5.1. Changes in Lake Morphology and Water Resources
The increase in air temperature in Poland in the second half of the 20th century slightly exceeded 0.2 °C per 10 years [
74,
75]. However, the increase within the study area was higher compared to other regions and amounted in Toruń to 0.31 °C per 10 years [
76]. In the study area, as in other parts of Poland [
77], no reduction in annual sums of precipitation was observed in the second half of the 20th century or the first two decades of the 21st century. However, Radzka [
37] indicates an increasing frequency of negative climatic balances in the growing seasons of 1971–2005, which increased the extent of Poland’s dry zone, those areas characterised by climate balance ranges from −90 mm to −120 mm [
77]. Meanwhile, calculations by Meresa et al. [
78] showed a large discrepancy between the results of calculations of drought indicators that only take climate parameters into account, and the Standardised Runoff Index, which can be used to identify hydrological drought [
78]. The calculations by the cited authors indicate that, in the Narewka lowland catchment in 1971–2000, the drought index calculated on the basis of outflows indicated hydrological drought in most of the studied years from the period 1984–2000. These results differ significantly from those for drought calculated based on precipitation, or on precipitation and evapotranspiration. The occurrence of hydrological drought in recent decades has also been confirmed by studies of other lowland basins of Poland, which recorded that underground outflow fell in the second half of the 20th century [
79], and total outflow fell within smaller catchment areas [
80] as well as in the lowland left-bank catchment of the Vistula River [
81].
Similar to those areas above-mentioned, Lake Borówno and its catchment are subject to considerable transformations in terms of hydrological conditions. As demonstrated in this study, a significant reduction in surface area, paired with a lowering of the water table, was recorded in Lake Borówno as well as in the neighbouring Lakes Kusowo and Dobrcz. This process intensified particularly in the second decade of the 21st century, when the reduction in surface area was comparable to that of the previous 45 years and the drop in water table more than doubled. Comparison of the obtained values with the rate of lake disappearance in different regions of Poland indicates that the decrease in the surface area of Lake Borówno is similar, while Lakes Kusowo and Dobrcz displayed larger-than-average decreases in area over a period of about 50 years, as calculated by Choiński [
82], amounting to 9.6% and 9.98% for the Pomorskie Lake District and the Mazurskie Lake District. The rate of lake surface area decline in Poland is very diverse, and in the perspective of the next 100 years may, in extreme cases, reach even several dozen percent of the initial area [
29,
39,
83,
84,
85]. The authors above-mentioned emphasise that the process of lake surface reduction depends on many natural and anthropogenic factors, and therefore each lake should be analysed individually. In the case of many lakes, the process of progressive disappearance associated with a decrease in surface area and water table lowering has accelerated significantly in the 21st century. It is related to an increase in air temperature [
39,
84], which causes higher potential evaporation [
76]. Annual potential evaporation in the research area in the years 1952–2018—measured at the meteorological station in Toruń—increased by 183.9 mm [
76]. The authors found that the highest increase occurred in the spring (March–May) and summer (July–September). Moreover, an increase in the differences in evaporation between the Baltic sea coast and inland regions was observed in the years 1952–2018. Therefore, the lakes located further away from the Southern Baltic Coastlands such as Lake Borówno, may be highly exposed to climate-driven water level changes.
A graphic representation of the differences in depth of Lake Borówno between 1964 and 2018 (
Figure 5) indicates that shallowing has occurred together with a lowering of water level. The lake shallowed most along the south-eastern shore (i.e., where it borders agricultural areas), single-family housing areas, and allotment gardens. On this shore, there are three tributaries that can supply mineral material and organic matter from agricultural areas to the eastern and north-eastern parts of the lake. The results presented in
Figure 5 compared with field observations performed in 2018 indicate that beaches may constitute the source of supply of sandy material, which caused shallowing of the selected sections of the lake’s shore. This phenomenon was observed both in the area of holiday resorts and the so-called “wild” beaches used by local residents and allotment garden owners. Along the shore sections used as public beaches, there are zones of considerable size that are devoid of vegetation cover, which may explain the supply of mineral material to the lake basin. The map of depth differences (
Figure 5) indicates that the shallowing of the shore zone in comparison to 1964 also occurred near the beach in the proximity of other beaches. The accumulation of material can be also observed in deeper parts. It is possible that this is due to the movement of material from the shore zone to the deeper parts of the lake basin as a result of works to maintain bathing areas. Another potential source of sandy material accumulated in the lake derives from the relatively rapid exposure of lakeshore caused by the lowering of its water level. It should be emphasised that the intense tourist pressure and heavy use of beaches on Lake Borówno in recent decades is also conditioned by climatic factors. Summer heat waves intensify the use of beaches and lakes by weekend tourists.
