Possibilities of Improving Water Quality of Degraded Lake Affected by Nutrient Overloading from Agricultural Sources by the Multi-Point Aeration Technique

Abstract

This research concerns the restoration of the strongly eutrophied Lake Łajskie (Masurian Lake District, Poland) that received pollutants from the agricultural catchment. It is a shallow (max depth 7.4 m) and small (area 48 ha) reservoir with a wide profundal zone characterized by complete deoxidation in summer. Due to its important natural and social role, the lake has undergone a restoration process. Artificial oxygenation is the main method of improving water quality. Due to unfavorable morphometric conditions, the necessity to use multi-point aeration was assumed. An experimental installation based on eight reactors selectively oxygenating only the over-bottom waters was launched in 2019. In 2021, spatial studies of the effectiveness of aerators’ work were carried out at 96 measuring points grouped into 12-test fields around each reactor. The investigations were performed three times during the summer season. It was shown that, in the water of the deepest layers of Lake Łajskie, the oxygen concentration around all reactors increased to an average level of 1–2 mg L⁻¹. The oxygenation efficiency was varied and related to the distance from the aerator and the location in relation to the direction of oxygenated water outflow. The results of the research indicate the legitimacy of designing multi-point oxygenation systems in the restoration of waterbodies degraded as a result of the impact of agriculturally transformed catchments.

1. Introduction

Of all types of inland surface waters, lakes are the most sensitive to eutrophication. Occupying the lowest place in the field, they are predisposed to capture nutrients and organic compounds flowing from catchment areas. Even in flow-through lakes, the hydraulic retention time resulting from horizontal water exchange is usually sufficient for lakes to have an advantage of accumulation of pollutants over their neutralization. These relationships have been described by the precursors of modern limnology since the middle of the last century [1,2,3], when lake ecosystems were called ephemerides on a geological scale. Despite the well-known causes of lake degradation, economic development in most countries intensifies anthropopressure on these ecosystems. The result is the ever-increasing problem of their eutrophication [4].

The main factors determining the rate of lake degradation are the supply of nutrients and the morphometric features of the lake basin. One of the most serious sources of eutrophication of surface waters is agriculture [5,6,7,8]. The catchments, transformed by cultivation and breeding, become efficient suppliers of biogenic and organic matter to the lakes [9]. The rate of nutrient inflow depends on the hydrological situation in the catchment [10,11]. The denser the river and drainage network, the faster pollutants reach water reservoirs. Non-flow lakes located in agricultural landscape resist degradation for the longest time. In lake districts with a temperate climate, deep lakes—thus stratified in summer—generally undergo eutrophication more slowly than shallow lakes. Due to the restriction of trophogenic processes to the surface layers (usually the epilimnion), they can withdraw some of the indigenous matter through sedimentation to the deeper layers of water. In this way, they reduce the symptoms of eutrophication as the growing season progresses. Shallow lakes, especially polymictic ones, are characterized by greater dynamics of matter circulation and greater productivity. Nutrient resources deposited in the bottom layers and in the sediment are more accessible to primary producers [8].

The possibility of periodic retraction of matter to deeper water layers facilitates the self-purification of surface waters, but also affects the risk of deoxidation of the water. This leads to the activation of the phenomenon of “internal supply” of phosphorus and nitrogen mobilized from bottom sediments in conditions of reduced redox potential [8,12,13]. The impact of these processes on the ecosystem is so strong, it may negate the effect of protective measures taken in catchments and accelerate eutrophication, despite the elimination of external sources of pollution [14]. Therefore, the key goal of the restoration of water reservoirs is to improve the oxygen balance. One of the most commonly used restoration methods is artificial aeration [15,16].

Artificial aeration is carried out in two ways: (I) with thermal destratification (complete mixing); and (II) without destruction of thermal layers (hypolimnion oxygenation). The first group of methods is aimed at eliminating the thermocline separating the warm waters of the epilimnion from the cold, stagnant layers of the hypolimnion, which consequently unifies the physical and chemical conditions in the entire water column. Destratification of water can be achieved by various methods, but the simplest and least energy-intensive is to discharge compressed air above the bottom in the deepest part of the lake. This method has been used in many lakes around the world [16]. The mechanism of destratification consists of repeated pushing of the bottom waters to the surface by compressed air bubbles. This, in turn, results in temperature equalization and mechanical mixing of the entire mass of water. Air is mainly a transport factor here—the effectiveness of oxygen diffusion into water during the rise of the generated air bubbles is estimated at only a few percent. The basic installation of the aeration system consists of a compressor located in a room located on the lake shore, a transmission line, and an air discharge system, e.g., a diffuser.

