THE HUMIC LAKE ACIDIFICATION EXPERIMENT (HUMEX) - IMPACTS OF ACID TREATMENT ON PERIPHYTON GROWTH AND NUTRIENT AVAILABILITY IN LAKE SKJERVATJERN, NORWAY

The aim of the Humic Lake Acidification Experiment (HUMEX) was to stud y the role of humic substances in the acidification of surface waters, and the impacts of acid deposition on chemical and biological propert ies of humic water. The dystrophic Lake Skjervatjern, Norway, was sepa rated into two basins by a plastic curtain in 1988, and acid treatment of one basin with sulphuric acid and ammonium nitrate started in 1990 . Shortly after the onset of the acid treatment, an extensive growth o f filamentous green algae occurred in the acidified basin. Clay flower pots filled with nutrient diffusing agar were used to study periphyto n growth. Additions of ammonium, nitrate, and bicarbonate, as well as two parallel releases of phosphate were tested. The control basin (B) proved to be N limited for most of the growth season. Consistently hig her accumulation of chlorophyll a per area in the acidified basin (A) than in the control, is suggested to be due to N fertilization by the acid treatment (addition of NH4NO3). Both the untreated and acidified basins showed pronounced seasonal and temporal variations in nutrient limitation. The assumption that low bioavailability of DIC limits prim ary production in acidified water was not confirmed for acid humic wat er. A large increase in areal chlorophyll a in the acidified basin in 1994 occurred simultaneously with increased nutrient supply, particula rly ammonia and TP. This is hypothesized to be caused, at least partly , by changed metabolism in the acidified environment, probably by redu ced/disrupted nitrification.