Analytical and statistical study of a lake system under various processes of eutrophication

L'étude des phénomènes d'eutrophisation se heurte essentiellement à deux difficultés qui sont inhérentes d'une part, à la diversité spatio-temporelle des eaux superficielles, et d'autre part, à l'action qu'exercent en retour les phénomènes d'eutrophisation sur ces eaux.- La diversité spatio-temporelle des systèmes aquatiques s'accompagne d'une diversité des phénomènes d'eutrophisation qui en sont le siège et a pour conséquence de rendre difficile sinon impossible l'établissement de critères de comparaisons entre les différents systèmes aquatiques. Dans ces conditions il est peu aisé de faire le choix objectif d'un état de référence universel qui seul permettrait de déterminer de manière absolue le niveau trophique d'une eau.- La deuxième difficulté, et sans doute la plus grande, est liée à l'action qu'exercent les phénomènes d'eutrophisation sur les variables analytiques du système. Cette action a pour conséquence de rendre les valeurs intrinsèques de ces variables impropres à l'étude de ces phénomènes et de leur évolution. Ces variables étant liées simultanément aux causes et aux conséquences des diverses manifestations des phénomènes d'eutrophisation.Les 10 lacs que nous avons étudié ici, présentent l'avantage peu courant d'être alimentés par les mêmes eaux traversant une zone géographique restreinte et par conséquent géologique et climatique identique. Ils sont soumis à l'influence d'activités agricoles ou urbaines différentes mais bien identifiées, entraînant pour ces eaux des caractéristiques trophiques différentes. Dans un tel cas, il aurait pu sembler a priori possible d'établir une classification des différents états trophiques des lacs. Cependant, même dans une situation aussi idéale, et essentiellement à cause des deux difficultés mentionnées précédemment, nous tentons de montrer ici qu'il n'en est rien. Cela amène les commentaires d'ordre général suivants:- Si pour le système lacustre étudié ici, probablement parmi les plus idéaux, il est difficile à partir des critères généralement retenus dans la plupart des études, (nutriments azotés, phosphorés , chlorophylle, oxygène dissous, etc.) d'établir un classement trophique, il semble a fortiori peu probable que cela puisse être réalisé sur des systèmes moins idéaux.- La littérature actuelle fait clairement apparaître ce problème majeur. Cependant pour y remédier, elle propose généralement de faire le choix d'autres indicateurs physico-chimiques ou même biologiques qui, pour les mêmes raisons, semblent également voués à l'échec.En conclusion de cette étude, nous proposons donc d'abandonner de tels indicateurs liés au milieu et de choisir de nouveaux critères qui prendraient en compte les perturbations apportées par le phénomène aux interactions liant les variables analytiques: les différents états trophiques d'une eau étant probablement mieux caractérisés par les différents états des relations liants les variables analytiques que par les valeurs intrinsèques de ces variables.One of the problems water managers and researchers have had to deal with for a long time is that of identifying the symptoms of eutrophication, with all its multiple facets. The search for a simplified model of this complex process does not seem, so far, to have attained this objective. In order to identify and classify the various trophic states of waters (lakes or rivers), two main types of trophic indicators have been and are still being used, those belonging to the biocenosis (biological factors) and those belonging to the biotope (physical-chemical factors).- The aim of the biological approach to eutrophication is to measure its impact on the environment's biodiversity. Thus, several classification indices have been drawn up, for example: the Biotic Index, which is one of the oldest; the General Quality Biological Index, which has been improved to become the Normalised Global Biological Index (NGBI) method; and more recently the Trophic Diatom Index. Working with such indices requires quite complex analyses since it is necessary to identify the local fauna and flora. Furthermore, these indices only apply to rivers, not to lentic environments.