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Jun 9, 2006 - Journal of Paleolimnology 35, 289–303. V. Valpola, S.E. and Salonen, V.-P. The paleolimnological development of the twin lakes. Etujärvi and ...
Lake sediment research as a part of lake management – case studies and implications from southern Finland Samu Valpola

Academic Dissertation To be presented with the permission of the Faculty of Science of the University of Helsinki, for public criticism in the Auditorium D101 of Physicum, Kumpula, on June 9th, 2006, at 12 o’clock noon

Publications of the Department of Geology D007 Helsinki 2006

PhD-thesis No. 188 of the Department of Geology, University of Helsinki

Supervised by: Professor Veli-Pekka Salonen Department of Geology University of Helsinki Reviewed by: Professor Matti Tikkanen Department of Geography University of Helsinki Finland Doctor Atko Heinsalu Institute of Geology Tallin University of Technology Estonia Opponent: Professor Lauri Arvola Lammi Biological Station University of Helsinki Finland

Cover photo: Lake Kaljasjärvi, late November 2005

ISSN 1795-3499 ISBN 952-10-2605-7 (paperback) ISBN 952-10-2606-5 (PDF) http://ethesis.helsinki.fi/ Helsinki 2006 Yliopistopaino

Samu Valpola: Lake sediment research as a part of lake management – case studies and implications from southern Finland, University of Helsinki, 2006, 33 pp., University of Helsinki, Publications of the Department of Geology D007, ISSN 1795-3499, ISBN 95210-2605-7, ISBN 952-10-2606-5 (pdf-version).

Abstract In Finland one of the most important current issues in the environmental management is the quality of surface waters. The increasing social importance of lakes and water systems has generated wide-ranging interest in lake restoration and management, concerning especially lakes suffering from eutrophication, but also from other environmental impacts. Most of the factors deteriorating the water quality in Finnish lakes are connected to human activities. Especially since the 1940’s, the intensified farming practices and conduction of sewage waters from scattered settlements, cottages and industry have affected the lakes, which simultaneously have developed in to recreational areas for a growing number of people. Therefore, this study was focused on small lakes, which are human impacted, located close to settlement areas and have a significant value for local population. The aim of this thesis was to obtain information from lake sediment records for ongoing lake restoration activities and to prove that a well planned, properly focused lake sediment study is an essential part of the work related to evaluation, target consideration and restoration of Finnish lakes. Altogether 11 lakes were studied. The study of Lake Kaljasjärvi was related to the gradual eutrophication of the lake. In lakes Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi and Iso-Roine the main focus was on sediment mapping, as well as on the long term changes of the sedimentation, which were compared to Lake Pääjärvi. In Lake Hormajärvi the role of different kind of sedimentation environments in the eutrophication development of the lake’s two basins were compared. Lake Orijärvi has not been eutrophied, but the ore exploitation and related acid main drainage from the catchment area have influenced the lake drastically and the changes caused by metal load were investigated. The twin lakes Etujärvi and Takajärvi are slightly eutrophied, but also suffer problems associated with the erosion of the substantial peat accumulations covering the fringe areas of the lakes. These peat accumulations are related to Holocene water level changes, which were investigated. The methods used were chosen case-specifically for each lake. In general, acoustic soundings of the lakes, detailed description of the nature of the sediment and determinations of the physical properties of the sediment, such as water content, loss on ignition and magnetic susceptibility were used, as was grain size analysis. A wide set of chemical analyses was also used. Diatom and chrysophycean cyst analyses were applied, and the diatom inferred total phosphorus content was reconstructed. The results of these studies prove, that the ideal lake sediment study, as a part of a lake management project, should be two-phased. In the first phase, thoroughgoing mapping of sedimentation patterns should be carried out by soundings and adequate

corings. The actual sampling, based on the preliminary results, must include at least one long core from the main sedimentation basin for the determining the natural background state of the lake. The recent, artificially impacted development of the lake can then be determined by short-core and surface sediment studies. The sampling must be focused on the basis of the sediment mapping again, and it should represent all different sedimentation environments and bottom dynamic zones, considering the inlets and outlets, as well as the effects of possible point loaders of the lake. In practice, the budget of the lake management projects of is usually limited and only the most essential work and analyses can be carried out. The set of chemical and biological analyses and dating methods must therefore been thoroughly considered and adapted to the specific management problem. The results show also, that information obtained from a properly performed sediment study enhances the planning of the restoration, makes possible to define the target of the remediation activities and improves the costefficiency of the project. Keywords: Lake sediments, eutrophication, lake restoration, sediment mapping, phosphorus, diatoms, chrysophycean cysts, Finland.

Contents 1. Introduction

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2. Location of studied lakes, materials and methods

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2.1. Echo soundings and sampling

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2.2. Sediment lithostratigraphy and physical properties of the sediments

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2.3. Chronology

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2.4. Chemical analyses

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2.5. Diatom and chrysophyte cyst analyses

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3. Overview of the papers

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3.1. Paper I

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3.2. Paper II

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3.3. Paper III

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3.4. Paper IV

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3.5. Paper V

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4. Discussion

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4.1. Recommendations for practice

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5. Conclusions

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7. Acknowledgements

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8. References

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List of original publications This thesis is based on the five publications listed below:

I

Kauppila, T. and Valpola, S.E. 2003. Response of a shallow boreal lake to recent nutrient enrichment – implications for diatom-based phosphorus reconstructions. Hydrobiologia 495, 47–58.

II

Valpola, S.E. and Ojala, A.E.K. Post-glacial sedimentation rate and patterns in six lakes of the Kokemäenjoki upper watercourse, Finland. Boreal Environment Research (In press).

III

Valpola, S.E., Forsell, J. and Salonen, V.-P. Comparison of phosphorus cycling in two basins of eutrophied Lake Hormajärvi, southern Finland. Submitted to Lake and Reservoir Management.

IV

Salonen, V.-P., Tuovinen, N. and Valpola, S. 2006. History of mine drainage impact on Lake Orijärvi algal communities, SW Finland. Journal of Paleolimnology 35, 289–303.

V

Valpola, S.E. and Salonen, V.-P. The paleolimnological development of the twin lakes Etujärvi and Takajärvi in Askola, southern Finland – implications for lake management. Bulletin of the Geological Society of Finland (In press).

Author’s contribution to the publications: In Paper I both authors were responsible for planning of the study and took part in coring. S. Valpola was responsible for the echo sounding, sedimentary phosphorus and spherical carbonaceous particles (SCP) analyses. T. Kauppila did the diatom and sedimentary pigment analysis, and performed diatominferred lake-water total phosphorus (DITP) reconstructions. The results of the study were interpreted and the paper was written in cooperation.

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Paper II was planned and sampling and echo soundings were carried out by both authors. S. Valpola did the laboratory analyses and the interpretation of the soundings and A. Ojala carried out and interpreted the paleomagnetic dating. The results of the study were interpreted and paper written in cooperation. In the study of Paper III the planning was mainly done by S. Valpola and V.-P. Salonen. Sampling was carried out by S. Valpola and J. Forsell. Phosphorus fractions and grain size analysis, as well as SCP-analysis were done by J. Forsell, and diatom analysis by S. Valpola. DITP-reconstruction was provided by T. Kauppila. The interpretation of the results and writing of the paper were done mainly by S. Valpola and V.-P. Salonen. The Lake Orijärvi study (Paper IV) was mainly planned by V.-P. Salonen. Sampling and fieldwork were done by all authors. N. Tuovinen carried out the diatom analysis and S. Valpola the chrysophycean cyst analysis. The results were interpreted and the paper written by all authors. The study of Paper V was planned by V.-P. Salonen and S. Valpola, who were also mainly responsible for the fieldwork and sediment descriptions. Phosphorus fractions, grain size analysis and SCP analysis were done by J. Forsell. S. Valpola carried out the diatom analysis and the DITP reconstruction was provided by T. Kauppila. The results were interpreted by S. Valpola and V.-P. Salonen, and they also wrote the paper.

