Finnish Lakes Research Articles

Increasing lake water and sediment oxygen levels using slow release peroxide

The sediment and hypolimnion of many Finnish lakes suffer from anoxia due to increasing nutrient loading. The aim of this research was to develop a method for increasing the oxygen level using granulated calcium peroxide (CaO2) as a slow oxygen releasing compound. This compound releases oxygen (O2) in a reaction with water during 5 to 7months. The method was tested in both laboratory and field conditions. In the field test granulated CaO2 were then spread manually from a rowing boat over the whole surface of the test pond. The granules sink onto and into the sediment. No mixing was needed. The dissolved oxygen concentration increased significantly during a laboratory experiment with a CaO2 amendment of 75gm−2 and in a pond experiment with a CaO2 amendment of 50gm−2. In the pond experiment, the effect was visible for the entire 40-week experiment. In the laboratory, the abundance of aerobic bacteria increased in the sediment after CaO2 addition, while the pond experiment gave more mixed results. The organic matter content of the sediment did not change during the experiment in the control pond, but decreased from 18% to 4% in the pond with the CaO2 amendment. This was possibly due to enhanced microbial activity in the test pond. Although the results show improved oxygen concentrations and effects on the sediment organic matter following CaO2 amendment, the usability of this method in larger lakes remains to be tested.

Past, Present, and Future Exceedance of Critical Loads of Acidity for Surface Waters in Finland

A critical load is a deposition limit below which harmful effects for a given ecosystem do not occur; the approach has underpinned European sulfur (S) and nitrogen (N) effects-based emission reduction policies during the last two decades. Surface waters are an important resource in Finland, as such the development of models and determination of critical loads has played a central role in supporting their recovery from acidification or preservation of ecosystem health. Critical loads of acidity for Finnish lakes were determined using the steady-state First-order Acidity Balance (FAB) model in conjunction with comprehensive national surveys of surface waters (headwater lakes; n = 1066) and soils. In the 1980s almost 60% of the study lakes were exceeded, impacting brown trout and perch populations. The steep decline in emissions and acidic (S and N) deposition during the last two decades has reduced exceedance to <10%, and by 2020 exceedance is predicted to reach preindustrial (1880) levels. In concert with these reductions, chemical and biological recovery has been observed. The critical load approach has been instrumental in assessing impacts to surface waters in Finland and directing effects-based emission reduction policies.

Subfossil chydorid (Cladocera, Chydoridae) ephippia as paleoenvironmental proxies: evidence from boreal and subarctic lakes in Finland

Chydorids (Cladocera, Chydoridae) have two reproductive strategies: asexual reproduction that prevails during favorable environmental conditions and sexual reproduction that is induced by environmental stimuli associated with seasonal or aperiodic environmental stresses. These modes of reproduction can be recognized in the subfossil sedimentary records as parthenogenetic shells of females (asexual reproduction) and by ephippia (sexual reproduction). We studied the interrelations between subfossil chydorid ephippia and environmental variables by analyzing surface sediment samples obtained from 76 Finnish lakes across a latitudinal gradient (60–70°N). The results showed that the total chydorid ephippia (TCE) increases along the climate gradient from ~2 to 3% in the south to ~25% in the north and suggested a significant dependence (r ~ −0.8, P < 0.001) with several climate factors, especially that of mean July air temperature. We used this relationship to create a model for reconstructing past mean July air temperatures. A linear regression of the log10 transformed TCE as a single independent variable explained 76% (SE ± 0.76°C) of the variance of the observed mean July air temperatures. Accordingly, we propose that this novel tool may be highly suitable for reconstructing paleotemperatures in cold-temperate environments.

