Variations in the trophic state of the mesotrophic Ińsko DuŜe Lake ( )

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1 Limnological Review 4 (2004) Variations in the trophic state of the mesotrophic Ińsko DuŜe Lake ( ) Agricultural University of Szczecin, Kazimierza Królewicza 4, Szczecin, Abstract: The study describes changes in the trophic state of Ińsko DuŜe Lake, the cleanest body of water in West Pomerania, between 1970 and The lake was found to show middle to high resistance to degradation. The moderate vulnerability of the catchment area in terms of nutrient load discharge as well as the high resistance of the lake resulted in a moderate rate of eutrophication of Ińsko DuŜe Lake. The strong resistance of the lake was able to level the adverse catchment are conditions. The studies have revealed that the lake, initially of a mesotrophic status, showed increasing parameters of the trophic state from the 1980s until mid-1990s, which was followed by a return of the lake to its previous mesotrophic status at the end of the 1990s. The latest surveys (2002) have shown that the water quality of the lake have improved; the water, previously of the 2 nd purity grade, has gained the characteristics typical for the 1 st grade waters. Some quality indicators (e.g. Secchi depth) have recently reached the values typical for oligotrophic lakes. The improvement in water quality and the return to the lake's mesotrophic status is, on the one hand, due to changes in the use of arable soils located within the catchment are that have undergone for the last years, but, on the other hand, also due to improved waste-water management of the town of Ińsko. Key words: Ińsko DuŜe Lake, water quality, oligotrophic lakes, mesotrophic lake. Introduction and objective In West Pomerania, like in other regions across Poland, excessive run-off loads of nutrients and organic matter have affected surface waters for decades (Tadajewski et al., 1980; Kubiak et al., 1997, 1999; Zdanowski, 1999). Increased level of nutrient concentrations, as well as its consequences, force us to find ways to slow down, stop, or even revert this adverse process or, at least, to find a remedy against its negative outcomes (Lossow, Gawrońska 1998; Lossow, 1998). In order to chose adequate protective measures, a lake should be thoroughly studied for the rate and direction of changes in its trophic state (Kudelska, 1979; Bakiewicz-Grabowska, 1981, 1990; Lossow, 1996). Long-term studies are necessary which would show whether the changes in the lake's water quality are a result of temporary changes in the habitat, arise from local factors related to the catchment area, or reflect wider changes undergoing in the environment (Jańczak, 1995). Considering (1) the necessity to monitor and protect Poland's inland waters, which results from the integration of Poland with the European Union, (2) multiple areas of importance of lakes for West Pomeranian economy, and (3) the effect of eutrophication on the water quality of the lakes, this study was aimed to describe changes in the trophic state of Lake Ińsko DuŜe occurring from 1970 until The studied body of water is located within Ińsk Lakeland neighbouring with the sandur Drawsko Plain to the east, with moraine Nowogard Plain to the west, and with Drawsko Lakeland to the north-

2 116 east (Mikołajski, 1966; Kondracki, 2000). Ińsko DuŜe lake is placed in the area previously heavily utilised by so called large-scale agricultural production, characterised by industrial animal farming, application of intensive mineral fertilising, etc. (Rocznik..., 1995). The parameters describing the morphometry of the studied body of water and its geographic location are presented in Table 1. Table 1. Location and morphometry of Ińsko DuŜe Lake Location Community Ińsko County Stargard Sz Ina river basin Latitude 53 o 26.6' Longitude 15 o 31.6' Height a.s.l Maximum length (m) 5400 Maximum width (m) 2100 Mean width (m) 901 Shoreline (m) Basin Source: Filipiak, Raczyński (2000), Czarnecka et al., (1989) Computed acc. to Choiński (1995) Source: Czarnecka et al., (1989) Islands 2875 Maximum depth (m) 41.7 Mean depth (m) 12.9 Relative depth by Halbfass Depth index 0.31 Shelter index Content index 0.13 Shoreline development K Shape index 0.31 Maximum effective length (m) 3500 Maximum effective width (m) 900 Total catchment area km Active catchment area km Water table area ha Islands area ha 22.3 total area ha Volume (thou. m 3 ) Unit run-off (l km -2 s -1 ) 3.4 Material and methods The study was based on the results of our own research ( and ), unpublished data ( ) of the Department of Hydrochemistry and Water Protection, Agricultural University of Szczecin, as well as the results by Nguyen (1972). Field methods The surveys were carried out in 1970, 1971, 1978, 1982, 1987, 1992, 1996, 1997, 1998, 1999, and 2002, samples being collected from the surface and bottom parts of the water column at the deepest part of the lake during the spring turnover and summer stagnation. Laboratory methods The following indicators of the trophic state and water quality were estimated: organic matter content, with BOD 5 ; chemical oxygen demand, dichromate method; orthophosphates and total phosphorus, (after mineralisation) in unfiltered water samples, colorimetrically with molybdenum method (tin chloride II or ascorbic acid as reducing agent); ammonia nitrogen, direct nesslerisation or indophenol blue; nitrate nitrogen, phenol disulfonic acid or on a copper-cadmium column after being reduced to nitrites; mineral nitrogen, as a total of the nitrate and ammonia forms; total nitrogen, after mineralisation with potassium persulfate (in alkaline environment) and further procedure like that with nitrates. The following were also analysed: seston dry mass content (by weight), chlorophyll a content, (colorimetrically, after extraction with acetone), dissolved oxygen content (Winkler test, or using a WTW oxygen meter), and percent oxygen concentration. The techniques applied are commonly recommended (Standard Methods 1995). Assessment of water quality and lake's trophic state The criteria we applied in the assessment of water quality have been published as guidelines for basic lake monitoring (Kudelska et al., 1994). The analysis of the lake's trophic status was based on the recognised and commonly used methods: that adopted by the Organisation for Economic Co-operation and Development (OECD) (Vollenweider, 1989); the method proposed by Zdanowski (1983), modified by Kajak (1983), as well as that given by Carlson (1977). The appraisal of the lake's vulnerability to degradation was based on methods published by Kudelska et al., (1994) and Bajkiewicz-Grabowska (1990); eutrophication progress rate was estimated according to Bajkiewicz-Grabowska (1990).

3 Variations in the trophic state of the mesotrophic Ińsko DuŜe lake ( ) 117 Results Vulnerability to degradation of Ińsko DuŜe Lake The vulnerability to degradation of the lake, the role of the catchment area in the transport of matter to the lake, and the rate of its eutrophication was reported by Kubiak (2003) basing on data from The data has been supplemented with the results of the surveys carried out in Applying the rules for lake monitoring (Kudelska et al., 1994), it has been found that the body of water remained highly resistant to degradation throughout the entire analysed period; only in 1992, due to poor stratification of the water column, the lake's vulnerability to degradation increased beyond the average. According to the criteria by Bajkiewicz-Grabowska (1990), the studied lake was characterised by an intermediate resistance to degradation over the entire period ( ). An analysis of the ecological system: catchment area lake, which allowed determination of eutrophication intensity for Ińsko DuŜe Lake, has revealed a moderate progress of this process in the lake over the entire period. This flow-through lake is surrounded by adverse catchment area conditions: the catchment area is intensively used by agriculture and water-retaining areas are scarce, which facilitates the discharge of nutrient loads with the run-off. This is compensated for with the lake's strong resistance resulting mainly from a considerable stratification of the water column (eumictic and, periodically, bradymictic type of mixing; Kubiak 2003). The lake contains a relatively large volume of water and has a poorly developed shoreline. Water quality assessment Oxygen content, which is the chief indicator of the intensity of matter circulation in bodies of water, represents an important component in the picture of a lake's trophic state. Lange and Maślanka (1994) suggest that analysis of oxygen distribution during the summer stagnation is of special value in the appraisal of the trophic status. Conclusions that arise from an analysis of oxygen conditions, which refer to the lake's vulnerability to degradation, describe the actual course of the process of eutrophication, not a possible development. During its summer stagnation, Ińsko DuŜe Lake showed a constant level of oxygen in the bottom part of the water column and, in most cases, dissolved oxygen content exceeded 4.0 mg O 2 dm 3, while saturation was higher than 30% (Tab. 2). The lowest dissolved oxygen content in Ińsko DuŜe was recorded from late-july until the second half or the end of September. During this period, the best oxygen conditions were found near the surface (110% 129% and mg O 2 dm 3 ). Also, the waters of the whole epilimnion were oversaturated with oxygen, and the level of compensation in both bodies of water was located below 6 m depth. The average gradient in the oxycline was 1.8 to 1.9 mg O 2 dm 3 m 1. The sharpest drops in oxygen content were recorded within the layer between 5 and 10 m. During the winter stagnation, surface waters were also considerably better oxygenated than those near the bottom; oxygen saturation near the surface was higher than 90% (11 12 mg O 2 dm 3 ), while in the benthic layer higher than 70% (more than 8.0 mg O 2 dm 3 ). During the climaxes of both mixing seasons, surface water was still better oxygenated, the differences in oxygen saturation reached 20 30% and were larger during the spring turnover. During the winter, the oxygen conditions were similar to those during the autumn turnover (Tab. 3). Over the entire period of the studies, the oxygenation of surface water during summer stagnations changed; during the 1970s, it ranged within %, during the 1980s, remained in the range %, and during the 1990s, it remained at a level of 115%. In 2002, oxygen saturation reached 129%. In the same period, the water above the bottom had 55-% oxygen saturation in the 1970s, 25 32% from 1978 to 1993 with the minimum in 1978 (26.1%). In the second half of the 1990s and in 2002, the indicator distinctively increased reaching the values close to 50%. Organic matter content, as estimated indirectly via COD Cr and BOD 5, increased in the summer within the surface layer, for the former parameter changing from 26.4 (in 1971) to 32.0 mg O 2 dm 3 (in 1998), whereas for the latter from 2.8 to 4.2 mg O 2 dm 3. The highest concentrations of organic compounds were recorded in the end of the 1970s and the beginning of the 1980s, as well as during the last years of the investigations. In 2002, organic matter was found to decrease in the summer within

4 118 the surface layer. A similar trend was observed in BOD 5 of the benthic layers of the water column, which increased from 1.7 (in 1971) to 4.2 (in 1996) and dropped below 3.0 mg O 2 dm 3 during (Tab. 4). During the studied period, a falling trend in the concentrations of all the studied nitrogen and phosphorus forms was recorded in the lake (Tab. 4). In the summer, the concentration of phosphates in the bottom layer decreased from in 1971 to less

5 Variations in the trophic state of the mesotrophic Ińsko DuŜe lake ( ) 119 than mg P PO4 dm 3 in , while total phosphorus decreased from in 1978 to mg P dm 3 in In spring, the concentration of phosphates dropped from in 1970 to dm 3 in The content of mineral nitrogen decreased too, which was the most distinct between , when the decline was from to mg N dm 3, as well as between , respectively to mg N dm 3. Gradual decrease in total nitrogen was also seen in surface waters, from in 1979 to mg N dm 3 in 2002 (Tab. 4). Table 4. Water quality assessment in Ińsko DuŜe Lake based on the values of individual indices observed for the entire period of studies No. Index Season Layer 1 Mean hypolimnion oxygen content L bottom 3 COD L surface 4 BOD 5 L surface 5 BOD 5 L bottom Ińsko DuŜe Mean Max Min Phosphates W surface Phosphates L bottom 8 Total phosphorus L bottom 9 Total phosphorus Mineral nitrogen ( NNH 4+NNO 3) Ammonia nitrogen (NNH 4) 12 Total nitrogen 13 Conductivity 14 Chlorophyll 15 Seston dry matter 16 Transparency Class mean (W+L) surface W L surface bottom mean (W+L) surface mean (W+L) surface mean (W+L) surface mean (W+L) surface mean (W+L) surface mean II II II II II II II II II II I II II I Source: Kudelska et al. (1994); Kudelska, Soszka (1996). W spring, L summer, Mean (W+L) mean from spring and summer measurements, x/ numbers obove the dashed line denote values of the indices, below the line water qualiry class, Bad the most adverse conditions, Good the most positive conditions,

