Polish River Basins and Lakes – Part I

The condensed characteristics of the climate of Poland are presented with particular emphasis on the areas of lake districts. More detailed studies were provided on 19 lakes with daily hydrological and thermic observations in the calendar years 2000–2016. Climate warming is clearly observed in observational temperature data, with increases of 0.2–0.4°C per decade. Slightly lower air temperatures and a higher annual sum of precipitation in relationship to neighboring regions are specific for the Polish Lakelands. Mean annual air temperature in the eastern part is 1.5°C lower than in the western regions. Much greater variation of precipitation and climatic water balance was noted, where the Wielkopolskie and Lubuskie Lake Districts have a permanent water deficit that is not observed in the rest of the Lakelands. Polish postglacial lakes are characterized by generally small fluctuations in water level, whereas variations in artificially regulated lakes are higher than in lakes with a natural regime, as well as a higher frequency of decreasing water level trends. Only a few lakes have a statistically significant increase of lake water temperature, mostly in northeastern Poland. A simulation indicated that in year 2050 the annual lake water temperature will be 1.1–1.9°C higher than at present. We can expect more evident changes in lake functioning in spring and summer, shifting lakes toward higher trophy, but dystrophic lakes will remain more humic when their hydrology is stable.

This paper characterises rivers, the major hydrographic objects within the water network of Poland. The authors point out to the diverse nature of the river network resulting from the natural topography of Poland and the fact that the contemporary river network uses valleys inherited from the past. Systems of two largest Polish allochthonous (transit) rivers, the Vistula and the Oder, are characterised. The rivers of Przymorze and the Niemen basin are discussed in less detail. The paper shows that the stability of discharge in Polish rivers, resulting from climate conditions (precipitation exceeding field evaporation over the entire water year), is determined by the underground contributor to the discharge, which is a result of underground water drainage by the river channels. The contribution of underground runoff to the total discharge of Polish rivers is generally stable over the year, and the percentage of surface runoff contributing to the total discharge depends on the morphogenetic zone (mountains, highlands, lowlands) where these rivers flow. Finally, the types of regimes characterising Polish rivers are presented and their spatial differentiation is discussed. Polish rivers are characterised by complex regimes. Most rivers are characterised by the moderately developed nival regime with the high water period in the spring. Some of the lowland rivers have well-developed nival regime, while some lakeland rivers are characterised by the poorly developed nival regime. The regimes of mountain rivers vary.

This chapter contains an analysis of the geoecosystem types of 388 lakes of northern Poland determined by the method elaborated by E. Bajkiewicz-Grabowska, on the basis of data from the literature. The concept of lake geoecosystem is understood as a natural landscape system, composed of a lake with its catchment. The type of geoecosystem determines the rate of natural eutrophication of the reservoir’s water, and the rate is usually strongly modified by human activity. The method used to determine the geoecosystem type of a lake involves two elements: (1) the catchment as a supplier of matter to the lake and (2) the lake as a recipient of matter. Therefore, it considers the relationship between the lake and its alimentation area. It can therefore be used in different types of natural landscapes, and its core is the determination of (a) the vulnerability of the catchment to supplying matter into the lake, based on the properties of the physico-geographical environment of the catchment, and (b) lake resistance to the catchment impact determined on the basis of the limnological features of the lake. The combination of these two characteristics allows to distinguish four lake geoecosystem types with a specific natural eutrophication rate. It was shown that the majority (39%) of the analysed lakes have a geoecosystem of type 4 (low lake resistance to external pressure and high catchment vulnerability to supplying matter into the lake). The smallest group (18%) is composed of lakes representing geoecosystem of type 1, i.e. those characterized by low activity of the catchment in the supply of areal loads, as well as high lake resistance to external impact. The remaining lakes are characterized by a moderate eutrophication rate (geoecosystem of types 2 and 3).

The territory of Poland includes more than 7,000 lakes (1 ha or larger) with a total surface area of 281,377.0 ha, translating into the lake density index of 0.90%. The territory of Poland is evidently divided into the northern part, including 95% of all lakes, and the southern part poor in lakes. The situation is related to the last Scandinavian glaciation. Due to this, in terms of genesis of basins, the majority of lakes are of postglacial character, where the occurrence and course of channel lakes is an indicator of the maximum range of the ice sheet. Lakes fulfil a number of important functions in the environment. The functions refer among others to the conditions of water circulation (suppressing extreme situations, both floods and draughts), affect the biodiversity, shape climatic conditions, and are of key importance for the development and functioning of many branches of the economy (agriculture, energy engineering, tourism), etc. Both environmental and anthropogenic effects resulting from the occurrence of lakes can soon be lost as a result of the progressing process of their disappearance. The situation is determined by the natural process of evolution, accelerated by human activity related to the regulation of water relations, and an increase in productivity of agriculture, and consequently an increase in supply of nitrogen and phosphorus compounds to water. It is estimated that from the moment of development of lakes until today, approximately 60% of the surface area of lakes disappeared.

