Search

Wastewaters discharged to Lake Vesijärvi ruined its recreational value and demanded mitigation measures. In the mid-1970s, the diversion of wastewaters elsewhere reduced epilimnetic total phosphorus concentration by ~ 40% from > 100 mg P m−3 in 5 years, but this was not enough to eliminate cyanobacterial blooms. In 1979–1984, hypolimnetic oxygenation was applied to decrease internal nutrient loading, but pumping warm surface water to the hypolimnion probably intensified gas ebullition from the sediment, carrying nutrients to the epilimnion and intensifying cyanobacterial blooms. Intensive fish removal in 1989–1994 was more successful. Five years of summer trawling removed over three-quarters of roach and smelt stock, and after the two most intensive fishing years, TP and chlorophyll concentrations abruptly decreased by ~ 35%. During subsequent years fish removal continued at ~ 30% intensity and maintained chlorophyll and total nutrient concentrations at a lower level until the end of the study. At the same time, the frequency of cyanobacterial blooms decreased from annual to twice per decade. Larger-scale oxygenation after 2010 no longer resulted in ebullition, and its effect on phytoplankton was indistinguishable from natural variability. Consequently, it was abandoned. The intensity of fish removal needed to maintain the present status of the lake is still awaiting evaluation.

Aim We use lake phytoplankton community data to quantify the spatio-temporal and scale-dependent impacts of eutrophication, land-use and climate change on species niches and community assembly processes while accounting for species traits and phylogenetic constraints. Location Finland. Time period 1977–2017. Major taxa Phytoplankton. Methods We use hierarchical modelling of species communities (HMSC) to model metacommunity trajectories at 853 lakes over four decades of environmental change, including a hierarchical spatial structure to account for scale-dependent processes. Using a “region of common profile” approach, we evaluate compositional changes of species communities and trait profiles and investigate their temporal development. Results We demonstrate the emergence of novel and widespread community composition clusters in previously more compositionally homogeneous communities, with cluster-specific community trait profiles, indicating functional differences. A strong phylogenetic signal of species responses to the environment implies similar responses among closely related taxa. Community cluster-specific species prevalence indicates lower taxonomic dispersion within the current dominant clusters compared with the historically dominant cluster and an overall higher prevalence of smaller species sizes within communities. Our findings denote profound spatio-temporal structuring of species co-occurrence patterns and highlight functional differences of lake phytoplankton communities. Main conclusions Diverging community trajectories have led to a nationwide reshuffling of lake phytoplankton communities. At regional and national scales, lakes are not single entities but metacommunity hubs in an interconnected waterscape. The assembly mechanisms of phytoplankton communities are strongly structured by spatio-temporal dynamics, which have led to novel community types, but only a minor part of this reshuffling could be linked to temporal environmental change.

Climate change is a pervasive threat to biodiversity. While range shifts are a known consequence of climate warming contributing to regional community change, less is known about how species' positions shift within their climatic niches. Furthermore, whether the relative importance of different climatic variables prompting such shifts varies with changing climate remains unclear. Here we analysed four decades of data for 1,478 species of birds, mammals, butterflies, moths, plants and phytoplankton along a 1,200 km high latitudinal gradient. The relative importance of climatic drivers varied non-uniformly with progressing climate change. While species turnover among decades was limited, the relative position of species within their climatic niche shifted substantially. A greater proportion of species responded to climatic change at higher latitudes, where changes were stronger. These diverging climate imprints restructure a full biome, making it difficult to generalize biodiversity responses and raising concerns about ecosystem integrity in the face of accelerating climate change.The authors analyse four decades of distribution data for various taxonomic groups to understand the shift of species within their climatic niches and the changing influences of different climate factors. The diverse and diverging climate imprints raise concerns about future ecosystem integrity.

