Volume 2, numéro 4, 1989

FR :

La télédétection satellitaire est un outil employé couramment et avec succès en océanographie. Il n'en va pas de même en limnologie, où les applications sont encore rares.

Par le moyen d'une revue bibliographique, nous tentons d'en analyser les raisons. Après une brève description de l'outil et des satellites en service, l'on met en évidence la spécificité des cibles aquatiques, caractérisées par une réflectance basse et une profondeur d'investigation variable. Ces particularités, jointes à la composition complexe des eaux intérieures, rendent impossible l'extension pure et simple, à la limnologie, des algorithmes développés en océanographie.

Néanmoins, nous montrons que la télédétection a été utilisée dans l'étude du bassin versant des lacs, ainsi que pour la cartographie de leurs limites, de la végétation aquatique, des courants, de la thermique et de la couleur de l'eau. Des modèles empiriques, exprimant la matière en suspension ou les paramètres de qualité de l'eau, ont été calculés et appliqués avec succès dans certains lacs.

On établit ensuite une typologie des difficultés rencontrées dans l'application de la télédétection à la limnologie : intrinsèques (complexité de la composition), technologiques (capteurs actuels non adaptés aux cibles aquatiques) et institutionnels (coûts élevés et manque de professionnels de ta télédétection dans les cercles limnologiques).

Finalement, l'on présente quelques propositions pratiques dans la perspective des nouveaux véhicules spatiaux et capteurs des années 90, qui devraient permettre une exploitation de l'énorme potentiel de la télédétection en limnologie.

EN :

Remote sensing has been used successfully in oceanography for many years, whereas applications in limnology have been comparatively modest. We attempt to discover why from a review of the literature.

After a brief description of remote sensing and of satellites in operation, we stress the specificity of inland waters targets compared with "solid" targets : low reflectivity in the visible and near infrared, complex water composition (chlorophyll, mineral suspensoids and gelbstoff which are not covariant) and the variable depth of investigation. Hence, simple chlorophyll and suspended solid retrieval algorithms, developed for type-I waters, cannot be applied to inland waters.

Nonetheless, remote sensing may be, and has been applied effectively to the study and management of drainage basins and to the mapping of lake limits, aquatic vegetation, water masses, currents, thermal structures and water colour. Site-specific, regression-type models have been computed to express suspended solid concentration and water quality parameters as functions of radiante. So far, most of the applications have been performed on large lakes (Laurentian Great Lakes, Tahoe, Chad, Biwa, Balaton, Léman), where not only LANDSAT but also CZCS and AVHRR data have been in operation.

A typology of present difficulties in applying remote sensing to limnology is proposed : intrinsic (complexity of inland waters), technological (land resource satellite bands not designed for water sensing, revisit time too long) and institutional difficulties (cost of data and equipment, and scarcity of limnologists proficient in remote sensing techniques).

FR :

Le lac du Bourget, l'un des principaux lacs alpins situé en France, a fait l'objet en 1981 d'importants travaux d'aménagement en vue de ta restauration de la qualité de ses eaux. Une campagne de mesure portant sur les années 1988-89 a été mise en place afin de faire le point sur l'évolution du lac depuis la fin des travaux.

Un modèle thermique et biogéochimique (phosphore, oxygène, azote) sera utilisé pour synthétiser les connaissances, prévoir l'évolution de la qualité des eaux du lac ainsi que l'influence d'éventuels aménagements complémentaires. Les résultats présentés ici concernent la première étape du projet d'études, la modélisation thermique du lac du Bourget.

Le modèle utilisé est un modèle unidimensionnel, vertical, basé sur l'équation d'advection-diffusion.

L'expression des coefficients de dispersion selon la profondeur reprend celle d'un modèle du lac Léman (Tassin, 1986). Les équations utilisées distinguent l'épilimnion, le métalimnion et l'hypolimnion.

Les résultats présentés montrent que le modèle décrit de façon satisfaisante le cycle thermique annuel et l'évolution inter-annuelle des températures dans le lac du Bourget.

Les profils et les valeurs des coefficients de dispersion calculés sur le lac du Bourget sont proches de ceux obtenus sur d'autres lacs à partir de mesures fines de température ou de concentrations d'isotopes naturels.

Les coefficients de dispersion obtenus pourront donc être utilisés dans la modélisation des substances dissoutes dans le lac.

EN :

In 1981, important works including the diversion of the main sewers entering Lake Bourget (one of the largest French alpine lakes) were undertaken in order to restore acceptable water quality standards. A detailed water quality survey will be performed in 1988-89 to complete the data base which already covers ten years. It should help in quantifying the evolution of the lake since the 1981 restoration works. The following activities will be undertaken as part of the survey : measurements within the water column, a study of the bottom sediments, the setting-up of sediment traps and the coupling of Landsat satellite images with measurements performed at some stations at the lake surface. A thermal and biogeochemical (phosphorus, oxygen, nitrogen) model will be used to summarise the information available and to forecast the evolution of water quality and the effect of other restoration measures. This paper reports on the first part of the study : the thermal modelling of Lake Bourget. A one-dimensional, vertical model based on the advection-diffusion equation is used. This equation is solved using a finite difference, semi-implicit method. The resolution grid has 145 layers (1 meter high) and the time step is 3 hours.

The heat fluxes at the air-water interface are computed with the three-hourly meteorological data collected at the Chambéry airport station, at the south end of the lake. The formulas are empirical and well established in the literature.

The expression for the eddy diffusion coefficients is based on a model of the lake of Geneva (Tassin, 1986). Different equations are used for the epilimnion, metalimnion and hypolimnion. In the epilimnion, the eddy diffusion coefficient expresses, by the Richardson gradients number, the interaction between the shear stress of the wind and the water column stability. It depends on the value of the eddy diffusion in neutral conditions and on a stability function which includes the Richardson number. The vertical profile of the horizontal number. The vertical profile of the horizontal currents is computed following Ekman (1905) and Simons (1981) and is approximated by a gradient which decays exponentially.

In the metalimnion and the hypolimnion, the water layer stability is characterised by the Brünt-Väisälä frequency. In the hypolimnion, the eddy diffusion coefficient includes a corrective term, which is a function of depth; this term serves to reduce the dispersion near the bottom of the lake.

The thermal model also includes the mixing of the first layers by waves, the vertical advective transport induced by rivers inflows into the lake and by surface water withdrawal, as well as the thermal convection induced by local instabilities of the water column.

The eight parameters occurring in the heat flux equations and the eddy diffusion coefficients were estimated using data from 1981. The model has been validated over an 8 years period 1976-1983).

Results of the model agree with the observed seasonal and long-term evolutions of temperature in Lake Bourget. The main characteristics of the annual cycle are reproduced : set-up of the thermocline in spring, depth of the epilimnion, thermocline and metalimnion, value of the temperature gradient in the metalimnion, deepening of the thermocline in fall and winter overturn. Significant differences between observed and simulated temperatures occur in the metalimnion and they may be partially explained by internal waves which are dominant features at this level during summer stratification. This kind of mechanism cannot be accounted for in a one-dimensional vertical model.

Profiles and values of eddy diffusion coefficients calculated for Lake Bourget show a good agreement with those obtained in other large takes from measurements of temperature or natural isotopes concentrations.

In general, the thermal model gives a good account of heat transport mechanisms in Lake Bourget. Accordingly, it provides acceptable dispersion coefficients which can be used to model the distribution of dissolved species in the lake.

FR :

Les études entreprises depuis 1982 sur un petit cours d'eau pollué par une forte charge organique, ont servi de base au développement d'un modèle mathématique destiné à appréhender le fonctionnement des mécanismes d'auto-épuration.

L'écriture du modèle a débuté par la description de l'écoulement du cours d'eau et des mécanismes de transports associés. Deux sous-modèles ont été utilisés dans ce but : un sous-modèle hydrodynamique dérivé du modèle de Saint-Venant et un sous-modèle de transport, constitué par l'équation classique de convection-dispersion. Ces deux sous-modèles sont appliqués à un écoulement monodimensionnel, non uniforme et non stationnaire.

Dans les petits cours d'eau le compartiment benthique joue un rôle prépondérant du point de vue de l'ensemble des processus détritiques. Le modèle a donc été appliqué, dans un premier temps, à la simulation de la dynamique du carbone organique dissous et des microorganismes benthiques.

Les mécanismes de biodégradation sont modélisés, ici, en utilisant l'analogie entre les processus détritiques et l'activité des biomasses des procédés de traitement des eaux usées. Le modèle de biofilm développé permet, par sa structure, d'englober tous les mécanismes physiques et biologiques qui entrent dans les phénomènes de disparition et de dégradation de la matière organique dissoute.

Ce modèle est utilisé, sous sa forme actuelle, en tant qu'outil cognitif destiné à accroître la connaissance des mécanismes d'auto-épuration de la matière organique dissoute dans les petits cours d'eau.

EN :

Several studies have been performed since 1982 on a small organically polluted stream. Their results have been used to conceive a mathematical model with the purpose of simulating the self-purification mechanisms.

The physical part of this model includes a hydrodynamic component derived from Saint-Venant's equations which is coupled with a transport model based on the classical convection-dispersion equation. The model is applied to unidirectional, non-uniform, unsteady flow conditions.

As the main detritic processes which occur in small streams take place in the sediment, the model has been applied, first, to simulate the dynamics of both dissolved organic matter and benthic microorganisms.

The simulation of the biodegradation mechanisms is based, in this work, on the analogy between the detritic processes in streams and the activity of the microbial biomass in wastewater treatment plants. From the structure of the biofilm model proposed, it was assessed that nearly all mechanisms involved in the removal and degradation of dissolved organic matter may be considered.

In these studies, the model is used as a cognitive tool intended to increase the understanding of self-purification mechanisms applied to dissolved organic matter is small streams.

