Undersea Life on an Arctic Cruise

The nature of the Arctic Ocean, which is an expansive, unrestricted, and generally stable marine environment, means that the species diversity is lower and much less complex than the temperate and tropical marine environments around the world. In other words, the Eurasian sector of the Arctic Ocean is continuous and very similar to the North American sector, insofar as species diversity is concerned, whereas the tropical IndoPacific Ocean is very different from the tropical Caribbean Sea. 

Most polar ecosystems are highly seasonal, which is a result of extreme annual cycles of light and dark and variations in the extent of sea ice coverage. As a result, there is also an annual cycle in marine productivity. An important generalization may be made concerning the species diversity of polar regions in that the harsh conditions, i.e. cold water temperature, equates to a low number of species. However, the environmental effects of high gaseous content and long daylight hours in the Summer season assure a rich biomass. Cold water holds dissolved gases, such as oxygen and carbon dioxide, more efficiently than does warm water (think of the effects of warm and cold temperatures on carbonated drinks), and up to 24 hours of sunlight per day during the Summer months means more time for photosynthesis is available. Both the zooplankton and the phytoplankton take advantage of these conditions, and of course the animals dependent upon them can attain huge concentrations. In fact, biological productivity in polar regions in Summer is generally much greater (as much as 10 times greater) than that of other regions.   

The Svalbard archipelago (located between 74º and 81º N latitude) is well north of the Arctic Circle, so its marine ecology is polar in nature. However, it is not as simple as one might assume, because the geography of this location complicates matters considerably. Its location near the boundary between the shallow Barents Sea and the much deeper Fram Strait, places the islands in a region that is at the southern margin of the annual sea ice coverage. 

The western region of the Svalbard Archipelago is bathed by the relatively warm waters of the West Spitsbergen Current, which is the northern-most extension of the Gulf Stream (which is known as the North Atlantic Drift in European waters). It generally has a temperature of more than 3ºC (36ºF), and brings Atlantic Ocean waters to this very isolated region. The North Atlantic Drift continues on to the coast of mainland Norway, several hundred miles to the south, and as a result the two marine environments show a strong relationship. It is interesting that the eastern and northern regions of Svalbard are isolated from the effects of the warm current and maintain a year round cold, Arctic tour environment with different ice conditions and species makeup. In addition, the aforementioned seasonality is quite variable from one region of the archipelago to another. The western region is warmer and wetter and therefore has greater variability than the colder and drier northern and eastern regions.

Biologists categorize the life zones of marine ecosystems into horizontal and vertical distributions. The Horizontal oceanic distributions are Tropical (surface waters are 20C [68ºF] or above), Temperate (surface waters are between 20º and 10ºC [68º and 50ºF]), and Polar (surface waters are 10ºC [50ºF] or less). The Vertical distributions utilize the same terminologies and temperatures, but are not so easily discernible on a map. The Vertical Tropical distribution is quite shallow, extending down to a depth of between 300 and 400 meters (1,000 to 1,300 ft) near the equator, and this in turn sits atop the Vertical Temperate distribution, which connects temperate waters in the Northern Hemisphere directly with temperate waters in the Southern Hemisphere. Even deeper are the Vertical Polar distributions which extend all the way from the Arctic waters to the Antarctic waters, allowing some species to inhabit both polar regions, as well as the deep, cold waters throughout the tropical and temperate latitudes in between. 

Of course, the physical aspects of the Arctic Ocean (temperature, light, chemical composition in the water, currents, and substrate) determine what life forms can exist and where. The marine environment is broadly divided into Pelagic (open ocean) and Benthic (sea floor) ecosystems, but the polar regions also contain a third category… Sympagic (under sea ice). Pelagic marine life is further divided into two main categories, plankton (organisms which drift with currents) and nekton (organisms that are capable of swimming actively against currents). Phytoplankton (including diatoms and dinoflagellates) is made up of tiny, plant-like organisms which have the ability to photosynthesize and form the base of the marine food web. Phytoplankton is eaten by zooplankton, tiny animals such as arrow worms, ctenophores, copepods, and a variety of larval crustaceans and fishes.

Benthic organisms live on the bottom of the sea floor. These creatures can either be attached directly to a substrate or be free living and able to moving across or within the bottom. Benthic organisms makeup the majority of Svalbard’s marine species. The coastal waters, particularly along the west coast and inside the larger fjords, tend to be quite deep and rocky. Strong currents tend to sweep away the sediments, which create steep, often vertical, rocky buttresses and walls. In the euphotic zone, shallow water where sunlight penetrates (generally above 30 m or 100 ft in depth in Svalbard), the most conspicuous organisms are brown macroalgae, otherwise known as kelp. This ecosystem also contains vast numbers of brittle stars and anemones. In the regions where sediment accumulates, particularly on the low angle bottoms of the large fjords, muddy or silty habitats form, which are colonized by bivalve mollusks, polychaete worms, and shrimp. 

Only about 60 species of fish have been recorded in Svalbard, which demonstrates the relative low species diversity typical of polar seas. Many of the fish found here are demersal species, that is, they inhabit bottom waters close to shore, while others are seasonal visitors from more southerly Atlantic waters. Very few species of large pelagic fish are able to succeed in the frigid waters around Svalbard, and although commercially important species such as Atlantic cod, herring, and capelin are commonly found here, they are not commercially viable.

 Svalbard lies within the region of annual sea ice cover, although parts of the west coast may remain ice-free year round. In some areas, ice can be an important limiting factor for marine life, because of its scouring action along the shorelines and on shallow bottoms.  Sea ice plays a very important role in the Arctic environment and has produced another ecosystem known as the Sympagic, or ice-associated community. This unique ecosystem has its basis in the growth of various species of algae on the edges and lower surface of the ice, where sunlight can penetrate. This algae, which accounts for as much as 25% of annual marine productivity in the Arctic Ocean, begins growing, or blooming, in early Spring and creates a huge food source for a considerable diversity of herbivores, particularly crustaceans such as amphipods and krill. These, in turn, attract predators, the most important of which is the polar cod. The polar cod is well-adapted to the challenging environmental conditions, such as low temperatures and frequent changes in salinity within and near the sea ice, and is considered a keystone species in the Arctic marine food web. Polar cod are the main food source for harp seals and other marine predators.

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