Studies on eutrophication of some Norwegian inland waters

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Abstract
Most rivers and lakes in Norway have a water of oligotrophic nature. The use of land in the catchment areas for agriculture and human settlement leads to an enrichment of the water. Phytoplankton investigations during the last five years have demonstrated recent changes in the trophic nature of several inland waters. Species of Oscillatoria have previously in a few cases only been reported as the cause of algal blooms. At least four oscillatorians are now common in such developments of plankton vegetation. They are: Oscillatoria Agardhii var. isothrix Skuja, Oscillatoria sp., Oscillatoria cf. rubescens D. C. and Oscillatoria Agardhii Gom. (fig. 2, 3, 4, 5 and 6). The three lakes L. Steinsfjord, L. Gjersjøen and L. Årungen (fig. 1) cover a range from a low grade of eutrophy to extreme eutrophy. Data about the lakes are listed in table 1, and an example of their hydrography at the end of summer stagnation are given in fig. 7. The vegetation of blue green algae and the state of eutrophication of the lakes are discussed. Laboratory cultures of algae have been used to demonstrate the effects of fertilization by domestic sewage on recipient water. The experimental procedure used is briefly described. Results from a regional survey where algal cultures were used to express changing growth qualities of the water flowing down a drainage basin are presented (fig. 8). An experiment is described where the results obtained (fig. 9) indicated that the qualities of the recipient water may be as important for the effect of pollution with respect to consequences for algal growth, as the amount and composition of the sewage itself.

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