The McMurdo Region Inland Lakes

Location The McMurdo Dry Valleys (MCM) (77°30' S) of southern Victoria Land have a surface area of —4000 km2, representing the largest and most southerly ice-free area on the Antarctic continent (Figure 3). The MCM comprise three large valleys (Victoria, Wright, Taylor) along with many adjoining areas and consist of a mosaic of landscape features including glaciers, ephemeral streams, perennially ice-covered lakes, and exposed bedrock and soils. The region is ice-free because the Transantarctic Mountains block the flow of ice from the Polar Plateau and the warm katabatic winds flowing from the Polar Plateau to the sea through the east-west trending valleys lead to relatively high rates of ablation and associated ice loss. The largest lakes in this

Table 3 Characteristics of the major lakes within the Bunger Hills based on watershed type, and the geochemistry and physical properties within the lakes

Lake type Lake character Examples

Low water retention resulting from input from Low conductivity, typically isothermal Algae Lake, Lake Dalekoje glacial melt

Lakes of glacial origin with some through flow Low to moderate conductivity, melt water Lake Dolgoe, Lake Dolinnoje dominated by land sources

Lakes with a marine origin, often isolated from High conductivity, generally closed Lake Polest, Lake Vostochnoye other hydraulic input

Lakes with marine incursions (epishelf lakes) Low salinity, tidal Transkriptsii Gulf, Lake Pol'anskogo

Modified from Klokov V, Kaup E, Zierath R, and Haendel D (1990) Lakes of the Bunger Hills (East Antarctica): Chemical and ecological properties. Polish Polar Research 11: 147-159.

Epishelf Lake

Figure 2 Conceptual diagram of the mixing dynamics in an epishelf lake. (a) High inflow rates keep seawater from entering the lake basin; (b) Low inflow allows seawater to enter the lake basin producing a two layer system with freshwater overlying a marine layer. The double dashed line depicts the interface between seawater and freshwater. Modified from Gibson JAE and Anderson DT (2002) Physical structure of epishelf lakes of the southern Bunger Hills, East Antarctica. Antarctic Science 14(3): 253-261.

Figure 2 Conceptual diagram of the mixing dynamics in an epishelf lake. (a) High inflow rates keep seawater from entering the lake basin; (b) Low inflow allows seawater to enter the lake basin producing a two layer system with freshwater overlying a marine layer. The double dashed line depicts the interface between seawater and freshwater. Modified from Gibson JAE and Anderson DT (2002) Physical structure of epishelf lakes of the southern Bunger Hills, East Antarctica. Antarctic Science 14(3): 253-261.

Mackay Glacier Tongue

Mackay Glacier Tongue

.'iAfounf

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Marble point

Marble point

20 Miles

20 Kilometers

Figure 3 Locator map of the McMurdo Sound area showing the McMurdo Dry Valleys to the east (brown areas) along with the locations of the Wright and Taylor Valleys. The coastal lakes areas on Ross Island (Capes Byrd, Evans and Royds) are shown to the west. The red boxes denote regions containing lakes discussed in the manuscript.

area lie at the bottom of these valleys. The average annual temperature (—18 °C) and precipitation (<5 cm water equivalents per year) make this ecosystem the coldest and driest of all lake regions in Antarctica. As a result, all lakes in the area are permanently ice-covered, except for a single pond (Don Juan Pond) where salinity is more than 18 times that of seawater.

Formation and diversity The McMurdo Dry Valleys contain eight relatively large lakes (Vida, Vanda, Fryxell, Hoare, Bonney, Joyce, Miers, and Trough) all of which have permanent ice covers ranging in thickness from —4 to 7 m (Vida is an exception with a 18 m thick ice cover overlying saline - 5x seawater - water) and possess unique physical and geochemical attributes. Except for Lake Miers, all these lakes have closed basins with no surface outflow. Studies on these lakes began in 1957 as part of the IGY, and today, lakes in the Taylor Valley (78° S) form the centerpiece for the US National Science

Foundation's MCM Long-Term Ecological Research (LTER), which has collected an extensive set of data on three of the lakes (Fryxell, Hoare, and Bonney) and the surrounding ecosystem, since 1993.