Interactions between stressors in freshwater ecosystems, including those associated with climate change and nutrient enrichment, are currently difficult to detect and manage. Our understanding of the forms and frequency of occurrence of such interactions is limited; assessments using field data have been constrained as a result of varying data forms and quality. To address this issue, we demonstrate a statistical approach capable of assessing multiple stressor interactions using contrasting data forms in 3 European catchments (Loch Leven Catchment, UK: assessment of phytoplankton response in a single lake with time series data; Pinios Catchment, Greece: macroinvertebrate response across multiple rivers using spatial data; and Lepsämänjoki Catchment, Finland: phytoplankton response across multiple rivers using spatiotemporal data). Statistical models were developed to predict the relative and interactive effects of climate change and nutrient enrichment sensitive indicators (stressors) on indicators of ecological quality (ecological responses) within the framework of linear mixed effects models. In all catchments, indicators of nutrient enrichment were identified as the primary stressor, with climate change-sensitive indicators causing secondary effects (Loch Leven: additive, total phosphorus [TP] × precipitation; Pinios: additive, nitrate × dissolved oxygen; Lepsämänjoki: synergistic, TP × summer water temperature), the intensity of which varied between catchments and along the nutrient stressor gradient. Simple stressor change scenarios were constructed for each catchment and used in combination with mechanistic models to explore potential management responses. This approach can be used to explore the need for multiple stressor management in freshwaters, helping practitioners navigate a complex world of environmental change.

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.

Environmental DNA (eDNA) and other molecular based approaches are revolutionizing the field of biomonitoring. These approaches undergo rapid modifications, and it is crucial to develop the best practices by sharing the newest information and knowledge. In our ongoing project we: assess the state-of-the-art of eDNA methods at Finnish Environment Institute SYKE; identify concrete next steps towards the long-term aim of implementing eDNA methods into environmental and biomonitoring; promote information exchange on eDNA methods and advance future research efforts both within SYKE and with our national and international partners. assess the state-of-the-art of eDNA methods at Finnish Environment Institute SYKE; identify concrete next steps towards the long-term aim of implementing eDNA methods into environmental and biomonitoring; promote information exchange on eDNA methods and advance future research efforts both within SYKE and with our national and international partners. Scientific background Well-functioning and intact natural ecosystems are essential for human well-being, provide a variety of ecosystem services and contain a high diversity of organisms. However, human activities such as eutrophication, pollution, land-use or invasive species, are threatening the state and functioning of ecosystems from local to global scale (e.g. Benateau et al. 2019; Reid et al. 2018; Vörösmarty et al. 2010). New molecular techniques in the field and in the laboratory have enabled sampling and identification of much of terrestrial, marine and freshwater biodiversity. These include environmental DNA (eDNA, e.g. Valentini et al. 2016) and bulk-sample DNA metabarcoding approaches (e.g. Elbrecht et al. 2017) and targeted RNA-based methods (e.g. Mäki and Tiirola 2018). The eDNA technique uses DNA that is released from organisms into their environment, from which a signal of organisms’ presence in the system can be obtained. For example, in aquatic ecosystems, eDNA is typically extracted from sediment or filtered water samples (e.g. Deiner et al. 2016), and this approach is distinguished from bulk DNA metabarcoding, where organisms are directly identified from e.g. complete biological monitoring samples (e.g. Elbrecht et al. 2017). Despite the demonstrated potential of environmental and bulk-sample DNA metabarcoding approaches in recent years, there are still significant bottlenecks to their routine use that need to be addressed (e.g. Pawlowski et al. 2020). Methods and implementati on The project is divided into three work packages: WP1 Gathering existing knowledge, identifying knowledge gaps and proposing best practices, WP2 Roadmap to implementation and WP3 eDNA monitoring pilot. Please see more details in the Fig. 1

Food quality is one of the key factors influencing zooplankton population dynamics. Eutrophication drives phytoplankton communities toward the dominance of cyanobacteria, which means a decrease in the availability of sterols and long-chain polyunsaturated fatty acids (EPA and DHA). The effects of different restoration measures on the nutritional quality of the phytoplankton community and subsequent impacts on zooplankton biomass have rarely been considered. We analyzed the nutritional quality of phytoplankton in the eutrophic Lake Vesijärvi in southern Finland over a 37-year period, and studied the impacts of two restoration measures, biomanipulation and hypolimnetic aeration, on the abundance of high-quality phytoplankton. We found that biomanipulation had a positive impact on the abundance of taxa synthesizing sterols, EPA, and DHA and, concurrently, on the biomass of the keystone species Daphnia. In contrast, hypolimnetic aeration did not result in such a beneficial outcome, manifested as a decrease in the abundance of Daphnia and frequent phytoplankton blooms dominated by cyanobacteria suggesting reduction in the nutritional quality of food for Daphnia. Our analysis shows that the determination of the nutritional value of algae and the contribution of essential fatty acids and sterols is an effective method to evaluate the success of various restoration measures.

