EFFECTIVE CONTROL OF NUISANCE CYANOBACTERIAL BLOOMS BY BIOMANIPULATION

Eutrophication is a general problem of small and large lakes. Excessive growths of algae especially cyanobacteria in natural and man-made lakes regularly present problems for recreational. industrial and municipial users of freshwaters. In freshwaters cyanobacteria cause the most serious deterioration of waters due to their hepatotoxin and neurotoxin production. Methods of controlling algae by removal of nutrients with tertiary sewage treatment is quite expensive. Thus there is a need for algal control method that is environmentally acceptable and inexpensive. The food and feeding of silver carp (Hypophthalmicthys molitrix Val.) attracted significant interest in the 1970s because this fish species was considered a potential tool for controlling eutrophication. Studies in lakes, ponds and experimental enclosures led to contradictory results. In some cases silver carp decreased the phytoplankton biomass, however other studies showed that silver carp did not reduce the phytoplankton biomass. Analysis of the food ingestion and the feeding selectivity of silver carp demostrated that its food selectivity is a passive function of the filter morphology. The lower limit for available food particles is about 10 µm. A logical consequence of the above-mentioned result is that silver carp can not control the total algal biomass, but will modify the size structure of algal communities. In vitro experiments with digestive enzymes of this species resulted in a very fast disintegration of non-mucilaginous cyanobacteria, digestion of diatoms and cryptophytes was also effective, but mucilaginous cyanobacteria and green algae (Chlorococcales) proved practically indigestible. A large hypertrophic shallow reservoir (Marcali reservoir, Hungary) was stocked with 1+ fish (approximately 500 kg ha^-1) consisted of 80% of silver carp. Size-selective filtration of the dense silver carp population completely inhibited the growth of filamentous nitrogen fixing cyanobacteria.