Manuel d’installation
Contents
1. apr HE me SD1 vp PL DE sp SAL 4 Vo Ph 1 c ez zZ Ce HE m 2 Da Vo D 2 Xp G mDi gaAr gcC1 Gh Pi dP v PN m D2 yp p rPs T dN SIE Xue mDinp gaAinp g8cC1np dN _ gt S2 Vs at m Danp Wn V N ONNs v S SOL max Osa Op Ba 80 Goxm D1 DL O ORE Goxin D2 Ds 2 On In sediment dDs _ 302 a YD Gz MDs ds weP2 m Ds pPs dN S YynN2 m Dsnp QnNs dt 6 aD2 az 7 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 181 Table 3 Model variables Tableau 3 Les variables du mod le Forcing variables Ti Tg Vi V2 Vs Xn Xp E Si Mean solar irradiance in the upper layer Temperature of the upper lower layer Volume of the upper lower layer Volume of the sediment layer Concentration of variables N and P in the inflow Inftow in top layer Outflow from layer f i 1 2 State variables TN O gt z 2 2 Phytoplankton grazed by herbivorous zooplankton i Cyanobacteria i 1 Herbivorous zooplankton Fish Detritus i 1 2 Dissolved inorganic phosphorus i 1 2 S Dissolved inorganic nitrogen i 1 2 S Dissolved oxygen i 1 2 top layer tom layer sediment layer Table 4 Processes represented in the model Tableau 4 Les processus repr sent s dans le mod le Phytoplankton growth Effect of light ifI gt o 4 S_ x 1 20 fiso 02. in collaboration with the Myriad firm which contribu ted the graphics and the user interface It exists now in the French version and will soon be available in English for use on PC compatible computers 3 1 2 and 5 1 4 diskettes and on Atari ST It may be ordered from the Mission Environment for FF 95 ELECTRICITE DE FRANCE Mission Environnement 26 rue de fa Baume 75008 Paris MYRIAD S A R L 4 rue de Bordeaux 31200 Toulouse References Andersen V amp Nival P 1989 Modelling of phytoplankton population dynamics in an enclosed water column J mar biol Ass U K 69 625 646 Bierman V J Jr 1976 Mathematical model of the selective enhancement of blue green algae by nutrient enrichment In Canale R P Ed Modelling biochemical processes in aqua tic ecosystems Ann Arbor Sciences Michigan 1 31 Canale R P De Palma L M amp Vogel A H 1976 A plankton based food web model for Lake Michigan In Canale R P Ed Modelling biochemical processes in aquatic ecosys tems Ann Arbor Sciences Michigan 33 74 Droop M R 1974 The nutrient status of algal cells in conti nuous culture J mar biol Ass U K 54 825 855 Droop M R 1975 The nutrient status of algal cells in batch culture J mar biol Ass U K 55 541 555 Eilers P H C amp Peeters J C H 1988 A model for the rela tionship between light intensity and the rate of photosynthe sis in phytoplankton
3. Simulation 1 An oligotrophic lake with no hydroelectric power generation Annual variation in plankton communities consu mable algae Cyanobacteria and zooplankton a and in dissolved oxygen in the epilimnion and hypolimnion b Dissolved oxygen is expressed as a percentage of the concentration under conditions of saturation Fig 2 Simulation 1 Cas d un lac oligotrophe sans production hydro lectrique Evolution annuelle des communaut s planctoniques algues consommables cyanophyc es et zooplancton a et de l oxyg ne dissous dans l pilimnion et l hypolimnion b L oxyg ne dissous est exprim en pourcentage de la concentration saturation 184 J M THEBAULT M J SALENCON 10 Fig 3 Simulation 2 Annual variation in plankton communities consumable algae Cyanobacteria and zooplankton a in dissolved oxygen in the epilimnion and hypolimnion b and in nutrients P and N in the epilimnion c Dissolved oxygen is expressed as a percentage of the concentration under conditions of saturation The high nutrient concentrations and the absence of hydraulic management foster the development of Cyanobacteria Fig 3 Simulation 2 Evolution annuelle des communaut s planctoniques algues consommables cyanophyc es et zooplancton a de l oxyg ne dissous dans pilimnion et l hypolimnion b et des nutriments P et N dans l pilimnion c L oxyg ne dissous est exprim en pourcentage de la concentration s
