Ecological Responses to Climate Change on the Antarctic Peninsula
The peninsula is an icy world that's warming faster than anywhere else on Earth, threatening a rich but delicate biological community
Forests of the Antarctic
Dive beneath the water's surface near Palmer Station and you enter a surprisingly lush and diverse benthic community. The steep rock slopes are densely populated with sessile invertebrates such as sponges and corals, along with a variety of grazing mollusks and other bottom-dwelling animals. The plant community is equally impressive. Some 90 percent of the sea floor is covered by large algae, whose waving fronds form dense stands much like the kelp beds of temperate seas. These are the forests of Antarctica.
All of these organisms are obviously well adapted to life in frigid water. How will they respond to the climatic changes expected in the coming decades and centuries?
The giant algae could in some respects be beneficiaries of warming. Their growth is limited mainly by access to sunlight, since they are shaded by sea ice for part of each year. As warming continues to erode the extent of the ice, the undersea forests are likely to expand into territory previously unavailable for colonization. But the further effects of changes in the plants' environment are hard to predict. With more energy available for photosynthesis, the algae may invest more of their resources in chemical defenses to prevent herbivores from consuming their tissues. The result could be a fundamental change in the dynamics of the community of organisms supported by the algae.
Among the benthic invertebrates, one potential trouble spot is in the timing of embryonic and larval development. Data gathered for species from various latitudes show a striking correlation between water temperature and time to maturity. The developmental processes are much slower in the Arctic and Antarctic, where reaching adulthood can take four or five times as long as it does in the tropics. Moreover, at the cold end of the temperature scale the slope of the graph is very steep, so that even slight temperature shifts correspond to a substantial change in development time. This finding suggests that environmental warming could shorten the embryonic and larval stages of life. The consequences of any change in the duration of development could be disastrous for those species that synchronize their breeding cycle with seasonal planktonic blooms. The eggs would hatch and the larvae would emerge into a sea that had insufficient resources to support them.
Early studies of polar ecology suggested that synchronization should not be a critical issue, because most immature offspring were expected to be "brooded"—supported by yolk or other parental resources rather than feeding on their own. This idea is so widely accepted that it has a name: Thorson's Rule, after the Danish zoologist Gunnar Thorson. However, more recent studies have revealed that some of the most ecologically dominant Antarctic benthic invertebrates have larvae that do indeed feed on plankton. As seawater temperatures along the Antarctic Peninsula continue to rise rapidly, these species may have their life cycle disrupted. Of course some of them may also be adversely affected in more direct ways by the temperature change. From a human perspective it may seem incongruous to speak of "thermal stress" in water a few degrees above freezing, but that is indeed a hazard for cold-adapted life forms, and some of them may not survive.
Another prospective community-altering impact was vividly brought to our attention recently when adult specimens of the spider crab Hyas araneus were dredged up from waters off King George Island, near the northern tip of the Antarctic Peninsula. This crab species is native to sub-Arctic northern waters and probably reached the Antarctic by traveling as larvae or juveniles in ship ballast water. What's most surprising is that the crabs were able to survive and grow in the colder waters of the Antarctic.
Crabs run into serious difficulty at very low temperature. As in many other animals, their activity level is reduced in the cold, but in addition they face a peculiar physiological challenge. Crabs cannot cleanse their bloodstream of magnesium, which has a narcotic effect. The magnesium concentration is no greater in cold water, but the narcosis is more severe there because the animals are already slower-moving. Below a threshold temperature, the crabs are immobilized and die.
Warming trends along the peninsula are removing this physiological barrier. The specimens discovered at King George Island are not the only evidence. In 2007 a population of large, deep-water king crabs (Paralomis birsteini) was discovered at a depth of 1,100 meters on the Antarctic continental slope. At these depths, the temperature is slightly higher than it is in shallow water, allowing the crabs to overcome the magnesium narcosis.
An invasion of crabs would present a significant threat to benthic invertebrates that lack defenses against crushing predators. And, as with the planktonic shelled pteropods, ocean acidification magnifies the risk. As absorbed CO2 continues to lower the pH of seawater, benthic invertebrates whose larvae or adults rely on calcified skeletal elements may either be killed outright or, with weakened shells, become increasingly vulnerable to durophagous predators. Evidence from temperate latitudes indicates that larvae of such key invertebrates as oysters and sea urchins can suffer significant decalcification-related mortality when exposed to seawater with even modestly lowered pH. Chemical and physical properties of southern seawater are known to inhibit calcification, and so it seems likely that the biota of this region will be among the first to show the effects of global ocean acidification.
» Post Comment