Limnological Review Directions 7, 2: 79-85 of Limnological Succession of Lobelia Lakes of Bory Tucholskie 79 Directions of Limnological Succession of Lobelia Lakes of Bory Tucholskie Karolina Pacewicz, Kamil Nowiński University of Gdańsk, Faculty of Biology, Geography and Oceanology, Institute of Geography, Department of Lakeland Geography, Dmowskiego 16a, 80-264 Gdańsk, e-mail: kmundo@wp.pl, geokamil@univ.gda.pl Abstract: In August 2006 a series of the expeditionary field research was performed. 17 lobelia lakes situated, according to the borders marked out by Kowalewski (2002), in the north-western part of Bory Tucholskie, were selected as the research subject. The selected reservoirs are characterized by small areas, below 20 ha and depths up to 10 m. They are also characterized by small catchments and, in terms of hydrology, they demonstrate an outflow-less character, with an exception of periodically flow-through Lake Kłodzko Małe. The aim of the paper was an assessment of the current condition of their water environments as well as of the direction and rate of limnological succession occurring in these small forest reservoirs. Measurements were made in places of maximum depths and covered vertical distributions of: temperature, specific conductivity, oxygen saturation, relative irradiance, ph reaction and chlorophyll a. Additionally, water transparency was measured using the Secchi disc, and water samples for laboratory analyses were taken in order to determine main nutrients content. The obtained results were presented in the form of graphs and processed using the Carlson method (TSI). On this basis, the following lakes were considered eutrophic: Kłodzko Małe, Moczydło, Nowoparszczenickie, Długie, Żabionek and Trzemeszno Małe. The ß-mesotrophic status was assigned to lakes Zmarłe Duże, Linówko, Piecki, Kły, Sosnówek, Chińskie and Bardze Małe. Level α-mesotrophy still occurs in lakes: Czarne near Zapceń and Sporacz, and oligotrophy level in Nawionek and Moczadło. In the examined group of the lobelia lakes of Bory Tucholskie it was established that, irrespective of the size of water tables, lakes of maximum depth up to 6 m have already achieved the eutrophic stage, while these with depths above 6 m, reveal considerable trophic diversity, dependent mainly on the size and the use of their catchments. Key words: lobelia lakes, Bory Tucholskie, the limnological succession, Carlson, Trophic State Index Introduction Lobelia lakes are a small group of exceptionally naturally valuable limnic systems, usually representing the initial stage of limnological succession. In Poland these reservoirs are distinguished on the basis of the floristic and phytocenotic criterion, i.e. the occurrence of at least one of three relict indicator plants: water lobelia (Lobelia dortmanna), lake quillwort (Isoëtes lacustris) and shoreweed (Littorella uniflora) (Szmeja 1996). These species are regarded as phyto-indicators of lakes with best preserved primeval natural value. On the territory of Poland there are 175 documented lobelia lakes (Gos and Bociąg 2003), in majority situated in the Pomorskie Lakeland, of which 17 (Table 1), situated within Bory Tucholskie, were selected as research objects. The aim of the paper was an assessment of the current condition of their water environments as well as of the direction and rate of limnological succession occurring in these small, forest reservoirs. In August 2006, during the summer thermaldensity stratification, a series of expeditionary field research was carried out. Its purpose was the investigation of the internal structure of reservoirs and the present trophic state. The measurements were made from a pontoon boat, along columns corresponding to maximum depths of lakes and their scope covered vertical distributions of: temperature, specific conductivity, oxygen saturation, water lighting, ph reaction and chlorophyll a. For comparative purposes, water transparency was measured using the Secchi disc. Also surface and near bottom water samples were taken for laboratory analyses (nitrogen and phosphorus compounds). Additionally, land mapping was performed together with a survey of possible pollution emitters.
