ARTIFICIAL MIXING PREVENTS NUISANCE BLOOMS OF THE CYANOBACTERIUM MICROCYSTIS IN LAKE NIEUWE-MEER, THE NETHERLANDS

1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successf ul in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcyst is per litre and also per m(2) was lower than in the two preceding con trol years. Hardly any nuisance scums of Microcystis occurred in the l ake. 2. The phytoplankton shifted from a cyanobacteria-dominated commu nity in summer to a mixed community of flagellates, green algae and di atoms. Reduced sedimentation losses in the mixed lake, probably in com bination with a lower pH, favoured nonbuoyant algae, while the entrain ment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy. 3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences betw een the years. The chlorophyll content integrated through depth (m(-2) ) increased in the artificially mixed lake. 4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 result ed in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction o f 75% of the energy costs, while the mixing was still sufficient to pr event stratification. 5. Determination of the buoyancy state of the co lonies on a sunny and calm day showed that the buoyancy loss was low c lose to the bubble plumes, and high at some distance from these plumes . This suggests that Microcystis could escape the mixing at some dista nce from the plumes, and could synthesize more carbohydrates during it s stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy sta te appeared to be a good and simple method to investigate the extent o f entrainment of colonies in the turbulent flow.