Abiotic manifestations of eutrophication of water reservoirs in cases of extreme agricultural and industrial anthropopressure

Transkrypt

1 Limnological Review 6 Abiotic (2006): manifestations of eutrophication of water reservoirs 179 Abiotic manifestations of eutrophication of water reservoirs in cases of extreme agricultural and industrial anthropopressure Robert Machowski, Marek Ruman, Mariusz Rzętała University of Silesia, Faculty of Earth Sciences, Department of Physical Geography, Będzińska 60, Sosnowiec; maro@plusnet.pl Abstract: e water reservoirs of Dzierżno Duże (50 22 N, E) and Turawa (50 43 N, E) function in totally different conditions existing within their river basin. e former is loaded with industrial anthropopressure, whereas the latter with agricultural anthropopressure. e carried our research allowed for the comparison of abiotic manifestations of eutrophication in both reservoirs. In case of extreme industrial anthropopressure the load with biogenic substances is much greater, therefore ecological conditions in the Dzierżno Duże reservoir are much worse. Oxygen deficiencies last longer and occur at much smaller depths than in the case of Turawa reservoir. Water reaction in Dzierżno Duże is more stable than in the case of Turawa reservoir where, during mass growth of algae, these changes are much greater. However, during water blooming, in both cases, water transparency decreases to a great extent. Key words: eutrophication, anthropopressure, Turawa, Dzierżno Duże, Silesia Lowland, Silesia Upland Introduction e increase in lake water fertility caused mainly by phosphorus and nitrogen compounds is called eutrophication (Wiśniewski 1994). is process shows the evolution of barrage water reservoirs because, as a result of supplying them with both organic and mineral matter, the water region becomes more shallow, and in the long run the hole of the lake completely disappears (Choiński 1994). e wholeness of the eutrophication processes disturbs the proper functioning of the water ecosystem making it impossible, to a great extent, to make full use of the water area (Kajak 1979). e deterioration of the quality of the retained water identified with eutrophication causes undesirable changes in the water ecosystems resulting in lower water transparency and oxygen deficiency in deeper parts of the reservoir, the result of which being a threat to the lives of oxygen organisms and overoxygenation of the epilimnion water. Moreover, improper sanitary condition of the water makes it difficult to use the shore zone for recreational purposes and the lush development of phytoplankton influences the decrease in water purification plant efficiency worsening the taste and smell of water. Currently the presence of biogenes in the limnic water of the Silesia Upland area and its periphery should be mainly connected with anthropogenic sources such as communal and industrial sewage disposal and sewage run off from agricultural areas. e development of eutrophication results, to a lesser degree, from natural processes such as leaching of bioelements from rocks occurring in the area of the river basin. e scope, aims and methods of the research Currently, practically every natural component of biological environment of the Silesia Upland has, to a great extent, been transformed due to human industrial activity. e occurrence in the area of natural resources rich in minerals such as hard coal, zinc and lead ores has brought about the development of extractive and processing industries. e greatest changes can be observed in the configuration of the area and water relations, that is why there are a few thousand artificial water reservoirs in the area of the Silesia Upland and adjoining it eastern part of the Silesia Lowland out of which some belong to the group

2 180 Machowski R., Ruman M., Rzętała M. of the biggest anthropogenic water reservoirs in Poland (Fig. 1). Such a great collection of the above mentioned objects allows for defining this area as an anthropogenic lakeland (Jankowski 1986; Czaja 2003; Jankowski, Rzętała 2004). e reservoirs existing here function under diverse anthropopressure (Tab. 1). However, according to M. Rzętała (1998), the reservoirs functioning in the system of Mała Wisła, Soła and Skawa rivers are characterized by the most favourable ecological conditions as far as chemical properties are concerned. Among them there are the Goczałkowice reservoir (49 55 N, E) and Dziećkowice reservoir (50 07 N, E) which are the source of water for the central part of the Silesia Upland. e water in the reservoirs of Kozłowa Góra (50 25 N, E) and Przeczyce (50 26 N, E) formed on the Brynica and Czarna Przemsza rivers are characterized by similar properties. However, in case of these reservoirs one can observe symptoms showing a great extent of eutrophication of their water (Kasza 1995; Jaguś, Rzętała 2003). Jankowski and Rzętała (1998) give examples of water reservoirs such as Pogoria I (50 21 N, E), Pogoria III (50 21 N, E), Sosina (50 14 N, E), Chechło (50 28 N, E) located in the area under great anthropopressure, but within which the increase in the fertility of the retained water is much lower. Such a situation is Fig. 1. Water reservoirs in the eastern part on the Silesia Lowland: 1 rivers, 2 canal, 3 water reservoirs (1 Turawa, 2 Dzierżno Duże), 4 towns. Table 1. Some physicochemical water properties in the selected anthropogenic reservoirs in the area of Upper Silesia Ostrava industrial region in the years (acc. to M. Rzętała 2001). Reservoir or location Reaction Electrical conductivity HCO 3 Cl SO 4 2 NO 3 PO 4 3 [ ph ] [µs/cm ] [mval/dm 3 ] [mg/dm 3 ] [mg/dm 3 ] [mg/dm 3 ] [mg/dm 3 ] Przeczyce Pogoria I Pogoria III Świerklaniec Balaton Sosina Dziećkowice Paprocany Goczałkowice Žermanice Terlicko Reservoir in city Hlucin Nezmar Dolni Benesov Reservoir in city Antosovice Bohumin Vrbice right bank of the Odra river Zbiornik Rybnicki Dzierżno Duże Dzierżno Małe Pławniowice