- For the physical-chemical approach, the aim is to quantify the trophic state of an aquatic environment by measuring a number of physical-chemical parameters. This approach is easier to implement and the results provided are those currently used by water managers.It is obvious that the two approaches are linked, since the biodiversity of an aquatic environment is conditioned by the physical-chemical quality of its water. However, the study of the eutrophication process in surface waters faces two main difficulties, which are inherent in the very nature of the system and the phenomena under study:1. Since aquatic systems naturally differ one from the other, it is obviously difficult if not impossible to establish a reference state to determine the trophic level of a water with absolute accuracy.2. The second difficulty lies in the choice that must be made among the analytical parameters, to select those that are the most appropriate to describe the phenomenon. Although it is currently admitted that the nitrogen, phosphorus and phytoplankton parameters, among others, cannot be ignored, their intrinsic values are not sufficient to completely describe the process of eutrophication. Indeed, the values of most of these parameters are linked both to the causes and the effects of eutrophication, and as a consequence they cannot be interpreted unambiguously. The same restriction applies to correlations that are established between these parameters and the plant biomass. Because of the originality of the lake system with which we are dealing, the present study offers the opportunity to better understand the reasons for both these problems.The ten lakes we have studied (number 1 to 10) present the rare advantage of being supplied by the same streams, running across a restricted geographical zone that is geologically and climatically similar. However, the trophic characteristics of these waters have been altered by their passage through different agricultural and urban zones. Thus, lakes 1 to 4 are located in an area of low urban density and are colonised by phytoplankton, the density of which decreases from Lake 1 to Lake 4; macrophytes are not present in these lakes.Lake 5, located in the centre of the town, receives domestic waste water. This lake was, over a long period of time, entirely covered with water hyacinths (Eichhornia crassipes), very invasive floating macrophytes, and with lotuses (Nelumbo nucifera) and rooted macrophytes. During the study period, following the manual removal of the macrophytes in July 1995, the water was strongly colonised by algae, as can be seen from the high concentration of chlorophyll-a, close to 200 µg/l.Lake 6 presents a similar situation; it was almost entirely covered by Eichhornia crassipes until July 1997, and was then manually cleared. As the elimination of the water hyacinths profoundly modified the characteristics of this lake, we will show the two periods of time separately.Lake 7 receives the waste water from a densely populated area. After being cleared in July 1997 of a thick layer of various plants that had covered it for several years, the lake was invaded by water lettuce (Pistia stratiotes), a floating macrophyte, which has dominated the lake since December 1997.Finally lakes 9 and 10 are almost completely covered by lotuses (Nelubo nucifera), which are rooted macrophytes, along with a few Pistia stratiotes, while Lake 8 is periodically colonised by water lilies (Nymphea lotus) and algae.To follow changes in the water quality of the 10 lakes studied, 21 sampling stations were chosen, usually at the entrance and exit of each lake. The mean values for the physical-chemical parameters for any lake gave a reasonable representation of the lake water under study. Sample collection in the ten lakes lasted two hours (between 8 and 10 a.m.). The sample collection, and the analysis of the 19 physical-chemical parameters taken into account, was carried out between April 1996 and April 1998 (twice a month in the rainy season and once a month otherwise), which represented about 15 000 measurements. At each location, a litre of water was taken, 50 cm below the surface, with a polyethylene bottle fixed on a 5 m bamboo pole. Portions of 250 ml were then transferred into a brown glass bottle, for later analysis of chlorophyll. After the in-situ analyses, the bottles were kept in the dark in a cooler.Because of the unique sampling situation, it might have been anticipated that the comparison of the different trophic states on the basis of the physical-chemical variables would be possible, but this was not the case. On the contrary, this study tended to show that the physical-chemical characteristics of these waters are as much influenced by the feedback effect of eutrophication as by the external factors, so that the contribution of each of the variables to the various types of eutrophication cannot be clearly determined. This is particularly true in the case of parameters characteristic of nutrients or those reflecting the water response.Although it may appear trivial, this "feedback action" can naturally be generalised to all lake systems, in temperate or tropical climates, whatever the kind of biomass that colonises them. The trophic level of these waters must therefore be evaluated on the basis of new criteria, which will take into account all the relations that link the various parameters