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1. Introduction

and Lakso 2005). The roles of limnological and hydrological properties of the lakes, external loading, as well as the land use of the catchment area and their importance to lake management activities are well understood. The restricting of the internal phosphorus load from lake sediments is having an important role in Finnish lake management planning also (Lappalainen and Matinvesi 1990; Eloranta 2005). Eutrophication is one of the processes that have influenced the development of the paleolimnological research and development of lake restoration methods. Håkanson and Jansson (1983) gave a detailed presentation of the possibilities and methods of lake sedimentology. Already Einsele (1936) and Mortimer (1941, 1942) described the phosphorus release from bottom sediments by the reduction of iron and manganese, the problem, which has then been widely studied (e.g. Golterman et al. 1969; Boström et al. 1982). Among others, Pettersson et al. (1988) and Ruban et al. (2001) evaluated and developed methods for sedimentary phosphorus fractionations. Diatom based inference models for phosphorus reconstructions have been developed and are widely used (e.g. Anderson et al. 1993; Bennion et al. 1995; Kauppila et al. 2002a, 2002b; Kauppila 2002; Miettinen 2005). The development of dating methods suitable for different time scales and materials, e.g. 14Cmethod (Stuiver and Polach 1977; Björck and Wohlfarth 2001), 210Pb-method (Appleby and Oldfield 1978; Appleby et al. 1979), 137Csmethod (Pennington et al. 1973; Jaakkola et al. 1983; Kansanen et al. 1991), paleomagnetic dating (Ojala 2001) or spherical carbonaceous particles (SCP) based dating (Renberg and Wik 1984, 1985; Rose 1990) has provided means to evaluate the results of paleolimnological studies more accurately. Many in-lake restoration techniques of eutrophied lakes are directly focused on lake sediment and methods for sediment manipulation have also been developed. A classic case is the removal of nutrient-rich sediments through suction dredging in Lake Trummen, Sweden, in the early 1970’s (Björk 1972). Chemical precipitation of sediment phosphorus and

The deterioration of lake water quality has been a common environmental concern throughout the world during the latter half of the 1900’s. For example acidification, loading caused by raw material acquisition or industry and eutrophication with their consequences have become a serious problem which harms, for example, water supply, fishing and recreational activities. At present, eutrophication is the most actual problem in Finnish lakes. The eutrophic lakes have been mentioned in lake classifications since the early stages of the 1900’s (Thienemann 1921), and they have had more weighting in the later categorisations and descriptions (OECD 1982; Harper 1992; O’Sullivan and Reynolds 2005). In Finland there exist altogether 187 888 lakes, of which ca. 56 000 are larger than 1 hectare (Raatikainen and Kuusisto 1990; Suomen Ympäristökeskus 2004). Lakes are a salient part of the Finnish environment and also have a great value as water resources for water supply and, for example, the forestry industry. At present the recreational values, such as fishing, of the lakes are substantial. There are over 380 000 active fishermen in Finland and at least two million Finns fish at least once a year (Kalatalouden Keskusliitto 2005). The majority of the 460 000 cottages in Finland are on the lake- or sea-side (Lehtoranta 2005). Therefore, the water quality and the general state of lakes are in the focus of a large number of people. The economic benefit obtained from lake restoration can be estimated, but the results are often very different depending on the evaluation method used (Majuri 2005). The problem caused by eutrophication is broad. Active farming areas and scattered settlement are often located near lakes and water systems, as is industry. The impaired condition of numerous lakes has increased the need of lake management. The problems and questions connected with lake restoration projects in Finland have discussed in several publications (e.g. Seppänen 1973; Ilmavirta 1990; Äystö 1997; Turunen and Äystö 2000; Salminen and Böhling 2002; Väisänen and Kuusela 2002; Ulvi

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inactivation with several different chemicals, for example with aluminium and iron compounds (Cooke et al. 1986, 1993, 2005; Oravainen 1990) has been used. In addition, for reduction of nutrient cycling between the sediment-water interface and improving the sediment binding capacity for phosphorus, in situ oxidation of anaerobic lake sediment by means of suitable oxidation agents has been applied (Ripl 1976). Alhonen (1985) also mentioned dredging as a method for removing permanent anaerobic sediment. Salonen and Varjo (1996, 2000) and Varjo (2001) developed a gypsum-based method for treatment of restricted sediment areas, i.e. stabile accumulation areas of eutrophied lakes. Clay has been tested for covering the anoxic, phosphorus releasing sediment (Kansanen and Pekkarinen 1996; Sommarlund et al. 1998). Application of detailed paleolimnological studies has become a part of lake management studies in Finland since the late 1960’s. Alhonen (1967) compared dystrophic, oligotrophic and eutrophic lakes from southern Finland, showing the change of Lake Kyrösjärvi from eutrophic to dystrophic and the presence of agricultural influence in the sediments of lakes Kyrösjärvi and Sarkkilanjärvi. In the study of Lake Gallträsket (Alhonen 1972), the artificial sewage water loading and eutrophication caused by it were detected from the lake sediment of a small, shallow lake. This work was one of the studies that formed a basis for the classification of the stages of cultural eutrophication of Finnish lakes (Alhonen 1979). According to this classification, the first phase of eutrophication is not necessarily to be seen in the nature of the sediment, but can be detected on the basis of sedimentary diatom, cladoceran or midge remains. In the second phase, anoxic conditions cause visible sulphide layers in the sediment; and in the third phase the accumulating sediment is black sulphide gyttja, showing permanent state of anaerobic hypolimnion (Alhonen 1979). Using the sediment records as indicators of the change caused by human impact then proved to be successful in different kind of lake environments in Finland (e.g. Simola et al. 1984; Kansanen and Jaakkola 1985; Vuorinen et al. 1986; Sandman et al. 1990;

Räsänen et al. 1992; Turkia et al. 1998). The importance of the sediment in lake restoration projects was emphasized by Alhonen (1985) and the use of paleolimnological studies has become more general, considering especially larger lakes such as Lake Päijänne or Lake Jyväsjärvi (Itkonen et al. 1999; Meriläinen et al. 2003) and known loading sources, such as industrial or municipal loading (Meriläinen et al. 2001; Kauppila et al. 2002b; Hynynen et al. 2004) or mining impact (Kauppila 2006). Detailed studies of smaller lakes, often eutrophied because of agricultural loading and/or cottage settlement have usually been made the context of lake management projects (Tolonen et al. 1990; Salonen et al. 1993; Itkonen and Olander, 1997). Small scale paleolimnological studies have also been carried out as a part of research programs of Finnish Regional Environment Centres (e.g. Valpola 2002a, b, c, d, e; Sihvonen and Valpola 2003a, b). However, the Water Framework Directive of the European Union (Directive 2000/60/ EC 2000) which at present can be considered as a most important legislative tool for surface waters protection and management, will increase the need of paleolimnological studies. The implementation of the Water Framework Directive also demands assessment of the reference i.e. natural background conditions of lakes that can be done by sediment-based methods, as is presented in Miettinen (2005) or Räsänen et al. (2006). In addition, the increasing number of local water protection associations, the activity of fisheries districts, and common ambition to start management activities on eutrophied lakes require reliable knowledge about the lakes’ development. Trustworthy paleolimnological data enable to derive how much water-quality change has been occurred and thus establish setting to the realistic target conditions of the lake restoration projects. The minimum amount of paleolimnological information required for a lake restoration project is case-specific. Knowledge of the sedimentation areas and lake bottom dynamics is, however, necessary in any case. In many cases the thickness and the areal distribution of

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2. Location of studied lakes, materials and methods

the sediment must be mapped. These preliminary studies make a reasonable sampling possible and focus the investigations and remediation activities to the areas where they have the most positive effect on the lake. Physical, chemical and biological analyses of the sediment, i.e. exterior features of the sediment, water content, loss on ignition, grain size and magnetic susceptibility; total analyses of the elements, extraction of the nutrients; and e.g. diatom, pollen and crysophycean analyses can provide a large amount of data which can be further analysed statistically and compared with water monitoring data. The dating of the sediment, if needed, can be carried out by several different methods adapted to different time scales. The selection of analyses must be carefully considered in the early stages of research and management projects. It is highly dependent on the stage of the lake, on the geological features and the land use of the lake catchment, on the possible loaders of the lake and on the aim of the study and the restoration as well. Also, the costeffectiveness of the analyses is often extremely important in local-scale research projects. Papers I–V related to this thesis consider local-scale paleolimnological studies of lakes related to different kinds of human induced problems and associated management needs. The usefulness of the analytical methods used will be discussed and recommendations for the best practice will be given. The main problems discussed in this study are how to optimize the application of sediment studies in lake management projects, how to define the minimum knowledge of the sediments required in lake management projects and to consider how to get the maximum benefit from the obtained results. Each lake management project is unique and strictly related to the problem, lakes sedimentary environment and, concerning the natural background state, development of the lake. This study was carried out to show that paleolimnological and lake sedimentological methods provide essential information for planning the lake management related in different kind of problems in different kind of lake environments.

