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Article

A Relationship Between Nutrients in a Mid-Forest Eutrophic Lake

by
Józef Antonowicz
1,
Michał Rybak
2 and
Tomasz Wróblewski
3,*
1
Department of Environmental Chemistry and Toxicology, Pomeranian University in Słupsk, Arciszewskiego St. 22b, 76-200 Słupsk, Poland
2
Department of Water Protection, Faculty of Biology, Adam Mickiewicz University, 61-614 Poznań, Poland
3
Department of Physics, Pomeranian University in Słupsk, Arciszewskiego St. 22b, 76-200 Słupsk, Poland
*
Author to whom correspondence should be addressed.
Water 2025, 17(19), 2913; https://doi.org/10.3390/w17192913
Submission received: 4 September 2025 / Revised: 30 September 2025 / Accepted: 7 October 2025 / Published: 9 October 2025
(This article belongs to the Section Biodiversity and Functionality of Aquatic Ecosystems)
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Abstract

In 2023, studies were carried out on the aquatic environment of the forest lake Łętowskie. The studies covered the horizontal and vertical planes and seasonal dynamics. Lake Łętowskie is a lake with an area of 402 ha, which distinguishes it from other lakes in Pomerania due to its large area. In three quarters of the lake shore border forests, changes in surface and volume have been observed in the lakes over the last century, which has affected the chemistry of the water. The aims of this study were to determine the dependencies between the concentration of biogenic substances in the near-bottom layer and subsurface water and analyze the dependencies between chemical parameters in the water of the mid-forest lake Łętowskie. In the water samples obtained, including the surface layer (SW) and the near-bottom layer (NBL), the concentrations of N-NO3, N-NO2, N-NH4, N-tot, N-org, P-PO4, P-tot, P-org, and O2, electrolytic conductivity, pH, Ca, and Mg were determined. Statistical analyses were carried out, including tests and multidimensional PCA and cluster analysis. A significant effect of forests on the chemical composition of lake water was observed. The conducted studies of Łętowskie Lake indicate that the NBL experiences seasonal dynamics, where phosphorus and nitrogen compounds are transformed, which causes trophic changes in the lake. Based on multidimensional cluster analysis, differences between the SW and the NBL were shown. In Łętowskie Lake, the level of biogenic substances in the water is significantly influenced by processes occurring inside the lake as a result of the exchange of matter between the NBL and bottom sediments. This exchange in shallower areas of the lake is influenced by winds, especially in exposed locations: this was observed for P-tot, P-PO4, P-org, Ca, N-NO3 and N-NH4, N-tot, and N-org. The conducted studies are important for supporting the protection of the Landscape Area “Łętowskie Lake and the vicinity of Kępice” to preserve the existing values of the natural environment and maintain the ecological balance of natural systems. Current scientific publications on the hydrochemical data of Łętowskie Lake are currently lacking, and the available data needs to be updated.

1. Introduction

The level of nutrients and consequently the trophic state of lakes are influenced by a number of factors. The decisive role is played by the inflow of chemical substances from the catchment area and the processes occurring inside the lake, including the exchange of matter between water and the bottom sediment [1,2]. The concentration of nutrients, including phosphorus, in lake water depends on how much is supplied from external sources, how much is discharged with the outflow and carried away with migrating organisms, and how much is sedimented into the bottom [3]. The eutrophication processes that have taken place over millennia have been slow and are a manifestation of the natural evolution of water bodies [4]. Currently, accelerated eutrophication is being observed, which is related to the intensification of anthropopressure [5,6]. Chemical impacts depend mainly on the hydrological and morphological conditions of the water reservoir [7]. Anthropogenic impacts are often reflected in excessive concentrations of nitrogen and phosphorus compounds in surface waters. This is reflected in changes in the physicochemical and biological properties of water quality in water reservoirs [2].
The release of biogenic substances to lake water from bottom sediments occurs through the transition zone that is the near-bottom layer (NBL). This layer is a dynamic area taking part in the regulation of the exchange of chemical substances and organisms between the bottom sediments and the water column [8]. In the lowest areas, directly in contact with the sediments, there are sudden changes in the dynamics of physicochemical factors [9]. Bottom sediments accumulate significant amounts of chemical substances [10,11,12], which can be re-released to the NBL and above water layers [13]. The bottom sediments can be equated to the storage of chemical substances which, under unfavorable conditions, can be released into the NBL and then into the water column. In such conditions for the lake, for example, as oxygen deficits, high temperatures at the bottom, and a low pH, phosphorus and nitrogen compounds and hydrogen sulphide may be released into the water column, leading to eutrophication of the lake [1,2]. Nutrient compounds delivered to the water column are used by phytoplankton [14,15]. At high nutrient concentrations in a lake, phytoplankton absorb them much more efficiently and, as a result, develop faster [16]. Available phosphorus and nitrogen compounds are absorbed by bacterioplankton, phytoplankton, and macrophytes and are accumulated in organisms throughout the growing season. After the growing season, phosphorus is still accumulated in dead plant and animal remains [3]. Further decomposition of these remains can lead to the release of phosphates into the water column, increasing the trophic level of the reservoir [3,16].
The aims of this study were (1) to determine the dependencies between the concentration of nitrogen and phosphorus components in the near-bottom layer and subsurface water of the mid-forest lake and (2) to analyze the dependencies between chemical parameters in both studied water layers of lake Łętowskie.

