The ecological status of lakes based on ichthyofauna, as defined by the Water Framework Directive, is assessed using intercalibrated methods. However, the methods adopted (in Poland, the Lake Fish Index LFI-EN method, based on results of one-off fishing with multi-mesh gillnets) are labor-intensive and do not allow for frequent repeat testing. Therefore, the concept of a simple model describing changes in the relative number of single traces in the vertical profile (according to the TS target strength distribution) in a lake is presented, as well as an index (the sum of deviations from such a model), enabling quantification of the similarity of TS distributions in lakes with this model. Preliminary analyses were conducted on acoustic data collected in Lake Dejguny. This lake—the condition of which could be estimated based on historical data using the relationships between LFI and the degree of lake eutrophication (expressed by Carlson’s TSI)—was assessed as having a good status in 2006, whereas in 2021, (based on LFI-EN) it had a moderate status. The study tested the TS distribution model, calculated as the arithmetic mean of the relative number of single traces in 2 m-thick layers. It was also shown that the proposed indicator can effectively signal deterioration of ecological status—the sum of the absolute values of the TS distribution deviations in 2021 (moderate status) from the model was more than seven times greater than the sum of the deviations of the distributions from which the model was built (good status). The obtained results confirmed the hypothesis about the possibility of determining a characteristic distribution of single traces in the vertical profile when the lake was classified as being in good condition.

The idea of the article was to verify the thesis that acoustic studies of ichthyofauna in lakes provide sufficient information to determine the necessity (or lack thereof) of carrying out a full procedure for assessing the ecological status based on fish. The manuscript compares acoustic data collected in 2008 and 2021 in Lake Dejguny (Poland) as a result of acquisition carried out with the same EY-500 echosounder, on the same routes (transects), and the echogram analyses were carried out using software dedicated to this echosounder. Target strength (TS) distribution was determined in the lake and in individual water layers with a thickness of 2 m, as well as, using the mapping method, changes in the distribution of fish in selected water layers. Large changes in TS distribution in the vertical profile were demonstrated between both years of the study, as well as a clear reduction in the space available for fish in waters below 24 m in 2021, which was associated with a reduction in the concentration of dissolved oxygen below this depth to a value lower than 2.5 mg L−1. It was suggested that it could be possible to develop a simple index enabling the assessment of changes in fish structure, based, similarly to the Large Fish Index, on the ratio of the number of large fish to small fish. The obtained results confirmed that the proposed methods of acoustic data analysis can be the basis for the decision on the necessity (or lack thereof) of conducting an assessment using the Polish national LFI-EN method based on the results of one-off fishing with Nordic multimesh gillnets.

Assessments of changes in the ecological state of aquatic ecosystems are always burdened with uncertainty, which results from environmental reasons and poor repeatability of measurement results of elements enabling the assessment. This study determines the uncertainty related to the elements of the assessment of the hydroacoustic structure of fish communities’ (1) vertical target strength distribution (TS) in two-meter layers of water and (2) changes in the area where fish were recorded (which was determined on the basis of maps of their distribution in 2 m deep water layers). The object of this study was a lake (depth: 27 m) in which at the end of June 2016 the O2 concentration was <1.4 mg L−1 below 8 m depth, which resulted in the accumulation of fish to a depth of 6 m. Hydroacoustic acquisition was carried out along transects arranged in the east–west (WE), north–south (NS), and zigzag (ZZ) directions in three repetitions. It was shown that the empirical probability of obtaining statistically different results was 2/9 when (1) Kendall’s τ coefficient, used to determine the similarity of the TS distribution, was less than 0.7—moderate correlation—and (2) fish occurrence areas in two cases (WE and ZZ on the third day of research) in layers 2–4 m and 4–6 m differed statistically significantly from the average area for all repetitions by 10–14% and 56–66% (p < 0.05), respectively. The obtained results indicate quite good repeatability of acoustic measurements; however, in order to reduce the uncertainty, it is recommended that tests be conducted in this type of lake in three series of measurements.