The location of the lakes studied in the separate papers is shown in Fig.1. The lakes are related to the papers as follows: Lake Kaljasjärvi; Paper I, lakes Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi, Iso-Roine and Pääjärvi; Paper II, Lake Hormajärvi; Paper III, Lake Orijärvi; Paper IV and lakes Etujärvi and Takajärvi; Paper V. The methods used are described in detail in each individual paper, but a general view in sampling, materials and analyses is given here to enable the easier comparison of the case studies.

2.1. Echo soundings and sampling Echo soundings were carried out for creating bathymetric maps and estimating the areal distribution of lake sediments at lakes Kaljasjärvi, Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi and Iso-Roine. Also, the possible lithostratigraphic boundaries and/or gaps in the sedimentation patterns were traced by using the echo sounding data. The soundings were carried out with SITEX Honda He 256 and Furuno FE 881 MK 2 echo sounders. The sounding data was checked by corings in every studied lake. The locations and frequencies of the survey lines were based on experience gained in previous studies in the glaciated boreal shield areas (Pajunen 2000; Pajunen 2004). The most used sampler in these studies was the Limnos-gravity corer with sectioning sample tube (Kansanen et al. 1991). It was used at lakes Kaljasjärvi, Hormajärvi, Orijärvi, Etujärvi and Takajärvi for short core (ca. 50 cm, a time period of under 200 years) samples, and at lakes Hormajärvi, Etujärvi and Takajärvi also for surface sediment samples. A crust freeze corer (Renberg 1982) was used at Lake Orijärvi for detailed description of the sediment structures as well as to collect sub-samples for diatom and chrysophyte cyst analyses. The Russian peat corer (Jowsey 1966) was used for test sampling or mapping of the sediment stratigraphy at lakes Kaljasjärvi, Orijärvi, Etujärvi and Takajärvi. At Lake Takajärvi a light, rod-operated piston corer

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25° Tampere

Rauma

JI

A

H

G

FE

FINLAND

D

Turku

C

B

Helsinki

60°

Papers I, III, IV and V A Lake Kaljasjärvi B Lake Hormajärvi C Lake Orijärvi D Twin lakes Etujärvi and Takajärvi

Paper II 61°10´N 60°17´N 60°13´N 60°32´N

21°39´E 24°00´E 23°34´E 25°35´E

E Lake Pääjärvi F Lake Ormajärvi G Lake Suolijärvi H Lake Lehee I Lake Pyhäjärvi J Lake Iso-Roine

61°04´N 61°05´N 61°08´N 61°10´N 61°10´N 61°13´N

25°07´E 24°57´E 24°48´E 24°47´E 24°42´E 24°34´E

Figure 1. Locations of the studied lakes

Water content (W) was determined from all the studied short core samples and surface sediment samples according to Håkanson and Jansson (1983). In the surface sediment samples it is a salient parameter for the evaluation of bottom dynamics and the sedimentation environment of certain bottom areas. Loss on ignition (LOI) was measured from all the studied short core and surface sediment samples, long core samples of lakes Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi, Iso-Roine, Pääjärvi and from the long core Taka10 from Lake Takajärvi. Determinations were done according to Håkanson and Jansson (1983) and Boyle (2004). The results were used to describe the content of organic matter in the studied lake sediments and, in Paper II, to evaluate the carbon content of the sediment. The magnetic susceptibility of the long core samples was determined from lakes Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi, Iso-Roine, Pääjärvi and Takajärvi to evaluate and locate the changes in the composition of the sediment sequences. Changes in the catchment land use, erosion,

also was used. A heavy piston corer (Putkinen and Saarelainen 1998) was used for long core sampling of lakes Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi, Iso-Roine and Pääjärvi.

2.2. Sediment lithostratigraphy and physical properties of the sediments The lithologic nature of the sediment may, if described in detail, provide valuable information for the sampling and pinpoint significant changes in sedimentation even without help of further physical, chemical or biological analyses (e.g. Aaby and Berglund 1986). The sediment lithostratigraphy of the short cores was described in detail in studies of lakes Kaljasjärvi, Hormajärvi, and Orijärvi adapting the method of Troels-Smith (1955). In the case of lakes Etujärvi and Takajärvi, the sediment lithostratigraphy was described from the long cores, as was that from lakes Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi, Iso-Roine and Pääjärvi.

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lake water level and sedimentation environment affect the grain size of the accumulating mineral matter causing detectable changes in the grain size distribution of the sediment. The grain size analysis of the sediment (Agrawal et al. 1991; Arnaud 2005) was done from the short core samples of lakes Hormajärvi, Etujärvi and Takajärvi, as well as from the surface sediment samples of the lakes Etujärvi and Takajärvi.

were dated with a paleomagnetic dating method (Thompson and Oldfield 1986; King and Peck 2001). Natural remanent magnetism (NRM) was measured at the Geological Survey of Finland and the obtained NRM curves were compared with the Nautajärvi reference curve (Ojala and Tiljander 2003). This comparison was made by connecting the characteristics of declination and inclination of the curves (Creer et al. 1976).

2.3. Chronology

2.4. Chemical analyses

Dating is one of the most important issues in paleolimnological research. In most of the studies related to lake management it is necessary to be able to connect the analysed indications of the loading history to time, for example, to enable the examination of the general changes in land use or to establish the beginning of the point source loading. In this study following methods were used: SCP were determined from the short cores of lakes Kaljasjärvi, Hormajärvi, Etujärvi and Takajärvi by the method of Rose (1990). The results of the soot particle counting were then compared with the data of annual combustion on fossil fuels in Finland. The 137Cs-activity was measured from the short cores of Lake Kaljasjärvi at the Geological Survey of Finland (Äikäs et al. 1994). It was measured also from the short core of Lake Orijärvi at the Accelerator Laboratory of the Department of Physics, University of Jyväskylä, as was the 210 Pb-activity. The 210Pb age was obtained by the C.I.C. (constant initial concentration) model. For establishment of longer temporal scale three accelerating mass spectrometry (AMS) datings were done, one from a long core of Lake Etujärvi and two from the long cores of Lake Takajärvi. Samples were treated in the Dating Laboratory of the Finnish Museum of Natural History according to Slota et al. (1987), and then dated in the Uppsala Tandem Laboratory (Possnert 1984, 1990). The radiocarbon ages obtained were then calibrated using the CalPal© program and the CalPal_SFCP_2005 calibration curve (Weninger and Jöris 2004). The long cores of lakes Ormajärvi, Suolijärvi, Pyhäjärvi, Iso-Roine and Pääjärvi

The sedimentary phosphorus fractionations were analysed to evaluate the changes in phosphorus accumulation and the changes in both external and internal load. Analyses were done with the method of Hieltjes and Lijklema (1980) from short core samples and the surface sediment samples from lakes Kaljasjärvi, Hormajärvi, Etujärvi and Takajärvi. A total phosphorus digestion after Bengtsson and Enell (1986) was carried out from the same samples. The determination of the phosphorus from digestions was done after Murphy and Riley (1962). The refractory phosphorus was then calculated as a subtraction of analysed fractions from the contents obtained in the total digestion. The sedimentary plant pigment concentrations were measured from the short core of Lake Kaljasjärvi after Bengtsson and Enell (1986) and represented as pigment units (Sanger and Gorham 1972). The results were used to evaluate the changes in the composition in the accumulated organic material. The Lake Orijärvi short core was analysed for Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Sr, Ti and Zn by ICPAES at the laboratory of the Geological Survey of Finland. These measurements were used to describe the accumulation of elements, mainly acid main drainage (AMD) derived heavy metals in the lake.

