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Cyanobacteria may limit recreational use of waters and have negative impacts on ecosystem services. The aim of this study was to determine phosphorus (P) thresholds for cyanobacterial taxa, which form toxin-producing blooms (mass occurrences) in boreal lakes. These thresholds help to set robust P reduction targets for restoration measures. We used long-term (> 40 years) June–August data from > 2000 Finnish lakes to evaluate the total P (TP) thresholds for the most important bloom-forming cyanobacterial genera and selected Microcystis and Dolichospermum species. The analyses were carried out separately for oligohumic, mesohumic and polyhumic lakes. TP threshold values varied between 10 and 61 µg l−1. The values were lower in oligohumic (water colour −1) than mesohumic (water colour 30–90 mg Pt l−1) lakes. The highest TP threshold (50 µg l−1) was observed for Microcystis in polyhumic lakes, and the lowest (10 µg l−1) for Planktothrix in oligohumic lakes.

Past occurrence and quantities of Anabaena cyanobacteria in Lake Pyhajarvi, SW Finland, were investigated using sediment and phytoplankton records. A short sediment core covering the past 20 years was examined for Anabaena resting spores (akinetes) in order to assess the utility of akinetes as a paleolimnological proxy. Sedimentary akinetes confirmed the past existence of Anabaena in water, but did not show a direct correlation with the amount of Anabaena spp. in water samples. The amount of planktonic Anabaena spp. correlated with total phosphorus and nitrogen concentrations in Lake Pyhajarvi, but the number of akinetes was considered to have a relationship with low nutrient concentrations and dominant bloom-forming cyanobacteria. Akinetes are probably suitable for low-resolution and long time scale paleolimnological investigations, where they provide information of past cyanobacteria that cannot be directly attained otherwise.

A mesocosm experiment in 24 enclosures (6 m3) started at the end of June 1996 in a highly eutrophic shallow lake, Lake Koylionjarvi (SW Finland). The original factorial design with nutrient, fish and macrophyte treatments was lost due to strong winds causing leakages. However, after the walls were made leak-proof again on July 11, the planktonic communities developed in divergent ways. On July 31 there was a tenfold variation in total crustacean biomass among the enclosures and the lake (40.2–417.5 μg C l−1), and chlorophyll a varied from 9.5 to 67.0 μg l−1. Here, the single-day data on the 25 planktonic communities is analysed by means of correlation and factor analysis in order to identify factors controlling the protozoans, with particular emphasis on ciliates. The data set comprised: total phosphorus, nitrogen, chlorophyll, bacteria, autotrophic picoplankton, heterotrophic flagellates, abundance and species composition of ciliates, phytoplankton and metazooplankton. The results indicate that although the total ciliate abundance (ranging from 16.2 to 95.0 ind l−1) was controlled by food resources, the observed differences in ciliate community structure could be attributed partly to differential predation by metazooplankton. The effect of Daphnia cucullata, the dominant daphnid cladoceran, was stronger than that of other metazoans.