2. Material and Methods

2.1. Biological and Morphometric Characteristics of Łętowskie Lake

Lake Łętowskie is located in northern Poland in the Pomerania region. It is an important part of the Protected Landscape Area “Lake Łętowskie and the vicinity of Kępice” (Journal of Laws of the Słupsk Voivodeship No. 9, item 23). The Protected Landscape Area “Lake Łętowskie and the vicinity of Kępice” is characterized by high forest cover, amounting to 76% [17]. Around most of the marshy shores of the lake, there is an ash–alder riparian forest and an alder forest [18].
In the areas of the open raised bog in the north-eastern part, there are sites of species subject to species protection: Drosera rotundifolia L., Carex limosa L., Rhododendron tomentosum Harmaja, syn. Ledum palustre L. 1753 [18].
The water surface of lake Łętowskie covers an area of 402 ha, and the maximum depth is 18.7 m [19]. More morphometric features of the lake are presented in Table 1. Despite the increase in the area of lake Łętowskie from 1900 to 2000, its volume decreased. As a result, the volume of the lake decreased from 37 mL m3 from 1900 to 33.1 million m3 in 1960 [20]. According to Brodzińska et al. [19], the lake area has decreased by about 9 ha since the 1960 figure from Ptak [20].
Currently, the main anthropogenic influences are the village of Łętowo and the bathing and recreational areas located on the north-eastern side of the beach.
Research on the lake, especially in terms of the availability of nutrients, is particularly important because the purpose of the established legal protection of the Protected Landscape Area “Łętowskie Lake and the vicinity of Kępice” is to preserve the existing values of the natural environment and maintain the ecological balance of natural systems [17].

2.2. Water Sampling

Water samples were collected in 2023 in three seasons, spring, summer, and autumn, from five research sites (st.1—depth circa 2.5 m; st.2—9 m; st.3—15 m; st.4—6.5 m; st.5—18 m) and simultaneously from two layers. Subsurface water (SW) samples were collected from a depth of 20–30 cm below the surface and from an estimated depth of 1–5 cm above the bottom (NBL) (Figure 1). Water samples were collected using a scoop. The scoop used was connected to a pneumatic pump and was anchored to the bottom using a weight with a silicone hose attached to it so that the hose inlet was approximately 2 cm from the bottom. This ensured a comparable depth of water collection from the NBL. At each research site, the hose was completely rinsed with the NBL from the appropriate research site before sampling. Water samples were siphoned into PET containers. The containers were chemically clean before use and rinsed with the first portion of water from the tested research site to prevent possible contamination [21]. Surface water (SW) samples were collected by immersing the container to a designated depth of 20–30 cm. Water samples, approximately 300 cm3 in volume, were collected into chemically clean PET containers. Each container was rinsed directly at the test site with the first portion of lake water. The samples were stored frozen until analysis.

2.3. Physicochemical Analyses

Water samples were analyzed for dissolved inorganic phosphorus (P-PO4) and dissolved inorganic nitrogen forms (N-NO2, N-NO3, N-NH4) after filtration through membrane filters (0.45 μm, Sartorius) using a flow injection analyzer (FIA compact, MLE GmbH) according to ISO 15681-1:2003 [22], ISO 13395:1996 [23], and ISO 11732:2005 [24], respectively. Total phosphorus (TP) was analyzed according to ISO 15681-1:2003 [25], with digestion carried out using an optimized version of the procedure. The total nitrogen (TN) analysis method was based on ISO 29441:2010 [25].
P-org was calculated from the difference in concentrations between P-T and P-PO4. Similarly, N-org was calculated by subtracting the mineral forms from N-T according to APHA [21]. N-min is the sum of N-NO3, N-NO2 and N-NH4.
Calcium and magnesium were analyzed using a flow injection analyzer (FIA compact, MLE GmbH). Calcium was analyzed with o-cresolphthalein complexone, using 8-hydroxyquinoline as a masking agent for magnesium. Magnesium concentrations were determined through reaction with o,o’-dihydroxy-azo dye (xylidyl blue), with calcium being masked as a chelate complex with ethylene glycol-bis-(2-aminoethyl)-N,N,N’,N’-tetraacetic acid (EGTA), and an excess of barium ions.
Dissolved oxygen content was determined through the potentiometric method. The oxygen meter was conditioned with respect to the atmospheric oxygen concentration. Water pH, electrolytic conductivity, and water temperature (°C) were determined using potentiometric methods according to APHA [21]. The pH meter was calibrated against pH standards (Elemtron), and the correctness of the electrodes for measuring electrolytic conductivity was checked using a reference standard (Milwaukee). The thermometer was calibrated against a laboratory thermometer.