Restoration carried out in this way changes the conditions both in the waters of the surface layers (lowering temperature and pH of the water, reducing biomass of phytoplankton, and lowering over-oxygenation) and deeper waters (water oxygenation, reducing the amount of organic matter and nutrients, while significantly increasing water temperature).

Avoiding the mixing of water masses (hypolimnion oxygenation) requires the use of special devices and systems preventing the destruction of thermal layers [15,16,17]. This is achieved by a mechanical aeration system (a system with a pulverization unit placed on the shore), with partial water lift (Limnox aerator) or with total water lift (e.g., “ekoflox” and “stratiflox” tubular aerators, pulverization aerator). Modern constructions enable the simultaneous introduction of a phosphorus-binding coagulant, which can improve the efficiency of the method. The simplest aerators consist of two coaxially mounted pipes—the inner one, to which compressed air is supplied from the compressor located on the shore; and the outer one, reaching above the water table. The water pushed out by the inner tube undergoes partial oxygenation and returns to the hypolimnion by the outer tube.

The technique of artificial aeration without destroying thermal layers is less invasive than destratification systems. However, it should be remembered that this technique only affects the bottom zone. Therefore, the improvement of the lake condition is achieved gradually—over the years, the release of nutrients from bottom sediments to water is successively inhibited during periods of water stagnation.

Most of the case studies of lakes restoration using artificial aeration described in the literature concern lakes with a clearly conical basin. Typically, aeration is based on the operation of one device located above the deepest part of the lake [15]. Especially in the aeration technique with destratification of thermal layers, the spatial efficiency of aeration significantly improves in lakes with a low depth factor (average depth/maximum depth). However, the efficiency of oxygenation, regardless of the technique used, is often unsatisfactory, even in lakes with favorable morphometry [18,19,20]. This may be caused by the insufficient intensity of the unit processes within the aerator, as well as the difficulty in underwater oxygen distribution beyond the immediate impact zone of the aerator [21]. Even more difficult conditions for effective oxygenation of tropholytic zones are in moderately deep lakes with a relatively flat bottom, with partial stratification or a residual, shallow hypolimnion. They are deep enough to show oxygen deficits at the bottom, but the unfavorable morphometry limits the spatial effectiveness of classic treatments based on single-point aeration.

One of such lakes is the hypertrophic Lake Łajskie (area 48 ha, maximum depth 7.4 m). Improving the quality of water in this lake is crucial for maintaining the welfare of one of the most valuable water reserves in the Warmia and Masuria region (Poland), located lower in the catchment area. Hence, a water restoration program has been implemented since 2019, including an innovative multi-point oxygenation system. The purpose of the program is to improve the oxygen conditions over the vide, flat area of the lake bottom. The aim of the work is a preliminary assessment of the effectiveness of oxygenation of the lower layers of the hypolimnion of Lake Łajskie by a network of aerators powered by pure oxygen.

2. Materials and Methods

2.1. Lake Łajskie

Lake Łajskie is a typical example of a reservoir degraded by inappropriate human management. It is a relatively small and shallow lake (

Table 1. Basic morphometric indicators of Lake Łajskie, based on contemporary measurements [26].

Parameter	Unit	Value	Bathymetric Surfaces
			Isobath	Area [ha]	Between	Area [ha]	Area [%]
surface area	ha	47.84	0 m	47.84	0–1 m	4.39	9.18
volume	thou. m3	1592.2	1 m	43.45	1–2 m	7.70	16.09
max depth	m	7.4	2 m	35.75	2–3 m	11.62	24.28
mean depth	m	3.3	3 m	24.13	3–4 m	8.75	18.29
relative depth	-	0.01	4 m	15.37	4–5 m	5.44	11.37
depth indicator	-	0.44	5 m	9.94	5–6 m	3.87	8.09
Max length	m	987.7	6 m	6.07	6–7 m	4.56	9.53
Max width	m	723.3	7 m	1.51	within 7 m	1.51	3.16
shoreline	m	3283
Shoreline development	m/ha	68.6

), hydrologically open, and fed by three tributaries. The lake’s catchment area has a great influence on this reservoir. It is used mainly for agriculture, which increases the supply of nutrients. The research carried out in the catchment area of this lake in 2008–2020 shows that watercourses draining agricultural catchment areas (about 18 km²) provide an annual load of pollutants at the level of 250–650 kg of phosphorus and 2600–12500 kg of nitrogen. Until recently, an additional problem was the discharge of insufficiently treated domestic and economic sewage to one of the lake’s tributaries. Historical and contemporary studies [22,23,24,25] have shown strong eutrophication of this reservoir. A characteristic feature of the lake is the fact that, despite its insignificant depth, it suffered from long-term strong oxygen deficits during periods of summer water stagnation and hydrogen sulfide was present in the waters near the bottom.