2.5. Diatom and chrysophyte cyst analyses The diatom analysis was carried out from the short core samples of lakes Kaljasjärvi, Hormajärvi, Orijärvi, Etujärvi and Takajärvi.

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The sample treatments and slide preparations were made according to Battarbee (1986) and Battarbee et al. (2001). The reference literature used was mainly Krammer and Lange-Bertalot (1986–1991a,b). The reconstructions of DITP based on diatom analyses of lakes Kaljasjärvi, Hormajärvi, Etujärvi and Takajärvi were done according to Kauppila et al. (2002a). The results of diatom analyses themselves and the DITP particularly were used for the determination of the eutrophication development of the lakes. In the case of Lake Orijärvi the diatom analysis was carried out especially to show the impact of the metal load on the lake ecosystem. Chrysophycean cyst analysis was carried out from the short core of Lake Orijärvi. Pre-treating of the samples was done according to Battarbee et al. (2001) and SEM preparations were done applying Duff et al. (1995). Identification and classifications were mainly based on Duff et al. (1995) and Wilkinson et al. (2001). In the case of Lake Kaljasjärvi chrysophycean cysts were counted as a separate category for determination of the chrysophyte/diatom ratio. As well as the results of diatom analysis, the information obtained from chrysophycean cyst analysis was used to demonstrate the effects of the metal load, but also to evaluate the possible eutrophication of the lake.

3. Overview of the papers 3.1. Paper I The multi-proxy study of Lake Kaljasjärvi can be considered as a typical example of paleolimnological research that was done because of the local need for information. Originally, the deterioration of the water quality and changes in the macrophytes were noticed by the local people and brought forward by the Laitila Fisheries District and the Lahdenvainio Local Fishing Association. Water quality monitoring data shows that Lake Kaljasjärvi has for the most part been mesotrophic since the beginning of the monitoring in the late 1960’s. The epilimnetic total phosphorus concentration had been ca. 20– 25 μg P l-1, but, after the mid-1980’s, sporadic

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higher concentrations were measured, the highest, 63 μg P l-1, in September 1999. During the monitoring period the total increase in the phosphorus concentration trend has been ca. 10 μg P l-1. According to the findings of the sediment study, the eutrophication of Lake Kaljasjärvi has been two-pronged. The first phase of development occurred at the turn of the 1940’s and 1950’s. It is characteristic for this phase that the proportion of the allochtonous mineral matter in sediments increases, which can be seen in the physical properties and the visual appearance of the sediment. Simultaneously, the pigment concentrations of the organic matter and the proportion refractory phosphorus increase, which seems to result from the increase of the lakes own primary production. The most likely the factor that has caused the increase in productivity is the accelerated and more efficient agricultural land use in the catchment area. This first phase of the initial eutrophication does not seem to have had any significant effects on the phytoplankton of Lake Kaljasjärvi if considering the diatom inferred total phosphorus. The DITP remains stable during this phase of the lake’s evolution. There are changes in the diatom taxa, the most evident being the increase of Cyclotella stelligera, presumably a tychoplanktonic species (Cholnoky 1968; Lowe 1974), but these changes have been interpreted to be more connected to the increase of the macrophytes in the large littoral areas of Lake Kaljasjärvi than to the phytoplankton’s straightforward benefit from the nutrient enrichment. It is also shown that the role of the submerged vegetation in removing of phosphorus from lake water and transferring it to the pools can be significant (Brenner et al. 2006). The occurrence of increased growth of macrophytes is supported by the pigment analysis and by the composition of the sediment, too. The inference model used may predict low values of DITP if the impact of littoral areas is evident (Kauppila et al. 2002b), which explains the stability of the DITP during this first phase of eutrophication. The second stage of increased nutrient input can be detected in sediment depth, which corresponds to ca. 1980. Changes in diatom

the NUTRIBA project (Rask et al. 2004). Lake Pääjärvi, located on the other side of the local watershed, was chosen for a reference site. The NUTRIBA-chain lakes were echosounded and sampled for cores representing the sedimentation and carbon accumulation since the lake basins were isolated from the Baltic basin ca. 10 000–10 800 B.P. Furthermore, the thickness and volume of sediment were calculated and sedimentation rates were estimated by dating the cores using paleomagnetic method (Thompson and Oldfield 1986; King and Peck 2001). This kind of data was considered essential when evaluating the sedimentary dynamics of the basins themselves (Scholtz 2001) and secondly, the retention of nutrients (e.g. Rask et al. 2004). Lake Ormajärvi gives an example of lake in which the focus of the sedimentation has distinctly changed in time, most likely because of human impact. The agricultural areas as well as the impact of sewage plant and dairy have increased the rate of sedimentation in the southeastern part of the lake. The sediment thickness, 8 m, in this part of the lake compares with the thickness in the northern sedimentation pool, but on the basis of the silty horizon (sediment depth of 26 cm), which was probably caused by lowering of the lake in the 1820’s–1830’s (Anttila 1967), the recent sedimentation has been accelerated. The majority of the total amount of ca. 18 million m3 of post–glacial gyttja has deposited in the eastern part of the lake. The outlet area, which is shallow and restricted from the deep basins of the lake by a moraine ridge, represents an area of transportation and erosion. The basin of Lake Suolijärvi contains approximately 7.7 million m3 of post-glacial sediments, the maximum sediment thickness being ca. 10 m. The shape and bathymetry of Lake Suolijärvi are rather simple, and the sedimentation occurs mostly at the deepest part of the outlet end of the lake. Since the isolation from the Baltic basin ca. 10 200 B.P. the focus of the sedimentation has – due to land uplifting (Sauramo 1958; Saarnisto 1971), and the infilling of the basin as well – gradually transferred to the south-east along the basin. The shallow inlet end of the lake is a transportation area. Lake Lehee is the only sediment flow-

taxa are more distinct, as is the increase in the sedimentary refractory phosphorus, LOI and DITP. This eutrophication is most likely connected to the early 1980’s intensive cottage construction in the catchment area. A similar eutrophication in lakes affected by artificial activities is reported for example by Wilkinson et al. (1999) or Karst and Smol (1998). The obtained results may indicate further success of the macrophytes, which has maintained the stable, clear water state of the lake. This corresponds with the observations from Florida by Brenner et al. (1999) and from Canada by Karst and Smol (2000). A similar state of equilibrium is described also in Scheffer et al. (1993). The results of Paper I show that data obtained only from water quality measurement cannot provide adequate information for lake management. In the case of Lake Kaljasjärvi the ecological changes are not as extreme as could be caused by the changes in the lake-water phosphorus concentrations. However, the factors affecting the extremity of the ecological changes, determinable only by using paleolimnological methods, are essential to know. This is especially important in shallow lakes like Lake Kaljasjärvi, in which is possible that change in macrophyte dominance could cause severe changes in lake stability (Kenney et al. 2002; Waters et al. 2005). The conclusion of the sediment study of Lake Kaljasjärvi stresses, that not only actions to reduce the external load are indispensable, but also, that the proportion of macrophytes may not decrease. In planning and choosing methods for restoration activities this kind of information is essential.

