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Article

Human Impact on Water Circulation Patterns in Raised Bogs of the Baltic Type, Northern Poland

by
Zuzanna Lipińska
,
Roman Cieśliński
*,
Marlena Pawłowska
,
Izabela Chlost
and
Łukasz Pietruszyński
Department of Hydrology, Faculty of Oceanography and Geography, University of Gdańsk, 80-952 Gdańsk, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(16), 12277; https://doi.org/10.3390/su151612277
Submission received: 7 June 2023 / Revised: 1 August 2023 / Accepted: 8 August 2023 / Published: 11 August 2023
(This article belongs to the Section Sustainability in Geographic Science)
Browse Figures
Versions Notes

Abstract

:
Raised bogs of the Baltic type are unique because of their geomorphologic structure and hydrologic conditions. There are about 80 bogs of this type in Poland. All are affected by human impact, and their water circulation patterns are disrupted by artificial outflows. The aim of the study was to show the effects of human impact on water circulation patterns in raised bog on an example of the Wielkie Bagno peat bog. The main work consisted of field studies, during which measurements of groundwater fluctuations, flows at main outflows, and hydrographic mapping were performed. Data on precipitation and evaporation were also obtained from state institutions. Research has shown that fluctuations in groundwater levels in the Wielkie Bagno bog average 78 cm per year (hydrological years 2018–2019) and the bog is in poor condition. This is mainly due to climate change, which manifests itself in too little precipitation in summer and an increase in evaporation, a too dense drainage network causing the lowering of the groundwater level in the peat bog, and the close proximity of a peat mine causing significant drainage of water from the examined peat bog, as well as too large fluctuations of groundwater during the year and in individual months. In some areas, a near-surface layer of the bog was also detected, about 30 cm thick, which is a sign of bog desiccation (especially in summer). Water balance data were used to show that the quantity of water available in the bog is determined by atmospheric precipitation, which is too small in the warm period. As a consequence, there are months with a negative climatic balance. It was also observed that factors such as physical location in different parts of the bog and the degree of degradation also affect water circulation patterns in the study area. At the moment, the most important task in the peat bog is to raise the groundwater level, reduce its fluctuations, and reduce human pressure on the bog.

1. Introduction

Peat bogs are some of the most productive ecosystems in the world and provide a habitat for rare species of fauna and flora [1]. They also perform many hydrologic functions mainly related to the retention of water in the natural environment [2]. Peat bogs store about 10% of the world’s freshwater [3]. Only a small part of it takes part in the water circulation during the year. Peat bogs of various types stabilize the water cycle, which is an excellent protection against flooding. In turn, during drought, they feed river ecosystems. Wetlands (peatlands) can be natural treatment plants for industrial, agricultural, and municipal pollution [4]. They significantly influence the reduction in pollution with nitrogen and phosphorus compounds [5]. In addition, peat bogs store large amounts of carbon and directly affect the concentration of CO2 in the atmosphere [6,7,8]. The quantity of carbon stored in peat bogs is twice as high as that stored in all of the forests around the world [9]. Peatlands globally store 20–33% of soil carbon [10,11,12]. CO2 emissions from this source are responsible for 5–10% of the annual global emissions of this gas. Carbon absorption by peatlands is greater than its release (net balance 0.14 Gt·year−1), but it is expected that by the end of this century the balance will turn negative and peatlands will become a source of net carbon [13]. According to research by Panai et al. [14], carbon accumulation was higher in periods of wetter and warmer conditions, when sphagnum was dominant, and lower in periods of mixed growth of sphagnum and vascular plants (mainly sedges) in drier/unstable hydrological conditions.
Raised bogs are recharged by atmospheric precipitation and usually constitute a self-regulating hydrologic system relative to their surrounding hydrographic network [15,16]. Peat bogs are further characterized by negligible water exchange with groundwater aquifers and usually contain little groundwater.
Raised bogs include Baltic-type bogs. They fill depressions in relief, which results in the formation of a peat dome. The central part the plateau is generally flat. However, it has a specific micro-relief resulting from the tuft-valley structure of the high bog moss, where local differences in height reach up to 30–40 cm. Along with the distance from the central part, the surface of the land decreases, creating slopes in the dome, inclined to varying degrees. Dome-type peat bogs are found in countries that surround the Baltic Sea including Poland, where they are most often found in the Pomerania region—most often across a coastal belt about 30 km wide. Baltic peat bogs in Poland are at least several times smaller than raised bogs located farther north, e.g., in Estonia [17]. They are extremely valuable in terms of nature and, at the same time, unique in geographical space [18] and arise in conditions of high water levels and the dominance of precipitation [19]. A colder and more humid climate, which is a condition necessary for the raised bog peat accumulation and rapid expansion of bogs to the adjacent forest area, occurred only in the Subatlantic [20].
Irresponsible human impact has reduced the surface area of wetland areas in Europe [21]. Over the last century, the vast majority of peatlands in Europe have experienced substantial transformation as a result of drainage works that led to an imbalance in the natural hydrologic regime, as well as to changes in vegetation composition [22]. According to Grootjans and Wołejko [23], about 80% of natural peat bogs no longer undergo the peat formation process in Poland. The main reason for the decline of peat bogs today is the lowering of the groundwater level caused by man-directed drainage projects and commercial collection of peat [24]. According Taminskas et al. [25], cyclic peatland surface variability is influenced by hydrological conditions that highly depend on climate and/or anthropogenic activities. The disruption in water circulation patterns caused by the creation of artificial outflows from the peat dome leads to a drastic change in a natural system that now experiences peat desiccation and, subsequently, deposit collapse [26]. An artificial drainage network increases the depth and variability of the groundwater table in peat soil, which reduces its retention capacity [27]. In addition, the proportion between atmospheric precipitation and evaporation is changing. Evaporation is increasing because of the emergence of bodies of water in the form of ditches and canals, as well as water-filled post-extraction pits [19]. The lowering of the groundwater level by drainage works triggers succession, leading to a change in a bog’s species composition in the direction of forest vegetation [28]. The unfavorable conditions noted in peat bogs are exacerbated by ongoing climate change. Tarnocai [29] was able to show that accelerating rates of climate change are causing serious damage to peat bogs and triggering the release of large amounts of CO2 into the atmosphere. Peat bogs are becoming emitters of atmospheric carbon instead of functioning as its collectors [30,31]. This will only further accelerate climate change around the world.
The purpose of the study was to identify and assess the impact of the effects of human impact on water circulation patterns in the Wielkie Bagno peat bog and to determine the extent of the influence of peat mines on the groundwater level in the bog. The Wielkie Bagno peat bog is a degraded peat bog, where, unlike in natural bogs, the balance equation is expanded with new elements related to human activity. In light of the extent of human impact in the study area, it is hypothesized in the study that water circulation patterns and the quantity of available water itself are determined principally by anthropogenic factors. They relate to direct human activity (creation of a dense drainage network, peat extraction from the past, proximity to peat mines today, and changes in land use over the centuries), as well as indirect human activity—climate change, increase in precipitation pollution, change in the amount and dynamics of precipitation (increase in rainfall irregularity), and decrease in periods with snow cover. All this meant that the examined peat bog will not function independently, and without human intervention, it has no chance of survival. Due to the strong degradation, but also to the ongoing climate change, the peat bog has no chance to regenerate on its own and should be supported by human activities, which, however, does not guarantee success, but one should try because of the valuable ecosystem and the reduction in CO2 emissions. Due to the fact that it is not possible to increase the amount of precipitation and regulate its course, the main task of man should be to raise the level of groundwater, limit the escape of water outside the peat bog area, and increase the retention capacity of the peat bog. This is only possible through the damming of drainage ditches that impede water outflow, backfilling parts of ditches, and active treatments (restoration), such as cutting down trees in the peat bog or supporting the development of peat-forming plants (floating islands on reservoirs). As a result, this should be enough to improve the water conditions of the fen to such an extent that peat bog will function properly and the hydrological conditions will be appropriate. The currently functioning monitoring in the Wielkie Bagno peat bog is carried out primarily in terms of whether the dams installed on the peat bog fulfill their function. However, it is too early to draw any conclusions.
Therefore, you should ask yourself a few questions. The most important of them include the following. What kind of human activities were conducted in the study area? How do these activities affect the water circulation? Which components of water circulation are investigated? How is this investigation done?
All the assumptions made at the beginning of the work have been implemented. Various forms of human activity were analyzed, starting with the impact of the drainage network on water relations, through peat extraction in the peat bog in the past, to the impact of the peat mine in Krakulice, and ending with the analysis of the climate balance and its changes resulting from climate change. Only the data from the peat mine in Krakulice on the amount of drainage of the peat bog during the mine’s operation could not be obtained. In the case of water conditions, a number of field measurements were made related to groundwater and surface-water fluctuations, horizontal flow (inflow–outflow), determination of the size of various forms of retention, or hydrological mapping. Data on vertical exchange (precipitation and evaporation) were also obtained and were used to calculate the climatic water balance.
What is important is that this research is part of a broad research trend on degraded peatlands.

