The influence of catchment use on recent transformations. of the Bytów Lakeland

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1 Limnological Review 5 (2005) The influence of catchment use on recent transformations of water properties in chosen lobelia lakes of the Bytów Lakeland University of Gdańsk, Department of Limnology, Dmowskiego 16a, Gdańsk Abstract: The morphometric and hydrological predispositions of individual reservoirs, as well as the local geographical environment conditions of their catchments have a significant impact on water properties and on the direction and intensity of its qualitative transformations. Despite relatively small eutrophication intensity, the group of lobelia lakes covered by the study showed a diversity of water environment quality dependent on the catchment use structure. The smallest pressure is exerted by alimentation areas with the domination of forest terrains while agricultural and the most anthropogenically transformed catchments are the least favourable. Key words: lobelia lakes, catchment use, qualitative transformation of water. Introduction Lobelia lakes constitute nowadays a very small group of naturally valuable reservoirs with well preserved natural features and reflecting the initial stage of limnological succession. They are distinguished by the presence of isoetides and the Isoëto-Lobelietum dortmannae assemblage formed by them (Szmeja, 1996). The presence of at least one of the three following phytoindicators: water lobelia (Lobelia dortmanna), lake quillwort (Isoëtes lacustris) and shoreweed (Littorella uniflora) is the precondition in Poland to classify a given lake as a lobelia lake. Currently the existence of 175 lobelia lakes has been documented in Poland (Gos, Bociąg, 2003). Most of them are located in the Pomeranian Lakeland six of which, situated in the Bytów Lakeland, have been chosen as the study object (Fig. 1). These are lakes: Jeleń, Pipienek, Cechyńskie Wielkie, Cechyńskie Małe, Głęboczko and Stary Staw. The chosen reservoirs differ in the morphometric and hydrological respect. Significant differences occur also within the local environmental conditions of their small catchments. The aim of the paper is the physical-limnological characteristics and a diagnosis of the current water environment state of the group of 6 lobelia lakes of the Bytów Lakeland alongside with an attempt to predict the direction and intensity of further qualitative transformations. Study methods The study is based on an annual cycle of field research (September 2002 July 2003) which included four measurement series. In March 2004 supplementary measurements were performed. The da-tes were chosen with respect to the phases of annual thermo-dynamic variability of the investigated lakes. The measurements were carried out along constant measurement columns assumed

2 196 to be representative of the studied reservoirs and corresponding to their maximum depths. The range of physical-limnologi-cal survey included the measurements of temperature distribution, proper conductivity (ProfiLine LF 197 conductometric probe made by WTW), water lighting with the use of LI-COR measure unit (except January, when total ice cover of reservoirs ma-de the measure impossible), oxygen content (ProfiLine OXI 197 oxygen probe coupled with temperature measure unit made by WTW) and in spring and summer chlorophyll a (Minitracka II HB 176 probe made by Chelsea Instruments Limited). Ad-ditionally, measurements of water transparency with the use of Secchi disc were carried out each time. Water samples were taken each time from the surface and near-bottom layer for laboratory analyses, which covered: total and carbonate hardness, ph reaction, chloride, calcium, magnesium, sodium, potassium content, as well as phosphates (PO 4 ) and phosphate phosphorus (P PO4 ), nitrates (NO 3 ) and nitrate nitrogen (N NO3 ), nitrites (NO 2 ) and nitrite nitrogen (N NO2 ) and ammonia (NH 4 ) and ammonium nitrogen (N NH4 ) using PhotoLab Spektral spectrophotometer made by WTW. Fig. 1. Situation of investigated lakes: 1 Jeleń, 2 Pipienek, 3 Cechyńskie Wielkie, 4 - Cechyńskie Małe, 5 Głęboczko, 6 Stary Staw, a - lakes, b - lobelia lakes, c - investigated lobelia lakes, d watercourses General natural conditions of lakes and their catchments The catchments of lakes covered by the study are located in the central part of the Bytów Lakeland which constitutes the eastern part of the West Pomeranian Lakeland macroregion. In the hypsometric respect it is an area with diverse relative and absolute heights. It is formed mainly by push moraines and accumulation end moraines within which there are melt-out areas (Sylwestrzak, 1977). Boulder clays