3.2. Paper II The investigation of the sedimentation patterns in six lakes of the River Kokemäenjoki upper watercourse, South Häme, was done to determine the bottom dynamics and the distribution of the sediments of the lakes. The obtained information is important for better understanding of the retention of nutrients and suspended material in lakes. The studied lakes, Ormajärvi, Suolijärvi, Lehee, Pyhäjärvi and Iso-Roine constitute a lake chain, which as a research area is a part of

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significant accumulation area in the central part of the lake. Only a minor proportion of the total sediment of Lake Iso-Roine (ca. 39 million m3) is accumulated in the shallow areas outside the fracture zone. The sedimentation rate (mm yr -1) and carbon content (%) were evaluated from the long cores of lakes Ormajärvi, Suolijärvi, Pyhäjärvi, IsoRoine and the reference lake, Lake Pääjärvi. In the case of Lake Lehee determination of the sedimentation rate was not possible due to unstable accumulation conditions. The highest rate of sedimentation was in Lake Iso-Roine where it has fluctuated between ca. 0.4–4.0 mm yr -1. In the other NUTRIBA-lakes and in Lake Pääjärvi the sedimentation rate has ranged between ca. 0.3–2.0 mm yr -1, which is common in the lakes of the Finnish Boreal zone (Pajunen 2004). The fluctuations in the trends of the carbon accumulation in the sediments in NUTRBAchain lakes are not straightforward. In lakes Iso-Roine, Pyhäjärvi and Ormajärvi the carbon content increases from 5% at ca. 10 000 B.P. to over 10% at ca. 2250 B.P., then declines back to 5% at 1000 B.P. and then increases towards the present day value. In lakes Suolijärvi and Pääjärvi the carbon content increases after the isolation up to ca. 12–14% at 1350 B.P. and then declines to the present day value. The interesting feature is that the covariation of the carbon accumulation curves in these two lakes is extraordinary high. Considering that the lakes are located on opposite sides of the local watershed and that their catchment area, size, shape and sedimentation dynamics differ remarkably, there seems to be a regional factor, e.g. change in palaeohydrology, that regulates long term changes in sediment accumulation in the area. The determinations of sedimentation rates and time-related carbon accumulation were possible due to the good functionality of the paleomagnetic dating, although it is evident, that calculations carried out on single coring locations can only provide rough estimations. The results of this study show that paleomagnetic dating is a method to be reckoned with in studies concerning the long term changes and their impact in planning the lake management also.

through lake of the in the NUTRIBA-chain. The theoretical retention time of the lake is only ca. 35 days. The basin itself is shallow (maximum water depth 4 m), and it is filled with sediments to the maximum level enabled by bottom dynamics. The total amount of the sediment is ca. 1.6 million m3 and maximal sediment thickness ca 4.5 m. There is a continuous state of erosion, transportation and re-deposition in the lake and no stable accumulation occur. Therefore, the dating of the sediments of Lake Lehee was impossible. Lake Pyhäjärvi, as a larger lake than the upstream lakes of the NUTRIBA-chain, also has much more complex sedimentation dynamics. Several accumulation pools are formed into a bedrock fracture zone (Matisto 1976) along the north side of the lake. As in Lake Suolijärvi, the tilting of the lake basin has transferred the focus of the sedimentation, which at present is in the deepest basin of the north-eastern part of the lake. This pool is near the inflow of the lake. In the narrower, western part of the lake the fracture zone has been partially filled with shore deposits which have eroded from the northwest–southeast orientated esker located on the southern side of the lake. This esker crosses the lake and isolates the south-western corner of the lake to an independent sedimentation basin, which effectively collects the matter from the south-western inlets of the lake. The maximum thickness of the sediment detected in the main sedimentation pool is ca. 15 m and the total amount of the sediment is over 30 million m3. On the other hand, the mean thickness of the sediment accumulations in Lake Pyhäjärvi is only 3.4 m, which underlines the importance of the relatively small sedimentation pools of the lake. The studied part of Lake Iso-Roine is also characterised by the same bedrock fracture zone as Lake Pyhäjärvi. The deepest basin of the lake, 76 m, is in this fracture and collects the majority of the incoming matter from the NUTRIBAchain. The sedimentation rate in this pool is the highest in the entire lake chain and the thickness of the sediments is almost 16 metres. In addition to this single basin, the fracture zone continues to the north-west through the lake and is a

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phosphorus is at present extraordinarily high, over 12 mg g-1 DW, which is tenfold compared to the state before the rapid eutrophication of the lake beginning in the late 1960’s. On the other hand, in the eastern basin the phosphorus content of the sediment has not increased in proportion, although the consequences of the eutrophication for the overall condition of the lake have been much more severe. The calculations based on the results of this study show that the phosphorus storage of the surface sediment (a layer of 5 cm) in the main sedimentation pool of the western basin is approximately 7300 kg, whereas in the eastern basin it is only ca. 1400 kg. The majority of the phosphorus in the western basin is HClextractable and refractory phosphorus and the proportion of the NaOH-extractable fraction is also notable. In the eastern basin over 50% of phosphorus is refractory fraction and the proportion of the NaOH-extractable fraction is minor. The diatom flora in both basins reflects eutrophication. The taxa of the bottom samples of the cores are comparable and represent rather similar water quality in both basins, as does to the DITP (ca. 10 μg P l-1) also. In the upper part of the profiles species such as Stephanodiscus hantzschii and Fragilaria crotonensis, make up a remarkable high proportion of the taxa. At present S. hantzschii is the dominant species in the western basin, whereas in the eastern basin F. crotonensis is the most abundant species. The DITP shows the increase in surface water phosphorus content, being ca. 50 μg P l-1 in the western basin, and ca. 25 μg P l-1 in the eastern basin. Compared to the water quality monitoring data of the western basin, the values given by DITP are significantly higher. In the case of the eastern basin the water monitoring data is scattered, but the few obtainable measurements give the same kind of result. Considering the surface sediment analyses, the quality of the sediment outside the deepest basins is good. The total phosphorus content is ca. 2.5–4.5 mg g-1 DW, which is high, but there are no signs of anoxic conditions or gas ebullition. The results of this paper indicate that the western basin of Lake Hormajärvi has until now

Moreover, the importance of echo soundings as a preliminary study for the sediment sampling, as well as the knowledge they can provide concerning the sediment distribution, must be emphasized.

3.3. Paper III The Lake Hormajärvi study was conducted to determine the recent (< 100 years) changes in the eutrophied lakes’ phosphorus accumulation and to investigate the paleolimnological differences between the two basins of the lake. The eastern basin of the lake has suffered severe water quality problems for decades, whereas the water quality of the western basin has been only a minor nuisance for the recreational value of the lake. One short sediment sequence was cored from both the western and eastern basin of the lake. In addition, four surface sediment samples were taken from the western basin and two from the eastern basin to detect the surface sediment properties in different parts of the lake. The results obtained from these samples were compared with the water quality monitoring data to explain the differences in the response of the sediment to an increased phosphorus load. Distinct signs of anoxic conditions were detected from both basins’ sediments. According to SCP dating these sulphide gyttja layers deposited in the late 1960’s. Simultaneously, W and LOI start to increase significantly, and this increase continues up to the sediment surface, being at present ca. 20%. It must be taken into account that the values of LOI in Lake Hormajärvi are rather low (ca. 10%), below the mentioned sulphide gyttja horizons. The average grain size distribution of the sediment during last few decades is stable. The only significant changes in grain size occur at the sediment depth of ca. 16 cm in the western basin and ca. 22 cm at the eastern basin. These changes towards finer grain size have been interpreted to be caused by artificial lowering of the lake in the late 1800’s (Anttila 1967). The development of the sedimentary phosphorus accumulation is complex. In the western basin the concentration of total

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different units were identified. The lowest unit is undisturbed detritus gyttja, which continues up to the depth of 8 cm. Above this there exists a 2 cm thick, grey, fine-grained, sticky layer. From 6 to 2 cm the sediment is greenish, homogenous gyttja with some black mottling. The surface sediment is very loose, brown gyttja. According to the 210Pb and 137Cs datings the solid 2 cm thick layer was accumulated during the 1940’s and 1950’s. This interpretation is also in good agreement with an earlier SCP dating done by Salonen and Tuovinen (2001). This layer was deposited during the most intensive mining period of the Orijärvi mine. It contains exceptionally high concentrations of heavy metals, which are related to the past mining activities, such as zinc (3.4%), copper (0.39%) and lead (0.29%). The history of the metal load is, however, more complicated. The elevated concentrations of elements occur already from the 1910’s–1920’s, right after the introduction of the flotation technique. Especially, the sulphur concentration seems already to be high in the early stage of the use of flotation. On the other hand, concentrations of heavy metals, such as cadmium, lead and zinc in the sediment are still, 50 years after the closure of the mine, very high. Also, the water analyses show that the concentrations of these metals in the lake water are still increasing. The effects of the metal load on the lake ecosystem have been severe. In the diatom taxa the abundance of the planktonic species decreases, whereas the proportion of tychoplanktonic species such as Fragilaria capucina (and varieties) increases. An extensive abundance of the F. capucina group is reported also from mine-water polluted Lake Pidisjärvi, central Finland (Kauppila 2006). Moreover, the increase of the abundance of Achnanthes minutissima and Anomoeoneis vitrea is simultaneous with the increase of the metal load. Changes like these are comparable to changes detected in Lake Orta, northern Italy, where the heavy copper load has affected the lake (Ruggiu et al. 1998). The biodiversity of the upper part of the core is very low and only remains of littoral diatom species can be found. Because there are no signs of lowering of pH in Lake Orijärvi,

acted as an effective sink of phosphorus, whereas the eastern basin is undergoing continuous cycling of phosphorus. However, the water quality measurements, as well as the diatom taxa, show that in the western basin internal load may also be a frequent phenomenon if anoxic conditions occur. The information obtained from this study is essential for successful lake management, showing that the inactivation of the internal load in both basins by sediment treatment is necessary, although the immediate impact of treatment would be more effective in eastern basin. For a better understanding of the history of the lake, the study should be completed by data obtained from long core sampling.