3 Abiotic manifestations of eutrophication of water reservoirs 181 possible only when the holes of the reservoirs are effectively isolated from the inflow of great amounts of biogenic substances from the river basin. However, the great majority of reservoirs in the area which function under great industrial or agricultural anthropopressure are characterized by progressive eutrophication. e main aim of the carried out research into the water reservoirs functioning in the river basins with a completely different way of land development was to compare abiotic manifestations of the eutrophication process occurring in the water region. e majority of observations were made with reference to the object located in a typically industrial river basin and the reservoir in which river basin arable land dominates. One of such water basins, which functions under extreme load of the river basin with industrial anthropopressure, is Dzierżno Duże reservoir (50 22 N, E). e river basin of the biggest in Poland postexploitation reservoir extends over the area of western part of Upper Silesia Industrial Zone, where both extractive and processing industries developed on a large scale. ese areas are mainly dewatered by the Kłodnica river and its tributaries which flow into the Dzierżno Duże reservoir. All kinds of pollutants carried away from a number of plants run off into the reservoir together with the river water. e river basin of the Dzierżno Duże reservoir is characterized by a great rate of industrialization and town development, where the percentage of arable land is 42% out of which 33% is cropland, whereas forests cover about 20% of the area (Rzętała 2000). However, the Turawa reservoir (50 43 N, E) formed on the Mała Panew river is the object being under the influence of industrial anthropopressure. e industrial use of the area reaches much bigger rates in case of its river basin than in case of the Dzierżno Duże reservoir with minimization of industrial and forest areas. at is why the quality of the Turawa reservoir water is formed, first of all, by the inflow of washed away, from the used by agriculture areas, substances and brought in together with the Mała Panew and Libawa rivers water. In addition, pollutants from the cities in the northern part of the Silesia Upland and its periphery (modifying up to a point the influence of agricultural anthropopressure) are carried away to the Mała Panew river. e comparison of abiotic manifestations of eutrophication of the above mentioned water reservoirs was carried out basing on the Katowice OBiKŚ data in relation to the Dzierżno Duże reservoir and also basing on the own findings realized within the Turawa reservoir which covered the measurements of the water reaction by means of a ph-meter (CP-104 by Elmetron), water transparency by means of a Secchi disc as well as oxygenation by means of an oxymeter (WTW Oxi 330i by Elmetron), the determination of phosphates concentration using the spectrophotometric method and nitrates by means of potentiometric method using the ion-selective electrode. e research findings e concentrations of phosphates in the Dzierżno Duże reservoir keep changing in a great range from a tenth part mg/dm 3 to some mg/dm 3. Such a level of typical concentrations of phosphates prevails in the findings of the research carried out in the 1990s (Rzętała 2000) and they are confirmed additionally by fragmentary data from the years 1993 and 2001 (Tab. 2). However, in the case of the Turawa reservoir (Fig. 2) the maximum contents of phosphates are within the minimal range of concentrations reported for the Dzierżno Duże reservoir. In the case of nitrates there are great disproportions between the two reservoirs as well. e concentrations of these compounds in the water of the Dzierzno Duże reservoir, according to Rzętała (2000), vary in the range of a dozen or so to nearly 90 mg/dm 3, although a clear lowering of concentrations has been reported in recent years, which the amounts of nitrate nitrogen display (Tab. 2). In the case of the Turawa reservoir (Fig. 2) the contents of nitrates is kept at the level lower by half than the minimal values characteristic of the Dzierżno Duże reservoir (Ruman 2005). In both cases there are seasonal differences in the concentrations of biogenes connected with a natural circulation of nutritious matter in nature. However, the circulation of phosphates and nitrates is clearly disturbed by the inflow of these compounds together with the water from the main tributaries. As for the Kłodnica river flowing to the Dzierżno Duże reservoir, the concentrations of phosphates occur in lesser amounts than observed in the reservoir, while the levels of nitrates are generally the same (Rzętała 2000). In the case of the Turawa reservoir, the Mała Panew river (more than any other tributary) supplies the reservoir with biogenic matter (Fig. 3). e water of the river is characterized by greater concentrations of both nitrates and phosphates as well as by greater flows than the Libawa river. erefore, the total load

