McMurdo Dry Valleys

Published:
Oct 19, 1993
Issue: 
Network News Fall 1993, Vol. 14 No. 1
Section:
Site News
By: 
Robert A. Wharton

A Cold Desert Ecosystem

A Long-Term Ecological Research site for the Taylor Valley in the McMurdo Dry Valleys, approximately 100 km west of McMurdo Station, Antarctica, has been funded through the Office of Polar Programs at the National Science Foundation. McMurdo Dry Valleys LTER will launch its first field campaign October 1993 through February 1994.

 The McMurdo Dry Valleys are the largest ice-free area (-4800 km2) on the Antarctic continent and are located on the western coast of the Ross Sea (77°00’S, 162°52’E). They are among the most extreme deserts in the world, far colder and drier than any of the established LTER sites. The perennially ice-covered lakes, ephemeral streams, and extensive areas of soil within the valleys are subject to low temperatures, limited precipitation and salt accumulation.

The McMurdo Dry Valleys were formed by the advances and retreats of glaciers through the coastal ranges of the Transantarctic Mountains, which rise several thousand meters above sea level and act as barriers to the flow of ice from the Polar Plateau. Glacial and periglacial features are a major component of the landscape. Taylor Valley is approximately 33 km long by 12 km wide and contains three major lakes (Bonney, Hoate and Fryxell) fed by 15 glaciers. The valley bottoms are predominantly glacial till and the higher slopes consist of granites, dolerites, sandstones and occasional volcanics. Rock outcrops are typically highly-weathered by the föhn winds. Outlet, piedmont and alpine glaciers chain from the Polar Plateau and cirques onto the valley floors. The dry valleys have been predominantly ice-free for the past 4 million years. The lakes in the Taylor Valley are the remnants of a much larger lake, Glacial Lake Washburn, that existed 10,000 to 24,000 years ago.

The climate of the Taylor Valley is extreme. Annual precipitation is received as snow and averages < 10 cm yr-1. The low precipitation, low surface albedo, and dry föhn winds descending from the Polar Plateau result in extremely arid conditions. Mean annual temperature is ~ -20°C, average wind speeds are 5.0 m sec-1, mean relative humidities are < 50%, and solar flux (available energy) is < 100W m-2. This region of Antarctica has four months each of continuous sunlight followed by twilight and darkness.

McMurdo Dry Valleys landscapes are a mosaic of ice-covered lakes, ephemeral streams, arid soils, permafrost and surrounding glaciers. Materials are transported between sires by wind and water defining functional relationships between these landscape units. Water flows primarily from glaciers to streams to lakes (except in cases where glaciers are in direct contact with the lakes, e.g., Taylor Glacier/Lake Bonney and Canada Glacier/Lake Hoare), while wind disperses particulate matter throughout the valleys. The transport of these materials appears to enhance the overall productivity of the area. Were it not for the melting of surrounding glaciers, there would be no streams or lakes, the lakes would eventually freeze and sublimate. Furthermore, glacial meltwater and leaching from soils adjacent to streams provide solutes to both streams and lakes, enhancing production. Winds deposit sediments on and remove organic mats from the surface of lake ice. The productivity of soils appears to exceed site-specific photosynthetic capacity and may be due to allochthonous inputs of wind-borne organic carbon.

Transported materials can be divided into two categories: inorganic (solutes, water) and organic (bound carbon and nutrients). Transport media are abiotic (primarily wind and water) and the direction of transport is determined by topography and wind direction. Lakes receive materials from streams, glaciers and soils, through the actions of wind and water.

Groundwater inflow may also be important to the solute budgets of the lakes. Streams receive the majority of water from glacier melt and both sediments and solutes from glacial water and soils. Particulate organic matter input to streams is probably small compared to stream productivity.

Soils receive water from snow Indian Ocean melt, with spatial abundance determined by drifting patterns (wind-dependent) and, possibly, subsurface lateral flow. Some mineral inputs may be of significance to soils and organic materials appear to be provided by streams and lakes (including the erosion of buried glacial Lake Washburn sediments).

