Ecosystems on Land
Plants and animals show a variety of effects from increased UV radiation, though these effects vary widely by species. In the short term, a few species are projected to benefit, while many more would be adversely affected. Long-term effects are largely unknown. In addition to direct effects, animals will be indirectly affected by changes in plants. For example, pigments that are needed to protect plants against UV also make them less digestible for the animals that depend upon them. So while some plants can adapt to higher UV levels by increasing their pigmentation, there are often wider implications of this adaptation for dependent animals and ecosystem processes. Increased UV also has long-term impacts on ecosystem processes that reduce nutrient cycling and can decrease productivity.
Springtime in the Arctic is a critical time for the birth and growth of animals and plants. Historically, ozone had been at its highest levels in the spring, offering living things the heightened UV protection they needed during this sensitive time. Since ozone depletion due to manmade chemicals became a problem several decades ago, spring is now the time of year with the largest losses of stratospheric ozone. Longer daylight hours in springtime also add to UV exposure. Increases in UV also interact with climate change, such as the warming-related decrease in springtime snow cover, creating the potential for increased impacts on plants, animals, and ecosystems.
Birch Forests at Risk from Impacts of UV and Warming on the Autumn Moth. One example of a documented impact of increased UV that also has interactions with climate warming involves the autumn moth, an insect that eats the leaves of birch trees, causing tremendous damage to forests. Increased UV modifies the chemical structure of the birch leaves, greatly reducing their nutritional value. The moth caterpillars thus eat up to three times more than normal to compensate. Increased UV also appears to improve the immune system of the autumn moth. In addition, UV destroys the polyhydrosis virus that is an important controller of the survival of moth caterpillars. Increased UV is thus expected to lead to increased caterpillar populations that would in turn lead to more birch forest defoliation. At the same time, winter temperatures below -36˚C have previously limited the survival of autumn moth eggs, controlling moth populations. When winter temperatures rise above that threshold, caterpillar survival increases. Thus, observed and projected winter warming is expected to further increase moth populations, thus increasing damage to birch forests. The damaging impacts of climate change are likely to exceed the impacts of UV on birch forests.
Some freshwater species, such as amphibians, are known to be highly sensitive to UV radiation, though the vulnerability of northern species has been little examined. Climate-related changes are projected in three important factors that control the levels of UV that reach living things in freshwater systems: stratospheric ozone, snow and ice cover, and materials dissolved in water that act as natural sunscreens against UV. Reduced stratospheric ozone is expected to persist for several decades, allowing increased UV levels to reach the surface, particularly in spring.
More significantly for aquatic life, the warming-induced reduction in springtime snow and ice cover will decrease protection for plants and animals normally shielded by that cover, leading to major increases in underwater UV exposure. White ice and snow form significant barriers to UV penetration; just two centimeters of snow can reduce the below-ice exposure to UV by about a factor of three. This is especially important in freshwater systems that contain low levels of dissolved matter that would shield against UV.
Lakes and ponds in northern areas of the Arctic generally contain much less dissolved material than those in the southern part of the region, due mainly to the greater vegetation that surrounds water bodies in the south. Arctic waters also contain little aquatic vegetation. In addition to the low levels of dissolved matter and resulting deep penetration of UV in arctic lakes and ponds, many of these freshwater systems are quite shallow. For example, the average depth of more than 900 lakes in northern Finland and about 80 lakes in arctic Canada is less than 5 meters. As a consequence, all living things, even those at the bottom of the lakes, are exposed to UV radiation.
Some of the first impacts of warming will be associated with the loss of permanent ice cover in far northern lakes; these impacts are already taking place in the Canadian High Arctic. As the length of the ice-free season increases, these effects will be amplified. However, climate warming is expected to increase levels of dissolved matter in many arctic freshwater systems as warming increases vegetation growth. In addition, thawing permafrost could increase the amount of sediment stirred up in the water, adding protection against UV. These changes could partially offset the increases in UV due to reduced snow and ice cover and to decreased ozone levels.
Phytoplankton, the tiny plants that are the primary producers of marine food chains, can be negatively impacted by exposure to UV radiation. Severe UV exposure can decrease productivity at the base of the food chain, perhaps by 20-30%. Current levels of UV negatively affect some secondary producers of marine food chains; UV-induced deaths in early life stages and reduced survival and ability to reproduce have been observed. Damage to the DNA of some species in samples collected from depths of up to 20 meters has been detected. Some species suffer strong negative impacts while others are resistant, depending on season and location of spawning, presence of UV screening substances, ability to repair UV-induced damage, and other factors.
There is clear evidence of detrimental effects of UV on early life stages of some marine fish species. For example, in one experiment, exposure to surface levels of UV killed many northern anchovy and Pacific mackerel embryos and larvae; significant sub-lethal effects were also reported. Under extreme conditions, this experiment suggested that 13% of the annual production of northern anchovy larvae could be lost. Atlantic cod eggs in shallow water (50 centimeters deep) also show negative effects due to UV exposure.
UV-induced changes in food chain interactions are likely to be more significant than direct effects on any one species. For example, UV exposure, even at low doses, reduces the content of important fatty acids in algae, decreasing the levels of these essential nutrients available to be taken up by fish larvae. Since fish larvae and the chain of predators through the food web require these essential fatty acids for proper development and growth, such a reduction in the nutritional quality of the food base has potentially widespread and significant implications for the overall health and productivity of the marine ecosystem. Exposure to UV radiation has many harmful effects on the health of fish and other marine animals, notably the suppression of the immune system. Even a single UV exposure decreases a fish’s immune response, and the reduction is still visible 14 days after the exposure. This could cause increased susceptibility to disease by whole populations. The immune systems of young fish are likely to be even more vulnerable to UV as they are in critical stages of development, resulting in compromised immune defenses later in life.
Recent studies estimate that a 50% seasonal reduction in stratospheric ozone could reduce primary production in marine systems by up to 8.5%. However, as with freshwater systems, cloud cover, ice cover, and the clarity or opaqueness of the water will also be important factors in determining UV exposure.