2.4. Statistical Analysis

The statistical analysis was performed in the Statistica software (ver. 13.3) [26] and in the statistical software Past (ver 4.13) according to [27,28] to analyze basic statistical parameters and conduct multidimensional cluster analysis (Ward’s method, Euclidean distance) and PCA. With the aim of determining the type of distribution of the variables, the Shapiro–Wilk test was used. After this, appropriate parametric or nonparametric tests were chosen. If the sample had a normal distribution, then the ANOVA test with the RIR Tukey’s test were used. However, in the case of a lack of a normal distribution, the nonparametric equivalent was used—the Kruskal-Wallis test—in combination with Dunn’s post hoc test.

3. Results

Table 2 presents basic descriptive statistics for individual forms of biogenic substances—nitrogen, phosphorus, dissolved oxygen, calcium, and magnesium—as well as parameters such as pH, EC, and water temperature in the SW and NBL layers of lake Łętowskie. In both studied water layers, the concentration of total N was recorded at a similar level of 0.6 mg dm−3. However, in the case of individual forms, the distribution was dynamic. The concentration of N-NO3 was twice as high in the NBL (on average 46.2 µg dm−3) than that in the SW, while the concentration of N-NH4 was on average 10% higher in the SW. The concentration of N-NO2 was similar in both layers.
The concentration of P-tot in the NBL was more than twice as high as that in the SW. The concentration of P-org was 1.7 times higher and P-PO4 2.3 times higher in the NBL. In the case of Ca and Mg, the concentrations oscillated at a similar level in both water layers. In the NBL, the average oxygen saturation was almost 2.5 times lower than in the SW, but the minimum value in the NBL was only 5%—which indicates periodic oxygen deficits. The pH values in both water layers were comparable, while the EC value was 7% higher in the NBL.
Table 3 presents the statistical significance of the differences between the measured parameters in relation to the research sites. Using the Kruskal–Wallis and post hoc Dunn tests, statistically significant differences were found in the concentrations of N-NH4, N-min, and P-PO4 between spring and autumn, as well as between summer and autumn. In the case of the calcium concentration, statistically significant differences were noted between spring and summer and between summer and autumn. An analogous trend was observed in relation to EC. On the other hand, for the concentration of magnesium, a statistically significant difference was found between spring and summer. In relation to the pH value, a significant difference was found between spring and summer, as well as between spring and autumn, while for N-NO3 and P-tot, one was found between summer and autumn.
Figure 2, Figure 3 and Figure 4 present the concentrations of physicochemical parameters in both studied water layers, the SW and the NBL, in the horizontal plane, taking into account individual research stations located on lake Łętowskie. In the horizontal distribution of parameters such as P-tot, P-org, and EC, higher values were observed in the NBL than those in the SW at practically all studied stations. At stations adjacent to rural and recreational buildings (station 1 and 4), higher values of parameters such as P-PO4, Ca, N-NO3, N-NH4, and O2 were observed in the SW than in the NBL.
Research sites 2 and 3 were located in the area of Łętowskie Lake where the shores are adjacent to forests. At these sites, higher values of N-NO3, EC, P-tot, P-PO4, and P-org were found in the NBL.
Site 5, located in the central part of lake Łętowskie, was characterized by higher values in the NBL for all forms of phosphorus, Ca, Mg, EC, and nitrogen forms, except for N-NH4. In relation to site 5, even three times higher concentrations of such parameters as N-NO3, P-tot, and P-PO4 were found in the NBL than those in the SW, which reflects the differences between both water layers in relation to the tested parameters. Similarly, comparing site 5 with the remaining sites, the difference in the content of these parameters is significant.
Significant differences in concentrations between both the tested water layers and sites were noted for P-org (sites 2 and 3), where the concentration in the NBL was more than twice as high as that in the SW.
For N-NO2, very low concentrations were recorded at all tested sites in both water layers, except for site 1 in the NBL, where these values were even 65 times higher than those in the SW and those at site 3 in the SW.
Figure 5 shows the dependencies of the studied parameters in the SW and NBL between seasons in the conducted studies of lake Łętowskie. For the concentrations of N-tot, N-NH4, Mg, Ca, and N-NO3, a uniform distribution of values is visible in relation to the seasons, with the minimum values observed in the summer season, while the concentration of N-org was observed to be maximum in both layers in the summer season. For all forms of phosphorus, the maximum concentration was found in the NBL in the summer season, while the minimum concentration in this season was found in the SW.
In spring, the highest N-NO3 concentrations were observed in the NBL, with a significant decrease in N-NO3 concentrations in summer and a renewed increase in autumn. In SW, however, the concentrations were the lowest in spring and then systematically increased in summer and autumn. For N-NO2, the opposite distribution of concentrations to those for N-NO3 was noted. Here, the highest concentrations were observed in spring in the SW, with a decrease in summer and autumn, while in the NBL, the lowest concentration was noted in spring, with a systematic increase until autumn. The seasonal distribution of the pH was similar to the distributions of the Ca and Mg concentrations. The level of water oxygen saturation was lower in the SW, with a minimum in summer, while at the same time, the maximum values were observed in the SW, while the highest EC was observed in summer in the NBL and the lowest values were seen in spring in SW.