The lobelia lakes, unique and rare in Poland, are located mainly in northern part of country in moraine plateaus of Bytów and Kashubian Lakelands and outwash plains of Tuchola Forest and Charzykowy. In the middle of the twentieth century, there were over 190 lobelia lakes in Poland. Lobelia lakes are classified as so-called soft-water lakes due to low concentrations of calcium and magnesium. Their waters are weakly buffered and usually have acid reaction and also small amount of biogenic elements. Lobelia lakes are overgrown by specific plant species – isoetids – adapted to the poor habitat: Lobelia dortmanna, Isöetes lacustris, Littorella uniflora and several other accompanying species. The lobelia lakes in Poland are prone to degradation and loss of their unique values. The most serious threats include acidification, humification deposition, alkalinization, eutrophication by agricultural activities or recreational use and lowering of water level induced by climate changes.

The research covered three groups of lakes: urban lakes in Kartuzy in the East Pomeranian Lake District, a group of Barczewo lakes in the Olsztyn Lakeland, and a group of Leginy lakes in the Mrągowo Lakeland. The water bodies were evaluated in terms of water dynamics, oxygen saturation, mineral pollution, and trophic status. The lake complexes differed fundamentally in land-use types in the surrounding areas. The lakes from each group were different. Electrolytic conductivity was highest in urban lakes, especially in Lake Karczemne which has been a sewage receptacle for many years. In the group of urban water bodies, the trophic state index was highest in Lake Karczemne (TSI (SD) and (TP), within the limits of hypertrophy, R ² = 0.886). In this group of lakes, organic matter limited visibility (R ² = 0.943). In the lakes surrounded mainly by forests, TSI was lower and indicative of oligo/mesotrophy in Lakes Kierźlińskie and Dłużek. Lake Dłużek was characterized by extremely low electrolytic conductivity (43–87 μS cm⁻¹). TSI (TP) exceeded the reference standards in all examined lakes. In eight water bodies, primary production was limited by nitrogen (TSI (Chl) – TSI (TN) > 0 and TSI (TN) – TSI (TP) < 0, TN/TP < 10), which promoted cyanobacterial blooms.

An analysis of the chemical parameters of bottom sediments in selected Polish lakes was made, based on historical and recent data demonstrating the chemical composition of profundal sediments. The analysed lakes belong to a broad environmental spectrum, being different in the hydrological regime (seepage, flow-through), trophic state (mesotrophic, eutrophic, dystrophic), mixing type (polymictic, holomictic, meromictic), stratification (stratified, non-stratified) and sewage input presence (in the past or now). Moreover, some have been restored using technical methods (artificial mixing with and without destratification, phosphorus inactivation).

The results revealed that the main chemical components of the analysed sediments were silica, organic matter and calcium together with carbonates. Other sediment components occurred in low amounts, i.e. less than a few percent of d.w. Cluster analysis showed that the main factor which differentiated the sediments of the analysed Polish lakes seems to be their hydrological regime. The trophic state and stratification type of the lakes were less important factors affecting the character of bottom sediments.

The characteristics of the postglacial relief of northern Poland, which is responsible for the initial development of the water network in this area, were described. The dominant form of relief in this area is concave forms of the terrain, clusters of which form areas permanently or periodically excluded from river runoff. In the area of the basal moraine, in the beds of these depressions there are usually lakes of various sizes, including the smallest ones – postglacial ponds, some of which are periodically connected by short watercourses. On outwash plains only deep channels are occupied by lakes; all other concave forms, often with significant denivelations, are dry.

The main objects of the hydrographic network of the postglacial areas are primarily lakes, boggy areas and watercourses: those of the lower orders in the Strahler classification, usually short, often periodic, functioning as overflows, and those using valleys formed by postglacial rivers (channels, stagnant ice melt-outs, fluvioglacial valleys), which are the main drainage axis. It was pointed out that in the postglacial areas of northern Poland, three types of hydrographic network can be distinguished, differing in the degree of river runoff organization: the hydrographic network of endorheic areas, the river-lake network and the mature river network (lakeland rivers).

