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1 Mirror Lake is a clear-water lake in the Hubbard BrookValley of theWhite Mountains of New Hampshire.This beautiful lake has had several names, including Hobart’s Pond, McLellan’s Pond, Jobert’s Pond, Hubbard’s Pond, and Tannery Pond, since white settlers colonized the area in the middle 1700s. But on a crisp, clear morning, it is easy to see why this lake has its current name. It reflects its surroundings with the perfection of an expensive mirror (Fig.1-1).The cultural history of the lake and its drainage basin is interesting and diverse and has included small farms;family,children’s,and church camps; a dance hall; a “sugar bush”; a soda bottling operation; saw mills; and a large tannery (Likens 1972; Likens 1985c, pp. 72–83). Mirror Lake’s size and depth are as follows (Table 1-1, Fig. 1-2): Maximum effective length 610 m Average depth 5.75 m Maximum effective width 370 m Length of shoreline 2.247 km Area 15.0 ha Shore development 1.64 Maximum depth 11.0 m Volume development 1.57 Relative depth 2.5% 1 A LIMNOLOGICAL INTRODUCTION TO MIRROR LAKE gene e. likens a limnolog ical introduction to mirror lake 2 The single outlet is dammed,but at high lake stages,flow over the dam drains into Hubbard Brook, near the mouth of the Hubbard BrookValley .Three tributaries flow into the lake from a drainage basin of 103 ha (Winter 1985, pp. 40–53).These streams are simply called NE Tributary, NW Tributary, and W Tributary (Fig. 1-3). Glacial deposits cover most of the watershed and underlie most of the lake’s sediment.A knob of local bedrock (highly variable schist) is exposed along the northeastern shoreline and serves currently as the “swimming rock” for the lake.The maximum relief of the watershed is 268 m. The climate is humid continental with about 1.4 m of precipitation a year, some 30 percent of which is received as snow. Summer, although 0 100 METERS N Lake depth contour, in meters below lake surface Thermal survey location 1 EXPLANATION 9 1 0 11 8 7 6 5 4 3 2 1 Dam figure 1-2. Bathymetric map of Mirror Lake and locations of thermal surveys. 43°56.5'N, 71°41.5'W. (From Likens et al. 1985, p. 90.) Surface-water divide Trace of main tributary channel Land-surface contour—shows altitude of the land surface, in meters above sea level EXPLANATION 198 93 Mirror Lake H ubbard Brook 213.4 Outlet Dam Earthen Dam November 1969 W NW NE 457 427 396 366 335 305 2 7 4 244 229 229 216 2 1 6 213 198 400 300 200 100 0 500 METERS Mirror Lake Pemigewasset River NEW HAMPSHIRE N figure 1-3. Outline of Mirror Lake watershed, showing tributaries, subwatershed boundaries, and Interstate 93. 43°56.5'N, 71°41.5'W. The structure on the Northeast Tributary is an earthen dam built prior to the construction of I-93. (From Likens 1985c, p. 81.) a limnolog ical introduction to mirror lake 4 short, is usually hot, and winter, although long, is cold and snowy (Likens 1985a; Likens and Bormann 1995). In 1969–1971, an interstate highway (I-93) cut through the NE drainage basin of Mirror Lake, diverting the drainage water from about 18 ha of that subdrainage of the lake and thereby reducing the total surface drainage area for the lake to 85 ha (Fig. 1-3).The ecological impact of this interstate highway on the lake and its watershed has been primarily twofold: (1) reduction in water input via the NE Tributary and (2) large input of NaCl as contamination from road salt added to I-93 during the winter for snow and ice removal (Bormann and Likens 1985,pp. 436–444; Rosenberry et al. 1999; Kaushal et al. 2005). The lake formed from an ice block, a remnant of the retreating glacier. This ice block, buried in the glacial deposits, melted some 14,000 years ago, leaving a depression in the land surface, which then filled with water (Likens and Davis 1975; Davis 1985, pp. 53–65). Currently, Mirror Lake contains some 860,000 m3 of water (Table 1-1). Water in Mirror Lake is relatively clear and nutrient poor, and is therefore considered oligotrophic (50? Macrophytes 37 Pelagic bacteria >50?? Pelagic fungi >10??? Benthic bacteria >100?? Benthic fungi >10??? Pelagic zooplankton (includes Protozoa) >50? Benthic invertebrates...

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