3 The influence of catchment use on recent transformations of water properties in chosen lobelia lakes and fluvioglacial sands and gravels dominate among the surface formations. The development of lake basins is connected with the glacial past of the area. The discussed reservoirs have different morphogenesis starting from channel (Cechyńskie Wielkie) through channel-meltout (Jeleń and Pipienek), morainic-melt-out (Cechyńskie Małe) to melt-out (Głęboczko and Stary Staw). The location of the discussed lakes within the River Słupia drainage basin is within the zone adjacent to the watershed. They are located along the headstream sections of two left bank tributaries of the Słupia. Their catchments are not large, from 72.6 ha (Stary Staw) to ha (Cechyńskie Wielkie). The lakes are also the main elements of the hydrographic network of these areas. The catchments of the discussed lakes have heterogeneous hydrographical structure which is reflected by total 32.6% share of absorptive and evapotranspirative areas without outflow. The largest contribution of areas without outflow occurs in the catchment of Lake Pipienek 52.6%, of which 35.2% are evapotranspirative areas. These slightly hydrologically active elements occupy 99.5 ha (35.6%) in the supply area of Lake Jeleń. The differential catchments of other lakes are on the average in 26% co-vered by areas without outflow. The most important hydrological feature of the majority of the discussed lakes is their connection with exorheic drainage systems. A large part of watercourses has a periodical character, which results in the seasonal type of outflow from some of the reservoirs. Only Lake Jeleń is a reservoir without outflow being a part of an endorheic system. The catchments of the discussed lakes are characterised by a considerable diversity of the use struc-ture (Tab. 1). The most favourable form for lakes are forest habitats, which curb surface flow thus decrea-sing the soil erosion intensity as well as the washing out of substances that accelerate eutrophication (Ka-jak, 2001). They also considerably absorb nutrients dissolved in water. Least favourable are agricultural and built-up areas, which contribute to the increase in the rate of eutrophication transformations. Table 1. Use structure of direct catchments of investigated lakes Reservoir basin forests arable land meadows and grazing land built-up areas Waste land area in basin [%] Jeleń Pipienek Cechyńskie Wielkie Cechyńskie Małe Głęboczko Stary Staw The largest anthropogenic areas are located in the direct catchments of Lakes Pipienek and Stary Staw and in the total catchment of Lake Jeleń. In the alimentation area of Lake Pipienek arable areas constitute as much as 60% and in the case of Stary Staw 55%. In both catchments forest areas have the smallest share. A similar situation concerns the catchment of Lake Jeleń, in which agricultural areas are dominant too. The other lakes: Cechyńskie Wielkie, Cechyńskie Małe and Głęboczko are characterised by favourable ecological conditions with woodiness above 78%. Thus, it can be assumed that exactly these three reservoirs have catchments which naturally slow down the rate of their transformations. In terms of morphometry, the basins of the investigated lakes reveal significant diversity of size and shape (Tab. 2). Their areas range from 11 ha (Stary Staw) to 88.9 ha (Jeleń). Table 2. Basic morphometric data of investigated lakes according to bathymetric charts of the Institute of Inland Fishery in Olsztyn Lake Index Pipienek Cechyńskie Wlk. Cechyńskie Małe Głęboczko Stary Staw Jeleń P 0 [ha] V [10 3 m 3 ] H m [m] H s [m] D [m] S [m] W L [m]

4 198 K Wo P 0 water surface area, V lake volume, H m maximum depth, H s mean depth, D maximum length, S maximum width, W elongation, L shoreline length, K development of shoreline, W o exposure (openness) factor The depth relationships of the discussed lakes are clearly reflected by two criteria: maximum and mean depth. On the basis of these criteria the lakes can be divided into the following groups: very deep Głęboczko and Jeleń (H m over 20 m, and H s over 9 m), deep Pipienek, Cechyńskie Wielkie and Cechyńskie Małe (H m from 10 to 20 m, and H s below 10 m), and Stary Staw meeting the criteria of medium deep lakes (H m 5 10 m; H s 2 5 m). Abiotic properties of water In terms of the thermo dynamic system, the examined lakes represent the dimictic type with the 4-phase annual cycle with two periods of homother-my in spring and autumn and two stagnation periods in summer and winter (Fig. 2). The analysis of their thermal characteristics reveals that five of them (Pi-pienek, Cechyńskie Wielkie, Cechyńskie Małe, Głę-boczko and Stary Staw) represent the bradymictic regime type. Large vertical temperature gradients and differences in water density cause the stratification to form quickly and the hypolimnion to main-tain in summer a temperature of 4 C. Only lake Je-leń represents the tachymictic type of thermo-dynamic regime, in which the heating of near-surface waters occurs slowly and spring circulation lasts long. The measurements of Secchi disc visibility (KSc) performed in April and July 2003 in the waters of the investigated lakes revealed significant diversity and variability of light transmission conditions. In both periods the highest transparency characterised the waters of Lake Jeleń, yet in spring KSc visibility was by 50 cm smaller than in summer and was 4.1 m, which must have been caused by a large con-centration of suspensions in the circulation period. In the remaining lakes these values ranged from 1.8 to 3 m, which evidenced a large quantity of organic matter in water.

