Ladoga and Onego - Great European Lakes Observations and Modelling Leonid Rukhovets and Nikolai Filatov (Editors)

Ladoga and Onego - Great European Lakes Observations and Modelling

Published in association with ~ Praxis Publishing Springer Chichester, UK Editors Professor Leonid Rukhovets Professor Nikolai Filatov Institute for Economics and Mathematics Institute of Northern Water Problems at St. Petersburg Karelian Research Centre Russian Academy of Sciences Russian Academy of Sciences St Petersburg Petrozavodsk Russia Russia

SPRINGER-PRAXIS BOOKS IN ENVIRONMENTAL SCIENCES SUBJECT ADVISORY EDITOR: John Mason, M.B.E., B.Sc., M.Sc., Ph.D.

ISBN 978-3-540-68144-1

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Cover design: Jim Wilkie Project copy editor: Mike Shardlow Typesetting: Aarontype Limited

Printed in Germany on acid-free paper Contents

List of contributors...... IX

Preface Xl

Acknowledgements ...... xv

1 The Great European lakes: state of the art...... 1 1.1 Physiographic features and history of the formation of the lakes and their catchments...... 1 1.2 History of research of the lakes...... 9 1.3 Characteristics of temperature and currents 14 1.3.1 The thermal regime and limnic zones 14 1.3.2 Currents and circulations 23 1.4 The cycle of substances in Lake Ladoga and the dynamics of its warer~osy~em 31 1.4.1 Lake ecosystem phosphorus supply...... 31 1.4.2 Phytoplankton in the Lake Ladoga ecosystem 33 1.4.3 Bacterioplankton, water fungi and destruction processes 39 1.4.4 Zooplankton...... 41 1.4.5 The role of the zoobenthos in the ecosystem 42 1.4.6 Dissolved organic matter 44 1.4.7 The role of seston and bottom sediments in the lake phosphorus cycle...... 46 1.5 The cycle of substances in Lake Onego and its water ecosystem 47 1.5.1 Phosphorus supply to the Lake Onego ecosystem 48 1.5.2 Biological communities in the Lake Onego eutrophication state 51 vi Contents

1.5.3 Relation between the primary production and the destruction of organic matter...... 59 1.5.4 Peculiarities of Lake Onego eutrophication ...... 60 1.6 The main tendencies in the evolution of large, deep, stratified lakes. . .. 61

2 Hydrothermodynamics of large stratified lakes 67 2.1 Ensemble of thermo- and hydrodynamical processes and phenomena in lakes 67 2.2 Lake models: state of the art. Problem formulation for the simulation of lake hydrothermodynamics ...... 69 2.2.1 Introduction...... 69 2.2.2 Equations of geophysical hydrodynamics...... 70 2.3 A climatic circulation model for large stratified lakes ...... 73 2.3.1 General comments 73 2.3.2 Mathematical formulation...... 76 2.3.3 Realization of the model...... 80 2.3.4 Generalized formulations of the mathematical model ...... 80 2.3.5 About the discrete model ...... 83

3 Climatic circulation and the thermal regime of the lakes ...... 85 3.1 The climatic circulation in Lakes Ladoga and Onego from observational data and estimates...... 85 3.2 On the problem of simulating climatic circulation ...... 87 3.3 Setting of external forcing ...... 91 3.4 Simulation of the Lake Ladoga climatic circulation...... 97 3.4.1 Computational procedure...... 97 3.4.2 Description and analysis of thermal regime calculation results.. 99 3.4.3 Description and analysis of current calculation results...... 112 3.5 Simulation of the Lake Onego climatic circulation ...... 122 3.5.1 Computational procedure...... 122 3.5.2 The results of thermal regime modelling...... 122 3.5.3 The results of currents simulations...... 129

4 Estimation of the lakes' thermohydrodynamic changes under the impact of regional climate ...... 134 4.1 Climate change over the lakes' catchments...... 134 4.1.1 Climatic features and their variability ...... 134 4.1.2 Probable climate changes over the lakes' catchments...... 138 4.1.3 Estimates of potential changes in the thermal regime of the lakes by 2050...... 143 4.2 Modelling the thermohydrodynamics of the lakes under different climatic conditions...... 150 4.2.1 Modelling thermohydrodynamics: statement of the problem and numerical experiments...... 150 4.2.2 Analysis of the results of simulations...... 155 Contents vii

5 Three-dimensional ecosystem model of a large stratified lake ...... 163 5.1 Modelling the functioning of the lake ecosystems: state of the art 163 5.2 Aquatic ecosystem mathematical model 165 5.3 Discrete models ...... 168 5.3.1 Discretization of the solution domain 169 5.3.2 Reconstruction of transport, turbulent diffusion and the sedimentation of substances in the model ...... 171 5.3.3 Reconstruction of the transformation of substances...... 173 5.3.4 Total variation of the concentration of substances in additional division cells 173 5.3.5 Discrete analogue of the total substances content variation law in lake waters ...... 173 5.3.6 Changes in the discrete model with coarsening of the domain decomposition...... 175

6 Ecosystem models of Lakes Ladoga and Onego ...... 179 6.1 The history of the ecosystem modelling of Lakes Ladoga and Onego. . 179 6.2 Complex of Lake Ladoga ecosystem models ...... 182 6.3 Ecosystem model for Lake Onego, based on the turnover of biogens - nitrogen and phosphorus ...... 186 6.3.1 Ecological formulation of the model 186 6.3.2 Mathematical formulation of the model 188 6.3.3 The discrete model 192 6.3.4 Reproduction of Lake Onego annual ecosystem functioning ...... 197 6.4 Lake Ladoga phytoplankton succession ecosystem model ...... 206 6.4.1 Formulation of the model 208 6.4.2 The discrete model 212 6.4.3 Model verification, computation experiments 217 6.4.4 Reproduction of phytoplankton succession 219

7 Estimating potential changes in Lakes Ladoga and Onego under human and climatic impact ...... 227 7.1 Modelling changes in the Lake Ladoga ecosystem under different scenarios of climate change and anthropogenic loading ...... 228 7.1.1 Modelling changes in the ecosystem under different scenarios of climate change 228 7.1.2 Modelling changes in the ecosystem under different scenarios of climate change and changes in the level of anthropogenic loading...... 232 7.2 Modelling changes in the Lake Onego ecosystem under different scenarios of climate change and anthropogenic loading ...... 238 viii Contents

8 Lake Ladoga and Lake Onego models of fish communities ...... 247 8.1 Introduction...... 247 8.2 Model description ...... 249 8.3 The models study ...... 354

9 Natural resources of Lakes Ladoga and Onego and sustainable development of the region ...... 261 9.1 Water supply and management in the catchments. Legal and regulatory aspects of water use...... 261 9.2 Assimilation potential of lake ecosystems and sustainable development of the region...... 268 9.2.1 Introduction...... 268 9.2.2 Assimilation potential of the natural environment...... 271 9.2.3 Quantification of the assimilation potential of the ecosystems of Lakes Ladoga and Onego ...... 271 9.2.4 Economic quantification of assimilation potential...... 273 9.2.5 Mathematical economic model ...... 274 9.2.6 Computational experiments...... 276 9.2.7 Conclusions...... 280