2. Study Area

Wielkie Bagno is situated in Słowiński National Park in northern Poland (Figure 1). The study area consists of about 1300 hectares of peat bog. It is part of a larger wetland area that features peat bogs formed along the southern shoreline of Lake Łebsko—in the Łeba River Valley. A natural peat dome remains fairly intact in the study area in the direction of the town of Żarnowska. The surface area of the dome is about 55 ha. A peat extraction area is also found in the southern part of the studied bog—its surface area exceeds 40 ha. North of the site, one finds former peat extraction areas, now recultivated and filled with water. The elevation of the study area is at most 5 m above sea level. The northern fringe consists of a belt of old and deteriorated dunes. The southwestern boundary of the bog is marked by the floodplain of the Łeba River. The direction of surface water flow is closely linked to local relief. The drainage ditches in the studied area form a system designed to channel water away from the bog in all directions.
Prior to the commencement of drainage works, the studied bog consisted of expansive treeless wetland areas. The high rate of drainage system development caused a change in the habitat conditions in the study area leading to a transition from open bog to wetland forest areas. At present, the Wielkie Bagno bog is populated with manmade bodies of water such as post-extraction pits, canals, and drainage ditches. Water circulation patterns in the study area reflect more than 100 years of drainage work. The area where the peat structure has been disturbed is about 210 ha, which is 16% of the peatland area. Today, the total length of the ditch network equals 120 km. Over the last century, the density of ditches on the peat bog increased from 1.5 km·km−2 to 8 km·km−2. There is no hydrotechnical infrastructure in the areas adjacent to the study area that could alter the amount of water delivered to the bog.

