Oscillation of a shallow lake ecosystem upon reduction in external phosphorus load

A long-term study of eutrophication abatement in the Botshol Nature Reserve, the Netherlands, showed an intriguing response in this shallow lake. Beginning in 1988, the external nutrient load was reduced by hydrological segregation from agricultural areas and by chemical stripping of phosphorus from the water supply. A side effect of the hydrological segregation of Botshol from agricultural areas was an increase in chloride from 500 to 1000 mg l -1. In the first four years after the decrease in nutrient load, reductions were observed in phosphorus and chlorophyll a concentrations, as well as in the density of phytoplankton, zooplankton, and fish. Reduced phytoplankton density resulted in reduced turbidity and increased cover of Characeae from 2 to 80 %. Although the objective of re-establishing submerged macrophytes seemed to be attained, the clear water state appeared unstable. From 1993 onwards, the ecosystem alternated between turbid water with minor macrophyte production (1993-1995, 1999-2003) and clear water with abundant growth of aquatic plants (1996-1998). Phosphorus concentrations in Botshol also showed strong related fluctuations, despite a stable external phosphorus load.