6 120 Electric conductivity varied slightly, ranging between and µs cm 1. Seston content in surface waters also changed, from 12.0 in 1970 to 9.5 in 1978 to 15.0 in 1998, 11.0 in 1999, and 9.5 mg dm 3 in 2002 (Tab. 4). The lowest Secchi depth (the mean of spring and summer measurements) was recorded in 1978 (3.2 m). In the preceding years, Secchi depth reached 4.2 m in 1970 and 4.5 m in 1971, whereas recently (2002) as much as 5.5 m. Chlorophyll a content changed from 17.0 in 1978 to 11.5 in 1996 and 9.5 mg dm 3 in According to the criteria of basic lake monitoring, the water quality of the discussed body of water remained within the 2 nd grade of purity. Recently, i.e. in 2002, some waters of Ińsko DuŜe lake was found at the 1 st grade of quality. This has occurred due to reduced values of organic matter content (COD CR ) in surface waters during the summer, total phosphorus in bottom waters during the summer, and total nitrogen in surface waters (Tab. 4). Trophic state of Ińsko DuŜe Lake and its long-term changes Lake Ińsko DuŜe, which according to the criteria by Kajak (1983), Zdanowski (1983), and Vollenweider (1989) was initially a mesoeutrophic lake ( ), acquired the characteristics of a mesotrophic lake during the years that followed ( ). Changes in the trophic state were seen to occur also according the model by Carlson (1977). During the years 1996 to 2002, the lake became mesoeutrophic (Tab. 5); recently, Secchi depth has increased to the values considered typical for oligotrophic waters (Carlson 1977). Also according to the criteria of Vollenweider (1989), the lake's trophic state after 1998 approached and reached the level of mesotrophy (Tab. 5). Table 5. Trophic state of Ińsko DuŜe Lake based on the value of individual indices recorded over the entire studied period Carlson (1977) Kajak (1983), Zdanowski (1983) OECD Vollenweider (1989) Additional Basic indices indices Lake Year TSI SD TSI Chl TSI TP summer TP summer SD summer Clorophyll a (summer) TN summer; surface TN/TP TP-mean Ch a max.(summer) Ch a mean (spring-summer) SD max(summer) SD mean (spring-summer) %O2 - bottom Ińsko DuŜe ME D1-MS OL MS MS ME D1-MS OL MS EU ME 60 ME 62 EU D2-ME D2-ME D2-ME D3-EU D1-MS EU MS EU MS MS EU ME 59 ME 65 EU D2-ME D1-MS D2-ME D3-EU D1-MS EU MS EU OL MS EU ME 59 ME 65 EU D2-ME D1-MS D2-ME D3-EU D2-ME EU MS EU OL MS EU ME 59 ME 53 ME D1-MS D1-MS D2-ME D3-EU D1-MS MS MS EU OL MS EU ME 56 ME 59 ME D1-MS D1-MS D2-ME D3-EU D1-MS MS MS EU OL MS MS OL 58 ME 57 ME D1-MS D1-MS D2-ME D1-MS D1-MS EU MS EU OL MS MS OL 59 ME 56 ME D1-MS D1-MS D2-ME D1-MS D1-MS MS MS EU OL MS EU OL 59 ME 58 ME D1-MS D1-MS D2-ME D3-EU D1-MS MS MS EU OL MS EU OL 57 ME 52 ME D1-MS D1-MS D2-ME D1-MS D1-MS MS MS EU OL MS MS Mean 41 ME 59 ME 59 ME D1-MS D1-MS D2-ME D3-EU D1-MS EU MS EU OL MS EU Good 37 OL 56 ME 52 ME D1-MS D1-MS D2-ME D1-MS D1-MS MS MS EU OL MS MS Bad 45 ME 60 ME 65 EU D2-ME D2-ME D2-ME D3-EU D2-ME EU MS EU MS MS EU L summer, W spring, TSI trophic state index by Carlson (1977), SD Secchi depth, chlorophyll a chlorophyll a content, TP total phosphorus, TN total nitrogen,

7 Variations in the trophic state of the mesotrophic Ińsko DuŜe lake ( ) 121 % O 2 oxygenation, Good trophic state at best values of individual indices, Bad trophic state at worst values of individual indices, Mean trophic state at average values of individual indices, OL oligotrothy, MS mesotrophy, ME mesoeutrophy, EU eutrophy, HE hipertrophy, D I, D II, D III, according to Kajak (1983), in Zdanowski (1983) correspond to MS, ME, EU Discussion Lakes represent dynamic systems, changing with time, with their biological production becoming more and more diverse and intensified The transport, exchange, and redistribution of minerals originating from the biogeochemical cycles, as well as of organic matter which is produced by the ecosystem itself or is introduced into it as pollution, are the processes that shape the direction and intensity of a lake's evolution (Lossow and Więcławski, 