Hydrological functions of lakes in shaping the runoff of rivers that drain them were discussed on the basis of data from the literature. The discussion indicates the role of lakes in initiating river runoff, their role in water storage and runoff equalization of a river flowing through or out of the lake, as well as the role in shaping the runoff capacity of the river: its increase due to underground water drainage by the lake basin or decrease as a result of lake waters supplying the underground water. It was pointed out that there are lakes which regardless of the character of the hydrological year (wet, average, dry) constantly increase the runoff of the river which flows through them by incorporating into it the water which they receive from underground drainage. There are also lakes that constantly feed underground waters with their waters, which reduces the runoff of rivers flowing from them. There is also a group of lakes which, depending on the level of the storage of the basin, conditioned by the water content of the hydrological year, either drain underground waters and increase river runoff or aliment them thus reducing river runoff. The above hydrological functions of lakes were discussed on the example of outflow and flow-through lakes of northern Poland.

Total organic carbon (TOC) resources in Polish water reservoirs are presented as an important factor affecting water quality and ecosystem trophic state. The study is based on hydrochemical and biological data from 47 reservoirs from the years 2005–2017 and collected from the archives of the Polish National Monitoring Program, provided by the Chief Inspectorate of Environment Protection. The mean (by weight) TOC concentration in reservoirs is 6.3 mg dm⁻³, with a range from 2.3 mg C dm⁻³ in the mountains, the Czorsztyn and Sromowce reservoirs, up to 18 mg C dm⁻³ in the hypereutrophic, lowland Siemianówka reservoir, varying according to reservoir elevation. Although reservoirs are large and deep, there is a significant negative correlation between mean reservoir depth and TOC. Seasonality and national TOC dynamics were strongly related to the rate of precipitation, with maximal concentrations in late spring and minimal in autumn or winter. The first global warming symptoms of TOC changes in reservoirs are noted, which will manifest as increased TOC and greenhouse gas emissions. Increased water retention time, which promotes water eutrophication, increases TOC resources in most Polish dam reservoirs as well as in flooded areas. Mean TOC concentrations are related to certain biological reservoir water parameters, such as phytoplankton index or diatom index. In future planning, Polish reservoirs should be placed outside lowlands, and their capacity should provide a high water exchange, in less than 2 to 3 months.

The lakes of the Great Masurian Lakes System create the largest lake complex in Poland hydrologically divided into three basins – Northern, Central, and Southern. The water flow direction depends on the position of the watershed between Vistula and Pregoła Rivers which can vary depending on the water level and the magnitude of the water outflow from the Pisa and Węgorapa Rivers. The studies aimed to analyze the changes in the hydrological regime and the ecological state of these lakes in 2008 and 2010–2012. In June 2010, the bifurcation zone occurred in the central part of the system due to the high quantity of precipitation. A positive heterograde oxygen curve in summer was in less eutrophic lakes, while a negative heterograde oxygen curve in more eutrophic lakes and a clinograde oxygen curve in highly eutrophic lakes. Phosphorus content was typical of medium or highly eutrophic lakes; therefore classifications based on physicochemical elements indicated below good ecological potential and status of all lakes. The least intensive phytoplankton growth was in the Northern Basin whereas the most intensive in the Southern Basin. Biological assessments based on phytoplankton indicated maximum potential in two lakes and good potential and status in five lakes, whereas the remaining lakes had moderate, poor, and bad potential and status. In 2010, when oxygen conditions improved significantly, the classification based on PMPL indicated significant deterioration up to bad ecological potential and status, in the lakes within the bifurcation zone and in several lakes in the Southern and Northern Basins.

One of the greatest threats to water quality is accelerated eutrophication, resulting from human activity, like the high intensity of tourism, surface runoffs from fertilized fields, and municipal pollution. Water eutrophication manifests as excessive growth of phytoplankton caused by overabundant nitrogen, phosphorus, and other nutrient supply which causes deterioration of water quality related to the amount of bacterial biomass in eutrophicated water reservoirs. The Great Mazurian Lake System (GMLS) is a chain of lakes located in mesoregion of the Great Mazurian Lakes in the Northeastern Poland. All lakes of the GMLS are connected by natural or artificial channels built in the eighteenth and nineteenth centuries and nowadays create widely spilled, long (the easiest route from northern to southern edge is about 110 km) gutter unique on the scale of the continent. The lakes of GMLS are of glacial origin. During the last five decades, all lakes of the GMLS passed different levels of eutrophication, thus significantly changing their trophic states. This report describes past and present trophic conditions of lakes of GMLS and analyzes environmental factors responsible for eutrophication of their waters. Eutrophication processes are not only responsible for high nutrients levels in lakes, extensive growth of phytoplankton biomass and productivity, cyanobacterial predominance, etc., but eutrophication is also responsible and connected to several threats for water quality. Presence of pathogenic bacteria, as well as the potential presence of many antibiotic-resistant bacteria in lakes of the GMLS, is discussed.