4. Discussion

The trophic state of lakes is mainly influenced by the inflow of biogenic substances from the catchment area and the processes of transport and transformation of matter occurring inside the lake [1]. In the presented studies of lake Łętowskie, the level of biogenic substances in the water column is probably significantly influenced by processes occurring inside the lake which result in the exchange of matter between the bottom sediment and the NBL, especially in its shallower areas. In lake Łętowskie, higher concentrations of phosphorus compounds are visible at practically all of the studied sites in the NBL than those in the SW (Figure 2). This constitutes large potential for the release of these compounds into the water column in the vertical profile. According to Carlson’s TSI (TP) index [29], the average trophic level for the studied species corresponds to eutrophy. However, when examining TSI seasonally, the values of this index corresponded to mesotrophy in spring, olygotrophy in summer, and eutrophy in autumn.
Lake Łętowskie is characterized by a large surface area and wide shallows at the shores [18]. The ratio of the surface area to the depth of the lake favors the release of matter from the bottom sediments to the water column. Strong winds have a stimulating effect on this process, similarly to Lake Gardno [1]. The average annual wind speed was estimated at 4 m/s [30]. According to data for the city of Słupsk, approximately 25 km away, the main wind directions were most common: from the south (24.1%), west (18.8%), and southwest (17.3%) [31].
The processes of the internal release of biogenic substances, especially phosphorus from the bottom sediments of the lake, can cause its transport to the water column [2,3] and then accumulation in phytoplankton and macrophytes. As a result of internal nutrient loading in the lakes, a stream of biogenic substances carried from the bottom sediments is created [10]. Significant loads of organic carbon, mineral, and organic nitrogen and phosphorus compounds are accumulated in the bottom sediments of lakes. Their concentrations can be several thousand times higher than those observed in the water column [4,32]. This represents a potential source of these substances.
In lakes with a large surface area, strong winds cause water ripples and turbidity of the bottom sediments. As a result, phosphorus compounds are released into the water column. Studies conducted on Lake Gardno, located about 30 km from lake Łętowskie, indicate that strong winds caused mixing of water with particles of matter lifted from the bottom sediments [1]. In some lakes, the phenomenon of internal nutrient loading in lakes is significant and delivers such significant amounts of phosphates to the water column from bottom sediments that it may have a negative impact on the efficiency of lake restoration [2,33].
Trojanowski and Trojanowska [1], based on studies of Lake Gardno, showed that internal changes and especially the exchange of matter between the bottom sediment and water are much more intensive in shallower reservoirs characterized by a larger surface area and exposed to wind. In the presented studies of lake Łętowskie, an influence of winds on water mixing was observed in the case of sites 1 and 4 for such parameters as P-tot, P-PO4, P-org, Ca, N-NO3, N-NH4, N-tot, and N-org, where the concentrations in the NBL and SW were comparable. Due to the significant exposure to trees, the sites are susceptible to wind influence. Also, their small depth, according to Brodzińska et al. [19], favors the mixing of waters from both layers. Also important are the impacts of the catchment during rainy periods, with significant amounts of mineral substances provided from the catchment [34], which would explain the increase in the concentrations of mineral forms of nitrogen and phosphorus and the EC in lake Łętowskie in autumn (Figure 5).
The opposite trend, confirming the conclusion of Trojanowski and Trojanowska [1], was observed for site 5 (with a depth of about 18 m). For all of the parameters studied, except for N-NH4, the concentrations were significantly higher in the NBL than in the SW. For parameters such as P-tot, P-PO4, and N-NO3, these were increased several times over, which clearly distinguishes the chemistry of the SW and the NBL in the deep areas of the lake. On the contrary, in relation to the exposed sites (sites 1 and 4), sites 2 and 3, also located at a small depth but surrounded by forests from the shore and less exposed to direct wind gusts, showed higher concentrations of P-tot, P-PO4, P-org, and N-NO3 in the NBL than those in the SW. According to Bartoszek [3], the difference in concentrations between the NBL and interstitial water probably caused the transfer of phosphates from the bottom sediments to the NBL.
The oxygenation of the NBL of lake Łętowskie in the studied period was on average 41.3% (Figure 3). At sites 1 to 4 of lake Łętowskie, the saturation of the NBL water with oxygen was close to 50%, which could have promoted nitrification processes. The observed high value of N-NO2 at site 1 (10.8 µg dm−3) probably indicates high dynamics of nitrification processes. These constituents are intermediate products of the nitrogen cycle and are caused mainly by the oxidation of N-NH4 in the nitrification process [35].