1991). In a particular locality, these processes depend on a whole collection of factors, such as the climate, hydrology, hydrogeology, or soil structure of the catchment area. Furthermore, the morphometry of the lake is no less important, and so are the hydrochemical relationships within the body of water (Olszewski and Tadajewski, 1959; Kalff, 2002). The dynamics of water circulation in the lake (Olszewski and Tadajewski, 1959; Patalas, 1960), the functioning of its biocenoses, and the exchange of the elements between the solid phase and the solution (Olszewski and Tadajewski, 1959; Kalff, 2002) are of key importance too. To reasonably manage a lake's resources, one must learn all the above-mentioned characteristics of a given body of water, and identify the sources of pollution. The intensity of matter circulation in a lake, including the rate of re-activation of deposited nutrients, strongly depends on the water circulation dynamics (Patalas, 1960). It has been recognised that the process of eutrophication is slower in lakes with slower water circulation, and that such lakes are more resistant to human impact. Ińsko DuŜe has been described as a lake of intermediate (Bajkiewicz- Gra-bowska, 1990) or high (Kudelska et al., 1994) resistance to degradation, which is supported by the lake's considerable depth and its eumictic to bradymictic type of mixing. Small area of the bottom within the epilimnion and a large proportion of hypolimnion in the total lake's volume result in a reduced intensity of nutrient recycling between the deposits and the trophogenic zone; matter circulation and internal supply of nutrients is limited. Ińsko DuŜe lake demonstrates a high level of resistance. The strong resistance of the lake levels the less positive conditions of its catchment area, which show a moderate potential to discharge run-off loads of nutrients. The qualities of the catchment, which serve against eutrophication, include: low lake percentage, low density of stream network, mild slopes of the active basin, sandy-clay geological structure of the catchment bed, and a good ratio of catchment area to lake area. The adverse characteristics include the flow-through character of the lake and lack of blind drainage areas within the catchment. The entire catchment area, partly covered with forests, partly cultivated, with the housing, takes part in supplying nutrients to this flow-through lake; however, the basin shows an average level of activity in discharging the deposited loads of matter. The moderate susceptibility to nutrient load discharge of the catchment and the high resistance of the lake result in a moderate progress in eutrophication. Ińsko DuŜe has good quality of water, with the 2 nd or, most recently, the 1 st grade of purity (Tab. 4). The oxygenation of the lake's water has always been very good; even bottom waters were well oxygenated during summer stagnations. Summer oxygen stratification of the water column in Ińsko DuŜe lake fits well a negative heterograde oxygen curve; therefore, the lake should be rated as an alpha-meso-trophic one. In 2002, a slightly marked positive heterograde oxygen curve was also found. The patterns of oxygen concentration demonstrate a poor trophic state of the lake (Kalff, 2002). A low phosphorus content, low primary production expressed as chlorophyll a concentration in the surface layer, high water transparency, and a high nitrogen-to-phospho-rus ratio confirm a mesotrophic status of the lake. Recently ( ), it has been noted that water transparency during the summer increases, reaching the values recognised by Vollenweider (1989) as those typical for oligotrophic waters. Over the last years, changes in the use of the arable land within the catchment area and improved waste-water