Eutrophication (as an increase in total phosphorus [TP]) increases harmful algal blooms and reduces the proportion of high-quality phytoplankton in seston and the content of ω-3 long-chain polyunsaturated fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) in fish. However, it is not well-known how eutrophication affects the overall nutritional value of phytoplankton. Therefore, we studied the impact of eutrophication on the production (as concentration; μg L

Long-term monitoring data from two lakes located at different latitudes were used to test the hypothesis that phytoplankton communities respond differently to environmental changes (e.g., global warming and anthropogenic activities, mainly eutrophication). Lake Pyhajarvi (temperate area) and Lake Taihu (subtropical area) are both shallow and productive lakes. Presence/absence data indicated that phytoplankton taxa present did not change significantly in the two lakes over the last two decades. However, biomass data showed that dominance relationships of species changed in both lakes. Results of assemblage ordination indicated that climate change played a vital role in mediating phytoplankton assemblage dynamics in Lake Pyhajarvi, while eutrophication was the primary factor driving the phytoplankton assemblage dynamics in Lake Taihu. In Lake Pyhajarvi, the variability of the climatic variables was greater than in Lake Taihu. In Lake Pyhajarvi, increasing wind speed decreased Secchi depth, promoting shade-tolerant taxa such as Planktothrix . At the same time, increased mixing reduced the loss rate of the heavy taxa such as Tabellaria . Increased nutrient concentrations and water temperature would also promote both Planktothrix and Tabellaria . In the warmer Lake Taihu, the biomass of the dominant Microcystis , which has high optimum temperature for growth, was strongly related to conductivity and less to temperature. Our results showed that climate change potentially enhanced cyanobacteria dominance in lakes Pyhajarvi and Taihu but by different mechanisms. In Lake Pyhajarvi, the former dominant species were replaced by cyanobacteria; in Lake Taihu, cyanobacterial dominance was accentuated by climate warming.

To assess variability in carbon isotope signatures (d13C) between and within populations under natural conditions, with a particular emphasis on colony size, we repeatedly collected planktonic colonies of a freshwater cyanobacterium Gloeotrichia echinulata in two lakes, Pyhaj a rvi (southwest Finland) and Erken (southeast Sweden). Despite substantial differences in the average d13C signature of Gloeotrichia between lakes (26.9% in Pyhaj arvi and 220.7% in Erken), a similar, systematic increase in d13C with colony size was observed in both lakes (of 2-3% in Pyhaj arvi and 3-5% in Erken). This suggests declining isotope fractionation with increasing colony size, probably related to diffusion limitation of carbon availability. Temporal variation explained a minor fraction of total subsample variability (range d13C ,4% in Pyhaj a rvi and ,6% in Erken). Isotopic 13C fractionation in Gloeotrichia was likely affected both by carbon source and by colony size.

The composition and abundance of phytoplankton is an important factor defining ecological status of marine and freshwater ecosystems. Chemotaxonomic markers (e.g., pigments and fatty acids) are needed for monitoring changes in a phytoplankton community and to know the nutritional quality of seston for herbivorous zooplankton. Here we investigated the suitability of sterols along with fatty acids as chemotaxonomic markers using multivariate statistics, by analyzing the sterol and fatty acid composition of 10 different phytoplankton classes including altogether 37 strains isolated from freshwater lakes. We were able to detect a total of 47 fatty acids and 29 sterols in our phytoplankton samples, which both differed statistically significantly between phytoplankton classes. Due to the high variation of fatty acid composition among Cyanophyceae, taxonomical differentiation increased when Cyanophyceae were excluded from statistical analysis. Sterol composition was more heterogeneous within class than fatty acids and did not improve separation of phytoplankton classes when used alongside fatty acids. However, we conclude that sterols can provide additional information on the abundance of specific genera within a class which can be generated by using fatty acids. For example, whereas high C-16 omega-3 PUFA (polyunsaturated fatty acid) indicates the presence of Chlorophyceae, a simultaneous high amount of ergosterol could specify the presence of Chlamydomonas spp. (Chlorophyceae). Additionally, we found specific 4 alpha-methyl sterols for distinct Dinophyceae genera, suggesting that 4a-methyl sterols can potentially separate freshwater dinoflagellates from each other.

