The most typical phytoplankton taxa in four types of boreal lakes

The monitoring of phytoplankton quantities and species composition in Finnish lakes started in the early 1960s. In addition, a substantial number of physical and chemical variables have been measured and recorded from the monitored lakes. The purpose of the present evaluation is to differentiate the phytoplankton taxa of the lakes being classified as acidic, oligotrophic, dystrophic and eutrophic. At present, these four lake types are differentiated on the bases of concentration of total phosphorus (mu g l(-1)), water colour (mg l(-1) Pt), and Secchi-disc transparency. A total of 38 lakes with 163 samples from the years 1984 to 1994 were selected for this study. Cyanoprokaryota Merismopedia warmingiana (M. tenuissima) was abundant in clear and dystrophic acidic lakes whereas Dinobryon divergens (Chrysophyceae) was dominant only in the clear lake types. Oocystis rhomboidea (Chlorophyceae) was also an important species, especially in the dark acidic lakes. In oligotrophic lakes, the phytoplankton biomass was low because of the combined effect of low organism numbers and small cell size of the occurring species. Additional chrysophyceans like Uroglena americana, unidentified Ochromonadales and Dinobryon divergens were especially abundant, as well as Merismopedia warmingiana. In the dystrophic lakes flagellated taxa, such as Cryptomonas spp., Uroglena americana, Ochromonadales and Pseudopedinella spp. dominated. In eutrophic lakes, the cyanoprokaryotes Microcystis aeruginosa, M. viridis and M. wesenbergii, as well as Allabaena flos-aquae and Aphanizomenon flos-aquae dominated both cell numbers and biomass. Some identification problems are also discussed.