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The goal of this study was a harmonization of diatom identification and counting among diatomists from the Scandinavian and Baltic countries to improve the comparison of diatom studies in this geographical area. An analysis of the results of 25 diatomists following the European standard EN 14407 during an intercalibration exercise showed that a high similarity was achieved by harmonization and not because of a long experience with diatoms. Sources of error were wrong calibration scales, overlooking of small taxa, especially small Navicula s.l., misidentifications (Eunotia rhomboidea was mistaken for Eunotia incisa) and unclear separation between certain taxa in the identification literature. The latter was discussed during a workshop with focus on the Achnanthes minutissima group, the separation of Fragilaria capucina var. gracilis from F. capucina var. rumpens, and Nitzschia palea var. palea from N. palea var. debilis. The exercise showed also that the Swedish standard diatom method tested here worked fine with acceptable error for the indices IPS (Indice de Polluo-sensibilite Specifique) and ACID (ACidity Index for Diatoms) when diatomists with a low similarity (Bray–Curtis

Information on the structure of microalgal assemblages in the epiphyton and epilithon is necessary to understand the origin of phytoplankton in lowland rivers. To this end, we carried out concurrent investigations on phytoplankton, epiphyton and epilithon in 18 reaches of three Estonian rivers during the midsummers of 2002 and 2003. A total of 251 taxa was recorded, of which 192 were epiphyton species, 158 were epilithon species and 150 were phytoplankton species. Canonical correspondence analysis (CCA), based on the 31 most abundant taxa, indicated differences in the structure of the algal assemblages between the different biotopes (phytoplankton, epiphyton and epilithon) as well as between the studied rivers. The composition of the phytoplankton clearly differed from that of the other biotopes, with prevailing small flagellates, a chrysophyte (Synura uvella) and cryptophytes (Rhodomonas lacustris and Cryptomonas erosa). The epiphyton was characterized by a large number of diatoms, while the epilithic community contained filamentous cyanobacteria (Phormidium tergestinum and Planktolyngya sp.) and a green alga (Stigeoclonium tenue) in addition to diatoms. Based on redundancy analysis (RDA), phosphorous was the most relevant parameter determining the distribution of species in the phytoplankton assemblages. Shading by trees on the river bank, dissolved oxygen concentration and water temperature as well as river width determined the distribution of species in the epiphyton. The data set on the epilithon did not reveal any significant relationships between species distribution and the measured environmental parameters.

The material for the present phytoplankton study was collected from 427 reaches of 162 Estonian rivers during the period of 1991–1997. Sampling was carried out at the end of June and in July. A total of 402 taxa of algae were identified in phytoplankton. The most numerous were diatoms (180), followed by green algae (125) and cyanobacteria (45). The phytoplankton of the Estonian rivers is sparse and heterogenous: only 13 species of algae were recorded in more than 50% of streams. Pennate diatoms dominate most frequently in phytoplankton biomass, while tiny flagellates dominate in phytoplankton abundance. The nonparametric means (medians) of phytoplankton biomass, abundance and number of taxa per sample in the lower reaches of the Estonian streams were 0.2 mg l−1, 0.4×106 cells l−1 and 19, respectively; the highest mean values (0.3 mg l−1, 0.6×106 cells l−1 and 23, respectively) were observed in the rivers of the drainage basin of Peipsi-Vortsjarv and the lowest ones (0.1 mg l−1, 0.2×106 cells l−1 and 13, respectively), in the streams of Hiiumaa. Phytoplankton biomass was correlated with content of chlorophyll a according to Lorenzen (L chl a) (Spearman r=0.72) and Jeffrey & Humphrey (JH chl a) (Spearman r=0.70) at the confidence level P