FR :

La biomasse et la production primaire phytoplanctonique, ainsi que la biomasse et l'activité bactérienne hétérotrophe, ont été suivies au cours de deux cycles annuels dans le tronçon de la Meuse situé entre la frontière franco-belge et Dinant. Tous ces paramètres présentent des évolutions saisonnières marquées avec des minima hivernaux et des maxima durant l'été. L'analyse des résultats expérimentaux met en évidence le couplage entre la production primaire et la production bactérienne.

Un modèle microbiologique de la dégradation bactérienne de la matière organique en milieu aquatique a été développé; il est basé sur l'étude de la cinétique des étapes successives de l'interaction entre matière organique et micro-organismes qui la dégradent. Ce modèle de biodégradation a été appliqué au cas du tronçon de Meuse étudié en tenant compte d'une hydrodynamique simplifiée. L'ensemble des valeurs des paramètres pris en compte dans ce modèle, qui permet de calculer les variations saisonnières de la biomasse et de la production bactérienne, a été déterminé expérimentalement ou sur base de simutations d'expériences de biodégradation. Les résultats des calculs de ce modèle déterministe des activités microbiotogiques sont en assez bon accord avec tes résultats expérimentaux.

EN :

The phytoplanktonic biomass (chlorophyll-a) and primary production (incorporation of ¹⁴CO₂), as well as the bacterial biomass (enumeration by epifluorescence microscopy after acridine orange staining) and bacterial production (³H-thymidine incorporation) were followed for two years (1983 and 1984) in the section of the River Meuse located between the Belgian-French border and Dinant (figure 1). All these parameters show seasonal fluctuations with minimum values during winter and maximum values during summer (figure 2). The analysis of these data show the connection between primary production and bacterial activity in this area.

Despite recent progresses in microbial ecology, most models describing organic matter degradation in aquatic ecosystems still basically derive from the STREETER and PHELPS's (1925) model of organic matter pollution, in which the rate of decomposition is assumed to be first order with respect to the organic load. Although these geochemical approaches often yield quite good predictions, they consider organic matter degradation as a chemical property of the organic matter itself, without taking into account the activity of the micro-organisms. Based on a study of the successive stages involved in the interaction between organic matter and heterotrophic bacteria, a microbiological model of bacterial degradation of organic matter in aquatic systems has been developed (BILLEN and SERVAIS, 1988).

The main features of this model, called the H3SB model, can be summarized as follows (figure 3) : organic matter is mostly supplied by primary production and allochtonous inputs in the form of macromolecular biopolymers (H) which cannot be taken up directly by bacteria. Only some small molecules, recognizable by the bacterial permeases, can be used by bacteria. These compounds called direct substrates (S) are produced by exoenzymatic hydrolysis of the high molecular weight compounds. Among these compounds, we distinguish three classes : H₁, compounds which are easily and rapidly hydrolyzed into direct substrates; H₂, compounds which are slowly hydrolyzed, and H₃, the refractory organic matter. Once taken up by bacteria, direct substrates can be either catabolized and respired or used for the production of bacterial biomass (B). Once formed the bacterial biomass is subject to a mortality process (grazing by heterotrophic microzooplankton or lysis).

This view of the interactions between bacteria and organic matter is certainly idealized. Its usefulness however lies in the fact that excellent methods have been developed for directly measuring the rate of most of the basic processes listed above (exoenzymatic hydrolysis, uptake of direct substrates, bacterial production and bacterial mortality) and for studying their kinetics and control mechanisms. Mathematical relationships have been proposed for the H3SB model. Equations (1), (2), (3), (4) and (5) describe respectively the time fluctuations of H₁, H₂, H₃, S and B. Exoenzymatic hydrolysis and bacterial uptake of direct substrates are characterized by MICHAELIS-MENTEN kinetics, bacterial production is proportional to substrate uptake and bacterial mortality is represented by a first order term.

This basic model of biodegradation has been used to build a model of heterotrophic activity, in the section of the River Meuse studied, taking into account a simplified form of the hydrodynamical processes. Three types of organic matter inputs to the river have been considered : phytoplankton Lysis, domestic loads and soil leaching. The values of the parameters involved in this model have been determined by different experiments performed on the natural population of bacteria from the River Meuse (figure 4), or by fitting experimental data of biodegradation experiments performed on organic matter from various origins : for example, River Meuse organic matter (figure 5) or phytoplanktonic material (figure 6). This model of heterotrophic bacterial activity in a river has allowed the seasonal fluctuations of a bacterial biomass and production in the river to be assessed, giving the known hydro-dynamical parameters (wet section and variations of discharge), temperature, domestic load (calculated from the population density) and phytoplanktonic biomass. The data of the calculations of this determinist model of heterotrophic activity agree well with the experimental date (figures 7, 8, 9, 10).

This kind of bacterioplankton model could in the near future be used to construct a complete ecological mode of rivers leading to studies on the impact on mater quality.

FR :

Les flux de matière organique particulaire allochtone et autochtone ont été suivis dans la Lône des Pêcheurs. Leur estimation quantitative et qualitative rend compte de l'impact de l'écotone rives sur le fonctionnement du système aquatique. Dans ce bras mort, les apports allochtones semblent plus importants que dans d'autres systèmes, comme les ruisseaux forestiers mentionnés par la littérature.

Pour l'estimation des flux de matière organique particulaire nous avons utilisé, comme moyen d'échantillonnage, des bacs de plastique rectangulaires déposés sur le fond; en plus nous avons procédé à des prélèvements par benne.

Le plus grand apport de feuilles terrestres a lieu en novembre, le plus grand apport de bois et autres débris en avril et dans les mois qui suivent, probablement par suite des crues. Au niveau de la lône, on note une différence entre les parties amont et aval.

A surface égale, les apports grossiers (feuilles mortes et bois) sont plus abondants en amont, les éléments en voie de désintégration sont, au contraire, plus nombreux en aval. Cette différence est principalement due à la proportion des rives par rapport à la surface du plan d'eau et probablement aussi à la vitesse de dégradation de ces apports dans les deux stations. Ceci nous amène donc à penser qu'il existerait un gradient décroissant de matière organique grossière de l'amont de la lône vers l'aval.

La biocénose aquatique est alimentée d'abord par les macrophytes immergés à décomposition rapide (été, automne), puis par les feuilles terrestres et les macrophytes émergés (hiver, printemps).

L'analyse de la structure des invertébrés aquatiques montre la relation entre ces apports, particutièrement les feuilles mortes, et la biomasse des dilacérateurs et des collecteurs.

EN :

This study concerns a side arm of the River Rhone, 20 km upstream from the city of Lyons, southeastern France. The arm opens into the channel of the river downstream, but it is closed and fed by groundwater upstream. Terrestrial particulate organic matter enters the water through the ecotone of the banks. This flow and that of carbon, nitrogen and phosphorus were measured, using sediment traps and a grab, by the monthly sedimentation of allochthonous organic matter on the bottom, corrected for the time of exposure under the water. The input seemed to be somewhat greater than in the temperate woodland streams described in the literature.

The greatest input of terrestrial leaves occurred in November, the greatest input of wood and other debris in April, presumably as a consequence of the flooding of the undergrowth. The arm was narrower upstream than downstream (respectively 10-25 and 60-80 m in width). Though the banks were equally and densely forested, the bottom sediment received more coarse particulate matter upstream, and this matter was equally distributed. In contrast, less organic matter was deposited along the banks downstream and still less in the middle of the arm, where partially, decomposed matter was more abundant. These differences may be explained by the ratio between the banks and the water area, by a certain transport during the floods, and by the faster processing of particulate matter closer to the channel. Hence there seems to be a decreasing gradient of particulate organic matter as one goes downstream along the arm.

The heterotrophic communities in the arm were fed successively by the decay of autochthonous submerged macrophytes such as Potamogeton in summer and autumn, then by that of terrestrial leaves (mostly alder, poplar and willow with respectively fast, medium and slow processing velocities) and emergent macrophytes such as Phragmites, during winter and spring.

A comparison of invertebrate biomass and community structure shows that shredders and collectors are more numerous in winter and spring, white terrestrial leaves are being processed.

FR :

L'utilisation des méthodes biologiques et plus particulièrement l'étude des communautés benthiques pour l'appréciation de la qualité des eaux est actuellement d'un usage général. L'impact des perturbations des milieux peut être déduit de la composition et de la structure des peuplements qu'ils hébergent. En est-il de même pour les peuplements d'invertébrés ripicoles endogés humides, soumis moins directement à t'influence du milieu aquatique? Pour répondre à cette question une étude comparée (par analyse factorielle des correspondances) a été entreprise sur le réseau hydrographique d'une rivière de la région méditerranéenne, l'Arc, présentant des situations hydrochimiques variées.

L'A.F.C., à partir de l'axe I, sépare les prélèvements d'invertébrés benthiques des prélèvements d'invertébrés ripicoles. Sur l'axe II les couples, prélèvement benthique - prélèvement ripicole de chaque station, possèdent sensiblement la même valeur sur l'axe et s'ordonnent selon un gradient de trophie croissant de l'amont vers l'aval. Les communautés ripicoles endogées humides peuvent donc être considérées comme des bio-indicateurs de la chimie de l'eau de la même manière que les communautés benthiques. Par contre le facteur temporarité (axe III) a une influence plus importante sur les communautés benthiques que sur les communautés ripicoles.

L'examen de la répartition des taxons sur les axes factoriels a permis de déterminer un certain nombre d'espèces qui, tout en étant représentées dans les 2 milieux, sont caractéristiques de l'habitat rivulaire endogé humide. La prise en compte des peuplements ripicoles, malgré des difficultés d'ordre taxonomique, pourrait être envisagée lors de la détermination de la qualité des eaux.

EN :

Utilization of biological methods and particularly through the study of benthic communities in order to estimate the quality of the water is for the present time very common. The impact of the disturbances of the medium may be deducted from the composition and the structure of the populations living there. But is it altogether the same for endogenous riparian wet invertebrate populations less directly submitted to the influence of the aquatic medium?

