AbstractIlestadvannet is a dimictic lake in Andebu County, Norway, with its catchment area situatedin the counties of Andebu and Re. One pre-sample was taken in November 2002, while themain sampling was done from February 2003 to January 2004. Samples for qualitativeidentification of algae were also collected in 2004.The lake is eutrophicated, mesotrophic, and has circumneutral pH, low conductivity andalkalinity. A total of 251 algae species in 136 genera were observed, 178 of these in the lakeplankton. The divisions Cyanophyta, Cryptophyta, Chrysophyta, Bacillariophyta,Xanthophyta, Haptophyta, Dinophyta, Euglenophyta, Rhodophyta, Prasinophyta andChlorophyta were registered. The dominating divisions in winter and spring 2003 wereCryptophyta and Chrysophyta. In the summer the centric diatoms, especially Aulacoseiradistans, were the dominating species. Late summer and early autumn Chlorophyta andDinophyta were also represented. Late fall and winter Cryptophyta and Chrysophyta wereagain the dominating divisions.In the fall of 2004, a bloom of the cyanobacteria species, Anabaena lemmermannii,Microcystis aeruginosa and Woronichinia naegeliana occurred. An ELISA test confirmedthat they produced microcystins. The varying contribution of nutrients from Merkedamselvaduring flood may be one reason for these annual variations in the phytoplankton community.