In order to determine the nutrient balance two groups of the lakes were selected: ten flow lakes, namely, Mielenko, Karczemne, Klasztorne Małe, Klasztorne Duże, Pasłęk, Wymój, Sarąg, Łęguty, Isąg and Suskie (located in Olsztyn Lake District and Kashubian Lake District), and ten non-flow lakes, namely, Czarne, Długie, Podkówka, Starodworskie, Sukiel, Track, Tyrsko, Kepijko, Paskierz and Podąbek (located in Olsztyn Lake District and in Iławskie Lake District).

The annual phosphorus load introduced into flow lakes ranged from 17.2 kg P (Mielenko) to 7,754.5 kg P (Isąg) and nitrogen from 202.6 kg N (Mielenko) to 81,876.5 kg N (Isąg). Nutrient load was mainly introduced with surface water inflow (46–96%). In the group of flow lakes, Mielenko, Karczemne and Suskie were fed primarily with surface runoff from the basin. The surface watercourses flowing into them were periodic, and the amount of water flowing through them was small. Annual load of phosphorus introduced into non-flow lakes were from 4.4 kg N (Czarne) to 169.8 kg N (Track) and nitrogen from 89.4 kg N (Czarne) to 2,311.8 kg N (Track).

In case of non-flow lakes, the main source of supply was usually the runoff from the direct catchment (from 40% to 97%).

It was found that the majority of flow lakes (except Lakes Isąg and Suskie) showed negative retention in relation to nutrients. Non-flow lakes retain the charge of phosphorus and nitrogen, which is introduced from various sources.

We hypothesized that the variability of P internal loading in lakes and reservoirs depends on the trophic state of analysed waterbodies, as well as spatial (different water depths) and temporal (season) aspect in particular waterbodies. Additionally, year-to-year changes of P loading in lakes restored with variable methods (sustainable restoration, P inactivation with iron compounds and magnesium chloride, nitrate treatment and effective microorganisms (EM) application) were expected. To verify these assumptions, we have analysed the process of internal P loading in 40 waterbodies situated in Western Poland, based on the ex situ experiments on intact sediment cores, collected from different water depths in various seasons. Additionally, basic sediment characteristics (TP and its fraction content, organic matter, SRP and TP in pore water and water above sediments) were studied. The most intensive P release from sediments into water column was noted in summer, especially at greater depths and in heavily eutrophicated lakes. Internal P loading in restored lakes usually decreased, apart from lakes in which EM were used.

Hydrochemical research conducted in the dam reservoirs often show strong eutrophication of their waters. This usually results from high supply of the mineral forms of nitrogen and phosphorus from the river waters. The reduction of the supply of nutrients to the reservoir below the level causing water quality deterioration can be limited in the case of the agricultural land use in the catchment. In order to reduce nutrients input and to improve the ecological state in the one of the reservoirs in the Eastern Poland, it has been proposed solution: a change of the functioning of the reservoir from dammed to lateral and construction of an additional preliminary reservoir above the existing one.

The Konin lakes are a system that has been artificially heated by power stations since 1958 and has continuously been the subject of ecological research. The coolest of the lakes remains Lake Ślesińskie, while the warmest is Lake Licheńskie. Both of the lakes are eutrophic, and their ecological potential is decidedly below good. Low water retention in the lakes facilitates the increase of easily absorbed organic matter flowing in from the catchment area, and the higher temperatures of the waters intensify processes of their decomposition. Summer phytoplankton blooms are noted in both lakes; they are more intense in Lake Licheńskie. A significant increase in primary production, higher cyanobacteria biomass, and the limited development of large cladocerans and copepods and the domination of small rotifers are confirmed in this lake. The lake system is inhabited by approximately 100 alien species. These frequently include invasive plant and animal species such as Vallisneria spiralis, Dreissena polymorpha, Sinanodonta woodiana, and Pseudorasbora parva, which contribute to the structure and functioning of the lakes. The best water quality is in the post-mining water basin, which was created in 2010 by inundating it with Early Cretaceous and Tertiary waters, the physicochemical and biological aspect of which indicate good or maximum ecological potential.