Even at the deepest level of 5, the average oxygen saturation in the NBL was about 24% (Figure 3), which could also have contributed to the occurrence of relatively high N-NO3 concentrations at this site. According to Trojanowski and Trojanowska [1], a high oxygen concentration in the NBL contributes to the occurrence of mineral nitrogen in the form of N-NO3 and also enables the processes of the mineralization of organic matter into N-NH4, as observed in the results obtained in lake Łętowskie. Moreover, denitrification is influenced by a higher water temperature [14], which was relatively high in the NBL of lake Łętowskie.
As a result of turbulence, the bottom sediments mix with water, and then they are well oxygenated and the oxygen concentration in the NBL is high [1]. Hence, in lake Łętowski, at site 1 (an exposed site), the oxygenation was the highest, at around 54.2%, in the NBL, while at site 2, covered on three sides by trees, the lowest oxygenation was observed—at around 36.8%.
As indicated by Bartoszek [3], the accumulation of organic matter in the bottom sediments often leads to the formation of oxygen deficits in the NBL and in eutrophic lakes to the replacement of aerobic bacterial metabolism by anaerobic fermentation. In lake Łętowskie, even in the deepest areas (site 5), only in the summer period was an oxygen concentration below 10% observed in 2023 (minimum value: 3.4%; site 5; Table 2). In the bottom sediments, especially at site 5, the smell of hydrogen sulphide was noticeable, which indicated the occurrence of periodic oxygen deficits. In previous periods, these deficits were more frequent, which is confirmed by the studies of Korzeniewski et al. [36], and the bottom sediments were rich in biogenic substances [37]. Currently, the level of biogenic substances present in lake Łętowskie is comparable to that in the nearby lakes of Pomerania (Table 4). The data collected in Table 4 from lakes located around the Baltic Sea basin indicate the eutrophic nature of lake Łętowskie.
An analysis of the data analysis graph for the multidimensional PCA provides important information. It is visible in Figure 6 that the obtained concentrations of P-PO4, P-org, N-tot, N-org, N-NH4, N-min, N-NO3, and P-tot, EC, Ca, and Mg correlate with the water samples from the NBL. This is probably related to their higher concentrations in the NBL than those in the SW. Using the PCA, a strong relationship was shown between the concentrations in the water of Łętowskie Lake of P-tot and P-PO4. This is probably related to the seasonal dynamics of oxygenation observed in Łętowskie Lake, together with periodic oxygen deficits and pH, factors which affect phosphorus transformations in the NBL–bottom sediment system [33]. The above information obtained from the PCA diagram can be explained by the processes of matter transformation at the water–bottom sediment boundary: sedimentation, as well as the release of biogenic substances stored in the bottom sediments in earlier periods, which is consistent with the data presented in the studies by Trojanowski et al. [37] and Korzeniewski et al. [36]. Biogenic substances may be released from bottom sediments into the water column, posing a potential threat to the entire aquatic ecosystem in the future [4]. The binding of chemical compounds by lake bottom sediments is not a permanent phenomenon. Under unfavorable conditions, they are released into the water due to such factors as changes in oxygenation, redox potential, pH, temperature, the chemical composition of the bottom sediment, and the concentration gradient between the NBL and interstitial water [38].
The seasonal dynamics of nutrients provides further data (Figure 5). Based on the analysis of the seasonal dynamics of phosphorus compounds in the NBL, it can be assumed that P is bound in lake Łętowskie during spring and autumn. The studies of lake Łętowskie show a minimum concentration of P-tot and P-PO4 in the NBL in the spring season and a low value in autumn despite high values in the SW at the same time (Figure 5). Bartoszek [3] explains similar observations according to the release of phosphorus compounds from the bottom sediments to the water column. The increase in the amount of nutrients in the lake in the spring period may be caused by intensive surface runoff from the direct catchment area at that time [16]. Moreover, the transport of nutrients and matter produced by plankton in the water column and turbid sediment occurs simultaneously from top to bottom [1]. In lakes, the main process of phosphorus withdrawal from biological circulation may be its sedimentation to the bottom sediments of the lake [33].
Table 4. Concentrations of biogenic substances in water of chosen lakes.
Table 4. Concentrations of biogenic substances in water of chosen lakes.
ReferencesECpHO2MgCaP-PO4P-orgP-totN-minN-NH4N-NO2N-NO3N-orgN-totLake Country
µS cm−1omg dm−3mg dm−3mg dm−3µg dm−3µg dm−3µg dm−3µg dm−3µg dm−3µg dm−3µg dm−3mg dm−3mg dm−3
[39] 10.5 ± 1.3 11.7 ± 3.911.6 ± 423.2 ± 8.6 18.4 ± 6.5 24.9 ± 8.3 0.61 ± 0.34 *Gardno,
Poland
[40]89.096.64 76.2147.08123.3 Dołgie Wielkie, Poland
[15] 8.22, 8.66 35.4, 75.9113.4, 136.0152.1, 199.0 22.9, 70.0 37.9, 43.60.82, 1.060.89, 1.11Łebsko,
Poland (low and high value from mean on st. 1–5)
[41] 5.02 0.320.8 3.1 1250.1350.261Lakes South of Norvay (1995 year)
[41] 5.38 0.250.7 3.1 710.1590.23Lakes South of Norvay (2019 year)
[42] 49 (21–100) 0.637 (0.410–1.200)Peipsi,
Estonia **
[42] 102 (52–180) ** 1.114 (0.64–1.80)Lämmijärv,
Estonia **
[42] 21 (54–220) ** 1.189 (0.930–1.700)Pihkva,
Estonia **