Paleolimnological evidence for increased sexual reproduction in chydorids (Chydoridae, Cladocera) under environmental stress

To investigate the extent to which anthropogenic perturbations such as eutrophication and trace metal pollution (i.e., environmental stress sensu Odum 1985) influence the reproductive modes of cladoceran populations, we analyzed the abundance of subfossils of the chydorids Alonella nana (Baird, 1850) and Alona affinis (Leydig, 1860) in sediment cores from three Finnish lakes. Reconstruction of lakes' pollutant history and the biological response of chydorids indicate that in two of the lakes the proportion of individuals reproducing sexually increased with environmental stressors. More specifically, A. nana responded to eutrophication in Lake Hampträsk with greater production of ephippia, while A. affinis responded to aluminum pollution or acidification in Lake Pieni Majaslampi. In contrast, the reference lake, Lake Iso Lehmälampi, showed no radical changes in sexual reproduction over the twomillennium long sediment record. We conclude that chydorids may use sexual reproduction as a strategy for overcoming unexpected environmental stresses.

Effects of mining on testate amoebae in a Finnish lake

Sediment geochemistry and testate amoebae (thecamoebians) were used to track the spatial extent and temporal evolution of mine loading and its impact on aquatic biota in Lake Pyhajarvi, central Finland. Pyhasalmi is an operating Zn-Cu–S mine, located by Lake Pyhajarvi. Mine loading to the lake comes mainly from wastewater. Highest metal loading occurred in the 1970s and 1980s. Present loading consists mainly of Ca and S that nearly saturate dense wastewaters with gypsum. This affects oxygen concentration in the lake’s northern basin, where effluent is presently discharged. Of 25 short sediment cores retrieved from Lake Pyhajarvi, 10 came from sites ‘downstream’ and 15 from ‘upstream’ of the mine. Three sample levels from each core were selected for further investigation after exploratory XRF metal analysis. Samples were chosen to represent lake conditions before the mine impact, during the peak impact phase, and at present. Results from pre-disturbance deposits show that the two lake basins are naturally different. The ‘downstream’ basin (Junttiselka) has lower oxygen and a higher sedimentation rate, which was revealed by concentrations of redox-sensitive and clastic-related elements, as well as faunal distributions. The peak mine impact is reflected in the high concentration of S and metals Cu, Zn, Cd, Pb, which spread widely in the lake and increased > 100% at sites near the tailings area. These concentrations have since decreased, but not to pre-disturbance levels. The faunal response to peak metal concentrations was weak, spatially limited and masked by larger responses to other environmental factors, including eutrophication, oxygen concentration, and increased clastic input generated by land-use changes. These environmental factors and changes in mine water composition probably account for the reason that faunal assemblages did not return to the pre-mine situation. Instead, the modern fauna reflects a third assemblage type that developed after the peak-loading phase. Geochemical and thecamoebian analyses show that the mine has affected the two basins of the lake differently during the lake history. Results suggest that mining impacts could be mitigated with good planning and management.

Equilibrium sampling of environmental pollutants in fish: Comparison with lipid‐normalized concentrations and homogenization effects on chemical activity

Equilibrium sampling of organic pollutants into the silicone polydimethylsiloxane (PDMS) has recently been applied in biological tissues including fish. Pollutant concentrations in PDMS can then be multiplied with lipid/PDMS distribution coefficients (D(Lipid,PDMS) ) to obtain concentrations in fish lipids. In the present study, PDMS thin films were used for equilibrium sampling of polychlorinated biphenyls (PCBs) in intact tissue of two eels and one salmon. A classical exhaustive extraction technique to determine lipid-normalized PCB concentrations, which assigns the body burden of the chemical to the lipid fraction of the fish, was additionally applied. Lipid-based PCB concentrations obtained by equilibrium sampling were 85 to 106% (Norwegian Atlantic salmon), 108 to 128% (Baltic Sea eel), and 51 to 83% (Finnish lake eel) of those determined using total extraction. This supports the validity of the equilibrium sampling technique, while at the same time confirming that the fugacity capacity of these lipid-rich tissues for PCBs was dominated by the lipid fraction. Equilibrium sampling was also applied to homogenates of the same fish tissues. The PCB concentrations in the PDMS were 1.2 to 2.0 times higher in the homogenates (statistically significant in 18 of 21 cases, p < 0.05), indicating that homogenization increased the chemical activity of the PCBs and decreased the fugacity capacity of the tissue. This observation has implications for equilibrium sampling and partition coefficients determined using tissue homogenates.