Article, Fish are an important source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) for birds, mammals and humans. In aquatic food webs, these highly unsaturated fatty acids (HUFA) are essential for many physiological processes and mainly synthetized by distinct phytoplankton taxa. Consumers at different trophic levels obtain essential fatty acids from their diet because they cannot produce these sufficiently de novo. Here, we evaluated how the increase in phosphorus concentration (eutrophication) or terrestrial organic matter inputs (brownification) change EPA and DHA content in the phytoplankton. Then, we evaluated whether these changes can be seen in the EPA and DHA content of piscivorous European perch (Perca fluviatilis), which is a widely distributed species and commonly consumed by humans. Data from 713 lakes showed statistically significant differences in the abundance of EPA- and DHA-synthesizing phytoplankton as well as in the concentrations and content of these essential fatty acids among oligo-mesotrophic, eutrophic and dystrophic lakes. The EPA and DHA content of phytoplankton biomass (mg HUFA g− 1) was significantly lower in the eutrophic lakes than in the oligo-mesotrophic or dystrophic lakes. We found a strong significant correlation between the DHA content in the muscle of piscivorous perch and phytoplankton DHA content (r = 0.85) as well with the contribution of DHA-synthesizing phytoplankton taxa (r = 0.83). Among all DHA-synthesizing phytoplankton this correlation was the strongest with the dinoflagellates (r = 0.74) and chrysophytes (r = 0.70). Accordingly, the EPA + DHA content of perch muscle decreased with increasing total phosphorus (r2 = 0.80) and dissolved organic carbon concentration (r2 = 0.83) in the lakes. Our results suggest that although eutrophication generally increase biomass production across different trophic levels, the high proportion of low-quality primary producers reduce EPA and DHA content in the food web up to predatory fish. Ultimately, it seems that lake eutrophication and brownification decrease the nutritional quality of fish for human consumers., published version, peerReviewed

Article, Analyses of carbon stable isotopes are often used to estimate the contributions of allochthonous and autochthonous dietary resources to aquatic consumers. Most pelagic food web studies assume that all phytoplankton taxa have a similar δ13C value. We studied pelagic food web compartments (dissolved inorganic carbon [DIC], phytoplankton, bacteria, seston, cladoceran zooplankton) in 12 small (< 0.1 km2) lakes in southern Finland. These lakes were classified as oligotrophic, mesotrophic, eutrophic, and dystrophic based on their concentrations of total phosphorus and dissolved organic carbon. Additionally, we studied phytoplankton photosynthetic carbon fractionation (εp) in laboratory conditions. The photosynthetic fractionation in 28 phytoplankton cultures from nine different phytoplankton classes varied significantly at the class level, and fractionation correlated significantly with the DIC concentration of the growth media. In small boreal lakes, the δ13C values of different phytoplankton taxa, as directly measured or estimated from the δ13C values of biomarker fatty acids, varied greatly (−18‰ to −44.5‰). Phytoplankton δ13C values varied significantly by lake type and were most depleted in dystrophic lakes even though the δ13C values of the DIC was similar to mesotrophic lakes. Further within-taxa variation was found between lakes and between different depths within a lake. Vertical samples from dystrophic lakes also showed lower εp in the phytoplankton from meta- and hypolimnion, possibly as a result of reduced light intensity. Altogether, in nine of the 10 sampled lakes, the δ13C values of cladoceran zooplankton were between the minimum and the maximum phytoplankton δ13C value of each lake, and thus, phytoplankton alone could explain zooplankton δ13C values. We conclude that stable isotope mixing models should take into account carbon variation among different phytoplankton taxa., published version, peerReviewed

