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A method of the simultaneous reduction of external and internal load of phosphorus as a new solution for lake restoration is presented. A flow-through, stratified lake, with diverse morphometry was selected for the study. In all three parts of the lake, an oxygen deficit was noted at the lake bottom at peak summer stagnation, with a simultaneous increase in mineral forms of phosphorus (the concentration of orthophosphates at sites I, II and III reached 0.301, 0.318 and 0.395 mg P l−1, respectively). Additionally, a large load of nutrients originating from the main inflow (annual load of 14395.9 kg P y−1 and 16 4829.8 kg N y−1), indicates that the conditions present in the lake are conducive to more intensive eutrophication. In this situation, reduction of the main loads of nutrients can be achieved by the simultaneous employment of pipelines delivering oxygenated water to the lake bottom during summer stagnation and a pipeline withdrawing deoxygenated and nutrient-rich water from the bottom in the vicinity of the outflow from the lake. It has been calculated that during the period of summer stagnation (VI-IX), the operation of delivering pipelines can reduce the phosphorus load from the main inflow by 5800.5 kg (40.3%). Using the pipeline withdrawing (site III), the amount of phosphorus removed from the lake would increase by 30.4% (from 2259.6 kg to 3247.8 kg). An additional solution, can be dosing the precipitating substances into the pipes delivering water to the lake, which will enable precise inactivation of phosphorus.

The aim of the study was to assess the efficiency of mineral aggregates in orthophosphate (OP) removal from hypolimentic water (HW) withdrawn from eutrophic lakes. Four P reactive materials were tested in sorption experiments: light weight expanded aggregate (LECA), zeolite (ZL), dolomite (DM) and limestone (LS). The two materials proved to be most effective in OP binding were LS and LECA. For these agents we conducted further investigations to reveal the effect of variable pH and temperature in the range commonly found in HW withdrawn from eutrophic lakes (6.0–8.0 and 5–15°, respectively). Kinetic studies under varying OP concentrations (0.1–2.0 mg PO43− L−1) in synthetic solutions and HW were involved to check influence of hypolimnion chemistry on OP fixation. LS was the most efficient in OP removal, followed by LECA, dolomite and zeolite. OP sorption by LS and LECA increased with decreasing pH and increasing temperature and was faster and higher in HW as compared to standard solutions. The results indicate that OP fixation onto these materials would be most efficient in case of lakes with acidic and relatively warm bottom layer. Hypolimnion warming, which is often observed in restored lakes, will further enhance OP removal. Use of LS mixed with LECA was recommended for a potential treatment of HW withdrawn from eutrophic lakes. OP retention before hypolimnion transport downstream can mitigate side effects of hypolimnion removal by preventing eutrophication of receiving water bodies which can be regarded as substantial improvement of this lakes` restoration method.

The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity1,2, nutrient biogeochemistry3, greenhouse gas emissions4, and the quality of drinking water5. The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity6,7, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification8,9 or oxygen may increase as a result of enhanced primary production10. Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world’s oceans6,7 and could threaten essential lake ecosystem services2,3,5,11., The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity, nutrient biogeochemistry, greenhouse gas emissions, and the quality of drinking water. The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification8,9 or oxygen may increase as a result of enhanced primary production10. Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world’s oceans and could threaten essential lake ecosystem services.

In this study, pressure driven ceramic membranes for microfiltration (MF) and ultrafiltration (UF) were used to improve the quality of waters in lakes. Four urban lakes located in the city of Olsztyn and belong to the Masurian Lake District: Track (53°47′21″N; 20°32′24″E), Podkowka (53°47′2″N; 20°27′1″E), Tyrsko (53°48′18″N; 20°25′21″E), and Redykajny (53°48′55″N; 20°25′4″E) were examined. The effects of the composition of lake water from different photic zones and of the type of membrane on the efficiency of removal of total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were investigated. The average efficiencies of rejection were 71.6 ± 5.8% (TOC), 16.0 ± 5.7% (TP) and 19.6 ± 9.3% (TN), independently of the composition of the feed and the type of membrane. Due to the similar average permeate fluxes of 2.2 ± 0.1 L/(m2 min) and water recoveries of above 85% in MF and UF, the use of MF is suggested because it has lower operational costs than UF. Because membrane filtration of waters from the zones with the highest chlorophyll contents was very effective both in terms of efficiency of pollutant rejection and hydraulic parameters of membranes, this technology can be used for selective treatment of parts of water with the highest phytoplankton abundance.