The lakes in the present day MCM evolved as the result of changing climate conditions since the last glacial maximum. Data from the Taylor Valley have shown that over the past —20 000 years, lake levels within the MCM have varied considerably in relation to climate. Glacial Lake Washburn filled the entire Taylor Valley from the last glacial maximum to the early Holocene as a result of an ice dam formed at the base of the valley by the advancing West Antarctic Ice Sheet. As the climate became warm, the West Antarctic Ice Sheet retreated and Lake Washburn drained to McMurdo Sound, leaving behind smaller lakes in the lowest portions of the valley. Little is known about the limnology of these lakes but recent isotopic measurements show that many of the lakes in the MCM lost their ice covers and evaporated to small brine ponds or disappeared completely

^1200 years ago. A warmer climate since this period produced a flush of glacier melt that overflowed the brine ponds, producing the chemically stratified lakes we see today. The legacy of the ancient lake stands is now evident and relict resources left behind by these systems drive many of the biological processes within the MCM. For example, as the lakes rose, the soils inundated and became organic rich lake sediments, which became part of the terrestrial landscape as the lakes fell. The organic matter deposited during the period of inundation fuels much of the present day heterotrophic activity in the dry valley soils. These ancient, climate-driven lake-level changes also led to concentrated brine pools containing high levels of dissolved organic carbon, inorganic nitrogen, and inorganic phosphorus, which now form in the deep waters of many of the present lakes. The upward diffusion of these ancient nutrients has been shown to drive contemporary phytoplankton and bacterio-plankton productivity. Owing to low rates of annual primary production resulting from the long polar night and low light penetration through the thick permanent ice covers during the austral summer, annual primary production to respiration ratios in Lake Bonney (and presumably other lakes in the area) are less than unity. Hence, these ancient nutrient pools are essential to contemporary life in the lakes -without them, biology would cease.

The deep-water salts in the lakes and ponds of Wright Valley (e.g., Lake Vanda, Don Juan Pond) are comprised of CaCl2 whereas NaCl dominates the brines of lakes in the other valleys. The large differences in salinity and ionic composition of the lakes (Figure 4) are related, in part, to how the lakes have responded to temperature changes through the Holocene. Specifically, the difference in brine composition among lakes is related to the eutectic properties of NaCl and CaCl2. The permanent ice-covers, low advective stream inflow (stream flow is low and exists for 4-6 weeks each year), and strong vertical chemical gradients that result from relatively young freshwater overlying ancient brines suppress vertical mixing in these lakes to the level of molecular diffusion. As a consequence, they have not mixed completely for thousands of years. The deep saline waters also trap and store solar energy in the chemically stratified lakes, producing deep warm waters that exceed 20 °C in Lake Vanda.

Biological measurements on these lakes reveal a truncated food web with relatively few metazoans

Temperature (°C)

Temperature (°C)

Figure 4 Temperature, conductivity, and chlorophyll a profiles for Lakes Fryxell, Bonney (west and east lobes), and Vanda in the McMurdo Dry Valleys. The chlorophyll a profiles, obtained with a spectral fluorometer, depict values for four algal groups as well as total chlorophyll a.

(primarily rotifers); the lakes completely lack crusta-ceous zooplankton and fish. The vertical zonation of phytoplankton reflects the lack of vertical mixing and the presence of strong chemical gradients. Figure 4 shows the strong vertical stratification of biomass and species composition through the water column, and the relationship between the deep-chlorophyll maxima and upward diffusion of ancient nutrient pools. A statistical comparison of the phytoplankton diversity among the lakes reveals that surface groups differ from the deep-living populations, and that the phytoplanktons in the lakes of the Taylor Valley differ from those in Lake Vanda in the Wright Valley. This pattern reflects the chemical evolution of these lake ecosystems.

Unlike the temperature regime on Signy Island, air temperatures in the MCM have come down at an average rate of 0.8 °C per decade over the past two decades. This cooling trend has led to the formation of thicker ice covers on the MCM lakes and decreased light penetration to the water column (0.055mol photons per square meter per day annually in west lobe Lake Bonney). Because phytoplankton photosynthesis in the lakes is light limited, phototrophic primary production has decreased by 50% over the past 10 years in response to higher light attenuation by the thicker ice covers (Figure 5). The increasing trend in chlorophyll a after 2001 is the result of nutrient enrichment following an unusually warm year. Continued cooling in this region will clearly

Figure 5 Long-term trends in depth integrated primary productivity and chlorophyll-a in the west lobe of Lake Bonney. Black circles represent data from all dates where measurements were made; the solid red line shows the long-term trend in average values for November and December. The solid and dashed green lines denote the mean and 95% confidence intervals around the November and December trend.

Figure 5 Long-term trends in depth integrated primary productivity and chlorophyll-a in the west lobe of Lake Bonney. Black circles represent data from all dates where measurements were made; the solid red line shows the long-term trend in average values for November and December. The solid and dashed green lines denote the mean and 95% confidence intervals around the November and December trend.

produce a cascade of ecological changes within the MCM lake ecosystems as phototrophic primary production decreases even further.

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