Évaluation de l'état d'une eau de rivière par analyse multidimensionnelle utilisant certains paramètres caractéristiques de la matière organique dissoute

L'évaluation de la qualité de l'eau de rivières pose un certain nombre de problèmes non encore résolus. Parmi ceux-là, nous en avons retenu un qui porte sur la méthodologie généralement adoptée pour réaliser cette évaluation et qui est à la base de nombreux autres problèmes. Actuellement en France, ainsi que dans de nombreux autres pays européens, la qualité de l'eau est évaluée à partir de grilles de valeurs limites de paramètres physico-chimiques ou biologiques. L'usage de telles grilles sous-entend que les paramètres qui la composent sont indépendants les uns des autres alors qu'il n'en est rien. En effet, il est indispensable de considérer les milieux aquatiques comme étant des systèmes chimiques, physiques et biologiques plus ou moins éloignés de leurs états d'équilibre et pour lesquels les paramètres qui les décrivent, considérés à un instant donné, sont liés les uns des autres par des relations qui évoluent avec leurs états.En plus du problème précédent, il faut rappeler que toute évaluation de l'état d'une masse d'eau (au sens de la Directive Cadre européenne sur l'Eau (DCE)) nécessite que soit précisé au préalable un état de référence. La diversité, hors anthropisation, des milieux aquatiques pose alors le problème du choix de telles références.Ce travail qui a pour cadre un cours d'eau du centre de la France (la Vienne) s'inscrit donc en partie dans cette double problématique.S'appuyant sur un suivi analytique mensuel en huit points, réalisé au cours d'une période de 27 mois entre 1999 et 2002, nous avons tenté de montrer que la méthode bien connue de l'analyse en composante principale (ACP) avec étude des corrélations, pouvait, moyennant quelques préalables, être un outil permettant de préciser avec une bonne objectivité l'état, ainsi que l'évolution spatio-temporelle de ces eaux.Nous avons également tenté de montrer que l'un de ces préalables était la prise en compte dans l'ACP de variables caractéristiques de la matière organique dissoute (MOD). Les paramètres classiques actuellement utilisés, en particulier par le système français d'évaluation de la qualité de l'eau (SEQ-Eau), qui sont essentiellement minéraux, ne suffisent pas à eux seuls pour expliquer et mettre en évidence les effets liés aux apports anthropiques.Dans cette étude, nous nous sommes donc attachés de différentes manière à montrer sur ce milieu, que les représentations ACP prenaient en compte les principales observations connues comme liées aux rejets polluants.Satisfactory systems for evaluating river water quality have not yet been established. In France, as in many other European countries, water quality is evaluated using physico-chemical or biological parameter grids. This method implicitly assumes that these parameters are independent from each other and that a universal state of reference exists. In fact, when taking into account the great diversity of rivers it is clear that any evaluation of water quality requires that reference conditions be specified for each water system. Although it is one of the objectives of the new European Water Framework Directive, this problem has not yet been resolved. In addition, it is important to note that the state of water is characterised by dependent analytical parameters, and that the relationships linking these parameters evolve with the state of the water; this variable dependence must be taken into account. Lastly, it is important to note that organic matter is generally underestimated by current water quality evaluation systems.In the present study an attempt was made to evaluate the water quality of the Vienne River situated in western France. This study provided us with the opportunity to verify the accuracy of the current system. A monthly analytical monitoring program at eight study sites was carried out during the period 1999-2001. It allowed us to demonstrate that the well-known method of the principal component analysis (PCA), together with correlation analysis, was an appropriate tool for this purpose. More specifically, we showed that when using parameters characteristic of organic matter, this tool is well adapted for monitoring spatial and temporal changes in water quality. As this method is graphical, it made the choice of a reference point possible.This study showed that traditional parameters currently used, which are primarily inorganic, were not sufficient to identify anthropogenic contributions. For this reason we used unconventional organic variables, formerly used to study the structure of the dissolved organic matter (DOM) in surface waters, such as:- the distribution of the various organic fractions, such as humic (or hydrophobic) substances and non humic (or hydrophilic) substances, after filtration through macroporous XAD8 and XAD4 resins, under well-defined conditions;- the distribution of the apparent molecular weights, by ultrafiltration through membranes with molecular weight cut-offs of 30, 10 and 3 kDaltons;- fluorescence spectrometry;- the ratio UV-absorbance at 254 nm/dissolved organic carbon (SUVA).It was noted in this study that these organic variables, coupled with those classically analysed (Chemical Oxygen Demand, COD; Biological Oxygen Demand, BOD5), were often mainly responsible for the variations in the water quality. Principal Component Analysis was used to extract the main factors associated with water quality variability. The graphical representation obtained underlined that the greatest variability of the points characteristic of the stations could be observed on the first principal component. This component was linked to the organic variables. The second principal component was linked to inorganic variables indicative of the geological substrate. In addition, it was shown that the temporal dispersion of stations in the PCA graphical representation can be used to evaluate (either from the variance of the PCA factors, or from the box plots of these same factors) the disturbances related to the contributions of anthropogenic origin. The proportion of variability in water quality due to hydrological conditions could be also highlighted by PCA

