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Importance of Landscape Position and Legacy: the Evolution of the Lakes in Taylor Valley, Antarctica

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Importance of Landscape Position and Legacy: the Evolution of the Lakes in Taylor Valley, Antarctica Freshwater Biology (2000) 43, 355±367 Importance of landscape position and legacy: the evolution of the lakes in Taylor Valley, Antarctica W. BERRY LYONS*, ANDREW FOUNTAINy, PETER DORANz,JOHNC.PRISCUx, KLAUS NEUMANN* AND KATHLEEN A. WELCH* *Department of Geology, University of Alabama, Tuscaloosa, AL 35487-0338, U.S.A. yDepartment of Geology, Portland State University, Portland, OR 97207-0751, U.S.A. zBiological Sciences Center, Desert Research Institute, Reno, NV 89506-0220, U.S.A. xDepartment of Biology, Montana State University, Bozeman, MT 59717, U.S.A. SUMMARY 1. The major factor influencing the chemical composition and evolution of the major lakes in Taylor Valley, Antarctica, is their location within the landscape. Present-day microclimatic variation and its manifestation over the past 6000 years have led to the differences observed in these lakes today. 2. Geographical and topographical variables within the Taylor Valley magnify subtle changes in the hydrological balances of these lakes. Even short-term variation of the surface temperatures and albedo greatly impact the run-off into the lakes, leading to positive or negative water balances. 3. The legacy of past climatic changes has had a profound effect on the ecology of the lakes today. Keywords: Antarctica, chemical composition, evolution, lakes, landscape Introduction ence the variability observed in the lakes (Fig. 2). These variations are great and the three major lakes in The limnology of the perennially ice-covered lakes in Taylor Valley are different in many aspects. The most Taylor Valley of Southern Victoria Land, Antarctica obvious difference is in their chemistry. Lake Hoare is (Fig. 1), has traditionally been evaluated lake-by-lake the freshest of the lakes throughout the water column, with little focus on comparative relationships. With Lake Fryxell has a brackish monimolimnion and the the arrival of the McMurdo Dry Valleys (MCM) Long- monimolimnia of both lobes of Lake Bonney are Term Ecological Research (LTER) site, a more syn- hypersaline (Fig. 3). In addition, the major ionic ratios thetic approach to studying the aquatic ecosystems in of the Taylor Valley lakes, even in their fresh surface these unusual environments has developed (Priscu, waters, are quite different (Lyons et al., 1998a). The 1998). For example, because the major source of water lakes are different in their nutrient characteristics to the lakes in Taylor Valley is from seasonal glacier (Priscu, 1995), as well as their chlorophyll and algal melt, a detailed knowledge of climatic variation and distributions (Lizotte & Priscu, 1998). Primary pro- glacier dynamics was needed to establish a quantita- duction rates are also different in the lakes (see tive understanding of the hydrological balances of below). These lakes are a portion of a suite of various these lakes. types of lakes which exist throughout the McMurdo Over the past 6 years of LTER investigations in region. Since the initiation of the study of these lakes Taylor Valley, it has become clear that landscape (Angino, Armitage & Tash, 1962; Angino & Armitage, characteristics, both past and present, greatly influ- 1963), the reason for these differences has remained unclear. Their proximity and similar climate and basin geology have left Antarctic limnologists at a loss in Correspondence: W. Berry Lyons, Department of Geology, University of Alabama, Tuscaloosa, AL 35487±0338, U.S.A. explaining their extraordinary chemical differences. E-mail: [email protected] We believe that the current climatic conditions and ã 2000 Blackwell Science Ltd. 355 356 W. B. Lyons et al. Fig. 1 Location map of Taylor Valley, southern Victoria Land, Antarctica. their past response to climate has been similar at each be explained completely, the degree to which the location. individual lakes interact with their surrounding envir- Clearly, the position of these three individual lakes onment, especially the small variations in climate within the landscape of Taylor Valley has led to some within the Taylor Valley, are a major key to under- of the major differences observed in these lakes today. standing their present and past development. Although not all of the lake variability observed can Although the emphasis of this paper will be on the physical and chemical variations in the lakes within Taylor Valley, there is little doubt that subtle climate differences within the landscape also may account for the biological variations among the lakes. In this paper, we will argue that subtle differences in climate, as dictated by location within Taylor Valley, have a profound effect on the geochemistry and biogeochem- istry of these lakes. In addition, subtle climatic effects of the past also greatly direct present-day ecological processes within the lakes. Taylor Valley, part of the McMurdo Dry Valleys system, is a polar desert with a mean annual Fig. 2 East±west profile of Taylor Valley showing locations of the temperature of approximately ±20 °C (Clow et al., lakes (from Spaulding et al., 1997). 