Effects of climate change on marine mammals

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The effect of climate change on marine life and mammals is a growing concern. Many of the effects of global warming are currently unknown due to unpredictability, but many are becoming increasingly evident today. Some effects are very direct such as loss of habitat, temperature stress, and exposure to severe weather. Other effects are more indirect, such as changes in host pathogen associations, changes in body condition because of predator–prey interaction, changes in exposure to toxins and CO2 emissions, and increased human interactions.[1] Despite the large potential impacts of ocean warming on marine mammals, the global vulnerability of marine mammals to global warming is still poorly understood.[2]

It has been generally assumed that the Arctic marine mammals were the most vulnerable in the face of climate change given the substantial observed and projected decline in Arctic sea ice cover. However, the implementation of a trait-based approach on assessment of the vulnerability of all marine mammals under future global warming has suggested that the North Pacific Ocean, the Greenland Sea and the Barents Sea host the species that are most vulnerable to global warming.[2] The North Pacific has already been identified as a hotspot for human threats for marine mammals[3] and now is also a hotspot of vulnerability to global warming. This emphasizes that marine mammals in this region will face double jeopardy from both human activities (e.g., marine traffic, pollution and offshore oil and gas development) and global warming, with potential additive or synergetic effect and as a result, these ecosystems face irreversible consequences for marine ecosystem functioning.[2]

Potential effects[]

Marine mammals have evolved to live in oceans, but climate change is affecting their natural habitat.[4][5][6][7] Some species may not adapt fast enough, which might lead to their extinction.[8]

Ocean warming[]

The illustration of temperature changes from 1960 to 2019 across each ocean starting at the Southern Ocean around Antarctica (Cheng et. al., 2020)

During the last century, the global average land and sea surface temperature has increased due to an increased greenhouse effect from human activities.[9] From 1960 to through 2019, the average temperature for the upper 2000 meters of the oceans has increased by 0.12 degree Celsius, whereas the ocean surface has warmed up to 1.2 degree Celsius from the pre-industrial era.[10]

Marine organisms usually tend to encounter relatively stable temperatures compared with terrestrial species and thus are likely to be more sensitive to temperature change than terrestrial organisms.[11] Therefore, the ocean warming will lead to increased species migration, as endangered species look for a more suitable habitat. If sea temperatures continue to rise, then some fauna may move to cooler water and some range-edge species may disappear from regional waters or experienced a reduced global range.[11] Change in the abundance of some species will alter the food resources available to marine mammals, which then results in marine mammals’ biogeographic shifts. Additionally, if a species cannot successfully migrate to a suitable environment, unless it learns to adapt to rising ocean temperatures, it will face extinction.

Sea level rise is also important when assessing the impacts of global warming on marine mammals, since it affects coastal environments that marine mammals species rely.[12]

Primary productivity[]

Changes in temperatures will impact the location of areas with high primary productivity. Primary producers, such as plankton,[13][14][15][16] are the main food source for marine mammals such as some whales. Species migration will therefore be directly affected by locations of high primary productivity. Water temperature changes also affect ocean turbulence, which has a major impact on the dispersion of plankton and other primary producers.[17] Due to global warming and increased glacier melt, thermohaline circulation patterns may be altered by increasing amounts of freshwater released into oceans and, therefore, changing ocean salinity. Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as upwelling.[18]

Ocean acidification[]

Change in pH since the beginning of the industrial revolution. RCP 2.6 scenario is "low CO2 emissions" . RCP 8.5 scenario is "high CO2 emissions", the path we are currently on. Source: J. P. Gattuso et al., 2015

About a quarter of the emitted CO2, about 26 million tons is absorbed by the ocean every day.[19] Consequently, the dissolution of anthropogenic carbon dioxide (CO2) in seawater causes a decrease in pH which is corresponding to an increase in acidity of the oceans with consequences for marine biota.  Since the beginning of the industrial revolution, ocean acidity has increased by 30% (the pH decreased from 8.2 to 8.1).[19] It is projected that the ocean will experience severe acidification under RCP 8.5, high CO2 emission scenario, and less intense acidification under RCP 2.6, low CO2 emission scenario. Ocean acidification will impact marine organisms (corals, mussels, oysters) in producing their limestone skeleton or shell. When CO2 dissolves in seawater, it increases protons (H+ ions) but reduces certain molecules, such as carbonate ions in which many oysters needed to produce their limestone skeleton or shell.[19] The shell and the skeleton of these species may become less dense or strong. This also may make coral reefs become more vulnerable to storm damage, and slow down its recovery. In addition, marine organisms may experience changes in growth, development, abundance, and survival in response to ocean acidification.

