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Eutrophication history of Lake Arendsee (Germany)1
ELSEVIER
Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998) 85–96
Eutrophication history of Lake Arendsee (Germany) 1 Burkhard W. Scharf * UFZ – Umweltforschungszentrum Leipzig-Halle GmbH, Sektion Gewa¨sserforschung Magdeburg, Am Biederitzer Busch 12, D-39114 Magdeburg, Germany
Abstract At the maximum depth of Lake Arendsee (49.5 m) only a thanatocenosis could be found in a freeze core with a length of 52 cm. According to varve-counting, the fauna changed between 1960 and 1972, characteristic of mesotrophic to eutrophic state. The cause for eutrophication could be correlated with the sewage loading of the town of Arendsee and the drainage of Lake Fauler See into Lake Arendsee. Lake Arendsee did not yet recover from the loading because of the long retention time of 114 years. The investigation of the living meio- and macrobenthos of Lake Arendsee shows that the lower profundal is not colonized. 1998 Elsevier Science B.V. All rights reserved. Keywords: eutrophication; palaeolimnology; Arendsee; Germany; varves; precipitation; ostracoda; Fauler See
1. Introduction Lake Arendsee is located in the eastern part of Northern Germany (Fig. 1). Before the formation of Lake Arendsee, another lake, Lake Wendischer See, existed in the northern part of Lake Arendsee. A relic of this lake is the marl bench in Lake Arendsee in front of the village Zießau. The marl with an age of at least 4000 years is much older than Lake Arendsee (Ro¨nicke, Trettin, unpublished). The present lake was formed due to the solution of a salt dome in the underground (sinkhole). The main collapse of the earth surface was in the year 822 A.D. and the second smaller one in 1685. The littoral zone is quite narrow with steep slopes due to the nature of its formation (Fig. 1). An exception is the marl Ł Fax:
C49 391 8507555; E-mail: [email protected] This paper is dedicated to my supervisor Prof. Dr. Harald Sioli in honour of his 85th birthday (25 August 1995).
bench in the bay at the northern shore. The central part of the lake bottom is not flat, probably because the earth’s surface was broken into plates during the sinking. At Lake Arendsee two limnological restoration measures to control eutrophication were carried out: the first in 1976 by a hypolimnetic withdrawal (Klapper, 1991, 1992) and the second in 1995 by flushing of naturally deposited marl (Ro¨nicke et al., 1995). In this respect it is interesting to know when the eutrophication started and what were the causes for eutrophication. In the present paper, the problem of eutrophication is probed by a palaeolimnological investigation of the meiobenthos, especially of the Ostracoda and Cladocera. In addition the distribution of the living species in Lake Arendsee was studied for interpreting the palaeolimnological results.
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Fig. 1. Bathymetric map of Lake Arendsee with the location of the freeze core sample site (FC).
2. Methods The methods for studying the living and fossil ostracods and cladocerans have been described in Scharf et al. (1995). The distribution of living ostracods were analysed from the years 1992 to 1995. The core for dating the sediment (Fig. 4) was taken with a gravity corer according to Niederreiter, Mondsee, Austria (similar to the sampling equipment described in Danielopol and Niederreiter, 1990) at the same place at which the freeze core was taken for studying the fossil Ostracoda and Cladocera (Scharf et al., 1995).
For dating, the varves were counted using the white layers. The white layers, identified as calcite layers, are formed by autochthonous calcite precipitation, each year in the months June and July, as we know by sediment trap experiments (Hentschel, unpubl. data). Therefore, it was easy to date the core. The result was confirmed by the determination of 137 Cs. The peak of the Chernobyl accident in 1986 was at a depth of 6 cm (Hupfer et al., in prep.). The analysis of the white layers in the sediment cores was performed by using SEM (DSM 942, Zeiss) with a micro-analysing system (EDX, Oxford).
PLATE I Living and fossil Ostracoda from Lake Arendsee. The arrows point at the anterior part of the animal. (1) Limnocythere (Limnocythere) inopinata, female, carapace, dorsal view, length 0.68 mm. (2) Limnocythere (Limnocythere) inopinata, female, carapace, dorsal view, length 0.70 mm. (3) Limnocythere (Limnocythere) inopinata, female, carapace, ventral view, length 0.72 mm. (4) Limnocythere (Limnocythere) inopinata, female, carapace, lateral view, length 0.64 mm, height 0.37 mm. (5) Cytherissa lacustris, female, carapace, lateral view, length 0.95 mm, height 0.59 mm, fossil. (6) Metacypris cordata, female, carapace, ventral view, length 0.57 mm. (7) Candona candida, female, carapace, lateral view, length 1.10 mm, height 0.62 mm, fossil. (8) Fabaeformiscandona protzi, male, right valve, lateral view, internal, length 1.04 mm, height 0.55 mm, fossil. (9) Fabaeformiscandona protzi, female, carapace, dorsal view, length 1.08 mm, width 0.42 mm, fossil. (10) Fabaeformiscandona protzi, female, right valve, lateral view, internal, length 1.02 mm, height 0.53 mm, fossil.
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The names of the Ostracoda are written according to Kempf (1980, 1991).
