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Article

Maintaning the Durability of the Effects of Urban Lake Restoration—New Challenges

by
Jolanta Katarzyna Grochowska
* and
Renata Augustyniak-Tunowska
Department of Water Protection Engineering and Environment Microbiology, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, St. Prawocheńskiego 1, 10-720 Olsztyn, Poland
*
Author to whom correspondence should be addressed.
Water 2025, 17(19), 2893; https://doi.org/10.3390/w17192893
Submission received: 7 August 2025 / Revised: 26 September 2025 / Accepted: 2 October 2025 / Published: 5 October 2025
(This article belongs to the Special Issue Water and Society: Challenges for Freshwater Quality Under a Climate Change Scenario)
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Abstract

The main aim of this study was to analyze the excessive biomass of invasive alien aquatic plants reducing the water quality of a lake which was restored in the past. This study was conducted on Długie Lake (26.8 ha, 17.3 m, Masurian Lake District, northeastern Poland), which was completely degraded by raw wastewater inflow. After the long-term restoration (1987–2003) and recovery of submerged macrophyte meadows, the invasion of Elodea nuttallii—an invasive alien aquatic plant (IAAP)—was observed due to the increasing water temperature in recent years, impairing the functioning, biodiversity, and ecosystem services of this urban lake, as well as causing the deterioration of lake water quality. Therefore, an excessive biomass of E. nuttallii has been removed from the lake since 2022. The analysis of physico-chemical water quality parameters showed that consecutive excessive biomass macrophyte gradual removal (three times during the growing season) helps to limit the excessive growth of E. nuttallii and also removes nutrient loads from the ecosystem. Removing excess aquatic vegetation also helps maintain the lake’s aesthetic and recreational value. Currently, the total phosphorus concentration in lake water did not exceed 0.3 mg P/L and total nitrogen did not exceed 2.0 mg N/L. Chlorophyll a contents oscillated in the range of 5 to 9 µg/L, and Secchi disk visibility exceeded 3 m.

1. Introduction

The increased eutrophication of lakes, which began over 100 years ago, is associated with many unfavorable phenomena affecting the management of water resources and constitutes a limitation to civilization’s development [1]. This phenomenon is primarily caused by the transformation of lakes into recipients of sewage, the conversion of adjacent areas into fertilized arable land, manure runoff from animal farms, and the removal of forests and grasslands in lake basins in favor of urbanized areas. This degradation manifests itself in several ways [2,3]. First, water quality declines, as evidenced by increased concentrations of nutrients. The surface water layer experiences hyperoxia due to increased photosynthesis, while the bottom layer experiences oxygen deficiency as oxygen is consumed by the decomposition of excessive amounts of organic matter. Under anaerobic conditions, an additional process of nutrients’ internal loading begins. Water transparency decreases, and phytoplankton and cyanobacteria blooms become visible [4,5]. Secondly, there is a decline in biodiversity in water bodies and the disappearance of species with high environmental requirements, which are now called bioindicators [6,7]. Another symptom of lakes’ degradation is the decline in their ecosystem services [8].
Accelerated lake degradation has forced the search for effective ways to reverse or slow down this process and its unfavorable consequences. According to Søndergaard et al. [9] the first concepts for lake restoration methods were put forward by Thomas in the early 1940s, proposing numerous biological and technical methods capable of halting the rate of eutrophication, but the first attempts at the practical application of these methods were undertaken only in the mid-1950s. Since then, numerous remediation methods have been implemented and their effectiveness described in Poland and worldwide [10,11,12,13,14,15].
Despite many successful protection and restoration efforts, the problem of the increased eutrophication and degradation of surface waters remains. According to data from the General Directorate for Environmental Protection (GDEP), over 90% of lakes in Poland still require radical restoration action because they are in poor ecological condition and fail to meet the Water Framework Directive guidelines. The situation is exacerbated by climate change and the invasion of alien plant and animal species. According to Gizińska and Sojka [16], Wang et al. [17] and Huang et al. [18] global warming increases water temperature and worsens oxygen conditions in lakes.
In a study by Woolwey et al. [19,20] and Zhang et al. [21], oxygen concentrations declined by 0.049 mg/L per decade, which exceeds the rate of decline observed in oceans (0.022 mg/L per decade) and rivers (0.038 mg/L per decade). This decline is alarming and suggests that lakes are more vulnerable to the effects of climate change than other water bodies. Warmer water has a lower capacity to dissolve oxygen, and as temperatures rise, vertical mixing decreases, hindering oxygen transport to the deeper layers of the lake. A high water temperature promotes stronger algal blooms, which increase oxygen production during photosynthesis. However, the intense decomposition of these algae at other times causes serious oxygen deficiencies. This decrease in oxygen levels has far-reaching ecological consequences, causing stress for many fish species and reducing their growth and food consumption [22,23].
Another serious problem is the emergence of invasive alien organisms (IAOs) in water bodies, i.e., plants and animals that are not native to ecosystems and can cause environmental or economic damage. Invasive species, in particular, negatively impact biodiversity, including by reducing populations or eliminating native species through food competition, predation, or the transmission of pathogens [24]. Due to the lack of natural enemies, greater tolerance to unfavorable conditions, and faster reproduction rates, invasive species gain an advantage in the competition for food, light, and sites for development and breeding [25,26]. It should be emphasized here that the biodiversity of underwater meadows restored through reclamation may be threatened by IAAP.
An example of an invasive alien plant is the Elodea nuttallii, which began aggressively spreading in many water bodies several years ago, massively displacing native species. Elodea nuttallii demonstrates the ability to grow in masses and produce enormous amounts of biomass [27,28], can overgrow the entire water column, or form detached, free-floating mats that block light from the deeper zones of the water body. By competing with native species for food resources and light, this plant causes them to decline until they are completely displaced. The dense tissues of Elodea nuttallii create difficulties in the recreational and economic use of water bodies, impair the operation of boat engines, and hinder fishing and commercial fishing.
While the impact of invasive alien species on biodiversity is relatively well understood, there is a limited number of reports on their impact on the durability of lake restoration efforts and water properties in the context of changing climate. Furthermore, the impact of vegetation removal on water chemistry has not been studied, particularly in the context of nutrient depletion.
This study aims to track the changes in the values of indicators defining the lake’s trophic status and parameters indicating the level of primary production in Lake Długie, which was restored using artificial aeration and phosphorus inactivation. Fifteen years after the end of the lake’s restoration, which brought about a spectacular improvement in water quality, the presence of the invasive plant Elodea nuttallii has been detected in the ecosystem. This plant is harvested from the water by mowing.