8 122 management of the town of Ińsko have resulted in improved water quality of Ińsko DuŜe Lake and allowed the lake to return to its mesotrophic status. References Bajkiewicz-Grabowska E., 1981, The influence of the physical geographic environment on the biogenous matter delivery to the lake. J. Hydrol. Sci. 8 (1 2), Bajkiewicz-Grabowska E., 1990, Stopień naturalnej podatności jezior na eutrofizację na przykładzie wybranych jezior Polski (Degree of natural vulnerability of lakes to eutrophication exemplified with selected Poland's lakes), Gospod. Wod. 12: Carlson R. F., 1977, A trophic state index for lakes. Limnol. Oceanogr. 22 (2), Choiński A., 1995, Zarys limnologii fizycznej Polski (Basics of Physical Limnology of Poland), Wydaw. Nauk. UAM, Poznań. Czarnecka H., Bialuk J., Hołdakowska J., Marcinkowska Z., Woroncow T., Majewska I., 1989, Ocena ilościowa i fizyko-chemiczna zasobów wodnych jezior województwa szczecińskiego. Cz. 4.. Zlewnie jezior województwa szczecińskiego (Quantitative and physiochemical assessment of water resources of Szczecin voivodship lakes. Part 4. Szczecin voivodship lake catchment areas), IMGW, Warszawa, (mscr.). Filipiak T., Raczyński M., 2000, Jeziora zachodniopomorskie zarys faktografii (West Pomeranian Lakes a catalogue outline). Wydaw. AR, Szczecin. Jańczak J., 1995, ZałoŜenia i cele monitoringu reperowego jezior polskich oraz niektóre uwagi po 4 latach jego funkcjonowania (Bases and objectives of the benchmark monitoring of Polish lakes and some observations after 4 years of its activity), Gaz. Obs. IMGW, 2, Kajak Z., 1983, Dependences of chosen indices of structure and functioning of ecosystems of different trophic status and mictic type for 42 lakes. Ecological characteristics of lakes in northeastern Poland versus their trophic gradient. Ekol. Pol. 31, Kalff J., 2002, Limnology. Prentice Hall Ltd., New Jersey. Kondracki J., 2000, Geografia regionalna Polski (Regional Geography of Poland), PWN, Warszawa. Kubiak J., 2003, Największe dimiktyczne jeziora Pomorza Zachodniego. Poziom trofii, podatność na degradację oraz warunki siedliskowe ichtiofauny (The largest dimictic lakes of West Pomerania. Trophic state, vulnerability to degradation, and state of fish habitats), Rozprawy 214, AR, Szczecin, Kubiak J., Chojnacki J., Tórz A., Sroka E., Nędzarek A., 1999, Spatial patterns of nutrients in the Odra Estuary from Widuchowa to the Pomeranian Bay, In: Hydrodynamic and ecological aspects of nutrient forecasting for Odra and Ebro Estuaries. Proc. 5 th International Scientific Conference, Szczecin 5 7 May 1999, MIR, Gdynia, Kubiak J., Mutko T., Tórz A., 1997a, Trends of water chemistry changes in lakes of the West Pomeranian region during the last twenty years, In: Proc. 3 rd Meeting of Internat. Center of Ecology PAS, Szczecin, December 9 12,1997, Szczecin, 1 3. Kudelska D., 1979, Klasyfikacja jezior dla potrzeb ich uŝytkowania i ochrony (Lake classification for their utilisation and protection), Wiad. Ekol. 25 (3), Lange W., Maślanka W., 1994, An attempt to evaluate the natural trends in the Masurian lakes (Próba oceny naturalnej tendencji jezior Pojezierza Mazurskiego), In: Problemy hydrologii regionalnej. Materiały Ogólnopolskiej Konferencji Hydrograficznej, Karpacz, , Karpacz, Lossow K., 1996, Znaczenie jezior w krajobrazie młodoglacialnym pojezierza Mazurskiego (Importance of lakes for the young-glacial landscape of the Masurian Lakeland), Zesz. Probl. Post. Nauk. Rol., 431, Lossow K., 1998, Ochrona i rekultywacja jezior teoria i praktyka (Protection and revitalisation of lakes; theory and practice), Idee Ekolog. Ser. Szkice 13 (7), Lossow K., Gawrońska H., 1998, External input to Lake Wadąg Effective and Estimate Loadings. Pol. J. Envir. Stud. 7 (2), Lossow K., Więcławski F., 1991, Migracja podstawowych pierwiastków poŝywkowych z gleb, uŝytkowanych rolniczo do wód powierzchniowych (Migration of basic nutrient elements from cultivated soils to surface bodies of water), Biul. Inf., ART, Olsztyn, 31, Mikołajski J., 1966, Geografia województwa szczecińskiego (Geography of Szczecin voivodship), Pr. Szczec. Tow. Nauk. 12, Nguyen Van T., 1972, Studia nad chemizmem wód jezior o róŝnym stopniu troficznym (Studies on water chemistry in lakes of various trophic state), Szczecin, (mscr). Olszewski P., Tadajewski A., 1959, Wpływ zlewni na Ŝyzność jezior (Effect of catchment are on lake fertility), Zesz. Nauk. WSR w Olsztynie, 4, Patalas K., 1960, Mieszanie wiatrowe jako czynnik określający intensywność krąŝenia materii w róŝnych morfologicznie jeziorach okolic Węgorzewa (Wind mixing as a factor determining the intensity of matter circulation in morphologically different lakes near Węgorzewo), Rocz. Nauk Rol. Ser. B 77(1), Rocznik Statystyczny (Statistical Yearbook), 1998, Wydaw. GUS, Warszawa. Standard methods for the examination of water and wastewater, 1995, Amm. Publ. Health Ass., New York.