Afterword...... 281 References ...... 283 Index 299

The colour plate section appears between pages 144 and 145. List of contributors

G. P. Astrakhantsev Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, S1. Petersburg N. N. Filatov Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk A. V. Litvinenko Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk V. V. Menshutkin Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, S1. Petersburg T. R. Minina Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, S1. Petersburg L. E. Nazarova Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk N. A. Petrova Institute of Limnology, Russian Academy of Sciences, S1. Petersburg V. N. Poloskov Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, S1. Petersburg L. A. Rukhovets Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, S1. Petersburg A. V. Sabilina Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk Ju. A. Salo Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk A. Yu. Terzhevik Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk T. M. Timakova Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, Petrozavodsk Preface

Problems of environment pollution and depletion of natural resources have become global. One such problem is the shortage of potable water in many parts of the world. Although Russia is one of the world's richest countries in terms of water resources, some problems with public potable water supply do exist here too. The region of the Great European Lakes is very rich in surface and ground waters, and the water factor does not limit economic development of northwest Russia. The Great European Lakes - Ladoga and Onego - attract the continuously increasing attention of both researchers and end-users. The importance of the Great European Lakes proper, to drinking purposes, recreation, transport, and energy, together with the use of bioresources and the impacts of the pulp-and-paper industry and the discharge ofwaste from cities and towns located on the shores and in the catchment areas, will require the working out of scientifically substantiated recommendations for the rational use and the protection of the resources. A serious problem for the Great Lakes of Europe, as for the other very large lakes of the world, is anthropogenic eutrophication. The necessity ofminimizing the impact of anthropogenic eutrophication and water pollution, which have reached a global scale and jeopardize the quality of already limited freshwater resources, has triggered quite a number and variety of studies in limnology, mathematical modelling, and economics, with view to the conservation, restoration, and efficient use of the resources of large stratified lakes. The authors undertook to develop a set of mathematical models that help to rework available knowledge about hydrophysical, chemical and biological processes in large stratified lakes into adequate reconstructions of circulation, temperature regime and function of the ecosystems. This set of models is meant to be a tool for handling the tasks of managing water use and conservation of the natural resources of large stratified lakes, the prime consideration being water quality. This monograph is based on the authors' work in the development of mathematical models of the hydrothermodynamics of deep stratified lakes and ecosystem xii Preface models, as well as in the application of the models to reproducing circulation, temperature regime and function of the lake ecosystems. An equally important component of the book is the description of the results of long-term limnological studies of Lakes Ladoga and Onego implemented by researchers from the Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences, and Institute of Limnology, RAS, and development of mathematical models in the Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, including the authors of this monograph. This book Ladoga and Onego - Great European Lakes: Observations and Modelling addresses the contemporary state of the largest lakes of Europe and their catchment under anthropogenic and climate changes, with special emphasis placed on feedforward and feedback interactions between aquatic ecosystems, watershed hydrology and the economy of the region. To investigate the responsive- ness of both environments to the respective counter-impacts, as well as to regional and global climate change, data analysis of multi-year field observation numerical modelling are exploited. This book is a first attempt to apply a quantitative approach to the assessment of changes occurring at present and anticipated in the future to dynamic relationships between the anthropogenic impacts, climate change and water ecosystems of both of the largest lakes of Europe. Thus, the book is primarily a synthesis of multifaceted interdisciplinary studies conducted by a team of experts working in a wide spectrum of natural and human sciences. Indeed, it is a synthesis of limnology, mathematics, hydrobiology, hydrochemistry, thermohydrodynamics, aquatic ecology, and economics. The book consists of nine chapters. Chapter 1 addresses a wide range of issues related to the geographical position, origin and palaeogeological background of Lakes Ladoga and Onego catchments. The knowledge of the physical geography of the catchments is essential for understanding the fundamental features of the lacustrine environments. Discussed here are the most reliable and recently updated data on the hydrodynamics ofthe lakes. The dynamics ofhuman impact on the lakes and their catchments is analysed. Special attention is paid to investigations of material cycles in Lakes Ladoga and Onego and dynamics of the ecosystems. The chapter contains detailed information on the chemistry and biology of the lakes. The last paragraph of the chapter is devoted to the main tendencies in the evolution of great deep lakes Ladoga and Onego. Chapter 2 discusses the range of water movements in the lakes, circulation patterns and currents, as they are influenced by atmospheric forcing. The approaches to choosing the hydro thermodynamic model are explained. The state ofthe art ofthe lake models is reported. Geophysical hydrodynamics equations and their applications for description and simulation of lake hydrothermal dynamics are presented. The main objective of modelling the dynamic hydrothermal regime in our monograph is to offer ecological models with information on abiotic environment factors, first of all hydrophysical processes, which to a large extent control the functioning of aquatic ecosystems. Chapter 3 is devoted to the reproduction of climatic circulations in lakes. The problem statement is given, and the issues of setting the external parameters Preface xiii are discussed. The central issue in the chapter is the results of modelling water dynamics in the lakes. Chapter 4 is dedicated to data analysis of long-term observations of the hydrometeorological regime in the catchment. Estimates of regional climate changes are made, climatic data are put together and analysed to reveal tendencies in climate change in the lakes' catchment. Climatic fluctuations in the region evidence ongoing warming. Possible climate scenarios are estimated using global climate models (ECHAM-4) and IPCC scenarios for Lake Ladoga and Onego basins. Based on these analyses, numerical simulations were performed in order to explore the options offuture alterations to the regional climate against the background of global climate change scenarios. In the second part of the chapter, the hydro thermodynamic model for large stratified lakes is applied to estimate potential changes in the lakes' temperature regime and currents until the year 2050. Chapter 5 formulates the 3D mathematical model ofthe aquatic ecosystem ofan abstract waterbody, represented as a system of nonlinear differential equations in partial derivatives. The chapter postulates requirements to the structure of the descriptors of the processes of biochemical transformation of matter in lake ecosystems. If these requirements are satisfied, long-term (many-year) calculations can be performed by the models. The ecosystem model for Onego is an adaptation of the model produced for Ladoga (Astrakhantsev et al., 2003). The biotic part of the model is based on the model developed by Menshutkin and Vorobyova (1987). The model of Lake Ladoga ecosystem is the phytoplankton succession model the most advanced one of all the ecosystems models developed for Ladoga. Chapter 6 focuses on the development of ecosystem models. The history of Ladoga and Onego ecosystem modelling is briefly described in this chapter. Also, the models of the lakes' ecosystems developed by the authors in recent years and representing, in fact, an integrated complex, are reviewed. Major attention in this chapter is paid to two models, that is, the Lake Onego ecosystem model, based on the nitrogen and phosphorus cycles, and the latest one developed by the authors: the model of phytoplankton succession in Lake Ladoga. The coupled thermohydrodynamic and ecosystem models for Lakes Ladoga and Onego have been developed to study the contemporary situation, to understand the main mechanisms of the ecosystem transformation, and to learn what may happen in future under the varying anthropogenic impact and climate change. Models are developed that enable simulation of hydrodynamics, phytoplankton, zooplankton communities, distribution and transformation of dissolved oxygen, distribution and transformation of substances/pollutants, evolution of the lakes' ecosystems, and reliable quantitative estimation of eutrophication in Lakes Ladoga and Onego. Descriptions of the models are followed by examples of their application for the present day and for hind- and forecasting. Chapter 7 analyses the dynamics of the lakes' water ecosystems under climate change (warming and cooling) and anthropogenic impacts relying on observed and modelled data. The advanced mathematical model of phytoplankton succession including nine species of phytoplankton was developed. Special attention here is given to the feedforward and feedback interactions in these lakes and the xiv Preface catchment under various scenarios of regional climate change and anthropogenic nutrient loading. The results of analysis of observed data and numerical experiments are presented. Chapter 8 tells about the state of the art in modelling fish populations and their variability. In fish community models, active migration plays a dominant role in fish movements from one region to another within Lakes Ladoga and Onego. The main idea of constructing fish community models consists in separate description of trophic, population, and fishery processes that take place in the fish community. This study deals with the succession of the fish community species composition under eutrophication. Chapter 9 is devoted to the analysis of water supply problems, economy of the regions of the Great European Lakes, and their sustainable development. It describes modern systems of water management for the large lakes of the Russian Federation. The environment assimilation potential (EAP) is the ability of an environment to restore itself with regard to matter and energy loading as the result of economic activities. The authors suggest that economic estimates of EAP are obtained using the iterative procedure based on the 'trial and error method'. We combined ecosystem models with economic and mathematical models of the enterprises that use water resources in the catchment areas of the lakes. The main goal of economic estimation of EAP is definition of the fees for discharges of nutrients and pollutants to ensure conservation of the resources and aquatic ecosystems of the largest lakes of Europe. The book offers useful answers and tools for decision-makers. In the Afterword the authors show that the important feature of this book devoted to the study of the Great European Lakes is a combination of traditional limnological research with numerical modelling. A satisfactory correspondence between the results of numerical modelling and observational data collected in Lakes Ladoga and Onego, especially well-reproduced successive stages of the lake ecosystem transformation, allows us to conclude that the main patterns ofecosystem functioning are reliably described by numerical models. This means that it is possible to use the models developed as a powerful tool in decision-making on the management of water use of the great lakes, and also for cognitive purposes. This book was written by the team of authors under the editing professors L. A. Rukhovets and N. N. Filatov. G. P. Astrakhantsev took part in Chapters 2 to 7; N. N. Filatovin the Preface, the Afterword, Chapters 3,4 and 9 and sections 1.1,1.6, 2.1, 6.1 and 7.2; A. V. Litvinenko in section 9.1; V. V. Menshutkin in Chapters 6 and 8; T. R. Minina in Chapters 6 and 7; L. E. Nazarova in section 4.1; N. A. Petrova in Chapters 6 and 7 and sections 1.4 and 1.6; V. N. Poloskov in Chapters 3, 4, 6 and 7; L. A. Rukhovets in the Preface, the Afterword, Chapters 2 to 7 and 9 and sections 1.1 and 1.6; A. V. Sabilina in section 1.5; Ju. A. Salo in section 4.1; A. Yu. Terzhevik in Chapter 4 and sections 1.3, 2.1, 3.1 and 6.3; T. M. Timakova in sections 1.5 and 1.6. Acknowledgements