The biological systems in the McMurdo Dry Valleys are relatively simple. For example, there are no vascular plants or vertebrates and very few insects. Trophic interactions and biogeochemical nutrient cycles are largely limited to microbial populations and microinvertebrates. Species diversity and abundance are low as would be predicted for such extreme environments. Despite this simplicity, complex interactions among species and between the biological and physico-chemical environment exist in the lakes, streams and soils. Furthermore, interactions between various components of the ecosystems enhance the overall productivity of the dry valley landscapes.

Physical factors largely control biological processes in the dry valleys. Stream ecosystems are controlled by the quantity and timing of glacial meltwater during the austral summer (October through February). Discharge from these streams and thickness of perennial ice cover are the primary factors regulating the lake environment. Soil communities are controlled by moisture availability, salt concentrations, and allochthonous carbon inputs. The most productive sires (aquatic) are those buffered from the most severe, short-term climatic fluctuations. Aeolian transport of organic carbon from these sites may form the base of the food chain on the most exposed sites (terrestrial soils). In turn, nutrients are carried by water and wind from glaciers and soils to the streams and lakes.

Hypotheses

All ecosystems are shaped to varying degrees by climate and material transport, but nowhere is this more apparent than in the McMurdo Dry Valleys. The LTER project focuses on two hypotheses that embody this theme.

• Hypothesis 1. The structure and function of the Taylor Valley ecosystems are differentially constrained by physical and biological factors.

The Grime’s Hypothesis predicts the relative importance of environmental and biotic interactions in structuring plant communities and is highly relevant to dry valley ecosystems. Grimes defined disturbance as any event that reduces standing biomass, and stress as reducing productivity. For example, freeze-thawing and flooding might remove organisms and therefore constitute a disturbance. In contrast, consistently low temperature, nutrient availability or light levels may simply reduce productivity below full potential, representing environmental stress. The importance of biological interactions would be greatest in systems of minimal stress and disturbance. Competitive interactions between producers and overall primary and secondary productivity would be greatest under these conditions. In the dry valleys, major differences exist both in the relative importance of abiotic and biotic factors in structuring ecosystems and the sensitivities of these systems to environmental changes.

• Hypothesis 2. The structure and function of the Taylor Valley ecosystems are modified by material transport.

 Biological activities of dry valley ecosystems are higher than expected, given the extreme climatic conditions. This may result from the transport of materials between ecosystems. Glacial melt is the primary source of water to aquatic systems, carlying dissolved solutes and sediments from the glaciers and soils. In addition, high winds (up to 160 km hr-1) disperse sediments and organic materials throughout the valleys that variously affect recipient ecosystems. Our efforts will focus on the integration of the biological processes within, and material transport between, the lakes, streams and terrestrial ecosystems comprising the Taylor Valley landscape.

All ecosystems are shaped to varying degrees by climate and material transport, but nowhere is this more apparent than in the McMurdo Dry Valleys

The McMurdo LTER project will address these hypotheses through systematic environmental data collection, long- term experiments, and model development. During the first six years, studies will be concentrated within the Taylor Valley, site of several non-integrated ecological studies since the 1 960s and presently a focal point for terrestrial and limnetic research by the United States and New Zealand. Generally representative of all Antarctic ice- free areas, Taylor Valley is 40 minutes from McMurdo Station by helicopter (weather permitting), and contains three relatively large lakes, numerous streams and a variety of terrestrial ecosystems.

Senior personnel include:

  • Robert A. Wharton, Jr. (principal investigator, limnology)
  • Andrew Fountain (glacial hydrology)
  • Diana W. Freckman (soil ecology)
  • Jordan T. Hastings (data and information management)
  • W. Berry Lyons (geochemistry)
  • Diane M. McKnight (hydrology)
  • Daryl L. Moorhead (systems ecology)
  • John C. Priscu (microbial ecology)
  • Cathy M. Tate (stream ecology)

For more information:

Robert A. Wharton, Jr Biological Sciences center, Desert Research Institute, P. O. Box 60220, Reno, Nevada 89506 702-673-7492, rWharton@LTERnet.edu.