Biomanipulation in shallow lakes in The Netherlands: an evaluation of 18 case studies

Eighteen shallow lakes in The Netherlands were subjected to biomanipulation , i.e. drastic reduction of the fish stock, for the purpose of lake restora tion. The morphology and the nutrient level of the lakes differed, as did t he measures applied. In some lakes biomanipulation was accompanied by reduc tion of the phosphorus loading. In all but two lakes, the Secchi disk trans parency increased after the fish removal. Eight lakes (no phosphorus loadin g reduction, except for one lake) showed a strong and quick response to the measures: the bottom of the lake became visible ('lake bottom view') and t here was a massive development of submerged macrophytes. In eight other lak es the water transparency increased, but lake bottom view was not obtained. In the biomanipulated lakes the decrease in total phosphorus and chlorophy ll a and the increase in Secchi disk transparency were significantly strong er than the general trend occurring in Dutch lakes where no measures had be en taken. The improvement in the Secchi depth and chlorophyll a was also st ronger than in lakes where only the phosphorus load was reduced. The critic al factor for obtaining clear water was the extent of the fish reduction in winter. Significant effects were observed only after > 75% fish reduction. Success seems to require substantial fish manipulation. In such strongly b iomanipulated lakes, wind resuspension of the sediment never prevented the water from becoming clear. No conclusion can be drawn with respect to the p ossible negative impact of cyanobacteria or Neomysis on grazing by Daphnia and consequently on water clarity. In all lakes where there had been a high density of cyanobacteria or years with a high density of Neomysis other fa ctors such as insufficient fishery may explain why lake bottom view was not obtained. In all lakes with additional phosphorus loading reduction the fi sh stock has been reduced less drastically (15-60%). In these lakes the eff ects on transparency were less pronounced than in the lakes with > 75% fish removal. Daphnia grazing seems responsible for spring clearing in all clea r lakes but one. In three lakes the reduction of benthivorous fish also inc reased the transparency. The factors that determine water clarity in summer are less obvious. In most clear lakes a low algal biomass coincided with a macrophyte coverage of more than 25% of the lake surface area. However, it was not clear what mechanism caused the low algal biomass in summer, altho ugh inorganic nitrogen concentrations were regularly found to be very low. Daphnia grazing in open water seemed to be of little importance for suppres sing the algal biomass in summer. Although in most lakes the total phosphor us concentration decreased after the biomanipulation, the dissolved phospho rus concentration remained too high to cause phosphorus limitation of the a lgal growth. In four out of six clear lakes for which there are long-term d ata the transparency decreased again after 4 years. In one lake with lower nutrient levels the Secchi disk transparency increased over the years. Howe ver, the number of lakes with low nutrient levels is too small for conclusi ons to be drawn regarding the impact of nutrient levels on the stability of the clear water state.