80 Karolina Pacewicz, Kamil Nowiński Table 1. Geographical situation and basic morphometric data of examined lakes No Name φ λ Area Depth [ N] [ E] [ha] [m] 1 Żabionek 53 50 17 37 30 7.0 2.4 2 Sosnówek 53 49 17 37 10 4.1 9.0 3 Długie 53 48 30 17 37 30 13.6 3.9 4 Moczadło 53 49 17 38 3.6 10.0 5 Trzemeszno Małe 53 50 17 45 2.3 4.9 6 Bardze Małe 53 46 10 17 26 20 6.0 8.2 7 Sporacz 53 46 50 17 25 40 11.5 6.1 8 Linówko 53 46 45 17 22 10.5 3.5 9 Kłodzko Małe 53 44 20 17 22 18.0 3.9 10 Piecki 53 54 50 17 33 6.0 6.6 11 Nawionek 53 54 17 32 12.5 8.5 12 Czarne n. Zapceń 53 58 30 17 28 8.5 7.4 13 Nowoparszczenickie 53 54 17 28 2.3 4.9 14 Zmarłe Duże 53 56 17 48 8.6 6.2 15 Kły 53 56 17 47 18.7 6.2 16 Moczydło n. Brusy 53 56 30 17 47 1.4 2.8 17 Chińskie 53 52 17 50 4.4 9.8 Fig. 1. Location of examined lakes. a lakes, b examined lobelia lakes, c rivers, d roads, e railway line, f border of the Bory Tucholskie National Park, g border of the Zaborski Landscape Park. Lakes numbers as in Table 1
Directions of Limnological Succession of Lobelia Lakes of Bory Tucholskie 81 Study area 17 lobelia lakes (Fig. 1) situated, according to the borders marked out by Kowalewski (2002), in the north-western part of Bory Tucholskie, within the Pomeranian province, were selected as the research subject. According to the physical-geographical division of Poland by Kondracki (1994), the research area covers fragments of three mesoregions: Bory Tucholskie (lakes: 3, 4, 5, 17), the Charzykowska Plain (lakes: 1, 2, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17) and Krajeńskie Lakeland (lakes: 8 and 9). In hydrographic terms, all the lakes, apart from No. 17 (Chińskie, also called Cyrkowe) situated in the River Wda drainage basin, are located in the Brda drainage basin. Both mentioned rivers are left-bank tributaries of the Vistula River. The research area is made of the Brda outwash and patches of moraine plateaus (Nowaczyk 2006) formed during the Pomeranian Stadial of the Baltic Glaciation. The lake basins situated there demonstrate a diversity of size and shapes, corresponding mainly to their channel, melt-out or evorsion origin, and location in hydrographical units differing in the water resources. The investigated reservoirs have small areas (Table 1), below 20 ha (the biggest lake Kły 18.7 ha) and depths up to 10 m (Moczadło). Their catchments are also small, because of the outflow-less character of the discussed lakes (an exception is periodically flow-through Lake Kłodzko Małe) and, except for the catchment of Lake Kłodzko Małe, are in the 100% covered with forests, mainly coniferous (Kraska 1994). Alimentation from the catchment is in such conditions marginal and usually limited to a disorganized surface flow, slowed down by forest covering of the land, and possibly groundwater drainage, which on the one hand is favourable for the maintenance of the natural value of the explored water environments, and on the other, hampers the exchange of waters, which occurs here mainly in the evaporation-precipitation way. Results In order to assess the current stage of the limnological succession, the results of author s own research results were used. They were presented, among others, using the Carlson s TSI method (Trophic State Index) which takes into consideration such properties of the water environment as: the scope of Secchi disc visibility, total phosphorus concentration and chlorophyll a concentration. Reservoirs for which TSI value does not exceed 30 are considered oligotrophic, whereas the border between mesotrophy, and eutrophy is here value 50 of the index. The highest general TSI value (Fig. 2), of 61.6, was observed in a relatively big and shallow Lake Kłodzko Małe, the only flow-through reservoir in the examined group, with a relatively large alimentation area, including anthropogenic terrain. The value of the remaining indicators also exceeds the eutrophy border here. Quite an advanced trophy is confirmed by the extent of the photic zone reaching only 3 m (Fig. 5) and oxygen saturation distribution with supersaturation of about a dozen percent within the 2-metre near-surface layer (Fig. 3). From 2 m of depth a rapid decrease in oxygen saturation was observed to 2% (0.19 mg l -1 ) above the bottom. Specific conductivity of water also differs considerably here from the re- Fig. 2. Trophic State Index according to Carlson. a TSI (SD), b TSI (Chl), c TSI (TP), d TSI
82 Karolina Pacewicz, Kamil Nowiński Fig. 3. Vertical oxygen saturation distribution of examined lake waters Fig. 4. Vertical distribution of chlorophyll a concentration