2. Material and Methods

2.1. Description of Research Area

Długie Lake (DL) is located in Olsztyn, in the Masurian Lake District (northeastern Poland, latitude 20°27.8′ E, latitude 53°47.2′ N). The DL has a surface area of 26.8 ha and a maximum depth of 17.3 m (Figure 1). The lake has an elongated shape in the north-south (N-S) direction. The depth index of 0.3 and a relative depth of 0.0334 indicate that the lake is deeply embedded in the substratum, despite shallower sections in the southern and northern parts of the lake. DL has a relatively well-diversified shoreline, as evidenced by the shoreline development value of 2.23 (Figure 1). The DL is devoid of outflow, fed primarily by direct surface runoff and springs. The lake’s catchment area covers an area of 115.4 ha.

2.2. Restoration Phases

Between 1956 and 1976, municipal sewage from the residential area located on the western shore of the lake was discharged into the DL through an emergency overflow at a rate of 350 to 400 m3/day. The lake was degraded, as confirmed by physicochemical tests of the water [30]. According to Gawrońska et al. [31], the degradation of the DL was evidenced by the overoxidation of surface waters reaching 290% oxygen saturation, the deoxidation of waters from 2 m depth, and high concentrations of nutrients. Total nitrogen concentrations in the bottom waters reached 30 mg N/L, and total phosphorus exceeded 12 mg P/L. BOD5 values reached 75 mg O2/L, and COD reached 42 mg O2/L.
Studies conducted after the sewage inflow was cut off showed that the effects of lake degradation are irreversible, and that improving the environmental conditions of the lake is only possible through the use of appropriate restoration methods [31].
From 1987 to 2000, the lake was restored using artificial aeration with thermal destratification in two stages (stage I 1987–1990; stage II 1991–2000) (Figure 2).
When improving water quality in the DL through artificial aeration was no longer possible due to the low sorption capacity of bottom sediments resulting from low iron and manganese contents, a phosphorus inactivation method was employed. This method involves precipitating phosphorus from the water column and blocking it in the bottom sediments by introducing iron or aluminum salts into the lake water, which bind phosphorus. After applying a coagulant, the bottom sediments have an improved sorption capacity. Phosphorus in the DL was inactivated using the aluminum coagulant PAX 18 (Figure 3). For the first time in Poland and worldwide, a new-generation coagulant, in liquid form, was used to inactivate phosphorus.
After the completion of long-term lake restoration (13 years of artificial aeration with thermal destratification and 3 years of phosphorus inactivation), the maximum concentration of total phosphorus in the water was 0.144 mg P/L (bottom layer), total nitrogen 2.0 mg N/L (bottom layer), BOD5—3.5 mg O2/L, and the amount of chlorophyll a—5.3 µg/L. Secchi disk visibility reached over 7 m, with an average of 3.5 m [32].

2.3. Data Collection

The study of Lake Długie was carried out at monthly intervals in the vegetation period from April to September 2000, 2003, 2018, 2020, 2022, and 2024. According to the guidelines used in limnological research, water samples for laboratory analysis were collected at the lake’s deepest point (17.3 m), which was located according to a bathymetric map and GPS. At the measuring station, water samples (432) were collected using a 3.5 L Ruttner sampler (Geomor Technik, Szczecin, Poland) into 2 L plastic bottles from 1 m below the surface and 1 m above the bottom. At the measuring station, at every meter of depth, water temperature, oxygen content, and chlorophyll a were measured using a YSI 6600V2 multiparameter sensor (Yellow Springs Instruments Company, Yellow Springs, OH, USA). Water transparency was measured using a Secchi disk (KC Denmark Research Equipment, Silkeborg, Denmark).

