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Microbial mats are common in polar streams and often dominate benthic biomass. Climate change may be enhancing the variability of stream flows in the Antarctic, but so far studies investigating mat responses to disturbance have been limited in this region. Mat regrowth was evaluated following disturbance by experimentally scouring rocks from an ephemeral McMurdo Dry Valley stream over two summers (2001–02 and 2012–13). Mats were sampled at the beginning and resampled at the end of the flow season. In 2012–13, mats were additionally resampled mid-season along with previously undisturbed controls. In 2001–02 rocks regained 47% of chlorophyll a and 40% of ash-free dry mass by the end of the summer, while in 2012–13 rocks regrew 18% and 27%, respectively. Mat stoichiometry differed between summers, and reflected differences in biomass and discharge. Oscillatoria spp. were greatest on scoured rocks and Phormidium spp. on undisturbed rocks. Small diatoms Humidophila and Fistulifera spp. increased throughout the summer in all mats, with the latter more abundant in scoured communities. Collectively, these data suggest that mats are variable intra-annually, responsive to hydrology and require multiple summers to regrow initial biomass once lost. These results will aid the interpretation of long-term data, as well as inform Antarctic Specially Managed Area protocols.
Liquid water is scarce across the landscape of the McMurdo Dry Valleys (MDV), Antarctica, a 3800 km2 ice-free region, and is chiefly associated with soils that are adjacent to streams and lakes (i.e. wetted margins) during the annual thaw season. However, isolated wetted soils have been observed at locations distal from water bodies. The source of water for the isolated patches of wet soil is potentially generated by a combination of infiltration from melting snowpacks, melting of pore ice at the ice table, and melting of buried segregation ice formed during winter freezing. High resolution remote sensing data gathered several times per summer in the MDV region were used to determine the spatial and temporal distribution of wet soils. The spatial consistency with which the wet soils occurred was assessed for the 2009–10 to 2011–12 summers. The remote sensing analyses reveal that cumulative area and number of wet soil patches varies among summers. The 2010–11 summer provided the most wetted soil area (10.21 km2) and 2009–10 covered the least (5.38 km2). These data suggest that wet soils are a significant component of the MDV cold desert land system and may become more prevalent as regional climate changes.
Water tracks are zones of high soil moisture that route shallow groundwater down-slope, through the active layer and above the ice table. A water track in Taylor Valley, McMurdo Dry Valleys, was analysed for surface hydrogeological, geochemical, and biological characteristics in order to test the hypothesis that water tracks provide spatial structure to Antarctic soil ecosystems by changing the physical conditions in the soil environment within the water tracks from those outside the water tracks. The presence of the water track significantly affected the distribution of biotic and abiotic ecosystem parameters: increasing soil moisture, soil salinity, and soil organic matter within the water track relative to soils outside the water track, and reducing soil phosphate, soil pH, and the population of nematodes and other invertebrates in water track soils relative to off track soils. These results suggest that water tracks are distinct and extreme ecological zones in Taylor Valley that provide long-range (kilometre to multi- kilometre) structure to Antarctic hillslope ecosystems through physical control on soil moisture and solute content. Contrary to expectations, these high soil-moisture sites are not hotspots for faunal biological activity because high soil salinity makes them suitable habitats for only the most halo-tolerant organisms.
Streams in the McMurdo Dry Valleys, Antarctica, flow during the summer melt season (4–12 weeks) when air temperatures are close to the freezing point of water. Because of the low precipitation rates, streams originate from glacial meltwater and flow to closed-basin lakes on the valley floor. Water samples have been collected from the streams in the Dry Valleys since the start of the McMurdo Dry Valleys Long-Term Ecological Research project in 1993 and these have been analysed for ions and nutrient chemistry. Controls such as landscape position, morphology of the channels, and biotic and abiotic processes are thought to influence the stream chemistry. Sea-salt derived ions tend to be higher in streams that are closer to the ocean and those streams that drain the Taylor Glacier in western Taylor Valley. Chemical weathering is an important process influencing stream chemistry throughout the Dry Valleys. Nutrient availability is dependent on landscape age and varies with distance from the coast. The streams in Taylor Valley span a wide range in composition and total dissolved solids and are surprisingly similar to a wide range of much larger temperate and tropical river systems.
