Abstract

A baseline study on a temperate, oligotrophic North Patagonian lake (Lake Chapo, Southern Chile) was made prior to the installation of a hydroelectric power station. Throughout one year (September 1986-October 1987) the physical and chemical properties of the lake were investigated monthly from the surface to a depth of 40 m. Lake Chapo is a deep, transparent (Secchi depth: 17-25 m), glacial lake located at 41°27.5? S and 72°30? W. It has a maximum depth of 298 m, mean depth of 183 m, surface area of 45.3 km 2 and water volume of 8.296 km3. The theoretical residence time of the water was 5.5 years. The temperature regime is monomictic with the mixed temperature between 8.1-8.8°C. Maximum temperature at the surface was 18.7°C during thermal stratification in summer when the epilimnion had a thickness of about 20 m. The conductivity was low (20.3-23.8 ?S cm-1) as was the buffering capacity of a predominantly CO 2-carbonate system. The predominant cations were Ca+2> Na+>Mg+2>K+. The phosphorous and nitrogen contents were very low (soluble reactive ortophosphate: 0-1.5 ?g P l-1, total phosphorus: 0.3-4 ?g P l-1 and nitrate: 0-35 ?g N l-1), which is typical of North Patagonian lakes.