This book is based on the results of the cooperations of the authors during the teamwork at the Institute of Limnology, Russian Academy of Science and those under realization ofjoint projects of the Russian Fund for Basic Research (RFBR) by the teamwork of the Institute for Economics and Mathematics at S1. Petersburg, Russian Academy of Sciences, and Northern Water Problems Institute, Karelian Research Centre, Russian Academy of Sciences. We involved some research results obtained by the Northern Water Problems Institute (NWPI), Karelian Research Centre, Russian Academy of Sciences, and the Institute of Limnology, Russian Academy of Sciences, and some published data of the Hydrometeorological Service. The authors of the book thank the projects of Basic Research supported by the Department of Earth Sciences of the Russian Academy of Sciences. The authors of the book thank their colleagues from NWPI, Drs N. Kalinkina, N. Belkina, P. Lozovik, M. Sjarki , T. Tekanova and Dr G. Raspletina from the Institute of Limnology, for kindly provided data analysis and useful recommendations and Dr R. Zdorovennov from NWPI for help. The authors also express their gratitude to Dr V. Podsechin for very important help, Mrs M. Bogdanova for preparing and redrawing figures for the present book. Special thanks go to academician S. Inge-Vechtomov and academician O. Vasiliev and Dr T. Florinskaja for support of our work. The authors extend their sincere gratitude to Dr T. Podsechina and O. Kislova for the translation of the book. The authors thank Mr I. Georgievsky for fine pictures of Lakes Ladoga and Onego. 1

The Great European Lakes: state of the art

1.1 PHYSIOGRAPIDC FEATURES AND IDSTORY OF THE FORMATION OF THE LAKES AND THEIR CATCHMENTS

Lake Ladozhskoe and Lake Onezhskoe (Ladoga and Onego respectively) are the greatest lakes in Europe. Another geographical object in Northern Russia has a similar name: the Onega River. At first the definition for the Great European Lakes (GEL) like Ladoga and Onego by analogy with the Great American Lakes (GAL) was used in a book written by Gusakov and Petrova, In front of the Great Lakes (1987). The authors called these European lakes 'Great', because of their size, their dimensions are larger than those of any other lake in Europe (Fig. 1.1, Table 1.1). From the point of view of geophysical hydrodynamics the Large Lakes of Europe (Ladoga and Onego) are the largest because the baroclinic Rossby radius of deformation is too small RR < L if compared with the lakes' horizontal dimensions. RR = Cdfwhere C, is the phase speed andfis the Coriolis parameter. In these lakes, the baroclinic Rossby radius of deformation RR during summer stratification is several kilometres, i.e, smaller by several orders than the lakes' horizontal dimension (RR < L); epilimnion thickness (hI) is much smaller than hypolimnion thickness (h2), hI < h2. That is why the effect of the Earth's rotation on water hydrodynamics is so essential. The Burger number Si, is defined as the ratio of the internal (baroclinic) Rossby radius deformation, RR' to a length scale, L, that characterizes the basin dimension. In the large lakes of Europe and North America this parameter in summer time is about 0.03-0.05 and in other large European lakes - Vennern, Geneva, Saimaa, Constance and others - this parameter is about 0.2-0.6. GEL as GAL represents system ofa unified lake. Lake Ladoga is connected with Lake Onego via River Svir, which is 224 km in length, with Lake Saimaa via River Vuoksa (Burnaya) and Lake Ilmen via River Volkhov (Fig. 1.I(b)). Lake system is linked with the Gulf of Finland in the Baltic Sea via River Neva. The surface area of Lake Ladoga is 17 891 km2 and volume is 902 krrr', it ranks among the top fifteen world's freshwater lakes and is comparable with surface area 2 The Great European Lakes: state of the art [Ch. I

