Lutvann - an unusual Oslo lake : a study of its limnology 2005 - 2006 and of its recent historic development

Abstract

Lutvann lake, a headwater in a forested suburban area of Oslo, Norway, was sampled between February 2005 and June 2006. Phytoplankton, ciliate and zooplankton populations were assessed and the basic chemical parameters of the water were analysed. Results were compared with those of former studies of the lake. Human habitation in the catchment area is almost absent. Surface-water inlet streams to the lake are insignificant and Lutvann is strongly dependent on supplies of ground water. The stratification regime of the lake was found to be Hutchinson´s type 1. The maximum Secchi depth was 17.5 m and the water colour was classified as blueish green. The vertical distribution of oxygen was more or less clinograde throughout the sampling period. Generally, oxygen values have decreased slightly since the 1940s, probably because of an increase in the supply of organic matter to lakes in the area that has occurred during the past decade. Alcalinity, pH, specific conductivity and ion concentrations were generally even throughout both the water column and the sampling period. Nitrogen and phosphorous concentrations were low. The calcium concentration was among the highest recorded in any of the lakes overlying precambrian rock in Oslo and the surrounding area due to sublacustrine marine deposits. Lutvann is well buffered against pH changes. Phytoplankton volumes were generally low in the epilimnion. In the metalimnion, volumes were consistently higher. Ciliates were present throughout the year but total concentrations were low. 13 ciliate morphotypes were identified. Nutrient concentrations, ciliate concentrations and phytoplankton volumes confirmed that Lutvann is an oligotrophic lake, bordering on ultraoligotrophic. 12 species of cladocerans, copepods and rotifers were identified and compared with those found in Lutvann in the late 1800s by Georg Ossian Sars. An increase in fish predation has occurred since the time of Sars´ studies and current fish predation of the lake´s zooplankton is intense. This is reflected in the number of small, transparent zooplankton morphotypes.