Due to the fact that the outflow from Lake Łajskie supplies other, very valuable water ecosystems (nature reserve—Lake Kośno, area 550 ha, max depth 45 m), achieving water quality improvement has become a key problem for local authorities and communities.

Lake Łajskie has been included in the protection and restoration program since 2019. Apart from activities related to the reduction of external pollution loads (arrangement of water and sewage management in the catchment area by the local self-government, filtration zones on tributaries), a multifaceted restoration was also started. To reduce the supply of phosphorus in the water, the method of phosphorus inactivation using an aqueous solution of iron chloride was used (first stage 2019–2020, several applications each year), as well as artificial aeration (oxygenation) of the lake (from 2019). In addition, an intensive program of fishing biomanipulation was launched—restocking predatory fish (pike and zander) and eliminating the excess of small cyprinids. Each of these activities is ultimately aimed at limiting the primary production in the lake and eliminating strong cyanobacterial blooms, which so far, occur practically throughout the summer season. A multi-threaded, holistic set of activities brings results—the quality of water in Lake Łajskie improves year by year. The entity implementing the restoration is the local ecological association.

2.2. Oxygenation System

One of the mentioned methods of improving the water quality of Lake Łajskie is artificial oxygenation. The oxygenation system was developed individually to meet the needs of the lake’s morphometry. According to the developed rehabilitation concept, the use of several aerators (specifically, oxygenation reactors) was proposed to improve the oxygen conditions in the lake. Eight devices of this type were located in the central, deepest part of the lake, so that their range of impact could cover the area from a depth of 5–6 m to the bottom, which corresponds to the range of the lake’s hypolimnion. The system supplies oxygen to the bottom layers of the lake to a depth of about 0.5 m above the bottom. Oxygen is distributed through the underwater main line connecting the central aerator platform (reactor no. 7—Figure 1) with the oxygen store located on the eastern shore of the lake. Supply hoses were led from the central platform to the remaining reactors.

Each reactor consists of a column system and a mixing container (Figure 2). Water is taken from the bottom of the lake, and the movement of water inside the reactor’s pipe system is forced by the difference in the density of oxygenated and drawn water. After oxygenation, the water returns to the bottom by one of the reactor pipes and is further distributed through three outlets of an underwater grate with a span of 13 m, suspended 0.5 m above the sediment. The columns and the reactor container are thermally insulated to prevent heating of the bottom waters during the oxygenation process. In fact, the developed aerators are a modification of the previously known solutions based on a total air lift system [15]. However, the factor improving the redox conditions in the over-bottom water is pure oxygen, not compressed air.

The grates discharging oxygenated water are oriented in a way that facilitates the creation of an all-round movement of water within the deep water (Figure 1). It was assumed that such a system would facilitate the distribution of oxygenated over-bottom water over a larger area. At the current stage of the project, the oxygenation efficiency is being tested with a consumption of approximately 50 tons of oxygen per season.

2.3. Methods

The analysis of changes in the water quality of the Łajskie Lake tropholytic zone subjected to experimental oxygenation was carried out in 2021, during the summer thermal stratification of waters. It was the third season of the system’s operation, which was built and launched in 2019.

Three measurement campaigns were carried out in May, June, and August 2021. The first measurement was made before the oxygenation system was launched, and the second and third were made during the operation of the system in 2021 growing season. The lower layers of the lake’s tropholytic zone (0.5 and 1.0 m above the bottom level) were tested.

For the purposes of assessing the operation of the lake restoration system, it was assumed that each of the 8 oxygenation reactors would be the focal point of an individual set of 12 measuring stations (Figure 3). They were located in two transects: latitudinal (points 1–6) and meridian (points 7–12). The span of the transects was 100 m, which means that evenly spaced points on each transect were separated by a distance of 20 m. The distance of the oxygenation reactor column from the central points in a given research area (points 3, 4, 9, and 10) was about 6–8 m. The grid of points, planned in this way, allowed researchers to maintain the assumed accuracy of the measurement methodology and to standardize the method of testing each field object.