4 182 Machowski R., Ruman M., Rzętała M. Table 2. e Dzierżno Duże reservoir the assessment of the selected physical and chemical properties and chemical properties of the Kłodnica river water in the years 1993 and 2003 (acc.to Rzętała, in print, simplified) The name of the indicator inflow into the reservoir outflow out of the reservoir inflow into the reservoir outflow out of the reservoir average range average range average range average range value of the parameter value of the parameter infolow =100% value of the parameter 1993 = 100% value of the parameter Reaction [ph] Dissolved oxygen [mg O2/dm 3 ] BOD 5 [mg O 2 / dm 3 ] COD Mn [mg O 2 / dm 3 ] Chlorides [mg Cl/ dm 3 ] Sulphates [mg SO 4 / dm 3 ] Soluble substances [mg/ dm 3 ] Total suspension [mg/ dm 3 ] Ammonium nitrogen [mg N/ dm 3 ] Nitrite nitrogen [mg N/ dm 3 ] Nitrate nitrogen [mg N/ dm 3 ] Phosphates [mg PO 4 /l] Source: own study on the basis of the data from Research and Control Center in Katowice inflow =100% 1993 = 100% Fig. 2. Contents of phosphates (1) and nitrates (2) in the Turawa reservoir in the hydrological year of of biogenes carried by the Mała Panew river into the reservoir is much greater than in the case of the Libawa river whose water has a diluting influence on the concentrations recorded in the reservoir. Nitrates and phosphates concentrations present in both reservoirs repeatedly exceed the critical values at which water blooming may occur. Generally, the levels of biogenic matter concentrations and other pollutants in the water of the compared reservoirs show polytrophy in the case of the Dzierżno Duże reservoir (Tab. 2) and eutrophy in the case of the Turawa reservoir (Fig. 2). e fact of supplying some amounts of biogenes from the bottoms is also significant. e liberation of the gathered on the bottom nutritious matter

5 Abiotic manifestations of eutrophication of water reservoirs 183 Fig. 3. Concentrations of nitrates and phosphates in the water of outlet parts of Mała Panew and Libawa rivers into the Turawa reservoir in the hydrological year of may occur in oxygen-free conditions and is periodically observed both in the Dzierżno Duże and Turawa reservoirs. As for the former reservoir oxygenation considered as normal saturation (60-100%) occurs sporadically, usually in the cool season. However, in summer oxygen conditions in water worsen distinctly to the point that below the depth o 4-7 meters (depending on the sector of the reservoir) there is total disappearance of the diluted in water oxygen (Fig. 4). However, a little better oxygen conditions are to be found in the Turawa reservoir (Fig. 5) because periodical lack of oxygen is revealed only at the depths twice as big as those observed in the case of the Dzierżno Duże reservoir. Eutrophication processes, thus intensive growth of algae as well, contribute to a distinct increase in water reaction in summer. e growing algae absorb great amounts of carbon dioxide causing simultaneous increase in water reaction. Seasonal change in water reaction in individual reservoirs is very distinct. In the Turawa reservoir there is a gradual increase in water reaction beginning with values a little lower than 7 up to maximum values which occur in summer and ph reaches 9,49 (Fig. 6). However, in the Dzierżno Duże reservoir water ph changes to a lesser degree and fluctuates around the value considered to be neutral reaction. Such a condition is determined by the presence of allochtonic pollutants supplied by the Kłodnica river (Rzętała 2000). One of abiotic manifestations of eutrophication is clear drop in the transparency of the retained water. Systematic research carried out in many objects located in the Silesia Upland and its periphery allow for a statement that, especially in summer, there is a great worsening of light conditions. e direct cause of this condition is uncontrolled growth of algae called water blooming. e average transparency of the Dzierżno Duże reservoir is close to 3 meters which in reference to the rate of pollution in the reservoir can be considered favourable. Of course, there are periods and spots in the reservoir where transparency is smaller. It takes place, especially, in the eastern part of the water region being strongly influenced by the polluted water from the Kłodnica river, where transparency o en drops below 150 cm. However, in the case of the Turawa reservoir, according to Ruman (2005), during mass growth of phytoplankton the visibility drops even to 45 cm. In other seasons the decrease in transparency is the result of the supply of the reservoir with suspended matter washed away during spring thaw and torrential rain carried into the reservoir by tributaries flowing into it.