2.4. Analysis

The scope of the water analysis included phosphorus and nitrogen compounds, and BOD5 (biochemical oxygen demand). Chemical analyses of water were performed under standard methods [33]. All chemical analyses were performed in triplicate, adhering to all standards. The coefficient of variation (CV) for the repeated analysis was 2%.
The results of research from 1984 were obtained from the Archives of the Department of Water Protection Engineering and Environmental Microbiology, University of Warmia and Mazury in Olsztyn.
The obtained results of selected water quality parameters were statistically analyzed (one-way ANOVA, p = 0.05, Tukey’s HSD) using the Statistica 13.3 software package [34]. The alternative tested hypothesis presumed the presence of significant differences in the content of the selected parameters of water between the control year 1984 (before restoration) and experimental years 2000 (the last year of artificial aeration), 2003 (the last year of phosphorus inactivation), 2018 (before the growth of an invasive plant), 2020 (the appearance of the invasive plant), 2022, and 2024 (biomanipulation by harvesting the invasive plant) (Table 1).

3. Results

DL is a holomictic reservoir with weak water dynamics, which was confirmed by the distinct temperature variations in the water column that appeared in spring. The water temperature in the 2–3 m-thick layer varied widely in the selected study years: from 9 °C to 17.8 °C (Figure 4). Deeper, the temperature dropped rapidly, reaching 4.2 °C at the bottom in 1984 and 2003, and approximately 6 °C in 2022 and 2024.
At the peak of summer stagnation, the epilimnion layer was typically 3 to 5 m thick and heated between 18.9 °C and 25.5 °C. Beneath the epilimnion was a metalimnion with a maximum gradient of 6.6 °C/m and a hypolimnion with a temperature above the bottom ranging from 4.9 °C to 6.4 °C (Figure 4).
The exception is the results from 2000, when the lake was artificially aerated and the water temperature was basically equal from the surface to the bottom due to the aeration system activated in spring, which caused water circulation.
High oxygen concentrations in spring were recorded in the water layer up to 5 m deep, and below that the amount of this gas gradually decreased to analytical zero in tests performed before restoration, and in subsequent years to the range of 2.4 to 7.7 mg O2/L (Figure 5). During artificial aeration, the oxygen content in the surface water layers was 16.5 mg O2/L in spring, while at the bottom it was 13.5 mg O2/L (Figure 5).
During the peak summer stagnation period, water oxygenation was less favorable. In the epilimnion, oxygen concentrations ranged from 8.5 to 10.6 mg O2/L. In the metalimnion, the amount of this gas decreased rapidly, and the hypolimnion water was deoxygenated or contained trace amounts of oxygen (2024). In 2000, during aeration, oxygen concentrations were not high in the water column and ranged between 2.1 and 4.2 mg O2/L (Figure 5).
The statistical analysis revealed significant differences in the mineral phosphorus content in the entire volume of DL water (Table 1). In the 1980s, the mean content of phosphate in the surface water layer was 0.079 ± 0.086 mg P/L, and 2.280 ± 0.275 mg P/L in the near-bottom water layer. Restoration techniques, such as artificial aeration and P inactivation, caused a very high reduction in mineral phosphorus content (to a mean value 0.002 ± 0.003 mg P/L in surface water and 0.019 ± 0.019 mg P/L in the near-bottom water). Fifteen years after the end of the restoration procedure, the average concentration of phosphates in the surface water layer was 0.010 ± 0.008 mg P/L and 0.056 ± 0.043 mg P/L in the over-bottom water layer (Figure 6). When the presence of invasive plants was noted (2020), the mean values decreased to 0.007 ± 0.003 mg P/L in the upper water layer and increased to 0.108 ± 0.079 mg P/L near the bottom. In the following research years, a small increase in phosphorus amounts, to approximately 0.008 ± 0.002 mg P/L at the surface and up to 0.115 ± 0.005 mg P/L in the over-bottom water (Figure 7), was observed. In the DL water, significant differences were noted in the content of organic phosphorus too (Table 1). In 1984 the mean value in the surface water layer was 0.199 ± 0.044 mg P/L and 0.734 ± 0.197 mg P/L (Figure 6). After the last application of coagulants, the content of this form of phosphorus was 0.058 ± 0.006 mg P/L in the surface water and 0.086 ± 0.025 mg P/L in the near-bottom water. In 2018, the concentration of the organic P form in the surface water layer was 0.048 ± 0.021 mg P/L, and near the bottom, 0.098 ± 0.032 mg P/L (Figure 7). In 2024, these values were changed to 0.075 ± 0.006 mg P/L in the surface water layer and 0.092 ± 0.004 mg P/L in the over-bottom water (Figure 7).
The data analysis showed very high differences in the content of total phosphorus in the entire water column (Table 1). In 1984, before restoration, the mean TP was 0.279 ± 0.126 mg p/L in the surface water layer and 3.011 ± 0.126 mg P/L in the near-bottom water. The long-term restoration of the lake by artificial aeration and P inactivation caused a very clear improvement of water quality. In 2003, the mean value of TP was 0.059 ± 0.030 mg P/L in the surface water and 0.105 ± 0.006 mg P/L in the near-bottom water. As a result of the massive growth of IAAP, the mean values of total phosphorus in the surface water layer slightly increased from 0.058 ± 0.019 mg P/L (in 2018) to 0.086 ± 0.010 mg P/L (in 2024). Total phosphorus concentrations in the over-bottom water layer increased 1.5 times, from 0.155 ± 0.045 mg P/L (in 2018) to 0.215 ± 0.048 mg P/L (in 2024) (Figure 7).