Sea ice changes[]

Sea ice, a defining characteristic of polar marine environment, is changing rapidly which has impacts on marine mammals. Climate change models predict changes to the sea ice leading to loss of the sea ice habitat, elevations of water and air temperature, and increased occurrence of severe weather. The loss of sea ice habitat will reduced the abundance of seal prey for marine mammals, particularly polar bears. Initially, polar bears may be favored by an increase in leads in the ice that make more suitable seal habitat available but, as the ice thins further, they will have to travel more, using energy to keep in contact with favored habitat.[20] There also may be some indirect effect of sea ice changes on animal heath due to alterations in pathogen transmission, effect on animals on body condition caused by shift in the prey based/food web, changes in toxicant exposure associated with increased human habitation in the Arctic habitat.[21]

Increase frequency of Hypoxia Occurrence in the entire Baltic Sea calculated as the number of profiles with recorded hypoxia relative to the total number of profiles (Conley et. al., 2011)

Hypoxia[]

Hypoxia occurs in the variety of coastal environment when the dissolved of oxygen (DO) is depleted to a certain low level, where aquatic organisms, especially benthic fauna, become stressed or die due to the lack of oxygen.[22] Hypoxia occurs when the coastal region enhance Phosphorus release from sediment and increase Nitrate (N) loss. This chemical scenario supports favorable growth for cyanobacteria which contribute to the hypoxia and ultimately sustain eutrophication.[23]  Hypoxia degrades an ecosystem by damaging the bottom fauna habitats, altering the food web, changing the nitrogen and phosphate cycling, decreasing fishery catch, and enhancing the water acidification.[23] There were 500 areas in the world with reported coastal hypoxia in 2011, with Baltic Sea contains the largest hypoxia zone in the world.[24] These numbers are expected to increase due to the worsening condition of coastal areas caused by the excessive anthropogenic nutrient loads that stimulate intensified eutrophication.  The rapidly changing climate in particularly, global warming, also contributes to the increase of Hypoxia occurrence that damaging marine mammals and marine/coastal ecosystem.

Species impacted[]

Polar bears[]

A polar bear waiting in the Fall for the sea ice to form.

Polar bears are one of many Arctic marine mammals at risk of population decline due to climate change.[25] When carbon dioxide is released into the atmosphere, a greenhouse like effect occurs, warming the climate. For polar bears and other Arctic marine mammals, rising temperature is the changing the sea ice formations that they rely on to survive.[25] In the circumpolar north, the Arctic sea ice is a dynamic ecosystem. The levels of sea ice extent varies by season. While some areas maintain year-round ice, others only have ice on a seasonal basis. The amount of permanent sea ice is decreasing with global temperature increases. Climate change is causing slower formations of sea ice, quicker decline and thinner ice sheets. Polar bears and other Arctic marine mammals are losing their habitat and food sources in result of the sea ice decline.[26]

Polar bears rely on seals as their main food source.[27] Although polar bears are strong swimmers, they are not successful at catching seal underwater, therefore polar bears are ambush predators.[28] When they hunt seals, they wait at seal breathing hole to ambush and haul out their prey onto the sea ice for feeding. With slower sea ice formations, thinner ice sheets and shorter winter seasons, polar bears are having less opportunity for optimal hunting grounds. Polar bears are facing pressures to swim further to gain access to food. This requires more calories spent to obtain calories to sustain their body conditions for reproduction and survival. Researchers use body condition charts to track polar bear population health and reproductive potential.[29] Trends suggest 12 out of 19 sub populations of polar bears are declining or data deficient.[30]

Polar bears also rely on sea ice to travel, mate and female polar bears usually choose to den up on the sea ice during denning season.[31] The sea ice is becoming less stable, forcing pregnant female polar bears to choose less optimal locations for denning.[26] These aspects are known to result in lower reproduction rates and smaller cub years.