5 The influence of catchment use on recent transformations of water properties in chosen lobelia lakes Fig. 2. Vertical temperature distributions in investigated lakes in January 2003 (A), April 2003 (B) and July 2003 (C), 1 Jeleń, 2 Pipienek, 3 Cechyńskie Wielkie, 4 - Cechyńskie Małe, 5 Głęboczko, 6 Stary Staw The comparison of the author s own measurements of July 2003 with those published by Kraska penetration conditions in the majority of lakes occur during spring circulation and at the et al. (1994) reveals a considerable deterioration beginning of the formation of summer of Secchi disc visibility indicating an increase in the stratification. This is caused by a high seasonal trophy of the lakes in the last 12 years. concentration of both autochtho-nous and A more objective measure of transparency is the photic zone scope determined by the depth of the transmission of 1% of light absorbed by the water surface. Fig. 3 shows that the worst light allochthonous suspensions, coming from thaw. Most lakes in spring had rather similar light-ing conditions. The photic zone reached in them a depth of about m.

6 200 Fig. 3. Distributions of relative irradiance in April (A) and July 2003 (B), 1 Jeleń, 2 Pipienek, 3 Cechyńskie Wielkie, 4 Cechyńskie Małe, 5 Głęboczko, 6 Stary Staw With the stabilisation of the summer thermaldensity stratification, the differences in relative lighting of waters of particular reservoirs rose considerably. In July in Lake Jeleń still the best optical conditions occurred, the photic zone covered a layer of almost 10 m, and total transmission was by al-most 13% bigger than total extinction. Such condi-tions indicate a good state of lake waters, low trophic level, and consequently rather low primary production. Dramatically worse radiation penetration was observed in Lake Cechyńskie Wielkie. The photic zone scope there fell by 80 cm in comparison with spring, and in the one-metre surface layer over 86% of light was absorbed. Such conditions result from strong eutrophication of the lake and its high organic production. In Lake Głęboczko the photic zone scope increased by over 1m to 6 m, and the extinction in the near-surface layer clearly di-minished. In Lake Cechyńskie Małe the conditions of light penetration remained basically unchanged, probably due to a decrease in the concentration of mineral suspension, and an increase in organic sus-pension, and also due to an inflow of humic substances in spring, which considerably limited light penetration. The vertical diversity of oxygen saturation of the investigated lobelia lakes in different phases of the thermal-oxygen regime is presented in fig. 4. In the winter stagnation phase all the discussed reservoirs were characterised by a relatively good oxygen saturation, despite complete ice cover. In all the lakes oxygen occurred to the bottom, which certainly was the effect of a good mixing of water dur-ing autumn circulation. Due to the bradymictic character of the majority of the investigated reservoirs, the April measurement corresponded to the conditions of the initial phase of the development of summer thermal-oxygen stratification. Only in the case of Lake Jeleń the observed distribution was close to homooxygeny, caused by the occurrence of strong and long-lasting winds. In the remaining lakes, supersaturations up to 120% were observed in layers to a depth of 5 m. In none of the lakes oxygen saturation at the bottom fell below 60%.

