Effects of an extreme precipitation event on water chemistry and phytoplankton in the Swedish Lake Mälaren

Extreme events can often be ecologically more relevant than fluctuations in the mean climate. We studied the effects of an exceptionally rainy period in 2000 on 13 physical and chemical variables and on the phytoplankton biomass and composition in Lake Mälaren, the third largest lake in Sweden. The rainy period caused a distinct increase in the chemical loading, in particular the loading of organic carbon, into Lake Mälaren. As a consequence water color, measured as light absorption of filtered water, was up to 3.4 times higher in 2001 than ever recorded since 1965. In addition, reactive silica concentrations doubled, particle concentrations were the highest and conductivity was the lowest registered. The changes in physical and chemical conditions of Lake Mälaren in 2001 showed temporal and spatial differences depending on runoff processes in the catchment, water residence times and the type of variable. It is suggested that especially the increase in water color was responsible for the observed doubling in the cryptophycean biomass in Lake Mälaren in May 2001. The increase in water color and cryptophycean biomass was also observed in a nearby lake. Such increases required alterations in the treatment process of raw water from Lake Mälaren for the drinking water supply of Stockholm city.