4. col Appi 1 91 134 Rhee G Yull 1978 Effects of N P atomic ratios and nitrate limitation on algal growth cell composition and nitrate uptake Limnol Oceanogr 23 10 25 Riley M J amp Stefan H G 1988 MINLAKE a dynamic lake water quality simulation model Ecol Modelling 43 155 182 Salen on M J Merle G amp Sabaton C 1984 Le r seau de mesures hydrobiologiques de la retenue de Grangent Loire analyse partielle des r sultats Cahiers du laboratoire de Mon tereau 15 7 12 Salen on M J amp Capblancq J 1987 Etude de la retenue de Pareloup Bilan des travaux r alis s en 1986 dans le cadre de la convention EDF Minist re de Environnement Rap port HE 31 87 5 Electricit de France Paris 26 p Salen on M J Th bault J M amp Capblancq J 1988 Etude de la retenue de Pareloup Bilan des travaux r alis s en 1987 dans le cadre de la Convention EDF Minist re de l Environ nement Rapport HE 31 88 23 Electricit de France Paris 43 p Salen on M J Th bault J M amp Capblancq J 1989 Etude de la retenue de Pareloup Bilan des travaux r alis s en 1988 dans le cadre de la Convention EDF Minist re de l Environ nement Rapport HE 31 89 13 Electricit de France Paris p Salen on M J Th bault J M amp Capblancq J 190a Etude de la retenue de Pareloup Bilan des travaux r alis s en 1989 dans le cadre de la Convention EDF Minist re de l E
5. amp Salen on M J Simulation model of a meso trophic reservoir Lac de Pareloup France Biological model Ecol Modelling in press Zohary T amp Breen Ch 1989 Environmental factors favou ring the formation of Microcystis aeruginosa hyperscums in a hypertrophic lake Hydrobiologia 178 179 192 15 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 189 LA VIE DU LAC Electricit de France Myriad Mode d emploi Allumez votre ordinateur avec une disquette syst me MS DOS version 3 ou 5 dans le lecteur A Le logiciel ne peut fonction ner avec la version 4 de MS DOS Une fois le syst me charg enlevez la disquette syst me et remplacez la par la disquette La vie du lac Tapez LAC suivi de retour chariot Une fois la version du jeu choisie et le logiciel charg la disquette peut tre retir e du lecteur La vie du lac peut galement tre copi e dans un r pertoire du disque dur Si vous poss dez une souris celle ci doit tre install e avant de lancer La vie du lac se reporter au manuel d installation de votre souris Le logiciel n cessite une m moire d au moins 512 Ko pour fonc tionner Les cartes graphiques Hercules CGA et EGA sont auto matiquement reconnues LAKE LIFE Electricit de France Myriad Instructions for use Start up your computer with MS DOS system disk in disk drive A Once the system is loaded eject the system disk and replace it with the Lake Life dis
6. and nutrient concentration We have chosen a similar equation to represent the effect of light and tempe rature introducing a threshold value below which there is no growth These functions are less sophis ticated than those used in more complete models Steele 1962 Parker 1974 Eilers amp Peters 1988 Tal bot et al 1991 in particular they do not simulate the drop in growth rate at high temperatures or under conditions of photo inhibition which is less important in this model They are on the other hand far simpler to calculate Real growth is defined as the product of a maxi mum growth rate and the three limiting factors The two groups of algae need phosphorus and nitrogen for development The first group is limi ted by the least abundant element Leibig law Droop 1974 1975 Rhee 1978 Cyanobacteria are limited only by a phosphorus deficit they are capable of fixing atmospheric nitro gen when mineral nitrogen is lacking 178 J M THEBAULT M J SALEN ON 4 Factors in disappearance predation by zooplank ton and sinking are the factors for disappearance of the first group of algae At least two causes may be found for major variations in sinking speed One is physical related to turbulence in the medium and cannot be simulated here the other depends on numerous factors such as cell size temperature age of the population or lack of nutriment We chose this latter factor for our model linking sinking s
7. de fonctionnement du mod le biologique 5 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 179 zooplankton disappearance is directly proportional to the fish population Fish We assume that the population is stable over time neither increasing nor decreasing under normal conditions of oxygenation Below a first threshold for concentration of dissolved oxygen the most fra gile fish the Salmonidae die If concentrations of dissolved oxygen continue to drop no fish survive Detritus and nutrients Detritus settles at a constant rate Phosphorus and nitrogen are mineralized from detritus in what is no more than transformation from an organic to a mineral form the speed of which is temperature dependent We do not distinguish the different forms of nitrogen or phosphorus A fraction of these elements is stored in sediment and later released into the hypolimnion when it is anoxic insufficient oxygen Dissolved oxygen Dissolved oxygen is produced as a result of pho tosynthesis attributable to phytoplankton and gaseous exchange at the air water interface or re aeration Such exchange is faster from autumn to spring the period during which wind speed is gene rally higher 2 4 Numerical considerations As certain growth rates are extremely high a high value for the derivative one needs to have a small integration time step We usually use the Runge and Kutta fourth order method for numerical int