Evaluation of approaches for measuring taxonomic completeness of lake profundal macroinvertebrate assemblages

Summary 1. Benthic macroinvertebrates (MI) are commonly used to assess freshwater ecosystems with the reference condition approach. Such assessments necessitate control for natural community variation, either by categorical typologies or by predictive models that have been widely and successfully developed for running water biota but not previously for lake profundal invertebrates. 2. We evaluated four modelling techniques [multivariate regression tree (MRT), limiting environmental differences, nonparametric multiplicative regression (NPMR) and River Invertebrate Prediction And Classification System (RIVPACS) and the operative Finnish lake typology for assessing taxonomic completeness (observed-to-expected number of taxa, O/E) of profundal MI assemblages. We used data from 74 and 33 minimally disturbed reference lake basins for calibration and validation of the approaches, respectively, and 72 test basins subject to various anthropogenic pressures to evaluate sensitivity to detect impact. Either all predicted taxa (threshold probability of capture Pt = 0+) or only those predicted to be captured with ≥0.25 probability were used to calculate O/E. 3. With Pt = 0.25, all four modelling approaches were accurate (mean O/E = 0.966–1.053) but imprecise (SD of O/E = 0.279–0.304) in predicting the fauna actually observed in validation sites. All models were subtly more precise than a null model (mean 1.038, SD 0.343) or the typology (1.046, 0.327). The taxon-specific NPMR model was slightly more precise than the other three models based on site groupings. 4. The O/E values correlated relatively weakly (r = 0.55–0.86) among the approaches, which thus produced contrasting lake-specific assessments, despite their seemingly comparable performances. Indeed, typology, suggesting that MI assemblages were impaired in 56% of test sites, was more sensitive than the other approaches (26–46%) as an indicator of human-induced deterioration. However, this greater ostensible sensitivity seemed to be biased, as lake morphometry, a main driver of natural community variation, remained uncontrolled by the typology. 5. Generally, our exercise illustrates the inconclusiveness of the common validation criteria for the assessment methods. The apparent poor predictability of the profundal fauna, irrespective of the method, may partly stem from large observation error, which could be alleviated by more intensive sampling. However, instead of an O/E-taxa index, some other metric encompassing quantitative aspects might be preferable for assessing these species-poor communities.

Bacterioplankton community composition in 67 Finnish lakes differs according to trophic status

The bacterioplankton composition of 67 Finnish lakes was characterised by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments, and subsequent sequence analyses of major fragments. The DGGE patterns grouped as a function of environmental characteristics describing the trophic status of the lakes, such as total phosphorus (TP), total nitrogen (TN), TN:TP, chlorophyll a, Secchi depth and water colour. Most sequences retrieved represented Verrucomicrobia, Actinobacteria and Cyanobacteria, while those representing Alpha- and Gamma- proteobacteria, Chloroflexi and Acidobacteria were less frequent. The presence of several sequences could be linked to the trophic status of the lakes, while that of others was more common and thus unrelated to the trophic status. These results suggest that individual responses towards environmen- tal factors may occur among the bacterioplankton at the level of phyla as well as phylotypes.

Long-term trends and regional differences of phytoplankton in large Finnish lakes