To answer to this question, a comparative study (Factorial Analysis of Correspondences) has been carried out on the hydrographic network of the Arc River (Mediterranean region) which exhibits diversified hydrochemical situations. From and after axis I, the F.A.C. separates the benthic invertebrate samples from the riparian invertebrate ones. On axis II, the couples "benthic samples-riparian samples" of each station appreciably posses the same value on the axis and are arranged according to a trophic gradient ascending from upstream to downstream. The wet endogenous riparian communities can therefore be considered as bioindicators of the chemistry of the water in the same way as benthic communities. In contrast, the temporality factor (axis III) has a more important influence on the benthic communities than on the riparian ones.

Examination of the distribution of the taxons on the factorial axis leads to identify a certain number of species which, although present in both mediums are characteristic of the wet endogenous riverside habitat. Taking into account of the riparian populations, in spite of taxonomic difficulties, should be considered as a valuable method for the determination of the quality of the waters.

FR :

Plusieurs études relevées dans la littérature expliquent l'augmentation de la biomasse du périphyton aux bas pH par la diminution de la prédation par le benthos, les larves herbivores paraissant être les plus sensibles à l'acidification. Cependant cette hypothèse n'a pas reçu de vérification expérimentale directe. La présente étude analyse l'évolution de la biomasse et de la production périphytique en présence et en absence de macrobenthos à deux valeurs de pH, soit 6,3 ± 0,1 et 4,2 ± 0,2. Pour ce faire nous avons travaillé en conditions semi-naturelles, dans des canaux artificiels alimentés par un ruisseau. Des plaques de céramique non vernie étaient introduites dans des cylindres de plexiglas transparent. Après le début de l'acidification, la moitié des cylindres étaient fermés avec du filet de Nytex à maille de 160 µm. Bien que ce dispositif d'exclusion ne se soit pas avéré totalement efficace, des différences notables sont observables entre cylindres ouverts et fermés en condition acidifiée. Après un mois d'acidification, il y a dans les cylindres fermés une accumulation de biomasse, une réduction de la production et un rapport P/B très faible; situation caractéristique d'une réduction de la pression de broutage selon le modèle de Lamberti et Moore. L'évolution de la biomasse et de la production périphytique en condition acidifiée semble donc être dépendante de la réduction du broutage.

EN :

Many studies attribute the increased biomass of periphyton at low pH to a decrease in grazing by benthic invertebrates. Herbivorus larvae appear to be the most sensitive to acidification. However this hypothesis bas not received direct experimental verification. The present study analyses the evolution of periphytic biomass and production in the presence and absence of macrobenthos at two pH values, 6,3 ± 0,1 and 4,2 ± 0,2. The study was carried out in semi-natural conditions, using artificial troughs fed by a nearby stream. Unglazed ceramic tiles were placed in transparent plexiglass cylinders. Once acidification had begun, half the cylinders were closed with 160 µm mesh nytex screen. Although this exclusion device was not completely effective, notable differences were observed between the open and closed cylinders in the acidified conditions. After a month of acidification the closed cylinders showed an accumulation of biomass, a decrease in production and a very low P/B ratio; these conditions are characteristic of a reduced grazing pressure according to the Lamberti and Moore model. The evolution of periphytic biomass and production in acidified conditions would thus appear to be determined to some extent by reduced grazing.

FR :

Les auteurs donnent les résultats d'une étude physico-chimique et biologique portant sur quatre torrents situés sur le versant lorrain des Vosges. Les stations étudiées sont placées en amont de toute agglomération et dans des secteurs non perturbés par l'activité agricole. L'assise rocheuse de nature siliceuse est granitique. La différence dans la composition des granites et leur altération pourraient être à l'origine des différences physico-chimiques observées entre torrents situés dans un même secteur.

La présence permanente ou temporaire d'eaux acides appartenant au crénon et au rhitron a été établie. Cette acidité s'accompagne de concentrations élevées en aluminium (Al total > 200 µg/l). Une enquête piscicole et la réalisation de pêches électriques ont révélé l'absence de salmonidés dans les cours d'eau acidifiés en permanence, alors que leur présence est connue antérieurement. L'existence d'épisodes acides lors de la fonte des neiges et des pluies printanières a été démontrée; les auteurs retiennent les retombées acides comme cause de l'acidification des eaux de surface. Ces événements d'ordre hydrométéorologique sont caractérisés par l'augmentation des concentrations en H⁺, NO₃^-, SO₄^2- et aluminium, ainsi que par une baisse des ions HCO₃^-, dans les eaux torrenticoles. Les macroinvertébrés benthiques intègrent ces épisodes et l'étude comparative de leurs populations montre des modifications apparaissant dans les secteurs acidifiés par rapport aux secteurs non perturbés. L'intégration par les invertébrés benthiques des conditions physico-chimiques se traduit notamment par la disparition des Ephéméroptères, et par un appauvrissement de la diversité taxonomique des Plécoptères, Trichoptères et Coléoptères.

EN :

A biological and physico-chemical survey of four streams has been conducted in the Vosges Mountains. The areas investigated have granitic bed rock and soils in the process of podzolisation; they are regarded as sensitive to inputs of strong acids. The streams drain forested catchments that are affected by decline of the forests Coniferous trees (Abiea alba, Picea abies) are dominant.

The sampling sites were located above constructed and agricultural areas (645-685 metres above sea level) thus avoiding organic pollution. Samples of invertebrate fauna in the streams were collected using a surber-type sampler. The presence of fish (Salmo trutta fario) was determined by electro-fishing.

To determine water chemistry, pH, Ca²⁺ , Mg²⁺ , K⁺, SO₄^2-, NO₃^-, total dissolved aluminium, alcalinity (T.A.C.) and HCO₃^-/SO₄^2- were measured. The permanent or occasional presence of acidic streamwaters was established. Acidification occurred in all streams during snow melting and rainstorms.

During these hydrometeorological events, characteristically low pH levels (pH < 5.5), low alkalinity associated with high concentrations of aluminium, nitrates and sulfates, occurred in the water. These are critical periods for aquatic organisms owing to the toxicological effects of low pH and high concentrations of aluminium.

Two streams (the "Rouge-Rupt" and the "Grand-Rupt") did not contain trout.

Diversity and abundance of benthic macroinvertebrates are reduced with increasing acidity. Only 17 taxa were found in the most acidic stream (mean pH : 4.68), whereas 51 taxa were found in the one with less acidity. Likewise, in the less acidic river, abundance was more than three times higher (933 ind/m²) than in the one with most acidity (295 ind/m²). Ephemeroptera were absent when the pH was below 5.9. This order of aquatic insects seems to be the most sensitive to acidification.

The benthic macroinvertebrate community also contained less Trichoptera. Only some Limnephilidae and the genera Plectrocenemia and Rhyacophila were common in acidic streamwaters and seemed to tolerate the acidification process.

The Plecoptera dominated benthic communities in acidified streams. Nemouridae, Capniidae and Leuctridae were common and abundant. However, apart from Chloroperla sp., the Setipalpia were absent.

A decrease in the diversity and abundance of Coleoptera was observed when the pH was lower than 5.9. The only beetle found in all the streams was Heldes sp.

Diptera, Oligochaetes and Flatworms occurred in all four streams and did not present any difference in their distribution when the pH was low.

Knowledge of the biology and ecology of the species appears essential in order to understand the impact of acidification on macroinvertebrates.

The possibility that some species can carry out their development cycle in periods of no acid stress, thus being protected from the harmful effect of acidification, is suggested. In contrast, other species are undoubtedly partly or totally eliminated, owing to their presence at a sensitive development stage during periods of intense acidification.

FR :

L'étude radioécologique du Rhône permet d'évaluer qualitativement et quantitativement les radionucléides présents dans le fleuve. Les études menées in situ posent des questions concernant les modalités de transfert des radionucléides. Dans ce travail des expériences sont mises au point, afin d'analyser les mécanismes de bioconcentration dans les écosystèmes aquatiques. Pour le césium-137 les échanges entre l'eau, le sédiment et divers organismes aquatiques ont permis d'élaborer un modèle mathématique que l'on peut confronter aux valeurs mesurées sur le terrain. En ce qui concerne le Cobalt-60 les auteurs décrivent des expériences permettant l'évaluation de la contribution relative de l'eau et de la nourriture dans l'accumulation du radionucléide par un poisson.

EN :

The radioecology of the Rhone Basin has been studied for the last 15 years. This has been an opportunity to make a quantitative and qualitative evaluation of radionuclides as a function of their different sources. Special attention is given to ¹³⁷Cs (present both in fallout and liquid wastes) and ⁶⁰Co, which characterize the liquid wastes of pressurised water reactors. In order to assess the transfer and bioconcentration of these two radionuclides in freshwater ecosystems, several experimental studies were undertaken.

The ¹³⁷Cs transfer studies were carried out with a 5-component experimental ecosystem and the data were included in a mathematical model. For ⁶⁰Co, the experimental study concerns the relative contribution of water and food in the accumulation of the radionuclide by Cyprinus carpio.

Water, sediment, plants and fishes were taken from ⁶⁰ sampling stations set up along the river (figure 1). Water was filtrated, then percolated on resin columns. Sediment, plants and fishes were dried and burnt to ashes in an oven at 500° C. Radioactivity was measured by gamma spectrometry and radiochemistry.

¹³⁷Cs experimental transfers were studied between water, sediment, midge larvae, daphnid and carp. These components were taken in pairs in order to estimate the radionuclide transfer from one to the other. Thus ten experiments were carried out (figure 2).

In order to study the relative importance of food and water as ⁶⁰Co sources for the carp, an experiment was carried out simultaneously on three homogeneous groups of ten juvenile fishes. The individuals of the first group were maintained in separate aquaria and offered 45 daily rations of labelled food over a 63-day period. Bach carp of the other two groups was placed in a compartment of a large tank with contaminated water. One group was fed with radioactive food, the other with non-radioactive food (table 1).