3. Research Methods

The article shows the state of water conditions caused by centuries of human impact. Particularly noteworthy is the monitoring created, which consists of a large number of piezometers with divers (measurements of groundwater and surface-water levels), measurements of water outflow with main linear objects, measurements of peat thickness and its hydration, morphometric studies of post-mining reservoirs, and conditions of vertical exchange (precipitation, evaporation, and climate balance). As a result, it allowed for a detailed analysis of hydrological conditions in various parts of the peatland (different because of natural conditions and the force of human impact). It was possible to determine the water shortage in certain parts of the peat bog and to plan dams that would improve the water conditions of the Wielkie Bagno peatland. Thus, the monitoring was carried out not only for research, but also for introducing restoration works.
Fieldwork was conducted in the study area in the hydrologic years 2018 and 2019—these were characteristic years in terms of precipitation and thermal conditions (Figure 2) according to the burbot quantile classification.
The work consisted of monthly measurements of water discharge at 8 gauging sites using a Valleport 801 hydrometric flow meter (Figure 3 and Figure 4). The measurement accuracy for this equipment is 0.01 m3·s−1.
Groundwater level changes were determined by employing data from a total of 24 automated piezometers (TD-diver) (Figure 5) and 11 gauges recording water levels in post-extraction pits and canals draining the studied bog. The devices were adjusted to record data twice a day, at 0:00 a.m. and 12:00 p.m. Their measurement accuracy is 0.1 cm. In addition, a baro-diver was located in Gać, which recorded atmospheric pressure. This parameter is essential for the correct reading of groundwater levels. Divers were read once a year, after the end of the hydrological year in November.
At the same time, data were read from a meteorological station located in Gać, on the northwest side of the bog. In addition, hydrographic surveys were done once per three months in different seasons of the year to capture the effects of different hydrometeorological conditions on the peat bog water level. Maps were created illustrating the quantity of surface water present in the studied bog following verification of field survey data using available groundwater level data. During the field mapping, channels were measured in terms of width and depth, which made it possible to categorize the ditches and calculate the volume of water retained by them. In addition, the depth of post-mining pits was measured, and bathymetric plans of the reservoirs were made. GPS was used to locate the measurement points. The volume of the reservoirs was calculated using the truncated cone method as the sum of the volumes of individual water layers bounded by successive isobaths and the volume of water between the deepest isobath and the maximum depth [32]. After verifying the water level in the reservoirs with the limnigraph readings, the volume of the reservoirs at minimum, average, and maximum water levels was calculated.
In the course of the study, meteorological data from the synoptic station in Łeba were used for the three decades from 1990–2019. Based on daily precipitation totals and average daily air temperature, precipitation and thermal conditions were determined for the hydrological years 2018 and 2019, as well as for each month. Thermal and precipitation characteristics were performed using the quantile method [33]. The amount of field evaporation was calculated using the Konstantinov method adapted to Polish conditions by Debski [34]. This method makes it possible to determine the seasonal variation of evaporation. It consists of two parts: a graph of the dependence of daily field evaporation totals on air temperature and water vapor pressure and a graph of seasonal corrections of air temperature and water vapor pressure. The method is adapted to the meteorological conditions of peatlands in Poland, which are a unique ecosystem between land and water. The values of field evaporation and precipitation on a daily basis allowed the calculation of the climatic water balance, thanks to which it was determined which months are characterized by a water surplus or deficit [35].
In addition, measurements of peat hydration (percentage of water content in peat) were performed using the roasting method. The analysis of a single sample consisted of collecting 5 cm3 of peat, weighing the wet sediment, and drying it in a dryer for 24 h at 105 °C. After drying, the samples were weighed, and based on the differences in weight before and after drying, the hydration of peat was calculated using the formula:
H2O (%) = (MM – MS)/MM · 100
where H2O—sludge hydration (%), MM—weight of wet sludge (g), and MS—weight of dry sludge (g).
For this purpose, peat cores were collected using an Instorf peat sampler (Eijkelkamp) (Figure 6).
The work also included a map of the spatial location of the groundwater level. For this purpose, the method of cartographic interpolation using a triangle grid (TIN) was used. It consists in determining the value of a point based on other points with known values using computer algorithms (ArcGIS).
Various calculations of retention (ground, trough, reservoir, and surface) were performed in this work. Reservoir retention was calculated as the volume of the water layer contained between the current and the highest permissible water levels. Surface retention was calculated as the capacity of water appearing periodically on the surface of the peat bog. For this purpose, the water surface in the peat bog (except for rivers and reservoirs) and the average depth were measured. Ground retention was defined as the amount of water contained in the peat deposit. For this purpose, peat thickness probing, a specific peat bog surface, and peat hydration were used. In turn, channel retention was defined as the amount of water that is contained in the channel, especially during periods of higher water levels in the river valley.
It was calculated using the river retention capacity graph.

4. Results

4.1. Changes in the Hydrographic Network

The area of the Wielkie Bagno peat bog is located in the Gardneńsko-Łebska Lowland. It shows all the changes in the hydrographic network that took place in the 19th and 20th centuries. To better recognize these changes in the Wielkie Bagno area, it was decided to limit the description to the Gardneńsko-Łebska Lowland. The reasons for the changes in the original hydrographic system of the Gardno-Łeba Lowland were the need to acquire new land for agricultural purposes and the introduction of effective protection against flood events. The first significant irrigation works in the area were carried out already in the late 18th century. Canals and ditches of general and detailed land irrigation were created at that time (Figure 7A,B). Although the human activity in the aquatic environment was significant even then, it did not contribute to serious consequences of changing the hydrographic system in that time. This is evidenced by the record of the wetlands. Almost the entire lowland was marshy (Figure 8A). A detailed analysis of cartographic material from the first half of the 20th century revealed the preferences for sites to be drained (Figure 8B). At the turn of the 19th and 20th centuries, very intensive drainage system works took place. They concerned the beds of both the main waterways’ valleys and smaller river valleys, particularly those subject to regulation. Large areas of wetlands located to the southwest of Lake Gardno (the Wielkie Bagno peat bog), as well as around Lake Łebsko, were drained. As a result of the engineering works carried out at the turn of the 19th and 20th centuries, the surface extent of drained areas increased greatly. A forced-water circulation caused increased volumes of discharge, especially from waterlogged areas. As a result, the groundwater table lowered, and consequently, water resources were depleted, and the extent of permanently damp areas was reduced (Figure 8B). In connection with opencast peat mining on an industrial scale (peat mine in Krakulice), the hydrographic system of the Wielkie Bagno peat bog situated southeast of Lake Łebsko was also transformed. This resulted in the formation of water bodies in the extraction pits.

