Restoration of Lake Veluwe, The Netherlands, by Reduction of Phosphorus Loading and Flushing

In the shallow and hypertrophic Lake Veluwe the cycling of phosphorus between water and sediments was mainly determined by a continuous bloom of the blue-green alga Oscillatoria agardhii. For the restoring of water quality it was considered necessary to reduce the external P-loading as well as to suppress the release of phosphorus from the sediments. The P-loading of the lake decreased from approx. 3 to 1 g P/m² a year in 1979. Additionally the lake has been flushed during winter periods with water low in phosphorus. Flushing was primarily aimed at interrupting the continuous algal bloom. After four winters of intensified flushing Oscillatoria still is the dominant alga but the diversity of species is increasing. In 1982 Oscillatoria disappeared for several months.

The results show a sharp decline of summer values of total phosphorus from 0,40-0,60 mg/1 to approx. 0,15 mg/1. In the trend of various parameters correlated with algal biomass a striking discrepancy is observed.

Chlorophyll-a, ashfree-dryweight and particulate-phosphorus show a considerable and approximately similar decrease (50-60%). On the other hand the decrease of algal biomass in terms of freshweight (viz. biovolume of Oscillatoria) and reciprocal Secchi-disc values is relatively small (approx. 25%). Obviously the O. agardhii filaments contain much less organic matter, chlorophyll-a, phosphorus and nitrogen per unit of biovolume after implementation of remedial measures.

Transparency appears to be determined by number and volume of O. agardhii more than by chlorophyll-a and ashfree dryweight.

Intensified flushing will be continued for the next few years.