Stable isotope signatures of filter-feeding freshwater mussels have been established as a convenient baseline measure at the primary consumer level for comparisons of pelagic food webs. We studied this idea with four unionid mussel species Anodonta sp. (A. anatina), Pseudanodonta complanata, Uniopictorum and U. tumidus in four Finnish lakes. δ 15 N and δ 13 C signatures of mussels were compared with surface sediment and summer averages of their potential food sources and planktonic herbivores. Isotope compositions of Anodonta, Pseudanodonta and Unio differed significantly between lakes but were roughly similar within lakes, except in the smallest study lake. There was considerable deviation in the δ 13 C of mussels relative to that of particulate organic matter (POM) in water or sediment, suggesting selective use of food sources. The δ 13 C of POM also showed considerable temporal variation. Enrichment in δ 15 N mussels over that of POM was less than expected, but the mussel signatures matched closely with those of herbivorous Daphnia. We suggest that unionid mussels can provide a reasonable baseline estimate for pelagic food web comparisons with much less sampling effort than other suggested primary consumers.

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.

Summary 1. The variability in the stable isotope signatures of carbon and nitrogen (δ13C and δ15N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south-west Finland. The lakes were sampled on nine to 16 occasions over 2–4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon-specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both δ13C and δ15N. The values of δ13C ranged from −34.4‰ to −5.9‰ and were lowest in chrysophytes (−34.4‰ to −31.3‰) and diatoms (−30.6‰ to −26.6‰). Cyanobacteria were most variable (−32.4‰ to −5.9‰), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (−14.4‰ to −5.9‰). For δ13C, the taxon-specific amplitude of temporal changes within a lake was usually

The impact of nutrient enrichment on the phytoplankton community structure, and particularly cyanobacteria, was studied in a 3-week mesocosm experiment conducted in August 2001 in the Archipelago Sea, a part of the northern Baltic Sea. The factorial design experiment included daily additions of nitrogen (N) and phosphorus (P) at two mass ratios, 1N:1P and 7N:1P, respectively, additions of iron (Fe) and a synthetic chelator, ethylenediaminetetraacetic acid (EDTA). The floating enclosures (400 l) were sampled for analyses of phytoplankton biomass and community structure, phytoplankton primary production, chlorophyll a , nutrients, and hepatotoxins. Chlorophyll a concentration, phytoplankton biomass and primary production increased most in the 7N:1P treatment. The increase was mainly due to an abundant growth of chlorophytes ( Dictyosphaerium subsolitarium , Kirchneriella spp., Monoraphidium contortum , and Oocystis spp.), pennate diatoms (especially Nitzschia spp.), dinophytes and the chroococcalean cyanobacterium Synechococcus sp. The nutrient enrichments had no effect on the total biomass of N 2 -fixing cyanobacteria. Nevertheless, the biomass of Anabaena spp. was highest in the enrichments with a low N/P ratio. Chlorophyll a concentration and total phytoplankton biomass were not affected by Fe or EDTA, but Fe alone had a positive effect on the chlorophyte Kirchneriella sp. The N 2 -fixing cyanobacteria Aphanizomenon sp. responded positively to Fe alone and to both Fe and EDTA added together. The hepatotoxin concentration increased during the experiment, but no clear responses to nutrient enrichments were found. Our study showed species-specific responses to nutrient enrichments among the N 2 -fixing cyanobacteria. Although the total phytoplankton production was not Fe-limited; the availability of Fe clearly affected the phytoplankton community structure.

The spatial distribution of phytoplankton and picocyanobacteria was studied in Lake Tanganyika in relation to environmental factors. Sampling was conducted within three weeks during the wet season of 1998 when the water column stratification was most stable. The secchi depth varied between 11 and 16 m, except off the river Malagarasi (3 m). The depth of the euphotic zone was 33 to 56 m. Altogether, 218 phytoplankton taxa were identified. Their total biomass varied between 13 and 88 mg m−3 fresh weight. Picocyanobacteria were present at very high densities (104 to 6 × 10 5 cells ml−1). The influence of the river Malagarasi appeared to spread along the water surface because of the lower density of the river water. The phytoplankton species number and biomass were particularly high near the river mouth. Excluding that area, there were no statistically significant differences in the species composition among study sites. Nevertheless, differences in water column mixing, indicated by variable temperature gradients, seemed to affect the horizontal distribution of at least some phytoplankton species.