3.4. Paper IV The first mining operation using the flotation enrichment technique in Finland, Cu (Pb, Zn) mine of Orijärvi area (in operation during 1911–1955), has had a severe impact on nearby situating Lake Orijärvi. The lake ecosystem has changed drastically due to mining activities. The goal of the Lake Orijärvi sediment study was to describe in detail especially the impact of the past mining activity on the lake. The area of Lake Orijärvi itself is 1.7 km2, the volume 14.8 million m3 and the lake has a residence time of 4 years. The average depth of the lake is 8.5 m and the maximum depth 21.4 m (Vogt 1998). The tailings area located near the north-western corner of the lake is ca. 5 ha in size and contains approximately 400 000 tons of mine waste. It has been estimated that on the average the topmost 50 cm of this mine waste has been affected by weathering. Furthermore, the metal load from the tailings area to the lake can be estimated at 300 tons of lead, 800 tons of copper and 5400 tons of zinc. To study the effect of this load, two short cores were lifted from the 11 m deep pool located ca. 800 m from the tailings area. A crust freeze core (Renberg 1982) was taken for accurate sediment stratigraphy, diatom analysis and chrysophycean cyst analysis. The Limnos gravity corer (Kansanen et al. 1991) was used to collect parallel cores for elemental analysis and datings. In the sediment lithostratigraphy four

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artificial lowering of the lake in the late 1800’s (Anttila 1967). A condensed paleolimnological study was carried out in both lake basins to obtain natural background information for lake restoration purposes. During the preliminary sediment mapping of this study an exceptional sediment stratigraphy of the lakes was discovered and the occurrence of Trapa natans was detected. The original sampling, four surface sediment samples and a short core sample from both lakes, was then extended with 16 long core samples to investigate the detailed sedimentation history of lakes Etu- and Takajärvi, including water-level fluctuations during the Holocene and the recent eutrophication history. According to the results obtained from the long core studies the lakes partially dried up rather soon after their isolation ca. 9500 cal. B.P. This is evidenced by observed overconsolidated soil and remains of plant roots in the basal sediment. Three 14C AMS datings were done to determine the ages of the different stages of the indicated water level fluctuations during the Holocene. The change towards wetter conditions in these lakes occurred ca. 8700 cal. B.P., although the water level did not rise as high as it was before the driest phase. The alternating peat and gyttja deposits show a fluctuation in water levels during the Mid-Holocene. The occurrence of T. natans in Lake Takajärvi is closely connected to the water level changes and their corollations in the lake. Two separate layers with high amounts of T. natans remains were detected at levels 32.15 and 31.60 m a.s.l. The AMS-dated remains of water chestnut, taken from the upper of the described horizons, has a calibrated age of 7590±50 B.P. This age shows that the most extensive abundance of water chestnut in Lake Takajärvi occurred in the rather early stages of the Holocene climate optimum (e.g. Heikkilä and Seppä 2003). The disappearance of T. natans from Lake Takajärvi is straightforwardly connected to the water level fluctuations and the infilling of the habitat caused by extensive peat accumulation. The last, continuous rise in the water level took place from ca. 2500–2000 cal. B.P. onwards. After this point, accumulation of gyttja has been uninterrupted in the central parts of the lakes,

these changes in diatom taxa can be interpreted as a result of the metal load only. The chrysophycean cysts are impacted by the metal load as well. Although the abundance of cysts itself has not changed tremendously, the changes in cysts are evident. In the most polluted section of the sediment, only less than 1% of the detected cysts are identifiable. The great majority of the cysts are severe corroded and/ or damaged. Also, the fine grained metallogenic matter (< 1μm), which could not be removed during the preparation of the SEM samples partially prevents identification. For this reason, the results of chrysophycean cyst analysis must be interpreted with caution. However, it seems that the changes in the relationships in cysts preferring different ecological conditions are smaller than in the case of diatoms. Cysts do not disappear even during the most severe loading phase, and there after the relationships of different ecological groups are rather similar to before loading. The only permanent change is the total decrease in the number of cysts preferring oligotrophic conditions. To conclude, the AMD-derived metal load has affected the ecosystem of Lake Orijärvi in the most severe way. The high concentrations of heavy metals in the sediment are exceptional. The almost complete disappearance of diatoms, changes in taxa, in chrysophycean cysts and properties of the sediment itself, are clear evidence of a load, which has continued for decades and will continue if proper remediation activities in the tailings area are not carried out. Moreover, the results show that a well planned paleolimnological study is a useful and often the only tool to detect the impact and to date changes caused by a single, significant load source.

3.5. Paper V The twin lakes Etujärvi and Takajärvi are artificially impacted, small and shallow (the maximum depth 5 and 4 metres, respectively) headwater lakes located near Askola municipality, southern Finland. The lakes are mesotrophic, largely surrounded by peat accumulations and separated from each other by a thin, peat covered isthmus only. This isthmus was formed after the

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content are in general alike. The phosphorus content has an increasing trend. It is noteworthy that in the bottom sample of Lake Takajärvi the total phosphorus content is twofold compared to the bottom sample of Lake Etujärvi. This difference diminishes in the upper part of the profiles, and the total phosphorus content is ca. 1.7 mg g-1 DW at the sediment depth of 4–2 cm in Lake Etujärvi and ca. 2.0 mg g-1 DW in Lake Takajärvi. However, the phosphorus content of the sample 2-0 cm in Lake Takajärvi is only 0.9 mg g-1 DW, whereas in Lake Etujärvi it is the same as in the sample 4–2 cm. Refractory phosphorus is the most significant fraction in both profiles. A single aberration in the general increase of phosphorus content is detectable in both profiles; the sample 10–8 cm in Lake Etujärvi and 8–6 cm in Lake Takajärvi have significantly lower phosphorus content. The results of the diatom analyses of the profiles are different. In the profile of Lake Etujärvi there are clear changes in the taxa. Certain species common in the lower part of the profile, such as Fragilaria brevistriata and Pinnularia interrupta, decrease or disappear in the topmost 10 cm of the core, and species like Aulacoseira granulata and Cyclotella meneghiniana appears and increase in term. In the profile of Lake Takajärvi the changes in the taxa are minor. The same species, such as A. granulata, A. ambigua and Tabellaria flocculosa are common in the whole profile. The diatom inferred total phosphorus in Lake Etujärvi has an increasing trend. The lowest values in the bottom of the profile are around 15 μg P l-1, whereas the value at the sediment surface is ca. 30 μg P l-1. In the middle part of the profile, at the sediment depth of 12–6 cm the values of DITP do not fit in the general trend, but are ca. 10 μg P l-1 higher than values in samples below and above. In Lake Takajärvi the situation is different. The increase in the level of DITP during the profile is smaller, from ca. 20 to ca. 25 μg P l-1. The same kind of peak as in Lake Etujärvi exists at the sediment depth of 16–12 cm, where the values are ca 20 μg P l-1 higher than the general level of DITP. The results of the short core analyses confirm the eutrophication of lakes Etujärvi and

