Original paper

Effects of destratification and fish removal on the genus Microcystis in a shallow hypertrophic lake

Lepistö, Lisa; Sammalkorpi, Ilkka; Jokipii, Reija; Niemelä, Maija


The boreal Lake Tuusulanjärvi is hypertrophic due to high diffuse and internal loading maintained by hypolimnetic oxygen depletion and a high density of cyprinids. The phytoplankton community, especially of cyanophyta (cyanobacteria, cyanoprokaryota) in the lake is highly diverse, and Lake Tuusulanjärvi has been characterized as a "botanical garden" of the genus Microcystis. The abundant cyanophyta, producing nuisance algal blooms, has generated a need for intensive restoration activities. Since the autumn of 1997, effective destratification and biomanipulation have decreased the internal loading. As a result of lower nutrient levels the phytoplankton biomass, especially of cyanophyta, has decreased and transparency has increased. The biomass of cyanophyta was lowest in summers after the highest removal of fish biomass. During the study period 69 taxa of cyanophyta were identified, High nutrient concentrations clearly explained the occurrence of some morphotypes of Microcystis. M. wesenbergii, M. aeruginosa and M. viridis, which are embedded in a specific kind of mucilaginous sheath, were considered as morphotypes occurring during different phases of stratification. M. wesenbergii and M. viridis were the most resistant species to biomanipulation and destratification.


biomanipulationdestratificationphytoplanktoncyanophytamorphotypesresting stagemicrocystis