15.6: Polar Regions

The marine ecosystems of the north and south polar regions differ from one another physically and biologically because of the configuration of the landmasses and because they are separated by warm tropical waters through which most cold-adapted marine species cannot transit. Nonetheless, the two ecosystems have a number of environmental similarities, including extreme seasonal variation in light availability, generally low surface water temperatures, and seasonally variable sea-ice cover.

In both polar regions, cooling of surface ocean waters and vigorous wind mixing caused by storms formed at the polar front (Chap. 7) prevent the formation of a permanent thermocline and promote vertical mixing. Therefore, nutrients are generally abundant, especially in surface waters of the upwelling region between the Antarctic Convergence and the near-coastal east-wind drift current that flows around Antarctica. Primary productivity in this region apparently is limited by the availability of micronutrients, particularly iron, even when light is ample, and nitrogen, phosphorus, and silica are abundant. Iron concentrations are low in this region because there is a very low supply of river inputs and atmospheric dust in this region.

Special Characteristics of Arctic Marine Environments

In the Northern Hemisphere, nutrients are abundant in surface waters of the marginal seas that surround the Arctic Ocean, particularly the Bering, Norwegian, and North Seas. Nutrients are less abundant in seasonally ice-free surface waters of the Arctic Ocean itself, because freshwater runoff and ice exclusion lower surface salinity and establish a strong halocline that inhibits the vertical mixing of higher-salinity, nutrient-rich deep water into the surface layer. In addition, the permanent ice cover of much of the Arctic Ocean and its location in an atmospheric downwelling zone (zone of weak winds) minimize wind-induced vertical mixing.

In coastal regions of the Arctic Ocean, nutrients are generally available in relatively high concentrations during summer because they are supplied to some extent in freshwater runoff. In addition, nutrients are returned to the water column through decomposition during the darkness of winter, when these regions are covered by ice. Hence, nutrients are readily available during at least the early part of the short spring–summer ice-free season.

Common Characteristics of Arctic and Antarctic Marine Environments

In both the Arctic and the Antarctic, substantial populations of microscopic ice algae live, or form resting phases, within liquid pockets in the ice or on the underside of the ice. During spring and summer, ice algae can grow in or under the ice as the light increases in intensity and begins to penetrate the ice. As the ice melts, the resting spores of many species of phytoplankton are released into the water column, where they grow rapidly in the nutrient-rich and now well-illuminated open water. Both the Arctic and the Antarctic have a zone of maximum productivity that coincides with the edge of the floating ice. Each spring, this zone moves poleward with the melting ice edge.

Several important characteristics of polar ecosystems determine the species that are able to live there. First, because nutrients are generally available, phytoplankton usually grow quickly and are abundant when light is also available. Second, the extreme seasonal variation of light intensity and duration, and in some areas the extent of sea-ice cover, limit the period of ideal conditions for phytoplankton growth to only a few weeks or months in summer. Third, upwelling and wind mixing, which supply nutrients to surface waters and at the same time affect the residence time of phytoplankton in the nutrient-rich photic zone, are particularly variable because of eddies, turbulence induced by seafloor topography, and especially changes in weather and climate.

As a result of the unique physical characteristics of their environment, many animals that live in polar regions must migrate seasonally or be able to obtain all the food they need for the year during the short productive summer period. In addition, they must be capable of ensuring the survival of their species during years when climatic extremes may result in a partial, or even total, loss of their food supply. The evolutionary response to these constraints is for the animals to be long-lived, to reach sexual maturity late, and to bear only one or a few offspring each year for several years. This response ensures that the species will survive if one or more successive years are poor years in which none of the offspring survive. In addition, the animals must be able to store the food energy needed to survive the winter when food is unavailable or to migrate to warmer regions for the winter. The food energy is stored as fat. Many species that live in polar ecosystems, including certain fishes, marine mammals, and birds, have these characteristics.

Many of the characteristics shared by polar animals make them more susceptible than the marine species of lower latitudes to the adverse effects of certain chemical compounds that can cause contamination. Because fat-soluble chemical compounds are concentrated in an animal’s fatty tissues, they can accumulate for several years before polar animals reach reproductive age. This buildup maximizes the possibility of carcinogenic, mutagenic, and teratogenic effects in the species or its offspring (CC18). Fortunately, the Arctic and Antarctic marine ecosystems are substantially less contaminated than marine ecosystems at lower latitudes.

