First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands.

Lake Breukeleveen is a compartment of the eutrophic Loosdrecht lakes system. In Lake Loosdrecht (dominated by filamentous cyanobacteria), due to water management measures taken from 1970-1984 (sewerage systems, dephosphorization) the external P load has been reduced from 1.2g m-2y-1 to 0.35g m-2y-1. Water transparency (Secchi-depth c30cm), however, has not improved. In March 1989 the standing crop of planktivorous and benthivorous fish populations was reduced by intensive fishery, from 150kg ha-1 to 57kg ha-1. The lake was made inaccessible to fish migrating from the other lakes and it was stocked with large-sized daphnids and 0+ pike Esox lucius, water transparency did not increase in the following summer and autumn 1989. The main explanations for the negative outcome in Lake Breukeleveen are: 1) the rapid increase of the planktivorous fish biomass and carnivorous cladocerans, predating on the zooplankton community; 2) suppression of the large daphnids by the high concentrations of filamentous cyanobacteria; 3) high turbidity of the lake due to resuspension of bottom material induced by wind, unlike in smaller lakes, and thus inability of submerged macrophytes to develop and to stabilize the ecosystem. -from Author