3. Results The living Ostracoda of Lake Arendsee are shown in the Plates I–III, their distribution on the northern slope in relation to the water depth in Fig. 2. The occurrence of some fossil ostracods is shown in Fig. 3. The lowest depth on the slopes at which living ostracods and harpactids could be found was 23 m. In comparison to the slopes the sediment on the hills in the middle of Lake Arendsee was colonized by living Ostracoda (Table 1), Harpacticoida and Tardigrada up to a depth of 34 m. This can be explained by different sediments. On the hills very little limnetic sediment was found, mainly sand, probably from the ancient earth’s surface. The limnetic sediment was very possibly transported into deeper parts of the lake during storms in the circulation periods. On the hills there was much less organic matter in the sediment; therefore no sulphur bacteria were found, in contrast to the slopes below 23 m. It is probably that the sulphur bacteria layer prevents the colonization of meio- and macrobenthos at the lower profundal (Wilhelmy and Scharf, 1996). For the ecological interpretation of the findings of ostracods on the slopes and in the freeze core the following comments are given to the special species. Limnocythere (Limnocythere) inopinata (Plate I) prefers sand as a substrate. With the eutrophication of Lake Arendsee the nutrient conditions improved and the population increased. Later on, the substrate conditions deteriorated so that the
population of L. inopinata diminished. Cytherissa lacustris (Plate I) is in central Europe characteristic of the profundal of oligotrophic and mesotrophic lakes. The subfossil occurrence in Fig. 2 is greater than the present one. This means that the living population is a relic. We find the same feature in Fabaeformiscandona protzi (Plate I). The valves of Cytherissa lacustris are so heavy that they are not transported into deep water and therefore are lacking in the freeze core samples. The same feature of distribution could be observed in Lake Laacher See (Kempf and Scharf, 1980). C. lacustris was not found in the lower part of the freeze core, indicating that the sediment was not appropriate for this species at that time (compare Lo¨ffler, 1969). Only a few living specimens of Metacypris cordata (Plate I) were found. They have a heavy carapace and live in the upper littoral. Therefore there is no fossil record in the freeze core. Candona candida (Plate I) lives in the littoral and profundal of oligo- and eutrophic lakes. In the freeze core there were only 2 valves of adult C. candida (at a depth of 46–48 and 26–28 cm), but some larval valves which could be easily transported by currents during the circulation. This means, that C. candida did not colonize the lower profundal in the last decades. Only larval valves of Fabaeformiscandona protzi (Plate I) were frequently detected in the freeze core samples for the same reason. A few valves of adult F. protzi were found in the freeze core at depths below 38 cm. Pseudocandona compressa (Plate II) is today abundant in the littoral; however, in the freeze core there were only a few larval valves at depths between 44 and 26 cm. Living and subfossil specimens of Fabaeformiscandona caudata (Plate II) were often found, but were not found in the freeze core.Cypria ophtalmica is able to swim very well and colonizes a lake
PLATE II Living and fossil Ostracoda from Lake Arendsee. The arrows point at the anterior part of the animal. (1) Pseudocandona compressa, female, juvenile, carapace, dorsal view, length 0.89 mm. (2) Pseudocandona compressa, female, carapace, lateral view, length 0.95 mm, height 0.57 mm. (3) Pseudocandona compressa, male, juvenile (?), carapace, lateral view, length 0.89 mm, height 0.55 mm. (4) Fabaeformiscandona caudata, female, carapace, lateral view, length 1.22 mm, height 0.59 mm, fossil. (5) Fabaeformiscandona caudata, female, right valve, lateral view, internal, length 1.21 mm, height 0.58 mm, fossil. (6) Physocypria kraepelini, male, carapace, lateral view, length of the left valve 0.61 mm. (7) Physocypria kraepelini, female, carapace, lateral view, length 0.62 mm, height 0.42 mm. (8) Ilyocypris decipiens, right valve, lateral view, external, length 1.03 mm, height 0.54 mm, fossil. (9) Ilyocypris decipiens, detail of (8), muscle scars.
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Fig. 2. Vertical distribution of Ostracoda species at the northern slope of Lake Arendsee. Black rectangle D occurrence of living individuals of a species; white rectangle D occurrence of single valves or empty carapaces (modified from Scharf et al., 1995).
