Long-term changes in the phytoplankton of large lakes in response to changes in nutrient loading

In Sweden the recovery process of the four largest lakes (<500 km²) has been followed since the mid-1970's when the phosphorus loading was reduced by approximately 50%. A ten-year investigation period before that acts as a basis of comparison. The response of the dominating species in the phytoplankton communities in the eutrophic Lake Mälaren and the oligotrophic Lake Vättern is discussed also in relation to a pristine state of the lakes. The long-term investigations with monthly sampling make a natural phytoplankton biomass variation range possible to establish - a range that is mainly due to effects of weather fluctuations. In order to get a tool for water management where disturbing and highly disturbing total phytoplankton volumes can be anticipated from a certain phosphorus concentration, a model is suggested based on experiences from more than 50 Swedish lakes investigated during a series of years.