Directions of Limnological Succession of Lobelia Lakes of Bory Tucholskie 83 maining discussed lakes and is as high as 120 µs cm -1, with about 20-40 µs cm -1 recorded in the majority of lakes in the examined group. The remaining lakes, for which general TSI value was over 50, are characterized by depths to 5 m and are classified as reservoirs with polymictic type of mixing; these are: Trzemeszno Małe, Żabionek, Długie, Nowoparszczenickie and Moczydło (an exception in this depth group is Linówko considered ß- mesotrophic though close to eutrophy). Oxygen saturation distributions have a shape close to clinograde, with small supersaturations mainly in the 3-metre near-surface layer and with a rapid saturation decrease close to 0 at the bottom. The shallowest extent of the euphotic zone, reaching slightly below 2 m, was observed in Lake Żabionek. This was caused by a high primary production, which is confirmed by TSI (Chl) and vertical chlorophyll concentration distribution. An influence on the considerable decrease in water transparency of Lake Żabionek is also exerted by the brownish colour of water caused by an inflow of humic substances from the catchment. Equally poor light transmission occurred in acidic Lake Nowoparszczenickie and in Moczydło (No. 16), with a neutral reaction. In both lakes, relatively high total phosphorus concentrations as well as chlorophyll a concentration were recorded. Lake Długie had the highest chlorophyll concentration among all the 17 examined reservoirs and consequently the highest TSI (Chl), and its vertical distribution (Fig. 4) was characteristic of eutrophic lakes (Kajak 2001). Lakes in which the scope of Secchi disc visibility is not over 2 m are also considered eutrophic. According to these characteristics, besides lakes mentioned earlier, the following lakes were considered eutrophic too: Bardze Małe and Kły (Fig. 6) which on the basis of the general Carlson s index were categorised as ß- mesotrophic. The remaining reservoirs, for which general TSI value was between 40 and 50, are: Chińskie, Sosnówek, Piecki, Linówko and Zmarłe Duże. They are still characterized by quite good conditions of water environment. The best oxygen conditions were observed in Sosnówek where up to 8 m oxygen saturation was over 100%, chlorophyll concentration distribution indicated the succession stage on the level of early α -mesotrophy, the same as TSI (SD) and TSI (TP), whereas TSI (Chl) of eutrophy. Low trophy was here confirmed by the extent of the euphotic zone reaching the bottom (9 m). Slightly shallower photic zones covered 6 m in lakes Bardze Małe and Kły, 5 m in Zmarłe Duże and Chińskie, and 4 m in Piecki. In the Fig. 5. Relative irradiance of examined lobelia lakes waters
84 Karolina Pacewicz, Kamil Nowiński last two, vertical chlorophyll distribution was close to that characteristic of mesotrophic waters of dimictic lakes (Kajak 2001), with the maximum of the primary production in metalimnion, with the availability of biogenic substances and good photic conditions. Lakes Sporacz and Czarne near Zapceń, acidic reservoirs of total depths between 6 and 8 m and areas about 10 ha, on the basis of general TSI were classified as α -mesotrophic because of the complete lack or scarce amounts of total phosphorus. In both cases TSI (Chl) again indicates a higher succession stage, however there are good photic conditions here and their vertical chlorophyll concentration distributions are characteristic of waters of low trophy. The lowest degree of the advancement of limnological succession, on the oligotrophy level (according to general TSI value), characterized lakes Moczadło and Nawionek. The first is a lake of a very small area (3.6 ha) and the largest depth (10 m) among the 17 examined lakes. Features of stratified dimictic lakes with an exceptionally long circulation cycle as for such a small, forest reservoir were observed here. The second one is slightly shallower (8.5 m) and bigger (12.5 ha), and polimictic. The oxygen saturation distribution in both cases takes a form of positive heterograde which indicates an intensified primary production within the layer of the biggest thermal jump, with a high water transparency allowing for photosynthesis and consequently oxygen production. According to Kajak (2001), such a situation is observed in oligo- or mesotrophic reservoirs. Low trophy is confirmed by a large extent of euphotic zone in Nawionek embracing the entire water mass, and in Moczadło reaching 9 m, as well as an even profile and very low chlorophyll concentration in the entire water column. Specific conductivity of water of Lake Moczadło was 35 µs cm -1 and of Nawionek 73 µs cm -1, which confirms low contents of all dissolved mineral compounds. Despite establishing a good state of