8. mouse in hand on a discovery of lake ecosystems it is at the same time a simplified version of an actual compu ter code developed in the course of scientific research The approach followed in developing this pro gram is identical to that followed in development of ecosystem models used in lacustrian ecology studies The modeled processes are relatively simplified though the model s response is on the whole most satisfying 13 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 187 There are many potential uses for this software as a teaching aid in sensitizing hydraulic manage ment specialists in informing the general public etc As the user retains control over pollutants dischar ged hydraulic management and stocking with young fish he can visualize the repercussions of his deci sions on the ecosystem as a whole In this way he discovers the mechanisms which lead to eutrophi cation as well as the difficulties one encounters in trying to find remedies To find out more In addition to the authors mentioned in this text a number of works dealing with modeling aquatic ecosystems might be consulted Bierman 1976 Canale et al 1976 Jorgensen 1976 J rgensen et al 1978 Kremer amp Nixon 1978 Garcon 1981 Spain 1982 Salencon et al 1984 Th bault 1984 Riley amp Stefan 1988 Andersen amp Nival 1989 Acknowledgements Lake Life was developed for the EDF Mission Environ ment
9. which can easily be assimilated by the user Only the predo minant system mechanisms have therefore been selected With apologies to the specialists we have simplified to the extreme the more complex mechanisms For all these reasons this model can absolutely not be used as a management tool and even less as a research tool The software runs on Atari or PC compatible computers simulation of one year requires approxi mately two minutes calculating time This is between 1000 and 5000 times faster than the time it would take similar computers to run an actual management model of the Pareloup type 2 Description of the model 2 1 Physical structure The physical structure of the system is one of the elements in the program which required the grea test simplification essentially for reasons of com puter memory The water mass is divided vertically into two homogeneous layers which represent sea sonal thermal stratification The surface layer the epilimnion is separated from the bottom layer the hypolimnion by a thermocline The bottom layer is in contact with sediment Each layer is characterized by its temperature volume depth and mean illumination The dyna mics of the two water masses are simulated in two phases one phase of homogeneity in winter when only the epilimnion exists and one phase of strati fication in summer when the thermocline level is constant It was impossible to introduce the tran s
10. Annis Limnol 28 2 1992 175 189 From the numerical model to the educational software Lake Life J M Th bault M J Salengon2 Keywords Ecosystem medel educational software hydroelectric reservoir Lake Life is a software program designed to introduce the lay person to lacustrian ecology and to the basic con cepts of hydraulic management Having become familiar with the dynamics of the trophic system as well as the mecha nisms leading to eutrophication the user may experiment with managing the reservoir of his choice Change over time in the principal elements in the ecosystem is calculated by a mathematical model This paper first presents the successive steps in development of the model choice of variables three plankton groups fish nutrients oxygen representation of complex mechanisms for example the vertical structure simulated in two layers transcription into equations This recapitulation will initiate the reader into the problems of modeling an ecosystem We shall then analyse a few simulated situations variations in plankton groups in three situations with increasing trophism and one example of the impact of turbining on the fish and planktonic populations and on oxygenation of the hypolimnion The quite realistic behavior of the simulations makes this software an excellent teaching tool Du mod le num rique au logiciel didactique la vie du lac Mots cl s Mod le d cosyst me