Antarctic Communities

The pelagic food web of the Southern Ocean is described in Chapter 12 (Fig. 12-12). It consists of many species of whales, seals, and penguins that feed on the abundant krill and other zooplankton (many of which can be found at the ice edge feeding on phytoplankton populations). Most of the penguin and seal species live year-round in Antarctic waters and haul themselves out on the continent or one of the nearby islands to breed and bear their offspring (Fig. 15-16). Like many polar species, most of these mammal and bird species mature only after they are several years old, normally have one or two offspring per year for several years, and build up heavy layers of fat during summer, when food is abundant.

Figure 15-16. Some examples of seal species that inhabit the Antarctic ecosystem. (a) Crabeater seals (Lobodon carcinophagus, Antarctic Peninsula) have specialized teeth (not shown in the image) to filter out krill, their principal food source. They are somewhat misnamed since they do not eat crabs. (b) Antarctic fur seals (Arctocephalus gazella, South Georgia Island), heavily hunted for their fur during the 18th and 19th century, feed mostly on krill squid fish and penguins, (c) Leopard seal, (Hydrurga leptonyx, Antarctic Peninsula) a voracious predator that will eat krill, fish, squid, penguins and other seals.

Although fat layers act as insulation against the Antarctic cold, particularly when the animal is on land, their most important function is to provide energy during the period when the animal is not feeding. For example, the majority of whale species in the Antarctic are baleen whales that visit the region only during summer, when food supplies are abundant. The huge store of fat that they build up during this time is used to supply them with energy during the remainder of the year, when most of these species migrate to breeding grounds in the tropical or subtropical ocean. During the migration and breeding season, the whales feed little or not at all.

Human hunting has dramatically reduced the populations of many seal and whale species in the Antarctic (CC16). These species are now protected, and most populations are showing signs of slow recovery.

Although fishes of the Antarctic ecosystem are much less well studied than the marine mammals and birds, we know that a high proportion of these species are present only in Antarctica and appear to be adapted to the cold and seasonally variable food supply in much the same way that Antarctic marine mammals are. Some species have evolved a unique blood chemistry that enables them to live at temperatures below freezing. The natural antifreeze of these fishes is the subject of considerable research because it could have commercial and medical applications.

Arctic Communities

The biological populations of the Arctic region are similar in some ways to those of the Antarctic, particularly in their concentrations of marine mammals. However, species found in the Arctic are different from those found in the Antarctic. Many seal species live year-round in the Arctic and its adjacent seas and haul themselves out on land to breed, as other seal species do in the Antarctic. Most northern whale species migrate between their Arctic feeding grounds and tropical or subtropical breeding grounds. Penguins and leopard seals are not present in the Arctic. However, the Arctic is populated by polar bears and walrus (Fig. 12-27d), which are not present in Antarctica. Polar bears are voracious predators and superb swimmers, but they are land animals. However, because they range across the sea ice to hunt seals, they are primarily dependent on food from the marine environment, and so they are considered a part of the marine ecosystem.

Susceptibility to Climate Change

Global climate models all predict that climate changes will be amplified in the polar regions, and this conclusion is supported by historical data and by observations during the past several decades. The reasons for this special susceptibility are many and complex, but they include the positive feedback (CC9) due to reduced snow and ice cover. A covering of snow and ice is a good reflector of the sun’s energy. Warming reduces snow and ice cover, which increases the amount of the sun’s energy absorbed by land and ocean, causing further warming.

The polar climate has warmed during the past 30 years, resulting in substantial reduction in the area of permanent sea ice in the Arctic Ocean, and causing ice sheets to be reduced in size in the Antarctic, and glaciers to retreat in both polar regions. If sustained, this trend will not only cause sea level to rise worldwide, but will also have profound effects on polar biological communities. For example, polar bear populations are already adversely affected and are likely to be devastated because the bears hunt mostly on floating summer sea ice, which is steadily reducing in area and retreating farther from the coast. Similarly, many species of seals and penguins are already and likely will be much further affected by changing ice and snow cover in their traditional breeding grounds on the Antarctic coast. Furthermore, changes with unknown effects are likely to take place because a reduction in ice cover favors phytoplankton production over ice algae production in both polar regions. Profound changes in both the Arctic and Antarctic marine ecosystems already appear to be well underway.