# Trends of Changes in Minimum Lake Water Temperature in Poland
## 1 Introduction  
The response of the hydrosphere to the effect of processes and phenomena occurring in its vicinity is evident due to the characteristic properties of water. Changes in its physical state, high capacity for accumulating energy, or facility for transporting and depositing matter are properties that make water ecosystems subject to continuous, and sometimes impulsive transformations [1, 2]. Lakes play a considerable role in the natural environment. Through their high water retention capacity, they serve as a specific buffer mitigating extreme hydrological phenomena such as floods [3, 4]. Moreover, freshwater lakes are a stable source of water resources in a given catchment area or region [5]. Water temperature is a key parameter determining the physical, chemical, and biological properties of these ecosystems. Its distribution and course, shape, among others, the course of ice phenomena [6], oxygen conditions [7] or the species composition of organisms [8]. The issue of the thermal conditions of inland waters is therefore among frequently discussed research problems, as evidenced by the abundant literature in the scope of [9, 10, 11, 12, 13, 14, 15]. So far, in the long term, papers referring to average temperatures have been dominant [16, 17, 18, 19]. Maximum water temperatures in lakes have been investigated to a lower degree [20, 21]. Less focus is also put on multiannual changes in minimum temperatures. [PERSON] et al. [22] conducted research for eight lakes in Europe. They determined an increase in their annual minimum temperature at an average rate of 0.35 \({}^{\circ}\)C decade\({}^{-1}\). In the case of Lake Tonle Sap (Cambodia), research conducted by [PERSON] et al. [23] showed a decrease in its annual water temperature at a rate of 0.91 \({}^{\circ}\)C/decade. In this context, the research presented in this paper corresponds with the broad issue of the thermal regime of lakes, simultaneously referring to the so-far marginally discussed issue concerning minimum water temperature. The value is important, among others, in the context of potentially limiting the growthof some fish species [24]. When the surface water temperature reaches 0 \({}^{\circ}\)C, any further cooling results in latent heat removal and ice formation [25]. Therefore, changes in the minimum water temperature characteristic of the liquid state will determine the duration of the ice season, and further consequences for lake ecosystems in the remaining seasons.  
Proper interpretation of the scale and scope of changes in the thermal regime of lakes provides the basis for the understanding of the entirety of the processes responsible for the functioning of these ecosystems. The availability of detailed information on its transformation in the times of global warming is therefore of utmost importance. It should be emphasised that extreme situations are critical in many aspects, and exceeding a specific threshold can cause a long-lasting and irreversible response [26].  
The objective of the paper is the analysis of long-term changes in minimum water temperatures in selected lakes in Poland. The analysis was performed in the context of minimum momentary air temperatures and minimum daily air temperatures in the years 1972-2021. Moreover, the relationship between changes in water temperatures and air temperatures was determined.