The data analysis showed significant differences in the average content of nitrogen compounds in the DL water column, except for the organic form in the bottom water layers (Table 1).
In the 1980s, the mean content of ammonia in the surface water layer was 0.688 ± 0.871 mg N/L and 20.718 ± 3.195 mg N/L in the near-bottom water layer. In 2003, these values were lower: 0.048 ± 0.108 mg N/L in the surface water and 2.183 ± 1.746 mg N/L in the near-bottom layer. Subsequently, 15 years after the end of restoration (2015), the average amount of ammonia in the surface layer of the water was 0.029 ± 0.061 mg N/L, and in the near-bottom water layer, 1.775 ± 1.041 mg N/L. After the appearance of the invasive plant (2020), the concentration of ammonia nitrogen decreased to 0.009 ± 008 mg N/L in the surface water layer and to 0.736 ± 0.416 mg N/L in the bottom water layer (Figure 8). In the next years, the average ammonia concentration at the surface was 0.084 ± 0.069 mg N/L, and at the bottom amounted to 0.683 ± 0.388 mg N/L. Before restoration, the mean content of NO3 in the surface water layer was 0.148 ± 0.041 mg N/L, and in the near-bottom water was 0.110 ± 0.049 mg N/L. Furthermore, 15 years after the end of restoration and before invasive plants grew (2018), the average amount of nitrates oscillated around 0.144 ± 0.019 mg N/L in the entire volume of DL water (Figure 9). The appearance of the invasive plant and their subsequent removal caused a decrease in nitrate nitrogen concentrations to 0.029 ± 0.007 mg N/L in the surface water layer and to 0.071 ± 0.023 mg N/L in the bottom (Figure 9).
In 1984, the content of the organic form of nitrogen in the surface water layer was 2.077 ± 1.125 mg N/L, and in the near-bottom water layer was 2.238 ± 1.742 mg N/L. The restoration techniques applied in DL did not cause statistically significant changes in organic nitrogen. In 2018 (15 years after the end of restoration), the average amount of organic nitrogen fluctuated between 1.94 ± 0.73 mg N/L (Figure 9) in the upper water layer and 1.29 ± 0.15 mg N/L in the near-bottom layer. After the appearance of invasive plants and after the implementation of their harvesting, a significant decrease in organic nitrogen concentrations was noted with 0.895 ± 0.141 mg N/L in the surface water layer and with 1.06 ± 0.47 mg N/L at the bottom.
In 1980s, in the water of DL, there were very high values of total nitrogen. In 1984, the average content of TN in the upper water layer was 2.913 ± 0.418 mg N/L and 23.067 ± 2.518 mg N/L in the near-bottom water layer. Restoration by artificial aeration and P inactivation caused radical changes in TN content in the water of the analyzed lake. In 2018, 15 years after the end of restoration and before the appearance of the invasive plant, the average amount of total nitrogen in the surface layer of lake water was 2.11 ± 0.67 mg N/L and in the over-bottom water layer, 3.22 ± 1.14 mg N/L (Figure 9). The harvesting of invasive plants decreased the total nitrogen in the surface water layer to 1.09 ± 0.13 mg N/L and 1.81 ± 0.80 mg N/L in the near-bottom water layer (Figure 9).
The data analysis showed significant differences in the average content of autochthonous organic matter in the entire water volume of DL between the analyzed research years. Before restoration, the mean value of BOD5 in the upper water layer was 6.9 ± 2.3 mg O2/L and 68.2 ± 8.3 mg O2/L in the near-bottom water layer (Figure 10). Artificial aeration and P inactivation methods caused a decrease in the primary production processes and a reduction in organic matter content. In 2018, 15 years after the end of restoration and before the invasive plant growth, the mean value of BOD5 in the surface layer of lake water was 1.05 ± 0.33 mg O2/L and 1.33 ± 0.19 mg O2/L in the bottom layer. In the following years, a clear increase in organic compound content was noted with 5.72 ± 0.69 mg O2/L in the surface water layer and 6.93 ± 4.13 mg O2/L in the over-bottom layer (Figure 11).
By reducing nutrient concentration, indicators of primary production, such as chlorophyll a and water transparency, also improved. The comparison of data showed significant differences in the average content of chlorophyll a in the water of DL and water transparency between the analyzed research years. In the 1980s, the mean content of chlorophyll a was 62.90 ± 4.58 µg/L, and the mean value of Secchi disk visibility was 0.5 ± 0.2 m (Figure 12). In 2018 (15 years after the end of restoration and before the invasive plant growth, the average amount of chlorophyll a in the water was 1.68 ± 1.11 µg/L, and the mean water transparency was 5.3 ± 1.5 m (Figure 13). The appearance of invasive plants caused an increase in the average content of chlorophyll a to 19.03 ± 6.78 µg/L and a decrease in mean water transparency to 3.2 ± 1.05 m. The subsequent harvesting of plants caused a slight improvement in water transparency to 3.8 ± 1.00 m and a decrease in the average content of chlorophyll a to 4.04 ± 2.07 µg/L (Figure 13).
The PCA revealed positive correlations between phosphorus compounds and BOD5 and chlorophyll a, and a negative correlation between phosphorus compounds and water transparency (SD) (Figure 14, Figure 15 and Figure 16). The two main analyzed factors explained more than 90% of the variability.
During one action of Elodea nuttallii removal, the amount of biomass which was withdrawn from the lake was c.a. 50 tons of wet weight. Taking into consideration three actions per year, this gives c.a. 150 tons of hydrophytes.