11. Ecol Modelling 42 199 215 Gar on V 1981 Mod lisation num rique d un cosyst me aquatique Application au r servoir de Grangent sur la Loire Th se de 3 cycle Universit Paris 7 Paris 230 p Gentil S 1982 Analyse de syst me en cologie Une tude de cas Lac d Aiguebelette In Mod lisation math matique et simulation des syst mes de l environnement Dominante eau et v g tation Editions du CNRS Paris 17 62 J rgensen S E 1976 An eutrophication model for a lake Ecol Modelling 2 147 165 Jergensen S E Mejer H amp Friis M 1978 Examination of a lake model Ecol Modelling 4 253 278 Kremer J N amp Nixon S W 1978 A coastal marine ecosys tem Simulation and analysis Ecological Studies 24 Springer Verlag Heidelberg 217 p Legras J 1971 M thodes et techniques de l analyse num ri que Dunod Paris 323 p Monod J 1942 Recherches sur da croissance des cultures bac t riennes Hermann Paris 210 p Mullin M M Stewart E F amp Fuglister F J 1975 Ingestion by planktonic grazers as a function of concentration of food Limnol Oceanogr 20 259 262 Parker A 1974 Empirical functions relating metabolic pro cesses in aquatic systems to environmental variables J Res Board Can 31 1550 1552 Pourcher A M amp Salen on M J 1990 Mod lisation du planc ton dans une retenue oligotrophe Sainte Croix sur le Ver don Hydro
12. Effect of temperature ifT gt Or I _ 1 87 Kr T 207 ifTSG 1 1 0 Effect of nutrients Ip aN In N XN N Kkp P t ly lp for Cyanobacteria ly MinG 1x for other algae Growth rate g g h ly ly Cyanobacteria mortality T lt Om MOT pon T Dm ifT gt 0 mA 0 Sedimentation EMSs salm Vos 1 m Bs ifl gt O Sally 0 Zooplankton Effect of temperature on maximum ingestion rate Tmax T er Ingestion rate FA gt Ga TA max PA Ka 20 ifASG 1 A 0 Fish Ingestion rate ifZ gt 0z peZ q 2 Kz Z 20z ifZS p Z 0 Nutrient regeneration from detritus m T a e amp T 182 J M THEBAULT M J SALENCON 8 Table 5 Model parameters Tableau 5 Les param tres du mod le Phytoplankton parameters Half saturation coefficient for limitation by respectively light temperature phosphorus and nitrogen Kp Kr Kp Ky CAT Light and temperature threshold for growth Maximum growth rate for algae Oa Temperature threshold for Cyanobacteria mortality Haas maximum mortality rate for Cyanobacteria np N P ratio for all variables Sedimentation parameters Vines Maximum sedimentation rate D Nutrient threshotd below which sedimentation begins Vo Constant sedimentation rate for detritus Vp Vy Uptake of P and N by sediment Op Py Release of P and N from sediment Animal parameters Tz Tp Maximum ingestion rate at optimal temperature for respecti
13. ae need phospho rus We have introduced nitrogen as it may be a limi ting factor It is consumed by both groups of algae but is not indispensable to Cyanobacteria Zooplankton our experience at Pareloup led us to distinguish a single group of herbivorous zoo plankton as the impact of carnivorous zooplank ton is limited in comparison with predation by plankton grazing fish The fish prey on zooplankton We assume here that the population is homogeneous over time which permits introduction of a single variable Detritus is composed of algae and dead animals phytoplankton which has settled and matter not assimilated by animals Bacteria are not simulated in this model We assume that they are constantly effective in mineralizing organic matter Oxygen is an important indicator of water qua lity and determines the possibility for survival of most fish species This variable is therefore inclu ded in the model 2 3 Modeled processes The diagram of the model dynamics is summari zed in Figure 1 all equations and variables in Tables 1 2 3 4 and 5 Phytoplankton The factors for phytoplankton production include light temperature and nutrient concentration all factors which can curb the phytoplankton growth rate especially in ranges of values far off optimum values We have used Monod s equation 1942 a hyper bolic function which is well suited to description of the relationship between phytoplankton growth