...... """-.,.- .....----.-..J---

~r lIt. r'

I. ~ · - --- ...

I' . .. "':- r ' Russi - - ...... ;..... r ~

- -- """ Germ n - ' . ' • .,...... J'

•...... , -- ~... .., '.... • r-J / j.~ .... ~. -..." , - I I ) (a)

Fig. 1.1. Largest lakes on European map (a) and catchment of Lake Ladoga (b). ofLake Ontario (Figs. 1.2 and 1.3; Table 1.1). Lake Onego's surface area is 9600 km2 and its volume is 292 krrr' (Chernyaeva, 1966).1 The water renewal time is 11 years for Lake Ladoga and 14 years for Lake Onego and indicates that the lake ecosystems are rather conservative. The catchment ofthe Great European Lakes is 258 000 km2 and extends through northwestern European Russia and the eastern part of Finland, including the large lakes Onego, lImen and Saimaa (see Fig. 1.1). Lakes Ladoga and Onego are

1 Other dimensions of the lakes are published in a paper by M. A. Naumenko in Lake Ladoga. At/as, St. Petersburg, Nauka, 2002. Sec. 1.1] Physiographic features and history of the formation of the lakes 3

(b)

Fig. l.l(h). an important link in the Caspian-Baltic-White Sea waterway system. Ladoga and Onego are also a key section in the drainage basin of the Baltic Sea, which at present is receivingvery deep interest from the research, monitoring and protection communities. Both systems of Great Lakes in North America and Northern Europe have certain common features due to a similar genesisand the similar geological evolution of their hollows and catchments. They both were generated between contrasting crystalline shields (the Canadian in North America and the Fennoscandian (Baltic) in Europe) and plates: North American and Russian respectively, composed of Palaeozoic rocks. In the area of crystalline shields the ancient Earth rocks - granites 4 The Great European Lakes: state of the art [Ch. I

Table 1.1. Physiographic parameters of the largest European Lakes.

Lake Square, km2 Above sea Maximum Average Volume, level m depth, m depth, m km3

Ladoga 18000 4 230 51 910.0 Onego 9840 33 120 30 295.0 Vennem 5648 44 106 27 152.5 Pskovsko-Chudskoe 3558 30 15 7 24.9 (Peipsi) Yettem 1856 89 128 40 74.2 Saimaa 1800 76 58 20 36.0

:ero I 91 Q,~ ~ ~ a) :,>;, i1 © .I ~~ , t

=""'1l :co J "J. -.__. ~-:::_~_ . ~-~--~..__ .-/ ! ;: 3 z '5 E:

,....------, '

Fig. 1.2. Volume in km? (a) and area in km2 (b) of the large lakes of Europe: I, Ladoga; 2, Onego ; 3, Vennem; 4, Pskovsko-Chudskoe (peipsi); 5, Yettem; 6, Saimaa. and gneisses - are found at the surface with an age up to 2 billion years. The regions of neighbouring plates are presented mainly by limesstone, dolomites and sandstones with an age of less than 570 million years. The northern parts of the basins and catchments of Lakes Ladoga and Onego, as well as Lakes Superior and Huron, were formed on crystalline shields in places of ancient tectonic cracks, and the southern parts of these lakes, as basins of Lakes Michigan, Erie and Ontario were formed in Sec. 1.1] Physiographic features and history of the formation of the lakes 5

La On

Fig. 1.3. Bathymetry of Lakes Ladoga and Onego.

sediments during the last glacial period. The last Ice Age ofthe Quaternary period, at a maximum approximately 25-10 thousand years ago, was the most important factor affecting the basins and catchments of both lake systems. At their maximum stage glaciers completely covered the northern parts of North America and Europe, including the regions occupied by the modern Great American and European Lakes. The highest degree of glacial erosion took place in the boundary shield regions, where vast systems of near-glacial lakes were formed (Kvasov, 1975; Gusakov and Petrova, 1987; The History ofLadoga and Onego ... , 1990). The history of the development of Lakes Ladoga and Onego is linked with the initial stage of the formation of the Baltic Sea - the Baltic system of near-glacial lakes. The territories of Lakes Ladoga and Onego were covered with ice at that time (Kvasov, 1975). About 11.8 thousand years ago with regression of the ice the Baltic glacial lake was formed, which occupied the major part of the modern Baltic Sea basin. At that time the near-glacial lakes system was generated. The main waterbody of this system was the Southern Baltic glacial lake, which existed approximately 12-13 thousand years ago. This lake water level was higher than the ocean level and saline water did not penetrate into it. At that time Lake Ladoga was a bay of the Baltic Lake, connected with it via a narrow strait in the northern part ofthe Karelian 6 The Great European Lakes: state of the art [Ch.l