The field work consisted of marking out previously designed points and determining their depth (Garmin echoMAP 50 s echosounder with a built-in GPS receiver, verification of the correctness of indications with a plate depth gauge). Positioning accuracy was 2–5 m on average. Then, at the measuring points, the oxygen content of the above-bottom water was tested (0.5 m and 1.0 m above the bottom level) using the YSI Exo 2 multi-parameter probe. To ensure the best accuracy of readings, the calibration process of the multi-parameter probe was based on own standards prepared each time before field tests. They were prepared from lake water using the classic Winkler method [27]. The deoxygenated water standard was obtained by incubating the sample in a sealed glass vessel until oxygen depletion, without the use of reducing reagents.

The Statistica 13.5 software (StatSoft, Inc., Tulsa, OK, USA) was used to compare the water oxygenation levels around the individual reactors. The multiple regression analysis was performed for the identification of location factors, which are significantly connected to oxygen concentration. That analysis allowed to obtain linear models of type:

Y = B₀ ± B₁ X₁ ± B₂ X₂ ± ⋯ ± B_i X_i + E_ij

where:

• Y—dependent variable (oxygen concentration);
• B₀—constant (intercept);
• B₁…B_i—regression coefficients;
• X₁…X_i—independent variables;
• E_ij—residual component;
• R—multiple correlation coefficient;
• R²—multiple determination coefficient.

Before analysis, data were subjected to log transformation (oxygen values were log (x + 1) transformed) in order to approximate to the normal distribution.

Two independent variables were tested: distance from reactor (D_i) and direction in which reactor released oxygen (D_r).

3. Results and Discussion

3.1. Changes in Oxygen Distribution in Near-Bottom Waters during the Research Period

The results of the research on oxygenation in the tropholytic zone in Lake Łajskie, carried out around the devices of the oxygenation system, are presented in

Table 2. Oxygen concentrations [mg/L] at measuring points 0.5 m above the bottom level. Roman numerals indicate months of research.

Point No.	Reactor 1			Reactor 2			Reactor 3			Reactor 4
	V	VI	VIII	V	VI	VIII	V	VI	VIII	V	VI	VIII
1	0.00	0.18	0.22	0.11	0.24	0.22	0.14	0.24	0.24	0.21	0.20	0.23
2	0.06	0.63	0.49	0.11	0.50	0.55	0.11	0.51	0.56	0.14	0.21	0.21
3	0.07	5.48	6.02	0.09	3.93	5.26	0.09	2.92	5.75	0.11	0.33	0.41
4	0.19	0.53	0.47	0.09	0.41	0.47	0.08	0.44	0.38	0.11	4.56	5.53
5	0.89	0.55	0.41	0.09	0.24	0.14	0.06	0.22	0.23	0.07	0.53	0.47
6	2.89	1.32	0.59	0.20	0.32	0.25	0.00	0.01	0.14	0.02	0.24	0.26
7	0.09	0.27	0.27	0.00	0.23	0.22	0.04	0.2	0.23	0.09	0.24	0.18
8	0.09	0.23	0.16	0.06	0.55	0.63	0.05	0.51	0.66	0.09	0.47	0.54
9	0.11	0.42	0.48	0.10	4.25	5.53	0.09	4.70	6.18	0.11	4.92	5.26
10	0.18	4.96	5.84	0.11	0.38	0.35	0.09	0.33	0.36	0.11	0.42	0.43
11	0.86	0.95	1.06	0.21	0.32	0.24	0.10	0.20	0.18	0.12	0.20	0.20
12	3.34	1.28	1.14	0.74	0.66	0.23	0.12	0.23	0.20	0.12	0.21	0.14
Point No.	Reactor 5			Reactor 6			Reactor 7			Reactor 8
	V	VI	VIII	V	VI	VIII	V	VI	VIII	V	VI	VIII
1	0.13	0.00	0.00	0.00	0.13	0.14	0.00	0.14	0.14	0.05	0.44	0.47
2	0.12	0.00	0.06	0.00	0.12	0.14	0.00	0.36	0.51	0.02	0.22	0.31
3	0.12	0.31	0.38	0.00	0.31	0.45	0.00	5.10	6.35	0.00	5.38	5.74
4	0.10	5.33	6.35	0.00	5.02	6.26	0.00	0.40	0.52	0.00	0.36	0.41
5	0.10	0.58	0.70	0.08	0.58	0.53	0.00	0.18	0.26	0.00	0.00	0.00
6	0.11	0.26	0.20	0.10	0.22	0.16	0.04	0.48	0.48	0.00	0.00	0.00
7	0.64	0.32	0.14	0.10	0.09	0.08	0.00	0.03	0.08	0.00	0.26	0.38
8	0.20	0.20	0.14	0.05	0.14	0.14	0.00	0.31	0.28	0.00	0.43	0.47
9	0.11	0.33	0.37	0.00	0.33	0.32	0.00	4.02	5.20	0.00	5.36	5.43
10	0.10	5.18	5.56	0.00	5.31	5.86	0.00	0.35	0.38	0.00	0.41	0.57
11	0.06	0.63	0.69	0.00	0.57	0.63	0.00	0.03	0.06	0.00	0.06	0.22
12	0.00	0.26	0.41	0.00	0.14	0.20	0.03	0.00	0.06	0.00	0.08	0.14

