Eutrophication and occurrences of nuisance cyanobacterial blooms in Lake Onuma, and possible environment-friendly control strategies of blooms using cyanobactericidal bacteria on water plants

Lake Onuma, Hokkaido, has a history of eutrophication accompanied by the developments of agriculture and cattle breeding in the catchment area. Non-point source loadings of nutrients caused the eutrophication and resultantly frequent occurrences of nuisance cyanobacterial blooms there. The existence of cyanobactericidal bacteria and growth-inhibiting bacteria against the toxic cyanobacterium Microcystis aeruginosa was newly discovered on the water plants such as the feed Phragmites australis and floating plant Trapa japonica in Lake Onuma. These effective bacteria were found with densities of the order of 105-107 CFU (colony-forming units) g ^-1 wet weight, and 10 ³ -10 ⁴ CFU mL ^-1 in the adjacent waters. The absence and/or lower densities of M. aeruginosa was closely related to the abundant presence of these effective bacteria detected from the water and biofilms of water plants. These facts provide new insights into the ecology of cyanobactericidal bacteria and growth-inhibiting bacteria, and suggest that the water plants provide an environment that reduces the abundance of cyanobacterial blooms in freshwater ecosystems. The Sato-Umi concept was proposed to maintain good conditions of Lake Onuma with decreased incidents of nuisance cyanobacterial blooms under the adequate control managements of biomass and diversity (species composition) of water plants.