Isthmus. About the same time the southern and the eastern part of pre-Onego Lake basin were released from ice. As a result the Vytegorskoye and Vodlinskoye near- glacial lakes were formed. Later these lakes were connected, forming the Southern Onego glacial lake, which was not then a connected system with Lake Ladoga with outflow towards the basin of the White Sea. Only after glacier regression, did the River Svir carry its waters to Ladoga Bay of the Baltic glacial lake. Nearly 10 thousand years ago the Baltic Sea water level dropped by more than 20 m and equalled the ocean level. Oceanic waters penetrated into the Baltic Sea and created a waterbody, which was named the Ioldy Sea. The consequence of declination of the Baltic glacial lake level was that Lake Ladoga was no longer a bay and for the first time became a separate waterbody. The river appeared where the strait was, which carried its waters from Lake Ladoga to the Ioldy Sea. Since the Lake Ladoga level was at that time considerably higher that the Ioldy Sea level, brackish sea waters did not penetrate into the lake, where water remained fresh. During the same years which saw regression and melting of the ice, the formation of Lake Onego continued, of which the northern part was much larger than it is nowadays. After complete release from glacial cover about 4.5 thousand years ago, the northern shores of the lake underwent an isostatic rise, which has continued at a low rate up to the present day, and Lake Onego acquired something close to its modern shape. During the isostatic rise of Scandinavia the level of the ancient Baltic waterbody started to grow again and a freshwater body was formed out ofthe brackish water of the Ioldy Sea, named Lake Antsylovoye. At its maximum the water level of this lake exceeded the threshold magnitude for Lake Ladoga inflow and, as a result, in the northern part a shallow strait was formed. Lake Ladoga became a bay, this time of Lake Anstylovoye. About 8.4 thousand years ago Lake Antsylovoye formed a new threshold flow mark in the region of the modern Danish straits and, under the influence of erosion processes, the flow threshold mark decreased and the level of Lake Antsylovoye declined by 12m. Sea waters later penetrated into the ancient Baltic waterbody and for some time the Littorinovoye Sea existed there but, even at the stage of its maximal development, the level of Lake Ladoga was always higher, which prevented the spreading of saline sea waters. During the whole formation period Lakes Ladoga and Onego remained freshwater bodies. Thawing glacial waters brought to forming lakes a large amount of coarse and fine mineral particles and dissolved substances, including biogens that were stocked earlier in the glacier. These waters, comprising the basis of income water balance, were relatively rich in biogens; the lakes ecosystem productivity was low for several reasons: first, incoming glacial waters had low transparency and light was one of the limiting factors for phytoplankton development, and, second, low temperatures and a short vegetation period, typical for the late post-glacial period, influenced biota development (Davidova and Subetto, 2000). Thus, at the end ofthe Pleistocene, pre- Ladoga and pre-Onego were oligotrophic cold water bodies. Lake ecosystems were formed under the influence of gradual climate warming. The beginning of the Boreal period and climate warming resulted in further development of forest vegetation on the lakes' catchments and was characterized by deposition in the deep lakes of Sec. 1.1] Physiographic features and history of the formation of the lakes 7 homogeneous clays with high biogenic concentrations. The climate warming and high humidity over the territory led to growth on the catchments of mixed conifer and broadleaved forests with an admixture of oak, lime, elm, and maple, which provided the growth of the organic component in bottom sediments (Davidova and Subetto, 2000) and higher accumulation rates of sludge. Approximately 4-5 thousand years ago the formation of the modern Baltic Sea was practically complete. By this time Lake Onego had reached its modern shape, while the Lake Ladoga basin continued to transform. About 5 thousand years ago (Saarnisto et al., 1995) on the edge of atlantic and sub-boreal periods, the isostatic rise in the near-Ladoga region modified the hydrographic network, redirecting flows and turning the flow of the Saimaa lake system through River Vuoksi to Lake Ladoga, which increased by approximately one-third the income component of the lake water balance. The northern coast of Lake Ladoga experienced considerable isostatic rise and, as a result, the outflow from the lake to the northern part of the Karelian Isthmus nearly stopped. The lake level started to rise, and that affected mostly the southern coastal part. The highest lake water level was reached about 2 thousand years ago. When the lake water level reached the height of the watershed dividing the Mga River, inflowing to Lake Ladoga, from the Tosna River, inflowing to Gulf of Finland, Lake Ladoga waters washed away the narrow isthmus of the watershed and the Neva River was formed. After the generation ofthe new threshold flow value, the level of Lake Ladoga started to decline. The northern flow had stopped completely and the relatively short, 74 km long, Neva River became the only outflow from Lake Ladoga to the Baltic Sea. Its average discharge equalled 2500m3/s, and it has not changed in fact since that time. The final formation of the Neva River and the whole system of Great European Lakes took place less than 2 thousand years ago. By this time the level ofLake Ladoga had attained its modern magnitude (about 4-5 m above sea level). At the same time erosion processes were intensified on the catchment, which created favourable conditions for natural but fairly gradual lake ecosystem eutrophication processes. The peculiarities of the Ladoga and Onego lake basin and catchment generation and evolution had created such specific features of the coastline as its high embayment in the north and its regular smoothness in the southern, southeastern and southwestern parts. A high degree of similarity is observed in the northern parts of the lakes' coasts - a large amount of narrow long coves, bays and fiords, elongated from north to south or from northwest to southeast, and the existence of skerries. Lakes Ladoga and Onego in general are very similar to the Great American Lakes in their origin, coast types, and bottom relief and in the contrast between the northern and the southern parts of their basins. Lake Onego is the upper one in the system of Great European Lakes. Comparing the two largest lake systems of the North American and European continents, it could be mentioned that they are relatively young by origin and reached their modern shapes only a few thousand years ago. The ecosystem dynamics of the Great Lakes in Europe and America have a lot in common, but there also exist some individual features specific to each. 8 The Great European Lakes: state of the art [Ch.l

(a)

(b) (c) Fig. 1.4. Satellite images of the lakes in winter. Ladoga: (a) satellite 'Resource 01'; Onego (b) cold year, 04 FEB 2005, and (c) warm year 05 FEB 2007, satellite 'Terra'. Sec. 1.2] History of research of the lakes 9

Summing up, it is possible to point out that, regardless of the initial similarity in the limnological characteristics of GAL and GEL, the contemporary evolution of these lake ecosystems over recent centuries has proceeded differently. Moreover, even within each lake system the processes of eutrophication and toxic contamina- tion differ quite distinctively. These differences are promoted by specificity in limnogenesis, in morphometry, in thermohydrodynamic processes and in catchment changes, as well as in legislation, practical water resources management and invest- ments in nature preservation. Current and future tendencies in climate change are allegedly bound to drive the evolutionary paths ofthe lake systems even further apart. Though Lakes Ladoga and Onego are located more towards the north than American lakes, climatic peculiarities of their catchments are to a certain degree similar. Air masses ofa different origin collide there and unstable climatic conditions develop with frequent changes of weather conditions. Air masses, coming from the Atlantic Ocean over the catchments of Lakes Ladoga and Onego, bring intensive snowfalls and thaws in winter, and in summer rainy and windy weather. The intrusion of arctic air masses causes abrupt cooling, sometimes below -40°C. The invasion of continental air masses from the east and southeast leads to dry and hot weather in summer and to clear and frosty weather in winter over the lakes' catchments. The Great American Lakes never freeze completely, while Lakes Ladoga and Onego are completely covered with ice during cold winters; in warm winters the ice covers only part of lakes (Fig. 1.4). Ice cover thickness on Lake Ladoga and Lake Onego may reach 1m and even more in some years. On Lake Ladoga during the winter period quasi-steady polynyas exist, dividing the ice cover of the coastal area (fast ice) from that of the central part of the lake. In years when the lake is not completely covered with ice, the ice mass in the central part drifts, depending on prevailing wind direction. This ice mass is usually fractured (Tikhomirov, 1982). Ice cover destruction usually takes place in May, but in a cold spring floating ice may be observed in June. The same origin of the Great American and Great European Lakes basins and the geological peculiarities of their catchment are revealed in their similarity of morphometric features and thermal regime formation. Lakes Ladoga and Onego belong to the so-called dimictic lake type (Ryanzhin, 1994).

1.2 HISTORY OF RESEARCH OF THE LAKES

Lakes Ladoga and Onego have been studied during the last hundred years, both before the pronounced influence of human activities and during the period of catastrophic ecosystem change under increased anthropogenic pressure in 1970s and 1980s. Taking into consideration the common features of the abovementioned lakes, we will show in this work that mathematical models developed and tested for Lakes Ladoga and Onego, contain formulations of the problems and algorithms suitable for the ecosystem modelling of other large lakes located in temperate latitudes. Lakes Ladoga and Onego, located between 59°54'Nand 62°55'N are among the northernmost of the world's great lakes. The tremendous catchment area of Lake 10 The Great European Lakes: state of the art [Ch.l