The lowering of the water table by 0.94 m between 1912 and 2018 at Lake Borówno shows that water resources in its catchment area are diminishing. The significantly greater lowering of the water table observed at the two remaining lakes—Kusowo and Dobrcz—results from their smaller depth and lack of contact with the main usable aquifer supplying Lake Borówno. Lakes of greater depths act as hydrogeological windows, allowing surface waters to contact deeper aquifers [
86,
87,
88]. Despite its more favourable supply conditions associated with deeper drainage, Lake Borówno lost 10.8% of its surface area and 14.4% of its water volume between 1964 and 2018. In the light of the research regarding changes in the water table within the two usable aquifers in the area [
66], it can be assumed that the deteriorating climatic conditions coincided with intensified use of groundwater (
Figure 3). In particular, the depletion of the main usable aquifer, which does not have a high capacity to recover, may permanently change water relations in the study area. This aquifer is intensively exploited as a result of the expansion of municipal waterworks, and estimates show that it will be several years before precipitation water infiltrates back into this aquifer [
66]. Since the 1970s, there has been a local lowering of the water table in the main aquifer of several dozen centimetres (up to a maximum of 1.4 m). This shows a steady trend of diminishing resources as well as indicating large-scale and long-term exploitation of groundwater in this usable aquifer [
66]. Groundwater resource depletion can also be related to the use of these waters in individual intakes; for example, for watering allotment gardens or greenhouse crops [
89]. In the case of Lake Borówno, an additional adverse factor increasing the risk of water resources depletion and lake water level decrease is the catchment area’s location near the edge of the Vistula River valley. As indicated by hydrological surveys carried out in the catchment area of the Wda—a left-bank tributary of the Vistula River—sections of this catchment area located near the edge of the Vistula River valley are characterised by a lower outflow volume than the areas located further from this edge [
90,
91]. This is due to the draining influence of the deep Vistula River valley. Similar conclusions were reached by Glazik and Pius (2007) [
92] in their study pertaining to the Vistula River section between Toruń and Tczew. The cited authors found that the Vistula River is fed by deep circulation groundwaters in this section, and the edges of the valley are a place of heavy drainage of these waters. The location of Lake Borówno in the edge zone may additionally influence the stronger reaction of groundwater to changes in climatic conditions and the increase in exploitation.
A decrease in water level in lakes resulting from increased evaporation caused by heat waves recorded in the summer of 2018 was also observed in other parts of Europe including England [
93]. However, instead of focusing on the losses in water resources
per se, a number of European countries became alarmed with the coinciding losses in agriculture [
94], which translated, among other things, into less restrictive approaches to protecting water resources and their use for irrigation [
95]. Toreti et al. [
96] indicated that the drought of 2018 proved to affect a considerable portion of Europe. However, having analysed seven climate scenarios, the authors anticipate that droughts of comparable severity will occur more frequently in the 21st century, and may in fact become the norm in the 2050s. At the same time, the increased demand for potable water and prioritisation of agriculture in the course of the drought of 2018 [
95,
97] suggest that the observed reaction of lakes to drought may constitute a combined result of diminishing water resources and increased use of basin water. Lake Borówno demonstrated a considerable drop in the water table in the second decade of the 20th century and, in light of the scenarios presented by Toreti et al. [
96] and Meresa [
98], we may anticipate that such hydroclimatic conditions will be recurring. The map presented by Feyen and Dankers [
35] shows that the research area and the whole territory of Poland is at the edge of an area covering southern and south-western Europe, which in the 21st century will suffer from an outflow deficit. Furthermore, taking into consideration that Lake Borówno is located in the suburban area of Bydgoszcz, it constitutes an attractive location for settlement as well as for seasonal migration of citizens during hot summer days, which is particularly true for the owners of nearby allotment gardens. Therefore, we may expect further growth of anthropopressure and increasing water consumption. Water use has been indicated as a factor intensifying hydrological drought by, among others, Wada et al. [
40] as well as Wanders and Wada [
41]. Taking into consideration the accelerated problems with drought caused by forecasted climate warming and accompanying water demand, Van Loon argues that future studies on droughts should concentrate on four key tasks, one of which involves “moving to including the human aspects of hydrological drought”. He also advocates the concept of “socio-hydrology”, a new science of people and water introduced by Sivapalan et al. [
99]. Moreover, in the second decade of the 21st century, the International Association of Hydrological Sciences (IAHS) identified a new important field of research related to the link between hydrological drought and society [
100]. As noticed by Arnell et al. [
101], the current state of knowledge pertaining to the impact of climate change on water management is largely insufficient. Therefore, in recent years, an effort has been made in Poland to develop a six-year nationwide programme known as the “Drought Effects Counteracting Plan” (2021–2027) [
102]. In this programme, several dozen of retention works are planned to increase the retention in areas most exposed to drought including central Poland.