from the littoral to the profundal. In Lake Arendsee it was abundant in the living and subfossil samples, and frequent in the freeze core at depths between 52 and 16 cm, as adults and instars. This species was perhaps the unique species which lived in the lower profundal of Lake Arendsee some decades ago. But it is more probable that the remains of C. ophtalmica were carried to this depth because C. ophtalmica is usually associated with Candona candida which did not live in this area as shown above. Cypria exsculpta which is less tolerant than the more common C. ophtalmica was not found in the samples with the living and subfossil animals. This species was abundant in the freeze core between 52 and 28 cm. Until now C. exsculpta was found only in the littoral and the upper part of the profundal in Europe. Therefore, it is probable that the carapaces and valves of this species
represent a part of the thanatocenosis at the maximum depth of Lake Arendsee. Only a few specimens of Physocypria kraepelini (Plate II) were found living in the littoral. Adults and instars of Cyclocypris laevis were frequent in the lower part of the freeze core. C. laevis prefers waters that are rich in vegetation. It probably did not live in the profundal, but the carapaces and the valves could very easily be transported by currents. When C. laevis disappeared it was followed by C. ovum. The reason for the change from C. laevis to C. ovum is unknown. The reduction of the most submersed macrophytes probably accounts for the disappearance of these two species. Three fossil remains of Ilyocypris decipiens (Plate II) were detected in the freeze core at a depth of 12–14 cm. Only a few individuals of this species were found in the samples with the living and the subfossil
PLATE III Living and fossil Ostracoda from Lake Arendsee. The arrows point at the anterior part of the animal. (1) Notodromas monacha, female, carapace, lateral view, length of the left valve 1.05 mm. (2) Notodromas monacha, male, carapace, lateral view, length of the left valve 1.14 mm. (3) Herpetocypris reptans, female, carapace, lateral view, length of the left valve 2.50 mm, height of the left valve 1.18 mm. (4) Isocypris beauchampi, female, left valve, lateral view, internal, length 1.17 mm, height 0.61 mm, fossil. (5) Cypridopsis vidua, female, carapace, ventral view, length of the right valve 0.66 mm. (6) Plesiocypridopsis newtoni, female, carapace, right valve, length 0.84 mm, height 0.56 mm. (7) Potamocypris smaragdina, female, carapace, lateral view, length of the left valve 0.81 mm, height of the left valve 0.46 mm. (8) Potamocypris unicaudata, female, carapace, lateral view, length of the left valve 0.83 mm, height of the left valve 0.47 mm.
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Fig. 3. Distribution of single ostracod species in a freeze core from Lake Arendsee (total number of ind./2 cm freeze core) (modified from Scharf et al., 1995).
B.W. Scharf / Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998) 85–96 Table 1 Distribution of Ostracoda at the northern slope and on the hills in the middle of Lake Arendsee Species
Slope
Limnocythere inopinata Cytherissa lacustris Metacypris cordata Candona candida Pseudocandona compressa Fabaeformiscandona caudata Fabaeformiscandona protzi Cypria exsculpta Cypria ophtalmica Physocypria kraepelini Cyclocypris laevis Cyclocypris ovum Ilyocypris decipiens Darwinula stevensoni Notodromas monacha Herpetocypris reptans Bradleystrandesia spec. Isocypris beauchampi Cypridopsis vidua Potamocypris smaragdina Potamocypris unicaudata Plesiocypridopsis newtoni
ž ž ž ž ž ž ž Ž ž ž Ž Ž ž ž ž ž Ž Ž ž ž ž ž
High in the middle of the lake
ž
ž ž
ž D living and fossil, Ž D only fossil.
animals. Only living Notodromas monacha (Plate III) was found in the reed zone of the lake. It was absent from freeze core samples. Herpetocypris reptans (Plate III) lives in the upper littoral, especially among macrophytes. Typically, only larval valves could be detected in the freeze core, at a depth between 48 and 12 cm. The carapaces and valves of adults of this big and heavy species are not far transported. The same observation was made at Lake Laacher See (Kempf and Scharf, 1980). Only two subfossil specimens of Isocypris beauchampi (Plate III) were found. This is important, because this species was very rare in Germany after the Second World War. Obviously, the colonization of this species was not successful in Lake Arendsee, although now this species is frequent in many waters in Germany, especially in young waters. Cypridopsis vidua (Plate III) also prefers to live among macrophytes. Its disappearance in the sediment of the freeze core is a hint at the extinction of the submersed macrophytes. Plesiocypridopsis newtoni (Plate III) and Potamocypris unicaudata (Plate III)
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are characteristic of water with a higher salinity (e.g. Hollwedel and Scharf, 1988). These two species were found in the freeze core at a depth between 18 and 10 cm, only two valves of Plesiocypridopsis newtoni at 16 to 14 cm. This could be an indication that at this time the salinity of the lake was higher than previously and later on. But the probability of a short-term change of the salinity of the whole lake is not great because of the long retention time of more than 100 years (Scharf et al., 1995). In this lake there are some springs with a higher salinity because of the location on a salt dome. It is more probable that these springs had a greater flow or higher salinity for a short time, so that in the vicinity more halophilic specimens could live. Due to the large volume of the lake the total salinity would not be changed significantly. P. smaragdina (Plate III) follows P. newtoni and P. unicaudata in the younger sediments of the freeze core. Living specimens of P. smaragdina were absent at the locations of Lake Arendsee where P. newtoni and P. unicaudata could be detected and vice versa. The Cladocera remains in the freeze core show a change in the community from the oldest part to the younger one (Scharf et al., 1995). By counting the varves it was possible to date the youngest sediment (Fig. 4). The ostracod fauna changed drastically at a depth of 18 to 10 cm of the freeze core. This corresponds with the years 1960 to 1972 (Fig. 4).
4. Conclusions Halbfass (1896) described oligotrophic features of Lake Arendsee (the definition of the trophic states were introduced later). At the beginning of the second half of this century the community was typical for a mesotrophic lake (Klapper, 1991, 1992). This can be proved, e.g. by the occurrence of many submersed macrophytes and Daphnia species. Since that time the trophic state of Lake Arendsee has changed dramatically. This change can be confirmed by zoological and palaeolimnological studies (compare Scharf et al., 1995): (1) The living fauna contains elements of a mesotrophic, eutrophic and polytrophic lake, the mesotrophic ones only as relic populations.
