ARTIFICIAL MIXING - A THERAPY MEASURE COMBATING CYANOBACTERIA IN LAKENIEUWE-MEER

In Lake Nieuwe Meer near Amsterdam, the bloomforming cyanobacterium Mi crocystis aeruginosa dominates the phytoplankton in late summer and fo rms scums that can be a big nuisance. In Lake Nieuwe Meer the water co lumn is stratified in summer. Microcystis can stay in the euphotic zon e due to its buoyancy. Deep mixing of the lake abolishes this advantag e of buoyancy of Microcystis and favours non-buoyant algae as they wil l have a reduced sedimentation loss. The technique of artificial mixin g has been applied in 1993 and 1994 applied in Lake Nieuwe Meer in ord er to reduce the growth of Microcystis. The application was successful : there has been no nuisance due to Microcystis scums. The percentage cyanobacteria of the total phytoplankton in summer decreased from +/-9 0% without mixing to less than 5% with mixing. Characteristics of the equipment used for destratification are presented.