Ladoga, located in different landscapes, determines the wide diversity of natural and anthropogenic factors affecting the lake ecosystem dynamics. The considerable size of the lake and the slow water exchange (the ratio of the lake volume to annual inflow, that is the conventional water exchange coefficient, equals 0.08) are the reasons for ecosystem conservativity. The complicated morphometry of the lake basin and its large proportions account for heterogeneity of hydrophysical, hydrochemical and hydrobiological processes in different parts of the waterbody. The variability of limnetic parameters, typical for deep, large lakes, is especially intensified under the heavy and essentially always non-uniform anthropogenic impact. The studies of this waterbody are typical, since all changes that take place in Lake Ladoga are reflected in the water quality of the Neva river, and thus in the water supply of St. Petersburg, and influence the water quality of the Neva Bay of the Gulf of Finland. The initial studies of Lake Ladoga date back to the end of the nineteenth and the beginning of the twentieth centuries (Andreev, 1875; Molchanov, 1945). The significant results ofthat period are generalized in the monograph written by I. V. Molchanov (1945). Knowledge of the ice regime helped in the building and maintaining of a temporary road (the so-called 'road of life') over the ice cover across the southern part of Lake Ladoga during the Second World War. This road provided a connection with Leningrad city during the blockade period. It helped to maintain a temporary road on the ice which was used to evacuate refugees and to organize the delivery of supplies to Leningrad city. The bibliography of Lake Ladoga studies includes several hundreds of titles (Bibliography . .., 1997). Comprehensive investigations of the limnological processes in Lake Ladoga were conducted by the specialists of the Laboratory of Limnology of the USSR Academy of Sciences (now the Institute of Limnology of the Russian Academy of Sciences) in 1956-1963. The results of these studies were published in eight monographs from 1961 to 1968 (Hydrological Regime ... , 1966). In this period of time, precise spatial and seasonal characteristics of the main hydrochemical and hydrobiological processes, which appeared as a result of increasing anthropogenic loading, were obtained. Until the mid-1960s industrial activities in the catchment had very little influence on the state of the ecosystem and the water quality in Lake Ladoga. The lake preserved its status as an oligotrophic waterbody, as it had at the beginning of the century. Insignificant bacterial pollution was registered only in the vicinity of the wastewater outlets of the paper mill establishments. Noticeable changes in the lake ecosystem were related to increasing phosphorus loading in the waterbody, mainly from the wastewaters of the Volkhov aluminium plant and agricultural activities. The increase of phosphorus loading led to the development of the process of anthropogenic eutrophication. The first step in studying this process, which nowadays has the central role in the evolution of the ecosystem of Lake Ladoga, is related to the period 1975-1980. The research was carried out by the Limnological Institute ofthe Russian Academy of Science and the results were published in the monograph ofPetrova (1982). The main features of the anthropogenic eutrophication were described and the reasons for its existence were Sec. 1.2] History of research of the lakes 11 established. The next step in this research, dating from 1981 to 1990, made it possible to formulate a series of theoretical concepts, necessary for understanding and forecasting the tendencies of waterbody development (Lake Ladoga. Atlas, 2002). Estimates were made of the principal difference of anthropogenic eutrophication in large deep lakes compared with its natural evolution, of the effect of morphological homogeneity in the lake basin on the formation oflimnological processes, and ofthe dangerous consequence of eutrophication - the decrease of oxygen content in water. An important result of these studies was the comparison of alteration scales in the ecosystem under the influence of anthropogenic activities with the natural diversity, and the selection of an optimal number of parameters - ecological criteria - which can be used during analysis, modelling and forecasting of the state of the lake. The beginning of the development of a mathematical ecosystem model dates to this period In all studies attention was mainly paid to the interaction of phosphorus with the carbon cycle in the lake ecosystem, which defines production-destruction relations, and as a result the rate ofits destabilization (Modern states . . . , 1987; Lake Ladoga ... , 1992). It was pointed out that anthropogenic eutrophication is a phenomenon comparable to the scale ofnatural processes. Furthermore a number of approaches for the estimation of the degree of pollution and the potential for such pollution in different parts of the waterbody was developed. The starting point was the analysis of the lake processes which determined the development of significant consequences of anthropogenic impact. Unfortunately the studies carried out in the 1990s were less regular and were limited mainly to the summer periods. Let us mention the international research on the lake which was conducted in these years (Viljanen and Drabkova, 2000). The monitoring of the main lake processes was carried on and the results could be found in the proceedings of three international Lake Ladoga symposiums and in the collective monographs (Lake Ladoga, 2000; Lake Ladoga. Atlas, 2002; Rumyantsev and Drabkova, 2006). In these years a number of substantial studies, started earlier, were fulfilled: the conclusions of the long-term analyses of the generation of lake organic substances pool were drawn (Kulish, 1996), and the phosphorus fluxes at the water-bottom interface were estimated (Ignatieva, 1997). There were studies of the role of humic complexes in lake organic substances during the accumulation process, conservation and recirculation in the lake phosphorus cycle. This analysis made it possible to understand the mechanism of lake ecosystem stability and the reasons for its transformation during long-term anthropogenic pressure (Korkishko et al., 2002). During the period 1996- 2005 the phosphorus loading on Lake Ladoga noticeably decreased. Its mean value during this period did not exceed 4000 tons of phosphorus per year. Such low levels of phosphorus loading on the lake had not been registered in more than 20 years, from 1975 until 1995. The following publications devoted to the analysis of the process of anthropogenic eutrophication from the 1960s until 2005: Petrova et al. (2005) and Rumyantsev and Drabkova (2006). Generalizations of the data obtained over several decades are presented in Lake Ladoga. Atlas (2002). Geographical descriptions of Lake Onego, performed by travellers and researchers were rare and incomplete before the nineteenth century. In ancient times Lake Onego was called Anizskoye (from Finnish Aiininen jdrvi; iiiini means 12 The Great European Lakes: state of the art [Ch.1 voice, sound) and later Onego. Academician Ozeretskovky, who visited Lake Onego in 1785, first published a review about the lake at the end of the eighteenth century (1792). In the nineteenth century several expeditions were organized there to study water level, thermal regime and sediments; a general geographic description of the lake was prepared by Bergshtresser, Stabrovsky, Keppen, Andreev and Drizhenko, Sovetov (see in Molchanov (1946) and in Lake Onego. Atlas (2009)). From the 1870s, water level recording and collecting observation data was organized at meteorological stations on Lake Onego and its catchment. Beginning from the end of the nineteenth century Russian Geographical Society initiated systematic investigations on Lake Onego. The first water-temperature data recordings on the lake were obtained in 1903 by N. A. Pushkarev. In 1914 the expedition under the leadership of S. A. Sovetov measured water temperature and transparency at 15 deepwater stations from a steamboat, and took sediment samples to fulfil mechanical, chemical and biological analyses. During the period 1924-1933 the complex Onezhskaya expedition of the State Hydrological Institute under the guidance of S. A. Sovetov conducted studies on the lake; the results were generalized in monograph by Molchanov (1946). Since 1933, after the construction of Belomorsko-Baltyisk Canal, the lake has been connected with the White Sea and joined the united system, linking the White, the Baltic and the Caspian seas. In 1953 on the River Svir, connecting Lake Ladoga and Lake Onego, an Upper Svir (Verhnesvirskaya) hydropower station was constructed, since then the lake water regime was regulated and the Upper Svir reservoir was formed. In 1964 the complex Onezhskaya expedition was organized to study the lake pollution problems and their influence on water quality and biological productivity (Ecosystem of Lake Onego ... , 1990). The expedition combined the Limnological Institute of the USSR Academy of Sciences, the Department of Water Problems of the Karelian Branch of the USSR Academy of Sciences, SevNIORH and the Petrozavodsk hydrometeorological observatory of UGMS. The studies were organized according to a special programme in the areas of the highest pollution. In 1970-1971 real-time measurements were conducted on a hydrophysical polygon in Bolshoye Onego Bay with different biotopes. Investigations were carried out by specialists of the Department of Water Problems of the Karelian Branch of the USSR Academy of Sciences, the Institute of Zoology of the USSR AS, and the Computing Centre of the USSR AS (Limnological Investigations ... , 1982). In 1981-1985 the Department of Water Problems of the Karelian Branch of the USSR AS performed complex studies of the lake all over the waterbody including measurements at 200 stations in connection with possible redistribution of water resources on the European territory of the USSR starting from the White Sea, via the lake system ofArkhangelsk and Vologodsky regions, through Lakes Ladoga and Onego to the south of the USSR (Ecosystem of Lake Onego ... , 1990). But the diversion of river flow from the north to the south with abstraction of the waters of Lakes Ladoga and Onego was not realized. At the end ofthe 1980s and the beginning ofthe 1990s the Limnological Institute of the USSR AS and the Department of Water Problems of the Karelian Branch of the USSR AS conducted a unique hydrophysical experiment 'Onego-89' using three Sec. 1.2] History of research of the lakes 13 research vessels, autonomous buoy stations, an airborne laboratory and three satellites. The aim of this experiment was the development of operational methods for checking water quality parameters (Lake Onego ... , 1999; Filatov, 1991). In the volume and spread of its observations it could be compared with the International Field Year of Great Lakes (IFYGL) on the Great American Lakes (Mortimer, 1974). In 1991 the Department of Water Problems was reorganized into the Northern Water Problems Institute of the Karelian Research Centre of the RAS. Since 1992 NWPI Karelian RC RAS has started regular observations on Lake Onego in accordance with a programme of complex monitoring (Ecosystem of Lake Onego ... , 1990). Since 1991 the Hydrometeorological Service of the RF has started to diminish the observation network and meteorological stations, so, the measurements of hydrophysical parameters of Lake Onego using research vessels and an airborne laboratory were stopped. At the present time, monitoring on the lake and its catchment are performed by the NWPI and the Karelian Hydro- meteocentre of Roshydromet. All information about the origin of the lake and its hydrophysical, hydrobiological and hydrochemical processes, collected during the last 50 years, are generalized in Lake Onego. Atlas (2009). Appreciating the results for Lakes Ladoga and Onego of all researches mentioned above, it is worth mentioning that as a rule obtaining generalized conceptions about the processes in ecosystems and its quantitative estimates were conducted without using mathematical models of waterbody ecosystems. The application of mathematical models for quantitative estimates of phosphorus fluxes and of the input of different hydrobiotic complexes in the regulation of matter and energy exchange in the ecosystem, of matter fluxes in the water/atmosphere and water/ bottom interfaces and for forecasting calculations appears to be not only useful but necessary. The main reasons for that were on the one hand the absence of relative mathematical models and on the other hand the lack of cooperation between specialists in ecological modelling and so-called naturalists (limnologists, biologists, ecologists, hydrologists etc.). The first mathematical model of the Lake Ladoga ecosystem, developed by V.V. Menshutkin and O.N. Vorobjova (1987) for the study of Lake Ladoga ecosystem response to the increase of phosphorus loading, is exclusion. Regarding the research into the hydrothermal regime of Lakes Ladoga and Onego, its development and its model applications, it started quite a long time ago (Okhlopkova, 1966; Tikhomirov, 1982; Akopyan et al., 1984; Astrakhantsev et al., 1987; Beletsky et al., 1994; Podsetchin et al., 1995). Reviews of studies devoted to the problem of modelling lake dynamics can be found in Filatov's books (1983, 1991) and in Kondratyev and Filatov (1999). Lake Ladoga and Lake Onego research results based on mathematical model applications carried out during the last two decades by the authors are presented in this monograph. It is worth mentioning that this monograph presents the development of conceptions expressed in the previous monograph, where were reflected mainly the studies of Lake Ladoga (Astrakhantsev et al., 2003). For the first time the results of Lake Onego studies applying mathematical modelling are presented in this monograph. 14 The Great European Lakes: state of the art [Ch.1