[43]352.42 ± 5.78.35 ± 0.0610.69 ± 0.81 0.02 ± 0.02 *** 0.15 ± 0.06 *** 0.65 ± 0.07 *** 1.35 ± 0.99Plitvice Lakes, Croatia
data of spring period
[44]17.77 (5.56–39.20)5.42 (4.61–7.40) 0.258 (0.034–2.354)2.551 (0.241–7.378) 0.138 NH4+ (0.002–1.159) 0.688 NO3− (0.001–1.925) lake waters in the Tatra National Park, Poland
[45]540–8357.0–8.90–17.96 0.02–0.71 *** 0.04–0.85 ***0.9–10.57 ***0.53–7.44 ***0.0–0.02 ***0.0–9.96 ***0.55–5.082.76–13.07Swarzęckie,
Poland **
[46]85.4 ± 6.27.7 ± 0.93 7.7 ± 0.90.037 ± 0.01 0.159 ± 0.020.53 ± 0.110.3 ± 0.0740.001 ± 0.00040.24 ± 0.0662.36 ± 1.33 Jeleń,
Poland (2018 year)
[47]321–3688.12–9.02 7.2–14.049.98–57.83 0.16–1.30 *** Giłwa (Poland)
* N-Kiejdahl. ** minimum value and maximum. *** mg dm−3.
A multivariate cluster analysis was carried out (Figure 7). Based on this analysis, it is visible that the studied SW and NBL differ fundamentally in terms of chemistry and show different correlations between the studied chemical components. The multivariate cluster analysis provides information that the near-bottom waters (NBL) are connected by matter transport processes with the bottom sediments. The P-tot and P-PO4 compounds are closely connected with each other, which may result from the release of these compounds from the bottom sediments of the lake (Figure 7). Also, mineral forms of nitrogen, N-NH4 and N-NO2, N-NO3, and Ca and Mg formed a common cluster, which probably results from the mineralization processes of organic matter at the bottom of the lake.
The chemical composition of the waters of lake Łętowskie is particularly influenced by its largely non-outflowing nature. According to Trojanowski et al. [37], only small streams supplying matter from forests flow into the lake, while in the north of the lake, there is a narrow stream where water flows away slowly. Biogenic substances can be supplied to lake Łętowskie from the land by surface runoff, which affects relatively short distances and is significantly dependent on the terrain. Undoubtedly, a significant source of the chemicals supplied to the lake is the surrounding forests with locally occurring marshes. This constitutes about three quarters of the shores of lake Łętowskie. This affects the median pH value. The highest pHs of 7.5 and 7.6 were observed near site 4 in the SSL and the NBL, respectively. This is related to the nearby ash–alder forests and alder forests where deciduous trees predominate [18]. At sites 1 and 2, lower pH values of 7.2 and 7.3 were observed, which corresponds to the nearby pine forests [18] that grow on acidic soils, and additionally, these areas are supplied by small forest streams [37]. In lake Łętowskie, the calcium level was 33.3 mg dm−3, which is lower than that in nearby lakes but prevents a significant decrease in pH due to the forest catchment area. For comparison, in nearby Lake Człuchowskie, it is almost 60 mg dm−3 [46], while in Lake Giłwa, it is 54.98 mg dm−3 [47]. On the other hand, nearby lobelia lake Jeleń also has a forest catchment area of only 7.7 mg Ca dm−3, according to Klimaszyk et al. [48]. In lakes with a low water pH in the Tatra National Park, the calcium concentrations were even lower, ranging only from 0.24 to 7.38 mg Ca dm−3 [44], as they were in the lakes of South Norway, with acidic waters—0.7 mg Ca dm−3 [41].
In lake Łętowskie, the weight ratio of N:P was 18.9. In lakes, the main source of nitrogen is surface runoff but also inflows of biogenic substances, along with rivers and atmospheric deposition and resuspension from lake bottom sediments [49]. The direct catchment area of lake Łętowskie is forest-based. This may significantly stimulate a high N:P ratio. The effect of surface runoff in lake Łętowskie is intense due to the elevation of the surrounding forests relative to the water level. Within 500 m of the lake, the forests grow at elevations ranging from a few meters to several dozen meters, while on the eastern side, the elevations are even higher, reaching nearly 30 m above the water surface [50]. The average rainfall in the study area in 2023 ranged from 600 to 800 mm [51]. Furthermore, streams carrying chemical substances from forest areas flow into the lake: from the west, south, and east (connecting the lake waters with the forest marshes) and from the north-east [50]. A forested catchment can significantly increase the amount of nitrogen [52,53], especially in the form of nitrate nitrogen, by supplying surface runoff with these compounds [53]. This information corresponds to the higher N-NO3 concentrations observed in the southern part of lake Łętowskie surrounded by dense forests. According to Downing and McCauly [45], lakes receiving primary runoff from forests should have a higher TN:TP ratio than that in lakes with significant inflows of groundwater or rivers. The second reason for the high N:P ratio may be biogenic substances released from the bottom sediments accumulated there in earlier periods. As indicated by Potasznik et al. [49], a high amount of nitrate nitrogen may indicate pollution of agricultural origin within the catchment area. As indicated by Trojanowski and Trojanowska [1], domestic–municipal and industrial pollution are suggested. However, currently, the lake is not a direct recipient of such sewage from outside. With good oxygen conditions in the NBL, phosphorus compounds are bound in the bottom sediments, and their release to the water column is slow [33]. Therefore, nitrogen compounds are probably released at a faster rate than phosphorus, increasing the N:P ratio.