6 184 Machowski R., Ruman M., Rzętała M. Fig. 4. Seasonal concentrations of oxygen in the water of the western part of the Dzierżno Duże reservoir (location of the vertical profile: N, E). Fig. 5. Changes in average contents of oxygen diluted in the water of the Turawa reservoir together with the depth from July to September 2004 (location of the vertical profile: N, E). Eutrophication of water reservoirs e Dzierżno Duże reservoir water is characterized by very bad quality. e research findings presented by Rzętała (2000) show that water supplying the reservoir contains much greater amounts of biogenic matter than water in other rivers even those flowing across industrial areas (e.g. the Mała Panew river supplying the Turawa reservoir). It is also the case of quite a few much smaller reservoirs occurring in the Silesia Upland and its periphery. e development of eutrophication processes is characteristic of the Turawa reservoir as well and its intensity has been growing over the past few years. e great flood of 1997 during which great amounts of biogenic substances were carried into the hole of the reservoir from the river basin can be regarded as the main cause of such a condition. Since then there has been a yearly intensive growth of algae in the water area especially in the summer months (Boroń 2002). e uncontrolled growth of algae begins in June and lasts up till the end of September, but it appears in the eastern part of the water region first. Jankowski and Rzętała (1998) mention that a similar phenomenon with a parallel scale of intensity can be observed in other big reservoirs located in the Silesia Upland area and its periphery i.e. the Goczałkowice, Dzierżno Duże, Pławniowice, Kozłowa Góra and Przeczyce reservoirs. e increase in fertility of limnic water is the cause of a series of unfavourable changes from the economic point of view. One of abiotic manifestations is great fluctuation, deficiency and sometimes even complete disappearance of oxygen in the hypolimnion and at the same time overoxygenation of the epilimnion water. Such a phenomenon can be observed each year in the biggest, as regards retention, anthropogenic water reservoirs located in the Silesia Upland and its periphery (Jankowski, Kuczera 1992; Jankowski 1995; Rzętała 2000; Kostecki 2003). Water ecosystems, within which symptoms of eutrophication processes can be observed, are characterized by a great modification of natural decomposition of oxygen in water. Spontaneous growth of great amounts of algae in summer in a well lit layer of epilimnion causes a big increase in oxygen at small depths. e oxygen production by algae has a great influence on the supersaturation by the gas of the above mentioned parts of the reservoir. However, there is very serious shortage of oxygen in water at greater depths because of the oxidation processes of

7 Abiotic manifestations of eutrophication of water reservoirs 185 Fig. 6. Changes in water reaction of the Turawa reservoir in the hydrological year of the deposited pollutants. e course and intensity of such processes are dependent on the amount of oxygen available in water. Another limitation is very little sun light which cannot get through a thick layer of algae into deeper parts of the reservoir. As a result of such an ecological condition of the reservoir the photosynthesis processes are stopped in the hypolimnion. at is why, according to M. Rzętała (1998), there is deficit saturation in a lot of reservoirs and as a result complete disappearance of oxygen, which causes the liberation of biogenic substances gathered in bottoms in the process of the so called internal feed (Wiśniewski 1995). Discussion Different management of water reservoirs river basins has significant influence on creating the qualitative and ecological conditions of the retained water. As it has been shown on the examples of