The study describes thermal regimes of thirty selected Polish lakes in the spring season. The author used 35-year series of daily measurements of surface water temperature in the years 1961-2005 and the measurements of the vertical distribution of water temperature taken in tens of selected water bodies. The diversified pace of the increase in surface water temperature (SWT) during the spring warming period makes it possible to distinguish two thermal phases: the early and late phases of spring warming. The limits of those phases are marked by the dates of the disappearance of ice cover and the dates when the SWT stays well over the threshold values, which amount to 4°C and 15°C respectively. The SWT increase in the lakes (April and May) causes changes in the water’s vertical thermal structure (the formation of epi- and metalimnion) and considerable dynamics of its descriptive parameters, such as water temperature, thermal stratification coefficient, thermal gradients, heat resources, etc.
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.
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.
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.
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 were 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, Sławskie) 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.
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.
Mieczysław Kunz, Rajmund Skowron and Szymon Skowroński
Morphometry changes of Lake Ostrowskie (the Gniezno Lakeland) on the basis of cartographic, remote sensing and geodetic surveying
The authors studied changes in the morphometry of Lake Ostrowskie over the period of 123 years on the grounds of cartographic materials, remote sensing data, land surveying, and data referring to the water stages. The changes that occurred in the lake up to the 1960s were related to the regulation and melioration works conducted in the entire area of the Gniezno Lakeland. On the other hand, drainage of the open pits of brown coal resulted in the considerable decline of the water table by another 2.5 m in the years 2002-2009. As a result of the decline of water down to 96.4 m a.s.l. the present lake was divided into two water bodies (the eastern one and the western one) joint with a channel. This led to the decline in the lake area from 346.7 ha in 1887 to 242.0 ha, and its volume by 28.9%, out of which by 18.3% only in recent twenty-eight years.
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.
This paper presents an attempt to model water-level fluctuations in a lake based on artificial neural networks. The subject of research was the water level in Lake Drwęckie over the period 1980-2012. For modelling purposes, meteorological data from the weather station in Olsztyn were used. As a result of the research conducted, the model M_Meteo_Lag_3 was identified as the most accurate. This artificial neural network model has seven input neurons, four neurons in the hidden layer and one neuron in the output layer. As explanatory variables meteorological parameters (minimal, maximal and mean temperature, and humidity) and values of dependent variables from three earlier months were implemented. The paper claims that artificial neural networks performed well in terms of modelling the analysed phenomenon. In most cases (55%) the modelled value differed from the real value by an average of 7.25 cm. Only in two cases did a meaningful error occur, of 33 and 38 cm.
The aim of this paper was to present the possibility of integrating LIDAR data with bathymetric measurements in order to assess the changes in water resources and morphometric parameters. The area of study was Lake Kiedrowickie (Poland). A change in most of the morphometric parameters has been observed. Especially important was the drop in water resources which amounted to more than 30%. A significant decrease (13.4%) was also observed in case of the lake surface. In conclusion we state that the integration of valid bathymetric measurements with the LIDAR data gives the full capability to model the changes of lake morphometric parameters with unprecedented accuracy. A significant complementarity and synergy of applying both techniques has been stressed.