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Fig. 4. Occurrence of calcite layers in a sediment core from Lake Arendsee, taken on the 15 August 1995. The dating is based on the counting of the calcite layers, precipitated in the period of June to July each year.
(2) Only a thanatocenosis is present at the maximum depth of Lake Arendsee. (3) Faunal elements typical for a mesotrophic lake are frequent only in the oldest sediments of the freeze core, e.g. Acroperus elongatus among the Cladocera. (4) With the eutrophication of the lake the number of specimens of single species increased and decreased later on. The faunal community changed within the freeze core. A dramatical change in the thanatocoensis happened at a depth between 18 and 10 cm, corresponding the time between 1960 and 1972. (5) In hard water lakes easily visible calcite layers are formed by eutrophication (Koschel, 1990). The dramatic change of the thanatocenosis was in the period between 1960 and 1972. At that time the loading with sewage (Table 2) and the artificial draining of Lake Fauler See (Fig. 5) were the most important eutrophicating sources of Lake Arendsee. It can be expected that at the end of the drainage of Lake Fauler See a lot of sediment had been transported from Lake Fauler See to Lake Arendsee. This may be the reason for the thick varves in 1971 and 1972. Due to the very long retention time of 114 years, Lake Arendsee did not recover from the loading (Table 2). Additionally, the present input of nutrients has not stopped completely. During heavy rains the lake is loaded with sewage due to the overflow of stormwater drainage. The catchment area is enlarged by the drainage area of the ancient Lake Fauler See which is now used for agriculture. The fishery of
Table 2 Eutrophication sources and oligotrophication measures at Lake Arendsee (adapted from Klapper, 1991, 1992) Date
Eutrophication sources
until 1970
loading of the lake with communal and industrial sewage of the town of Arendsee drainage of Lake Fauler See into Lake Arendsee (input of loaded water, enlargement of the catchment area, input of drain water of the agriculturally used area of the ancient Lake Fauler See) intensification of agriculture in the catchment area intensification of the fishery (high stocking of whitefish)
1960–1970
since 1960 1960–1970 1976 1995
Oligotrophication measures
construction of a canal for the town of Arendsee installation of a hypolimnetic withdrawal flushing of marl
B.W. Scharf / Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998) 85–96
Fig. 5. Historical map of Lake Arendsee and Lake Fauler See of the year 1786 (from Meußling, 1993). 95
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Lake Arendsee is intensified and has impacts on the food web (see Scharf and Ehlscheid, 1993). From the data collected it can be concluded that the first limnological restoration measure was only partly successful. The hypolimnetic withdrawal may be a measure to control the eutrophication in a lake with a very short retention time (Scharf et al., 1992). For that reason in 1995 a new method of lake restoration by flushing of the naturally deposited marl was performed to accelerate the oligotrophication (Ro¨nicke et al., 1995).
Acknowledgements I thank Dr. Dietmar Keyser, Zool. Inst. Univ. Hamburg, for the SEM-pictures of the Ostracoda, Dr. Thomas Neu, UFZ Gewa¨sserforschung Magdeburg, for the critical reading of the manuscript, Prof. Dr. Helmut Klapper, UFZ Gewa¨sserforschung Magdeburg, for the information on the loading of Lake Arendsee, Dr. Michael Hupfer, Institut fu¨r Gewa¨ssero¨kolgie und Binnenfischerei Berlin, for discussing the results, and Dr. Linda MacDonald, Canadian Forest Service, Sault Ste. Marie, Ontario, for improving the English text.
References Danielopol, D., Niederreiter, R., 1990. New equipment and extraction methods for meiobenthic organisms. Bull. Inst. Bassin d’Aquitaine, Bordeaux 47, 277–286. Halbfass, W., 1896. Der Arendsee in der Altmark. Petermanns Mitt. 42, 173–187. Hollwedel, W., Scharf, B.W., 1988. Su¨ßwassercladoceren und -ostracoden (Crustacea) auf den niedersa¨chsischen Nordseeinseln Mellum und Memmert. Drosera 88, 341–369.