4.2. Vertical Water Exchange

The precipitation total for the hydrologic years 2018 and 2019 was 704 mm and 665 mm, respectively (Table 1). Both years were classified as normal in terms of precipitation amounts. It was observed that the beginning of the year 2018 consisted of extremely wet months, while the highest precipitation total was observed in 2017 in November at 119 cm (Figure 9). On the other hand, the lowest precipitation total for the study period was noted in April 2019—at only 9 mm—which led to its classification as a month characterized by extremely dry conditions. The average annual air temperature in both hydrologic years 2018 and 2019 equaled 9.7 °C, which made both years extremely warm (Table 2). This was more than 1 °C higher than the annual average temperature for the last 30-year period. Both years were characterized by a high number of warmer-than-usual months. Such high temperatures are undoubtedly associated with climate change. Both 2018 and 2019 were found to have a positive climatic water balance. The precipitation excess over evaporation was equal to 151 mm for 2018 and 105 mm for 2019.
The annual evaporation total for the 2018 hydrological year was 553 mm. In the winter season, the evaporation value reached 119 mm, which accounted for 27% of the evaporation of the entire year. In the hydrological year 2019, it was slightly higher than the previous year and amounted to 565 mm. In the winter semester, the evaporation value reached 159 mm, which accounted for 28% of the evaporation for the entire year. In both years, the highest value was recorded in June, forming 101 mm (2018) and 110 cm (2019).
From April to August, the predominance of evaporation over precipitation was observed. This was due to high temperatures during the summer months and the occurrence of the longest rain-free periods in late June and early July. Other months of the year were characterized by a positive climatic water balance. November 2017 was exceptional humid (excess precipitation over evaporation about 108 mm). This was the month with the highest total precipitation in the entire period and, at the same time, low evaporation. Thus, the essential retention of peatlands is formed in winter, which is lost in spring with the start of the growing season. In contrast, during summer, despite high precipitation, the climatic water balance reaches a minimum because of high temperature, evaporation, and transpiration of plants.

4.3. Horizontal Exchange

The network of drainage ditches in the area in question is decentralized; the drainage of water from the bog is in all directions. Annual runoff in the hydrologic year 2018 was 1.4 million m3 (Figure 10A). On the other hand, annual runoff in the hydrologic year 2019 was much lower at 0.57 million m3 despite a similar annual precipitation total (Figure 10B). This situation was caused by high precipitation totals and high surface-water levels in the months preceding the 2018 hydrologic year. On the other hand, the very dry summer of 2018 led to reduced runoff in the first few months of the 2019 hydrologic year. The most important points draining the studied bog were Q1, Q3, Q5, and Q8. On average, total runoff from these four points constituted 86% of total annual runoff. The subcatchments of these points are characterized by a well-developed drainage network. These are areas that are strongly degraded and covered with forest. Catchment point Q2 had the lowest runoff in both years. The area is not forested and is mainly post-mining areas where peat has settled heavily, allowing water to accumulate and stagnate. Drainage from them is therefore impeded by barriers in the form of dykes between the various pits. It can therefore be concluded that the slowest water drains from areas of the peat bog that are in the best condition and characterized by high retention capacities, while water “runs away” the fastest from areas heavily transformed by man. It should also be stated that a significant part of the peat bog (480 hectares) is drained through the mine site by means of a dense network of drainage ditches and culverts, followed by a final collecting canal that drains into the Łeba River.
The Wielkie Bagno peat bog is characterized by a variable hydrographic network over the course of the year. Most canals and drainage ditches in the area experience water flow between October and March. All canals in the study area examined between June and September were dry in both hydrologic years 2018 and 2019. The largest runoff in the studied bog occurred in November 2017 when it reached 480,000 m3, constituting 30% of total annual runoff (Figure 11). An increase in runoff can also be observed in March during the spring snowmelt. In spite of relatively high precipitation and low evaporation in September and October of 2018, a long period of drought in the summer months led to negligible runoff from the bog. This means that precipitation first infiltrates dried layers of peat to reach the groundwater table, while its runoff occurs after a significant delay.
In the case of horizontal inflow to the Wielkie Bagno peat bog, there are no places of such inflow.
The dense hydrographic network contributes to an increase in degradation in the form of the disappearance of characteristic vegetation and the process of peat mucking, which is a consequence of the drying of the site. Therefore, the revival of the peat-forming process is not fully possible, so it should be supported by active protection measures, such as the construction of embankments or removal of tree overgrowth. For this purpose, baffles blocking the outflow of water were installed on the peat bog. The idea was to make the baffles so that they would serve to initiate gradual over-growth of the ditches. The location of damming on the ditches was determined taking into account the ModFlow modeling. With the help of modeling, calculations of water level changes were made for normal periods (assumption of 600 mm rainfall). It is predicted that it is likely to be possible to achieve improvements in water conditions with the installed dams. The rise in groundwater levels would include the areas located in the western, northwestern, and central parts, where there are a considerable number of canals and ditches (Figure 12).

4.4. Fluctuations in the Groundwater Level

The groundwater level in the peat bog fluctuates significantly. The average difference between the lowest and highest water levels in 2018 was 85 cm, and in 2019, it was 69 cm.
The least variable groundwater level recorded by piezometers in the bog area was noted at sites Z1, Z7, Z10, and Z11, all of which were situated on the peat dome or in post-extraction areas (Figure 13).
The depth at which water is found in the studied bog varies from site to site. In the study period, the depth of the groundwater table as measured by piezometers was greatest near the extraction site and in western fringe areas—at −45 cm. On the other hand, the smallest average, maximum, and minimum depths at which groundwater is found in both studied years was recorded by piezometers Z1 and Z7. A significant lowering of the groundwater table was observed at all piezometers in the summer months both in 2018 and 2019 (Figure 14). The water level in the early part of the 2019 hydrologic year did not return to near its initial state after suffering a large decrease in the summer period. The initial state is defined as that in November 2017. The difference between the water level in November 2017 and November 2018 equaled 30 cm, on average.
The water conditions of the bog vary depending on the location within the site. During the study period in the bog area, groundwater was deepest in piezometers located near the peat mine and in the extreme part of the bog on its eastern side (Figure 15). The extent of the mine’s influence on the lowering of the groundwater level was estimated at about 1.2 km. On the eastern side of the site, the area has been and continues to be degraded as a result of the drying of the peat and, thus, the beginning of the peat mucking process. As a result, its water retention capabilities are impaired, which is reflected in the low groundwater table values also observed on the northern side, where there is a strip of inland dunes. The best hydrated areas, where the average groundwater table does not exceed −10 cm, are located in the central part of the bog. These are mainly areas with selected peat or broken turf and most of the dome of the bog.