The article presents a novel, practical, sustainable and multi-purpose conceptual method for urban lakes water treatment. It is based on the separation of the inflowing water from the lake's main body with an impermeable barrier and a construction of enhanced littoral zone. The impermeable barrier alongside the shoreline creates a new littoral zone between the land and the lake water. Inflowing water as well as nutrient-rich seepage water from a direct catchment will be pretreated in a specially constructed biotic and abiotic zones. The naturally formed community of macrophytes in the littoral zone ensures biological pretreatment. The remaining phosphorus load will be eliminated in the abiotic part of the system composed of natural, cost-efficient and easily accessible sorption material. Based on nutrients budget calculated for a few urban lakes, we estimate that the diversion of the main nutrient-rich river inflow and the cutoff of the littoral zone will reduce a load of nitrogen and phosphorus by more than 80% and 90% respectively.

The attempts to restore Lake Wolsztynskie, by direct dosing to the water iron sulphate (PIX 113) and chloride polyaluminium (PAX 18) in 2005–2006, and then iron chloride to the lake's bottom sediments in 2012–2013, resulted in only short-lasting improvement of water quality. In 2016, the physicochemical and biological parameters have shown that the lake should be classified into eutrophic and even hypertrophic, primarily because of the high concentrations of phosphorus and chlorophyll a as well as low visibility. Although in theory, the methods of restoration were selected correctly, in practice, the improvement of water quality in a flow-through lake without eliminating the influx of water from allochthonous sources of pollution was impossible. The total load of nutrients arriving in Lake Wolsztynskie from the direct catchment, that is point and atmospheric sources, was 1877 kg P and 17 137 kg N, respectively. At the moment, the actual load of phosphorus in Lake Wolsztynskie is about 12-fold higher than the one deemed as tolerable by the ecosystem. About 86% of the phosphorus load is delivered from the main river (Dojca River), thus, elimination of this source of pollution is the necessary condition for attaining lasting improvement of the water quality in this lake. Analyzing the hydrological conditions, landscape relief as well as economic and environmental conditions, a possible solution of cutting the Dojca River waters from Lake Wolsztynskie was given.

The authors acknowledge COST Action ES 1105 "CYANOCOST Cyanobacterial blooms and toxins in water resources: Occurrence impacts and management" and COST Action Global Change Biology ES 1201 NETLAKE -Networking Lake Observatories in Europe" for contributing to this study through networking and knowledge sharing with European experts in the field. We acknowledge the members of the Global Lake Ecological Observatory Network (GLEON) for their collaborative spirit and enthusiasm that inspired the grassroots effort of the EMLS. E.M. was supported by a grant from the Swiss State Secretariat for Education, Research and Innovation to Bas Ibelings and by supplementary funding from University of Geneva. We thank Wendy Beekman for the nutrient analysis. We thank Pieter Slot for assisting with the pigment analysis. We thank Dr. Ian Jones for valuable feedback on an earlier version of the manuscript. We thank the Leibniz Institute of Freshwater Ecology and the Aquatic Microbial Ecology Group for logistic and technical support of J. Fonvielle and H.-P. Grossart, and the Leibniz Association for financial support. H.P. was supported by the US National Science Foundation (1840715, 1831096). A.C.'s work was funded by the Spanish Agencia Estatal de Investigacion and EU funds through the project CLIMAWET (CGL2015-69557-R). The collection of data for Lough Erne and Lough Neagh were funded by the Department of Agriculture, Environment and Rural Affairs, Northern Ireland. We are grateful to Kristiina Vuorio from the Freshwater Centre of the Finnish Environment institute for her help in organizing, collecting and analysing samples by the University of Jyvaskyla and to Gerald Dorflinger from the Water Development Department of Cyprus for his assistance with the sampling in Cyprus and for granting the CUT team permission to use WDD's equipment. Finally, we would like to thank the numerous other assistants that helped realizing each local survey. Open access funding provided by Universite de Geneve., To determine the drivers of phytoplankton biomass, we collected standardized morphometric, physical, and biological data in 230 lakes across the Mediterranean, Continental, and Boreal climatic zones of the European continent. Multilinear regression models tested on this snapshot of mostly eutrophic lakes (median total phosphorus [TP] = 0.06 and total nitrogen [TN] = 0.7 mg L-1), and its subsets (2 depth types and 3 climatic zones), show that light climate and stratification strength were the most significant explanatory variables for chlorophyll a (Chl a) variance. TN was a significant predictor for phytoplankton biomass for shallow and continental lakes, while TP never appeared as an explanatory variable, suggesting that under high TP, light, which partially controls stratification strength, becomes limiting for phytoplankton development. Mediterranean lakes were the warmest yet most weakly stratified and had significantly less Chl a than Boreal lakes, where the temperature anomaly from the long-term average, during a summer heatwave was the highest (+4 degrees C) and showed a significant, exponential relationship with stratification strength. This European survey represents a summer snapshot of phytoplankton biomass and its drivers, and lends support that light and stratification metrics, which are both affected by climate change, are better predictors for phytoplankton biomass in nutrient-rich lakes than nutrient concentrations and surface temperature., European Cooperation in Science and Technology (COST) ES 1105 ES 1201, Swiss State Secretariat for Education, Research and Innovation, University of Geneva, Leibniz Association, National Science Foundation (NSF) 1840715 1831096, Spanish Agencia Estatal de Investigacion, European Commission CGL2015-69557-R, Department of Agriculture, Environment and Rural Affairs, Northern Ireland