A procedure is proposed to assess the impact of various relationships found in the literature and is used to convert acoustic target strengths (TS) to fishes’ total length (TL) with respect to the compatibility of fish length data obtained from vertical hydroacoustics and gillnets. The study used one set of data collected with a 120 kHz echosounder across the mesotrophic, dimictic Lake Dejguny. Four general multi-species TS–TL relationships were tested for the maximum dorsoventral characteristic: (1) a relationship developed using mainly West Atlantic marine and brackish water fish for various frequencies, (2) a relationship developed using fish from the Salmonidae, Percidae, and Cyprinidae families at 120 kHz, as well as the relationship shown by two generalized equations for representatives of (3) the Cyprinidae family (200 kHz) and (4) the Percidae family (200 kHz). In addition, two other equations were developed for (5) perch (Perca fluviatilis) and (6) roach (Rutilus rutilus). The procedure for selecting the most appropriate TS–TL ratio began by determining the TS threshold that would eliminate small fish that were ineffectively caught with gillnets. Depending on the TS–TL relation, the threshold ranged from −48.5 dB to −45.5 dB, and the corresponding TL was in the range of 62.3–93.0 mm. Then, using linear regression, the relationship between the percentage of caught fish organized in length classes (TL), whose boundaries were determined using the tested TS–TL relationships, and the share of fish recorded acoustically in the corresponding TS classes (with a 1.5 dB interval) was examined. The fit of the regression model to the data (percentage) was assessed using the coefficient of determination r2, the mean absolute error (MAE), the Nash–Sutcliffe model efficiency coefficient (NSE), and root mean square error (RMSE). For the data from Lake Dejguny, the most similar distribution of fish echo proportions and the corresponding distribution of total length (TL) for fish larger than 62 mm were obtained using the TS–TL relation developed using fish from the Salmonidae, Percidae, and Cyprinidae families (2), and for fish larger than 74 mm, the relation was developed for the family Pericidae (4). No evidence was found to unambiguously verify the meanings of different sound frequencies (120 and 200 kHz) for which the TS–TL relationships used in the analysis were derived. The proposed procedure can be used to select the optimal regression equation.

We examined the influence of sampling frequency and the elimination of selected metrics from the Phytoplankton Multimetric for Polish Lakes (PMPL index) on lake ecological status classification. The effect of sampling frequency was analysed by calculating the official PMPL index using data from four sampling periods (848 lake-years; 478 Polish lakes), and compared the Pmpl-based classification to that assessed from PMPL variants calculated using one, two and three sampling periods. The effect of metric elimination was analysed by comparing the official three-metric PMPL classification with two variants, a one-metric PMPL including only the concentration of chlorophyll-a and a two-metric variant including total phytoplankton biomass and summer cyanobacterial biomass. Correlation strength between the official PMPL and simplified variants weakened by reducing sampling frequency but varied by period combinations. Uncertainty of ecological status classification increased in both simplification types but it was greatest when using the one-metric (chlorophyll-a based) variant. Sampling reduction from four to three periods resulted in 0.5–6.5% of lakes being over- or under-classified (total lake-year misclassification: 3.7–6.7%). Highest uncertainty of classification to at least good status occurred when the spring sampling period was excluded (6.5%). The risk of misclassification increased to 4.2–14.9% when two sampling periods were used. When classification was based only on a one-period (late summer) PMPL, 12.1% of lake-years were misclassified to at least good and 9.3% to below-good ecological status. The risk of misclassification to at least good ecological status was 2% when the two-metric (biomass-based) PMPL index was used and 28.9% when the one-metric (chlorophyll-a based) was used. Lowest misclassification error (0.5%–5.2%) occurred when a three-period (spring-August-October), three-metric PMPL index was used, but as this 5.2% would result in substantial status under-classification, we recommend further research to possibly optimize a three-period PMPL variant to reduce the costs of lake ecological monitoring in Poland.

The purpose of the study was to ascertain the effect of cyanobacterial abundance and its taxonomic structure on the results of measurements made by a fluorometric device designed to detect in situ chlorophyll a and phycocyanin. A multiparameter water quality probe was tested at 10 lakes located in the Wel River catchment. We found a strong correlation between the chlorophyll a concentration determined by laboratory procedure (CHL-a) and that obtained as a result of the probe measurements (YSI CHL-a) (R=0.78) as well as between the YSI CHL-a and the total phytoplankton biomass (R=0.73), whereas YSI CHL-a was not a good predictor of cyanobacterial biomass (R=0.24). The phycocyanin recorded by the probe was proportional to the total biomass of cyanobacteria (R=0.86); however, this cyanobacterial taxonomic structure influenced the fluorometric signal. Nevertheless, our study showed significant differentiation of phycocyanin measurement distribution at different levels of cyanobacterial abundance (<2 mg L−1, between 2 and 10 mg L−1, and >10 mg L−1), which indicates that the PC-YSI measurements are valuable in the detection of increased risk of exceeding health alert thresholds recommended by the WHO.