4.5. Water Retention

The most important function of peatlands is the possible periodic retention of water in its area (retention). One form of retention is ground retention, i.e., determining the amount of water stored in the peat layer. The average hydration of peat in 2018 and 2019 was 86.5%, so the water resources present in the peat deposit were determined to be 26 million m3. Hydration was analyzed in different months, but there were no significant differences between the wet and dry semesters. The peat bog is filled with numerous objects of anthropogenic origin, such as drainage ditches and post-mining reservoirs, which can store significant amounts of water. At the average water level, their volume is 72,143 m3. In the peat bog, there are six water-filled post-peat pits. At high water levels, together, they are capable of storing a maximum of 430,000 m3. During dry periods, the volume of water in the reservoirs decreases and then amounts to only 138,000 m3 (Figure 16).
Surface retention is another form of retention, strongly linked to hydrometeorological conditions. There are times when water is directly on the surface, flooding significant areas of peatland. A near-surface layer of marsh was noted at piezometers Z9, Z12, Z13, Z14, Z15, Z17, Z23, and Z24 caused by years of drainage work across the Wielkie Bagno bog. The thickness of the layer ranged from 20 to 50 cm, which helps to substantially reduce the retention capacity of the studied bog. Surface-water retention across peat bogs is closely linked with the relief of the land and specifically with the presence of post-extraction pits. Levees found between the extraction pits prevented the outflow of the stagnant water. The highest water levels are noted during precipitation events and periods of low evaporation and after the spring snowmelt.
All ditches are filled with water during these periods. More than 30% of the bog (450 ha) is covered with water at this time, while 40% is a wetland (540 ha) (Figure 17A). In the summer the ditches dry out. Only a few canals and ditches retain some stagnant water, and their total length is reduced to 40 km (Figure 17B). The water level in post-extraction pits declines significantly, while the peat dome dries out.

4.6. The Water Balance

The data described above were used to calculate the water balance for the studied bog. Vertical water exchange was found to be the dominant form of water exchange in the said water balance (Figure 18). Precipitation constituted 100% of the inflow of water, while the outflow was divided into evaporation (82%), runoff (10%), and retention (8%). The amount of data on the inflow and outflow of groundwater was insufficient to make categorical inferences; instead, it was assumed that groundwater inflows and outflows offset each other, and there is no need to factor them into the calculations in the study. The low water retention rate observed herein in the study period may be explained by the presence of a well-developed hydrographic network in the study area that leads to excessive peat drying. The existing network of ditches and the degradation of the peat deposit is responsible for the lack of proper retention and generates rapid drainage of water outside the peatland during periods of surplus.

4.7. The Hydration Status of the Peat Bog

Human activity is visible through the drying of the top layer and its mineralization (state of decomposition). The average hydration of the entire peat deposit in the Wielkie Bagno peat bog is 88% (Figure 19). Smaller variation of water content variability with depth occurs on the peat overgrown by the forest; greater variability is visible on the open peat, on the dome (Figure 19). The lower hydration near the surface is due to the loss of retention caused by peat mineralization in muck. In deeper parts, slopes are marked in places where peat is gytinated or in contact with mineral substrate or in organic formations with pieces of wood.

4.8. The Influence of Drainage on the Phytocoenosis of a Peat Bog

In the Wielkie Bagno peat bog, the drainage network is still in operation (Figure 20). Although some of the ditches have overgrown and disappeared, the others still drain water at least periodically, worsening the water balance. In a large area of peatlands, the groundwater level is lowered as a result of drainage. The ongoing process is, therefore, the decay of the surface layers of peat and the associated release of carbon dioxide and other greenhouse gases. The biocenotic effect of peat drainage and rotting are the processes of degeneration of phytocenoses, unfavorable from the point of view of biodiversity, including the encroachment of trees and shrubs and expansive herbaceous plants such as Molinia coerulea. These plants displace moisture-loving and light-loving peat bog vegetation. The most valuable bog phytocoenoses, including the tussock moss with marsh heath (Erico-Sphagnetum medii association) [37], are primarily at risk (Figure 21). Under the conditions of reduced groundwater levels in the patches, the expansion of trees and gradual transformation of moss phytocoenoses into forest phytocoenoses are observed. Drainage also has a negative impact on the possibility of regeneration of bog vegetation in areas previously exploited. The vegetation of the Wielkie Bagno bog indicates drying, i.e., a violation of the natural water conditions of raised bogs. The reason for this state is human activity, in particular drainage of the peat bog.

4.9. The Impact of the Created Post-Peat Reservoirs on the Functioning of the Peat Bog

In the southern part of the Wielkie Bagno peat bog, the Krakulice peat mine operates. There are three extensive post-peat excavations here, where peat extraction was completed about 10 years ago. These excavations currently take the form of three large (16–20 ha) rectangular water reservoirs (Figure 22). Due to their surfaces, wind undulations within them practically prevents the succession of vegetation and, thus, the activation of peat-forming processes. The development of reed and bog vegetation is possible only at their shores, while the surface of the open water table is not able to be inhabited by reed plants or nympheids.