A brief description of the evolution of Lakes Ladoga and Onego ecosystems in the process of anthropogenic eutrophication and of observation data is presented below. The focus is on the main processes causing ecosystem destabilization and the new trophic state of the lake. Wide national and international experience within scientific programmes concerning the problem of lake ecosystem anthropogenic eutrophication shows that many processes in these lakes are similar to those which took place, or have been observed at the present time, in other lakes ofthe temperate zone. Furthermore, according to all their characteristics, Lakes Ladoga and Onego provide classical examples of large stratified lakes in the temperate zone.

1.3 CHARACTERISTICS OF TEMPERATURE AND CURRENTS

1.3.1 The thermal regime and limnic zones

The complicated morphometry of Lake Ladoga basin determines the spatial heterogeneity of processes in the Lakes Ladoga and Onego waterbodies. So, the difference in depths within the waterbody leads to significant inhomogeneity in the heating and cooling of water masses. According to the Hutchinson classification (Hutchinson, 1975), Lakes Ladoga and Onego belong to the dimictic type of lake, where the complete mixture of the waterbody takes place twice a year - in spring and in autumn. The thermal cycle is divided into two periods: heating (hydrological spring and summer) and cooling (autumn and winter). Long spring and winter seasons strongly affect variations of limnetic processes. Due to specific freshwater density distribution, spring and autumn thermal heterogeneity initiates the formation of a unique phenomenon: the so-called 'thermal bar' (Tikhomirov, 1982). The early studies of the thermal bar in Lake Ladoga and Lake Onego by Tikhomirov (1963) along with Rodgers (1971) publications on the Great American Lakes are the classical studies on this subject. The thermal bar is a zone of intensive lake water mixing; the resulting effect is that water temperature in this zone reaches the temperature of maximal density +4°C all through the waterbody. This frontal zone, extending along the coastal line, divides the lake into two regions: the warmer coastal and the deep, colder central one. In the frontal zone, from both warm and cold directions, appear steady vertical downwelling movements (Fig. 1.5). In the bottom layer water starts moving aside from the thermal bar: in the coastal zone to the shore, in the central zone towards the open parts of the lake. Besides vertical downwelling water movements, density flows along the thermal bar front are formed. Circulation density flow in the coastal region is of a cyclone type (oriented anti-clockwise); in the central part it is of anticyclone type. This flow pattern along the thermal bar additionally supports its sustainable state. Due to cyclonic coastal flow, tributary waters are spread far away from the mouth, not mixing with the lake waters of the central region. This phenomenon is especially significant for Lake Ladoga: the chemical compound of the Volkhov river water flowing into the southeastern part of the lake differs considerably and it spreads along the eastern coast far away to the north. The Volkhov waters are formed within the Lake Ilmen Sec. 1.3] Characteristics of temperature and currents 15