Major nutrients (N and P) and phytoplankton from 19 large lakes from southern (61°) to northern (69°) Finland were analyzed to detect long-term trends and regional differences. The data sets from June, July and August cover the period from the early 1980s to the present. Altogether >700 phytoplankton and >4000 N, P and Chl a results were used for the study. In 40% of the lakes, the total phosphorus (TP) concentration decreased significantly and in >25% of the lakes a significant reduction was found in the total nitrogen (TN) concentration. At the same time, the phytoplankton biomass declined only in 15% of the lakes and the long-term trends in chlorophyll a more often increased than decreased. A clear gradient from southern to northern Finland and western to eastern Finland was found in the phytoplankton biomass. During the study period, the biomasses of cyanobacteria and centrales (diatoms) decreased whilst there was an increase in the biomass of pennales (diatoms) in one-third of the lakes. The proportion of chlorophytes in the total biomass also increased in >20% of the study lakes. In southern and western Finland, the total biomass and the contribution of cyanobacteria were higher. Centrales made a higher contribution to the total biomass in the north. Pennales and chlorophytes were less abundant and chrysophytes more abundant in the east. Differences in the community composition reflected the gradients in the total nutrients, and particularly in TP. The observations support the assumed role of phosphorus as the key limiting nutrient in large Finnish lakes irrespective of lake′s location. The N:P ratio proved to be a poor predictor of cyanobacteria occurrence in the study lakes.

Quality and status of fish stocks in lakes: gillnetting, seining, trawling and hydroacoustics as sampling methods

All sampling methods give selective or biased estimates of fish species abundance, distribution and size structure. This creates problems, e.g. in regard to the Water Framework Directive of the European Union, which demands evaluation of the quality and status of fish stocks in lakes. We compared fish sampling by means of Nordic multimesh gillnets, seining, trawling and hydroacoustics in two Finnish lakes in summer 2007 and 2008. Sampling methods were used ‘as such’, i.e. no special design was implemented for method comparison. In the shallow eutrophicated lake the species’ composition of gillnet sampling and seining were very different. The biomass-% of percids dropped from gillnet (61%) to seining (9%) and that of cyprinids grew from 39 to 90%, respectively. In the deep pelagic area of the oligotrophic lake, vendace and smelt predominated in trawl catches. The number of fish caught by gillnetting in that area was too small to make any conclusions about the species composition. In the eutrophicated lake, the combined length distribution for all fish species differed significantly between gillnetting and seining. In the oligotrophic lake, the gillnet catches were too small for any comparison of fish size. The difference in the length distribution of fish between trawl and echosounding was significant in most analysed depth layers. In upper depth layers acoustics sampled larger fish than trawling, and in deeper layers smaller fish. Using a combined acoustic-trawl method, the pelagic fish biomass was estimated to be approx. 17 kg ha−1 in the deep and oligotrophic lake. We conclude that in large and deep-water areas, the use of active gear is enough in fish sampling to evaluate the quality and status of fish stocks. Gillnetting together with seining is an appropriate method to work out the quality and status of fish stocks in shallow and littoral areas of large lakes. Variation in the catch selectivity of fish sampling gear requires a discrete ecological classification for each type of gear.

Ecological classification of large lakes in Finland: comparison of classification approaches using multiple quality elements

Ecological classifications according to the Water Framework Directive (WFD) are presented for a set of 32 large (surface area >75 km2) Finnish lakes. We compared three different approaches: classification according to the strictest biological quality element (One-out, All-out approach, OoAo); numerical integration of biological quality elements (BQE) to determine median scores; and the national classification based on weight-of-evidence (WoE) framework. We also examined the sensitivity of eutrophication metrics to phosphorus concentrations. The WoE based classification proposed that the ecological status in most lakes is high (12/32) or good (17), whereas the integration of BQEs ranked more lakes (18) to high status. Of the biological elements, macrophytes and phytoplankton indicated the status closest to those given by the national WoE classification whereas generally fish indicated higher and macroinvertebrates lower status. Compared to the OoAo classification, the national WoE approach gave similar, downgraded or upgraded status classes. Downgrading was due to the higher weight given to water quality and eutrophication pressures. Upgrading was due to the lower weight given to a single macroinvertebrate metric with poorly represented data. In our opinion, the classification based on the WoE approach produces more realistic status estimates than the OoAo classification. Nevertheless, in practical lake management the evidence from the strictest classification metrics still needs to be thoroughly considered.

