For European lakes, how clean is clean enough? - Science And Society

Science News, August 9, 2003

Decreases in water quality are often associated with modern farming practices, including the use of artificial fertilizers and the practice of keeping many animals in small areas. However, new research on lakes in Denmark suggests that agriculture has been affecting water quality there for more than 5,000 years.

The finding could help determine the background levels of various water pollutants. That's important because the European Union is now developing lake-water quality standards that will be implemented by 2015, says Emily G. Bradshaw, a paleoecologist at the Geological Survey of Denmark and Greenland in Copenhagen.

Bradshaw and her colleagues looked at sediments from Dallund So, a 12-hectare lake in central Denmark. That area has been one of the most densely populated and intensively cultivated regions of Europe for at least 6 millennia, says Bradshaw.

The rate of sedimentation in Dallund So increased dramatically about 1,000 years ago, a date that roughly corresponds with the founding of a town nearby and a significant change in agricultural practices throughout Denmark. Another spurt in silt accumulation around 2,500 years ago matches a period of rapid deforestation by area settlers. The team's analyses suggest some deforestation also occurred around the lake as many as 6,000 years ago.

Even though modern agricultural practices have caused plant-nutrient concentrations in Dallund So to skyrocket, the researchers discovered that the lake has been extremely nutrient-rich for the past millennium. Many algae-nourishing substances probably entered the lake via an ancient agricultural practice known as retting. In this procedure, farmers submerged bundles of hemp and flax in the lake during the winter so that the plants would soften and partially decompose, easing the recovery of fiber from the stalks.

A significant boost in zooplankton and aquatic plants in sediments deposited just after the rapid deforestation 2,500 years ago suggests that the nutrient concentrations in the lake jumped considerably at that time. Chemical analyses of sediments deposited immediately after the deforestation hint that each liter of lake water then contained about 50 micrograms of phosphorus, more than twice the amount the water contained only a century before. Today, the lake's phosphorus content varies between 65 and 120 [micro]g/1.

Bradshaw says that the earlier concentrations shouldn't be used as thresholds for the proposed European water standards because the only way to meet those targets would be to depopulate the region.--S.P.