14. an ecosystem nutrients phyto and zooplankton fish etc It also describes the principal mechanisms at work thermal stratification interaction of various kinds among the links in the trophic chain and the dynamics of eutrophication Lake Life is a software program designed to describe for a lay public the manner in which a lacus trian ecosystem reacts to disturbances whether natu ral or related to human activities Making use of 1 Laboratoire d Hydrobiologie URA 695 du CNRS Univer sit Paul Sabatier 118 Route de Narbonne 31062 Toulouse Cedex France n revie 2 Electricit de France Direction des Etudes et Recherches Once these fundamental concepts have been D partement Environnement 6 Quai Watier 78401 Chatou Cedex France wed the user is asked to choose a lake type charac terized by the period required for renewal of water Article available at http www limnology journal org or http dx doi org 10 1051 limn 1992015 176 J M THEBAULT M J SALEN ON 2 and to assume control over its future by managing Nutrient concentrations parameters of hydraulic management withdrawal levels and period as well as any stocking with young fish Critical periods during which water is unfit for swimming fish sur vival etc are indicated by alarms The user s goal is of course to fight eutrophication while integra ting those mechanisms which contribute to maintai ning water quality In ord
15. aturation Les apports importants de nutriments et absence de gestion hydrauli que favorisent le d veloppement des cyanophyc es n 11 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 185 12 PHYTO CYANO M A M J M A M J A S OND Fig 4 Simulation 3 An eutrophic lake Annual variation in plankton communities consumable algae Cyanobacteria and zooplank ton fa and in dissolved oxygen in the epilimnion and hypolimnion b Dissolved oxygen is expressed as a percentage of the concen tration under conditions of saturation Fig 4 Simulation 3 Cas d un lac eutrophe Evolution annuelle des communaut s planctoniques aigues consommables cyanophyc es et zooplancton a et de l oxyg ne dissous dans pilimnion et l hypolimnion b L oxyg ne dissous est exprim en pourcentage de la concentration saturation hypolimnion than does turbining through the upper outlet by fostering exchange between epilimnion and hypolimnion It would be unwise to turbine through the lower outlet at the end of summer when the hypolimnion is anoxic oxidation and precipitation of iron and manganese together with the potential effects on downstream river quality are clearly pointed out to the novice hydraulic manager We must remember that the model takes into account the accumulation of nutrients in the sedi ment where concentrations may become very high When the h
16. e are two possible sources of phosphorus and nitrogen concentrations from upstream and any discharge of water containing some concentrations of these same elements The residence time deter mines the annual inflow outflow rate All variables with the exception of fish are withdrawn by turbining 2 2 The variables Forcing variables These variables are directly entered into the model and are therefore not calculated respective volumes of the two layers and of the sediment inflow and outflow rates concentration of N and P in inflow mean light energy in the epilimnion represen ted schematically by a sine curve with a period of 365 days It is supposed that light energy in the hypo limnion is too low to permit algal growth the temperature of each layer also represen ted by a sine curve During stratification the hypo limnion temperature remains low and stable State variables in the model The model is based on a phosphorus budget which is to say that all variables with the exception of nitrogen and dissolved oxygen are expressed in phosphorus units Phytoplankton are divided into two groups algae which may be consumed by the zooplankton and Cyanobacteria capable of fixing atmospheric nitro gen and not consumed by the zooplankton In order to limit the number of variables we do not simulate silica and therefore do not distinguish a diatom group Nutrients both groups of alg
17. egra tion Legras 1971 in solving differential equations in biological models While this method is highly simple it calculates the totality of the equations and related subroutines four times per time step The finite difference method is quicker as calculation is performed once only but less precise for an iden tical time step Error due to numerical integration and to a lesser degree to computer errer can be esti mated by calculating a mass budget A compromise between acceptable error for this type of software and speed in calculation led us to solve equations by means of finite differences with a time step of 1 10 of a day Table 1 Sedimentation process Tableau 1 La s dimentation aking aX a oz OXi _ sy OXi HSM Sedimentation in top layer A ES aX At to bottom layer Sedimentation in bottom layer V2 ARS Sx 2X1 Sx aX2 At from top to sediment layer Sedimentation into sediment layer V AXs _ Sx aX2 At for X A C D a mean area of the lake In order to simplify expressions sedimentation corresponding to each layer i and variable X has been written in table 2 X Sx z 180 J M THEBAULT M J SALENCON Table 2 System of differential equations Tableau 2 Le syst me des quations diff rentielles In water layers OAL pi SLA oZ sa OAL x7 Ba VA t7Z Sa 3z a c mc ne Qzrz Uz Sz prF dE Qppp Hr F OL eC ape pZ