5. Discussion

Peat bogs have been subject to intense human impact for centuries because of their economic value to humankind, especially in the last millennium (drainage and extraction). Today, many peat bogs in the world are exposed to the negative impact of human activities. The sample research by Lomnicky et al. [38] in the USA shows that from 10% to 75% of peatlands (depending on the region) have been degraded to a greater or lesser extent. The drainage of bogs and peat extraction lead to the destruction of the water retention system in bogs because of the presence of drainage ditches in wetland areas and overall changes in water circulation patterns. Hence, the calculation of a water balance for peat bogs requires consideration of new sources of recharge, as well as surface drainage. In addition, the effects of climate change occurring over the last millennium require a better understanding of the climate–hydrology–bog relationship, as the existence of peat bogs is strongly dependent on climate conditions.
The proper groundwater level and fluctuations in peat bog models in the research literature [39] are determined by relief and micro-landforms in the local area [40]. Under natural conditions, fluctuations in groundwater levels are rather low throughout the year. On the other hand, all long-term declines in the level of groundwater lead to an increase in the density of peat, which then leads to a lowering of the peat surface, resulting in the groundwater level being closer to the surface and, at the same time, to the total volume of water available to the peat bog being smaller [41]. This problem is observed on most raised bogs, including the Wielkie Bagno peat bogs. The depth at which groundwater is found is affected by the local hydrographic network. In addition, the depth at which groundwater is found is affected also by land use. The lowest groundwater tables are found in the part of the peat bog covered with trees, while the highest water levels are detected in open, treeless areas [40]. The same type of situation is found in the Wielkie Bagno peat bog.
Degraded peat bogs are characterized by low water retention caused by increased surface runoff via a network of drainage ditches [27]. Luscombe [42] showed that a manmade drainage network increases the depth at which groundwater is detected and increases variances in the groundwater level in peat soils. In the Wielkie Bagno area, this was particularly easy to observe near the ditch channeling water away from the peat extraction area—the level at which groundwater was detected was −45 cm, on average. In order for peat-forming processes to occur in raised bogs, it is necessary to maintain a high water level that does not fluctuate a lot from one season to another. Peat mosses grow at an appreciable rate when the water level is found at a depth of 1 to 22 cm below the bog surface [43]. On the other hand, the depth at which groundwater is found in the dome section of nondegraded raised bogs in Europe is 30 to 40 cm, on average [44]. The mean monthly groundwater level in the studied bog suggests that only high retention conditions are capable of facilitating the proper growth of peat-forming vegetation. This is confirmed by the studies of Chlost et al. [45] and Chlost and Cieśliński [46], who indicate a high dynamics of changes in the groundwater level, especially in the summer. Chlost and Cieśliński [46] indicate that, in the Baltic-type raised bogs in the warmer months, water level fluctuations can be up to 1.5 m per week. This is due to the fact that there are dry and wet seasons in the peat bog, which follow each other quickly.
In addition, the relationship between peat surface collapse caused by degradation and changes in the groundwater level expressed in absolute height values is important [47]. However, this angle of analysis is in need of more detailed verification. Lamentowicz et al. [48] determined the critical point for proper bog functioning at 11.7 cm under the bog surface. In turn, according to Regan et al. [49], maintaining a high water level with small fluctuations during the year is important for the peat-forming process in raised bogs. In a well-preserved top of a raised bog, the water level should be no deeper than 10–20 cm below the ground level (and the fall of its summer level to 40 cm below the ground level is considered a critical level for the growth of raised bog communities) [50]. According to Taminskas et al. [25], for Lithuanian peat bogs, this value is 25–30 cm. Below this critical point, the bog stops accumulating carbon and becomes a carbon emitter. In the studied bog, the average depth at which groundwater was found in the study period was 30 cm below the bog surface. None of the piezometers used detected this level, while the maximum water level detected near the peat extraction site during the study period did not reach the critical level. This suggests a strong degree of desiccation in the bog and the onset of marsh formation, which prevents the proper functioning of a peat bog.
Poor functioning of peat bogs is also associated with large fluctuations of groundwater and high water level amplitude during the year. Unfortunately, groundwater fluctuations were very high for the analyzed bog. In Wielkie Bagno, fluctuations ranged from 43 to 113 cm below ground level. As a result, we observe amplitudes of 70 cm. These are large values compared to those from Estonian peat bogs, where the groundwater level amplitude is 3–22 cm in the bog domes and 3–14 cm in the forested lagg zones [51]. Drewnik et al. [22] made similar observations, the results of which show that peat bogs exhibit a relatively high stability of groundwater table levels despite meaningful changes in weather conditions. The most visible response of peat bogs to weather conditions was observed in the summer and autumn. According to the authors, degraded peat bogs experience the largest decrease in groundwater table levels and more frequent fluctuations. However, when compared with other degraded Polish raised bogs, the values obtained on the Wielkie Bagno bog are similar. For example, at Czarne Bagno and Łebskie Bagno, the peat bogs’ annual amplitudes of fluctuations in the water table was in the range of 28 to 78 cm (Łebskie Bagno) and of 46 to 105 cm (Czarne Bagno). As a result, we observe amplitudes of 50–60 cm in these peat bogs [52].
Summarizing, it should be said that human impact has significantly altered every major biogeochemical cycle [53]. It should be remembered that degradation of groundwater-dependent ecosystems has raised a need for their restoration, but ecological responses to restoration are largely unknown [54]. An example would be two renaturalized Irish peat bogs, where in one case was observed improvement of hydrological conditions and, in the second, a worsening of these conditions [55]. Even so, please note that drained organic soils are a source of substantial emissions of carbon dioxide (CO2) into the atmosphere [56]. Rewetting these soils may not only reduce greenhouse gas emissions but also create a favorable environment for the return of the basin function, which is characteristic of well-functioning organic soils [57].
At the moment, the most important thing is to create an aquatic monitoring program that is imperative for the functioning of the environmental impact assessment process [58]. This statement is confirmed by the research of Jácome et al. [59], who indicate that constant water quality and quantity monitoring is essential for a better understanding of the patterns occurring in this ecosystem.