Hupfer, M., Ga¨chter, R., Ro¨nicke, H., Treutler, H.-C., in prep. Using sediment core investigations to study early diagenesis and release of phosphorus. Kempf, E.K., 1980. Index and bibliography of nonmarine Ostracoda. Sondervero¨ff. Geol. Inst. Univ. Ko¨ln 35, 1–188; 36, 1–180; 37, 1–204; 38, 1–180. Kempf, E.K., 1991. Index and bibliography of nonmarine Ostracoda. Sondervero¨ff. Geol. Inst. Univ. Ko¨ln 77, 1–232. Kempf, E.K., Scharf, B.W., 1980. Lebende und fossile Muschelkrebse (Crustacea: Ostracoda) vom Laacher See. Mitt. Pollichia 68, 205–236. Klapper, H., 1991. Control of Eutrophication in Inland Waters. Ellis Horward, New York, 337 pp. Klapper, 1992. Eutrophierung und Gewa¨sserschutz. Gustav Fischer, Jena, 277 pp. Koschel, R., 1990. Pelagic calcite precipitation and trophic state of hardwater lakes. Arch. Hydrobiol. Beih. Ergebn. Limnol. 33, 713–722. Lo¨ffler, H., 1969. Recent and subfossil distribution of Cytherissa lacustris (Ostracoda) in Lake Constance. Mitt. Int. Ver. Limnol. 17, 240–251. Meußling, O., 1993. Marginalien zur Arendsee-Forschung. Wissenschaftliche, historische und kuriose Messungen der Seetiefen. In: Meußling, O., Blatt, H. (Eds.), Der Arendsee anno 2000? Erste See-Symposium. Presse-Druck und Verlag, Augsburg, pp. 27–32. Ro¨nicke, H., Beyer, M., Tittel, J., 1995. Mo¨glichkeiten zur Steuerung der Blaualgendynamik in eutrophierten stehenden Gewa¨ssern durch Maßnahmen zur Seenrestaurierung. Limnol. Aktuell 8, 133–156. Scharf, B.W., Ehlscheid, T., 1993. Extensivierung der Fischerei — ein Beitrag zur Oligotrophierung von Seen. Nat. Landschaft 68, 562–565. Scharf, B.W., Bernhardt, H., Ehlscheid, T., Lu¨sse, B., 1992. Restoration. In: Scharf, B.W., Bjo¨rk, S. (Eds.), Limnology of Eifel maar lakes. Arch. Hydrobiol. Beih. Ergebn. Limnol. 38, 239–262. Scharf, B.W., Hollwedel, W., Ju¨ttner, I., 1995. Fossil (Holocene) and living Ostracoda and Cladocera (Crustacea) from Lake Arendsee, Germany. In: Riha, J. (Ed.), Ostracoda and Biostratigraphy. Balkema, Rotterdam, pp. 321–332. Wilhelmy, H., Scharf, B.W., 1996. Makrozoobenthos des Arendsees, Sachsen-Anhalt. Braunschw. Naturkd. Schr. 5, 85–90.
Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998) 85–96
Eutrophication history of Lake Arendsee (Germany) 1 Burkhard W. Scharf * UFZ – Umweltforschungszentrum Leipzig-Halle GmbH, Sektion Gewa¨sserforschung Magdeburg, Am Biederitzer Busch 12, D-39114 Magdeburg, Germany
Abstract At the maximum depth of Lake Arendsee (49.5 m) only a thanatocenosis could be found in a freeze core with a length of 52 cm. According to varve-counting, the fauna changed between 1960 and 1972, characteristic of mesotrophic to eutrophic state. The cause for eutrophication could be correlated with the sewage loading of the town of Arendsee and the drainage of Lake Fauler See into Lake Arendsee. Lake Arendsee did not yet recover from the loading because of the long retention time of 114 years. The investigation of the living meio- and macrobenthos of Lake Arendsee shows that the lower profundal is not colonized. 1998 Elsevier Science B.V. All rights reserved. Keywords: eutrophication; palaeolimnology; Arendsee; Germany; varves; precipitation; ostracoda; Fauler See
1. Introduction Lake Arendsee is located in the eastern part of Northern Germany (Fig. 1). Before the formation of Lake Arendsee, another lake, Lake Wendischer See, existed in the northern part of Lake Arendsee. A relic of this lake is the marl bench in Lake Arendsee in front of the village Zießau. The marl with an age of at least 4000 years is much older than Lake Arendsee (Ro¨nicke, Trettin, unpublished). The present lake was formed due to the solution of a salt dome in the underground (sinkhole). The main collapse of the earth surface was in the year 822 A.D. and the second smaller one in 1685. The littoral zone is quite narrow with steep slopes due to the nature of its formation (Fig. 1). An exception is the marl Ł Fax:
C49 391 8507555; E-mail: [email protected] This paper is dedicated to my supervisor Prof. Dr. Harald Sioli in honour of his 85th birthday (25 August 1995).
bench in the bay at the northern shore. The central part of the lake bottom is not flat, probably because the earth’s surface was broken into plates during the sinking. At Lake Arendsee two limnological restoration measures to control eutrophication were carried out: the first in 1976 by a hypolimnetic withdrawal (Klapper, 1991, 1992) and the second in 1995 by flushing of naturally deposited marl (Ro¨nicke et al., 1995). In this respect it is interesting to know when the eutrophication started and what were the causes for eutrophication. In the present paper, the problem of eutrophication is probed by a palaeolimnological investigation of the meiobenthos, especially of the Ostracoda and Cladocera. In addition the distribution of the living species in Lake Arendsee was studied for interpreting the palaeolimnological results.
1
c 1998 Elsevier Science B.V. All rights reserved. 0031-0182/98/$19.00 PII S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 0 3 3 - 9
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B.W. Scharf / Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998) 85–96
Fig. 1. Bathymetric map of Lake Arendsee with the location of the freeze core sample site (FC).
2. Methods The methods for studying the living and fossil ostracods and cladocerans have been described in Scharf et al. (1995). The distribution of living ostracods were analysed from the years 1992 to 1995. The core for dating the sediment (Fig. 4) was taken with a gravity corer according to Niederreiter, Mondsee, Austria (similar to the sampling equipment described in Danielopol and Niederreiter, 1990) at the same place at which the freeze core was taken for studying the fossil Ostracoda and Cladocera (Scharf et al., 1995).
