, Stirling I, Lowry LF, Wiig O, Heidi-Jorgenson MP, Ferguson SH (2008) Quantifying the sensitivity of arctic marine mammals to climate-induced habitat change. Ecol Appl 18:97-125

Wassmann P, Duarte CM, Agustí S, Sejr MK (2011) Footprints of climate change in the Arctic marine ecosystem. Glob Chang Biol 17:1235-1249

Sea-ice loss boosts visual search: fish foraging and changing pelagic interactions in polar oceans

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By Nicole Suren, SRC Intern

Light availability is one of the most important factors in the success of visual foraging. It can be controlled by many variables such as turbidity or weather, but in polar ecosystems ice cover and seasonality are the primary controls for light availability. Climate change has had and will continue to have a huge effect on polar ecosystems through temperature and weather changes, but one of the most notable side effects examined in this study is how increased light availability caused by receding ice and reduced snow cover will affect the success of polar visual foragers. The study modeled the success of planktivorous, visually foraging fish, with the underlying principle of the model being that increased ambient light will increase visual range, thereby making prey detectable at a larger distance and increasing foraging efficiency. The results showed that declines of polar sea ice would boost the visual search of planktivorous fish, but only seasonally. While light availability related to ice cover can be variable due to climate change, the long dark periods caused by polar seasonality are factors independent of climate, and therefore will still place limits on visual foraging during those seasons.

Figure 1

(a) The blue line shows how sea ice extent has decreased over the past decades, and below is a diagram demonstrating that prey will become more likely to be visually detected as the thickness of sea ice decreases. (b) A variety of factors influence prey detection, including the nature and angle of incoming light. Predator, prey, and visual range are not drawn to scale. (Langbehn & Varpe, 2017)

The models predict that several things will change due to light availability caused by loss of ice cover. First, primary productivity may increase, depending on nutrient availability. Second, seasonal feeding migrations into the poles are expected due to the removal of the limiting factor of lack of light for visual foragers. This prediction has already been verified by real-world data; increased feeding forays by Atlantic Salmon, Atlantic Mackerel, and Atlantic Herring have been recorded, and these coincide with decreasing ice cover over the past 35 years. More generally, mobile, fast-swimming predators are predicted to take advantage of these foraging opportunities the most. However, increased light availability can also increase the likelihood of planktivorous predators being spotted and predated upon by larger visual predators in a higher trophic level. This means that not only will the ideal user of these seasonal foraging grounds be mobile and fast-swimming, but they will either be apex predators or schooling fish, which can use the technique of schooling to forage in relative safety despite being visible.

Figure 2

The extent of sea ice is averaged from 2010-2015 in (a) and (b), and (c) and (d) show how visual range correlates with these averages. Data from the Bering Sea and the Barents Sea are shown. (Langbehn & Varpe, 2017)

No matter how efficiently visual foragers learn to take advantage of increased light availability at the poles during the summer months, the darkness of winter will still be a significant limiting factor in regards to permanent habitat expansion. Polar winters will always be long and dark, even if climate change alters the ice cover in that time. This means that the permanent inhabitants of the poles will likely remain the only permanent inhabitants due to their specialized adaptations for living in darkness, while trophic interactions are likely to change during the summer.

Work Cited

Langbehn, T. J., & Varpe, Ø. (2017). Sea-ice loss boosts visual search: Fish foraging and changing pelagic interactions in polar oceans. Global Change Biology, (November 2016). https://doi.org/10.1111/gcb.13797

Polar Bears are Vulnerable to Loss of Sea Ice

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By Rachael Ragen

Figure 1

Polar Bear, https://sealevel.nasa.gov/ system/news_items/main_images/ 74_polarbear768.jpeg

Polar bears are currently facing a major problem: declining sea ice. As greenhouse gases continue to increase due to anthropogenic factors causing temperatures to rise and ice to melt. Since polar bears rely on sea ice as they search for prey, the decline in sea ice makes hunting much more difficult. The current population of polar bears is estimated to be 26,000 with 19 subpopulations in 4 ecoregions (Figure 2). It is very difficult to properly assess each subpopulation of polar bears as they live in extreme environments. Therefore, no global assessment has been done and the status of some subpopulations is unknown. The study by Regehr et al. aimed to look at the effect of sea ice decline on polar bears by determining the generation length, forming a standardized sea ice metric, and then using statistical models and computer simulations.