5. Conclusions

During the study period, phosphorus compounds in lake Łętowskie were usually found in higher concentrations in the NBL than in the SW. This suggests the internal loading of phosphorus compounds from the bottom sediments into the NBL. The positive information is that significant amounts of oxygen were observed in the NBL, which certainly slowed down the processes of the internal release of phosphorus into the NBL. This is a good prognosis for the improvement of the lake’s condition, as is the fact that low concentrations of phosphorus forms were observed in the SW during the summer, as well as a low presence of nitrate nitrogen (V) in the NBL.
Organic nitrogen concentrations were comparable in both layers. It is likely that intensive processes associated with the release of matter along with surface runoff rich in nitrogen compounds supplied the lake with significant amounts of organic matter from the surrounding forests. The influence of the forest catchment area was confirmed by the high N:P ratio observed in the waters of lake Łętowskie.
Winds mixing the lake waters also have a significant impact on the distribution of biogenic substances, mainly nitrogen compounds, in the lake, especially in shallower areas such as the northern part, which is most exposed to wind.
The protection of the ‘Łętowskie Lake and the vicinity of Kępice’ area has a beneficial effect on the good condition of the lake; improves water quality; and prevents the intensification of anthropogenic pressure. It is recommended to continue this form of lake protection and further hydrochemical research.