La mise à l'équilibre des eaux tropicales : conséquences sur la déferrisation

L'étude de l'équilibre calco-carbonique d'une eau souterraine de l'Afrique de l'Ouest par la méthode de LEGRAND et POIRIER, a conduit à la détermination des conditions de traitement pour neutraliser l'agressivité naturelle de l'eau, due essentiellement à une forte teneur en acide carbonique et réduire ainsi son caractère corrosif. le problème de l'évolution du fer présent en relation avec la mise à l'équilibre a été également étudiée.L'application réalisée sur un pilote de laboratoire a permis d'optimiser les principales étapes du traitement, c'est-à-dire l'addition d'une base forte, la précipitation et la filtration des hydroxydes ferriques qui se forment à la suite de la neutralisation de l'acide carbonique.Studies on calcium carbonate equilibrium of West African ground water, a sample of potable water distributed in Lomé (Togo), were carried out using the method described by LEGRAND and POIRIER. It is a standard graphic method in which calculations include all basic (Ca++, HCO3-, CO3--, CO2, H2O+, OH-), and characteristic (Mg++, Na+, K+, SO4--, Cl- ...) chemical parameters of natural water. Processing of a computer program for calculations enabled the equilibrium and saturation curve [CO2] = f[Ca++], to be obtained. The aggressivity to the lime deposit character, different conditions of treatment, aeration, and addition of an appropriate base to neutralise the nature aggressivity of water due to its high dissolved carbon dioxide content could also be defined.The problem of the presence of iron was studied; high concentration of iron in natural ground water lead to various problems : developing of tastes, staining and discoloration of clothes, and growth of iron bacteria in the distribution system. Its removal from water is accomplished by means of the oxidation of iron (II) to iron (III) followed by the precipitation of relatively insoluble ferric hydroxide. The oxidation rate is highly dependent on pH. Thus, the process of removal of carbon dioxide by stripping or neutralisation raises the pH of the water. If the pH is raised high enough, an insoluble form of iron will precipitate. Another important aspect of the chemical behaviour of iron is the formation of complex ions with inorganic as well as organic ligands; silica in water forms a relatively stable complex with iron (III). The difficulty in removing iron from water is often associated with this phenomenon.Continuous flow studies were carried out to improve important steps of treatment such as addition of a strong base, and precipitation and filtration of ferric hydroxide produced during carbonic acid neutralization. They show the weak stability of silicato-iron (III) complex at high pH value and prove that practically complete iron removal may be obtained

Ephemeroptera communities as bioindicators of the suitability of headwater streams for restocking with white-clawed crayfish, Austropotamobius pallipes

Currently, the distribution of the European native white-clawed crayfish (Austropotamobius pallipes) is restricted and fragmented over its range. Many European countries have active programmes for conservation of the species, including reintroduction programmes. Here, we have studied four brooks that were restocked with crayfish coming from population rescue from the Brissionnières after a drought in summer 2009. The aim was to validate the use of Ephemeropteran communities as potential bioindicators to select brooks for crayfish restocking. Restocked brooks were surveyed in 2010 and 2012. All brooks showed chemical and physical parameters in accordance with A. pallipes requirements. Three brooks which had high Ephemeropteran richness including Leptophlebiidae species (Habrophlebia lauta and Paraleptophlebia submarginata) have been restocked successfully. No crayfish were observed during the survey in Le Peu, which had only two Ephemeropteran species tolerant of reduced water quality and had no Leptophlebiidae species. Ecological characteristics of Ephemeropteran species and the limitations of their use as bioindicators to select brook for restocking are discussed

Physical and chemical water requirements of the white-clawed crayfish (Austropotamobius pallipes) in western France

