Thanks to Jenny Rock, Biological Sciences, Bangor University, Wales & Falk Huettmann, University of Alaska, Fairbanks.
South Orkney Composite courtesy of David Barnes, British Antarctic Survey
The polar regions remain one of the least studied and understood ecosystems on the planet, despite their influential role in driving many global climatic processes. The polar marine environment is a unique habitat rich in species adapted to extreme conditions. In both the Arctic and the Antarctic, physical isolation, marked seasonal cycles of photoperiod/nutrient availability, extreme climates and complex food webs have posed special challenges for marine life.
One of the most influential shaping forces in the evolution of polar organisms is of course their extreme low temperature environments. For instance, in the high latitudes of the Ross Sea in Antarctica, water temperatures currently have a constant annual mean of –1.87ºC +/- 0.09 sd . In this environment, most organisms are stenothermal with upper lethal limits of 6-8ºC. At slightly lower latitudes on the Antarctic Peninsula, water temperatures vary more throughout the year, but still
only by ~ 2-3ºC. The high Arctic is also characterized by low and relatively constant water temperatures (e.g. the coastal waters of Svalbard (~80º N) range from –2 to +3ºC).
Components of the Arctic food chain; thanks to ArCODiv
For many organisms, the polar regions are intensely productive, as exemplified for instance by the huge seabird and sea mammal colonies present in both regions. The effects of extended summer daylight conditions on different trophic levels of the food chain contribute to this situation, as do nutrient levels, temperature cycling and mixing of fresh sea ice with saltwater. The ‘Polar Gold’ is probably the zooplankton (krill) which feed on massive diatom blooms and fuel the polar food chains. Much of these ‘biological gold mines’ are patchy in time and space and located at marine hotspots, where upwellings and currents provide a mixing regime. They are frequently located on shelf edges. These hotpots not only attract biological diversity, but also human activity. In addition to these mixing regime hotspots, the Arctic is famous for its polynyas (or ‘polynia’), which are areas free of ice throughout the entire year; these have also historically attracted humans inhabiting the vast ice wilderness (e.g. the Canadian North Water polynya). Similar phenomenon exist for the Antarctic region, and such hotspots have been the focus of much of the fishing and whaling activity in the Southern Ocean.
Image from Research Council Norway
Endemism (species only occurring in a unique location) in polar waters can be very high. This is because species have evolved in these extreme conditions for thousands to millions of years. Contemporary issues in biodiversity are not only focused on high diversity areas, but even more so on areas of endemism and “Evolutionary Cradles”; of which the polar waters are clearly exemplary.
Distinct geological and oceanographic histories have shaped two different polar ecosystems. For example, the Antarctic and Arctic underwent glaciation during different geological epochs that differ by over 30 MY(Miocene and Pleistocene, respectively). The formation of the Antarctic Polar Front created a circum-Antarctic current and the subsequent near-complete physical and biological isolation of the Southern Ocean for the last 25 MY, with a cold water ecosystem stable for at least the last 8 million years. Today, the resulting Antarctic marine ecosystem has many more species and a greater biomass than the Arctic. (Although it is believed that over 200 species are shared in common between the poles.) The unique hydrological and geographic separation barriers in the Antarctic have resulted in extremely high levels of endemism at the species level. In contrast, the Arctic Ocean cooled much more recently (~2.5 MYA) and has experienced more recent degrees of contact with other major oceans, as well as multiple cycles of ice sheet advance and retreat. These events have dramatically altered the biogeography of Arctic marine organisms. Consequently, the evolution of marine species in the Antarctic has been driven by a long period of stable low temperatures that are considerably more severe, though less variable, than in the relatively young Arctic cold water ecosystem. Antarctic crustaceans, for example, are generally far less tolerant to increases in temperature, with upper lethal ranges often only a few degrees above their narrow ambient temperature range. Many Arctic ectotherms can adopt a degree of temperature compensation, whereas in their Antarctic counterparts the capacity for such short term adjustments is greatly reduced. In general, it appears that the
relatively young cold water Arctic ecosystem is more heavily colonized by eurythermal species.
Cuspidoserolis_sp Photo: David Barnes, British Antarctic Survey
Climate change and human exploitation
Polar marine organisms will be contending with the most rapid and significant aspects of climate change because they are largely stenothermal and in many cases intolerant of what might be seen as globally insignificant temperature increases. (See Physical Oceans page for additional information on ocean warming.) Rapid and locally catastrophic changes in species distribution and abundance have already been predicted within the next 50 years across the northern reaches of the Western Antarctic Peninsula region. Although much research has focused on the physiology of upper thermal limits of polar organisms (e.g. thermal tolerance/lethal limits), we are now also beginning to identify sub-lethal but pervasive effects on physiological performance which will impact on population dynamics and biogeographic distribution. For example, the duration of larval development in some marine organisms determines recruitment success because it allows dispersal over greater
distances. As the duration of development is directly affected by sea temperature, this presents a critical relationship between sea temperature, developmental rate, dispersal distance and recruitment success. Coupled bio-physical oceanographic modelling is currently being used to project connectivity of marine populations under varying environmental conditions in the Southern Ocean. In Arctic waters, these methodologies have already indicated that a temperature increase of 1-1.5ºC can been associated with recruitment failure in Arctic cod.
Henry Kaiser, NSF
The unique biota at the poles leaves polar ecosystems widely affected by human activity including climate change, industrial contamination and other effects of human development. Particularly in the last hundred years, the polar regions have suffered intensive human exploitation. This is specifically problematic because these regions have a low potential for resiliency, and human activities can be tracked in the environment for 100s of years. Further, the polar oceans are directly linked with the terrestrial ecosystem such that the marine biological environment further suffers from terrestrially-sourced contamination and pollution.
Oceanographic connectivity also means that chemical pollution does not only stem from local sources, but also from abroad by means of features like the ‘Great
Conveyor Belt’ which transports contaminants from many regions including Central Europe, North America and Asia into the sensitive polar regions. Further, much of the endemic flora and fauna is already infested with invasive species. This does not just include plant species, but also marine fauna, and many diseases and carriers , e.g. rabies, avian influenza and lice are spreading northwards in the Arctic; even malaria has been widely discussed as a new global scenario. Overall, studies dealing with species diversity and biodiversity inventories are rather incomplete in the polar seas and we are still learning a great deal about these wilderness regions, such that the precautionary principle of IUCN should be applied. Immediate protection, under comprehensive governance and management regimes such as Adaptive Management, is vital and necessitates urgent data collection and construction and interpretation of open-access databases.
Photo: British Antarctic Survey
crinoid photo: David Barnes, British Antarctic Survey
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Thursday, 05 February 2009 16:54
Polar Oceans: Biological OceanWritten by Nicola Munro
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