Phytoplankton (including plant-like, animal-like algae and Cyanobacteria) blooms have recently become a serious global threat to the sustenance of ecosystems, to human and animal health and to economy. This study focused on the composition and stability of blooms as well as their taxa-specific ecological sensitivity to the main causal factors (especially phosphorus and nitrogen) in degraded urban lakes. The analyzed lakes were assessed with respect to the trophic state as well as ecological status. Total phytoplankton biomass (ranging from 1.5 to 181.3 mg dm −3 ) was typical of blooms of different intensity, which can appear during a whole growing season but are the most severe in early or late summer. Our results suggested that steady-state and non-steady-state bloom assemblages including mono-, bi- and multi-species or heterogeneous blooms may occur in urban lakes. The most intense blooms were formed by the genera of Cyanobacteria: Microcystis , Limnothrix, Pseudanabaena , Planktothrix , Bacillariophyta: Cyclotella and Dinophyta mainly Ceratium and Peridinium . Considering the sensitivity of phytoplankton assemblages, a new eco-sensitivity factor was proposed (E-SF), based on the concept of Phytoplankton Trophic Index composed of trophic scores of phytoplankton taxa along the eutrophication gradient. The E-SF values of 0.5, 1.3, 6.7 and 15.1 were recognized in lakes having a high, good, moderate or poor ecological status, respectively. For lake restoration, each type of bloom should be considered separately because of different sensitivities of taxa and relationships with environmental variables. Proper recognition of the taxa-specific response to abiotic (especially to N and P enrichment) and biotic factors could have significant implications for further water protection and management.

We studied an urban, flow-through lake with a large total catchment area, in the context of implementing complex protection and restoration methods in the shoreline zone and within the lake’s basin. The study was found that the annual total load of nutrients was 4 688 kg P and 50 652 kg N, which – when converted to the lake’s unit area – corresponded to 8.2 g P m −2 y −1 and 88.4 g N m −2 y −1 . Taking into account the surface inflows alone, the current actual phosphorus load of the lake is over 20-fold higher than the permissible one. An additional source of phosphorus is the lake’s bottom sediments, which is confirmed by the negative balance of this element (−2 882.5 kg P y −1 ). Despite the polymictic type of the lake, oxygen deficits may appear near its bottom, which intensify the internal enrichment process. At present, the primary production in the lake is extremely large, which is indicated by a very high oxygen content (the maximum value of 230%) coinciding with a very high pH value (around 9.7 at both sites) as well as a high chlorophyll a and seston content (158 mg m −3 and 30.6 mg dm −3 at sites S-1, 176 mg m −3 and 33.4 mg dm −3 at sites S-2, respectively), and a low visibility of Secchi disc (around 0.5 m at both sites). At such advanced trophy of the lake and high content of phosphorus (TSI(TP) = 100), it is impossible to stop the rapidly progressing degradation of this water body without man’s intervention. The only possible way to eliminate allochthonous sources of pollutants is by constructing a system of pipelines and waterproof walls for transporting pollutants outside the lake’s catchment. Selection of adequate technological solutions must be preceded by a detailed analysis of hydrological and morphometric properties of the water body, parameters of its catchment and the potential use of lake water in the context of economic calculations. An innovative approach may involve self-oxygenation of lake waters by supplying some of a lake’s surface inflow water to the lake’s bottom. This method can assist in improving oxygen conditions, which to a large extent will reduce the internal supply. It can also be a supporting method in the process of phosphorus inactivation with the use of safe ferrous coagulants, whose efficiency of phosphorus precipitation and immobilization depends on the oxygen content.