International audienc

MANAGEMENT OF THE WHITE-CLAWED CRAYFISH (AUSTROPOTAMOBIUS PALLIPES) IN WESTERN FRANCE: ABIOTIC AND BIOTIC FACTORS STUDY

In France, the distribution of the white-clawed crayfish, Austropotamobius pallipes (Lereboullet, 1858), is restricted, fragmented and mainly located in headwaters. To preserve this indigenous species, it is necessary to characterize its ecological requirements (water and habitat quality). With this aim in view, a two-year study is being conducted in the Deux-Sèvres department (Western France) since November 2002. Nine brooks from four different catchments are monitored regularly; eight of the nine brooks harbour whiteclawed crayfish populations. Two sampling sites are surveyed per brook, the first being where the crayfish population is located and the second 2 to 3 km downstream. Physicochemical parameters (18) are measured twice monthly and biotic factors are estimated twice yearly. In this study, the I.B.G.N. (Indice Biologique Global Normalisé) protocol based on the determination of macroinvertebrates was used as a biotic index of biological water quality. Results of this preliminary study on two brooks (Thouet and Verdonnière) show that physico-chemical and biological data considered separately do not provide reliable information about A. pallipes ecological requirements. However, the use of multivariate analyses (Principal Component Analysis) to combine abiotic and biotic factors highlights a good correlation between these parameters. Organic matter appears to be a better discriminating factor than mineral matter affecting presence or absence of the whiteclawed crayfish

MANAGEMENT OF THE WHITE-CLAWED CRAYFISH (AUSTROPOTAMOBIUS PALLIPES) IN WESTERN FRANCE: ABIOTIC AND BIOTIC FACTORS STUDY

In France, the distribution of the white-clawed crayfish, Austropotamobius pallipes (Lereboullet, 1858), is restricted, fragmented and mainly located in headwaters. To preserve this indigenous species, it is necessary to characterize its ecological requirements (water and habitat quality). With this aim in view, a two-year study is being conducted in the Deux-Sèvres department (Western France) since November 2002. Nine brooks from four different catchments are monitored regularly; eight of the nine brooks harbour whiteclawed crayfish populations. Two sampling sites are surveyed per brook, the first being where the crayfish population is located and the second 2 to 3 km downstream. Physicochemical parameters (18) are measured twice monthly and biotic factors are estimated twice yearly. In this study, the I.B.G.N. (Indice Biologique Global Normalisé) protocol based on the determination of macroinvertebrates was used as a biotic index of biological water quality. Results of this preliminary study on two brooks (Thouet and Verdonnière) show that physico-chemical and biological data considered separately do not provide reliable information about A. pallipes ecological requirements. However, the use of multivariate analyses (Principal Component Analysis) to combine abiotic and biotic factors highlights a good correlation between these parameters. Organic matter appears to be a better discriminating factor than mineral matter affecting presence or absence of the whiteclawed crayfish

ECOLOGICAL CHARACTERISATION OF STREAMS IN TUSCANY (ITALY) FOR THE MANAGEMENT OF THE THREATENED CRAYFISH AUSTROPOTAMOBIUS PALLIPES COMPLEX

Any initiative aimed at the management of a threatened species needs a good knowledge of its environmental requirements. Aims of this study were to individuate suitable areas for the reintroduction of crayfish species belonging to the Austropotamobius pallipes complex and to evaluate the causes of the local extinction in Tuscany (Italy) of some populations. Between May 2003 and September 2004, we collected ecological data from 19 streams of 4 catchments, 9 watercourses where crayfish were present (WI) and 10 where they were present in the recent past and absent now (WO), and we compared them. Multivariate analyses were performed using chemico-physical and biotic parameters to examine the relationships between their values and the presence of crayfish. The results did not allow us to find significant differences between the two categories of streams, suggesting their suitability for crayfish reintroduction

ECOLOGICAL CHARACTERISATION OF STREAMS IN TUSCANY (ITALY) FOR THE MANAGEMENT OF THE THREATENED CRAYFISH AUSTROPOTAMOBIUS PALLIPES COMPLEX

Any initiative aimed at the management of a threatened species needs a good knowledge of its environmental requirements. Aims of this study were to individuate suitable areas for the reintroduction of crayfish species belonging to the Austropotamobius pallipes complex and to evaluate the causes of the local extinction in Tuscany (Italy) of some populations. Between May 2003 and September 2004, we collected ecological data from 19 streams of 4 catchments, 9 watercourses where crayfish were present (WI) and 10 where they were present in the recent past and absent now (WO), and we compared them. Multivariate analyses were performed using chemico-physical and biotic parameters to examine the relationships between their values and the presence of crayfish. The results did not allow us to find significant differences between the two categories of streams, suggesting their suitability for crayfish reintroduction