7 The influence of catchment use on recent transformations of water properties in chosen lobelia lakes Fig. 4. Vertical distributions of water oxygen saturation of investigated lobelia lakes in September 2002 (A), January (B), April (C) and July 2003 (D), 1 Jeleń, 2 Pipienek, 3 Cechyńskie Wielkie, 4 - Cechyńskie Małe, 5 Głęboczko, 6 Stary Staw The most favourable oxygen conditions during the summer thermal-oxygen stratification occurred in lakes: Jeleń, Głęboczko and Cechyńskie Małe. They were characterised by clinograde distributions without distinct depletions within near-bottom layers. The measurement performed in July 2003 in Lakes Głęboczko and Cechyńskie Małe revealed that they were in the initial phase of the development of the oxygen distribution called negative heterograde, which is manifested in the diagram presenting the state of September In Głęboczko the mean oxygen saturation of the hypolimnion reached as much as 47%, which according to this index enables classification of the lake as class one of water cleanness. In Cechyńskie Małe the mean oxygen saturation of the hypolimnion was 27%, which also re-flects quite good oxygen conditions. In the three re-maining lakes Pipienek, Cechyńskie Wielkie and Stary Staw there was an extremely clinograde distribution with a zero oxygen saturation of the hypo-limnion. This results from higher water fertility than in the first three lakes. The worst qualitative conditions of water were observed in Lake Cechyńskie Wielkie. The best oxygen saturation of water masses characterised Lakes Głęboczko and Jeleń, which was an effect of considerable volume, good oxygen saturation during circulation phases and small fertility, not causing substantial oxygen supersaturations or deficits (Lampert, Sommer, 1996). Only slightly worse oxygen conditions, being the result of a little more fertile water, occurred in Lake Cechyńskie Małe. The three remaining reservoirs represent a more advanced succession stage. Both mineralization and proper conductivity, dependent on it, of the water of the investigated lakes is very low. It ranges from about 39 mg/l (63 µs/cm) in Lake Jeleń to 65 mg/l (105µS/cm) in Pipienek. Such low concentrations of salts dissolved

8 202 in water may signify the domination of rain alimentation. The spring values of electrolytic conductivity corresponded to the conditions of a complete mixing of waters in reservoirs, which is evidenced by a lack of distinct gradients. During the spring circulation the similarities, relationships and influences among the discussed lakes became clear. Both lakes Cechyńskie were characterised by the same conduc-tivity and mineralization (75 µs/cm; 46 mg/l); Lake Cechyńskie Wielkie probably exerts strong influence on Lake Cechyńskie Małe situated lower in the lake system. A similar situation is the case for lakes Głęboczko and Stary Staw, where the salt concentration is identical (86 µs/cm; 53 mg/l). Stary Staw is intensely supplied with water from the preceding Lake Głęboczko. In terms of hardness, the discussed lobelia lakes should be classified as very soft water class, which is a specific feature of this type of reservoirs (Szmal, 1959). In none of them general hardness exceeded 1.5 mval/l (Tab. 3). The waters of the discussed lakes represent the most widespread and characteristic of the zone of forest belt bicarbonatecal-cium type. A low contribution of magnesium com-pounds with regard to calcium compounds was re-corded in them; their average ratio was from about 1:6 in Cechyńskie Małe to 1:26 in Głęboczko. Gen-erally in all the discussed lakes, the concentration of chlorides and sodium is low and does not cross the border of class one of water cleanness. Table 3. Chemical structure of water of investigated lakes Lake Jeleń Pipienek Cechyńskie Wlk. Cechyńskie Małe Głęboczko Stary Staw S surface, B bottom Cl Ca Mg Na K hardness [mval/l] [mg/l] general carbonate S B S B S B S B S B S B In the annual cycle, some regularities in the occurrence of nutrients were observed. Phosphates and phosphate phosphorus had very low concentrations or did not occur at all in January and April. Slightly higher concentrations of phosphorus compounds were recorded in July (Tab. 4) when waters flowed in from arable fields. The absence of phosphate ions most probably indicates their exhaustion in the biomass production, while their presence in surface layers of lakes indicates that they were not used up yet or that they were supplied from bottom sediments of the bottom active during circulation. Table 4. Concentration of main nutrients in waters of investigated lobelia lakes in July 2003 Lake Nutrient [mg/l] PO4-P NO3-N NH4-N NO2-N S surface, B bottom, n.r. not recorded Jeleń Pipienek Cechyńskie Wielkie Cechyńskie Małe Głęboczko Stary Staw S n.r n.r n.r B n.r S B n.r. n.r. n.r. 0.5 n.r. n.r. S n.r. n.r B n.r S B

