Artificial mixing to reduce growth of the blue-green alga Microcystis in Lake Nieuwe Meer, Amsterdam: an evaluation of 7 years of experience

Abstract

The problem of Lake Nieuwe Meer (area = 1.3 km2, max. depth 30 m, Ptot = 500 mg/m3) was extensive growth of Microcystis with disturbing scum forming. Since 1993 the lake has been artificially mixed in summer by a bubble plume installation. The result is quite successful since the mass of Microcystis is up to 20 times lower than in the years before mixing and no scum is present any more. The study in Lake Nieuwe Meer showed a shift from cyanobacterial dominance (mainly Microcystis) to flagellates, green-algae and diatoms when artificial mixing was applied. Total phosphorus and nitrogen concentrations did not change as a result of mixing and had apparently no effect on the shift in the phytoplankton composition. The chlorophyll-a concentration was much lower in the mixed lake as a result of dilution. The total algae biomass decreased. The transparency did not improve. The total heat energy of the lake is slightly higher than before mixing but still remains in the range of annual fluctuation. The temperature on the surface is approximately 2°C lower. In the whole water-body oxygen was always higher than 5 mg/l. Living space for fish is therefore wider. The installation in Lake Nieuwe Meer consists of flexible pipes near the sediment, built in a way to prevent sediment erosion and transport into the water. There are no constructions in the water-body. All mechanical parts are on land. The layout of the installation is shown in Fig. 1. Installed compressor energy is 85 kW. This is equivalent to an upper middle-class motor-car. The design was made specifically for this problem. It is based on the physical data of the algae and the plant. It would be beneficial to use this 7 year's experience for further applications e.g. elimination of toxic algae in drinking-water reservoirs.