The large raphidophyte Gonyostomum semen is a world-wide distributed flagellated that causes allergic reactions to people swimming in lakes. Its mass development was first recorded by Drouet & Cohen (1935) in Cedar pond, Massachusetts. Regional reviews of the distribution and ecology of G. semen are published for Sweden (Cronberg et al ., 1988) and Finland (Lepistö et al ., 1994). We analysed phytoplankton and water chemistry data collected from 269 Estonian small lakes since the 1950s. Being not found in Estonian lakes until the 1950s, this species is now common in 63 lakes with soft (HCO 3 - <25 mg l -1 ) brown water and high organic matter content (COD Cr >40 mgO l -1 ). It achieved maximum biomass (> 100 g m -3 ) in dystrophic lakes with extremely dark water (Secchi depth < 1 m; COD Cr 60-100 mg O l -1 ). During last years, the species has invaded some oligo- and semidystrophic lakes. Processes connected with lake acidification are considered the main reason for the expansion of G. semen. High phosphorus levels favour the alga. The vertical migration ability gives to the species a certain independence of the phosphorus deficiency during stratification.

Institute of Zoology and Botany,
Estonian Agricultural University,
Võrtsjärv Limnological Station,
61101 Rannu, Tartumaa, Estonia.