Restoration of Botshol (The Netherlands) by reduction of external nutrient load: The effects on physico-chemical conditions, plankton and sessile diatoms

Abstract

The nature reserve Botshol (Utrecht, The Netherlands), consisting of two shallow lakes, ditches and reedland, originated from excavation of peat by man in the 17th century. Up to 1960 Botshol was a clear-water Charophyte lake system. Since the sixties water quality deteriorated and phytoplankton concentrations increased, while the number and dispersion of Chara species decreased. Several restoration measures were attempted to restablish a Charophyte-dominated ecosystem. This paper reports the promising results of this restoration experiment and mentions some complications that arose in restoring the reserve to a less fertile state. The restoration measures have resulted in a sixfold reduction of the external phosphorus load, from 0.6 to 0.1 g m−2.y−1, and in a significant reduction of phosphorus levels at all locations. Moreover, the light climate improved and the phyto- and zooplankton compositions changed considerably Unexpectedly, a bloom ofPrymnesium parvum and a fish kill were observed during the last three months of 1990. Despite this fish kill the restoration of the lake is successful so far.