Biomass and species diversity (richness and evenness) of littoral organisms were explored in 27 sites in three basins of the large Lake Saimaa system in eastern Finland. The basins differed in degree of nutrient loading and trophic status. Six organismal groups, i.e., phytoplankton, periphyton, macrophytes, crustacean zooplankton, benthic macroinvertebrates and fish were studied. Factors affecting the biomass and diversity of these groups were explored by multiple stepwise regression analysis. The biomass of different groups was explained by the same variables, mainly nutrients, while diversity was associated with different environmental factors among the studied groups. The biomass of periphyton, phytoplankton, zooplankton, and fish correlated significantly with each other. There was also an apparent association between the biomass of macrophytes and that of benthic invertebrates. However, no significant correlations were found among the diversity of the studied groups. In accordance with previous studies, our results did not support the existence of species-rich hotspots or the possibility of using any surrogate taxon to reveal overall biodiversity. Thus, for conservation planning, biological surveys should include extensive collection of taxonomic groups and organisms at all trophic levels.

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Zooplankton usually behaves in complex and dynamic ways on various temporal and spatial scales and its spatial distribution is usually highly aggregated, as it possesses a large-scale spatial variability similar to that observed in the physical environment in general. We assess here the suitability of the Optical Plankton Counter for estimating zooplankton abundance, as well as the day-to-day temporal and spatial patterns in its distribution during the open water season. The influence of certain environmental variables on zooplankton abundances is also examined. Abundances of mesozooplankton-sized particles were measured with the Optical Plankton Counter monthly at five stations in a large mesohumic lake from May to September 2005 and agreed rather well with zooplankton abundances counted using a microscope. The poorest agreement was in autumn, when the Counter overestimated the zooplankton abundance, and for some samples in July, when it underestimated the abundance. Fluorescence and chlorophyll a had a significant effect on the Counter readings. Both the intercept and the slope of chlorophyll a varied randomly between depths, because the chlorophyll a decreased much more markedly in deeper waters than did the Counter particle counts. This emphasized the stratified nature of sampling. In addition to seasonal variations, there were also marked short-interval variations (day-to-day) in zooplankton abundances in all the sampling periods. These patterns are highly dynamic and can on some occasions change rapidly in response to fluctuations in the weather. There were no clear differences in zooplankton abundance between the sampling stations. The horizontal heterogeneity was less pronounced than the seasonal and short-interval heterogeneity, possibly because there were no trophic gradients in the basin. While the Optical Plankton Counter provided a rapid assessment of temporal and spatial patterns of zooplankton abundances, it does have limitations. At times it either overestimated abundances due to a large contribution from non-zooplankton particles, or underestimated abundances due to coincidence.

The wastewaters of an iron mining company in NW Russia have changed the water quality in some forest lakes to be hard with a high pH, in contrast to the soft water and low pH in natural lakes. Two impacted lakes and a reference lake were sampled once in early August in three successive years for water quality, plankton communities, and fish. The concentrations of potassium, lithium, and sulphate were high in the impacted lakes. Total phosphorus was low (~ 10 µg L -1 ) in all lakes. The primary producers' biomass and chlorophyll a content in the impacted lakes were high because of high densities of autotrophic picocyanobacteria (Synechococcus spp.). All planktonic communities showed a changed taxa composition and lower species richness. Zooplankton was predominantly Rotatoria in all the lakes. In the impacted lakes, Cladocera was represented by Bosmina, and Copepoda by small cyclopoids. The most obvious substance to be harmful to some planktonic species was potassium. The high pH and high mineral content of the water obviously lower both the toxicity and the bioavailability of heavy metals in the impacted lakes.

Key-words:boreal lake, chlorophyll, phytoplankton, Self-Organizing map, water temperature, zooplanktonThe Self-Organizing Map (SOM) proved to be the method of choice for analysing a large heterogeneous ecological dataset. In addition to distributing the data into clusters, the SOM enabled hunting for correlations between the data components. This revealed logical and plausible relationships between and within the environment and groups of organisms. The main conclusions derived from the results were: (i) the structure of early summer plankton community significantly differed from that of late summer community in Lake Pyhäselkä and (ii) plankton community in late summer was characterized by two functional groups. The first group was formed mainly by phytoplankton, rotifers, and small cladocerans, such as Bosmina spp., and driven by water temperature. The second group was formed by small copepods and the abundant generalist herbivorous cladocerans Daphnia cristata and Limnosida frontosa, which, in turn, associated with chlorophyll a concentration. Biomasses of Bosmina spp. and D. cristata showed decreasing monotonic trends during a 20-year study period supposedly due to oligotrophication. Versatile possibilities to cluster data and hunt for correlations between data components offered by the SOM decisively helped to reveal associations across the original variables and draw conclusions. The results would have been undetectable solely on the basis of unorganised values. RÉSUMÉAnalyse d'un large ensemble de données de surveillance à long terme de la qualité de l'eau et du plancton par classification SOM Mots-clés :lac boréal, phytoplancton, carte auto-organisatrice, La carte d'auto-organisation (SOM) s'est avérée être la méthode de choix pour l'analyse d'un large ensemble de données hétérogènes écologiques. En plus de distribuer les données en grappes, la SOM permet la recherche de corrélations entre les composantes des données. Cette étude a révélé des relations logiques et plausibles entre et au sein de l'environnement et des groupes d'organismes. Les principales conclusions tirées des résultats étaient les suivantes : (i) la structure de la communauté planctonique du début de l'été diffère significativement de celle de la communauté de fin de l'été dans le lac Pyhäselkä et (ii) la communauté planctonique en fin d'été a été marquée par deux groupes fonctionnels. Le premier

