Algedynamikk i Ilestadvannet

Abstract

Abstract Ilestadvannet is a dimictic lake in Andebu County, Norway, with its catchment area situated in the counties of Andebu and Re. One pre-sample was taken in November 2002, while the main sampling was done from February 2003 to January 2004. Samples for qualitative identification of algae were also collected in 2004. The lake is eutrophicated, mesotrophic, and has circumneutral pH, low conductivity and alkalinity. A total of 251 algae species in 136 genera were observed, 178 of these in the lake plankton. The divisions Cyanophyta, Cryptophyta, Chrysophyta, Bacillariophyta, Xanthophyta, Haptophyta, Dinophyta, Euglenophyta, Rhodophyta, Prasinophyta and Chlorophyta were registered. The dominating divisions in winter and spring 2003 were Cryptophyta and Chrysophyta. In the summer the centric diatoms, especially Aulacoseira distans, were the dominating species. Late summer and early autumn Chlorophyta and Dinophyta were also represented. Late fall and winter Cryptophyta and Chrysophyta were again 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 confirmed that they produced microcystins. The varying contribution of nutrients from Merkedamselva during flood may be one reason for these annual variations in the phytoplankton community.