the Dzierżno Duże and Turawa reservoirs, the river basin with extreme industrial anthropopressure has much greater importance in the eutrophication processes. Much bigger amounts of biogenic substances are carried away from the used in this way area than from areas managed by agriculture. Besides biogenic substances great amounts of other pollutants are flown into the reservoir influencing the general ecological condition of the reservoir the manifestation of which are even the very small changes in water reaction. at is why mass effective growth of algae can be observed in the Turawa reservoir which has much better quality water than the Dzierżno Duże reservoir. As a result of accumulation of a lot of organic matter in the holes of both reservoirs as well as its consequent mineralization in the oxidation process there is periodical total disappearance of oxygen in the hypolimnion in the water of both reservoirs. References Boroń S Zakwity wód, powstanie, zapobieganie i usuwanie jego skutków (Flourish of water, formation, preventation and elimination of effects). In: Czystość wody zbiornika Turawa gwarancją dobrego wypoczynku. Instytut Śląski, Turawa Opole: Choiński A Współczesne tendencje zmian powierzchni jezior polskich (Modern tendencies in changes in lake surface area in Poland). In: Tomaszewski J. T. (ed.), Problemy hydrologii regionalnej. IG UW, Oddział Wrocławski PTG, Wrocław (in Polish with English summary): Czaja S Zbiorniki i pojezierza antropogeniczne (Water reservoirs and anthropogenic lakelands). In: Szczypek T., Rzętała M. (eds.), Człowiek i woda. Polskie Towarzystwo Geograficzne Oddział Katowicki, Sosnowiec (in Polish with English summary): Jaguś A., Rzętała M Zbiornik Kozłowa Góra funkcjonowanie i ochrona na tle charakterystyki geograficznej i limnologicznej (Kozłowa Góra water reservoir functioning and protection against a background of geographical and limnological characteristics). Komisja Hydrologiczna Polskiego Towarzystwa Geograficznego, Warszawa (in Polish with English summary):

8 186 Machowski R., Ruman M., Rzętała M. Jankowski A. T Antropogeniczne zmiany stosunków wodnych na obszarze uprzemysłowionym i urbanizowanym (na przykładzie Rybnickiego Okręgu Węglowego) (Anthropological changes of water on industrialized and urbanized areas (on example of Rybnicki Cole Area). Wydawnictwo Uniwersytetu Śląskiego, Katowice (in Polish with English summary): Jankowski A. T Z badań nad antropogenicznymi zbiornikami wodnymi na obszarze górnośląskim (Research on antropogenic water reservoirs in the area of Upper Silesia). In: Wybrane zagadnienia geograficzne. Pamięci geografów Uniwersytetu Śląskiego Józefa Szaflarskiego i Piotra Modrzejewskiego. WNoZ UŚ, Oddział Katowicki PTG, Sosnowiec (in Polish with English summary): Jankowski A. T., Kuczera A Wpływ zrzutu wód podgrzanych na warunki termiczne, tlenowe i przezroczystość wody w Zbiorniku Rybnickim ( e effect of discharging heated waters to the Rybnik Reservoirs on the thermal and oxygen conditions and on water transparence). Wydawnictwo UŚ, Katowice (in Polish with English summary): Jankowski A. T., Rzętała M Eutrofizacja sztucznych zbiorników wodnych na Wyżynie Śląskiej i jej obrzeżach (Eutrophication of man-made reservoirs in the Silesian upland and its periphery). In: Lange W., Borowiak D. (eds.), Zagrożenia degradacyjne a ochrona jezior. Wydawnictwo DJ, Gdańsk (in Polish with English summary): Jankowski A. T., Rzętała M Stan badań limnologicznych w regionie górnośląskim (State of limnological research in the Upper Silesian Region). In: Jankowski A.T., Rzętała M. (eds.), Jeziora i sztuczne zbiorniki wodne funkcjonowanie, rewitalizacja i ochrona. Uniwersytet Śląski Wydział Nauk o Ziemi, Polskie Towarzystwo Limnologiczne, Polskie Towarzystwo Geograficzne Oddział Katowicki, Sosnowiec (in Polish with English summary): Kajak Z Eutrofizacja jezior (Eutrophication of lakes). PWN, Warszawa: Kasza H Symptomy eutrofizacji a sukcesja ekologiczna w Goczałkowickim zbiorniku zaporowym (Symptoms of eutrophication while ecological succession in Goczałkowice dam reservoir. In: M. Zalewski (ed.), Procesy biologiczne w ochronie i rekultywacji nizinnych zbiorników zaporowych. Biblioteka Monitoringu Środowiska, PIOŚ, WIOŚ, ZES UŁ, Łódź: Kostecki M Alokacja i przemiany wybranych zanieczyszczeń w zbiornikach zaporowych hydrowęzła rzeki Kłodnicy i Kanale Gliwickim (Allocation and transformations of selected pollutants in dam reservoirs of the hydro-junction of the river Kłodnica and Gliwice Canal). Instytut Podstaw Inżynierii Środowiska Polskiej Akademii Nauk, Zabrze (in Polish with English summary): Ruman M Abiotyczne przejawy eutrofizacji zbiornika Turawskiego (Abiotic indications of eutrophication in Turawski reservoir. In: Zasoby wodne triasu opolskiego i ekologia Jezior Turawskich. Opole Strzelce Opolskie: Rzętała M Zróżnicowanie występowania zbiorników wodnych na obszarze Wyżyny Katowickiej (Diversity of artificial water reservoirs occurrence in the area of Katowice Upland). Geographia. Studia et dissertationes, T. 22. Wydawnictwo Uniwersytetu Śląskiego, Katowice (in Polish with English summary): Rzętała M Bilans wodny oraz dynamika zmian wybranych zanieczyszczeń zbiornika Dzierżno Duże w warunkach silnej antropopresji (Water balance and dynamics of changes in selected pollutants of Dzierżno Duże water reservoir under conditions of strong human impact). Wydawnictwo Uniwersytetu Śląskiego, Katowice (in Polish with English summary): Rzętała M Niektóre możliwości wykorzystania sztucznych zbiorników wodnych na obszarze górnośląskoostrawskiego regionu przemysłowego (Selected possibilities of utilization of artificial water reservoirs in the upper silesian-ostrava industrial region). In: Změny geografického prostŕedí v pohraničních oblastech Ostravského a Hornoslezského Regionu. Ostravská Univerzita, Ostrava (in Polish with English summary): Wiśniewski R. J Rola zasilania wewnętrznego w eutrofizacji zbiorników zaporowych (Role of internal alimentation in eutrophication of dam reservoirs). In: Zalewski M. (ed), Procesy biologiczne w ochronie i rekultywacji nizinnych zbiorników zaporowych. Biblioteka Monitoringu Środowiska. WIOŚ, ZES UŁ, Łódź: Wiśniewski R. J Fosfor w zbiornikach zaporowych zasilanie, kumulacja, wymiana między osadami dennymi i wodą (Phosphor in dam reservoirs delivery, cumulation, exchange between bottom deposits and water. In: Zalewski M. (ed.), Zintegrowana strategia ochrony i zagospodarowania ekosystemów wodnych. WIOŚ, ZE S UŁ, Łódź: Streszczenie Dwa zbiorniki wodne we wschodniej części Niziny Śląskiej (ryc. 1) Dzierżno Duże (50 22 N, E) oraz Turawski (50 43 N, E) funkcjonują w skrajnie różnych warunkach panujących w obrębie ich zlewni (tab. 1). Pierwszy z nich obciążony jest antropopresją przemysłową natomiast drugi rolniczą (ryc. 2, 3). Przeprowadzone badania pozwoliły na porównanie abiotycznych przejawów eutrofizacji w obrębie tych akwenów. Znacznie większe obciążenie substancjami biogennymi występuje w przypadku skrajnej antropopresji przemysłowej, dlatego też w obrębie zbiornika Dzierżno Duże panują znacznie gorsze warunki ekologiczne. Deficyty tlenowe trwają dłużej i pojawiają się na znacznie mniejszych głębokościach (ryc. 4) niż w przypadku Turawy (ryc. 5). Odczyn wody na Dzierżnie Dużym jest mniej zmienny niż w przypadku zbiornika Turawa, w obrębie którego podczas masowego rozwoju glonów zmiany te są znacznie większe (ryc. 6). Natomiast podczas zakwitów zarówno w jednym, jak i drugim przypadku wyraźnemu zmniejszeniu ulega przezroczystość wody. Generalnie, poziomy stężeń substancji biogennych i innych zanieczyszczeń w wodach porównywanych zbiorników świadczą o politrofii w przypadku Dzierżna Dużego (tab. 2) i eutrofii w odniesieniu do zbiornika Turawa.