Natural radioactivity remained steady all along the river. It ranged around 1 Bq.l^-1 in water, 2250 Bq.kg^-1 DW in sediment, 1700 Bq.kg^-1 DW in aquatic plants, 110 Bq.kg^-1 WW in fish. The fallout impact was characterized by ¹³⁷Cs presence. PWR liquid wastes contained mainly, ⁵⁸Co, ⁶⁰Co, ¹³⁴ Cs, ¹³⁷Cs. The Chernobyl fallout gave an increase of Cs and the presence of 103Ru and 106RU+Rh specially during May and June 1986 which later decreased (tables 2, 3 and 4).

¹³⁷Cs transfer between water and sediment was very fast and important. Less than 2 % of the radionuclide was released from sediment into a non-radioactive water. During the transfer from water to chironomids the larvae radioactivity increased steadily (figure 3). Conversely, the transfer from the sediment to larvae did not seem to depend on the contact time. The transfer from water to carp was regular without any steady state during the 63 days of the experiment (figure 3). Then the fish concentration factor was less than 5. For 42 days, the transfer factor from sediment to carp was 3.6.10^-3. The retention factor from food to carp was 0.03 when fishes were fed with daphnids and 0.13 with chironomids. An experiment showed that the various ways of ¹³⁷Cs transfer could have an added impact. Thus the carp radioactivity was the sum of the separate transfers. Water was responsible for 4 % of the fish ¹³⁷ Cs concentration, sediment for 45 % and chironomids for 51 %.

It is possible to include the different kinetic equations in a mathematical model. If the radioactivity of one of the components is known, the nuclide concentration can be computed in others, as a relation of the contact time, the quantity and quality of ingested food, etc... This model gives a concentration factor for juvenile carp of 1000 in 180 days and 500 for 3-year old fish. Considering the field conditions (e.g. seasonal nutritive cycles) the computed concentration factor in fish was between 200 and 350. For a 1 mBq.l^{-1 137}Cs concentration in water, the model gave a concentration of 0.2 to 0.35 Bq.kg^-1 WW in carp, which was the ¹³⁷Cs radioactivity level measured in the Rhone fish before the Chernobyl accident.

During the ⁶⁰Co accumulation phase, the mean weight of the fish in the three groups increased exponentially and the resultant relative weight gain was 52-59 % after 63 days (table 5).

The ⁶⁰Co accumulation kinetics showed that the steady state should be reached after 165 days for fish exposed to ⁶⁰Co in food, 92 days for fish exposed to radiocobalt in water and 120 days for fish exposed to ⁶⁰Co in both sources (figure 4). According to the ⁶⁰Co concentration in the fish in the three treated groups, the accumulation from water accounted for 75 % of the total radioactivity and the accumulation of the radionuclide from both water and food was in addition.

Depuration of ⁶⁰Co from carp was a relatively intensive process reflecting a high Co turnover. Biological half-lives for loss from the long-lived compartment ranged from 35d in fish previously contaminated by food, to 87d in fish previously contaminated by food, to 87d in fish previously contaminated by water (figure 5).

¹³⁷Cs and ⁶⁰Co are the most concentrated radionuclides in liquid wastes of the pressurised water reactors, and they are often measured in the aquatic ecosystem components. Though it accounts for the highest fraction of total radioactivity in the liquid wastes, ⁶⁰Co cobalt is not the most concentrated radionuclide in fish. Experimental studies show that it is primarily transferred from the water so it is logical that its concentration in fish remains at a low level. Conversely the ¹³⁷Cs has a low concentration in water but as it is transferred simultaneously from water, sediment and food, its concentration in fish is still important. Moreover its 30 years half-life means that the cesium contamination of fish is a long and important process, all the more so as the source terms can add their own effects during time and space.

FR :

L'étude s'inscrit dans le cadre d'un programme de surveillance de la dispersion des substances toxiques dans le fleuve Saint-Laurent. Elle vise à identifier la nature et le cheminement des substances toxiques dans différents compartiments du lac Saint-Pierre. D'une superficie totale de plus de 400 km², ce lac est un élargissement du fleuve et son bassin de drainage inclut la région métropolitaine de Montréal et la région inter-nationale des Grands lacs. L'échantillonnage a porté sur l'eau, les matières en suspension, les sédiments de fond, les poissons adultes (entiers, chairs, foie, oeufs et gonades) et les poissons juvéniles (âge 0⁺). Des analyses chimiques ont été effectuées pour les métaux, les pesticides organochlorés, les biphényles polychlorés (PCB), les hydrocarbures aromatiques polycycliques (PAH) et les chlorobenzènes. Les résultats indiquent que la contamination du milieu aquatique par les substances toxiques est très significative. Les contaminants majeurs retrouvés dans les divers compartiments sont l'arsenic, le mercure, le plomb, les biphényles polychlorés (PCB) et les différentes formes de DDT. Les concentrations excèdent fréquemment les critères de qualité pour le mercure dans les chairs de poissons, le plomb dans l'eau et les sédiments et les PCB dans plusieurs compartiments analysés. Plusieurs pesticides organochlorés ont été détectés dans l'eau, grâce à la technique d'échantillonnage à grand volume. Cette technique permet d'estimer 1a concentration de fond et le bilan massique annuel d'un polluant. Les poissons entiers et le foie de lottes constituent par ailleurs des indicateurs de choix de la présence et de l'étendue de la contamination par les substances toxiques bioaccumulables, alors que les sédiments de fond et les poissons juvéniles permettent d'étudier leur répartition spatiale. Sauf pour le mercure, les concentrations dans les chairs sont un mauvais indicateur de la contamination du milieu; la moyenne et la variabilité des teneurs en PCB et pesticides organochlorés y sont très faibles.

EN :

As part of a monitoring program on pollutants dispersion in the St. Lawrence river, a study was conducted in 1986 on the fate and pathways of toxic substances in Lake St. Pierre. This enlargement of the St. Lawrence drains the Montreal metropolitan area and the international great lakes region. Water, sediments and fish were collected and analyzed. Water was sampled following the conventional method (1 l) or passed through an XAD-2 resin column (70 l of centrifuged water). Suspended sediments were collected by centrifuging 2000 l of water. Bottom sediments were collected by retaining the top layer of a grab sample taken using an Eckman dredge. Young-of-the-year forage fishes (Spottail Shiner and Yellow Perch) were captured using a seine whereas adult fishes (Northern Pike, Walleye, Brown Bullhead, Yellow Perch, White Sucker, Bowfin and Burbot) were captured using hoop-nets and gill-nets. All samples were analyzed for metals, organochlorine pesticides, PCBs, PAHs and chlorobenzenes. Fishes were analyzed as homogenate of whole individuals (young-of-the-year and adult) and as homogenate of fish flesh. Results indicate that the level of contamination by toxic chemicals is highly significant. The major contaminants found in the different media are arsenic, mercury, iron, PCBs and various derivatives of DDT. The concentration frequently exceeds the guidelines suggested for mercury in fish flesh, iron in water and sediments and PCBs in most compartments analysed. Several organochlorine pesticides have been detected in water using Large Volume Sampling technique. This technique allows estimation of background concentration and annual mass balance for specific pollutants. Whole fishes and Burbot's livers are good indicators of the presence and concentration range of bioaccumulating toxic substances. Bottom sediments and juvenile fishes are good indicators of spatial distribution of toxic chemicals in the lake. PCB's highest concentrations are found in adult whole fish samples (mean of 1030 µg/kg). Bottom sediments concentrations show a higher mean (112 µg/kg) than for Lake St. Louis, located upstream of Montreal. Some PAHs and chlorobenzenes are occasionally detected in various compartments.

FR :

Le Lac de Bastani (Corse - France), petit lac d'altitude (2089 m) à bassin versant rocheux, est dépourvu de toute implantation ou activité humaine. Ce lac est mésotrophe à eutrophe par certains de ses caractères alors que 4 autres lacs étudiés dans le même contexte sont oligotrophes. De plus, il était décrit au début du siècle comme oligotrophe.

Dans ce type de lacs, les apports atmosphériques constituent la majeure partie de la source externe de nutriments. L'apport d'azote inorganique dissous atmosphérique au bassin versant est important, de l'ordre de 0,8 g.N.m^-2 par an, soit l'équivalent du rejet d'une trentaine d'habitants permanents. Les trois-quarts sont relargués à la fonte, pendant environ un mois; cela représente l'équivalent des rejets d'environ 300 habitants sur le bassin qui, du fait de la faible activité biologique dans les sols à cette température, arrive pratiquement en entier au lac. Ces apports de nutriments à la fonte sont responsables du démarrage très précoce de la production primaire.

En l'absence d'un bilan complet des nutriments la comparaison du lac de Bastani avec les autres lacs oligotrophes et en particulier avec celui de Capitello, aux caractéristiques voisines, permet de cerner les facteurs responsables de la différence de leur niveau de trophie. Bastani et Capitello recevant une charge externe semblable, la source interne est probablement responsable de la situation de Bastani. Les concentrations estivales de N-NO₃ notables (0,2-0,3 mg.l^-1) des lacs oligotrophes sont un indice de la limitation de leur production primaire par le phosphore. Par contre à Bastani, l'azote étant pratiquement entièrement consommé en été le lac ne semble pas limité par le phosphore. Le vent violent et fréquent qui règne à Bastani semble le facteur déterminant de son niveau trophique supérieur : il assure le brassage de l'eau du lac et la remontée du phosphore (et aussi de l'azote) libéré des sédiments jusque dans la zone photique. Le phytoplancton consomme presque entièrement l'azote, alors que dans les lacs oligotrophes abrités du vent où le phosphore est limitant, les nitrates restent à des teneurs notables.

L'eutrophisation récente du lac de Bastani peut s'expliquer par l'accroissement des apports atmosphériques, les seuls qui aient varié. Le phosphore étant disponible la production primaire devait être limitée auparavant par l'azote. Les apports d'azote inorganique dissous ont été dans cette région multipliés par 2 au moins par la pollution à longue distance. Cette dernière pourrait entraîner à elle seule l'évolution trophique d'un lac de haute montagne, dans un site vierge de toute occupation humaine, lorsque le brassage par le vent assure la mobilisation du phosphore des sédiments.