The relationship between water quality and chironomid distribution in Finland—A new assemblage-based tool for assessments of long-term nutrient dynamics

Abstract Relationship between water quality variables and distribution, diversity and abundance of sedimentary chironomids (Diptera: Chironomidae) were examined in 51 limnologically different shallow lakes in Finland. The objective was to identify which of the water quality parameters influenced chironomids the most and to explore the potential of developing a palaeoecological tool to infer past changes in water quality for the use of lake management in conservation, preservation and restoration projects. In addition, taxon-specific optima and tolerances were calculated to identify indicators for ultraoligotrophic–hypereutrophic Finnish lakes. The statistical tests indicated that the chironomid assemblages were closely related to water quality. Of the examined environmental variables total phosphorus (TP), total nitrogen (TN), colour, turbidity and hypolimnetic oxygen were the most important explanatory variables. Because autumnal epilimnetic TP had the highest eigenvalue (λ1:λ2) ratio, which indicates the relative significance of particular variable in explaining the variance in the species data, and lowest statistical significance level, it was considered to have the greatest potential for the development of a calibration model for quantitative inferences of past limnological conditions. In addition to the complete model, a reduced model using 41 lakes was developed that is independent of covariance with temperature. Of the tested model types, weighted averaging-partial least squares (WA-PLS) had the strongest relationship between observed and inferred TP values together with lowest error of prediction and maximum bias showing favourable model performance. In general, the results of taxon-specific indicator value showed close agreement with previous data. The results provided coherent autecological and synecological data that can be used in qualitative assessments of water quality and ecological status of lakes and most importantly as a basis for the new quantitative tool to be used in palaeolimnological studies and evaluations of TP reference conditions.

Concentrations of organotin compounds in various fish species in the Finnish lake waters and Finnish coast of the Baltic Sea

Organotin compounds (OTCs) leaching from the antifouling paints used in boats and ships have contaminated many water areas worldwide. The purpose of this study was to obtain a general view of the organotin contamination in fish in Finnish lake areas and Finnish coast of the Baltic Sea using perch as the main indicator species. Perch sampling covered areas presumed as less contaminated and areas suspected as more contaminated. Besides perch, 12 other species were sampled from sites presumed as less contaminated. OTCs measured were mono-, di- and tributyltin, mono-, di-, and triphenyltin and dioctyltin. The sum concentration of OTCs (ΣOTCs) in perch in the least contaminated areas of the Baltic Sea were around 20 ng/g fresh weight (fw) and less than 10 ng/g fw in lake areas. In heavily contaminated areas of the Baltic Sea 150–500 ng/g fw in perch were detected. In lake areas the maximum ΣOTCs in perch was only 30 ng/g fw. With regard to the other species in the Baltic Sea, salmon, sprat, flounder, whitefish, vendace and lamprey contained low concentrations (ΣOTCs mainly less than 20 ng/g fw), whereas in pike, pike-perch, burbot and bream concentrations were higher. ΣOTCs in lake fish were generally lower than in the Baltic Sea. In a distance gradient study, ΣOTCs in perch decreased quickly from nearly 200 ng/g fw at a contaminated harbor area to 35 ng/g fw during a distance of 5 km. Further decrease was slower and reached 15 ng/g fw at 100 km. In a size dependence study triphenyltin showed better correlation with the fish length than tributyltin for all species studied, i.e. for perch (0.16 vs 0.26), pike-perch (0.13 vs 0.24) and roach (0.46 vs 0.80). High correlation for roach may be partly explained by smaller number of samples collected and small length range.

Fish‐based assessment of ecological status of Finnish lakes loaded by diffuse nutrient pollution from agriculture

Abstract The European Union Water Framework Directive stipulates that pressures on surface waters due to diffuse loading of nutrients have to be taken into account in determination of their ecological status. A Finnish data set of fish communities in 178 lakes was used to assess the effects of agriculture‐induced nutrient loads on the ecological status of lakes. The lakes were divided to reference (n = 100) and impacted sites (n = 78) based on water quality, land use and nutrient load data. Fish sampling was conducted by standardised gill net test fishing. A fish‐based classification tool of the four parameters (EQR4) was applied to assess the ecological status of lakes. The parameters included were: mean total biomass of fish per gill net night, number of fish individuals per gill net night, biomass proportion of cyprinid fishes and the presence of indicator species. The analysis resulted in a median EQR4 value of 0.78 (good status) for reference lakes and 0.56 (moderate status) for affected lakes. Thus, the classification tool gave an acceptable output for different nutrient‐loaded lakes.