The study aimed to assess the usefulness of mineral aggregates in orthophosphate (OP) removal from hypolimnetic water withdrawn from eutrophic lakes. Two low-cost and easily available reactive materials were tested: lightweight expanded clay aggregate (LECA) and crushed limestone (LS). Their performance regarding OP removal and the effect on the pH, Ca2+, Mg2+, N-NO3 and N-NH4 concentrations of treated water were investigated in a column experiment with four-filter beds made of LECA and amended with LS (additions of 0, 25, 50 and 75% of the bed volume). The highest OP removal (&gt, 50%) was achieved in LECA beds with high (75% by volume) amendments of LS. Neither LECA nor LS distinctly affected the pH (maximum pH increase, from 7.1 or 7.2 to 7.6, occurred in the case of the LECA bed). In real-life conditions, it is not feasible to install a full-scale bed made of these mineral aggregates on the outflow from a lake due to the large required size of such a bed. At the operation time set for 30 d, the size of a bed would need to reach between 6113.2 and 12,226.4 m3. The proposed bed should be just one of the elements of an integrated treatment system. Constructions consisting of sorption beds ought to be coupled with adequately designed zones of aquatic vegetation. Three conceptional solutions were proposed for in situ treatment of the withdrawn water, differing in arrangement and construction of the potential sorption bed. Application of such solutions should be regarded as a substantial improvement of Olszewski&rsquo, s method, as it can mitigate the pollution of downstream ecosystems.

We analysed total bacterial number and mean volume of cells at three sites in each of ten floodplain lakes in the Middle Basin of the Biebrza River, North-Eastern Poland to test bacterioplankton communities change according to the distance to the river. The composition of the bacterial communities was determined by fluorescent in situ hybridization method. Total number of bacteria in the lakes ranged from 4.0 to 7.48 cells × 10 6 mL −1 with dominance by Actinobacteria, the contribution of which was positively correlated with water level. Old river channels (side-arms) featured Alpha- and Gammaproteobacteria. The community of Betaproteobacteria was limited by concentration of dissolved organic carbon. Archaea, in spite of a minor role (

Lakes along with their surroundings are an attractive landscape component that play an important role in recreation and relaxation. In Poland, the Great Masurian Lakeland, visited by a million tourists every year, is considered to be the most attractive lake area. However, tourist pressure has several negative effects which cause degradation of the water environment. The aim of this study was to determine the trophic state of Lake Mikołajskie which is a hub for sailing and motorboating traffic. Values of the trophic status index were calculated from the content of total phosphorus, chlorophyll a and Secchi disc visibility, according to the data from the Voivodeship Inspectorate for Environmental Protection. In summer, the concentration of total phosphorus was 0.019–0.042 mg dm−3 in 2013 and 0.022–0.055 mg dm−3 in 2014. The concentration of chlorophyll a was in the range of 8.3–45.0 μg dm−3 in 2013 and 8.9–22.0 μg dm−3 in 2014. The maximum visibility of Secchi disc reached 2.0 m (2013) and 1.8 m (2014). Based on the calculated, Lake Mikołajskie can be classified as a eutrophic lake. In order to protect lakes from degradation tourists should be educated about the rational use of surface waters and the surrounding catchment. However, it is not only the massive influx of tourists, but also the activity of residents (dictated by economic reasons) that contribute to the pollution of lakes. Therefore, the development of tourism and the economy of the whole region should be harmonious and balanced so that the natural resources can serve us and future generations for as long as possible.

The experiment was established in a set of containers. Every container held a 10 cm thick layer of profundal sediments and water from a badly polluted lake. The following five preparations were tested (brand names): AQUAKALGON+, BIOGEN REMEDIANT, BIODENN K, ECOTABS 1(TABLETS), ECOTABS 2(POWDER). The biological preparations were applied twice: phase I lasted 56 days and phase II was continued for 21 days. The research results demonstrated that considerably higher values, compared to the control, were initially (day 3) obtained for TOC after the application of ECOTABS 1, for total nitrogen − after BIODENN K and ECOTABS 1, and for total phosphorus − after BIODENN K, ECOTABS 1 and BIOGEN REMEDIANT. Multiplying bacteria might have been an additional source of organic matter in water, which was subsequently deposited in the bottom sediments as implicated by a higher percentage (by 1 to 7% of d.w.) of organic matter in the sediments at the termination of the experiment. Our preliminary experiment did not provide any justification for using the tested preparations in lake restoration efforts. Subsequent studies should focus on the question whether mineral phosphorus released to the lake water or a drastic change in the environmental conditions would not lead towards the dominance of invasive and highly toxic cyanobacteria.

Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains. © 2018 by the authors. Licensee MDPI, Basel, Switzerland., The authors acknowledge COST Action ES 1105 “CYANOCOST—Cyanobacterial blooms and toxins in water resources: Occurrence impacts and management” and COST Action ES 1201 “NETLAKE—Networking Lake Observatories in Europe” for contributing to this study through networking and knowledge sharing with European experts in the field. Evanthia Mantzouki was supported by a grant from the Swiss State Secretariat for Education, Research and Innovation (SERI) to Bas Ibelings and by supplementary funding from the University of Geneva. We thank Clare Ahnlund, Ena Suarez and Irene Gallego for helping out with the Swiss survey. We thank Wendy Beekman and Els J. Faassen for the nutrient and toxin analysis.

Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includes in-situ lake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment., The authors acknowledge COST Action ES 1105 “CYANOCOST – Cyanobacterial blooms and toxins in water resources: Occurrence impacts and management” and COST Action ES 1201 “NETLAKE – Networking Lake Observatories in Europe” for contributing to this study through networking and knowledge sharing with European experts in the field. Evanthia Mantzouki was supported by a grant from the Swiss State Secretariat for Education, Research and Innovation (SERI) to Bas Ibelings and by supplementary funding from University of Geneva. We thank Wendy Beekman for the nutrient analysis and the University of Wageningen for covering the costs of this analysis from the personal funding of dr. Miquel Lürling. We thank Pieter Slot for assisting with the pigment analysis and the University of Amsterdam for covering the costs of the analysis through funding from the group of Prof. Jef Huisman and dr. Petra Visser (IBED). We would like to thank the Environmental Data Initiative for covering the cost of archiving the EMLS dataset. We would like to thank the Leibniz Institute of Freshwater Ecology and Inland Fisheries, Dept. of Experimental Limnology and the Aquatic Microbial Ecology Group for logistic and technical support of J. Fonvielle and H.-P. Grossart; and the Leibniz Association for financial support. The collection of data for Lough Erne and Lough Neagh were funded by the Department of Agriculture, Environment and Rural Affairs, Northern Ireland.

We investigated the effect of long-term inflow of untreated municipal sewage on the trophic status of the four urban lakes (Mielenko, Karczemne, Klasztorne Male, Klasztorne Duze) connected by the river into one system. We delineated the catchments and sub-catchments of the individual lakes, to determine their hydrological parameters as well as the physicochemical properties of water in the lakes, their surface inflow and outflow as well as water discharged to lakes from stormwater and drainage ditches. Results of the TSI demonstrated that Karczemne Lake and Klasztorne Male Lake were the most polluted of the analyzed lakes. The value of TSITP was extremely high: 103 and 85, respectively. The biggest hazard for the quality of waters in the lakes was: angling in Mielenko Lake (44.8%); point sources (42.1%) and the inflow from Mielenko Lake (33.5%) to Karczemne Lake; the inflow from Karczemne Lake (60.4%) and point sources (35.9%) for Klasztorne Male Lake; and the inflow from Klasztorne Male Lake (95.2%) to Klasztorne Duze Lake. According to Vollenweider’s hydraulic model, the actual phosphorus load introduced to Mielenko Lake is double the critical load (7.8 kg y −1 ); in Karczemne and Klasztorne Male Lakes, the critical load is exceeded by 700% (12.1 kg y −1 and 37.8 kg y −1 , respectively) and in Klasztorne Duze Lake, the actual load is 1000% higher than the critical one (the critical load—96.0 kg y −1 ). With such high phosphorus concentrations, natural inhibition of the lake’s severe degradation is impossible. To achieve permanent water quality improvement we recommend the comprehensive protective and restoration actions: constructing pipelines and perimeter drainage ditches with waterproof walls sealed with PVC vinyl sheet piling along the entire lake system; removal of the bottom sediments from Karczemne Lake and some of the bottom sediments from Klasztorne Male Lake; phosphorus inactivation in the lakes Mielenko, Karczemne and Klasztorne Male, along with supportive actions in the form of biomanipulation.

This study focused on the determination of the phytoplankton-based recovery requirement, including bloom intensity thresholds in human-affected lakes, helpful in the restoration. The phytoplankton and physicochemical analyses were carried out on seven urban lakes in Kartuzy and Skępe. The Phytoplankton Metric for Polish Lakes was used to assess the ecological status and the Trophic Level Index was used to determine the trophic state. Only one lake had a good ecological status (meeting the WFD requirements for at least good status), whereas other lakes had poor or bad ecological status. All of them were eutrophic or hyper-eutrophic and the summer phytoplankton assemblages were dominated mainly by bloomforming cyanobacteria. The lowest phytoplankton bloom threshold of 2.6 mm3 l-1 (seasonal maximum) and the following thresholds of 5 mm3 l-1 for classes I/II; 8 mm3 l-1 for classes II/III; 21 mm3 l-1 for classes III/IV; 100 mm3 l-1 for classes IV/V were proposed for urban lakes. This could directly refer to the bloom intensity for high, good, moderate, poor and bad ecological status. This all indicates a huge recovery requirement in the majority of urban lakes, and the proposed bloom intensity classification may be helpful in fulfilling the WFD targets for the ecologicallyrelevant lake status.