5.2. Factors Affecting Water Quality
The chemical status of the water in the lake is shaped by a number of factors that are related to the lake itself and its catchment area [
103]. The assessment of the susceptibility of Lake Borówno to eutrophication [
104] showed that in terms of catchment criteria, the lake represented the second group of catchment area susceptibility. This means that “the catchment area is characterised by a low susceptibility to activation of the load deposited in its area and a low possibility of it reaching the lake”. Such an assessment results, among other things, from the low slope gradients within the lake’s catchment area and the only periodical functioning of the small watercourses supplying them and draining the agricultural areas. However, the observed lowering of the water level in Lake Borówno may determine the ecological type of the lake and the rate of eutrophication. A change in the parameters of average depth, water stratification percentage, and active bottom surface area decreases the lake’s resistance to external factors [
105].
As shown above, in the second half of the 20th century, changes shaping the lake’s water resources occurred, which resulted in a decrease in the water level, surface area, and depth of the lake. These changes may be the cause of the observed shifts in the thermal stratification of Lake Borówno. According to Piasecki and Skowron (2014) [
29] and Magee and Wu (2017) [
106], changes in lake stratification, which depend mainly on the type of lake basin and its depth, are strongly related to climate change. Reduction of the maximum depth of Lake Borówno and lowering of the water table results in incomplete summer stratification (lack of hypolimnion) and the water temperature in the entire profile is higher in the 21st century than in the 20th century. Results presented by Woolway et al. [
107] in relation to 20 lakes in Central Europe show an increasing trend in lake surface water temperature in the past 50 to 100 years, with the most notable growth reported in the late 1980s. Studies concerning individual lakes show that, despite the fact that the trend concerning changes in surface water temperature in Europe appears to be uniform, the processes related to heat transport within lakes tend to vary on a lake-by-lake basis [
108,
109,
110]. Increased supply of heat to the surface layer of water accompanied by lower wind intensity may increase the temperature difference between the epilimnion and the hypolimnion [
108]. Lake Borówno displays a rise in temperature across the entire vertical profile; however, we also observed an increase in the difference between surface water temperature and hypolimnion. Moreover, similar to Upper Raduńskie Lake [
108], but unlike Lake Geneva [
109], the depth of the thermocline was observed to have decreased. However, it should also be noted that the shallow zone’s high share in total lake volume means that currently heated surface layers of 4–6 m deep and a temperature of 20–23 °C account for as much as 66.7% of water by volume, while only about 7% by volume are colder waters at depths of over 8 m, with a temperature reaching 7–11 °C during summer stratification (
Figure 7a,
Table 7). It is often emphasised that an increase in surface water temperature in mixed lakes results in greater stability of water masses within the lake related to briefer water mixing and a longer duration of stratification in summer [
106,
108,
111,
112]. Moreover, changes in vertical mixing within the water column constitute an important factor changing the availability of nutrients and light [
113,
114].