and

Table 3. Oxygen concentrations [mg/L] at measuring points 1.0 m above the bottom level. Roman numerals indicate months of research.

Point No.	Reactor 1			Reactor 2			Reactor 3			Reactor 4
	V	VI	VIII	V	VI	VIII	V	VI	VIII	V	VI	VIII
1	0.03	0.27	0.36	0.16	0.56	0.50	0.20	0.39	0.40	0.34	0.26	0.26
2	0.10	0.80	0.78	0.16	0.78	0.89	0.12	1.02	1.17	0.21	0.27	0.28
3	0.11	6.22	6.89	0.18	5.14	6.88	0.23	5.17	7.04	0.20	0.53	0.61
4	0.23	0.79	0.82	0.14	0.69	0.72	0.19	0.79	0.73	0.19	5.62	7.14
5	0.94	0.67	0.80	0.13	0.49	0.39	0.13	0.64	0.70	0.16	1.12	1.27
6	3.16	1.68	1.06	0.27	0.99	0.85	0.06	0.17	0.42	0.10	0.44	0.50
7	0.17	0.35	0.41	0.06	0.43	0.48	0.14	0.47	0.61	0.16	0.38	0.29
8	0.13	0.35	0.40	0.10	1.02	1.17	0.17	1.19	1.43	0.17	0.89	1.16
9	0.14	0.69	0.69	0.13	6.57	8.02	0.16	5.56	7.02	0.20	6.01	7.33
10	0.22	6.20	7.11	0.19	0.69	0.93	0.16	0.58	0.70	0.18	0.66	0.70
11	0.96	2.16	1.56	0.30	0.73	0.70	0.16	0.49	0.43	0.20	0.42	0.40
12	3.68	2.21	1.43	1.23	1.21	1.02	0.18	0.51	0.44	0.17	0.44	0.37
Point No.	Reactor 5			Reactor 6			Reactor 7			Reactor 8
	V	VI	VIII	V	VI	VIII	V	VI	VIII	V	VI	VIII
1	0.21	0.14	0.10	0.00	0.33	0.49	0.06	0.34	0.37	0.08	0.89	0.96
2	0.24	0.14	0.12	0.00	0.37	0.44	0.02	0.56	0.78	0.06	0.68	0.73
3	0.23	0.56	0.66	0.00	0.69	0.88	0.00	5.89	6.99	0.00	6.12	6.49
4	0.19	6.21	7.84	0.09	6.24	7.54	0.00	0.72	0.80	0.00	0.63	0.72
5	0.17	1.23	1.32	0.17	1.12	1.21	0.00	0.29	0.34	0.00	0.22	0.28
6	0.17	0.48	0.39	0.21	0.48	0.39	0.08	0.72	0.72	0.00	0.11	0.18
7	1.08	0.64	0.28	0.20	0.37	0.39	0.00	0.19	0.28	0.00	0.66	0.84
8	0.42	0.40	0.30	0.17	0.39	0.37	0.00	0.44	0.59	0.00	0.89	1.12
9	0.21	0.56	0.79	0.12	0.56	0.66	0.00	5.32	6.24	0.00	6.14	6.51
10	0.21	6.24	7.11	0.07	6.29	6.74	0.00	0.59	0.66	0.00	0.60	0.89
11	0.16	1.14	1.34	0.00	0.99	1.14	0.02	0.11	0.28	0.00	0.34	0.52
12	0.06	0.68	0.87	0.00	0.36	0.54	0.08	0.10	0.24	0.00	0.33	0.38

and Figure 4 and Figure 5.