9 Variations in the trophic state of the mesotrophic Ińsko DuŜe lake ( ) 123 Tadajewski A., Mutko T., Faberski Z., Górka-Niwińska E., Kubiak J., 1980, Aktualny stan zanieczyszczenia i eutrofizacji jezior Pomorza Zachodniego (Current state of pollution and eutrophication of West Pomeranian lakes), In: Stosunki wodne w zlewniach rzek Pomorza i dorzecza dolnej Wisły ze szczególnym uwzględnieniem gospodarki wodnej jezior, Materiały sesji naukowo-technicznej, Cz. 3, Słupsk, , IMGW, Słupsk, Vollenweider R. A., 1989, Global problems of eutrophication and its control. Symp. Biol. Hung. 38, Streszczenie Znaczenie gospodarcze jezior oraz konieczność ich ochrony są bezsporne. W niniejszej pracy przedstawiono wyniki wieloletnich badań hydrochemicznych, jakimi objęto jezioro Ińsko DuŜe, na Pojezierzu Ińskim Oceniono zmiany poziomu trofii jeziora Ińsko Du- Ŝe, najczystszego akwenu Pomorza Zachodniego, jakie następowały w latach r. Stwierdzono, ze badane jezioro charakteryzowało się średnią bądź duŝą odpornością na degradację. Umiarkowana podatność zlewni a uruchamianie ładunku biogenów oraz wysoka odporność akwenu powodują, Ŝe tempo eutrofizacji jeziora Ińsko DuŜe było umiarkowane. DuŜa odporność jeziora niwelowała niekorzystne warunki zlewniowe. Zdanowski B., 1983, Chemistry of the waters of 41 lakes. Ecological characteristics of lakes in northeastern Poland versus their trophic gradient. Ekol. Pol. 31, Zdanowski B., 1999, Eutrofizacja jezior Wigierskiego Parku Narodowego: zagroŝenia i ocena (Eutrophication of the lakes of the Wigry National Park: threats and evaluation), In: Funkcjonowanie i odnowa ekosystemów wodnych na obszarach chronionych, B. Zdanowski, M. Kamiński, A. Martyniak, (eds.), IRŚ, Olsztyn, Badania wykazały, Ŝe w jeziorze Ińsko DuŜe początkowo mezotroficznym, w latach i pierwszej połowie lat nastąpił wzrost poziomu trofii, a następnie z końcem lat badany akwen powrócił do stanu mezotrofii. Ostatnie badania (2002 r.) pokazały, Ŝe uległa poprawie jakość wód w badanym akwenie, wody dotychczas II klasy nabrały cech typowych dla wód klasy I. Niektóre ze wskaźników (widzialność) w ostatnim czasie miały wartości typowe dla wód oligotroficznych. Popraw jakości wody i powrót do stanu mezotrofii nastąpił na skutek zmiany w ostatnich kilkunastu latach w uŝytkowaniu gleb na obszarze zlewni a takŝe uporządkowanie gospodarki ściekowej miasta Ińsko. Omawiany akwen jest eumiktyczny, ze skłonnościami do bradymiksji.