Screening for biohydrogen production by cyanobacteria isolated from the Baltic Sea and Finnish lakes

Cyanobacteria are the only bacteria capable of performing oxygenic photosynthesis in which they harness solar energy and convert it into chemical energy stored in carbohydrates. Under specific conditions, cyanobacteria can use solar energy to produce also molecular hydrogen. Biodiversity among cyanobacteria for H 2 production has not been efficiently studied. Here we report the screening of 400 cyanobacterial strains isolated from the Baltic Sea and Finnish lakes for efficient H 2 producers. Approximately 50% of these strains produced detectable amounts of H 2. Ten strains produced similar or up to 4 times as much of H 2 as the hydrogenase mutants of Anabaena PCC 7120 and Nostoc punctiforme ATCC 29133 specifically engineered in different laboratories to produce higher amounts of H 2. All ten H 2 producers are N 2-fixing filamentous, heterocystous strains, seven of them are benthic and three are planktonic strains. Different culturing parameters, such as light intensity, cell density, pH and temperature had a pronounced effect on the H 2 production rates of the two good H 2 producers, Calothrix 336/3 and XPORK 5E strains. Notably, the culture conditions for optimal H 2 production varied between different cyanobacterial strains.

Palaeolimnological validation of estimated reference values for a lake profundal macroinvertebrate metric (Benthic Quality Index)

Modern assessment and monitoring of aquatic ecosystems is increasingly based on biota and the “reference condition” approach, in which the observed values (O) of biological variables are compared to those expected in the absence of human disturbance (E). To use this approach, correct estimation and validation of reference conditions are critical. Because appropriate modern or historical data are never available for this approach, palaeolimnological data offer an alternative. We used a calibration data set from 73 profundal sites in semi-pristine Finnish lakes to construct a regression model for estimating expected values for the chironomid Benthic Quality Index (BQI)—a macroinvertebrate metric widely used in bioassessment—from environmental variables that are insensitive to human disturbance. For comparison, reference values were estimated using the European legislative rationale based on a priori lake typology. Performance of the alternative approaches was assessed by internal ‘leave-one-out’ cross-validation using the calibration set and by external cross-validation using independent palaeolimnological data on BQI values representing the historical pristine status of 24 lake basins. Additionally, for 19 of these sites, which vary in their degree of human impact, the ratio of present BQI to that in pristine condition, which shows the degree of actual change, if any, was calculated from palaeolimnological data and compared with the O/E ratios based on the present chironomid data and estimated E. A linear regression model with mean depth and mean/maximum depth ratio as independent variables estimated the reference values of BQI much closer to the observed ones (r2 = 0.58, RMSEP = 0.65 and r2 = 0.71 RMSEP = 0.55; for internal and external cross-validation, respectively) than did the typology approach (r2 = 0.28, RMSEP = 0.86; r2 = 0.10, RMSEP = 0.97). The regression approach also yielded O/E ratios more similar to the actual ones (r2 = 0.79, RMSEP = 0.09) than did the typology approach (r2 = 0.62, RMSEP = 0.23). Our results strongly support the use of lake morphometric variables and modelling instead of categorical lake typology for the establishment of reference conditions for profundal macroinvertebrate communities and demonstrate the utility of palaeolimnological data in the validation of reference values and assessment methods.

