The aim of this investigation was to examine the recent history of Douglas Lake, with a focus on determining past levels of hypolimnetic oxygen. To this end, a paleolimnological study of Douglas Lake sediments was undertaken, using the stratigraphy of several chemical signals and the assemblages of larval Chironomidae (Diptera) as indicators of oxygen levels. Through the study of lake sediments that were deposited over time, paleolimnologists strive to reconstruct the history of a lake and its watershed. Analysis of biological remains, as well as physical and chemical properties of the sediments has led to insights concerning postglacial climate change, lake ontogeny, the process of eutrophication, and the impacts of pollution, acid deposition, and fluctuating lake levels. Paleolimnology can be a particularly useful tool when no historical data base exists and can provide models for understanding current conditions and future changes. The process of cultural eutrophication has become a central issue in aquatic research. Increases in nutrient input to surface waters and the resultant increases in primary productivity lead to changes in lake chemistry and biota, including shifts in species composition to organisms often considered undesirable by humans. In stratified lakes of the temperate zone, organic matter that reaches the hypolimnion is oxidized by microorganisms. Oxygen supplies to the hypolimnion are not renewed during stratification and oxygen depletion can result. Paleolimnological techniques have been used to study the process of eutrophication and increasing producitivity, but few attempts have been made to document the occurrence of oxygen deficits. Redox potential at the sediment-water interface affects the solubility of metals such as Mn and Fe, and the preservation of algal pigments. These chemical signals and the use of fossil remains of the Dipteran family Chironomidae were the main tools used to document oxygen levels in Douglas Lake during the time span covering European settlement up to the present. Larval Chironomidae have been used successfully as indicators of water quality, organic pollution, and eutrophication of surface waters. Their fossil remains in lake sediments can serve the same purpose.