The McMurdo Dry Valleys are the largest and one of the most southernly exposed terrestrial antarctic environments (Ugolini and Bockheim,2008) and have been a prominent analog environment for speculations about surface processes (Mahaney et al., 2001; Dickenson and Rosen, 2003; Marchant and Head, 2007) and potential biology (McKay, 1997; Wynn-Williams and Edwards, 2000) on Mars. The extremes in cold and aridity, the paucity of visually conspicuous life forms, and the undisturbed conditions of the McMurdo Dry Valleys make this region an obvious candidate for such comparisons. Recent discoveries of evidence demonstrating past and perhaps present availability of liquid water on the martian surface detected by the Mars Global Surveyor (Malin and Edgett, 2000; Baker, 2001) and the Spirit and Opportunity rovers (Squires et al., 2004a; Haskin et al., 2005) have extended the foundation of these comparisons beyond similarities in climate to surface geomorphology, geochemistry, and mineralogy (Chevrier et al., 2006; Marchant and Head, 2007; Amundson et al., 2008).
Water is the primary limitation to geochemical weathering and biological activity in the McMurdo Dry Valleys of Antarctica and other cold desert ecosystems where availability and movement of liquid water is limited by low temperatures (Kennedy, 1993; Convey et al., 2003; Barrett et al., 2008). This limitation of liquid water results in slow weathering and highly constrained biological activity contributing to relatively stable geochemical conditions in surface environments. Thus, in the McMurdo Dry Valleys, the legacy of paleo-aquatic environments is preserved in contemporary patterns of soil geochemistry.
The stream systems of the McMurdo Dry Valleys of Antarctica represent a relatively simple end member of terrestrial hydrologic systems. Many Dry Valley streams are prominent landscape features, especially in summer when they carry glacial meltwater from the alpine and outlet glaciers to the perennially ice-covered lakes on the valley floors (Fig. 5.1). Observations beginning in 1968 indicate that these channels carry water for 8–12 weeks each year, though some are only wetted in warm, high flow years, and others have been deactivated because of changes to flow routing. In addition to obvious channels incised in the landscape, smaller, less frequent fluvial features may become active in the Dry Valleys, such as small rivulets (shallow, broad gullies that are not wetted annually) carrying snowmelt or meltwater from buried ice down steep valley walls in particularly warm summers. Although these fluvial systems are relatively unique on Earth, the surface of Mars holds evidence of ancient fluvial features that are similar to snowmelt rivulets observed in the Dry Valleys.
In this chapter, we compare the contemporary status and function of streams of the Dry Valleys with those that may have existed on ancient Mars. Our current understanding of martian fluvial processes is limited to what can be inferred by the “leftover” drainages that are readily observed, some of which are quite large.
The McMurdo Dry Valleys (MDV) comprise a mosaic of habitats at scales ranging from micrometers to the kilometer scale. The varied landscape of the valleys, combined with strong physical and chemical gradients within and across the terrestrial and aquatic habitats, yields an ecosystem dominated by microbes that is both complex and diverse (Gordon et al.,2000; Smith et al., 2006; Mikucki and Priscu, 2007). The cold desert environment is analogous to icy conditions found on other icy worlds. For example, the low organic carbon, cold, arid soils of the MDV are similar to Mars' present-day terrestrial environment and the glaciers and ice-covered lakes of the MDV are comparable to conditions that existed on Mars in the past (Priscu et al., 1998; Wynn-Williams and Edwards, 2000; McKay et al., 2005). If there are extant or extinct life forms on Mars, they likely experience similar physical constraints and environmental challenges as do microbial communities in the MDV. Therefore, the MDV provide a unique earthly setting to gain insight into the diversity, adaptation, and function of life on other icy worlds. Here we describe the ecological processes and conditions that contribute to the microbial diversity observed in the MDV and relate these to potential life on Mars.
The McMurdo Dry Valley ecosystem
The MDV include a variety of unique habitats that are connected physically, chemically, and energetically (Fig. 8.1). Solar radiation and wind are the underlying forces that determine the existence and distribution of biota throughout the valleys (Dana et al., 1998; Nkem et al., 2006).
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