CRITERIA OF THERMAL CLASSIFICATIONS OF LAKES

Since the early twentieth century limnological literature has presented numerous classifications of lakes. Most frequently these classifications refer to genetic systematization. In the latter half of the twentieth century new divisions of inland water bodies were introduced with respect to mictic, trophic and thermal properties. These proposals, however, make up a very diverse and scattered material, particularly due to the considered criteria. The author of this article decided to verify opinions and criteria of these systematisations to provide a better understanding of the problem.

CHANGEABILITY OF THE ICE COVER ON THE LAKES OF NORTHERN POLAND IN THE LIGHT OF CLIMATIC CHANGES

The study is based upon instrumental observations of ice covers which formed on the lakes in northern Poland in the period 1956–2005 and records of air temperature measured at 9 meteorological stations in the period 1960–2005. Relations between mean dates of ice cover freeze-up, ice cover duration, maximum ice thickness, and also other properties of ice regime indicate obvious dependency upon air temperatures in winter months (December-February). Both air temperatures and main properties of ice covers revealed definite trends, showing the increase in air temperature in winter (0.04–0.06°C year-1), earlier disappearance of ice cover (0.5–0.6 day year-1), its shorter duration (0.6–0.7 day year-1), and decreases in maximum thickness of the ice cover (0.2–0.25 cm year-1). The author shows considerable statistical relations between main properties of the course of the ice cover, air temperatures in winter and the NAO winter indexes. Therefore, changeability of the ice covers on the lakes in northern Poland in the latter half of the twentieth century may be treated as another proof and an indirect indicator of climatic changes undergoing in this part of Europe.  

Water temperature in the lakes of Northern Poland during the bathing season

The study presents characteristics of the bathing season on the basis of stationary daily measurements of surface water temperature in the lakes in the period 1971-2015 conducted by the Institute of Meteorology and Water Management. These measurements are taken in the littoral zone (from bridges) of 28 lakes at 7:00 (6:00 GMT). In order to determine representativeness of these measurements, the author also documents the comparison of water temperature with its values at various points of the lake and its daily course. Stationary surface water temperature measurements provided the basis for the characteristics of the average, the earliest and the latest dates of the beginning and end of the bathing seasons, their duration and mean water temperatures in the summer months. Hence, a new parameter (tsum) is introduced to define the mean surface water temperature for the summer months (June, July and August), and compare water temperature in lakes over a larger area (the Baltic Sea catchment area). The most favorable conditions for bathing in Polish lakes are found in the western part of the Wielkopolskie Lakeland (lakelands: Łagowskie, Poznańskie, Slawskie and Myśliborskie) from the beginning of July to the end of August, when the surface water temperature in lakes generally exceeds 18°C. Furthermore, the best conditions for bathing in the water are from 10:00 to 18:00. When choosing a place to relax, holidaymakers should also consider bathing locations, infrastructure and safety conditions

Ice phenomena in investigations of Polish lakes

The paper presents nearly 150 years of history of ice research conducted on Polish lakes. In the first stage, these were observations and expedition studies that had an exploratory purpose. It was not until the 1920s that stationary measurements on several lakes were introduced. Unfortunately, the outbreak of World War II interrupted these observations. After the war, they were resumed in 1946, but the real beginning of investigations of ice phenomena on lakes is taken to be 1960. At present, stationary measurements of ice phenomena are conducted on about 70 lakes located mainly in northern Poland. Besides the purely explorative purpose, experimental research on ice phenomena on lakes has also contributed to the development of a measurement methodology. The author of this paper took part in numerous experimental studies conducted on over 30 lakes for which the ice results are partially presented below

Natural conditions for the development of lake tourism in Poland

There are about 6,800 lakes in the Polish Lowlands. They are a potential place for rest and recreation not only for the local population, but also for those coming from other regions of Poland and foreign guests. The work was based on the analysis of water temperature data in lakes in the years 1961–2020. The highest surface water temperatures in lakes occurred most often at the turn of the last decade of July and the fi rst decade of August, reaching values above 2 °C in most lakes. In the daily cycle, the highest water temperatures in the surface water layer (up to 1.5 m deep) were recorded mostly between 14:00 and 17:00 (13:00–16:00 GMT). The beginning of the bathing season on lakes in Poland usually falls between May 20 and 25, while the end is in the second half of September, reaching an average of about 69 days, characterized by a positive trend in its duration. According to the Chief Sanitary Inspectorate in Poland, in 2020 there were 602 active bathing areas, most of which were organized on lakes.There are about 6.8 thousand lakes in the Polish Lowlands. They are a potential place for rest and recreation not only for the local population, but also from other regions of Poland and foreign guests. The work was based on the analysis of water temperature data in lakes in the years 1961-2020. The highest surface water temperatures in lakes occurred most often at the turn of the last decade of July and the first decade of August, reaching values ​​above 21oC in most lakes. In a daily course, the highest water temperatures in the surface water layer (up to 1.5 m deep) were recorded mostly between 14:00-17:00 (13:00-16:00 GMT). The bathing season on lakes in Poland usually falls between May 20 and 25, and ends in the second half of September, reaching an average of about 115 days. According to the Chief Sanitary Inspectorate in Poland, in 2020 there were 602 active bathing areas, most of which were organized on lakes