4. Discussion

4.1. Restoration of Lake Długie

In the 1970s, Lake Długie was considered one of the most degraded reservoirs in Poland because, for 20 years, it received untreated domestic sewage. In addition to extremely high concentrations of total nitrogen (30 mg N/L) and total phosphorus (12 mg P/L), high amounts of organic substances (BOD5—75 mg O2/L) were recorded in the lake. Parameters indicating the level of primary production, such as chlorophyll a (500 µg/L), the overoxygenation of surface water layers (290%), and low water transparency—0.2 m—were also recorded in the lake. Raw sewage flowing into this reservoir also harmed the biodiversity of the ecosystem. According to Rodziewicz and Rybak [35], the phytoplankton species composition was poor, with a clear dominance of cyanobacteria and green algae, and submerged vegetation disappeared. Benthic macrofauna was represented mainly by Chaoborus flavicans—an indicator of increased eutrophy [36]. After cutting off the sewage flow, environmental conditions in the reservoir did not improve. An appropriate remediation method was necessary.
According to Lürling and Mucci [37], each restoration method is aimed at precipitating nutrients from the water column and blocking them in bottom sediments or removing them from the lake ecosystem. These activities can deplete the ecosystem of mineral salts and reduce its productivity. According to many researchers [38,39,40], the production of organic matter in lakes is determined by the abundance of mineral salts in their waters (especially nitrogen and phosphorus). The higher the concentrations of these elements in the water, the higher the trophic status of the reservoir and its ability to produce organic matter.
In the years 1987–2000, DL was restored using artificial aeration with thermal destratification. The effect of artificial circulation was a tenfold reduction in the amount of total nitrogen and total phosphorus (Figure 6 and Figure 8) [32]. The reduction in the phosphorus content in the water was achieved by improving the oxygenation of the bottom layers of the water, which enabled the binding of phosphates with iron or manganese, which were present in the bottom sediments. The decline in nitrogen content was related to the nitrification process, which had previously been limited by the lack of oxygen and low water temperatures in the lower reaches of the lake (Figure 4 and Figure 5). Nitrates formed during nitrification were transferred to the bottom sediments, where under anaerobic conditions, they were denitrified into molecular nitrogen. Denitrification was the main process leading to nitrogen losses from the ecosystem, because the molecular nitrogen produced in this process was released into the atmosphere [41]. DL productivity also decreased, primarily due to the destruction of the micro-layer needed for algae to live, an increase in carbon dioxide content, and a decrease in pH, which occurred due to the constant mixing of the entire water column. Such conclusions were also drawn by Visser et al. [42], who analyzed the results of studies on lakes and water reservoirs in many regions of the world that were subjected to the process of artificial circulation. Furthermore, water mixing forced algal cells into deeper layers, where they died due to a lack of light. Artificial aeration also improved the algal species composition, reducing the dominance of cyanobacteria in favor of green algae and diatoms. Cyanobacteria have a poor tolerance to increased N:P ratios. Before the restoration the N:P ratio in the surface water layer ranged from 5.5 to 12 and during artificial aeration—from 8 to 23.
It should be emphasized that thanks to the method of artificial aeration with thermal destratification, a significant reduction in the concentrations of both mineral and organic forms of nitrogen and phosphorus was achieved, the chlorophyll a values decreased to 30 µg/L, and the water transparency increased to 1 m, and despite this, the effect of restoration was not satisfactory (Figure 6, Figure 8 and Figure 12) [31,32].
According to Maberly et al. [43], the inhibition of phytoplankton blooms is only possible when the concentration of mineral phosphorus in water during the growing season does not exceed 0.03 mg/L. Over 13 years of artificial aeration, these values were not achieved. This was due to the properties of the bottom sediments, primarily their low sorption capacity, determined by low iron and manganese concentrations. Therefore, the bottom sediments of this lake were characterized by a deficit of elements capable of binding phosphorus. This situation was further exacerbated by the fact that a significant portion of the iron, which has the greatest affinity for phosphorus, was locked in the form of sulfides [44]. A huge pool of phosphorus was obviously locked in the sediments, but with such high concentrations remaining in the water due to sewage inflow, the capacity of the sediments was too low to further bind phosphorus to values that limited primary production. The significant role of phosphorus in the production of organic matter by algae is also evidenced by the results of the principal component analysis (PCA), taking into account the correlations between BOD5, chlorophyll a, and Secchi disk visibility and the content of phosphorus compounds in the lake (Figure 14, Figure 15 and Figure 16).
Then the decision was made about using a supporting method—phosphorus inactivation, which is based on the precipitation of phosphorus from the water column and blocking it in the deposits. Coagulant application (salts of metals binding phosphorus) improves the sorptive properties of sediment. Because aluminum is an element that forms a durable bond to P, even in anoxic conditions and with a low redox potential, it was decided to apply to DL the Al coagulant PAX 18 (polyaluminum chloride, which contains ca. 9% Al).
After a three-year, phased application of the phosphorus binding agent, the almost-complete precipitation of phosphates from the water column occurred (over 90%) and, as a result, a decrease in the total amount of phosphorus in the reservoir (over 90%) (Figure 6). After the completion of the restoration, the average phosphate content in the epilimnion layer was 0.001 mg P/L and the total phosphorus was 0.076 mg P/L. At the bottom, the average phosphate concentration was 0.011 mg P/L and the total phosphorus was 0.059 mg P/L. The studies showed that despite the fact that P inactivation does not directly affect the content of nitrogen compounds, a several-fold decrease in the concentration of total nitrogen was achieved in the lake, which remained at the level of 1.32 mg N/L in the epilimnion and 3.77 in the hypolimnion water (Figure 8). This can be explained by the fact that, as a result of phosphorus precipitation from the water column, there was a radical decrease in primary production and, as a result, there was also a decrease in the amount of organic nitrogen. In such a situation, the total amount of allochthonous organic matter (produced within the ecosystem) was smaller. As a result, the amount of organic matter that fell to the bottom of the tank after dying was much lower, and there was also a decrease in the amount of ammonium nitrogen, which is a product of its decomposition. The biochemical oxygen demand (BOD5), reflecting the amount of organic matter produced within the ecosystem, was only 2.2 mg O2/L (Figure 10).