18. er the hypolimnion is anoxic and the autumn mixing triggers a serious drop in oxygen concentrations throughout the water mass which may be fatal to certain fish species If high phosphorus concentrations are maintai ned over several years Simulation 3 Fig 4 one reaches a more pronounced situation of eutrophy with major declines in oxygen level even in the epilimnion The model results correspond closely to observa tions on a number of sites For example the first simulation could well apply to Sainte Croix an oli gotrophic lake Pourcher amp Salen on 1990 Varia tions in the plankton communities at Pareloup situate this particular reservoir somewhere between M A M J the first and second simulations depending on the year Salencon amp Capblancq 1987 Salencon et al 1988 1989 1990a 1990b The third simulation could apply to a highly eutrophicated reservoir Figure 5 shows the effect of two other possible types of hydraulic management the initial condi tions and inflow concentrations are identical to those in Simulation 2 summer turbining is through either the upper or lower outlets Here we note a major decrease in Cyanobacteria and a marked autumn peak for consumable algae and zooplankton The level of the turbining outlet has no major effect on variations in the epilimnic populations Turbining through the lower outlet on the other hand does permit better reoxygenation of the Fig 2
19. er for a simulation closely to approach rea lity it is necessary to use a numerical model which simulates the ecosystem dynamics The great num ber of possible responses the model may give depen ding on the user s choice and the history of the reser voir precludes use of pre established scenarii The model performs on line calculation of variations in the main ecosystem components before the user s eyes enabling him to visualize continually the reper cussions of his system management This approach closely parallels that made of decision support models and is for that reason a good introduc tion to one of the key functions of modeling The Lake Life model is a simplified version of the computer code perfected for study of Pare loup Lake Th bault amp Salen on 1992 This paper presents the concepts behind the model the choice of trophic network and variables as well as the simplifying hypotheses it was neces sary to make One of the major problems confron ting us was to find a compromise between the model s complex form of representation integrating to a high degree present hydrobiological know how in the field of reservoir ecology and a calculating time which seemed reasonable for a microcomputer The purpose of the model is above all pedagogi cal for this reason it is essential that the simula tions be characteristic even caricatures in a sense so as to constitute teaching cases
20. idly due to active consumption by the zooplankton In autumn cooling in the surface layers triggers mixing of the nutrient deficient epi limnion with the nutrient rich hypolimnion leading to a new development of plankton communities In the hypolimnion dissolved oxygen is consu med in decay of settled detritus The autumn mixing prompts the drop in mean oxygen values for the entire water mass As the demand for oxygen in the hypolimnion is not very high in this simulation we note no significant hypoxia following the mixing If we increase the phosphorus concentrations ups tream Simulation 2 Fig 3 the behavior of the springtime plankton is identical to that in the pre vious simulation but the biomass is greater As phos phorus is highly concentrated in inflow nitrogen becomes a limiting factor in the epilimnion before phosphorus This situation linked to the rise in tem perature and the decreased turbulence fosters the development of Cyanobacteria which assimilate excess phosphorus and fix dissolved atmospheric nitrogen During the autumn mixing the impact of Cyanobacteria is so great that other algae can no longer develop The Cyanobacteria decline when the medium is more turbulent and the temperatures lower In this case as the Cyanobacteria are not con sumed there is only one period of zooplankton development in the spring 9 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 183 At the end of summ
