Russian-U.S. Arctic Census 2012 Expedition Purpose
Why Are Scientists Exploring Arctic Marine Ecosystems?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers to how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
The Bering Strait is a narrow body of water that separates the western-most point of Alaska from the eastern-most point of Russia, and it provides the only connection between the Pacific and Arctic Oceans. Water flowing through the Strait brings heat, nutrients, and freshwater into the Arctic. Although the Bering Strait is relatively small (about 85 km wide and 50 m deep), this flow has a strong influence on the Arctic Ocean ecosystem, and may also affect the deep ocean thermohaline circulation (the “global conveyor belt” that connects all of Earth’s oceans; see More About the Deep Ocean Thermohaline Circulation, below). Despite its importance, relatively little is known about the processes that affect the Bering Strait throughflow, or about how these processes will respond to rapid changes now being observed in the Arctic climate. [Note: A “strait” is defined as a narrow, navigable channel of water that connects two larger navigable bodies of water. The key feature of a strait is that it provides a way for ships to sail past obstacles that separate two bodies of water. Usually, the obstacles are land masses, but may also be reefs, shallow water, or other features interfering with navigation. For a map of the Bering Strait and surrounding land masses, visit this page.
To improve our understanding of Arctic ecosystems and the impacts of climate change, the Russian-American Long-term Census of the Arctic (RUSALCA) was established in 2003 as a cooperative project of the National Oceanic and Atmospheric Administration (NOAA) and the Russian Academy of Sciences (“rusalca” means “mermaid” in the Russian language). The overall purpose of this project is to provide ways to detect and measure changes in Arctic Ocean ecosystems. In 2004, the first RUSALCA expedition began investigations of ecosystems in the Bering and Chukchi Seas. A key component of these investigations was the installation of instrument packages attached to moored buoys to measure chemical and physical properties of water flowing through the Bering Strait (this was part of a multi-year measuring program begun in 1990). In addition, RUSALCA scientists studied hydrothermal systems, atmospheric conditions, and Arctic marine life including fishes, plankton, bottom communities, and food webs. Links to reports and photographs from previous RUSALCA expedition are available online.
These studies showed that nutrients (needed by marine plants for photosynthesis) are highly concentrated in the western portions of the Bering Sea, with extremely low nutrient concentrations near the coast of Alaska. Rates of photosynthesis were highest just north of the Bering Strait and in the central Chukchi Sea. Farther north in the Chukchi Sea, photosynthetic rates declined; probably due to lower nutrient concentrations.
High nutrient levels provide a foundation for ecosystems that contain large amounts of living organisms. Such ecosystems are said to have “high biological productivity.” In addition to nutrient availability, the productivity of Arctic marine ecosystems is also strongly affected by the presence of sea ice, water temperature, and current patterns. Food webs in these ecosystems tend to be less complex compared to other marine ecosystems, so that changes near the bottom of the food web (primary producers and primary consumers) can quickly affect animals near the top of the food web such as whales, seals, walruses, and sea birds. Benthic (bottom-dwelling) animals include clams, snails, polychaete worms, amphipods, echinoderms, crabs, and fishes. Filter-feeders obtain food from particulate material in the water, while deposit feeders consume organic material from sediments and the remains of other organisms that settle to the bottom. Both groups are important to the recycling of nutrients from degrading organic matter back into the water column, and changes in the distribution of these species may be an indication of changing environmental conditions.
Particulate organic matter (POM) is a major food base for marine ecosystems in the study area. High nutrient concentrations in the western Bering Sea contribute to high levels of primary production, which in turn produces an abundant supply of POM some of which is consumed by benthic organisms. Food webs in this area tend to be less complex (primary producer > POM > benthic consumer), because there is an ample supply of food. In the eastern Bering Sea, nutrient concentrations are lower and there is less primary production and less POM. In this area, most of the POM has already been processed by pelagic organisms by the time it reaches the bottom, so food webs are more complex (primary producer > POM > pelagic consumer > pelagic consumer > benthic consumer). So, changes food web structure could be used to detect changes in nutrient content and other characteristics of the surrounding water.