In the period preceding the launch of oxygen dosing, in May 2021, clear deoxidation of the water was already noted in the bottom layers. Most of the measuring stations were characterized by the oxygen content not exceeding 0.1–0.2 mg L⁻¹, and virtually no oxygen was found around the deepest fragments of the bottom (Table 1 and Table 2). The area to the south-east of reactor no. one, located in the shallowest location, i.e., on the border of the 6 m isobath in the place of the fastest shallowing of the bottom (Figure 3), was characterized by slightly better conditions. The average oxygen content in the water for this area was 0.73 and 0.82 mg L⁻¹, respectively, 0.5 and 1.0 m above the bottom.

By starting the oxygenation process in the lake, the oxygen content around all aerators was increased. On average, the average oxygenation was at a level of 1–1.5 mg L⁻¹ in the layer 0.5 m above the bottom and 1.5–2.0 mg L⁻¹ at a distance of 1 m from the bottom sediment level (Figure 4). The highest average concentrations in both analyzed layers were also obtained around the shallowest reactor no. one. A more detailed analysis of the obtained results shows that the differences in oxygen concentrations at individual 12 measurement points around each device were high (Table 2 and Table 3). The highest values were found, without exception, at the stands closest to the aerator, but these were always stands opposite the oxygenated water outlets. At the points closest to the aerator, but opposite to the water intake, the oxygen content did not increase so rapidly, which is confirmed by the mechanism of oxygen transport along with the moving water masses. This was in accordance with the assumed directions of water discharge outlets indicated in Figure 1.

The results of the statistical analysis confirm these observations. Both tested variables (distance from reactor and direction in which reactor released oxygen) were highly significant (p < 0.000000). The assessed model showed good fit to experimental data (R² = 0.821, R = 0.675, n = 96). The obtained regression equation was as follows:

Y = 1.21 − 0.92 D_r − 0.60 D_i + E_ij

It means that both tested factors have influence on oxygen concentration in restored area of lake. Oxygenation decreased with distance from aerator, as well as direction of oxygen releasing shaped oxygen conditions.

For a more complete picture of the mosaic of oxygen conditions in the water of the lower layers in the tropholytic zone of the lake, a graphic summary of the results of research works in the peak period of the summer season was prepared (Figure 5).

The comparison of changes in the oxygen content at individual stations after the start of oxygenation (June) and after about 2 months of system operation made it possible to obtain valuable information about the changes taking place in the experimented environment. The general tendency was an increase in the average oxygen content around individual aerators in August (Figure 4); however, it was not a phenomenon accompanying the entire research area. The analysis of individual measuring points showed that in water at test stands located usually on the edge of the oxygenation area (e.g., stands 5–6, 11–12 around aerators no. 1 and no. 2, stands 7–8 around aerator no. 5), after the initial increase in oxygen concentration in June, deterioration of oxygenation conditions was noted in August. This indicates a reduction in system efficiency in areas remote from the aerator assembly and confirms their horizontally limited effectiveness. This effect was already predicted for deep-level aerators in previous studies [28,29]. The fact that, in most of the central stations, there was an improvement in oxygenation during the summer season seems to confirm the correct range of impact assumed during the design of the reactor foundation sites.

3.2. Multi-Point Aeration—Strengths and Weaknesses of Restoraation in Unfavorable Morphometry Conditions

The contemporary philosophy of lake restoration is based on a thorough diagnosis of the conditions for the functioning of the ecosystem and the selection of techniques and activities to suit the nature of the lake, primarily its hydrological system, morphometry, and water chemistry. Increasingly, rehabilitation programs are based on not one, but several complementary methods [30,31,32,33]. A holistic approach, not only taking into account the change in the water chemistry in the lake, but also the living conditions of biocenoses and strengthening the natural mechanisms of self-purification, gives the best results in counteracting eutrophication [34].

Artificial aeration is crucial for the success of both accompanying methods of restoration of Lake Łajskie, i.e., inactivation of phosphorus with iron coagulants and biomanipulation. The improvement of redox conditions supports the stability and durability of iron–phosphorus connections [35,36,37], thus increasing and extending the efficiency of phosphorus inactivation in the deeper zones of iron coagulant application. Increasing the range of oxygenated waters improves the living conditions of the population of settled predatory fish, especially zander, which prefer the penetration of deeper waters [38].

