Epilithic, epiphytic and epipelic diatoms were sampled at four sampling stations monthly from June to October in three boreal rivers in South Finland to examine the seasonal community persistence and stability of diatom communities in three habitats. The variation of successive samples in ordination space using Non-Metric Multidimensional Scaling was substantial and without an evident pattern; there were no clear differences between community stability on three different substrata. At two stations, Multi Response Permutation Procedures showed significant among-group differences indicating that diatom communities were distinctly different among the three substrata. According to Indicator Species Analysis, we found several species showing significant (p 5 0.05) specificity for, and fidelity to, certain substrata at two stations. The stability between sampling months in the rank abundance of diatom taxa was lowest among epiphytic communities. At each station, Spearman rank correlation decreased substantially until August indicating a distinct change in rank abundances. Communities tended to revert to their pre-disturbance state toward autumn after a major spate in the end of August. Using linear regression, we found that changes in total P were significantly (p 5 0.05) related to community stability in the epilithon. Species turnover seemed to be highest among epiphyton and lowest among epipelic communities. Although these differences could partly result from lower diversity in epiphyton, they probably reflect lower persistence among epiphytic communities in these boreal rivers.

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The freshwater red algal genus Batrachospermum has been shown to be paraphyletic since the first molecular studies of the Batrachospermales. Previous research, along with this study, provides strong support for the clade Batrachospermum section Helminthoidea. This study has found that heterocortication, the presence of both cylindrical and bulbous cells on the main axis, is an underlying synapomorphy of this clade. Based on support from DNA sequences of the rbcL gene, the COI barcode region and the rDNA ITS 1 and 2, along with morphological studies, the new genus Sheathia is proposed. Seven heterocorticate species were recognized from the molecular clades. Sheathia boryana and S. exigua sp. nov. appear to be restricted to Europe, whereas S. confusa occurs in Europe and New Zealand. Sheathia involuta is widespread in the USA and reported for the first time from Europe. Sheathia americana sp. nov., has been collected in the USA and Canada, and S. heterocortica and S. grandis sp. nov. have been collected only in the USA. Sheathia confusa and S. grandis can be distinguished based on morphological characters, whereas DNA sequence data are required to conclusively distinguish the other species. Sheathia fluitans and S. carpoinvolucra also are placed within this genus based on the presence of heterocortication. These data also hint at greater diversity among non-heterocorticate Sheathia than is recognized by the single species name S. arcuata.

Clearance rates of epizooic ciliates (Vorticella sp.) were measured together with their host, a planktonic cladoceran Daphnia longispina by using fluorescent latex beads as tracers of food. Vorticellans and their host graze on food of same size range (nanoplanktonic algae and bacteria). Individual clearance rates of Vorticella averaged 6.9 and 7.0 μl ind h and those of Daphnia 463 and 708 μl ind h for beads with diameter 2.00 and 3.92 μm. On the average, epizooic vorticellans together on the carapace of Daphnia cleared particles with rates representing 25-33% of that the host cleared, the maximum rates being 50-80%. In a steeply stratified polyhumic lake vorticellans take advantage of following Daphnia to food patches and they can severely compete for food with their host.

In the eutrophic L. Hiidenvesi, the spring biomass maximum of cladoceran zooplankton is missing and the highest biomass takes place in July-August. The factors behind the delayed biomass peak were studied in four different basins of the lake with concomitant data on cladocerans assemblages, density of the predatory cladoceran Leptodora kindti and food composition of fish. In all the basins, the