EN :

Lake Bastani (Corsica, France), a small alpine lake (2089 m), is located in a pristine rocky watershed. It presents some characteristics of eutrophic lakes, whereas four other similar lakes are oligotrophic or ultraoligotrophic. Moreover, at the beginning of the century it was described as being oligotrophic.

The lake is ice-covered from November to June. Intense phytoplankton activity begins before the ice melts completely and goes on until fall freezing. Consequently Secchi disk transparency is between 3.2 m in June and 1.8 m in August. The algal biomass, as expressed by chlorophyll-a content, is maximum at the bottom. Chlorophyll-a is about 4-5 mg-m^-3 at ice melting and reaches a maximum of 46 mg.m^-3 in August. Primary production is highest in the upper levels of water with 41 mg.C.m^-3.d^-1 at - 5 m. These values are 20 to 80 times that of the other Corsican lakes. Regarding the different trophic classifications this lake presents some features of eutrophic lakes.

In 1923 a survey of phytoplankton by Pestalozzi showed that the lake was oligotrophic. This trophic evolution is therefore recent; it is shown by the complete disappearance of Desmidiaceae, the near disappearance of Diatoms, the appearance of green algae with Oedogonium and Spirogyra dominant in summer and autumn, and the development of blue-green algae with Oscillatoria.

The seasonal pattern of phytoplankton composition displays diatom development at ice- and snow-melting with the green algae Oedogonium. During the summer warmth, green algae are dominant with the blue-green Oscillatoria. Green algae Oedogonium and Spirogyra increase until the fall turnover.

The lake is 2nd-order dimictic with two turnovers in spring and autumn, and two periods of thermal stratification. A strong and frequent wind stirs the lake water inducing homogeneisation of the epilimnion and sometimes the disappearance of the hypolimnion. Dissolved oxygen is always present even at the bottom where O₂ saturation is about 50% in winter. Nitrogen is very low - 0.01-0.04 mg.l^-1 N-NO₃ - and phosphorus (PO₄) is below the detection level in the epilimnion during summer, due to intense phytoplankton activity, The hypolimnion is richer in NH₄, NO₂ and PO₄. In spring when the snow begins to melt the surficial waters are enriched with N (NO₃ + NH₄) and P-PO₄, showing the contribution of concentrated snow water.

In fact atmospheric input is the dominant external loading in such a watershed. Nitrogen atmospheric input is extrapolated from data obtained at Lake Bavella 30 km southwards for 3 years - 1984 to 1986. The dissolved inorganic nitrogen input is about 0.8 g.m^-2 .y^-1 , equivalent to wastes from some 30 permanent inhabitants for the whole watershed, About 80 % of annual rainfall is stored in the snowpack, as are ¾ of the dissolved inorganic nitrogen. This nitrogen returns to the lake during a period of about one month which represents 100 kg and a loading of 80 mg.m^-2.d^-1 for 30 days; it is equivalent to wastes from some three hundred capita during one month. Moreover it is known that the first melwaters are considerably enriched with several ions, especially those of ecological concern, H⁺, SO^4--, NO₃¯, NH₄⁺. Using findings from literature we can estimate that in the first two weeks of melting, up to half the annual atmospheric input of dissolved inorganic nitrogen reaches the lake with meltwater concentrations of more than 1 mg.N.l^-1. The total dissolved nitrogen input is estimated at 1.2 g.m^-2.y^-1 and the total nitrogen input at 1.3 g.m^-2 .y^-1.

No data are available for the phosphorus atmospheric input but estimations from literature are about 10 mg.m^-2 .y^-1 for orthophosphate and 60 mg.m^-2 .y^-1 for total phosphorus. The release of P-PO₄ from the melting snowcover and from ice, added to the tremendous release of inorganic nitrogen, explains the start of vigorous phytoplankton production before the ice melts completely. By this early start primary production may avoid the flush of nutriments resulting from the flow of melting water.

Trophic conditions in Lake Bastani are a matter of question as the other four lakes studied with the same edaphic conditions are oligo- or ultraoligotrophic. A comparison between Lake Bastani and the ultraoligotrophic Capitello Lake, which has the same geomorphological and ecological environment, reveals the determining factors of this paradoxical situation, though a nutrient budget is still not available.

A low flushing rate (17 months) cannot be the explanation, as that of Lake Capitello is similar (12 months).

The external loading (from watershed and atmosphere)being the same, the difference between the two lakes is to be found in the internal loading.

The fact that Lake Capitelto and the other oligotrophic lakes present notable N concentrations in summer (0.2-0.3 mg.l^-1 ), when Bastani displays N- concentrations 10 times lower, is a sign that they are limited in phosphorus. Conversely Lake Bastani is not phosphorus-limited.

The determining factor in this situation seems to be windstirring. Lake Bastani is downwind of a pass in the axial range of Corsica and is submitted to strong frequent winds. Windstirring induces an epilimnion homogeneisation, the downward migration of the thermo- and chemoclines. Thus phosphorus from the sediments can be transferred to the photic zone and is not limiting for phytoplankton. Phytoplankton exhausts nitrogen which is very low at the end of summer. In Lake Capitello as in the other lakes, shielded from the wind by rockwall screens, no mixing occurs, phosphorus cannot be supplied from the sediments and is limiting : nitrogen is not used up and remains at relatively high levels of 0.2-0.3 mg.l ^-1.

Recent eutrophication cannot be explained by an increase of nutrient availability from the sediments due to windstirring, as no climatic change has occurred in the fast century. The probability that internal loading had reached a threshold by progressive accumulation of nutrients in the sediment is very slight if there is no change in the external loading (the unique source of "new" nutrients) and/or the trophic status of the lake. Thus an increase in the external loading seems to be responsible for the eutrophication of Lake Bastani. The atmospheric inupt is the only external reason for a change in the nutrient rate. An increase in the phosphorus atmospheric input by local (forest fires) and regional (long range pollution) human activities is suspected, but does not seem sufficient to change the trophic status of the lake as it has no effect on other lakes, which remain oligotrophic. Moreover, atmospheric P loading is well below the level of phosphorus release from sediments, as estimated from literature. Conversely, an atmospheric inorganic nitrogen input has increased at least twice due to long range pollution (agricultural, industrial and domestic activities). This was estimated from a typologic classification of rain events in Corsica. This increase in atmospheric nitrogen loading might explain the change in the trophic status of the lake, N having been limiting. In the other P-limited lakes the increased atmospheric N input has only increased the N-NO₃ concentration in lake water. This explanation being the most realistic, it is of great interest to note that long range atmospheric pollution could induce the eutrophication of a pristine oligotrophic lake.

FR :

Des échantillons d'eau de rivière ont été prélevés près de l'embouchure de la Venoge, affluent du Léman, au cours de cinq crues survenues entre octobre 1986 et novembre 1987. Les échantillons ont été extraits de grands volumes d'eau à l'aide d'une centrifugeuse à débit continu. La concentration de matières en suspension, et les formes du phosphore de ces échantillons ont été dosés. L'interprétation de ces données, basée sur une méthode de régression progressive, montre que la concentration en phosphore particulaire total peut être estimée par deux paramètres, la fraction de suspensions supérieure à 63 µm et la concentration en phosphore réactif dissous. Par ailleurs, puisqu'il existe des relations statistiques entre le phosphore particulaire total et les autres formes du phosphore particulaire, il est possible d'évaluer la concentration de ces dernières (formes de phosphore apatitique, non apatitique, organique et inorganique). Le phosphore total dissous peut être pareillement estimé en fonction du phosphore réactif dissous. L'estimation des formes du phosphore et ainsi que celle de leur charge en crue peut donc se faire même lorsque le volume des échantillons est limité, c'est-à-dire, lorsque la quantité de matières en suspension n'est pas suffisante pour l'analyse de toutes les formes du phosphore particutaire.

EN :

In order to gain a better understanding of phosphorus transport in a storm-dependent river system, water samples were collected near the mouth of the River Venoge, Switzerlang, during five storm events between october 1986 and november 1987. Suspended sediment (SS) was extracted from large-volume water samples by continuous flow centrifugation. Soluble and particulate forms of phosphorus were subsequently analysed with the centrifuged and filtered (0.45 µm) waters, and freeze-dried SS in a < 63 µm fraction.

However, the sampling usually performed at more than 4-hour intervals could not guarantee that no information had been missed, for example, the instantaneous fluctuation of soluble reactive phosphorus (SRP) during the events. Although more frequent sampling using an auto-sampler is feasible, the small volume of raw water cannot provide sufficient sediment for all the analyses of phosphorus forms. Thus, it would be useful to find a model capable of estimating phosphorus concentration in different forms, whenever the present measurements are not possible.

A multivariate progressive analysis of the measured phosphorus data set shows that total particulate phosphorus (TPP) concentration can be estimated as a function of two parameters, the percentage of a SS fraction coarser than 63 µm and the SRP concentration measured in filtered water. On the other hand, general statistical relationships exist between the various forms of phosphorus. Total soluble phosphorus (TSP) is dependent upon SRP. Organic phosphorus (OP) and non-apatite inorganic phosphorus (NAIP) can be approximately assessed from TPP, measured or calculated. Then, apatite phosphorus (AP), inorganic phosphorus (IP) and total phosphorus in raw water can be calculated by means of summation/substraction operations.

Modelling is apparently suitable to the storm events during which only a limited volume of water samples could be collected. It also provides a rapid way to estimate the partitioning of phosphorus loads in high flow periods of the river system, thereby reducing the field and laboratory work required.

FR :

L'eutrophisation due au phosphore a débuté à la fin des années 50. Les concentrations en phosphore étaient de 10 µg/l avant 1960 et ont atteint 90 µg/l vers 1976-1979. Depuis 1979, on note une diminution de 31 % (62 µg/l en 1988).