21. ient spring thermoclines variations in depth of the thermocline or the gradual deepening of the ther mocline in autumn Thus the onset of stratification and the autumn mixing are instantaneous Inflow and outflow of water are represented very schematically inflow is always localized in the sur face layer outflow due to turbining may be at the surface or on the bottom with the user free to deter mine the outlet level for each season The program proposes a choice among three types of reservoir characterized by the period required for water renewal a short residence time of from two weeks to a month Grangent for example from one to three months as for Chambon for exam ple and on the order of one year for reservoirs with a protracted residence time such as Pareloup and Sainte Croix The length of the period of stratifi cation and exchange between the two layers differ according to the type of reservoir in the first case no stratification is introduced in the second there is stratification but there is nonetheless a low but constant amount of exchange between the two layers in the third case stratification lasts from late spring to mid autumn and there is no exchange except in the event of turbining For the last two 3 FROM THE NUMERICAL MODEL TO THE EDUCATIONAL SOFTWARE LAKE LIFE 177 reservoir types turbining through the lower outlet triggers movement from the epilimnion toward the hypolimnion Ther
22. k Type LAKE followed by return the program will load and start up As soon as you choose the game level you want and the disk drive light goes off eject the disk and put it away carefully If you have a mouse and would Eike to use it you must first install it follow the instructions in your user s manual before running Lake Life Lake Life calls for at least 12 Ko of memory Your graphics card is automatically detected if it is a Hercules CGA or EGA
23. logiciel didactique retenue hydro lectrique La Vie du lac est un logiciel destin initier un public non averti l cologie lacustre et aux notions fondamentales de la gestion hydraulique Apr s avoir pris connaissance du fonctionnement de la cha ne trophique ainsi que des m ca nismes conduisant l eutrophisation l utilisateur a la possibilit de g rer une retenue de son choix L volution dans le temps des principales composantes de l cosyst me est calcul e par un mod le math matique Nous pr sentons ici les tapes successives qui ont permis l laboration du mod le le choix des variables trois groupes planctoniques poissons nutriments oxyg ne la sch matisation de m canismes complexes par exemple la structure verticale simul e en deux couches la mise en quations Cette d marche constitue une bonne initiation la mod lisation d un cosyst me Nous analysons ensuite quelques simulations l volution des communaut s planctoniques dans trois situations de trophie croissante puis un exemple de l impact du turbinage sur les populations et sur l oxyg nation de l hypolimnion Le comportement suffisamment r aliste des simulations permet d utiliser ce logiciel comme support de cours pour l enseignement 1 Introduction animated images a choice of information level and a user friendly mouse it gives an attractive presen tation of the various components in a lacustri
24. nviron nement Rapport HE 31 90 17 Electricit de France Paris 67 p Salen on M J Th bault J M amp Capblancq J 1990b Etude de la retenue de Pareloup Synth se des travaux r alis s dans le cadre de la Convention EDF Minist re de l Environnement mars 1986 mars 1990 Rapport HE 31 90 23 Electricit de France Paris 40 p Spain J D 1982 Basis microcomputer models in biology Addison Wesley Reading Massachusetts 354 p 188 J M THEBAULT M J SALENCON 14 Steele J H 1962 Environmental control of photosynthesis in the sea Limnol Oceanagr 7 137 150 Steinberg C E W amp Hartmann H M 1988 Planktonic bloom forming Cyanobacteria and the eutrophication of lakes and rivers Fresh Biol 20 279 287 Talbot P Th bault J M Dauta A amp De la No e J 1991 A comparative study and mathematical modeling of tempe rature and light on growth of three microalgae potentially useful for wastewater treatment Water Res 25 465 472 Th bault J M 1984 Mod lisation des premiers niveaux du r seau trophique p lagique marin Mise au point de modu les et simulation de s ries exp rimentales Th se de 3 cycle Universit Paris 7 Paris 94 p Th bault J M 1985 Etude exp rimentale de la nutrition d un cop pode commun Temora stylifera Dana Effets de la tem p rature et de la concentration de nourriture J Exp Mar Biol Ecol 93 223 234 Th bault J M