1988) and a total annual precipitation of £ 10 cm ã 2000 Blackwell Science Ltd, Freshwater Biology, 43, 355±367 Landscape position and legacy 357 and the waxing and waning of lacustrine environ- ments over the past few million years (Porter & Beget, 1981). Three types of glacier advances are documented: 1 advances of ice from the east as a result of the growth of the West Antarctic Ice Sheet; 2 advances of ice from the west which are thought to be related to thickening of the East Antarctic Ice Sheet (i.e. advance of the Taylor Glacier); and 3 advances of the alpine glaciers within the valley. Initial research suggested that the Taylor Glacier and the various alpine glaciers advanced during warmer, interglacial times, while the Ross Ice Shelf advances (from the east) were associated with cooler glacial periods (Hendy et al., 1977). More recent work suggests a more complicated pattern of events which are difficult to correlate to known glacial events (Campbell & Claridge, 1987). Because of these glacier movements, the valley floor contains a mosaic of tills of differing age and composition (PeÂweÂ, 1960; Stuvier et al., 1981; Burkins et al., 1999). The ages of the morainal materials in the region date to 2.5 million years (Brown et al., 1991). The eastern portion of Taylor Valley has been in direct contact with the ocean through time with fjord- Fig. 3 Chloride versus depth profiles of the Taylor Valley Lakes. like conditions until the early Pliocene (Porter & (Keys, 1980). Therefore, the MCM is the coldest and Beget, 1981). The entire valley has been modified by driest of all the current LTER sites, and is among the lacustrine sedimentation with a maximum lake high- coldest and driest terrestrial environments on earth. stand called Glacial Lake Washburn between » 11 000 Taylor Valley is a mosaic of ice-covered lakes, and 24 000 years ago (Denton et al., 1989). The lake ephemeral streams, soils and surrounding glaciers probably existed until » 6000 years ago and it was the (Fig. 1). Water flows from the glaciers to the three precursor to the present-day lakes in the Taylor Valley major lakes in the valley intermittently with flow (Doran, Wharton & Lyons, 1994). Lake levels reached generally beginning in late November to mid-Decem- their Holocene lows » 1000 years ago (Matsubaya ber and ending in mid-January to late February et al., 1979; Lyons et al., 1998b). The waxing and (Conovitz et al., 1998). Flow is highly variable, both waning of these lacustrine environments have had a daily and seasonally (Conovitz et al., 1998). Water is profound effect on the Taylor Valley ecosystem, and lost from the lakes through sublimation of the ice- the imprints of these changes are just beginning to be cover. During the past 20 years, there has been a net understood as important legacies to the present-day gain in water to the lakes as lake levels have been ecology (Priscu, 1995; Burkins et al., 1999). generally rising (Chinn, 1993). The presence of liquid water remains the primary Methods limiting condition for life in Antarctica (Kennedy, 1993), and therefore, the relationship of energy Detailed outlines of the analytical techniques utilized balance to liquid water availability, ecological func- in this work are presented elsewhere (Priscu, 1995; tion and biological diversity has been a major interest Welch et al., 1996; Lizotte & Priscu, 1998; Lyons et al., in MCM-LTER research. 1998a) and will not be repeated here. We urge the The valley's geomorphology has been modified by reader to see our web site for details (http://huey.color- the movement of glaciers, the inflow of ocean waters, ado.edu). ã 2000 Blackwell Science Ltd, Freshwater Biology, 43, 355±367 358 W. B. Lyons et al. Table 1 Characteristics of the Taylor Valley Lake District Characteristic Lake Fryxell Lake Hoare Lake Bonney Surface area (m2) 7.1 ´ 106 1.9 ´ 106 4.8 ´ 106 Volume (´ 106 m3) 25.2 17.5 64.8 Maximum depth (m) 21 34 40 Depth of oxycline (m) 9.5 28 20 Water temperature (°C) 0 1.0 0 4.0 ±2.0 7.0 Conductivity (mmho cm±1) 500±8600 400±800 500±156 000 Results The geochemistries of the monimolimnia of the three lakes are very different from each other Taylor Valley is 33 km long and contains three major (Table 2), also implying different developmental lakes: Lake Bonney, Lake Hoare and Lake Fryxell. histories and/or different sources of solutes. There Lake Fryxell is closest to the ocean, » 8 km from the are even subtle differences in the surface waters coast (Fig. 1). It is also the shallowest lake (Table 1). among the lakes (Table 2). The enrichment of Ca2+, Lake Hoare is » 15 km west of the ocean and it is relative to the other major cations in the bottom of the `held' in place by the Canada Glacier (Fig.
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