Les apports en phosphore particulaire par les affluents principaux suivent parfaitement l'hydraulicité, mettant ainsi en évidence leur origine principalement érosive. Par contre, les apports en phosphore dissous, qui sont plus liés à l'activité humaine, ont présenté une forte croissance au début des années 1960 et se sont maintenus à un niveau élevé jusqu'en 1975. Depuis cette époque ils sont en forte décroissance (- 56 % depuis 1975). Cette décroissance est liée à la mise en service des stations d'épuration pratiquant la déphosphatation et à la mesure d'interdiction des phosphates dans les produits de lavage textiles en Suisse (01.07.1986), qui a conduit à réduire de 40 à 45 % les quantités de phosphore rejetées dans les eaux superficielles par les réseaux d'assainissement.

La transparence moyenne estivale (mai-septembre) a régulièrement diminué depuis les années 1960. Lorsque les conditions climatiques sont favorables à la croissance du phytoplancton le Léman est susceptible de se comporter comme un lac eutrophe. Malgré la nette baisse des concentrations en phosphore dans le lac, il n'a pas encore été relevé de diminution de la production algale. Ceci est en accord avec les études effectuées sur d'autres lacs qui ont montré qu'il est nécessaire d'abaisser ces concentrations en-dessous de 20 à 30 µg/l pour limiter la croissance du plancton.

EN :

The water quality of Lake Leman (Lake of Geneva), relatively good at the end of the fifties, has rapidly deteriorated, due to an increase of nutrient inputs. Eutrophication, resulting from increased phosphorus inputs, reached a critical stage during the period 1976 to 1979. The phosphorus concentration increased from 12 µg/l in 1960 to 90 ug/l during this period. Since 1980, a decline in the concentrations has been observed, i.e. 62 µg/l (31 % less than the 1979 level).

Hydraulic paths of principle affluents seem to indicate that the major source of particulate phosphorus is erosion. However dissolved reactive phosphorus inputs, originating from anthropogenic sources, rose during the interval 1963 to 1975. On the other hand, from 1975, concentrations of dissolved reactive phosphorus have decreased by 56 %. This decline is attributed to tertiary wastewater treatment and to the ban on phosphates in detergents in Switzerland (01-07-1986), ca. 40 - 45 % being due to the latter.

The average transparency of water during the summer season (May - September) was found to diminish continuously since 1960. Under climatic conditions favorable for phytoplancton growth, the lake may behave like a eutrophic one.

The winter turnover seldom affects the entire water column of Lake Leman because of it great depth (309 m). Wear anoxia in the bottom waters was observed from 1976 to 1978 owing to incomplete turnover during the years 1972 to 1978 and to a substantial increase in the biomass in surface waters.

In spite of the decrease in phosphorus concentrations in the lake, no significant decrease in the algal production has been observed. These observations are in agreement with those in other lakes indicating that the phosphorus concentrations must be towered below 20 to 30 µg/l to inhibit plankton growth.

FR :

L'évolution comparée des biocénoses rotatoriennes du Léman et du Lac de Constance montre un parallélisme remarquable lors des changements continus de l'état trophique, tant avant qu'après le niveau maximal d'eutrophisation atteint par ces deux lacs. La comparaison avec un certain nombre d'autres milieux soumis également à un changement de leur niveau trophique met en évidence une variation très nette des effectifs des espèces méso-eutrophes et eutrophes en fonction de la progression ou de la régression de l'eutrophisation; ces changements sont beaucoup moins marqués en ce qui concerne l'association des espèces oligo-mésotrophes.

EN :

Eutrophication of a lake must be expected to cause both abiotic and biotic responses. The changes in the trophic state of a lake certainly occur at all trophic levels, but to different extents depending on the character of the ecosystem; the subsequent evolution of the rotifer biocenosis is one of the most pronounced features occurring as an indirect consequence of eutrophication.

In the last three decades, the rotifer community in Lake Geneva has presented important changes in structure, as a result of continuous changes in the trophism of the lake. This study covered a period of time sufficient to show possible true modifications of the community structure.

Lake Geneva has undergone anthropogenic eutrophication since the 1950's. The lake reached its highest nutrient level during 1979-1980, which has decreased though since 1981 with the improvement of phosphorus removal in sewage treatment plants. A similar trend was observed in Lake Constance, with more important quantitative changes connected with previous results dating from the oligotrophic stage of this lake.

Table 1 gives the variations in abundance of rotifer species observed during the eutrophication increase and during the decrease of the eutrophication level in some lakes of different trophic status.

Figure 1 indicates the quantitative changes of the main rotifer species in Lake Geneva and Lake Constance. Compared to the maximal trophic level (indicated by arrow), data from Lake Geneva for 1959-1987 are similar to those obtained in Lake Constance between 1963 and 1978, if the previous data obtained for this lake during its oligotrophic stage are excluded.

Figure 2 presents the general trend observed for the different trophic indicator groups during the increase or decrease of eutrophication. Throughout the eutrophication process, these groups exhibited a general increase for most of the different species, with the ratio "number of increasing species/number of decreasing species" growing from the oligo-mesotrophic group to the eutrophic one. During the decrease of eutrophication, the strength of meso-eutrophic and eutrophic indicator groups diminished more than that of the oligomesotrophic group.

Many species have appeared during the eutrophication increase, mainly belonging to meso-eutrophic and eutrophic indicator groups, together with the settlement of some oligo-mesotrophicspecies (Ascomorpha saltans, Synchaeta oblonga, Notholca caudata). The proportion of meso-eutrophic and eutrophic species increased in the two lakes, but there is a trend in populations of the oligo-mesotrophic species to decrease, leading to a precocious disappearance of Ploesoma hudsoni and later of Ascomorpha ovalis, Gastropus stylifer, Ploesoma truncatum in Lake Constance, and of Synchaeta tremula and Notholca foliacea in both lakes.

Decreasing eutrophication reduces the number of some mesoeutrophic (Trichocerca longiseta and T. pusilla in Lake Geneva, T. rousseleti in Lake Constance) and eutrophic species (Anuraeopsis fissa and Trichocera cylindrica in Lake Geneva, Pompholyx complanta in Lake Constance). However, the water quality has mot improved enough to allow a further increase in oligo-mesotrophic species, except for Conochilus unicornis in both lakes, and for Ascomorpha ecaudis, Kellicottia longispina and Synchaeta oblonga in Lake Geneva.

Since 1981, the oligo-mesotrophic indicator group is numerically dominant in Lake Geneva (BALVAY and LAURENT, 1989c), preceding the euryecious one (Asplanchna priodonta + Keratella cochlearis), white the strength of the meso-eutrophic group decreases and the eutrophic one remains almost steady.

FR :

Les variations spatio-temporelles à court terme des compartiments planctoniques ont été étudiées simultanément du 30 juillet au 5 août 1986 dans un lac humique du Bouclier canadien. L'abondance du bactérioplancton fluctue de 1,4 à 1,7.10⁶ cell. ml.^-1 (coloration DAPI) ou de 2,7 à 7,7.10⁶ cell. ml^-1; (coloration Acridine Orange). La production du bactérioplancton estimée par incorporation de ³H méthyl thimidine, varie de 4 à 24.10⁶ cell. l^-1 h^-1. Les valeurs d'activité hétérotrophe potentielle bactérienne estimée par assimilation de ¹⁴C glucose, s'échelonnent de 0,007 à 0,065 µg C.l^-1. h^-1. La biomasse pigmentaire (chlore. a et pheopigments) varie de 6,8 à 21,7 mg.m^-3 . La production primaire est très faible (max. : 10 mg C. m^-3 .h^-1; 20 mg C.m^-2 .h^-1 ) et décroît très rapidement avec la profondeur (25 % à 82 % dans le premier mètre). Le microzooplancton (Rotifères, Bosmina, nauplies) représente plus de 90 % du peuplement zooplanctonique et les taux de broutage global du macrozooplancton sont faibles (25 % j^-1). Les compartiments hétérotrophes prédominent dans le métabolisme du lac par rapport au compartiment autotrophe. Les patrons de variation spatiale reflètent la stratification verticale des compartiments et des processus autotrophes et hétérotrophes. Les maxima de photosynthèse, d'ATP et de production bactérienne se situent dans les eaux épilimnétiques tandis que les maxima d'abondance du bactérioptancton et des pigments se rencontrent dans l'hypolimnion. Le zooplancton est plus dense et broute d'avantage dans la strate 1-3 m. Ces variations spatiales semblent influencer l'activité hétérotrophe potentielle du bactérioptancton. Nos résultats montrent aussi des variations temporelles à court terme de la production primaire, de l'assimilation hétérotrophe et du broutage du macrozooplancton. Cette étude préliminaire met en lumière la nécessité de tenir compte des variations à court terme lors des études des relations trophodynamiques dans les écosystèmes planctoniques.

EN :

Short term spatial and temporal variations in planktonic compartments were studied simultaneously, from July 30th to August 5th, 1986, in a humic lake on the Canadian Shield. Abundance of bacterioplankton ranged from 1,4 to 1,7 10⁶ cell. ml^-1 (DAP1) or from 2,7 to 7,7 10⁶ cell. ml^-1 (Acridine Orange). Bacterial production, as measured by incorporation of ³H methyl-thimidine, was estimated at 4-24 106 cells. l^-1. h^-1 and potential heterotrophic bacterial activity ranged from 0,007 to 0,065 µg C.l^-1. h^-1, as estimated by ¹⁴C glucose incorporation. Pigments biomass (chloro. a and phaeopigments) varied from 6,8 to 21,7 mg m^-3. Primary production was low (max. : 10 mg C.m-3. h^-1; 20 mg C.m-2. h^-1) and decreased rapidly with depth (25-82 % in 1 m depth). Microzooplankton (rotifera, Bosmina, nauplii) accounted for more than 90 % of total numbers and macrozooplankton global grazing rates were low (25 % d^-1). The heterotrophic compartments play a greater rate in the lake metabolism than the autotrophic compartment. Spatial variations reflect the stratification of the heterotrophic and autotrophic organisms and processes with depth. Maximum levels in primary production, ATP and bacterial production occur in epilimnetic waters, while maxima in bacterial numbers and algal pigments occur in the hypolimnion. Zooplancton density and macrozooplankton grazing rates were higher in the 1-3 m strata. These spatial patterns seem to influence the vertical profiles of the bacterial potential heterotrophic activity. Our results also show short term temporal variations in primary production, potential heterotrophic activity of bacterioplankton and macrozooplankton grazing rates. This preliminary study stresses the importance of short term variations in the assessment of the trophodynamics of the planktonic food wed.