4.2. Phytoplankton and Macrophytes

The lake’s restoration brought about a spectacular improvement in water quality, which also led to an increase in biodiversity. Biological studies [45] of the reservoir conducted 15 years after the completion of its restoration process showed that the phytoplankton structure was dominated by Cryptophyta (Cryptomonas erosa, Plagioselmis nannoplantica, Cryptomonas curvata) and Chlorophyta (Chlamydomonas sp., Kirchneriella irregularis). A few cyanobacterial cells were also present (Anathece clathrata, Aphanocapsa delicatissima, Dolichospermum affine). The ecological status of the DL in terms of phytoplankton species and abundance was considered good. The presence of a number of macrophyte species in the littoral zone was also noted. Among the plants with submerged leaves, the following were present: Ceratophyllum demersum L., Ranunculus circinatus, Elodea canadensis, Myriophyllum spicatum L., Potamogeton lucens L., Potamogeton friesii Rupr., and Potamogeton crispus L; with floating leaves of Potamogeton natans L., Nuphar lutea, Nymphea x hybrida, and Polygonum amphibium L. The rush vegetation consisted of Scirpus lacustris L., Sagittaria sagittifolia L., Sparganium emersum Rehmann, Eleocharis palustris, Phragmites australis, Acorus calamus, Typha latifolia, Glyceria maxima, Carex acutiformis, Carex rostrata Stokes, Phalaris arundinacea L., Alisma plantago-aquatica, Glyceria fluitans, and Rumex hydrolapathum. Moreover, macroalgae were present in the DL, which were an indicator of very good water quality—Chara, Nitella flexilis.
According to Maemets and Freiberg [46], covering about 40% of the lake bottom with macrophyte vegetation allows the maintenance of good water quality and prevents phytoplankton blooms.