Radiation transfer and heat budget during the ice season in Lake Pääjärvi, Finland

Lake Paajarvi, a boreal Finnish lake, was investigated in winter for weather conditions, structure and thickness of ice and snow, solar radiation, and under-ice current and temperature. Heat budget of Lake Paajarvi in January–March was governed by terrestrial radiation losses of 20–35 W m−2 recompensed by ice growth of 0.5–1.0 cm day−1. In April, snow melted, albedo decreased from 0.8 to <0.1, and the mean ice melt rate was 1.5 cm day−1. Internal melting and surface melting were about equal. The mean turbulent heat loss was small. The heat flux from the water to ice was about 5 W m−2 in winter, increasing to 12 W m−2 in the melting season. The light attenuation coefficient was 1.1 m−1 for the congelation ice (black ice) in winter, compared with 1.5 m−1 for the lake water, and it was up to 3 m−1 for candled congelation ice in spring, and about 10 m−1 for superimposed ice (white ice) and snow. Gas bubbles were the main factor that reduced the transparency of ice. The radiation penetrating the ice heated the water body causing convective currents and horizontal heat transfer. This increased the temperature of the water body to about 3°C before the ice break-up. After the snow had melted, the euphotic depth (the depth of 1% surface irradiance) was estimated as 2.0 m, only two-thirds that in summer.

Transfer of 90Sr into fish in Finnish lakes

The long-term behavior of 90Sr was investigated from 1987 to 1997 in fifteen lakes in southern and central Finland following the 1986 Chernobyl accident. Both water and fish samples (perch, pike, vendace) were analyzed. 90Sr stays long in the freshwater ecosystem; the observed half-lives during the study period were generally around 10 years in water and even longer in fish. One lake exhibited very different behavior for 90Sr, with elevated levels in fish and water and very short observed half-life in fish, less than 2 years. Concentration factors of 90Sr in fish (Bqkg −1 f.w. in fish/Bqkg −1 in water) among the studied lakes significantly correlated with both the Ca concentration and electrical conductivity of the water. More 90Sr was transferred into fish in lakes with a low electrical conductivity and a low Ca concentration. Among other water parameters evaluated were pH, color, total nitrogen, and phosphorus.

Natural variation of profundal macroinvertebrate communities in boreal lakes is related to lake morphometry: implications for bioassessment

Modern biological assessment of aquatic systems is often based on the reference condition approach, which requires characterization of biota in undisturbed conditions. For this approach, it is essential to recognize the origins and degree of natural variation in communities. We used data from 55 minimally disturbed Finnish lake basins to investigate the natural variation of profundal macroinvertebrate community composition in relation to environmental factors. Partial canonical correspondence analysis showed that most (68% combined, 39% uniquely) of the total explained compositional variation (29%) was correlated with environmental variables insensitive to human activities, especially lake morphometry. In contrast, the unique contributions of geographical location and variables influenced by human activities (dissolved oxygen and total phosphorus) were substantially smaller (5.5% and 22%, respectively). All of the explained variation (38%–64%) of three widely used community metrics was also correlated with human-insensitive environmental factors. The results suggest that variation in undisturbed profundal invertebrate communities of reference sites is strongly dependent on variables insensitive to human activities and especially on those related to lake morphometry, suggesting that reference communities for bioassessment of impacted lakes should be rather predictable from these variables.

Application of optical classifications to North European lakes

Two classifications (rough and fine) based on the optical properties of water were used to classify 42 Estonian, Finnish, and Swedish lakes. The rough classification was based on the amount of optically active substances (OAS: chlorophyll a, suspended matter, and colored dissolved organic matter) in the water. The basic variables of the fine classification were chlorophyll a concentration, beam attenuation coefficient of light in the photosynthetically active radiation (PAR) region in the water (or corresponding Secchi depth), and the beam attenuation coefficient for filtered water at 380 nm. All optical classes were represented in the studied lakes by both classifications, when they changed their class depending on the seasonal and biological conditions. In a large lake, different parts of the lake belonged to different optical classes. The results obtained by both classifications were in good agreement. There was a concurrence between optical classes and water transparency by Secchi depth. Often the bio-optical variations of water properties are described from the measurements of all three OAS and Secchi depth because these parameters belong to routine monitoring datasets and have been carefully recorded. The water classes provide a method to summarize the influences of the different factors.