The 2004 RUSALCA expedition identified two groups of fishes that may also be useful as indicators of climate change: species that are native to the Chukchi Sea; and species that originate in the North Pacific or Bering Sea that are rarely found in the Chukchi Sea (native species are called “autochthonous” species, while those that originate elsewhere are called “allochthonous” species). To be useful as indicators of climate change, autochthonous species need to be fairly abundant (so that enough individuals can be captured to provide a statistically valid sample) as well as relatively non-mobile (so that changes in the distribution of the species can be easily detected). The Arctic Staghorn Sculpin Gymnocanthus tricuspis meets both requirements since it was the most abundant species found in samples collected by the RUSALCA 2004 expedition, and spends much of its time burrowed in the mud of the Chukchi Sea and is not particularly mobile (the Staghorn Sculpin together with the Shorthorn Sculpin, Bering Flounder, and Arctic Cod accounted for 80 percent of fishes captured). So if the range of the Arctic Staghorn Sculpin were to change, this would suggest a possible change in climatic conditions. Similarly, an increase in the numbers of allochthonous species in the Chukchi Sea could also suggest that waters in the Chukchi Sea had become warmer, making them more suitable for species that originate in more southern areas.
In addition to providing baseline information about fish distribution for comparison with future studies, the 2004 RUSALCA expedition identified physical characteristics that affect fish species composition and distribution. This is important to identifying ecosystem change, because physical characteristics can be measured and analyzed more quickly (for example, from moored buoys) than fish can be collected and identified. Understanding the relationship between fishes and physical characteristics of water masses provides important background for using information about physical characteristics to make inferences about ecosystems in the northern Bering and Chukchi Seas.
Major scientific questions guiding the Russian – U.S. Arctic Census 2012
- What are major physical and chemical properties of water flowing through the Bering Strait?
- What is the velocity of water flowing through the Bering Strait, and how does this velocity vary from month-to-month and year-to-year?
- What organisms are native to this area, and what species are migrating into the area from elsewhere?
- What are the optimum methods for monitoring environmental conditions and change in other Arctic regions?
The key technology component of the RUSALCA 2012 Expedition is moored instruments that measure physical and chemical properties of water flowing through the Bering Strait. Making these measurements is particularly challenging, because the Bering Strait region is covered by ice during the winter, and floating sea-ice can damage equipment. RUSALCA moorings are designed to survive encounters with sea ice by allowing the instruments to be knocked down by sea-ice or very strong currents, and then bounce back when the ice or currents are gone.
A typical mooring includes a large (about 1 m diameter) float attached to a length of chain that is anchored in place by a heavy weight (such as railroad wheels). The chain is connected to the weight by an “acoustic release” that will disconnect the chain from the weight when scientists transmit a coded sound signal from the research vessel. Instruments are attached at certain points along the chain to collect data from specific depths. Instruments used in the RUSALCA expeditions include current meters, pressure gauges (to measure variations in water depth), temperature and salinity sensors, nutrient analyzers, transmissometers (to measure water turbidity), fluorometers (to measure chlorophyll concentrations that provide a measure of photosynthesis), and CTDs (see below). For more information about RUSALCA moorings, visit http://psc.apl.washington.edu/HLD/Bstrait/bstrait.html#Basics.
A CTD is used to collect data on seawater conductivity, temperature, and depth. These data can be used to determine salinity of the seawater, which is a key indicator of different water masses. A CTD may be carried on a submersible, or attached to a water-sampling array known as a rosette that can be deployed from a research ship, or attached to a moored buoy. For more information on CTDs and rosettes visit http://oceanexplorer.noaa.gov/technology/
More About Studying Food Webs Using Isotope Ratios
RUSALCA scientists study food webs in Arctic ecosystems using a tool called “stable isotope analysis.” Isotopes are forms of an element that have different numbers of neutrons. For example, carbon-13 (13C) contains one more neutron than carbon-12 (12C). Both forms occur naturally, but carbon-12 is more common. When an animal eats food that contains both carbon isotopes, carbon-12 is selectively metabolized, so the ratio of carbon-12 to carbon-13 in the tissues of the animal is higher than the ratio of these isotopes in the food they consumed. In other words, carbon-13 is “enriched” in the animal’s tissues. If this animal is eaten by another consumer, the enrichment process will be repeated. So the ratio of carbon-13 to carbon-12 increases with each increase in trophic level (i.e., “each step up the food chain). For additional discussion of stable isotope analysis, see “Who Is Eating Whom?” (http://oceanexplorer.noaa.gov/explorations/
More About the Deep Ocean Thermohaline Circulation
The deep ocean thermohaline circulation is driven by changes in seawater density. Two factors affect the density of seawater: temperature (the ‘thermo’ part) and salinity (the ‘haline’ part). Major features of the THC include:
- In the Northeastern Atlantic Ocean, atmospheric cooling increases the density of surface waters. Decreased salinity due to freshwater influx partially offsets this increase (since reduced salinity lowers the density of seawater), but temperature has a greater effect, so there is a net increase in seawater density. The formation of sea ice may also play a role as freezing removes water but leaves salt behind causing the density of the unfrozen seawater to increase. The primary locations of dense water formation in the North Atlantic are the Greenland-Iceland-Nordic Seas and the Labrador Sea.