9 Limnological Review 5 (2005) The measurement in April revealed the lowest concentrations of nitrogen compounds. In none of the lakes nitrate nitrogen was recorded, and nitrite and ammonium nitrogen occurred only in trace amounts. In January only small concentrations of nitrate and ammonium nitrogen were recorded at the bottom. The presence of NH 4 -N may have resulted from decomposition of organic matter accumulated in bottom sediments with the participation of putrefactive bacteria, while NO 3 -N may have been released from sediments. In July all forms of nitrogen were recorded in the investigated lakes. The highest concentrations of ammonium nitrogen at the bottom were recorded in relatively strongly eutrophicated and surrounded by fields lakes Pipienek and Stary Staw, as well as in Lake Jeleń also surrounded by fields and additionally polluted by inflows from holiday centres. In the first two reservoirs the high NH 4 -N content was most probably the result of the decomposition of dead biomass accumulated above the bottom. Together with ammonium nitrogen there are also some quantities of nitrite nitrogen. As they constitute a transitory form, in surface layers most probably they came from the oxidation of N 2 molecules taken from the air. In the case of Lake Jeleń, ammonium nitrogen can come not only from the de-composition of matter produced in it but also from sewage inflowing from numerous summer houses around. Lake Cechyńskie Małe was the only reservoir in which nitrates were recorded in near-bot-tom layers, in anaerobic conditions they could be reduced to NO 2. Only there, as well, no ammonium ions were recorded at the bottom, which may indi-cate that the whole produced matter was decompo-sed in the water mass. In conclusion it can be stated that the investigated lakes are characterised by low concentrations of the main nutrients; their contribution does not exceed the values established for class one of the cleanness of inland surface waters. The waters of the investigated lobelia lakes can generally be classified as neutral or with slightly acidic reaction, and in the conditions of summer exhaustion of CO 2 in the surface layer, as slightly alkaline. Conclusions Concluding the results of the performed research, it can be assumed that the lowest trophic state characterises Lakes Jeleń and Głęboczko, as the features of their basins and waters accumulated in them enable the lakes to be classified as α-mesotrophic but close to β-mesotrophic. Such water conditions are confirmed, among others, by the lush population of all the indicator plants. Lake Cechyńskie Małe, due to its relatively high resistance to external pressure, as well as limited intensity of threats, has a relatively well maintained specific natural character at a level of β-mesotrophy slowly changing into eutro-phy. Stary Staw, Cechyńskie Wielkie and Pipienek, according to most classifications, can be treated as moderately eutrophic but still quite close to β-meso-trophy. Their eutrophication is evidenced by the low contribution of lobelia species in the whole vegetation population, by oxygen deficits covering a high percentage of water masses and relatively poor con-ditions of light transmission. In the light of the performed research it can be stated that the chosen lobelia lakes are characterised by the occurrence of numerous differences of the water environment in comparison to other lakes of Poland as well as of the region. Within the investigated group considerable differences were also observed resulting mainly from local environmental conditions and individual features of particular lakes. Although the natural character of the chosen lobelia lakes was observed to be quite well maintained, it should be remembered that together with the growing agricultural use of the catchments and lakes and their tourist exploitation, the burden on the reservoirs increases and the threat with intensified eutrophication occurs. For this reason, it is important for the areas surrounding such valuable limnological systems to be covered by protection and be used in a sustainable manner without the threat of degradation. References Gos K., Bociąg K., 2003, Uzupełnienie do listy jezior lobeliowych w Polsce, Bad. Fizjogr. Pol. Zach., 52, Kajak Z., 2001, Hydrobiologia Limnologia, Ekosystemy wód śród-lądowych, PWN, Warszawa. Kraska M., 1994, Jeziora lobeliowe, Charakterystyka, funkcjonowanie i ochrona, Idee Ekol., seria: Szkice, 5, t. 7, UAM, Poznań. Lampert W., Sommer U., 1996, Ekologia wód śródlądowych, PWN, Warszawa. Sylwestrzak J., 1977: Pojezierze Bytowskie, Monografia geograficzno-ekonomiczna, Ossolineum, Gdańsk, 247.