6. Conclusions

The results of the present study indicate that water circulation patterns in the Wielkie Bagno peat bog are characterized by large variances over the course of the year and are reliant mainly on meteorologic factors. The groundwater level in the bog is determined by atmospheric precipitation—and especially by its intensity and distribution in time. Runoff from some of the drainage ditches in the study area follows an episodic pattern—water appears only during high precipitation events. The water retention capacity of bogs is determined in the wintertime, while in the springtime, water is lost to growing vegetation. The bog dries out in the summer when the surface of the groundwater table drops substantially to more than −100 cm. Surface runoff stops at this time, and only some stagnant water remains in the largest of the drainage ditches. Such large water deficits are undoubtedly the result of high air temperatures and long periods without precipitation. Summers in both years were characterized by warmer-than-usual months and the biggest water deficits. Projections of climatic conditions for Poland in connection with climate change predict increases in air temperature and evaporation. Then, this results in drought and degradation of water conditions in peatlands.
As mentioned earlier, water supply in peatland is controlled by atmospheric precipitation. Therefore, it is necessary to consider how climate change will affect precipitation and, consequently, the hydrological conditions of the peatbog. In Central Europe, including Poland, the amount of precipitation will not change much, but the distribution of precipitation will change—in summer there will be less, in winter more—which will lead to more frequent droughts and torrential rains.
According to the IPCC reports, the amount of precipitation will mainly depend on the increase in the global average air temperature. For a temperature increase of 1.1 °C, precipitation will increase by 1.3 times; for a temperature increase of 2 °C, the amount of precipitation will increase by 1.8 times, and for a temperature increase of 4 °C, the amount of precipitation will increase by 2.8 times. In many regions, the proportion of days with high precipitation above the 95th percentile will increase. In Europe, the upward trend for intensive precipitation will prevail. In Poland, it is forecast that precipitation will occur more often and will be more intense than before. However, in the summer, there will be more days without precipitation, causing droughts. More frequent rainfall is forecast in autumn, winter, and early spring. The highest sums of precipitation occur in June and July and the lowest in the winter months—January, February, and March. Due to the higher air temperature in Poland, an increase in evaporation is expected and, thus, water shortages in the climatic water balance. As a result, groundwater supply will decrease. As a consequence, accelerated and definitely greater degradation of peatlands than we observe today should be expected.
In response to the question posed in the very beginning of the paper, the quantity of water available in the studied bog is determined mostly by atmospheric precipitation, although factors such as location in the bog and degree of degradation also substantially affect the water circulation pattern of the entire study area. The runoff rate is highest in areas most substantially affected by man, which are characterized by dense hydrographic networks and a near-surface layer of marsh. On the other hand, the lowest runoff rate was noted in areas least affected by human impact and characterized by the highest retention capacity. The very low groundwater level present in the dry season renders peat-forming processes unfeasible. This is especially easy to observe near the main drainage ditch channeling water away from the extraction site. Despite advanced degradation across 75% of the bog area caused by human impact, some live peat areas remain, which means areas home to peat-forming vegetation whose growth rate still exceeds losses because of mineralization. A very bad condition for the Wielkie Bagno peat bog is not only the low level of groundwater retention (especially in the summer) but also the dynamics of their changes, which may exceed 110–120 cm in a given measuring point during the year.
Hence, the Wielkie Bagno peat bog is an environmentally valuable area, which remains threatened by functioning drainage ditches and continuing deposit extraction and the observed climate change manifested by more extreme meteorological conditions.
Summing up, human activity determines the water circulation in the examined peat bog, and precipitation is the main water supplier. This is due to the fact that the drainage network is constructed in a way that leads only to the drainage of the peat bog in all directions, but at the same time this network determines the faster transfer of water outside the peat bog and affects the insufficient retention of rainfall.

Author Contributions

Conceptualization, Z.L. and R.C.; methodology, Z.L., R.C. and I.C.; software, Z.L. and M.P.; validation, R.C. and Ł.P.; formal analysis, Z.L., R.C. and I.C.; investigation, Z.L., R.C., M.P., I.C. and Ł.P.; resources, Z.L. and R.C.; writing—original draft preparation, Z.L. and R.C.; writing—review and editing, Z.L. and R.C.; visualization, Z.L., M.P. and I.C.; supervision, R.C.; All authors have read and agreed to the published version of the manuscript.