25. peed to growth limitation due to nutrients Natural mortality is considered to be a negligible factor in comparison to predation and sinking Cyanobacteria can generally not develop in a tur bulent medium and at low temperatures Steinberg amp Hartman 1988 Zohary amp Breen 1989 As we do not simulate turbulence we have linked mortality in this group of algae to temperature field observations at Pareloup indicate that generally Reaeration speaking agitation in the water mass is at its maxi mum level in winter and spring when the tempera ture is the lowest Zooplankton The rate of ingestion depends primarily in the short term on temperature and nutrient concentra tion We have simplified the model proposed by Th bault 1985 which permits simulation of their compounded effect The function linking ingestion to the quantity of available prey has been replaced by a hyperbolic function Mullin et al 1975 To this we have added a threshold below which fee ding is nil Only the maximum feeding coeffi cient is a function of the temperature This simpli fication permits a considerable reduction in the num ber of coefficients needed Rates of assimilation and natural mortality are constant Predation by fish the principal cause of Grazing egestion D 1 i 1 1 1 t i i L Fig 1 Diagram of the dynamics of the biological model Fig 1 Diagramme
26. vely zooplankton and fish Kz Half saturation coefficient for tpa a Threshold concentration of algae for zooplankton feeding GB Threshold concentration of zooplankton for fish feeding LALA Half saturation coefficient for r A and p Z Oz Ap Assimilation rate Uz He Mortality rate Nutrient regeneration and oxygen parameters a P Coefficients for m T Diffusion of oxygen at the lake surface Oxygen produced by photosynthesis Oxygen used in organic decay of detritus pre Saturated oxygen concentration 3 Simulation results The most interesting simulations from an educa tional point of view are those related to reservoirs where the residence time is seasonal or annual The management scenarii the user adopts here can signi ficantly modify the future of the reservoir unlike the case of the reservoir with a low residence time where it is impossible to exercise any influence on eutrophy in the river In our model when there is no hydroelectric power generation the flow left above the dam is equal to inflow from the watershed We can sup pose that in this case the lake behaves like a natu ral lake When the nutrient concentrations are low Simu lation 1 Fig 2 we note a springtime increase in phytoplankton followed by an increase in herbivo rous zooplankton In the summer when the lake is stratified nutrients become a growth limiting fac tor in the surface layer for the phytoplankton which decline rap
27. ypolimnion is anoxic phosphorus and to a lesser degree nitrogen through de nitrification is released into the water mass proportionally increasing the amount available to algae at the time of deepening of the thermocline Therefore when the lake is highly eutrophicated it would be a mis take to imagine that the situation will rapidly improve if concentrations are reduced The model represents well the inertia attribuable to the quan tity of nutriment stored in the sediment The most effective annual hydraulic management approach is to turbine at the surface in spring to eliminate a maximum of algae and zooplankton so as to decrease the fraction in the sediment and at the bot tom in summer to eliminate detritus which may have settled and oxygenate the hypolimnion as much as possible It may take several years to return to acceptable conditions 186 J M THEBAULT M J SALEN ON 12 Fig 5 Variation in plankton communities in the case of turbining during the summer a Impact of the choice of outlet level on oxyge nation in the water mass turbining through the lower b or upper outlet c Fig 5 Evolution des communaut s planctoniques dans le cas d un turbinage pendant l t a Effet du choix de la prise d eau sur l oxyg nation de la masse d eau turbinage par la prise de fond b ou de surface c 4 Conclusion While Lake Life is an educational software program permitting the user to embark
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