Polar Ice
Over the past century, sea level has slowly been rising. This is in part due to the addition of water to the oceans through either the melting of or the "calving" off of icebergs from the world's land ice. Many individual mountain glaciers and ice caps are known to have been retreating, contributing to the rising sea levels. It is uncertain, however, whether the world's two major ice sheets-Greenland and Antarctica-have been growing or diminishing. This is of particular importance because of the huge size of these ice sheets, with their great potential for changing sea level. Together, Greenland and Antarctica contain about 75% of the world's fresh water, enough to raise sea level by over 75 meters, if all the ice were returned to the oceans.The Greenland ice sheet is warmer than the Antarctic ice sheet and as a result, global warming could produce serious melting on Greenland while having less effect in the Antarctic. In the Antarctic, temperatures are far enough below freezing that even with some global warming, temperatures could remain sufficiently cold to prevent extensive surface melting.There are three distinct marine water masses located within the Arctic Ocean: the Arctic Surface Water (0–200 meters); the Atlantic Water (200–900 meters or 650–2,950 feet); and the Arctic Deep Water (900 meters–seafloor). The Arctic Surface Water is divided into three layers: the surface, subsurface, and lower surface layers. Each of these water layers has distinct salinity and temperature characteristics.
The Atlantic Water (AW) is located below the Arctic Surface Water (ASW) and above the Arctic Deep Water (ADW). The average temperature (3°C [37.4°F]) of the AW is warmer than both that of the ASW (−1.9°C to −1.0°C [28.6°F to 30.2°F]) and the ADW (−0.8°C to 2.0°C [30.6°F to 35.6°F]). The AW has a higher salinity range (34.8–35.1) than that of the ASW (28.0–34.0). The ADW, with a salinity range of 34.9 to 34.99, represents approximately 60 percent of the Arctic Ocean total water volume and is comprised of the Norwegian Sea, Greenland Sea, Eurasian basin, and the Canadian basin deep waters.
Ice and Productivity
In the polar oceans, ice exists primarily in the form of either icebergs (glacier fragments) or sea ice . Sea ice, the major form of ice in the polar oceans, is formed by a sequence of events that occur once suitably cold (−1.8°C [28.8°F]) conditions exist to freeze sea water.After sea water begins to freeze, frazil ice (small ice crystals) is formed. Frazil ice eventually accumulates to form grease ice (surface ice slicks), which in turn accumulates to form small ice chunks and floes that aggregate together to form a solid ice cover. Over time this solid ice cover will thicken into sea ice. Sea ice is present in the Arctic Ocean in three forms: the Polar Ice Cap, pack ice, and fast ice. Unlike Antarctica, the Arctic Ocean has no central landmass.
Polynyas.
Polynyas are large areas of open water surrounded by sea ice. Polynyas can range in size from a few square kilometers to more than 50,000 square kilometers (more than 19,000 square miles). Polynyas, which are of biological and physical interest, are produced by either the removal of sea ice or by the prohibition of sea-ice formation.Nearshore polynyas are generally formed by strong surface winds blowing sea ice offshore, leading to sea-ice removal, surface-water exposure, and, in some cases, new production of sea ice. Polynyas also exist in the open ocean as a result of convection, a process that allows warmer subsurface waters to rise above sinking colder surface waters.
Recurrent polynyas play significant roles in the marine ecosystem by triggering early and intense phytoplankton production. Additionally, polynyas serve as wintering grounds for marine mammals.
Primary Productivity.
Primary productivity is affected by the availability of sunlight, carbon dioxide, and inorganic nutrients (nitrates, phosphates, and trace elements). In the marine environment, nutrients are recycled from phytoplankton to animals to decomposers (such as bacteria) before returning to phytoplankton. One of the most effective pathways for nutrients to be re-incorporated into phytoplankton is through the upwelling of nutrientrich deep waters in which the bodies of marine plants and animals have previously decomposed.In the polar oceans, phytoplankton blooms (explosive population growth) occur during the summer months as a result of favorable light conditions which lead to short-term increased primary productivity. During these months, the Antarctic Ocean's upwelling zone exhibits some of the Earth's highest primary productivity.
In the Arctic and Antarctic Oceans, the sea-ice formation and melt processes also play important roles in primary productivity. Frazil ice is mixed with surface and subsurface water, entrapping phytoplankton between ice crystals that are eventually incorporated into pack ice. The phytoplankton (mainly diatoms ) will grow within the sea-ice brine channels, causing the pack ice to appear greenish-brown. During the yearly ice melt process, the diatoms are released back into the water, resulting in local increased primary productivity.