WATER TEMPERATURE AND ITS DIVERSITY IN THE DEEPEST LAKES OF THE TUCHOLA FOREST AND THE KASHUBIAN AND BRODNICKIE LAKELANDS

This study presents the results of monthly examinations of the vertical distribution of water thermal structure (2008–2011) carried out over a four-year period in the deepest lakes located in the Kashubian and Brodnickie Lakelands and the Tuchola Forest1. Three lakes were selected for examination (Raduńskie Górne, Zbiczno and Ostrowite). Their maximum depths slightly exceed 40 m, and their surface areas range from 121 to 362.5 ha. The results of the measurements show that, despite only minor differences in depth, water temperature varied significantly between the studied lakes. These differences were mainly apparent in the extent of the epilimnion, water thermal stratification, and in the water temperature in the bottom-most layers in summer and winter. The diversity in thermal stratification of the lakes is mainly determined by their morphometric properties, their location above sea level, and the dynamic influences of winds

Changes in lake area as a consequence of plant overgrowth in the South Baltic Lakelands (Northern Poland)

The authors carried out the analysis of changes in the plant overgrowth of the lakes based on cartographic materials. Among 6 793 lakes with the area exceeding 1 ha located in the lakelands of Northern Poland, 893 lakes were selected for the analysis. The lakes were selected on the grounds of the existing bathymetric plans and information on their overgrowth and depth relations. Over the last 50 years lake area declined by 1.9% (from 140 975.0 ha do 138 273.7 ha) and so did the lake area covered with emergent plants, i.e. by 0.27% (from 11 219.0 ha down to 10 637.2 ha). Emergent plants cover on average 7.69% of the lake area. In the case of the lakes of smaller areas (below 80 ha) or medium areas (80÷200 ha) the extent of plant overgrowth was 14.3 and 9.6% respectively. The article presents two indicators which determine plant overgrowth of the lakes. These are the coefficient of overgrowing the lakes (%) and the coefficient of overgrowing the shoreline (ha km -1). These coefficients make it possible to study the extent of lake overgrowing in the South Baltic Lakeland, regardless the direction of these changes

Spatial diversity of the ice cover on the lakes of the European Lowland in the winter season 2003/2004

In the middle of the winter period of 2003/2004 the authors carried out synchronous measurements of the thickness of the ice cover, snow cover and the vertical distribution of water temperature in 33 lakes located in the area of northern Germany, Poland and southeast Lithuania. The area spread over the area of approx. 1200 km (8.02 – 25.50 E), in the belt of approx. 250 km width (52.87 – 55.26 N). The lakes are of different areas (from 75 to 595 ha) and mean depths from 3.5 to 38.7 m. Ice thickness evidently increased from 3 – 8 cm in the western part of the Mecklenburg Lakeland to 27 – 31 cm in the Vilnius Lakeland, and corresponded to the course of the thermal winter. Water thermal conditions were mostly determined by local environmental conditions, particularly hydrological and morphometric ones

Thermal Structure of Water During the Summer in Lakes of the Polish Lowlands as a Result of their Varied Morphometry

The paper discusses the impact of lake morphometric parameters on the thermal structure of water during the period of summer stagnation. The summer period in the lakes is characterized by clearly expressed properties of the thermal structure of water, differentiating lakes from one another. The study was carried out on 141 lakes in northern Poland, for which at least 3 vertical water temperature measurements were taken in different years. They showed a significant influence of lake basin morphometry upon the thermal structure of water in lakes. In general, measurements of the vertical distribution of water temperature showed clear diversity, and depending on the depth also thermal layers (epi-, meta- and hypolimnion). The results of the analysis of 8 lake morphometric parameters and 10 thermal stratification parameters revealed the existence of significant dependencies between them. The best-preserved relations (statistically significant) occurred between the parameters characterizing the lake depth and meta- and hypolimnion properties, as well as the thermal stratification factor and heat content in a unit of volume

Water temperature in investigations of Polish lakes

Scientists became professionally interested in Polish lakes in the early 1850s. They focused predominantly upon the measurements of depth, observations of water stages, optical properties, and water temperature. The first systematic observations of surface water temperature were carried out in 1956. At present these measurements are conducted in 29 lakes. Investigations of the vertical distribution of water temperature were initiated in the interwar period and they contributed to a better recognition of the processes and factors conditioning dynamics of water masses. In general, measurements of water temperature have constituted fundamental observations with respect to the studies of yearly and daily courses of the temperature of surface water and the entire water mass, the influence of basin morphometry upon water thermal conditions, heat balance and heat resources, thermal conditions of bottom deposits and thermal classification of the lakes. The introduction of automatic gradient probes gave a new impulse to the investigations of water temperature in the lakes. The foundation of the Polish Limnological Society in 2001 and 18 national and international limnological conferences stimulated integration of the circle of limnologists. Specialist journals (Limnological Review, Studia Limnologica et Telmatologica) have presented around 40 publications with the leading theme of water temperature