4.3. IAAP

After a lengthy restoration process, the municipal Lake Długie, degraded by sewage, was restored to the public and once again able to provide ecosystem services in terms of aesthetic, natural, recreational, and tourist values. Walking and cycling paths, recreational infrastructure, viewpoints, a café, a restaurant, and a playground were built around the lake. The lake was increasingly used for fishing.
Unfortunately, due to increased anthropopressure, the lake has been infected with an invasive alien species—Elodea nuttallii. This plant could have been introduced by anglers, divers, birds, and aquatic animals, or by connecting it with Lake Ukiel via an artificial, underground canal. Studies by Kelly et al. [47] and Hrivn’ak et al. [48] have shown that the human factor is the most important factor in the spread of IAAP. Most invasions result from the trade and cultivation of ornamental aquatic plants, and the importance of the human factor in the spread of aquatic neophytes is evidenced by the positive correlation between the presence of IAAPs and inland navigation, fishing, tourism, and human populations [49,50]. Another crucial factor contributing to IAAP invasion is climate change. Ongoing global warming allows IAAP to occupy areas that were previously unsuitable for them [51,52,53]. The increased expansion of IAAP can be explained by the fact that many alien species originated from warmer regions of the world and therefore did not have adaptations to colder climates. Thus, rising air and water temperatures are enabling these species to colonize new regions that until recently were characterized by low winter temperatures and generally lower average temperatures throughout the year. These assumptions are also confirmed by studies of the thermal systems of the DL, which have shown a very significant increase in water temperature. In spring, the April water temperature in the DL increased from 7.8 °C in 1984 to 17.9 °C in 2024 in the upper parts of the lake and from 4.2 °C to 6.0 °C at the bottom (Figure 4). In turn, during the peak period of summer stagnation, the temperature increased from 21.0 °C to 25.0 °C in the epilimnion and from 4.9 °C to 6.4 °C in the lower hypolimnion (Figure 4). These conditions favor the growth of Elodea nuttallii. Another important factor is good light conditions (transparency ranging from 3 to 5 m) (Figure 13) and high sunlight, due to the fact that the lake is an urban reservoir and its edges are largely unforested.
The appearance of Elodea nuttallii worsened the chemistry of the lake’s water. The plant likely drew nutrients from the bottom sediment and spread massively. After the growing season and the mineralization of plant tissues, nutrients were introduced into the water in a form available to primary producers (Figure 7 and Figure 9). A similar situation was observed in studies by Chamier et al. [54] in reservoirs in South Africa. As a result, the concentration of phosphorus in the lake water increased compared with the values observed before the introduction of IAAP. Other indicators illustrating the increase in organic matter production in the reservoir were BOD5 and chlorophyll a, whose values doubled (Figure 11). Moreover, filamentous algae (Mougeotia sp., Spirogyra sp.) developed massively on the surface of Elodea nuttallii, forming troublesome mats. The constant expansion of vascular vegetation led to the increasing dysfunction of the water body in terms of aesthetics and recreational use. It is true that submerged vegetation is an indicator of good water quality, and from a scientific and ecological point of view, its presence is beneficial because it takes up phosphorus and nitrogen for growth, accumulating these elements in its tissues. Furthermore, this vegetation competes for nutrients with phytoplankton, which, due to a lack of nutrients in the water column, cannot reproduce and create algal blooms. However, as mentioned earlier, outside the growing season, fragments of vascular vegetation that fall to the lake bottom undergo mineralization, which enriches the water with mineral forms of phosphorus and nitrogen.
To improve the reservoir’s water quality and restore its aesthetic and recreational value, biomanipulation was necessary, involving the removal of excess macrophytes, along with plant tissues and nutrient compounds. Bartodziej et al. [55] report that the mechanical removal of vegetation is an excellent tool for improving the aesthetic and recreational value of IAAP-infected reservoirs, and an additional advantage of these treatments is the acquisition of material that can be used as a substrate for the production of feed and fertilizers, or as fertilizer directly applied to fields. Removing 3600 kg of dry matter of Elodea nuttallii from the reservoir results in 16.4 kg of P. According to Verhofsad et al. [56] and Hussner et al. [57], plant removal should be carried out as needed, at least twice during the growing season. An important principle when removing vegetation is to prevent sediment resuspension. Kohzu et al. [58] noted an increase in nutrient concentrations in Lake Biwa water as a result of their release from pore water and bottom sediments due to resuspension induced by plant removal.
At the DL, vegetation removal is carried out three times during the growing season. Removing plant biomass involves mechanically mowing aquatic plants to a depth of approximately 1.5 m above the bottom. A boat equipped with an underwater scythe with adjustable depth is used for this purpose. Elodea nuttallii has brittle tissue and is easily trimmed, so it is not necessary to use heavy equipment like specialized brushcutters used to control tough vegetation (e.g., reed beds). The vegetation, trimmed with an underwater scythe, floats to the surface, where it can be swept to the shore by watercraft. After the initial drainage of the lake water, it is collected as fresh plant matter in sealed containers and utilized. It provides valuable fertilizer and is used for agricultural purposes.
Experiments on DL have shown that the biomass should not be allowed to dry out in the heaps formed on the shores of the lake, because the degradation of plant cells and the runoff of the cellular content containing nutrients takes place just a few hours after the shoots are removed from the water. Therefore, biomass collection should be coordinated with on-water operations. A key aspect of the method employed is leaving at least 1 m of plant shoots rooted in the bottom, which prevents sediment disturbance and water turbidity and promotes the improved oxygenation of the bottom water due to the photosynthesis process carried out by the remaining aquatic vegetation. The method used to combat IAAP has improved water quality. Research conducted over five years has shown a decrease in phosphorus and nitrogen concentrations in the water.
BOD5 and chlorophyll a values also decreased, while water transparency increased (Figure 11 and Figure 13). Vascular vegetation absorbs nutrients and prevents algae growth, and removing excess vegetation from the ecosystem contributes to the removal of significant nutrient loads throughout the year. Repeating these treatments positively impacts water quality and biodiversity.

4.4. Future Perspectives

Long-term studies of Długie Lake have shown that it is very easy to destroy the natural ecosystem by transforming it into a sewage receiver, but restoring such a lake is an extremely difficult, lengthy, and costly process. The restored ecosystem is highly sensitive to all the changes occurring within its catchment area, as well as to climate change and the resulting changes in the circulation of matter within the ecosystem. Increased water temperatures favor the growth of invasive alien species, which displace native species. Invasive alien species, such as Elodea nuttallii, draw nutrients needed for growth from the bottom sediments of the water body, and after their tissues die off during the post-vegetation period, they cause a significant deterioration in water quality. Nutrients accumulated in the massively growing vegetation are released into the water during its decomposition and can provide food for phytoplankton in the next production season. Increased vegetation mineralization is favored by higher water temperatures and short-term ice cover or the complete absence of ice on the lake, which has been observed in recent years.
In this situation, the best solution is to gradually remove the part of the macrophyte vegetation, mainly invasive alien species. Studies conducted on LD have shown that the optimal solution is to remove excess macrophyte vegetation three times. Each action removes approximately 50 tons of wet biomass and reduces nutrient concentrations by 30% per year. Removing plants more frequently stimulates their excessive growth. The harvesting of aquatic plants helps to not only limit excessive growth but also removes nutrient loads from the ecosystem, which threaten water quality and deteriorate the reservoir’s trophic status. Removing excess aquatic vegetation also helps maintain the lake’s aesthetic and recreational values.
The remaining macrophyte vegetation (covering 70% of the bottom of the shallow part of the lake) plays a positive role in controlling water quality.
Due to climate change and the presence of invasive alien species being observed in an increasing number of water bodies, managers and owners of lakes should include plant removal in their annual maintenance schedules. It is recommended that water owners, in the case of municipalities and cities, allocate funds for invasive plant removal in their annual budget planning. Maintaining a lake is crucial to maintaining its ecosystem services and maintaining good water quality.