FR :

Les peuplements zooplanctoniques et piscicoles de la retenue de Sainte-Croix (lac oligotrophe profond) ont été suivis de la mise en eau (1974) à 1986.

La phase initiale de colonisation (1974-1975), caractérisée par le calanide Acanthodiaptomus denticornis et la dominance des espèces piscicoles peuplant antérieurement le Verdon lotique (Barbus fluvatilis, Leuciscus cephalus) a été de courte durée. Par la suite, les cladocères sont restés dominants avec cependant de profondes modifications dans la structure du peuplement. En 1977-78, Daphnia longispina était dominante tout au long de l'année (76 %). Plus récemment, cette espèce ne représentait plus que 34 %. des effectifs (1983-84), puis 24 % (1985-1986). La situation actuelle est caractérisée par la présence de Diaphanosoma brachyurum en été et l'abondance des petites formes (bosmines).

L'ichtyofaune s'est elle aussi modifiée avec, notamment, une importante population d'ablette (Alburnus cluburnus), exploitant la zone pélagique. Son régime alimentaire est largement zooplanctonophage mais inclut mollusques et insectes lorsqu'elle est capturée en zone littorale.

Cette augmentation de la pression de prédation a entraîné un net déplacement du spectre des tailles du zooplancton vers tes classes inférieures à 0,8 mm en 1986 (82 %) alors que les grandes formes ( > à 1 mm) représentaient 60 % des effectifs en 1977-78. Les densités de D. longispina et D. brachyurum décroissent rapidement entre 1983 et 1986.

Les conséquences de cette prédation sélective sont discutées (évolution prévisible du système) ainsi que les causes pouvant expliquer l'absence de régulation efficace (densité-dépendance) dans un tel système prédateur-proie.

EN :

The aim of this study was to identify relationships between changes that occurred, from 1974 to 1986, both in the limnetic zooplankton and the fish communities of the large reservoir of Sainte-Croix (Southern France).

This reservoir (figure 1) is characterized by a capacity of 767 hm³ and a long average renewal period (280 days). It was filled up in 1974. Changes in levels can attain 16 m, especially in winter. During summer, the lake is clearly stratified : surface mater temperatures are about 22-250°C whereas waters below the thermocline are cooler ( < 10° C) but well oxygenated (more than 50 % saturation). According to the mean annual total phosphorus (19 mg/m³) and chlorophyll a (ranging from 0.5 to 2 mg/m³), the lake is oligotrophic.

The zooplankton was collected with a Clarke-Bumpus devise sampler (125 µm mesh-size) during both the 1974-1978 and the 1983-1984 periods. More recently (1985-1986), vertical hauls of zooplankton were made with a 50 µm mesh-size aperture net. Only relative abundance of large species (excluding small rotifers) have been taken into account for comparisons between different periods (figure 2). For the last four years (1983-1986), a set of data on filtration efficiency and comparison between sampling methods provides information concerning the evolution of annual mean densities of zooplankton species (figure 3).

Successive inventories of the fish species were made in 1976, 1977, 1984 and 1987 at the same location with 15 mm, 27 mm and 50 mm mesh-size nets respectively. Figure 1 shows that abundant autochtonous species (Chondrostoma chondrostoma, Barbus fluviatilis) during the first period (1976-1977) are now relatively scarce, whereas the roach (Rutilus rutilus), an introduced species, is now largely dominant. Likewise, the relative abundance of the bleak, Alburnus alburnus, also increased, especially during the last period (1984-1987). Only this last species was caught by fine mesh-size nets in the pelagic epilimnion. Adults fed mostly on zooplankton but also took more molluscs in the littoral area (stomach analysis).

In 1974, one year after an overflow, the calanoid Acanthodiaptomus denticornis represented the majority of zooplankton collected but it disappeared quickly. Since 1976, cladocerans are largely dominant. General evolutive trends show a decrease in relative abundance of Daphnia longispina from 1978 to 1986 whereas Diaphanosoma brachyurum have been recorded since 1983. Quantitative data (figure 3), available for the fast 4 years (1983-1986) show that the population densities of both these two large cladoceran and cyclopoïd species (mainly Macrocyclops albius) declined radically. On the other hand, small species such as Bosmina longirostris, Bosmina coregoni and Asplancha priodonta exhibited the same population densities throughout the fast period. Comparisons of phytoplankton community structure from 1982 to 1986 do not show signicative trends. Thus, variations in resource abundance cannot explain the long-term evolution of zooplankton communities.

By comparing the mean annual length frequency distributions of zooplankton in 1977-78 and 1986 (figure 4), a marked decline of large bodied forms is seen. The individuals larger than 1.0 mm represented 60 % of total organisms in 1977-73 against 14.5 % in 1986. Mean annual lenghts were 0.8 mm in 1977-78 and 0.3 mm in 1986. These results are in good agreement with BROOKS and DODSON hypothesis that planktivorous fish can have a profound effect on zooplankton community taxonomics and size structure. They show that the control of structure and abundance of zooplankton by fish predation in new man-made lakes is a reality.

Most fish are general predators. As demonstrated by MURDOCH and BENCE (1987), this can explain the instability of the coupled predator-prey system in the pelagic portion of Sainte-Croix reservoir because of : i) the capacity of the bleak to use fat reserves and to change its diet when large zooplankton prey density is too low, ii) the generation time of the predator is much longer than that of planktonic preys. Thus, the classical density-dependence mechanisms do not stabilize the predator-prey system and the main planktonic prey tend to disappear (D. longispina, in our case).

Nevertheless, some potentially stabilizing mechanisms could interfere in the system before the local extinction of the prey : compensatory responses by the prey and significant changes in the fish community. There is reason to consider :

1) The existence of refuges in space. Vertical size distribution of D. longispina (unpublished data) show that there is a strong vertical size gradient during daylight hours. Larger-bodied reproductive forms (longer than 1 mm) are only present in the deeper layers (20-30 m) whereas, during the night all the organises migrate to the superficial layer.

2) The existence of refuge in time. In the past (1976-1978), D. longispina was dominant all year around. More recently (1983-1986), population densities were high only in spring (figure 5). From April to June, bleaks tend to assemble in the littoral zone for reproduction.

3) An increase in the bleak mortality rate due to a recent development of large predator populations, such as Esox esox, could permit a reduction or, at least, a stabilization of the bleak population growth rate.

FR :

La gestion est un terme qui doit être défini et précisé selon le plan sur lequel on se place : administratif, économique, écologique. Le poisson étant l'aboutissement ultime de la production naturelle d'un plan d'eau, toute intervention sur le milieu se répercutera sur la production de poisson, d'où la complexité de la gestion piscicole, dépendante de la gestion hydraulique, de la gestion économique du bassin versant, etc...

La gestion d'une ressource piscicole implique une connaissance précise et continue des "entrées" et "sorties" dans l'écosystème considéré : recrutement, mortalité naturelle, repeuplement, exploitation. Elle suppose également une bonne maîtrise de la dynamique des stocks allant jusqu'à l'établissement de modèles prédictifs nécessaires à un partage raisonné de la ressource. Une revue succincte et critique des différents modèles prévisionnels utilisables met en évidence la nécessité d'approfondir les recherches pour certaines phases du cycle vital des poissons en vue d'améliorer la fiabilité des modèles.

EN :

When planning fish-farming, it is important to clarify the approach chosen, which can be administrative, economical or ecological.

Fish being the final outcome of natural production in an aquatic environment, any intervention affecting the environment will be reflected in fisheries production. This explains the complexity of fisheries management, for it involves, among others, hydraulics, the efficient use of the watershed, etc...

A detailed and up-to-date knowledge of the input and output of the ecosystem concerned is essential, particularly regarding recruitment, natural mortality, stocking and exploitation. An effective control of stock dynamics is also required in order to set up predictive models to ensure the rational sharing of a resource.

A brief critical review of various predictive models which can be used points to the need for further research on certain stages in the life history of the fish, in order to make the models more reliable.

FR :

L'estimation des stocks de poissons que peut abriter un secteur de cours d'eau non pollué nécessite le calcul de sa capacité d'accueil en terme d'espace et de nourriture. Le premier volet a donné lieu aux Etats-Unis à la mise en place de modèles de comportement de différentes espèces de poissons en fonction de variables physiques et de modèles hydrauliques décrivant l'évolution temporelle de ces variables.

Les hypothèses sous-jacentes sont discutées. Une modification du calcul de la perte de charge linéaire est préconisée pour le modèle hydraulique dans le cas des rivières à truites, et l'utilisation de méthodes multivariées est proposée pour décrire avec une meilleure fiabilité les relations entre densités de poissons et variables physiques des cours d'eau.

Quelques cas d'application illustrent l'intérêt de cette démarche pour répondre à des questions posées par les gestionnaires, tant en matière de prévision des impacts piscicoles des aménagements que d'optimisation des repeuplements.

EN :

When evaluating the potential fish stock that an unpolluted reach of a stream can accomodate, the load capacity with regard to space and food available must be assessed. It is for this purpose that the Instream Flow Incremental Methodology has been developed in the United States.

The assumptions on which behavioural and hydraulic models are based are discussed and modifications are suggested to improve the efficiency of this methodology.

The use of the Manning equation to compute the energy loss is misleading in the case of trout streams and results from simulation are more reliable when using resistance equations designed for mountain rivers.

For biological monitoring, multivariate data analysis is an interesting alternative to drawing one-dimensional preference curves. Qualitative variables can be used and redundancy or dependence between parameters no longer distorts the results. The relations between physical variables of streams and a probability of the presence of fish are estimated with greater accuracy.