Water temperature is an important ecological variable that affects the functioning of lakes. Unfortunately, for many lakes there are no long-term observations enabling the assessment of changes in water temperatures. This makes it difficult to include this aspect in research into the biology, ecology and chemistry of such lakes. This paper presents a literature review related to changes of surface water temperatures in lakes and in particular describing the response of water temperatures and stratification to changing climate in Polish lakes. On this basis, a model based on the available data on water temperature in 931 Polish lakes in the years 1951–1968 was proposed, which allows to estimate the baseline water temperature on any day of the year. This model is calculated using the complementary error peak function on the 0–3 m water temperature dataset, which provides the best reduction of diurnal temperature fluctuations. It can be an alternative to the average temperature of surface waters, which are calculated on the basis of systematically collected data. Based on the average water temperature data obtained from 56 thermal profiles in 10 lakes in 2010–2019, the equation was analogically calculated. The average monthly water temperatures in June, July, August and September and the change in water temperature (0.24–0.30 °C decade−1) in the period 1951–1968/2010–2019 were estimated then. Similar regional or single lake trends have been found in studies by other authors covering a similar or longer period of time. The proposed method, which is suitable for simulating temperatures, especially in summer, enables the determination of the value of changes in surface water temperature in Polish lakes when only thermal profiles data from different dates are available, which can be especially helpful when analyzing hydrobiological results.

The Konin lakes, heated by power stations and invaded by alien organisms, are a natural laboratory in which we can study the impact of climate change on the native communities of aquatic organisms. The aim of our study was to assess the impact of water heating and the occupation of the littoral zone of the lake by invasive species Vallisneria spiralis on changes in the species structure of rotifer communities of plankton, epiphyton and psammon. The archival material was used from the years: 1970–1975, 1978 and 1983, and compared with the results of studies conducted in Licheń and Ślesin Lakes in the years 2010–2011 and 2017–2018. It has been shown that the heating of waters of the studied lakes, combined with the shortening of their retention time, as well as the invasions of alien species, have caused significant changes in the taxonomic and trophic structure of plankton rotifers. In inhabiting Vallisneria bed epiphytic rotifer communities, the share of alien species did not increase, but relatively high densities of uncommon sessile species still persist. Psammon communities in the lakes are dominated by monogonont species relatively common in this habitat in nonheated lakes, but they are nearly devoid of bdelloids, which are abundant in psammon of Masurian lakes.

This article presents and tests a new method for the retrospective assessment of ecological status assessment of the lakes in accordance with the Ecological State Macophyte Index (ESMI), which is formally used in biological monitoring in Poland. The proposed method is based on three metrics, the Z colonization index, the average maximum depth of lake vegetation Cmax, and the Secchi disk depth. Mathematical functions of ecological class were developed on the basis of the mean values of these three indicators in summer for different ecological status classes in 88 stratified lakes in northern Poland and the Łęczyńsko-Włodawskie Lake District and five lakes in the catchment area of the Wel River (published data). The new metrics were validated on the basis of literature data—ESMI, Cmax, Z and SD values from 11 lakes near Olsztyn (Poland). The obtained results are similar to those calculated based on macrophyte field surveys and can be an alternative of the Ecological State Macophyte Index (ESMI), which is formally used in biological monitoring in Poland. The proposed method makes it possible to compare long-term changes in the ecological state of lakes, because it enables an analogous assessment on the basis of data calculated from historical bathymetric maps showing the distribution of hydro macrophytes (parameters Z and Cmax) as well as contemporary data, collected, among others, during hydroacoustic research.

Abstract A total of 313 lakes with charophyte vegetation were identified based on the data presented in 1111 manuscripts under the shared title of “Assumptions for the fisheries management project in Lake (lake name)”, based on the results of an environmental inventory carried out by the Inland Fisheries Institute in 1953-1968. The lakes’ morphological characteristics were described, hydrophyte species that most frequently accompanied charophytes were identified, differences in summer water transparency were analyzed in 281 lakes with charophyte vegetation and the results were compared with the observations made in 657 other lakes, and the potential trophic state of lakes with charophyte vegetation was determined using Carlson’s trophic state index (1996). More than half (54%) of 171 dimictic lakes with charophyte vegetation were classified as mesotrophic and 31% as oligotrophic, whereas 50% of 110 polymictic lakes were classified as eutrophic, and 40% as mesotrophic. The frequency of taxa that contribute to the eutrophication (degradation) of water bodies, including Ceratophyllum spp. Myriophyllum spp. and Elodea canadensis, increased with a decrease in the percentage of charophytes in communities of submerged hydrophytes. Regardless of the proportion of charophytes in submerged hydrophyte communities, water in the lakes colonized by charophytes was more transparent than in the 659 lakes without charophytes. Water in many polymictic and dimictic lakes with charophyte vegetation, including lakes with a small contribution of charophytes, was more transparent than in lakes without charophytes, which suggests that charophytes are sensitive indicators of water quality and are components of ecological memory in aquatic ecosystems.