The choice of oxygenation technique is critical to the lake rehabilitation process. It is generally recognized that the method of water mass destratification is more effective [15,16]. However, it leads to the elimination of thermal stratification, which may be unfavorable for sensitive organisms (e.g., fish of the genus Coregonidae). Aeration with thermal stratification is usually more expensive and more difficult to implement. This is due to the need to use systems that isolate the over-bottom water during the oxygenation process, including column aerators, pulverization tanks and chambers, water supply lines, etc. [16,29]. Specific morphometric conditions determined the choice of the Lake Łajskie aeration method. The wide and flat area of profundal sediments posed a risk of a negative impact of increased temperature of bottom waters on the metabolism of sediments in the water mass destratification scenario. Hence, it was decided to use a method that preserves thermal stratification, while forecasting the need to use at least several oxygenation reactors.

Multi-point aeration is a solution that has already been used for lake renewal; however, scientific reports on this subject are scarce. Most often, one aerator is used in lakes, in the central deep, or only a few located in lake sub-basins. Such a solution was used e.g., in the Trzesiecko lake in Poland (three aerators) and in the Brno reservoir in the Czech Republic (five aerators) [39]. In Lake Tegel, in Berlin (400 ha, maximum depth of 16 m, with an extensive profundal with depth index of 0.4), a system of 15 deep-water aerators improved the oxygenation of the bottom waters on the surface of the central basin with an area of approximately 80 ha. It was assumed that this installation was not supposed to lead to the summer destratification of waters; however, in practice, it contributed to the destabilization of thermal stratification due to the types of limnox aerators used. These devices have an underwater gas exchange reactor and excess air escapes above the body in the form of a stream of bubbles, which favors the ascending movements of the hypolimnion waters and the mixing of thermal layers [40]. Contrary to the limnox deep-water aerators, column aerators were used in the reclamation project of Lake Łajskie. In this project, the reactor chamber is located near the surface and does not require degassing of the underwater installation components. Technical and economic possibilities made it possible to build eight such devices (Figure 1).

The choice of pure oxygen as a medium to improve the balance of this element in the lake was not accidental. Although compressed air is usually the cheapest carrier of oxygen, the restoration of Lake Łajskie, due to the morphometric determinants described above, presupposed the need to obtain the maximum possible efficiency of the devices due to the selection of techniques that do not cause thermal destratification of the lake in summer periods. The most effective gas exchange within a single reactor, with a short retention time in relatively low columns, was expected to ensure the greatest individual impact on a given fragment of the lake. The air only contains 21% of oxygen, and the gas exchange using this medium is dominated by nitrogen, which has been confirmed by previous studies [28,41,42]. Its migration to lake waters in gaseous form is not beneficial from the point of view of restoration goals [8]. In fact, the proposed solution is therefore not a technique of artificial aeration, but rather, oxygenation of a degraded lake. The greater environmental efficiency of using pure oxygen compensates for its higher cost compared to compressed air. The estimated cost of the project during the first three years of implementation was approximately EUR 200,000, with annual operating costs (purchase of technical oxygen) amounting to approximately EUR 15–20,000. It should be noted, however, that unlike artificial aeration systems using air compressors, the described installation is independent of electricity sources. This is a definite advantage of a system based on pure oxygen in the absence of power grids near lakes located among agricultural areas that require restoration.

The decision to locate oxygenation devices in a larger area of research was the result of collecting empirical experiences during previous experiments. In these experiments, it was assumed that the improvement of the environmental conditions of the tropholytic zone was done by redirecting the oxygenated waters of one of the tributaries to the bottom of the lake [43]. This concept was implemented in 2014–2018. The research carried out during that experiment showed the limited spatial effectiveness of the adopted solution [44]. The water of the watercourse with a higher temperature than that of the metalimnion and the vestigial hypolimnion of Lake Łajskie, flowing just above the bottom, obtained a lower specific gravity compared to the lake water which they fed. As a result, the stream of oxygenated waters was floating, and it was difficult to obtain an even distribution of oxygen in the over-sediment water. The phenomenon of loss of the original temperature of water exposed to aerators often accompanies deep-water aeration systems [40,45,46] and is mentioned as one of the most important reasons for reducing the efficiency of oxygenation. In the installation on Lake Łajskie, the problem of heating water drawn from above the bottom in the described reactors was minimized by isolating columns and averaging tanks. Nevertheless, an increase in the heat of the drained water was observed, usually by 0.4–0.7 °C. This was one of the reasons for the differentiation of the obtained oxygenation results at the depths of 0.5 and 1.0 m.