A thorough comparison is made of conventional and automated zooplankton sampling techniques. The findings offer tools for complementing netted and weighed zooplankton biomasses with estimates based on acoustic and optic techniques, and thereby fill a gap in our knowledge concerning the suitability of acoustic and optic techniques for freshwater zooplankton sampling. Plankton abundance in six large boreal lakes with varying characteristics was determined by four independent methods: one acoustic (Acoustic Doppler Current Profiler), two optic (laboratory versions of the Optic and Laser Optic Plankton Counter), and a simple seston dry mass (DM) method. The combined DM of Chaoborus and medium-sized zooplankton (300-500 μm) explained 63% of the original variability in the acoustic backscatter strength. The correspondence between the optical measurements and DM was especially good in the case of medium-sized zooplankton. Thus the LOPCderived biomass (ESD 351-500 μm) explained 90% of variation in the DM of zooplankton of size 300-500 μm. The medium-sized fraction consisted mainly of small Cyclopoida and copepodite stages of Eudiaptomus and Daphnia. Both optical devices, and particularly the LOPC, are reliable for analyzing preserved net samples of freshwater zooplankton, but in situ estimation of zooplankton abundances on the basis of ADCP results is not reliable without simultaneous net or optical sampling. Acoustic results can be calibrated by reference to DM or LOPC-derived biomass estimates. The combined use of acoustic and optical techniques clearly improves the spatio-temporal resolution of zooplankton measurements and can thus provide closer insights into ecosystem dynamics, including the forces which regulate plankton communities. *Corresponding author. E-mail: minna.rahkola@uef.fi AcknowledgmentsWe thank the skilful staff of the R/V Muikku for carrying out the sampling in the field, and all the ex-workers of the former Ecological Institute for their assistance in the laboratory. We are grateful to Greta Waissi-Leinonen, Irina Dushkina, and Matti Koistinen for running the OPC and LOPC routines and Kirsti Kyyrönen for drawing the map. We also thank professors (emeritus) Kalevi Salonen and Jouko Sarvala for their helpful comments and suggestions that improved the clarity of the manuscript. Malcolm Hicks kindly revised the English language of the manuscript.

Lake littoral environments are heterogeneous, and different organisms typically show specific responses to this environmental variation. We examined local environmental and spatial factors affecting lake littoral biodiversity and the structuring of assemblages of phytoplankton, zooplankton and macroinvertebrates within and among three basins of a large lake system. We explored congruence of species composition and species richness among the studied organism groups to evaluate their general indicator potential to represent spatial variation in other groups. We expected that effects of water chemistry on plankton assemblages were stronger than effects of habitat characteristics. In contrast, we anticipated stronger effects of habitat on macroinvertebrates due to their mainly benthic mode of life. We also expected that within-basin spatial effects would be strongest on macroinvertebrates and weakest on phytoplankton. We predicted weak congruence in assemblage composition and species richness among the organism groups. Phytoplankton assemblages were mainly structured by the shared effects of water chemistry and large-scale spatial factors. In contrast to our expectations, habitat effects were stronger than water chemistry effects on zooplankton assemblages. However, as expected, macroinvertebrate species composition and richness were mainly affected by habitat conditions. Among-group congruence was weak for assemblage composition and insignificant for richness. Albeit weak, congruence was strongest between phytoplankton and zooplankton assemblages, as we expected. In summary, our analyses do not support the idea of using a single organism group as a wholesale biodiversity indicator.

We observed small schools of vendace (Coregonus albula (L.)) which occasionally crossed the thermocline and migrated into the exceptionally warm (22 to 23 °C) epilimnion of a large Finnish lake in August 2010. This is against previous observations regarding diel vertical migration (DVM) in this species. The ascending schools were detected for a short period of time about an hour after sunset. The fi ndings were based on data gathered with a 120 kHz echo sounder, an Acoustic Doppler Current Profi ler, a Laser Optical Plankton Counter, a MultiNet plankton sampler and a mid-water trawl for the experimental sampling of pelagic fi sh. The study emphasises the importance of examining DVM with different kinds of samplers simultaneously. We are unable to state the ultimate reason why some vendace migrated from cool to warm water, but we can state that there was a lot of food for them in the epilimnion around sunset. It would have been impossible to detect the phenomenon using only mean or maximum abundances of fi sh, and we had to use very fi ne scales to detect the phenomenon on echograms. The ascension of macrozooplankton, mostly Chaoborus fl avicans (Meigen), followed a rhythm described by a third order polynomial function. Vendace were feeding in the evening near sunset within the ascending anti-predation window in which there was enough light for detecting zooplankton and shelter against predators, while during daytime the majority of stomachs of vendace (63%) and smelt (Osmerus erperlanus (L.)) (50%) were empty in mid-water depth layers of the lake where the main pelagic fi sh populations were detected. Based on our observations on fi sh movements during thermal stratifi cation in August, we conclude that vendace has a potential to tolerate by behavioural means warm water up to 23 °C.