Author Contributions

Conceptualization, J.K.G.; Investigation, J.K.G. and R.A.-T.; Methodology, J.K.G. and R.A.-T.; Software, J.K.G.; Supervision, J.K.G.; Writing—review and editing, J.K.G. and R.A.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by Olsztyn City Hall.

Data Availability Statement

Data are available at the Department of Water Protection Engineering and Environmental Microbiology.

Acknowledgments

The author would like to thank the Olsztyn City Office.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 02893 g001
Figure 1. Location of DL in Poland and its aerial photo [29].
Figure 1. Location of DL in Poland and its aerial photo [29].
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Figure 2. Scheme of artificial aeration technology of DL.
Figure 2. Scheme of artificial aeration technology of DL.
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Figure 3. Scheme of phosphorus inactivation technology in DL.
Figure 3. Scheme of phosphorus inactivation technology in DL.
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Figure 4. Spring and summer thermal settings in the analyzed research years.
Figure 4. Spring and summer thermal settings in the analyzed research years.
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Figure 5. Spring and summer oxygen settings in the analyzed research years.
Figure 5. Spring and summer oxygen settings in the analyzed research years.
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Figure 6. The average annual values of phosphorus compounds content (±SD) in the water of DL—all research years.
Figure 6. The average annual values of phosphorus compounds content (±SD) in the water of DL—all research years.
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Figure 7. The average annual values of phosphorus compounds content (±SD) in the water of DL—period from 2018 to 2024.
Figure 7. The average annual values of phosphorus compounds content (±SD) in the water of DL—period from 2018 to 2024.
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Water 17 02893 g008
Figure 8. The average annual values of nitrogen compounds content (±SD) in the water of DL—all research years.
Figure 8. The average annual values of nitrogen compounds content (±SD) in the water of DL—all research years.
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Figure 9. The average annual values of nitrogen compounds content (±SD) in the water of DL—period from 2018 to 2024.
Figure 9. The average annual values of nitrogen compounds content (±SD) in the water of DL—period from 2018 to 2024.
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Water 17 02893 g010
Figure 10. The average annual values of BOD5 (±SD) in the water of DL—all research years.
Figure 10. The average annual values of BOD5 (±SD) in the water of DL—all research years.
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Figure 11. The average annual values of BOD5 (±SD) in the water of DL—period from 2018 to 2024.
Figure 11. The average annual values of BOD5 (±SD) in the water of DL—period from 2018 to 2024.
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Water 17 02893 g012
Figure 12. The average annual values of chlorophyll a and SD visibility (±SD) in the water of DL—all research years.
Figure 12. The average annual values of chlorophyll a and SD visibility (±SD) in the water of DL—all research years.
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Figure 13. The average annual values of chlorophyll a and SD visibility (±SD) in the water of DL—period from 2018 to 2024.
Figure 13. The average annual values of chlorophyll a and SD visibility (±SD) in the water of DL—period from 2018 to 2024.
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Figure 14. Results of PCA for phosphorus compounds and BOD5.
Figure 14. Results of PCA for phosphorus compounds and BOD5.
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Figure 15. Results of PCA for phosphorus compounds and chlorophyll a.
Figure 15. Results of PCA for phosphorus compounds and chlorophyll a.
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Figure 16. Results of PCA for phosphorus compounds and SD visibility.
Figure 16. Results of PCA for phosphorus compounds and SD visibility.
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Table 1. Results of one-way ANOVA analyses for investigated variables in the DL water (S—surface water layer, B—bottom water layer).
Table 1. Results of one-way ANOVA analyses for investigated variables in the DL water (S—surface water layer, B—bottom water layer).
ParameterSum of Squares
Effect		F Coefficient in the Fisher DistributionP
Significance Level
NH4 S
NH4 B		2.173
2000.986		3.243
156.708		0.012
0.000
NO3 S
NO3 B		0.101
0.046		27.359
5.095		0.000
0.000
Norg. S
Norg. B		8.149
5.277		4.045
1.638		0.003
0.165
TN S
TN B		15.862
1911.321		15.403
158.022		0.000
0.000
PO4 S
PO4 B		0.027
24.813		4.24740
315.7112		0.002
0.000
Porg. S
Porg. B		0.105
2.072		21.1961
55.7330		0.000
0.000
TP S
TP B		0.230
41.179		13.9782
316.1568		0.000
0.000
BOD5 S
BOD5 B		150.130
21,336.540		10.333
243.269		0.000
0.000
Chlorophyll16,677.23042.1440.000
Visibility106.70921.6610.000
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Grochowska, J.K.; Augustyniak-Tunowska, R. Maintaning the Durability of the Effects of Urban Lake Restoration—New Challenges. Water 2025, 17, 2893. https://doi.org/10.3390/w17192893

AMA Style

Grochowska JK, Augustyniak-Tunowska R. Maintaning the Durability of the Effects of Urban Lake Restoration—New Challenges. Water. 2025; 17(19):2893. https://doi.org/10.3390/w17192893

Chicago/Turabian Style

Grochowska, Jolanta Katarzyna, and Renata Augustyniak-Tunowska. 2025. "Maintaning the Durability of the Effects of Urban Lake Restoration—New Challenges" Water 17, no. 19: 2893. https://doi.org/10.3390/w17192893

APA Style

Grochowska, J. K., & Augustyniak-Tunowska, R. (2025). Maintaning the Durability of the Effects of Urban Lake Restoration—New Challenges. Water, 17(19), 2893. https://doi.org/10.3390/w17192893

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