Food web structure of the pelagic community in Lake Tanganyika was studied using the stable nitrogen and carbon isotopes 15N and 13C. Size-fractionated seston, zooplankton, shrimps, medusae and fish were sampled in the northern part of Lake Tanganyika. Picoplankton fractions as well as cyanobacteria-dominated nano/microplankton fractions had very low nitrogen isotope signatures typical for nitrogen-fixing organisms. Fractions containing mainly dead organic matter (and associated bacteria) or nano/microalgae (chlorophytes and diatoms) had δ15N 2 to 4‰ higher. The low δ15N signatures of small cyclopoids and shrimps suggest they are feeding on nitrogen-fixing cyanobacteria (picoplankton or larger forms), while the higher δ15N signature of larger copepods suggest mixed feeding on large algae and small zooplankton and/or cyanobacteria. Medusae were slightly enriched in δ15N relative to large copepods. Among fish, the signatures of Stolothrissa and small Lates stappersi suggested feeding on large copepods, while Limnothrissa and larger Lates were slightly more enriched, indicating partial piscivory. The enrichment of 13C between the putative trophic levels (2 to 3‰) was higher, while that of 15N (2 to 3‰) was lower, than usual in isotope studies. Our results indicate that picocyanobacteria and possibly also larger cyanobacteria are important producers in the pelagic food web of Tanganyika.

SUMMARY 1. We studied the effect of the small crustacean zooplankton on heterotrophic micro-organisms and edible phytoplankton in a eutrophic lake during a cyanobacterial bloom. 2. Small (15 L) enclosures were filled with natural or screened (100 μm) lake water and incubated for 5 days in the lake. Screening removed crustacean zooplankton but the initial density of rotifers and phytoplankton remained the same in control and removal treatments. Changes in the abundance and biomass of bacteria, autotrophic picoplankton (APP), heterotrophic nanoflagellates (HNF) and ciliates were measured daily. 3. The crustacean zooplankton, dominated by the small cladoceran Chydorus sphaericus, did not affect cyanobacteria, the main phytoplankton group during the experiment. 4. The removal of the crustacean zooplankton induced a higher abundance of ciliates and reduced that of the HNF, indicating the importance of ciliates in controlling HNF in this system.

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.

Determinations of chlorophyll a and in vivo fluorescence of photosynthetic pigments were used to study vertical and horizontal distribution of phytoplankton in Lake Tanganyika (East Africa). Blue excited fluorescence (IVFb) was an approximate predictor of chlorophyll a at different depths and locations. Green excited fluorescence (IVFg), which reflects phycoerythrin in cyanobacteria, explained chlorophyll a variation equally well, and in combination with IVFb the degree of explanation was improved to 87% (n = 90). Particularly during the shallow stratification in March–May, the maxima of chlorophyll a, IVFb and IVFg were located within the thermocline. Such distribution may have resulted from the high penetration of UV light, often accentuated by very shallow daytime thermal stratification, leading to inhibition of phytoplankton near the surface. Because the decrease of chlorophyll a specific IVFb was less striking towards the surface, the decrease of IVFb was not caused by light inhibition only. In October–November, epilimnetic IVFb and chlorophyll a values seemed to be consistently higher than in April–May and often showed remarkable patchiness. The sometimes very dense phytoplankton blooms (Anabaena sp., Cyanobacteria) observed in the central and southern parts of the lake, suggest that local upwelling or mixing events may be important for the development of phytoplankton in Lake Tanganyika.

We studied the potential nutrient limitation of phytoplankton by means of seston nutrient stoichiometry and nutrient enrichment bioassays in the epilimnion of Lake Tanganyika. In most cases, the particulate carbon to phosphorus (C:P) ratio was high and indicated moderate P deficiency, while the respective C:N ratio mainly suggested moderate N deficiency. The N:P ratios of seston indicated rather balanced N and P supply. In three two-day enrichment bioassays in April—May 1995, a combined addition of P, N and organic carbon (glucose) always increased primary production in comparison to untreated controls. Primary production also slightly increased after the addition of phosphate-P, while the additions of single ammonium-N and glucose had no effect. Although the measured turnover time of P was short and our few nutrient enrichment experiments suggested that P may be the most limiting single nutrient, the particulate nutrient ratios and the strong stimulation of primary production after the combined addition of P and N mostly suggest that in the upper epilimnion of Lake Tanganyika plankton experience a restricted, but approximately balanced nutrient supply.

