In 1986 lake restoration was started in the Langeraar lakes, three small connected lakes in the western part of The Netherlands. In one of the lakes, Lake Geerplas, both the internal and external phosphorus load to the lake was reduced. Water quality parameters have been monitored six years after the measures were taken, in both Lake Geerplas and in one of the other Langeraar lakes. Water quality improved in Lake Geerplas during four years, indicated by decreased phosphorus concentrations and turbidity. At the end of the third fourth summer, phosphorus concentrations increased rapidly, presumably due to excessive phosphorus release from the lake sediment. Phytoplankton biomass and phytoplankton composition changed as well. In the sixth year phosphorus concentrations decreased slowly without any additional measures taken. Chlorophyll-a concentrations and turbidity are at the same level now as shortly after the measures were taken. At this moment, the effects of the measures are evaluated and additional measures are considered. Though the changes in water quality variables during the years are clearly related, the driving forces are not yet identified.

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