- The dense water sinks into the Atlantic to depths of 1000 m and below, and flows south along the east coasts of North and South America.
- As the dense water sinks, it is replaced by warm water flowing north in the Gulf Stream and its extension, the North Atlantic Drift (note that the Gulf Stream is primarily a wind driven current and is part of a subtropical gyre that is separate from the THC).
- The deep south-flowing current combines with cold, dense waters formed near Antarctica and flows clockwise in the Deep Circumpolar Current. Some of the mass deflects to the north to enter the Indian and Pacific Oceans.
- Some of the cold water mass is warmed as it approaches the equator, causing density to decrease. Upwelling of deep waters is difficult to observe, and is believed to occur in many places, particularly in the Southern Ocean in the region of the Antarctic Circumpolar Current.
- In the Indian Ocean, the water mass gradually warms and turns in a clockwise direction until it forms a west-moving surface current that moves around the southern tip of Africa into the South Atlantic Ocean.
- In the Pacific, the deepwater mass flows to the north on the western side of the Pacific Basin. Some of the mass mixes with warmer water, warms, and dissipates in the North Pacific. The remainder of the mass continues a deep, clockwise circulation. A warm, shallow current also exists in the Pacific, which moves south and west, through the Indonesian archipelago, across the Indian Ocean, around the southern tip of Africa, and into the South Atlantic.
- Evaporation increases the salinity of the current, which flows toward the northwest, joins the Gulf Stream, and flows toward the Arctic regions where it replenishes dense sinking water to begin the cycle again.
The processes outlined above are greatly simplified. In reality, the deep ocean THC is much more complex, and is not fully understood. Our understanding of the connections between the deep ocean THC and Earth’s ecosystems is similarly incomplete, but most scientists agree that:
- The THC affects almost all of the world’s ocean (and for this reason, it is often called the ‘global conveyor belt’);
- The THC plays an important role in transporting dissolved oxygen and nutrients from surface waters to biological communities in deep water; and
- The THC is at least partially responsible for the fact that countries in northwestern Europe (Britain and Scandinavia) are about 9ºC warmer than other locations at similar latitudes.
In recent years, changes in the Arctic climate have led to growing concerns about the possible effects of these changes on the deep ocean THC. In the past 30 years, parts of Alaska and Eurasia have warmed by about six degrees Celsius. In the last 20 years, the extent of Arctic sea ice has decreased by 5 percent, and in some areas, sea ice thickness has decreased by 40 percent. Overall, the Arctic climate is warming more rapidly than elsewhere on Earth. Reasons for this include:
- When snow and ice are present, as much as 80% of solar energy that reaches Earth’s surface is reflected back into space. As snow and ice melt, surface reflectivity (called ‘albedo’) is reduced, so more solar energy is absorbed by Earth’s surface;
- Less heat is required to warm the atmosphere over the Arctic because the Arctic atmosphere is thinner than elsewhere;
- With less sea ice, the heat absorbed by the ocean in summer is more easily transferred to the atmosphere in winter.
Dense water sinking in the North Atlantic Ocean is one of the principal forces that drives the circulation of the global conveyor belt. Since an increase in freshwater inflow (such as from melting ice) or warmer temperatures in these areas would weaken the processes that cause seawater density to increase, these changes could also weaken the global conveyor belt.
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Director, Education Programs
NOAA Office of Ocean Exploration and Research
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