PERIPHYTON INVESTIGATIONS IN HUMEX LAKE SKJERVATJERN IN 1992

During the first year of artificial acidification of the dystrophic La ke Skjervatjern with a combination of sulphuric acid and ammonium nitr ate, extensive growth of filamentous green algae occurred in the acidi fied half. Therefore, investigations studying the effect of acidified humic water on periphyton started early 1992. Quantitative measurement s in the two lake-halves (acidified-A, control-B) were carried out, us ing small clay flower pots as growth substrates. The pots were placed in the two lake halves at 0.5 and 2 m depth. During the first three mo nths, accumulated periphyton, measured as dry weight and chlorophyll a , was 50-600% higher in the acidified basin A than in B. Chlorophyll a per unit dry weight was higher in A than B, indicating higher photosy nthetic activity in the acidified half. C/N-ratios in freeze-dried mat erial from basin A were 10-11, indicating little or no N-deficiency du ring the first three months of the experiment at both 0.5 and 2 m dept h in the acidified half. In the control basin B, C/N-ratios between 15 and 19 indicated N-deficiency during the first three months of the ex periment at 0.5 m depth. At 2 m, there was no growth in basin B for th e first 56 d of the experiment. After three months, periphyton accrual at 2 m had approximately the same chlorophyll a per unit dry weight a nd C/N-ratios as in basin A. This indicated that conditions in basin 1 3 at 2 m depth changes from strong growth limitations early in the sum mer to high photosynthetic activity and little or no N-limitation late r in the year. The bluegreen alga Hapalosiphon fontinalis (Ag.)Born. i s known to fix elemental nitrogen. It proliferated in the control basi n, but was scarce in the acidified half. This supported the assumption of nitrogen limitation in the control basin.