For dating, the varves were counted using the white layers. The white layers, identified as calcite layers, are formed by autochthonous calcite precipitation, each year in the months June and July, as we know by sediment trap experiments (Hentschel, unpubl. data). Therefore, it was easy to date the core. The result was confirmed by the determination of 137 Cs. The peak of the Chernobyl accident in 1986 was at a depth of 6 cm (Hupfer et al., in prep.). The analysis of the white layers in the sediment cores was performed by using SEM (DSM 942, Zeiss) with a micro-analysing system (EDX, Oxford).
PLATE I Living and fossil Ostracoda from Lake Arendsee. The arrows point at the anterior part of the animal. (1) Limnocythere (Limnocythere) inopinata, female, carapace, dorsal view, length 0.68 mm. (2) Limnocythere (Limnocythere) inopinata, female, carapace, dorsal view, length 0.70 mm. (3) Limnocythere (Limnocythere) inopinata, female, carapace, ventral view, length 0.72 mm. (4) Limnocythere (Limnocythere) inopinata, female, carapace, lateral view, length 0.64 mm, height 0.37 mm. (5) Cytherissa lacustris, female, carapace, lateral view, length 0.95 mm, height 0.59 mm, fossil. (6) Metacypris cordata, female, carapace, ventral view, length 0.57 mm. (7) Candona candida, female, carapace, lateral view, length 1.10 mm, height 0.62 mm, fossil. (8) Fabaeformiscandona protzi, male, right valve, lateral view, internal, length 1.04 mm, height 0.55 mm, fossil. (9) Fabaeformiscandona protzi, female, carapace, dorsal view, length 1.08 mm, width 0.42 mm, fossil. (10) Fabaeformiscandona protzi, female, right valve, lateral view, internal, length 1.02 mm, height 0.53 mm, fossil.
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The names of the Ostracoda are written according to Kempf (1980, 1991).
3. Results The living Ostracoda of Lake Arendsee are shown in the Plates I–III, their distribution on the northern slope in relation to the water depth in Fig. 2. The occurrence of some fossil ostracods is shown in Fig. 3. The lowest depth on the slopes at which living ostracods and harpactids could be found was 23 m. In comparison to the slopes the sediment on the hills in the middle of Lake Arendsee was colonized by living Ostracoda (Table 1), Harpacticoida and Tardigrada up to a depth of 34 m. This can be explained by different sediments. On the hills very little limnetic sediment was found, mainly sand, probably from the ancient earth’s surface. The limnetic sediment was very possibly transported into deeper parts of the lake during storms in the circulation periods. On the hills there was much less organic matter in the sediment; therefore no sulphur bacteria were found, in contrast to the slopes below 23 m. It is probably that the sulphur bacteria layer prevents the colonization of meio- and macrobenthos at the lower profundal (Wilhelmy and Scharf, 1996). For the ecological interpretation of the findings of ostracods on the slopes and in the freeze core the following comments are given to the special species. Limnocythere (Limnocythere) inopinata (Plate I) prefers sand as a substrate. With the eutrophication of Lake Arendsee the nutrient conditions improved and the population increased. Later on, the substrate conditions deteriorated so that the
population of L. inopinata diminished. Cytherissa lacustris (Plate I) is in central Europe characteristic of the profundal of oligotrophic and mesotrophic lakes. The subfossil occurrence in Fig. 2 is greater than the present one. This means that the living population is a relic. We find the same feature in Fabaeformiscandona protzi (Plate I). The valves of Cytherissa lacustris are so heavy that they are not transported into deep water and therefore are lacking in the freeze core samples. The same feature of distribution could be observed in Lake Laacher See (Kempf and Scharf, 1980). C. lacustris was not found in the lower part of the freeze core, indicating that the sediment was not appropriate for this species at that time (compare Lo¨ffler, 1969). Only a few living specimens of Metacypris cordata (Plate I) were found. They have a heavy carapace and live in the upper littoral. Therefore there is no fossil record in the freeze core. Candona candida (Plate I) lives in the littoral and profundal of oligo- and eutrophic lakes. In the freeze core there were only 2 valves of adult C. candida (at a depth of 46–48 and 26–28 cm), but some larval valves which could be easily transported by currents during the circulation. This means, that C. candida did not colonize the lower profundal in the last decades. Only larval valves of Fabaeformiscandona protzi (Plate I) were frequently detected in the freeze core samples for the same reason. A few valves of adult F. protzi were found in the freeze core at depths below 38 cm. Pseudocandona compressa (Plate II) is today abundant in the littoral; however, in the freeze core there were only a few larval valves at depths between 44 and 26 cm. Living and subfossil specimens of Fabaeformiscandona caudata (Plate II) were often found, but were not found in the freeze core.Cypria ophtalmica is able to swim very well and colonizes a lake
PLATE II Living and fossil Ostracoda from Lake Arendsee. The arrows point at the anterior part of the animal. (1) Pseudocandona compressa, female, juvenile, carapace, dorsal view, length 0.89 mm. (2) Pseudocandona compressa, female, carapace, lateral view, length 0.95 mm, height 0.57 mm. (3) Pseudocandona compressa, male, juvenile (?), carapace, lateral view, length 0.89 mm, height 0.55 mm. (4) Fabaeformiscandona caudata, female, carapace, lateral view, length 1.22 mm, height 0.59 mm, fossil. (5) Fabaeformiscandona caudata, female, right valve, lateral view, internal, length 1.21 mm, height 0.58 mm, fossil. (6) Physocypria kraepelini, male, carapace, lateral view, length of the left valve 0.61 mm. (7) Physocypria kraepelini, female, carapace, lateral view, length 0.62 mm, height 0.42 mm. (8) Ilyocypris decipiens, right valve, lateral view, external, length 1.03 mm, height 0.54 mm, fossil. (9) Ilyocypris decipiens, detail of (8), muscle scars.