continuing even after the artificial lowering of the lakes. This lowering caused the separation of the lakes building up the isthmus nowadays covered with peat. Concerning the recent eutrophication history of the lakes, the results of the surface sediment analyses show that the in-lake sediment properties are at present rather uniform in Lake Etujärvi and in Lake Takajärvi. The LOI is around 20% in Lake Etujärvi and around 30% in Lake Takajärvi, except for the northernmost sample (ca. 90%) taken near the large peat accumulations. The median grain size in Lake Etujärvi is less than 10 μm except for the sample taken form the deepest pool (ca. 85 μm). In Lake Takajärvi the median grain size of the surface sediment samples is ca. tenfold, varying from 105 to 129 μm, except for the northernmost sample again (ca. 40 μm). The majority of the sedimentary phosphorus has deposited in refractory form, and the phosphorus content is not particularly high, varying from 1.3 to 1.7 mg g-1 DW in Lake Etujärvi and 0.9 to 2.2 mg g-1 DW in Lake Takajärvi. Despite the surface samples of the deepest basin, the sediment is fine detritus gyttja, rich in humic substances. Sulphide colouring and gas ebullition was observed only in the samples taken from the deepest pools of the lakes. According to the SCP dating of the short core samples the profile of Lake Etujärvi represents the time period from approximately the late 1940’s to the present, whereas the profile of Lake Takajärvi covers only the time since ca. the mid1960’s to the present day. During these periods, W of the sediment has increased from ca. 65 to 90% in Lake Etujärvi and from ca. 85 to 95% in Lake Takajärvi, whereas the change in LOI has been from ca. 11 to 25% in Lake Etujärvi and from ca. 22 to 31% in Lake Takajärvi. The grain size distribution of the sediment profile of Lake Etujärvi has two different phases: in the lowermost part the grain size is less than 200 μm, but in the upper part of the profile the proportion of the coarser, ca. 1000 μm, material is significant. In the profile of Lake Takajärvi there are no signs of the phase of finer material, but the grain size distribution as a whole is comparable with the upper part of the Lake Etujärvi profile. The changes in sedimentary phosphorus

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activities on the sediment to proper areas and establishes the case that certain restoration methods cannot be used in Lake Kaljasjärvi. The littoral areas must not be disturbed by water level changes or dredgings, and if the sediment will be treated (e.g. oxygenation, chemical stabilization), the activity must be restrict to the deepest pool of the lake only. In Paper II, thorough echo soundings and detailed sediment lithostratigraphical description are used as a tool to determine the sedimentation history of the lake basins since their isolation from the Baltic basin. The study showed that changes in larger scale environmental factors, e.g. climate, can control the overall sediment composition, but the sedimentation rate is more dependent on the development of an individual basin and its catchment. Moreover, the necessity of complete echo soundings as a basic tool in the first phase of a lake management project and the paleolimnological study related to it is evident. For the accurate direction of the activities the management of a complex lake sedimentation environment needs precise information of the morphometry and sedimentation patterns of the lake. However, the data obtained from the soundings and long core samples should connect to extensive surface sediment mapping with physical and chemical analyses. Only then is adequate information for successful management decisions available. The Lake Hormajärvi study (Paper III) gives an example of a defined, local-scale investigation of a eutrophied lake. Again, the water monitoring data alone have not been able to provide sufficient information for planning the lake management project. By contrast, a well planned paleolimnological study with a rather limited group of analyses provided information on the differences of the loading history of the basins, the accurate accumulation areas of the deteriorated sediment and the phosphorus storage of the sedimentation pools. This can already provide a basis for starting effective and cost-efficient management activities. However, for a better view of the lake’s history and for specifying a realistic target for remediation, a long core study of the lake’s basins should be carried out.

Takajärvi. Moreover, when the results are fitted to the SCP chronology, it can be seen that the eutrophication has been simultaneous, although the sedimentation in Lake Takajärvi has been faster. It is also evident that the extensive peat accumulation, which by wave erosion causes a heavy load of humic substances especially to Lake Takajärvi, is at least as severe a problem for the water quality as the present eutrophication. Overall, the study shows that small lakes with small catchment areas and minor groundwater inflow may have easily been affected by water level fluctuations caused by the climate changes. Such fluctuations have changed the properties and conditions in the lake ecosystem drastically. This, in association with infilling of the pool, was the factor behind the disappearance of the Trapa natans from Lake Takajärvi, as may have been the case in several former habitats of the water chestnut in southern Finland. This study also shows, that the management of these lakes is complicated: dredging of the peat accumulations would cause a state of unstable sedimentation and accelerated oxygen consumption and must therefore been carefully considered. Moreover, although the information of the short core studies is essential for determining the goals and tools for lake management, only the complete study of the history of the lake basin can give a full understanding for setting a target and the implementation of well planned restoration projects.

4. Discussion The results of papers I–V emphasize the importance of lake sediment research for lake management projects. In the case of Lake Kaljasjärvi the water monitoring data did not provide a reliable picture of the overall development of the lake. The perception of the two phases of eutrophication, the first caused by farming and the second by cottage settlement, was possible only using paleolimnological methods, as was the role of the macrophytes in the retention of phosphorus in Lake Kaljasjärvi. The echo sounding data shows the large littoral areas and rather restricted sedimentation pools of the lake, makes it possible to direct remediation

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The starting point of the study in the case of Lake Orijärvi was to describe the impact of a severe, well known metal load originating from a well known source, the tailing area of the Orijärvi mine. The sampling was targeted to the optimal sedimentation pool in consideration of the metal source and the analyses were chosen to describe the concentrations of accumulated elements themselves and, secondly, to show the changes in certain algal communities of the lake. With proper dating methods this study provided a detailed, time related description of the impact of mining on the lake during the different phases of the mine. This information has great value for the possible remediation activities both in the lake and in the tailings area, but also as a reference for the environmental control of comparable mining areas. In the Paper V a description of the development history of two, nowadays separated, lakes is described on the one hand as an example of the impact of the Holocene water level fluctuations on the small basins, and on the other hand as an example of lakes influenced by recent eutrophication. As an interesting detail, the disappearance of the water chestnut could be connected with the water level fluctuations mentioned above. This study shows the connection between the long term changes in the lake’s history and the water quality problems at present. The thick peat accumulations cause a heavy load of humic substances on the lakes. Connected with recent eutrophication this has led to a deterioration of the water quality. In this case the information obtained from the study shows that the goal of possible lake management must be very carefully considered; the natural state of the lakes has never been suitable for recreational activities. One of the main points in the results of papers I–V is the importance of sediment mapping. If a bathymetric map of the lake is available, it may be possible to focus the sampling on the areas, which are important for the ongoing study, e.g. sedimentation pools or areas of continuous resuspension. However, without echo soundings, ground penetrating radar (GPR) soundings or thorough coring of the lake, critical information of the sedimentation areas and patterns will be lost.

In contrast, if the mapping is carried out carefully with soundings and extensive sediment sampling, the bottom dynamics of the lake, the thickness of the sedimentation patterns, the areal distribution of the sediment and the areas of the deteriorated sediment can be determined. In many cases of restoration, i.e., dredgings or seinings, this kind of information may be satisfactory for the needs of planning or even implementation of the remediation activities. For further studies, such as chemical or biological analyses, information on the location of the sedimentation areas, their possible changes during the lake’s history, and the ongoing sedimentation processes, is almost mandatory. Moreover, without the soundings the existence of independent sedimentation basins and their relation to the load sources can hardly be determined. Eutrophication is probably the most common reason for the need of lake management. Hence, the fluctuations of the water phosphorus concentration as well as the sediment phosphorus content are often the most important subject of research. The water monitoring programmes in Finland started during the 1960’s (Keskitalo and Eloranta 1999), when eutrophication had already occurred in many water systems and lakes. Therefore, the usefulness of the water monitoring data series is often minor in the study of a detailed history of the lakes. Also, a comparison of the older data to the analyses done nowadays may be difficult because of the changes and development of the analytical methods. Diatoms have long been used in paleolimnological research in Finland as indicators of eutrophication (e.g. Alhonen 1972). During the last decade, the developments in obtaining quantitative data from paleolimnological analyses (e.g. Birks 1998) has made applications for reconstructing the total phosphorus content of lake surface waters possible (Kauppila 2002; Kauppila et al. 2002a; Miettinen 2005). This has clearly enhanced the ability to detect trophic level changes in lakes. The use of reconstructions is even more important because of the transient nature of the data obtained from water quality measurements. There again, the sedimentary phosphorus measurements may be difficult to interpret because of the post-depositional mobility of the phosphorus