Dolphins[]

Dolphins are marine mammals with broad geographic extent, making them susceptible to climate change in various ways. The most common effect of climate change on dolphins is the increasing water temperatures across the globe. This has caused a large variety of dolphin species to experience range shifts, in which the species move from their typical geographic region to warmer waters.

In California, the 1982-83 El Niño warming event caused the near-bottom spawning market squid to leave southern California, which caused their predator, the pilot whale, to also leave. As the market squid returned six years later, Risso's dolphins came to feed on the squid. Bottlenose dolphins expanded their range from southern to central California, and stayed even after the warming event subsided.[32] The Pacific white-sided dolphin has had a decline in population in the southwest Gulf of California, the southern boundary of their distribution. In the 1980s they were abundant with group sizes up to 200 across the entire cool season. Then, in the 2000s, only two groups were recorded with sizes of 20 and 30, and only across the central cool season. This decline was not related to a decline of other marine mammals or prey, so it was concluded to have been caused by climate change as it occurred during a period of warming. Additionally, the Pacific white-sided dolphin had an increase in occurrence on the west coast of Canada from 1984 to 1998.[33]

In the Mediterranean, sea surface temperatures have increased, as well as salinity, upwelling intensity, and sea levels. Because of this, prey resources have been reduced causing a steep decline in the short-beaked common dolphin Mediterranean subpopulation, which was deemed endangered in 2003. This species now only exists in the Alboran Sea, due to its high productivity, distinct ecosystem, and differing conditions from the rest of the Mediterranean.[34]

In northwest Europe, many dolphin species have experienced range shifts from the region’s typically colder waters. Warm water dolphins, like the short-beaked common dolphin and striped dolphin, have expanded north of western Britain and into the northern North Sea, even in the winter, which may displace the white-beaked and Atlantic white-sided dolphin that are in that region. The white-beaked dolphin has shown an increase in the southern North Sea since the 1960s because of this. The rough-toothed dolphin and Atlantic spotted dolphin may move to northwest Europe.[35] In northwest Scotland, white-beaked dolphins (local to the colder waters of the North Atlantic) have decreased while common dolphins (local to warmer waters) have increased from 1992-2003.[36] Additionally, Fraser’s dolphin, found in tropical waters, was recorded in the UK for the first time in 1996.[35]

River dolphins are highly affected by climate change as high evaporation rates, increased water temperatures, decreased precipitation, and increased acidification occur.[32][37] River dolphins typically have a higher densities when rivers have a lox index of freshwater degradation and better water quality.[37] Specifically looking at the Ganges river dolphin, the high evaporation rates and increased flooding on the plains may lead to more human river regulation, decreasing the dolphin population.[32]

As warmer waters lead to a decrease in dolphin prey, this led to other causes of dolphin population decrease. In the case of bottlenose dolphins, mullet populations decrease due to increasing water temperatures, which leads to a decrease in the dolphins’ health and thus their population.[32] At the Shark Bay World Heritage Area in Western Australia, the local Indo-Pacific bottlenose dolphin population had a significant decline after a marine heatwave in 2011. This heatwave caused a decrease in prey, which led to a decline in dolphin reproductive rates as female dolphins could not get enough nutrients to sustain a calf.[38] The resultant decrease in fish population due to warming waters has also influenced humans to see dolphins as fishing competitors or even bait. Humans use dusky dolphins as bait or are killed off because they consume the same fish humans eat and sell for profit.[32] In the central Brazilian Amazon alone, approximately 600 pink river dolphins are killed each year to be used as bait.[37] Another side effect of increasing water temperatures is the increase in toxic algae blooms, which has caused a mass die-off of bottlenose dolphins.[35]

Notes[]

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