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Fig. 2. Vertical distribution of Ostracoda species at the northern slope of Lake Arendsee. Black rectangle D occurrence of living individuals of a species; white rectangle D occurrence of single valves or empty carapaces (modified from Scharf et al., 1995).
from the littoral to the profundal. In Lake Arendsee it was abundant in the living and subfossil samples, and frequent in the freeze core at depths between 52 and 16 cm, as adults and instars. This species was perhaps the unique species which lived in the lower profundal of Lake Arendsee some decades ago. But it is more probable that the remains of C. ophtalmica were carried to this depth because C. ophtalmica is usually associated with Candona candida which did not live in this area as shown above. Cypria exsculpta which is less tolerant than the more common C. ophtalmica was not found in the samples with the living and subfossil animals. This species was abundant in the freeze core between 52 and 28 cm. Until now C. exsculpta was found only in the littoral and the upper part of the profundal in Europe. Therefore, it is probable that the carapaces and valves of this species
represent a part of the thanatocenosis at the maximum depth of Lake Arendsee. Only a few specimens of Physocypria kraepelini (Plate II) were found living in the littoral. Adults and instars of Cyclocypris laevis were frequent in the lower part of the freeze core. C. laevis prefers waters that are rich in vegetation. It probably did not live in the profundal, but the carapaces and the valves could very easily be transported by currents. When C. laevis disappeared it was followed by C. ovum. The reason for the change from C. laevis to C. ovum is unknown. The reduction of the most submersed macrophytes probably accounts for the disappearance of these two species. Three fossil remains of Ilyocypris decipiens (Plate II) were detected in the freeze core at a depth of 12–14 cm. Only a few individuals of this species were found in the samples with the living and the subfossil
PLATE III Living and fossil Ostracoda from Lake Arendsee. The arrows point at the anterior part of the animal. (1) Notodromas monacha, female, carapace, lateral view, length of the left valve 1.05 mm. (2) Notodromas monacha, male, carapace, lateral view, length of the left valve 1.14 mm. (3) Herpetocypris reptans, female, carapace, lateral view, length of the left valve 2.50 mm, height of the left valve 1.18 mm. (4) Isocypris beauchampi, female, left valve, lateral view, internal, length 1.17 mm, height 0.61 mm, fossil. (5) Cypridopsis vidua, female, carapace, ventral view, length of the right valve 0.66 mm. (6) Plesiocypridopsis newtoni, female, carapace, right valve, length 0.84 mm, height 0.56 mm. (7) Potamocypris smaragdina, female, carapace, lateral view, length of the left valve 0.81 mm, height of the left valve 0.46 mm. (8) Potamocypris unicaudata, female, carapace, lateral view, length of the left valve 0.83 mm, height of the left valve 0.47 mm.
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Fig. 3. Distribution of single ostracod species in a freeze core from Lake Arendsee (total number of ind./2 cm freeze core) (modified from Scharf et al., 1995).
B.W. Scharf / Palaeogeography, Palaeoclimatology, Palaeoecology 140 (1998) 85–96 Table 1 Distribution of Ostracoda at the northern slope and on the hills in the middle of Lake Arendsee Species
Slope
Limnocythere inopinata Cytherissa lacustris Metacypris cordata Candona candida Pseudocandona compressa Fabaeformiscandona caudata Fabaeformiscandona protzi Cypria exsculpta Cypria ophtalmica Physocypria kraepelini Cyclocypris laevis Cyclocypris ovum Ilyocypris decipiens Darwinula stevensoni Notodromas monacha Herpetocypris reptans Bradleystrandesia spec. Isocypris beauchampi Cypridopsis vidua Potamocypris smaragdina Potamocypris unicaudata Plesiocypridopsis newtoni
ž ž ž ž ž ž ž Ž ž ž Ž Ž ž ž ž ž Ž Ž ž ž ž ž
High in the middle of the lake
ž
ž ž
ž D living and fossil, Ž D only fossil.
