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LOCATION DESCRIPTION Mirror Lake is 15 ha in area, and an oligotrophic (low productivity), clearwater (Secchi transparency 6 to 7 m), natural water body adjacent to HBEF. The lake normally mixes in spring and fall, and is ice-covered from about 1 December to about 15 April each year. Numerous studies have been conducted on Mirror Lake since the mid-1960s, including extensive physical, chemical, biological, and paleoecological research which are described in detail in the book: An Ecosystem Approach to Aquatic Ecology (Likens, 1985).

SAMPLING DESIGN Water samples are collected in a boat from an anchored buoy station, or from the ice surface located at the deepest part (11 m at full pool) of Mirror Lake. Samples for ion analysis are collected in acid-washed, deionized water-rinsed, polyethylene bottles, using a manual closure, plastic VanDorn collector bottle (before 1990), triggered at discrete depths, or lifted to the surface through weighted vinyl tubing by peristaltic pump (since 1990). DO and DIC samples are collected in standard glass BOD bottles. Typically, water samples are taken from the surface (<0.5m), and at depths of 2, 4, 6, 8, and 10 meters. The usual sampling interval is 4 to 6 times each year, especially at times of maximum and minimum thermal stratfication. Temperatures are determined by thermistor probe (Whitney, YSI, or Digisense Models).

DATA DESCRIPTION Concentrations (mg/L) in lake water samples from discrete depths. Base cation, pH, and dissolved oxygen data are available since 1967. Ammonium, anions, phosphate, and dissolved silica have been measured routinely since 1970, although some data are available prior to these dates for each solute. Other standard data include temperature and specific conductance at each depth. Prior to 1990, not all records have complete solute arrays. since 1990, dissolved in-organic carbon (DIC) and acid neutralizing capacity (ANC; alkalinity) have also been measured on a routine basis, although prior data do exist.