Water chemistry, phytoplankton, zooplankton and fish populations were studied over several years in three shallow, non-stratified lakes with differing nutrient loadings and fish communities in southwest Finland. Lake Pyhajarvi was weakly mesotrophic in 1980–1996, Lake Koylionjarvi was highly eutrophic in 1991–1996, and Lake Littoistenjarvi was mesotrophic in 1993–1996 and eutrophic in 1992. In Lake Pyhajarvi, natural year-class fluctuations of vendace and smelt (range of combined biomass 5–28 kg ha−1) caused significant variation in planktivory. The very dense fish stocks of Lake Koylionjarvi (mainly roach, bream and smelt) were decimated from > 175 kg ha−1 in 1991 to about 50 kg ha−1 in 1996 by removal fishing. The roach stock of Lake Littoistenjarvi declined from about 71 kg ha−1 to about 28 kg ha−1 during 1993–1996. In Lake Pyhajarvi, strong stocks of planktivorous fish were accompanied with depressed crustacean zooplankton biomass, reduced role of calanoids and cladocerans, a low proportion of larger cladocerans (length > 0.5 mm), and a high chlorophyll level. In the lakes Littoistenjarvi and Koylionjarvi, zooplankton was dependent on both fish and phytoplankton: in spite of dense fish stocks, a high crustacean biomass developed in a phytoplankton peak year, but it was dominated by very small cladocerans. In Lake Pyhajarvi, late summer chlorophyll concentration was predictable from total phosphorus in water and cladoceran biomass (r2 = 0.68), both factors explaining roughly similar fraction of total variation. In combined data from all three lakes, chlorophyll was almost solely dependent on total phosphorus, while the cladocerans were regulated both from below by productivity and from above by fish. Our data from Pyhajarvi lend support to consumer regulation of late summer phytoplankton; low chlorophyll values prevailed when planktivorous fish biomass was below 15 kg ha−1. In large eutrophic lakes it may be difficult to reduce fish stocks to such a low level: in Lake Koylionjarvi, after six years of removal fishing, fish biomass still remained higher, and changes in plankton were accordingly small. Unexpectedly, in 1993–1996, phytoplankton biomass in Littoistenjarvi remained low in spite of low crustacean zooplankton biomass; submerged macrophytes probably regulated the water quality.

The vertical distributions of bacteria, picoalgae,protozoan and metazoan zooplankton, and phytoplanktonin the highly eutrophic Lake Koylionjarvi(SW Finland) were studied monthly during the period ofice-cover in January-April 1996. For comparison, wealso provide some data on the distributions of theplankton during the summer. The whole watercolumn remained oxic during the ice-covered period,although the near-bottom oxygen concentrations werealways very low. The heterotrophic nanoflagellateswere more abundant in winter than in summer, butciliates, picoalgae and bacteria were more numerous insummer. In general both zooplankton and phytoplanktonhad low biomass during the ice-covered period.However, the biomass of the diatom Aulacoseiraislandica ssp. islandica was high under the icein April. The calanoid copepod Eudiaptomusgraciloides was the dominant zooplankton species fromJanuary to March, but had almost disappeared by thebeginning of April and did not increase again until inJune. The dominant rotifer species in winterwere Keratella cochlearis, Filinia terminalis,and Filinia longiseta in the surface water andRotaria neptunia near the bottom.

Water level regulation related to hydroelectric power production and flood prevention is an important hydro-morphological pressure on many watercourses around the world. Fouling of fishing nets in autumn during the open water period and in winter under the ice is a common phenomenon in large Finnish lakes where the water level is regulated. This fouling of fishing nets can sometimes be so extensive that fishing has to be stopped. Based on the practical experiences of fishermen, the main cause for the under-ice fouling has been proposed to be the winter draw-down of water causing low water level and stronger currents in lakes, but no conclusive relationship between fouling and water level regulation has yet been demonstrated. Here we show, using long-term winter data from a boreal lake and short-term netting experiments, that fouling of fishing nets results from increased water draw-down during winter (January–March). Our results also show that fouling of fishing nets takes place only if the high water flow rates are accompanied by an intensive lowering of the water level. We also discuss the relevance of our results to other regulated lakes with ice cover. peerReviewed