maintenance of primeval limnological features of both reservoirs, again TSI (Chl) value was here much higher than of the remaining indices. In all the examined reservoirs low contents of biogenic substances were recorded, which results from their forest location and quite good isolation from areas transformed by man. Recent transformations of limnic environments proceed mainly in the direction of eutrophy, with a relative balance of the influence of environmental factors or of dystrophy, with a domination of some components of the limnic system, e.g. humic acids (Lange 1993). The location of the examined lakes on postglacial formations and in majority among pine forests, is theoretically favourable for maintaining the ph advantageous for the growth and the life of lobelia species, but in case of excessive, disturbing the Fig. 6. Scope of Secchi disc visibility Fig. 7. ph reaction of water of examined lobelia lakes
Directions of Limnological Succession of Lobelia Lakes of Bory Tucholskie 85 balance, inflow of humic substances, can lead to their dystrophication. On the basis of the data collected in summer 2006, it was established that 9 of the examined reservoirs had neutral ph reaction, i.e. from 6.5 to 7.5, and 8 had acidic reaction (Fig. 7). This can suggest that lakes Żabionek, Sosnówek, Długie, Bardze Małe, Sporacz, Linówko, Czarne near Zapceń and Nowoparszczenickie are ageing disharmoniously and undergoing dystrophication, whereas the remaining reservoirs are progressing in the direction of eutrophy. Conclusions On the basis of the obtained results it is possible to state that the main factors influencing the trophic state of the water environment of these lakes are basin morphometry and the size and use of the alimentation area. The slowest ageing occurs in lakes with relatively large depths but also situated in catchments anthropogenically unchanged, only moderately supplied with humic acids. In the examined group of lobelia lakes of Bory Tucholskich it was established that irrespective of the size of water tables, lakes with the maximum depth up to 6 m have already achieved the eutrophic stage, whereas those of depths over 6 m, demonstrate a considerable trophic diversity dependent above all on the size and use their catchment, and consequently on the supply of all substances hastening the increase in trophy. At present the biggest threat to the specificity of lobelia lakes of Bory Tucholskie is their tourist exploitation which consists above all in trampling to the ground and littering of shores, establishing bathing beaches, which leads to an increase in the delivery of biogenic substances, and consequently causes extinction of lobelia species. The presence of man also contributes to mechanical damaging of these plants occurring in shallow waters. The threats also come from all other anthropogenic forms of transformation in lakes surrounding, e.g. forest clearings, locating the arable land or pastures, holiday cottages, camping sites in their vicinity, fluctuations of water level caused by artificial damming of outflow etc. The increase in all forms of anthropopressure, increases the burden of the reservoirs and threat of intensified eutrophication. For this reason, it is particularly important that the areas surrounding so valuable limnic systems were used in a sustainable way, not threatening with degradation. References Gos K., Bociąg K., 2003, Uzupełnienie do listy jezior lobeliowych w Polsce, Bad. Fizjogr. nad Polską Zach., Ser. Botanika 52: 151-158. Kajak Z., 2001, Hydrobiologia-Limnologia. Ekosystemy wód śródlądowych, Wydawnictwo Naukowe PWN, Warszawa, p.360. Kondracki J., 1994, Geografia Polski, Mezoregiony fizyczno-geograficzne, Wydawnictwo Naukowe PWN, Warszawa, p. 340. Kowalewski G., 2002, Granice Borów Tucholskich, [in:] Banaszak J., Tobolski K. (eds) Park Narodowy Bory Tucholskie na tle projektowanego rezerwatu biosfery, Charzykowy: 121-138. Kraska M. (ed.), 1994, Jeziora lobeliowe. Charakterystyka, funkcjonowanie i ochrona, Idee Ekol. 6-7, ser. Szkice nr 4 i 5, UAM, Poznań. Lange W. (ed.), 1993, Metody badań fizycznolimnologicznych, Wydawnictwo Uniwersytetu Gdańskiego, Gdańsk, p. 175. Nowaczyk B., 2006, Geneza jezior na sandrze Brdy, [in:] Kowalewski G., Milecka K. (eds) Jeziora i torfowiska Parku Narodowego Bory Tucholskie, Oficyna Wydawnicza Forest: 43-52. Szmeja J., 1996. Rejestr polskich jezior lobeliowych, Fragm. Flor. Geobot. Ser. Polonica 3: 347-367. Tobolski K., 2006, Wprowadzenie do Parku Narodowego Bory Tucholskie, [in:] Kowalewski G., Milecka K. (eds) Jeziora i torfowiska Parku Narodowego Bory Tucholskie, Oficyna Wydawnicza Forest: 13-41.
86 Karolina Pacewicz, Kamil Nowiński