Changing trend of phosphorus and nitrogen cycling in lake Alpnach

Phosphorus limited Lake Alpnach was loaded with high amount of phosphorus from wastewaterduring the 1970s leading to eutrophication and water quality deterioration affecting the structureand function of the lake ecosystem. Later on advanced wastewater treatment and ban onphosphate detergents lowered the phosphorus input to the lake. This study assesses the change inthe load and trophic status of the lake and their impact on nitrogen and phosphorus cycling overthe period of 30 years. It estimates the load of various nitrogen and phosphorus species from thetwo rivers, Sarner Aa and Chly Schliere, based on concentration-discharge relationships andcontinuous discharge measurements. The nutrient export was determined from lake waterconcentrations and discharge readings. The results of the study showed that the phosphorus inputhas greatly decreased mainly due to improved wastewater treatment. The load of terrigenousphosphorus, however, did not change. In the same time in-lake nitrate concentration have greatlyincreased. As the lake is phosphorus limited, the decreased phosphorus load lowered theproductivity of the lake. At present the lake water is oxic at all depths throughout the year andthe net sedimentation of phosphorus has decreased. The lowered trophic state also affected thenitrogen cycling. It depressed nitrate consumption by the phytoplankton and net nitrogensedimentation. However, benthic denitrification did not change though the nitrate concentrationin the lake water has increased considerably. From this we conclude that benthic denitrificationis now controlled by the supply of settling organic matter whereas in the eutrophic lake it waslimited by the nitrate concentration of the water layer overlying sediment