animals. Only living Notodromas monacha (Plate III) was found in the reed zone of the lake. It was absent from freeze core samples. Herpetocypris reptans (Plate III) lives in the upper littoral, especially among macrophytes. Typically, only larval valves could be detected in the freeze core, at a depth between 48 and 12 cm. The carapaces and valves of adults of this big and heavy species are not far transported. The same observation was made at Lake Laacher See (Kempf and Scharf, 1980). Only two subfossil specimens of Isocypris beauchampi (Plate III) were found. This is important, because this species was very rare in Germany after the Second World War. Obviously, the colonization of this species was not successful in Lake Arendsee, although now this species is frequent in many waters in Germany, especially in young waters. Cypridopsis vidua (Plate III) also prefers to live among macrophytes. Its disappearance in the sediment of the freeze core is a hint at the extinction of the submersed macrophytes. Plesiocypridopsis newtoni (Plate III) and Potamocypris unicaudata (Plate III)
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are characteristic of water with a higher salinity (e.g. Hollwedel and Scharf, 1988). These two species were found in the freeze core at a depth between 18 and 10 cm, only two valves of Plesiocypridopsis newtoni at 16 to 14 cm. This could be an indication that at this time the salinity of the lake was higher than previously and later on. But the probability of a short-term change of the salinity of the whole lake is not great because of the long retention time of more than 100 years (Scharf et al., 1995). In this lake there are some springs with a higher salinity because of the location on a salt dome. It is more probable that these springs had a greater flow or higher salinity for a short time, so that in the vicinity more halophilic specimens could live. Due to the large volume of the lake the total salinity would not be changed significantly. P. smaragdina (Plate III) follows P. newtoni and P. unicaudata in the younger sediments of the freeze core. Living specimens of P. smaragdina were absent at the locations of Lake Arendsee where P. newtoni and P. unicaudata could be detected and vice versa. The Cladocera remains in the freeze core show a change in the community from the oldest part to the younger one (Scharf et al., 1995). By counting the varves it was possible to date the youngest sediment (Fig. 4). The ostracod fauna changed drastically at a depth of 18 to 10 cm of the freeze core. This corresponds with the years 1960 to 1972 (Fig. 4).
4. Conclusions Halbfass (1896) described oligotrophic features of Lake Arendsee (the definition of the trophic states were introduced later). At the beginning of the second half of this century the community was typical for a mesotrophic lake (Klapper, 1991, 1992). This can be proved, e.g. by the occurrence of many submersed macrophytes and Daphnia species. Since that time the trophic state of Lake Arendsee has changed dramatically. This change can be confirmed by zoological and palaeolimnological studies (compare Scharf et al., 1995): (1) The living fauna contains elements of a mesotrophic, eutrophic and polytrophic lake, the mesotrophic ones only as relic populations.
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Fig. 4. Occurrence of calcite layers in a sediment core from Lake Arendsee, taken on the 15 August 1995. The dating is based on the counting of the calcite layers, precipitated in the period of June to July each year.
(2) Only a thanatocenosis is present at the maximum depth of Lake Arendsee. (3) Faunal elements typical for a mesotrophic lake are frequent only in the oldest sediments of the freeze core, e.g. Acroperus elongatus among the Cladocera. (4) With the eutrophication of the lake the number of specimens of single species increased and decreased later on. The faunal community changed within the freeze core. A dramatical change in the thanatocoensis happened at a depth between 18 and 10 cm, corresponding the time between 1960 and 1972. (5) In hard water lakes easily visible calcite layers are formed by eutrophication (Koschel, 1990). The dramatic change of the thanatocenosis was in the period between 1960 and 1972. At that time the loading with sewage (Table 2) and the artificial draining of Lake Fauler See (Fig. 5) were the most important eutrophicating sources of Lake Arendsee. It can be expected that at the end of the drainage of Lake Fauler See a lot of sediment had been transported from Lake Fauler See to Lake Arendsee. This may be the reason for the thick varves in 1971 and 1972. Due to the very long retention time of 114 years, Lake Arendsee did not recover from the loading (Table 2). Additionally, the present input of nutrients has not stopped completely. During heavy rains the lake is loaded with sewage due to the overflow of stormwater drainage. The catchment area is enlarged by the drainage area of the ancient Lake Fauler See which is now used for agriculture. The fishery of
Table 2 Eutrophication sources and oligotrophication measures at Lake Arendsee (adapted from Klapper, 1991, 1992) Date
Eutrophication sources
until 1970
loading of the lake with communal and industrial sewage of the town of Arendsee drainage of Lake Fauler See into Lake Arendsee (input of loaded water, enlargement of the catchment area, input of drain water of the agriculturally used area of the ancient Lake Fauler See) intensification of agriculture in the catchment area intensification of the fishery (high stocking of whitefish)
1960–1970
since 1960 1960–1970 1976 1995
Oligotrophication measures
construction of a canal for the town of Arendsee installation of a hypolimnetic withdrawal flushing of marl
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Fig. 5. Historical map of Lake Arendsee and Lake Fauler See of the year 1786 (from Meußling, 1993). 95
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Lake Arendsee is intensified and has impacts on the food web (see Scharf and Ehlscheid, 1993). From the data collected it can be concluded that the first limnological restoration measure was only partly successful. The hypolimnetic withdrawal may be a measure to control the eutrophication in a lake with a very short retention time (Scharf et al., 1992). For that reason in 1995 a new method of lake restoration by flushing of the naturally deposited marl was performed to accelerate the oligotrophication (Ro¨nicke et al., 1995).
Acknowledgements I thank Dr. Dietmar Keyser, Zool. Inst. Univ. Hamburg, for the SEM-pictures of the Ostracoda, Dr. Thomas Neu, UFZ Gewa¨sserforschung Magdeburg, for the critical reading of the manuscript, Prof. Dr. Helmut Klapper, UFZ Gewa¨sserforschung Magdeburg, for the information on the loading of Lake Arendsee, Dr. Michael Hupfer, Institut fu¨r Gewa¨ssero¨kolgie und Binnenfischerei Berlin, for discussing the results, and Dr. Linda MacDonald, Canadian Forest Service, Sault Ste. Marie, Ontario, for improving the English text.
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