Author Contributions

Conceptualization: J.A. and T.W.; methodology: M.R., T.W. and J.A.; software: J.A.; investigation: J.A., M.R. and T.W., resources: J.A. and T.W.; data curation: M.R., T.W. and J.A.; writing—original draft preparation: J.A., M.R. and T.W.; writing—review and editing: T.W.; visualization: J.A. and T.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by an internal Pomeranian University in Słupsk grant No. 7.4.14.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the author/s.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A contour map of north Europe (A) and the sampling stations on lake Łętowskie—approximate location (B).
Figure 1. A contour map of north Europe (A) and the sampling stations on lake Łętowskie—approximate location (B).
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Figure 2. Concentrations of nitrogen forms and dissolved oxygen in the SW and NBL at the selected sampling stations at lake Łętowskie.
Figure 2. Concentrations of nitrogen forms and dissolved oxygen in the SW and NBL at the selected sampling stations at lake Łętowskie.
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Figure 3. Concentrations of phosphorus forms, calcium, and magnesium and water reaction in the SW and NBL at the selected sampling stations in lake Łętowskie.
Figure 3. Concentrations of phosphorus forms, calcium, and magnesium and water reaction in the SW and NBL at the selected sampling stations in lake Łętowskie.
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Figure 4. Electric conductivity in the SW and NBL at the selected sampling stations in lake Łętowskie.
Figure 4. Electric conductivity in the SW and NBL at the selected sampling stations in lake Łętowskie.
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Figure 5. Seasonal variations in concentration of biogenic substances, EC, pH, and temp in the SW and NBL of lake Łętowskie.
Figure 5. Seasonal variations in concentration of biogenic substances, EC, pH, and temp in the SW and NBL of lake Łętowskie.
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Figure 6. Principal component analysis ordering of parameters and variables on axes from Components 1 and 2.
Figure 6. Principal component analysis ordering of parameters and variables on axes from Components 1 and 2.
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Figure 7. Cluster analysis performed for concentrations of selected nutrients and in the SW and NBL in lake Łętowskie (parameters applied: Euclidean distance, Ward method).
Figure 7. Cluster analysis performed for concentrations of selected nutrients and in the SW and NBL in lake Łętowskie (parameters applied: Euclidean distance, Ward method).
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Table 1. The morphometric features of lake Łętowskie according to Brodzińska et al. [19].
Table 1. The morphometric features of lake Łętowskie according to Brodzińska et al. [19].
Parameter and UnitValue
Latitude and longitude54°16.2′ 16°49.7′
Surface area (ha)402
Volume (103 m3)33,128.50
Maximum depth (m)18.7
Average depth (m)8.2
Maximum length (m)2800
Maximum width (m)1900
Table 2. Concentrations of biogenic substances in SW and NBL of lake Łętowskie.
Table 2. Concentrations of biogenic substances in SW and NBL of lake Łętowskie.
SW NBL
ParameterUnitMeanMin.Max.SDMedianMeanMin.Max.SDMedian
N-totmg dm−30.60.40.80.10.50.60.40.90.20.5
N-orgmg dm−30.40.20.70.10.40.40.30.80.10.4
N-org %%75 73.8
N-NO3µg dm−3232.56325.2746.24342.585.715.5
N-NO2µg dm−32.9033.58.602.60328.10.5
N-NH4µg dm−3117.439205.565100.5104.425195.564.6106
N-minµg dm−3143.343.526284.6113.5153.229393109.7133.5
P-totµg dm−331.86.5682622.567.68.52726359.5
P-orgµg dm−39.14.5163.9815.44.5398.213
P-org %%28.6 22.7
P-PO4µg dm−322.70.559.526.2752.20.525761.544.5
Camg dm−333.321.340.1636.433.421.338.14.433.3
Mgmg dm−32.51.43.10.52.62.51.430.42.5
O2%104.369.6125.415104.241.33.4120.723.744.9
pH 7.46.68.50.67.17.46.68.30.67.1
ECµS cm−1235192.2250.217.5244.2257.5227.9341.327.7249.2
Temp.°C16.68.323.26.219.313.88.322.44.314.3
Table 3. The result of the Kruskal–Wallis test together with the post hoc Dunn test performed for obtained biogenic substance concentrations, for which the existence of a normal distribution was demonstrated (n = 27). Data with a normal distribution are demonstrated in the table, where the results of the ANOVA test together with the Tukey RIR test are presented (n = 27).
Table 3. The result of the Kruskal–Wallis test together with the post hoc Dunn test performed for obtained biogenic substance concentrations, for which the existence of a normal distribution was demonstrated (n = 27). Data with a normal distribution are demonstrated in the table, where the results of the ANOVA test together with the Tukey RIR test are presented (n = 27).
Kruskal–Wallis TestPost Hoc Dunn Test
ParameterHp
N-NO315.68***Su.–Au.
N-NO20.76nsns
N-NH416.85***Sp.–Au., Su.–Au.
N-tot7.34*Su.–Au.
P-PO411.95**Sp.–Au., Su.–Au.
P-tot9.32**Su.–Au.
Ca13.27**Sp.–Su., Su.–Au.
Mg7.51*Sp.–Su.
P-org5.89nsns
N-org0.19nsns
N-min16.83***Sp.–Au., Su.–Au.
N-org%13.82**Su.–Au.
pH19.5***Sp.–Su., Sp.–Au.
Temp.20.84***Sp.–Au., Su.–Au.
EC10.37*Sp.–Au., Su.–Au.
P-org%15.68***Sp.–Au., Su.–Au.
ANOVA testRIR Tukey test
ParameterFp
O20.05nsns
Explanations: *—p < 0.05; **—p < 0.01; ***—p < 0.001; ns—nonsignificant; Sp—spring; Su—summer; Au—autumn.
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Antonowicz, J.; Rybak, M.; Wróblewski, T. A Relationship Between Nutrients in a Mid-Forest Eutrophic Lake. Water 2025, 17, 2913. https://doi.org/10.3390/w17192913

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Antonowicz J, Rybak M, Wróblewski T. A Relationship Between Nutrients in a Mid-Forest Eutrophic Lake. Water. 2025; 17(19):2913. https://doi.org/10.3390/w17192913

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Antonowicz, Józef, Michał Rybak, and Tomasz Wróblewski. 2025. "A Relationship Between Nutrients in a Mid-Forest Eutrophic Lake" Water 17, no. 19: 2913. https://doi.org/10.3390/w17192913

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Antonowicz, J., Rybak, M., & Wróblewski, T. (2025). A Relationship Between Nutrients in a Mid-Forest Eutrophic Lake. Water, 17(19), 2913. https://doi.org/10.3390/w17192913

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