The study was conducted on Lake Długie, located in the city of Olsztyn, which for 20 years received raw domestic sewage (400 m3 per day). After preliminary conservation operations, the lake was restored by artificial circulation and phosphorus inactivation methods. During artificial circulation, water temperature in the whole lake volume was equalized. The disconnection of the compressor stimulated the return to typical thermal parameters in the lake. Phosphorus inactivation did not affect the thermal regime in the lake. Artificial circulation caused an increase in the oxygen content in the whole lake, lowered the oxygen-depletion rate during stagnation, and shortened the duration of anaerobic conditions in the near-bottom waters. Phosphorus inactivation did not directly affect the content of oxygen. However, after the coagulant was added to the lake, the oxygenation of the water was further improved owing to the depressed photosynthesis caused by drastically reduced availability of phosphate for primary producers.

We investigated the effect of withdrawal of nutrient rich hypolimnic waters on diel changes in water column physicochemical conditions, composition of pelagic communities and metabolic processes over a 5-day period. The study was done during stratification in the eutrophic Kortowskie Lake divided into a southern experimental part with removal of near-bottom water by a pipe directly to the outlet, and a northern control part unaffected by the pipe. Volumetric metabolic rates with depth were calculated in each basin using 6 daily vertical profiles of dissolved oxygen (DO). Results demonstrated that withdrawal of near bottom water increased hypolimnic water temperature by 1.3 °C decreasing the strength of thermal stratification. This increased phytoplankton biomass and primary production in the metalimnion of the experimental part of the lake, which became dominated by Cyanobacteria, and more net autotrophic compared to the photic zone of the control site. Daily variability in gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP = GPP–R) of the photic zone and metalimnion indicates the highly dynamic physical (mixing and light) and biological (pools of phytoplankton and bacteria) conditions occurring in the photic layers. In the control part of the lake, a multiple regression analysis showed that vertical patterns in GPP and R depended on phytoplankton biomass and total nitrogen and NEP followed changes in particulate organic carbon. In the experimental part of the lake, vertical patterns in NEP were only significantly related to changes in temperature. But as the temperature was positively correlated with bacterial abundance, this was most likely the main driver of variation in NEP with depth (relationship between R and medium cell volume (MCV), bacterial biomass (BB), respectively: r = 0.60; r = −0.63; n = 15; p

The objective of the study was to identify seasonal changes in phytoplankton taxonomic structure and development rates to characterize phytoplankton communities as determinants of the trophic and ecological conditions of Lake Kortowskie in northeast Poland which has been undergoing restoration with selective hypolimnetic withdrawal. The lake was designated as being of high trophic state and bad ecological status based on intense phytoplankton growth, the strong dominance and persistent occurrence of blue-green algae, and seasonal modifications in phytoplankton taxonomic structure and assemblage growth rates. The ecological status of the lake assessed in 2011 corresponds to that determined in 1987-1990 and 1999 and indicates that eutrophication in Lake Kortowskie is progressing.

We investigated changes in the spatial distribution of phosphorus (P) and nitrogen (N) in the deep, mesotrophic Lake Hańcza. The raw data collection, supported by global navigation satellite system (GNSS) positioning, was conducted on 79 sampling points. A geostatistical method (kriging) was applied in spatial interpolation. Despite the relatively small area of the lake (3.04 km2), compact shape (shore development index of 2.04) and low horizontal exchange of water (retention time 11.4 years), chemical gradients in the surface waters were found. The largest variation concerns the main biogenic element – phosphorus. The average value was 0.032 at the extreme values of 0.019 to 0.265 mg L−1 (coefficient of variation 87%). Smaller differences are related to nitrogen compounds (0.452–1.424 mg L−1 with an average value of 0.583 mg L−1, the coefficient of variation 20%). The parts of the lake which are fed with tributaries are the richest in phosphorus. The water quality of the oligo-mesotrophic Lake Hańcza has been deteriorating in recent years. Our results indicate that inferences about trends in the evolution of examined lake trophic status should be based on an analysis of the data, taking into account the local variation in water chemistry.