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The actual sampling of the study should then be carried out according to the results of the sediment mapping, and description of the sediment should be done in detail. The long core sample(s) should core from the main sedimentation basin(s) of the study lake and describe in detail. At least magnetic susceptibility and LOI should be determined and dating using, for example, the paleomagnetic method or AMS radiocarbon dating should be carried out. The natural background state of the lake can then be determined by using case-specific methods, e.g. sedimentary phosphorus content, diatom analysis and selected analyses of elements. For more detailed information on the recent, artificially impacted development of the lake, short-core and surface sediment studies should be carried out. The sampling should be focused on the basis of the sediment mapping, and it should represent all different sedimentation environments and bottom dynamic zones, considering the inlets and outlets, as well as the possible point loaders of the lake. The sedimentary pH and redox measurements should be carried out during the sampling. The subsampling of the cores must be carried out with the highest possible frequency. Magnetic susceptibility, W, LOI and grain size distribution should be determined from all samples. The total phosphorus and phosphorus fractions, sedimentary plant pigments and at least a few focal elements, such as C, N, Fe, Mn and Si (see e.g. Itkonen 1997) should be analysed. Diatom analysis with total phosphorus reconstruction from the short core should be carried out and a proper dating method, i.e. 137Cs, 210 Pb or SCP counting should be used. In ideal cases of research, other biological indicators such as chironomids, cladoceras or chrysophytes, as well as macrofossil, pollen, and useful chemical or microbiological analyses could be used if needed. If the study concerns problems other than eutrophication, the proper chemical analyses should be chosen case-specifically. In practice, the budget of the lake management projects of eutrophied lakes in Finland is usually limited and the resources focused on the research rather small. Therefore, only the most essential work and analyses can often be carried out (Mattila and Kirkkala 2005).

(Carignan and Flett 1981; Søndergaard et al. 1996; Itkonen 1997), even though a proper fractionation method (e.g. Hieltjes and Lijklema 1980; Psenner et al. 1988; Ruban et al. 2001) of the phosphorus would may been used. The information provided by diatoms is always a more stable indication of the prevalent state of the lake. However, the comparison of the water monitoring data, the results of the sedimentary phosphorus analysis and diatom inferred total phosphorus may be very informative, as shown in the papers of this study. As a whole, the structure of the sediment study is highly dependent on the nature of the lake and the problems, which have caused the need of management activities. The set of analyses must therefore thoroughly consider and match up the knowledge of the history and of the contamination sources of the lake. Moreover, for the optimal result, the supplementing of the study with additional analyses should be possible. For example, in papers IV and V, the widening of the original research with chrysophycean cyst analyses and thorough stratigraphical analyses of the sediments improved the studies substantially. The necessity of datings is highly dependent on the purpose of the study. If the goal is, for example, to evaluate the amount of the sediment for dredgings or to determine the heavy metal content of the removable sediment, datings are not necessary or even rational to carry out. On the other hand, a thorough study of the lake’s history or the determination of a recent eutrophication unquestionably demands a dating of the sediment sequence.

4.1. Recommendations for practice The ideal lake sediment study as a part of a lake management project should be two-phased. In the first phase, thoroughgoing sediment pattern mapping should be carried out using either echo soundings or GPR soundings. Adequate corings should be implemented both for surface sediment and long-core samples, first to ensure the interpretations of the soundings, and second for the approximate calculations of the amount of the sediments.

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areal distribution of the sedimentation patterns and zones with a different bottom dynamics. It also evidences the nature and condition of the sediment and shows the long term changes both in sedimentation rate and the composition of the sediment. Also, the recent history of the lake, i.e., instrumental water monitoring data (e.g. phosphorus concentrations) and fluctuations in sedimentary phosphorus content, as well as the phosphorus storage of the sediment can be considered. By extending the set of chemical analyses these parameters can be determined for heavy metals also. However, valid recommendations for lake sediment study proper in each lake can not be given. A successful lake sediment study can be planned only after a thorough familiarisation with the catchment area, its infrastructure and history, proper sediment mapping and preliminary investigations. The results of lake sediment studies are often reported to the consumer as unpublished reports in which not all the possibilities of the data and its modern processing are utilised. This procedure is not acceptable. If the needs of the requester of the study do not require confidentiality, the results should unquestionably be published at least in the national series, but preferably in international refereed journals.

Thorough sediment mapping and at least one long core sample from the central sedimentation basin with magnetic susceptibility, LOI determinations and, if possible, dating, should be performed in every study. This provides an opportunity to focus the study the critical areas of the lake and to evaluate the number and length of short cores and surface sediment samples needed. It also allows the possibility of evaluating the amount of sediment and the quantities of the different elements, e.g. phosphorus, in the sediment accumulations. The short core samples should be analysed for magnetic susceptibility, W, LOI and, if possible, grain size distribution. The sedimentary phosphorus determinations and fractionations, and the diatom analyses with phosphorus reconstructions are essential in studies related to eutrophication. For the evaluation of the recent sedimentation rate and to date the changes in the sediment properties the short cores should be dated with 137Cs or 210Pb. The SCP analysis can also be used, but the results may be difficult to interpret and the SCP method as an only dating method does not necessarily provide a detailed dating. The surface sediment samples must undergo the same analyses, except for the magnetic susceptibility and diatom analyses. This set of analyses provides knowledge of the

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5. Conclusions

carried out at least from one long core sample and the amount of the sediment in the lake basin is evaluated. The eutrophication development of the lake is described by means of sedimentary phosphorus content, phosphorus fractionation and diatom analysis with the total phosphorus reconstruction. These analyses are determined from a dated short core and combined with the physical properties of the sediment.

1. A lake sediment study should be a part of each lake management project. A properly performed sediment study enhances the planning of the restoration, helps to define the target of the remediation activities and thereby also improves the cost-efficiency of the project. 2. A thorough sediment mapping including echo/ GPR soundings and adequate corings should always be part of the lake sediment study. Without the information on the location of the sedimentation patterns and bottom dynamics the conceptualisation of the lake system is insufficient.

4. A lake sediment study must be planned and scheduled so that the extension of the study is possible during the lake management project. 5. The present state of the surface water, the data obtained from the surface sediment samples and the long core records must be integrated and used in their entirety to provide a general view of the lakes history, its natural background state and the magnitude of the changes caused by artificial impact.

3. A proper sediment study related to the managing of a eutrophied lake includes a detailed determination of the areal distribution and the nature of the sediments. The description of the lake’s history is

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7. Acknowledgements

cooperation and spurring. At the university of Helsinki Dr. Anu Hakala, Dr. Mia Kotilainen, Dr. Arto Miettinen, Dr. Minna Väliranta, Dr. Anu Kaakinen, Dr. Seija Kultti, Dr. Mia Tiljander, M.Sc. Mikko Haaramo, prof. Ragnar Törnroos, Kirsi-Marja Äyräs and Helena Korkka, among others, gave valuable advices and support whenever they were needed, thank you. Petrik and Sari, you are warmly thanked: the possibility to stay under your roof during weekends and hunting seasons gave the breaks that really were needed. Mother and Seppo, Paula and Kalevi; thank you for all kind of support that you have given during this work. M.A. Mikko Karjalainen and M.Sc. Jarno Laivola, directors of the Jägermeister-Kilta: all the scientific seminars, conferences and working committee meetings have been extraordinary valuable. I feel deeply honoured to have friends like you. And Salla, where would I be without you.

First of all, I would like to thank professor VeliPekka Salonen, whose patience and, there again, enthusiasm for new projects, made this work possible. Reviewers, Professor Matti Tikkanen and Dr. Atko Heinsalu are thanked for their constructive and thoughtful comments. It has been a privilege to work with co-authors like Dr. Tommi Kauppila, Dr. Antti Ojala, M.Sc. Jutta Forsell, M.Sc. Nanna Tuovinen and, professor Salonen, thank you. The Graduate School of Geology has supported this work financially. Old colleagues at the university of Turku, Dr. Eila Varjo, Licentiate Reijo Pitkäranta, Dr. Aki Artimo, M.Sc. Mika Mäenpää, M.Sc. Päivi Heikkinen, Licentiate Petri Siiro, Dr. Kirsti Korkka-Niemi, M.Sc. Sami Saraperä, Hannu Wenho and others; thank you for your

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