Figure 2

Map of Ecoregions, Regehr et al.

In order to determine the generation length, the authors looked at the age of female polar bears with a cub and found the average to be 11.5 to 13.6 years. Live capture data was used to determine these numbers. The upper level is used to account for variations in generation length.

A sea ice metric was determined using satellite data from 1979 to 2014. This data was used to establish the carrying capacity, which is the maximum amount of organisms the habitat can support, for the polar bears. Then the value found for K (carrying capacity) was used in linear models. This analysis generated predicted future values of ice as well, as the effect these values had on subpopulations. The ice decline was shown to affect all subpopulations.

The statistical models and computer simulations looked at the relationship between polar bear populations and sea ice over three generations using three different methods. First they assumed that changes in sea ice are directly proportional to changes in subpopulation abundance. This method was useful for populations with limited data. Second they looked at a linear relationship between ice and subpopulation abundance for subpopulations, although data was only available for seven of the nineteen. There was not shown to be a significant change due to variations in the status subpopulations as well as uncertainty in estimates of abundance. Lastly they again looked at a linear relationship between ice and population but for each of the four ecoregions. Some ecoregions showed a significant change, whereas others did not, showing that dynamics and biological productivity varies between subpopulations.

Figure 3

Table of data found, Regehr et al.

This study looked at the IUCN Red List’s guidelines for risk tolerance. The culmination of these studies showed that the first generation’s mean global population size was to decrease by 30%, the second by 4%, and the third by 43% (Table 1). Since there was shown to be a high risk of the population decreasing by 30% and a low chance of the population decreasing by 50% (Table 1), polar bears are classified as vulnerable.

Climate Change to Cause Polar Bear Population Declines

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By Laura Vander Meiden, SRC Intern

Over the next 35-40 years polar bear populations have the potential to decrease by more than 30% according to an assessment by the International Union for Conservation of Nature (IUCN). The report cites climate change and the resulting loss of sea ice as the cause of this probable decline.

Photo by Ansgar Walk vie Wikimedia Commons.

Photo by Ansgar Walk vie Wikimedia Commons.

 

Polar bears are specifically built to survive the harsh conditions of the arctic. Their adaptations include two types of insulating fur, a deep layer of fat to keep warm while in the water, bumps called papillae on the bottom of their feet for grip on ice, and feeding behaviors designed for living on the ice. Ironically it is these adaptations that make polar bears most vulnerable as the climate changes.

Scientist’s primary concern is the effect melting sea ice has on the eating habits of the bears. Though polar bears have been seen to opportunistically feed on a variety of organisms, their primary source of food is ring seals which live on the edge of the ice. The seals have a very high calorie content, particularly in their blubber, which is necessary for the polar bear’s frigid lifestyle. This allows the bears to build up large fat reserves which are critical as the bears can only hunt seals when there is ice. When seasonal ice melts in the summer, the bears typically must fast, living off their fat reserves, until the ice returns in the winter.

As climate change continues the ice will melt more quickly each summer and take a much longer time to return each winter. This extends the length of time polar bears must fast, resulting in higher chances of starvation. Melting ice and the subsequent reduced access to food can also lead to an overall decrease in body condition, reduced survival rates of cubs, loss of denning habitat and increased drowning as the bears attempt to swim between ice floes.

Polar bears are found on four different sea ice regions. The populations found in the region where ice is the most seasonal are at present in the most danger from climate change. Also vulnerable are populations in the divergent ice region where ice forms along the shore, but is not always connected to pack ice further out to sea. Safest are populations in the region where convergent ice connects the bears to pack ice and the archipelago region where ice remains year round. The latter region is expected to be the final refuge of the bears, but unless carbon dioxide emissions are reduced even this ice will be melted in 100 years.

Of 19 subpopulations 3 are declining, 6 are stable, 1 is increasing, and 9 have insufficient data to make a determination. Map via Norwegian Polar Institute.

Of 19 subpopulations 3 are declining, 6 are stable, 1 is increasing, and 9 have insufficient data to make a determination. Map via Norwegian Polar Institute.

While the situation for the polar bears appears dire, scientists have not completely lost hope. If significant reductions are made in greenhouse gas emissions, the amount of time before the sea ice melts could be extended. However scientists warn that action must be taken soon, since once a tipping point is reached sea ice will decrease rapidly and no amount of emission reduction will be able to stop the ice from melting.