10 204 Szmal Z., 1959, Badania hydrochemiczne jezior lobeliowych Pomorza Zachodniego, PTPN, Prace Kom. Biol., 19, Szmeja J., 1996, Rejestr polskich jezior lobeliowych, Fragm. Flor. Geobot. Ser. Polonica, 3, Streszczenie W okresie od września 2002 do marca 2004 przeprowadzono cztery serie pomiarów grupy jezior lobeliowych Pojezierza Bytowskiego (ryc. 1). Celem pracy jest charakterystyka fizycznolimnologiczna na tle aktualnego stanu ich środowiska wodnego wraz z próbą prognozy kierunku i nasilenia dalszych przemian jakościowych. Rozpoznanie fizycznolimnologiczne obejmowało pomiary rozkładów temperatury, przewodności właściwej, oświetlenia wody oraz zawartości tlenu. Pobrano próby do analiz laboratoryjnych, których zakres obejmował ogólną strukturę jonową, a takŝe zawartość głównych nutrientów. Badane zbiorniki charakteryzują się istotnym zróŝnicowaniem struktury uŝytkowania zlewni (tab. 1) oraz warunków morfometrycznych niecek (tab. 2). Pod względem ustroju termiczno-dynamicznego omawiane jeziora zaliczono do typu dymiktycznego, z których pięć (Pipienek, Cechyńskie Wielkie, Cechyńskie Małe, Głęboczko i Stary Staw) reprezentuje typ bradymiktyczny (ryc. 2), natomiast Jeleń, typ tachymiktyczny. Stwierdzono wyraźne róŝnice właściwości optycznych wody między badanymi jeziorami między sobą, jak i w obrębie poszczególnych zbiorników (ryc. 3). Najmniej sprzyjające warunki dla przenikania światła w większości jezior panują w czasie cyrkulacji wiosennej i na początku kształtowania się stratyfikacji letniej. Najlepszym natlenieniem mas wodnych odznaczały się jeziora Głęboczko i Jeleń (ryc. 4), natomiast najgorsze warunki tlenowe panowały w jeziorach: Cechyńskie Wielkie i Stary Staw. Przewodność właściwa oraz mineralizacja wody badanych jezior jest bardzo niska (39 mg/l 65 mg/l). Podstawową strukturę chemiczną prezentuje tab. 3. Wody omawianych zbiorników stanowią najbardziej rozpowszechniony typ wodorowęglanowo-wapniowy, a pod względem twardości zaliczono je do klasy wody bardzo miękkiej. Charakteryzują się one równieŝ niskimi stęŝeniami nutrientów (tab. 4). W świetle dokonanych badań stwierdzić moŝna, Ŝe wybrane jeziora lobeliowe charakteryzują się występowaniem licznych odrębności środowiska wodnego w stosunku do innych jezior Polski, a takŝe regionu. RównieŜ wewnątrz badanej grupy zaobserwowano znaczące róŝnice wynikające głównie z lokalnych warunków środowiska oraz indywidualnych cech poszczególnych zbiorników. Najbardziej naraŝone na zanik naturalnej specyfiki środowiska wodnego są jeziora małe, płytkie i dodatkowo zasilane nutrientami i zanieczyszczeniami ze źródeł obszarowych i punktowych, połoŝone w zlewniech ze znacznym udziałem obszarów rolniczych.