':IV 'j) I ..... -,

r' "I- M' ~- ~5 l...... W~ \ , u

\ r." ...... , i --...., )

Fig. 1.5. Mean perennial location of the spring thermal bar in Lake Ladoga (Lake Ladoga. Atlas, 2002). catchment where sedimentary rocks prevail. Those waters are the main source of biogenic elements in Lake Ladoga, phosphorus in particular. As the lake waters warm up, the thermal bar moves towards the deeper regions (Naumenko, 1994; Zilitinkevich et al., 1992). When water temperature exceeds +4 °C (exactly 3.98°C) all through the waterbody the thermal bar completely disappears. The thermal bar horizontal mixing speed in Lake Ladoga at the end of May is nearly 150m S-I, sometimes reaching 600m S-1 (Tikhomirov, 1982). The disappearance of the thermal bar occurs usually at the end of June - beginning of July, within 50-60m depth, and definesthe end of the spring period of the lake hydrological cycle. In the late 1980s-early 1990s, the theoretical, laboratory, and field studies of the thermal bar phenomenon in Lake Ladoga were continued. First, a theoretical model taking into account the horizontal heat transport from the warm to the cold zone was proposed (Zilitinkevich and Terzhevik, 1989), which was further developed in (Zilitinkevich et al., 1992). 16 The Great European Lakes: state of the art [Ch.1

In the years 1991-1992, the joint Soviet-Swedish field study ofthe thermal bar in Lake Ladoga was initiated to validate the theoretical parameterizations received. The surveys along two cross-sections with different bottom slopes perpendicular to the southern and western shores were performed in the spring 1991 and 1992 two and three times, respectively, to collect data on the vertical distribution of water temperature and currents. The results of the 1991 (Malm et al., 1993) and 1992 (Malm et al., 1994) field campaigns can be summarized as follows. The measured current velocity distribu- tions were found to be strongly dependent on wind conditions. The density-induced currents seemed to be of secondary importance compared to the observed currents, even during calm conditions. Estimates of the heat content change along cross- sections revealed the presence ofhorizontal heat transport from the nearshore warm zones to the thermal bar. The estimates of the thermal bar propagation rates based on observational data were compared with those received from the theoretical models. The model accounting for the horizontal heat transfer (Zilitinkevich et al., 1992) was found to better predict the propagation rate compared to earlier models (e.g. Elliot and Elliot (1970), and similar). The depth-integrated advective flux calculated from the temperature distribution observed and the along-section velocity component computed with a one-dimensional k-c model was found to be 100 times smaller than that estimated from heat content change calculations. The analysis of the satellite images (Kondratyev et al., 1988) clearly demon- strated the presence ofthe warm-water vortex trails on the cold side ofa thermal bar, which should accelerate the front propagation. The mechanism of these intrusions is not clear yet, but Zilitinkevich has suggested that such a phenomenon can occur due to baroclinic instability of the currents on the warm side of the thermal bar. As surface water warming proceeds, the horizontal thermal heterogeneity near the coasts becomes the vertical thermal stratification. Along with disappearance of the thermal bar the vertical stratification is formed in the deep regions, marking the beginning of the hydrological summer. The isotherm +4°C in every vertical cross-section defines the lower boundary of the heated layer, dividing it from the waterbody thickness, forming the cold water dome in the deep water part of the lake. By the moment of dense water dome formation the difference in the surface water temperature over the lake exceeds the maximal annual value and the cyclone circulation its utmost development. The drift flows start to lie over the cyclone circulation. The surface temperature over the whole lake gradually becomes even, under the influence of convective-wave mixing. The water dome and the limiting layer of rapid temperature decrease, the so-called thermocline or depth of metalim- nion, drops down. The upper layer of the lake becomes isothermal, forming a sustained epilimnion (the upper quasi-homogeneous layer). The water temperature in the lake exceeds its maximal value. The thicknesses of epi- and hypolimnion gradually increase and in the deep layers (hypolimnion) the water temperatures remain nearly +4°C. According to Tikhomirov (1982) classification Lakes Ladoga and Onego belong to the classification: hypothermal lakes - those lakes where, during the period of summer warming, the main water mass forms the hypolimnion. Sec. 1.3] Characteristics of temperature and currents 17

By the beginning of the hydrological autumn the dense water dome is observed at depths of more than 100m. Starting with water cooling in the coastal regions, the autumn horizontal thermal heterogeneity becomes settled. This phase is characterized by the coming into existence of the thermal bar, intensive water cooling in shallow coastal waters, ice cover formation and the long-term preservation (until December-January) of a water temperature of nearly +4°C in the deep parts of the lake. Complete ice cover of the lake occurs during cold winters only. Over the deepest regions ice cover is observed for short periods of time: 10-15 days. The deep part of the lake is not covered with ice during warm winters (Fig. 1.4(a)). Ice cover disappears in April- May (Tikhomirov, 1982). Water masses the thermal heterogeneity of water masses is responsible for the variation in most limnological features within the waterbody. Of greatest significance during the period of the hydrological spring is the horizontal thermal heterogeneity. The existence of the frontal thermal bar zone defines the accumulation of the initial water masses, the lake tributary waters, preserving their specific chemical, physical and other features. That is to say, the biogenic elements of the epilimnion mainly participate in the consumption cycle during the summer period - regeneration related to biological processes. That is why only here is recorded the reduction of dissolved mineral phosphorus and nitrogen concentrations, whereas in the hypolimnion its storage is preserved at the winter-spring level. Equally all changes of hydrophysical and hydrochemical indicators caused by the photosynthetic activities of phytoplankton (high values of pH, reduction of transparency, the increase of water oxygen concentration) are revealed in the epilimnion. The hypolimnion mainly is the zone of the development of destruction processes - the tropholytic area of the lake. The main process leading to essential changes of limnetic parameters in the hypolimnion is the decrease of oxygen concentration in water. The mixing of the whole water column during the periods of spring and autumn homothermy provides the equalizing of hydrophysical and hydrochemical parameters. As a result of long-term Lake Ladoga studies the lake was divided into four limnetic zones (Lake Ladoga ... , 1992) each of which plays a special role in ecosystem functioning and in general has its value from the point of water supply (Fig. 1.6). In Lake Ladoga the coastal zone I, where the depths are the shallowest and less than 15m, is subjected to the maximum influence of its catchment processes including anthropogenic impact. It is here that tributary waters, industrial and agricultural wastewaters, surface runoff, drainage waters of land reclamation systems and so on enter the lake. At the same time only in the coastal zone are located industrial and municipal water intakes, recreational areas and the most of the spawning-grounds. In spring and autumn months the thermal bar front prevents free water exchange with the deep central part of the waterbody. The flood waters of tributaries enriched with biogenic elements and allochthonous organic matter are retained in the near-shore zone for a long time. In summer time in the well-heated coastal zone the composition of water organisms is diverse and their production