Funding

This paper was funded by University of Gdańsk.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and/or analyzed during the current study are avail-able from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of study area.
Figure 1. Location of study area.
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Figure 2. Various hydrometeorological conditions during field research.
Figure 2. Various hydrometeorological conditions during field research.
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Figure 3. Location of measurement sites across the studied Wielkie Bagno peat bog.
Figure 3. Location of measurement sites across the studied Wielkie Bagno peat bog.
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Figure 4. Flow measurements in channels located in the study area.
Figure 4. Flow measurements in channels located in the study area.
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Figure 5. Examples of piezometers located in the Wielkie Bagno peat bog.
Figure 5. Examples of piezometers located in the Wielkie Bagno peat bog.
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Figure 6. For example, a collected peat core in the studied peat bog.
Figure 6. For example, a collected peat core in the studied peat bog.
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Figure 7. Hydrographic network of Gardno-Łeba Lowland in the early 19th c. (A) and 20th c. (B) (Adapted from Ref. [36]).
Figure 7. Hydrographic network of Gardno-Łeba Lowland in the early 19th c. (A) and 20th c. (B) (Adapted from Ref. [36]).
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Figure 8. Extent of wetlands in Gardno-Łeba Lowland in the early 19th c. (A) and 20th c. (B) (Adapted from Ref. [36]).
Figure 8. Extent of wetlands in Gardno-Łeba Lowland in the early 19th c. (A) and 20th c. (B) (Adapted from Ref. [36]).
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Figure 9. Precipitation totals, evaporation, and climatic water balance for hydrologic years 2018 and 2019.
Figure 9. Precipitation totals, evaporation, and climatic water balance for hydrologic years 2018 and 2019.
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Figure 10. Magnitude and direction of surface runoff in hydrologic years 2018 (A) and 2019 (B).
Figure 10. Magnitude and direction of surface runoff in hydrologic years 2018 (A) and 2019 (B).
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Sustainability 15 12277 g011
Figure 11. Monthly runoff data for the studied peat bog for hydrologic years 2018 and 2019.
Figure 11. Monthly runoff data for the studied peat bog for hydrologic years 2018 and 2019.
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Sustainability 15 12277 g012
Figure 12. Projected change in groundwater levels.
Figure 12. Projected change in groundwater levels.
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Sustainability 15 12277 g013
Figure 13. Average fluctuation range for groundwater levels in 2018 and 2019.
Figure 13. Average fluctuation range for groundwater levels in 2018 and 2019.
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Figure 14. Fluctuations in the groundwater level given for selected piezometers.
Figure 14. Fluctuations in the groundwater level given for selected piezometers.
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Figure 15. Average depth of groundwater determined on the basis of cartographic interpolation using a triangle mesh (TIN).
Figure 15. Average depth of groundwater determined on the basis of cartographic interpolation using a triangle mesh (TIN).
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Figure 16. Distribution of the various forms of retention.
Figure 16. Distribution of the various forms of retention.
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Sustainability 15 12277 g017
Figure 17. Surface water levels in December 2017 (A) and August 2018 (B).
Figure 17. Surface water levels in December 2017 (A) and August 2018 (B).
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Sustainability 15 12277 g018
Figure 18. Water balance for the Wielkie Bagno peat bog.
Figure 18. Water balance for the Wielkie Bagno peat bog.
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Sustainability 15 12277 g019
Figure 19. Peat hydration in the forest profile (left figure) and on the bog dome (right figure) in the Wielkie Bagno peat bog; S—standard deviation.
Figure 19. Peat hydration in the forest profile (left figure) and on the bog dome (right figure) in the Wielkie Bagno peat bog; S—standard deviation.
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Sustainability 15 12277 g020
Figure 20. The drainage infrastructure and post-exploitation water bodies (A–C) on the Wielkie Bagno peatland in relation to the digital terrain model (Adapted from Ref. [37]).
Figure 20. The drainage infrastructure and post-exploitation water bodies (A–C) on the Wielkie Bagno peatland in relation to the digital terrain model (Adapted from Ref. [37]).
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Sustainability 15 12277 g021
Figure 21. The biggest patch of peat moss vegetation with a bog heather (Erico-Sphagnetum medii) endangered because of the lowered level of groundwater in the peatland, covered by the practice of periodic tree and shrubbery removal (Adapted from Ref. [37]).
Figure 21. The biggest patch of peat moss vegetation with a bog heather (Erico-Sphagnetum medii) endangered because of the lowered level of groundwater in the peatland, covered by the practice of periodic tree and shrubbery removal (Adapted from Ref. [37]).
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Sustainability 15 12277 g022
Figure 22. Post-exploitation water body in the southern part of Wielkie Bagno (Adapted from [37]).
Figure 22. Post-exploitation water body in the southern part of Wielkie Bagno (Adapted from [37]).
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Table 1. Distribution of precipitation (mm) in the hydrological years 2018 and 2019 according to the quantile classification of Miętus et al. (Adapted from Ref. [33]).
Table 1. Distribution of precipitation (mm) in the hydrological years 2018 and 2019 according to the quantile classification of Miętus et al. (Adapted from Ref. [33]).
XIXIIIIIIIIIVVVIVIIVIIIIXXYEAR
20181199264113440222975469481704
20191776444075946188241111107665
>90 quantileextremely wet
70–90 quantilewet
30–70 quantilenormal
10–30 quantiledry
<10 quantileextremely dry
Table 2. Thermal conditions in the hydrological years 2018 and 2019 according to the quantile classification of Miętus et al. (Adapted from [33]).
Table 2. Thermal conditions in the hydrological years 2018 and 2019 according to the quantile classification of Miętus et al. (Adapted from [33]).
XIXIIIIIIIIIVVVIVIIVIIIIXXYEAR
20188.92.91.5−2.20.09.914.716.719.119.414.910.49.7
20195.22.70.53.25.28.011.119.417.218.614.510.59.7
>95 quantileextremely warm
90.01–95 quantileanomalously warm
80.01–90 quantilevery warm
70.01–80 quantilewarm
60.01–70 quantileslightly warm
40.01–60 quantilenormal
30.01–40 quantileslightly cool
20.01–30 quantilecool
10.01–20 quantilevery cool
5.01–10 quantileanomalously cool
<5 quantileextremely cool
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Lipińska, Z.; Cieśliński, R.; Pawłowska, M.; Chlost, I.; Pietruszyński, Ł. Human Impact on Water Circulation Patterns in Raised Bogs of the Baltic Type, Northern Poland. Sustainability 2023, 15, 12277. https://doi.org/10.3390/su151612277

AMA Style

Lipińska Z, Cieśliński R, Pawłowska M, Chlost I, Pietruszyński Ł. Human Impact on Water Circulation Patterns in Raised Bogs of the Baltic Type, Northern Poland. Sustainability. 2023; 15(16):12277. https://doi.org/10.3390/su151612277

Chicago/Turabian Style

Lipińska, Zuzanna, Roman Cieśliński, Marlena Pawłowska, Izabela Chlost, and Łukasz Pietruszyński. 2023. "Human Impact on Water Circulation Patterns in Raised Bogs of the Baltic Type, Northern Poland" Sustainability 15, no. 16: 12277. https://doi.org/10.3390/su151612277

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