The McMurdo Dry Valleys (MCM) represent the largest of the ice-free oases on the Antarctic continent (ca. 4800 km2). They are among the coldest and driest terrestrial environments on earth with an average annual temperature of ca. -20° C and total annual precipitation of 6 cm. received as snow during the winter. The dry valley landscape is a mosaic of perennially ice-covered lakes, ephemeral streams, soils and glaciers. Despite the extreme conditions, biological communities exist in these lakes, streams and soils.
Much of what is known about such extreme polar deserts has been discovered by work conducted in Antarctica because few comparable systems exist elsewhere. Much of our knowledge about the structure and function of these polar desert ecosystems has been discovered only recently resulting, in large part, from the first phase of the McMurdo Long-Term Ecological Research program (MCM-I 1993-99). In MCM-I, our research explored the physical constraints controlling the structure and function of this polar desert. We discovered that subtle changes in temperature, precipitation, and albedo have profound effects on the hydrologic cycle, biogeochemistry, productivity and biodiversity within the valleys. Moreover, local effects are modified by landscape position and topography.
The MCM ecosystem is sensitive to very small variations in climate because the change between solid and liquid phase of water is delicately poised in this environment. Thus, small changes in temperature and radiant energy regimes are amplified by large, non-linear changes in hydrologic budgets that can ramify throughout the system. The presence of liquid water remains the primary limiting condition for life in Antarctica, so the relationship of energy balance to liquid water availability, ecological function and biological diversity will continue to be a major emphasis of the McMurdo Dry Valley LTER renewal (MCM-II).
A second focus of MCM-I was generated by observations that biological activities in the dry valleys were affected by the transport of water, nutrients and organic carbon between landscape units (glaciers, streams, lakes and soil). Such linkages have been shown to considerably influence ecosystem structure and function, but are most evident in extreme environments, such as deserts and tundra. Perhaps the best example of a critical linkage among landscape units is represented by the glacier-stream-lake continuum, in which water and solutes move from glaciers to lakes by streamflow. This movement of water defines the streams in which biological activity is restricted to periods of liquid water availability. Water in streams stimulates weathering of minerals in streambeds and transformation of nutrients by stream communities. Thus, water reaching the lakes has a very different chemical signature than glacier melt water.
Within the context of an extreme sensitivity to variations in climate and significant linkages among landscape units, it has become clear that "legacies" of past events influence the structure and function of the modern ecosystem. Geomorphic evidence indicates that the MCM environment has been remarkably stable over the past few million years, with extremely slow rates of landscape modification in comparison to temperate regions. In addition, nutrient cycling has been slow because biological activity is limited by the harsh polar environment. As a consequence, legacies of the past, such as pools of soil organic matter deposited by paleo-lakes, and nutrients in the hypolimnion of modern lakes, directly impact present ecosystem function.
The term "legacy" refers to the carry-over or "memory" of the ecosystem of past events. A climatic legacy strongly imprints current ecosystem functions at MCM, including inundation of the entire Taylor Valley by Lake Washburn during the late Pleistocene (24,000-6,000 yr BP), and a subsequent cold, dry period ending about 1000 years BP. Legacy effects at other LTER sites usually are viewed as much shorter-term consequences of historic land-use patterns, such as forest clearing (HBR,HFR,CWT,AND), grazing and resultant desertification (JRN,SEV,SGS), and conversion to agriculture (KBS), or resulting from severe weather (e.g. hurricanes, LUQ) and fires (KNZ,AND).
CENTRAL HYPOTHESIS: Past climates in polar desert environments strongly overprint present ecosystem structure and function.
Results of MCM-I research suggest that "memory" exists in all landscape units in the dry valleys. A hydrological memory exists in lake and glacier volumes, a chemical memory resides in lakes, a geomorphic memory controls stream processes and an organic carbon memory is evident in soils. In addition, biological memory is manifest in the distribution of present-day organisms across the landscape (spatial pattern of biodiversity). Our working hypotheses are derived from three dominant legacies that are now apparent in the valley ecosystem (lake, stream, and glacier characteristics; pools of organic and inorganic compounds in lakes and soils; distribution and diversity of biota). The impacts of these legacies on modern ecosystem processes are determined partly by the linkages between the soils, streams, lakes, and glaciers, cast within an overall context of the modern climate.
BIOLOGICAL ACTIVITY AND DIVERSITY HYPOTHESIS: The distribution, abundance, community structure and activity of lake biota are controlled by the water balance and the availabilities of radiant energy, ancient and modern nutrient and dissolved organic carbon.
The distribution and activity of biota in the dry valleys are determined largely by the availability of water. Results of MCM-I indicated that other factors also constrain biological activities (e.g. temperature, nutrients, energy, etc.), but that linkages among landscape units can ameliorate some of these limitations. Legacies of nutrients and organic carbon that have accumulated through past biological and geochemical processes within ancient hydrological regimes may also drive modern communities. Thus, current patterns of biological activity and diversity partly reflect past distributions of water, nutrients, organic carbon and biota.
Biological activity in streams primarily is limited to microbial mats that are found on stable substrates, which are active when liquid water is present and nutrient availability is adequate. Primary production in lakes is limited by available light and nutrients, but sources of nutrients differ with respect to location in the water column. Surface waters receive nutrients from streams flowing into lakes, whereas phytoplankton in deeper water rely on the upward diffusion of ancient nutrient pools from deep water sediments. Also, the upward diffusion of ancient DOC from deep water and sediments apparently results in the current, negative values of net primary production observed in phytoplankton communities. The activities of soil communities are limited by a number of factors, including temperature and availabilities of water and organic carbon. Sources of organic carbon include in situ production, modern transport from remote sources, and ancient carbon residing in soil profiles.
BIOGEOCHEMICAL PROCESSES HYPOTHESIS: The chemistry of lake water is dictated by the (1) timing, duration, amount and chemistry of stream water, (2) biogeochemical processes occurring within the lake, and (3) retention of past inputs and products of biogeochemical processes by the lack of hydrologic flow through stable stratification and restricted interaction with the atmosphere.
Biogeochemical processes responsible for the immobilization and mineralization of nutrients, and geochemical weathering, require a medium of liquid water. Hence, the timing, duration and location of biogeochemical processes in the past and the present are controlled by water availability. Also, hydrologic linkages among landscape units provide both a medium for these processes and a conduit for transport of chemical moieties. It is for these reasons that legacies of carbon, nutrients and other chemicals are present throughout the modern dry valley landscape.