The possibility of doing a back assessment of the ecological status of a lake based on archival bathymetric maps indicating areas overgrown with rushes and aquatic vegetation was verified. This assessment was assumed to be in accordance with that performed with the official Polish macrophyte-based method for lake assessment (Ecological State Macrophyte Index, ESMI). The study was conducted on Lake Dobrąg located in the Warmian-Masurian Voivodeship (surface area - 108 ha, maximum depth - 27.9 m, mean depth - 11.6 m). It included the hydroacoustic distribution of submerged macrophytes along 85 evenly distributed belt transects (perpendicularl to the shore line), creating a bathymetric chart and maps of vegetation occurrence and identifying areas occupied by hydrophytes (Cmax) and the maximum depth of lake colonization (Z). Analogous data were read from archival bathymetric chart dating from 1964-1968. The values obtained were compared with the means (and their confidence intervals) of 83 stratified lakes in Poland in different ecological status classes. Analysis of changes indicated that the ecological status of the lake had deteriorated. In the mid-1960s, the status of the lake was less than “very good” while the current status borders between “good” and “moderate.” The results indicate that the proposed method could be useful when attempting to assess changes in ecological status using archival bathymetric charts showing areas overgrown with vegetation and the distribution of it in lakes.

This study focused on site-specific preferences of potentially harmful cyanobacterium Gloeotrichia echinulata to occur in lakes with different ecological and trophic conditions. Its pelagic growth was studied in six lakes from June to September in 1986-1988, 2000-2001 and 2009. In total, 78 samples were taken from the epilimnion (stratified lakes) or the whole water column (non-stratified lakes). Analyses of phytoplankton and environmental variables were performed according to standard methods.During summer, a distinct maximum of the Gloeotrichia growth was observed in July or August (the warmest period). Bloom events of G. echinulata occurred in lakes where the light and oxygen conditions were significantly inferior while the phosphorus content remained on a slightly elevated level. The distinct domination of this cyanobacterium (above 40% of the total phytoplankton biomass) was limited to lakes with a high, moderate or even poor ecological status, and to the mesa-eutrophic or eutrophic state of lakes. However, G. echinulata occurred in a broader range of ecological and trophic conditions of lakes. The historical approach to mass occurrence of G. echinulata, with its possible contribution to phosphorus translocation from sediment to the pelagic zone, suggested its importance as an indicator of progressive ecological and trophic deterioration of lakes. This indication should be very useful for establishment of main targets in water management.

The European Water Framework Directive (WFD) requires that the ecological status of waterbodies is assessed using multiple biological quality elements (BQEs) that are combined into a single status class. The recommended combination rule (the "one-out, all-out" rule; OOAO) has been criticised for being unreasonably conservative and for being sensitive to uncertainty. In this study, the objective was to compare the sensitivity to uncertainty of four different combination rules: (1) OOAO, (2) OOAO with exclusion of one element, (3) average and (4) weighted average. Index values for 5 BQEs (phytoplankton, phytobenthos, macrophytes, macroinvertebrates and fish) sampled from 10 lakes in the We! River catchment in Poland were used to classify the lakes according to the OOAO and the three alternative combination rules. Based on the mean and (where possible) standard deviation of these index values, we modelled the risk of misclassification by simulating 10,000 resamples for each BQEs in each lake, classifying each resample and calculating the proportion of misclassified resamples under each combination rule. For individual BQEs, the risk of misclassification increased both with higher uncertainty (standard deviation) and with the proximity of the index value to a class boundary. Under the OOAO rule, the risk of misclassification was more biased towards worse status ("underclassification") than towards better status. Furthermore, risk of underclassification was more affected by uncertainty under the OOAO rule compared with the alternative combination rules. This analysis has demonstrated the weaknesses associated with the OOAO rule for integration of BQEs for lake classification. However, the alternative combination rules are associated with other shortcomings, such as the need for subjective judgement, and involve a higher risk of not protecting the most sensitive BQE and thus the whole ecosystem. We recommend that future versions of instructions for WFD implementation consider alternatives to the OOAO combination rule, and provide guidelines for weighting of individual BQEs. (C) 2014 Elsevier Ltd. All rights reserved.