The decrease in the depth of the lake and its high stability of water masses also entails changes in relation to oxygen distribution. The anoxia observed in Lake Borówno at the depth of 6–14 m (
Figure 7b) may be a result of changes in heat distribution within the lake water as well as processes related to the decomposition of organic matter. An increase in water temperature results in a reduced capacity to dissolve oxygen, especially in relatively warm benthic layers [
115]. Moreover, a prolonged period of summer stratification and shorter period of water mass mixing could worsen conditions for oxygen delivery, and also exacerbate seasonal oxygen deficiency in the bottom water [
116]. In the case of the analysed period, one may note that in the years featuring the occurrence of bottom zone anoxia, the mean annual air temperature was relatively high, and in the years 1992, 2002, and 2017 amounted to 8.9 °C, 9.4 °C, and 9.2 °C, respectively (
Figure 3a). In the years 1984 and 2009, in turn, it was lower: 8.0 °C and 8.5 °C. It should be stressed that Wiśniewska and Paczuska also did not observe any oxygen deficit in Lake Borówno in the year 2009 [
117], when the mean annual temperature was lower than in the years when the above-mentioned occurrence was recorded. Due to the limitation of existing data, in order to provide a detailed analysis pertaining to the susceptibility of Lake Borówno to eutrophication and the potential risk of anoxia, further research is required, particularly in terms of the contents of carbonates in the spring–autumn cycle.
Transformations of the thermal character of the lake waters may also cause changes in water transparency (Secchi disk visibility) [
118]. This parameter is mainly related to the amount of organic and inorganic substances suspended in water and the content of chlorophyll
a and phytoplankton [
115,
119,
120]. In Lake Borówno, an increase in water transparency in the summer season was observed in the last three years of the study compared to the previous period. It can therefore be assumed that the increase in transparency during the summer season was due to the relatively high stability of the epilimnion, associated with high air temperatures during the summer months [
120].
Neither the amount and composition of the phytoplankton nor chlorophyll
a in the lake showed significant changes over time. The collected data prove the generally low capacity of Lake Borówno for intensive primary production. Low content of phytoplankton (1.23–5.74 mg·dm
−3), albeit slightly higher than in the presented study (0.25–1.39 mg·dm
−3), was observed by Wiśniewska and Paczuska in 2009 [
117]. The aforementioned authors recorded the highest contents of phytoplankton in the summer season (July–August), but the highest content of chlorophyll
a (36.1 mg·m
−3) was in May 2009. They classified the lake as mesotrophic. In our study, similar to the aforementioned study, higher values of chlorophyll
a were seen in spring than summer, though phytoplankton mass was higher in spring than in summer. The differences in these results may be associated with the higher sampling frequency in the research by Wiśniewska and Paczuska [
117].
Generally reduced phytoplankton blooms may result from low surface outflow in the lake’s catchment having caused low nutrient contents [
22]. The obtained results showed a decrease in total phosphorus and total nitrogen in the 21st century, especially in the summer (
Figure 10). Temporary enrichment in nutrients was observed in 2002, when tributaries were flowing during the sampling dates, showing that surface outflow may be a source of nitrates and organic matter. However, probably due to their periodic character and decreasing outflow in the last dozen or so years, the total supply of phosphorus and nitrogen compounds to Lake Borówno is not particularly heavy and has been decreasing in recent decades. A decrease in the flow of nitrogen and phosphorus compounds, along with a reduction of outflow were observed in many flowing waters [
121,
122]. The discharge in periodic tributaries of Lake Borówno is activated in spring, which explains the higher level of nitrogen and phosphorus in spring than in summer in the last three years of the study. It is difficult to determine whether such seasonality, associated with seasonal discharge of water, will be maintained. In the 21st century, on the one hand, a further increase in air temperatures is predicted, which will translate into high evaporation values [
85], but on the other hand, there is also a possibility of more frequent heavy rainfalls. Such precipitation under conditions of significant soil dryness may result in the activation of surface run-off [
123]. This may well increase the inflow of compounds of agricultural origin in some years, as can be concluded from high nitrogen concentrations in the hypolimnion of Lake Borówno in 2002 (2.58 mg·dm
−3), when the lake was intensively supplied by the inflowing watercourses. However, our results also suggest the possible activation of a mechanism of periodic internal enrichment of the lake with phosphorus secreted from bottom sediments in the lake. This indicates the importance of both allochthonous nutrients and nutrients previously stored in bottom sediments in the eutrophication of Lake Borówno. In this lake, the process of eutrophication can take place spontaneously with low external pressure.