Despite the location of individual aerators in a fairly even manner and the orientation of the water outflow so as to support the even distribution of oxygen along with the movement of water, it was not possible to ensure satisfactory oxygen concentration in the entire experimental area. In three sub-reservoirs, apart from reactors five, seven, and eight, the water of the layers directly in contact with the sediments (0.5 m above the bottom) was characterized by only trace amounts (<0.1 mg L⁻¹) of this element. The indicated locations are not directly exposed to the outflow of oxygenated water from the reactors. Therefore, in subsequent seasons of the system’s operation, it is reasonable to test larger doses of oxygen to central aerators, add additional distribution points, or modify the direction of water outflow from the aerators closest to the blind spots of the oxygenation area. Regardless of the encountered difficulties and limitations of the tested system, the obtained results indicate the correct assumption of the necessity to use multi-point oxygenation in the lake.

3.3. Environmental Benefits

Environmental monitoring studies conducted in recent years have the effectiveness of the restoration process [24,25]. The successive improvement of the oxygen balance observed since 2019 has enabled positive changes in the quality of the lake water. First of all, the process of internal release of nutrients from sediments to water has been limited. Before restoration, the content of total phosphorus in the interstitial waters of the upper sediment layer (0–10 cm) reached very high values, averaging 4.0–8.5 mg L⁻¹. The high concentration gradient between the sediment layer and the water column favored the migration of nutrients from the bottom sediments to the water. The observed concentrations of phosphorus in the near-bottom water (10 cm layer of water-bottom sediment interphase) during the summer stratification reached 3.5–5.0 mg L⁻¹; in the waters 1 m above the bottom, we recorded values of 2.5–3.0 mg L⁻¹; surface water was characterized by the richness of this element at the level of 0.3–0.4 mg L⁻¹. During the period of operation of oxygen reactors (2020 and 2021), phosphorus compounds were present in much lower amounts. The water column was characterized by the concentration of this nutrient at the level not exceeding 0.2 mg L⁻¹ (surface water); the over-bottom water contained phosphorus in an amount that did not excel 0.5 mg L⁻¹; the interphase waters, just above the sediment, had a phosphorus concentration below 1 mg L⁻¹ The slowdown of the phenomenon of nutrients internal loading from sediments was also evidenced by the reduction of phosphorus concentration in interstitial waters (a decrease of concentrations by about two-fold, to a level not exceeding 2 mg L⁻¹). Reducing the supply of phosphorus in the ecosystem caused a number of beneficial changes—the biological productivity of the lake was reduced, phytoplankton biomass decreased, the process of rebuilding submerged macrophytes began, and water transparency increased [25]. The year 2021 was the first year in the history of contemporary research on Lake Łajskie in which the presence of hydrogen sulfide was not observed in the bottom layers of water.

4. Conclusions

The innovative renaturation technique presented in this article does not require the use of electrical devices and can be used for the restoration of lakes located in agricultural landscape away from energy sources necessary for mechanical aeration.

The research carried out during the restoration of Lake Łajskie confirmed that obtaining uniform oxygenation of the over-bottom water with the use of aerators with maintain thermal stratification of the waters is difficult in conditions of unfavorable morphometry. The large area of the tropholytic zone at a relatively shallow depth determines the unfavorable ratio of vertical to horizontal dimensions of the reservoir and practically excludes the achievement of sufficient oxygenation of the bottom waters by means of a single-point injection of this element. The concept of multi-point oxygenation with a controlled outflow direction of oxygenated water, implemented in the lake under study, turned out to be effective in obtaining spatial improvement in oxygen conditions. At the same time, weaknesses of the system were demonstrated, including the possibility of the formation of zones with reactors that have an influence that is too weak, on key fragments of the lake profundal. In the first years of operation of the multi-point aeration system, it is reasonable to conduct a thorough monitoring of the efficiency of aerators. The analysis of spatial data makes it possible to adjust the functioning of the system to the actual lake conditions. In the case of the oxygenation system of Lake Łajskie, a recommendation that does not generate additional operating costs is to rotate the axis of some of the aerators.

The authors hope that this work will be a useful collection of guidelines for research teams dealing with the issues of reversing the effects of agriculture on surface waters. Especially when combining several techniques of lake restoration, considering the spatial efficiency of oxygenation as the guiding method can facilitate the planning process of the entire ecosystem recovery.