This paper outlines the Phytoplankton Multimetric for Polish Lakes (PMPL) an ecological status assessment method which can be used to implement the European Water Framework Directive (EC, 2000). The PMPL includes abundance parameters of phytoplankton: the metrics: "chlorophyll a", "total biomass" while the taxonomic composition is partly evaluated by the metric "biomass of cyanobacteria". All of these three single metrics as well as the final PMPL index values are the same scale and range from 0 to 5. The PMPL can be transformed to a normalized Ecological Quality Ratio (EQR) range from 0 (the worst status) to 1 (the best status). The PMPL method differentiates "stratified" and "un-stratified" lake types as well as subdivisions into lakes characterized by a high (>2) or low (<2) lake volume-to-catchment area ratio (VQ). The metrics used and the PMPL index respond to eutrophication pressure expressed by total phosphorus and total nitrogen concentration, the significance of the relationships depends on the tested parameter and the abiotic type of lake. From among the single metrics, the metric "chlorophyll a" was usually found to reveal the strongest significant correlation to both total phosphorus and total nitrogen (Spearman' s coefficient varied from -0.41 to -0.75 and from -0.34 to -0.56, respectively). The weakest correlation with nutrients was noted for metric "biomass of cyanobacteria" (R=-0.13 to -0.62). The PMPL correlated best with TP and TN in unstratified lakes (R=-0.49 to -0.70). The testing of the PMPL index shows a statistically important distinction between Good/Moderate ecological status and confirmed designated boundaries of High/Good and Good/Moderate states.

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Abstract The aim of the study was to perform an ecological status assessment based on the phytoplankton in the vendace-type Lake Dejguny in accordance with the requirements of the Water Framework Directive. The phytoplankton analyses were conducted during three growth seasons in 2006, 2007, and 2008. The phytoplankton multimetric Phytoplankton Metric for Polish Lakes (PMPL) was used to assess the lake’s ecological status. The average total biomass of phytoplankton ranged from 1.6 to 3.9 mg dm-3, while the average biomass of Cyanoprokaryota ranged from 0.4 to 1.4 mg dm-3, with the minimum noted in 2008. The phytoplankton assemblages were dominated mainly by filamentous Cyanoprokaryota and pennate Bacillariophyceae in 2006 and 2008, or exclusively by pennate Bacillariophyceae in 2007. The relatively low total biomass and Cyanoprokaryota biomass, as well as seasonal phytoplankton dynamics with the dominant taxa of Tabellaria flocculosa, Dinobryon sociale, and D. divergens confirmed the lake’s mesotrophic state and some of the features of reference conditions in this lake, whereas the predominance of filamentous species Planktothrix agardhii and Planktolyngbya limnetica was characteristic of eutrophic conditions. However, a clear tendency towards progressive eutrophication stemming from the significant domination of filamentous Cyanoprokaryota was observed, and according to the PMPL the assessment indicated that Lake Dejguny had a good ecological status in 2006 and 2007 and even a high one in 2008.

The epilimnetic and metalimnetic phytoplankton assemblages were compared in two mesotrophic lakes (L. Hańcza and L. Dejguny, northeastern Poland) and a “metalimnetic niche” is described. Phytoplankton analysis conducted during July and August 2006, 2007, and 2008 indicated that the total phytoplankton biomass in both lakes was relatively small (from 0.6 to 5.2 mg dm−3), with maximum values mainly occurring in the metalimnion. The lowest number of species and diversity indices values were usually observed in the metalimnetic layers. Lake Hańcza was dominated by diatoms (e.g. Cyclotella spp.) whereas Lake Dejguny was mainly dominated by cyanoprokaryotes (e.g. Planktothrix agardhii), with a distinct predominance of diatoms (e.g. Tabellaria flocculosa) in 2007. The common features of the phytoplankton assemblages in both lakes were a tolerance to nutrient and light deficiencies. A cluster analysis of the taxonomic patterns indicated large dissimilarities between the lakes. The phytoplankton assemblages developed under the influence of seasonal conditions, especially in Lake Hańcza. The distinction between metalimnetic and epilimnetic phytoplankton assemblages only appeared in Lake Dejguny.