Recent technological developments have increased the number of variables being monitored in lakes and reservoirs using automatic high frequency monitoring (AHFM). However, design of AHFM systems and posterior data handling and interpretation are currently being developed on a site-by-site and issue-by-issue basis with minimal standardization of protocols or knowledge sharing. As a result, many deployments become short-lived or underutilized, and many new scientific developments that are potentially useful for water management and environmental legislation remain underexplored. This Critical Review bridges scientific uses of AHFM with their applications by providing an overview of the current AHFM capabilities, together with examples of successful applications. We review the use of AHFM for maximizing the provision of ecosystem services supplied, by lakes and reservoirs (consumptive and non consumptive uses, food production, and recreation), and for reporting lake status in the EU Water Framework Directive. We also highlight critical issues to enhance the application of AHFM, and suggest the establishment of appropriate networks to facilitate knowledge sharing and technological transfer between potential users. Finally, we give advice on how modern sensor technology can successfully be applied on a larger scale to the management of lakes and reservoirs and maximize the ecosystem services they provide.

We experimentally manipulated the thermal environment in laboratory mesocosms to evaluate the effect of temperature on organic matter transformation in two systems of lower and higher ambient nutrient availability. We used a system of 6 mesocosms, equipped with heating and cooling systems; 3 were filled with water from a mesotrophic lake and 3 with water from a eutrophic lake. Each of the 3 mesocosms were maintained at different temperatures: ambient lake temperature (10 °C and 20 °C), ambient +5 °C, and ambient +10 °C and the experiment was replicated in twice, in the spring and summer seasons. We measured rates of physicochemical and bacteriological parameters changes over a 2-day period. Results of the study demonstrated that, irrespective of nutrient concentration, an increase in temperature resulted in enhanced the level of labile organic matter, indicated by significantly elevated concentrations of dissolved organic carbon (DOC) and low values of SUVA (specific UV absorbance = Abs 260·1000/DOC). The effect of temperature was stronger in the lower nutrient concentration in the spring experiment treatments elevated by 5 °C. The increase in temperature was additionally accompanied by significant increased total bacterial counts (TBC), biomass (BB) and heterogeneity of bacterioplankton. In the higher nutrient concentration, the increase in temperature exerted a greater influence on the dynamics of concentration of heteromorphic organic matter, shown by the negative correlation between particulate organic carbon (POC) and DOC content and TBC value. The accumulation of considerable quantities of refractory DOC and POC and low bacterioplankton activity may intensify the rate of degradation of eutrophic lake ecosystems.

Total organic carbon (TOC) can be one of the parameters used to determine trophic state in lakes, as shown by the statistically significant correlation between TOC and chlorophyll, total phosphorus and Secchi disk transparency. Calculation of the TSI index based on TOC according to the formula TSI(TOC) = 20.59 + 15.71 ln(TOC) and its introduction as an additional index to the classification by Carlson may optimize the comparative analysis of lakes and allow a measurable assessment of lakes with respect to the trophic gradient.

Hypolimnetic withdrawal as a restoration method comprises the withdrawal of the nutrient-rich hypolimnion by a pipe directly to the outlet, thus replacing the surface outflow. The method was first applied to Lake Kortowskie in 1956 and continues today. The direct effects of hypolimnetic withdrawal can be observed in the near-bottom waters during the pipe's operation (May-Sep). The treatment depends on the quantity of water withdrawn by the pipe. A high withdrawal rate increased the rate of hypolimnetic heating, diminished the water mass stability and shortened summer stagnation. Hypolimnetic withdrawal did not improve the oxygen conditions in Lake Kortowskie, although it shortened the duration of the anaerobic conditions near the bottom and diminished the spatial extent of oxygen deficiency. Increased release of phosphorus from the bottom deposits to the near-bottom water and finally the outlet via the withdrawal pipe has been observed, with a consequent impoverishment of the upper sediments. The...

The aim of this study was to compare the concentration of nitrogen and trace the dynamics of its changes in two lakes with different intensity of anthropogenic pressure. The dominant land use of Lake Paskierz catchment is built-up areas, while in Lake Sajmino, wasteland is the dominant land use. The total amount of nitrogen in Lake Paskierz ranged from 1.68 to 6.58 g Ntot m-3, while in Lake Sajmino it was from 1.03 to 1.84 g Ntot m-3. The organic fraction was a dominant form in the surface water layers of the examined lakes. A slightly different situation was found in near-bottom water layers of Lake Paskierz, where ammonium nitrogen was the dominant form in the summer stagnation. In other cases organic nitrogen was a dominant form in each of the reservoirs. Concentrations of nitrites and nitrates were low and did not affect essentially the overall amount of nitrogen in the studied lakes. Based on the results, it can be concluded that Lake Paskierz is overfertilized. The high concentrations of ammonia measured in near-bottom layers of the lake indicate that the internal supply may be a very important process affecting the trophic status. Lake Sajmino was characterized by significantly less nitrogen abundance, although the periodically increasing nitrogen concentration reveals the presence of adverse anthropopressure on the lake.

