BORTPUMPNING AV SJÖARS BOTTENVATTEN FÖR ATT MINSKA INTERNBELASTNINGEN AV FOSFOR Erfarenheter från Bornsjön / Hypolimnetic withdrawal to reduce internal phosphorus loading in lakes Experiences from Lake Bornsjön
Eutrophication of lakes is a serious problem, especially in agricultural areas. It is mainly due to external and internal loads of phosphorus (P), which increase the P concentration in the lake water. Lake Bornsjön is a reserve source of water supply for Stockholm and is located 25 km southwest of the city. Maximum yearly concentration of total P has increased during recent years to 920 µg l–1 during 2000–2004 compared to 700 µg l–1 during the 1990s. The lake seems to be in the beginning of a negative process with accelerating P concentrations and low levels of oxygen (O2) in the hypolimnion during stagnant periods.
During the summer of 2004, 480 000 m3 of water containing 158 kg P were pumped from the hypolimnion of the north-west basin of Lake Bornsjön. Pumping started in early June and lasted during the entire stratified period. The O2 depletion was quite high during June–August, 0.024 mg cm–2 dygn–1, and there was no clear effect on the P concentration of the hypolimnetic withdrawal. However, there was no additional O2 consumption in that year, in contrast to the significant trend observed in at least previous 18 years. Better results with improved O2 conditions would probably have been achieved with a more effective pump capable of 120 l s–1 or more compared with the 40 l s–1 employed here. Some alternative methods of managing the pumped water are discussed.