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.