Coagulation behaviour of polyaluminium chloride (PACl) was comparatively investigated in terms of the removal of turbidity and colour with surface water. The growth of flocs was also compared. The results show that the decrease in saturation and values of dissolved oxygen was ca. 20% for ALCAT and PAX 18 at both dosages and 15% for PAX XL 19H, respectively. The floc formation growth indicated that PACl coagulation occurred not only by charge neutralization but also by a form of sweep flocculation. Besides, flocs formed by ALCAT had better sizes than the flocs formed by PAX 18 and PAX XL19H. The rate of sedimentation was stable during coagulation with higher doses, and it was dependent on the substance used.

Sediments from hypereutrophic Lake Łajskie localized in northeastern Poland were examined on the basis of P-fraction. The sediments were collected at five sampling sites. In the investigated lake, the rank order of P-fractions was HCl-P > BD-P > NH4Cl-P > NaOH - nrP. The loosely sorbed phosphorus NH4Cl-P represented < 3% of the sedimentary inorganic phosphorus, while the reductant phosphorus (BD-P) ranged from 2 to 10%. The calcium bound phosphorus (HCl-P) showed considerable contribution (59-74%) to the sedimentary inorganic P-loads. BDP was the most reactive fraction in Lake Łajskie. Iron compounds and organic matter seem to play a significant role in regulating this labile P-budget.

The aim of this study was to determine the chemical composition of Lake Widryńskie bottom sediments (max. depth. 27.0 m, area 123.9 ha). The sampling of bottom sediments was conducted once, on 16 August 2010. Sampling was made in 10 specific areas. Among the 10 samples, 4 samples were taken from the littoral and sublittoral zones, while the rest were taken from the profundal zone. The dominant component of the sediments was silica and calcium carbonates were subdominant. Based on the survey, it was determined that silica occurred in greater numbers in littoral sediments, while in sublittoral sediments there was increased participation in the chemical composition of organic matter. The elements that build a capacity complex were a small percentage of the dry weight of sediment. Sediment from the vicinity of inflows contains higher amounts of silica, which confirmed the impact of the catchment on the chemical composition of sediments.

We evaluated the influence of habitat connectivity and local environmental factors on the distribution and abundance of functional fish groups in 10 floodplain lakes in the Biebrza River, northeastern Poland. Fish were sampled by electrofishing, and 15 physico-chemical parameters were recorded at three sampling sites at each lake in the period of 2011–2013. A total of 18,399 specimens, belonging to 23 species and six families, were captured. The relationships between environmental factors and fish communities were explored with the use of canonical correspondence analysis (CCA). Sampling sites were grouped based on fish communities using a hierarchical cluster analysis (HCA). Along a lateral connectivity gradient from lotic to lentic habitats (parapotamic–plesiopotamic–paleopotamic), the proportions of rheophilic species were determined as 10:5:1, whereas the proportion of limnophilic species was determined as 1:2:5. The predominant species were the roach (Rutilus rutilus), and pike (Esox lucius) in parapotamic lakes, rudd (Scardinius erythropthalmus) and pike in plesiopotamic lakes, and sunbleak (Leucaspius delineates) and Prussian carp (Carassius auratus gibelio) in paleopotamic lakes. The findings indicated that the composition and abundance of fish communities are determined by lake isolation gradient, physico-chemical parameters and water stage. Although intact riverine ecosystems may promote fish biodiversity, our findings suggest that lateral connectivity between the main channel and floodplain lakes is of utmost importance. Thus, the conservation of fish biodiversity requires the preservation of this connectivity.

Key-words: organic carbon, lakes, morphometry, catchment The study lakes are located in north-eastern Poland, in the same physiographic system. These lakes were formed during the last glacial period. The lakes differedin morphometry,catchment size and environmental conditions. The study included deep, clear water lakes that stratify in summer. The results from multiple linear regression equations indicate that the morphometric and catchment variables may influence mainly the particulate fraction of OC during summer stratification. No significant correlation was found between the examined parameters and the quantity (DOC) or the carbon-specific absorbance (SUVA260) of the dissolved fraction. The most important factors which determine POC abundance in lakes are the maximum depth (zmax), length and development of shoreline (L, DL), catchment area to lake volume (CA/V), and stratification percentage (“% strat.”), as well as epilimnion bottom area to epilimnion volume (AE/VE). Thus, in the summer the quantity of OC in lakes, especially of POC, will be determined by the in-lake production and consumption processes. Low productivity can be easily maintained in deep, strongly stratified lakes, and therefore should be protected and high water quality should be preserved.