Examples of the application of this methodology are presented with a view to assessing the effects of water abstraction on fish populations and to optimizing the restocking of trout streams.

FR :

Nous avons étudié la dispersion (vitesse, durée, distance) d'alevins de saumon atlantique à l'émergence dans un ruisseau expérimental.

Le rythme journalier de dévalaison suit étroitement le rythme d'émergence des alevins, ce qui montre une bonne corrélation entre les deux activités, tout au moins en début de période. Les histogrammes de capture des alevins échantillonnés tous les 10 m présentent des caractéristiques de forme similaires (durée, asymétrie et aplatissement). Jusqu'au pic des captures, près de 50 % des alevins dévalent en 5 jours. Ils se dispersent à partir de la frayère sur 50 m en 3 ou 4 nuits et forment une cohorte homogène d'après leur rythme de dévalaison et leur taille. Après le mode, les captures sont plus étalées dans le temps (environ 10 jours) et montrent plus de variabilité en fonction de la distance. Les alevins résidents ne sont pas distribués uniformément dans les cinq biefs : les densités, de même que les poids moyens, tendent à augmenter vers l'aval. Plus de 50 % de la population d'alevins survivants s'est établi dans les 50 m en aval de la frayère. Les résultats révèlent deux vagues de dévalants. La première vague de dispersion, aussitôt après l'émergence, est rapide et importante. Elle ne dépendrait pas directement de la compétition territoriale et de la densité, mais permettrait d'éviter des densités localement trop élevées et d'utiliser plus efficacement les zones productives situées en aval de la frayère. La deuxième vague d'alevins dévalants correspondrait aux émergents tardifs et aux poissons soumis aux effets de la compétition territoriale.

EN :

We have analysed the dispersal patterns (rate, duration, extent) of Atlantic salmon fry at emergence. The rate and duration of movement, and the distance travelled were measured in an experimental stream, located near S-Pée-sur-Nivelle, in SW France. A batch of 8 850 eyed eggs, from the grilse wild stock of the Nivelle River, was buried in the gravel substrate at the upstream end of a series of 5 sections, each 10 m long by 3 m wide. Shortly before emergence, drift nets equipped with fry traps were installed at the downstream end of each section. The nets sampled about 1/10 of the flow, except for those nets at the downstream end of the last section which collected all downstream moving fry. The traps were visited every morning and the fry enumerated. At the time of peak movements, samples were collected for length-weight measurements. At the end of the dispersal period, fry which had settled in the different sections (residents) were captured with electro-fishing gear and measured.

The pattern of downstream movement of fry in a set of drift nets was closely related to the pattern of emergence from an artificial redd upstream of the nets. Hence, emergence and downstream dispersion were well synchronized, at least during the first part of the dispersal from the redd. The time-frequency histograms of fry sampled every 10 m showed the same pattern and general shape (duration, skewness and kurtosis). Until the peak of captures, nearly 50 % of all downstream moving fry were caught within 5 days in each section. Dispersion from the redd over 50 m occurred within 3-4 nights. During this first period, the fry exhibited similar characteristics with respect to activity patterns and sizes. After the modal day of capture, catches were more evenly spread over time (about 10 days) and showed greater variability in relation to the distance travelled from the redd.

Resident fry were not uniformly distributed in the 5 sections : densities, as well as average weights, increased from upstream to downstream. Over 50 % of the surviving fry (75.3 % of planted eggs) settled within 50 m downstream from the redd.

Our results showed two waves of downstream dispersion. The first dispersal wave, occurring soon after emergence, was swift and implied large numbers of fry. It was not the result of territorial competition or density, since it occurred before the onset of aggressive behaviour. Rather, this first wave appeared as process to avoid the formation of clumps and allow for a more efficient use of the more productive zones, generally located downstream from the redd. The second wave of downstream moving fry corresponded to late emerging fry and to those fry which, 10-12 days after emergence, were displaced by territorial competition.

FR :

L'acoustique passive (étude des bruits propres du milieu environnant) constitue une méthode simple, sinon facile, d'étude de l'activité et du comportement de la faune aquatique.

Nous décrivons ici les systèmes de prise de son et d'analyse utilisés à cet effet à l'Institut de Limnologie (Thonon-les-Bains) et rapportons certains résultats d'observations réalisées sur les zones de fraie des corégones (Coregonus lavaretus) du lac d'Aiguebelette et des ombles chevaliers (Salvelinus alpinus du lac d'Annecy : l'activité de fraie des corégones est caractérisée par des bruits brefs (0,5 à 2 s) de type "stridulation"; certains bruits de "fouille" sur substrat d'une omblière semblent directement associés à l'activité prédatrice des lottes (Lota lota) sur la ponte.

EN :

Listening to underwater sound is a simple way of studying the activity and behaviour of aquatic fauna.

The material used in the Institute of Limnology (Thonon-les-Bains) for recording and analysing sounds is described (figure 2). Experiments have been performed on the spawning sites of coregonids (Coregonus lavaretus) in Lake Aiguebelette and of charrs (Salvelinus alpinus) in Lake Annecy. This paper refers mainly to observations made in December 1987 on the site of "Roc de Chère" in Lake Annecy; this is where the charrs spawn and the site is easy to survey (figure 1). Most of the work consisted in continuous visual and acoustical observations, in order to determine the origin of the sounds recorded. A great part of the period was noisy because of water disturbance (wind-induced waves).

The most interesting aspect is the study of sounds produced by fish (figure 3). We could not record the activity of the charrs, because of the inconvenient area studied : the observation field was limited to 1 sq.metre, which is very small in comparison to the spawning zone (figure 1). The presence of burbot (Lota lota) in the field of the video camera suggested that these fish came and ate the charr eggs which had been deposited on the bottom. This hypothesis was confirmed by the continuous digging activity (which was heard and recorded) due to the burbot feeding and by an analysis of the stomacal content of specimens, caught white SCUBA diving, which contained fertile eggs. Though acoustically evident, burbot activity was not analysed.

The coregonids found in Lake Annecy (1987) (C. species) have the same behaviour as those found in Lake Aiguebelette (1986) (C. lavaretus, a different species). Typical, short (0.5-2 s) stridulating noises are produced during courtship, when the male and the female rub against each other; these noises, nearly always the same, have been analysed (figure 4). The loudest sounds emitted are in the 100-300 Hz range.

Another aspect of fish activity has been studied : the noise made by coregonids swimming (figure 4). During courtship, fish is very active and swims energetically. The low frequency of the sound has been recorded by using a special electronic device.

All these observations are in accordance with the results presented in the literature. Acoustical observation is an interesting method which should be developed.

FR :

Ces dernières années, un ensemble de méthodes et techniques d'étude a été mis au point ou amélioré à partir de recherches réalisées sur les populations sauvages de salmonides. Dans un premier temps, ces nouvelles techniques peuvent être considérées comme des outils disponibles pour les gestionnaires; ce sont : la mise en place et l'homogénéisation de critères scalimétriques pour la détermination de l'âge des salmonidés migrateurs; la détermination du sexe par sérodiagnostic et à partir de critères morphométriques; l'estimation de la production en smolts de saumon à partir de l'habitat et des caractéristiques des peuplements automnaux. Dans un deuxième temps, l'application de certaines de ces méthodes dans le domaine écologique a permis d'obtenir des éléments essentiels pour la connaissance et/ou la gestion des populations sauvages de salmonides.

EN :

Turing the fast few years some new techniques for the study of wild populations of salmonids have been developed. The following procedures are intended to provide.

1) an improved method for determining age by scale-reading, particularly with migratory salmonids (Atlantic salmon and sea trout). A standard zone for removing scales, the presence of three different zones on each scale (river, run-out, sea) and some criteria for scale interpretation have all been defined. These criteria are mainly concerned with the structure of the scale during the sea-life stage and reproduction. On the other hand, a spawning mark has been determined for both sedentary and migratory salmonids.

2) an means of sex determination. Two techniques have been proposed :- the first one is based on the detection by immunoagglutination of the serum vitellogenin of females in active gametogenesis. This method concerns females only; it does not allow a distinction to he made between non spermating males and immature salmonids. It is a very reliable technique which can be used in field conditions, but only by experienced researchers;

- the second technique is based on the sexual dimorphism of the relative length of the upper jaw bone. It is very reliable (8 % to 12 % error) and can also be applied in field conditions, but only for large fish (fork length > 40-45 cm).

3) a way of estimating Atlantic salmon smolt production. This technique is based on :

- the estimation of a 0+ and 1+ salmon population in the whole river from data of density obtained in sectors representative of the different types of habitat ;

- the presence of growth bimodality in a 0+ salmon population in autumn, given the following hypothesis : one yearold smolts belong to the upper mode ;

- the estimation of mortality for the whole winter season.

This technique has been used in the River Oir and compared with a smolt-trapping experiment conducted for two years. A comparison of the results shows that :

- the one year-old smolts belong to the upper mode of bimodality;

- when based on data on habitat characteristics, this technique can be used in field conditions to evaluate the smolts with sufficient accuracy.

All three techniques are immediately available as useful methods for fisheries management. At a second stage, they can also serve in ecological research to advance knowledge on which to base further study and improved management of wild salmonid populations.

FR :

Le lac de Comabhio (Lombardie, Italie du Nord) est un lac peu profond (z^- = 4,4 m) en comparaison à sa surface (3,44 km²). L'eutrophisation naturelle du lac a été accélérée par l'apport d'effluents domestiques pas traités. Pour améliorer la situation, les effluents conduits par un collecteur circumlacustre, ont été recueillis à l'extérieur du bassin versant.

Les effets seront visibles seulement dans plusieurs années et ces interventions ne seront pas suffisantes pour ramener le lac à l'état de mésotrophie. Afin d'accélérer l'assainissement du lac, mais surtout pour éviter la forte mortalité du poisson périodiquement à la fin de l'été, on a pris la décision d'oxygéner artificiellement l'hypolimnion. Pour prévoir les effets de l'oxygénation on a fait une expérience dans le lac avec deux enceintes en PVC (diamètre : 40 m; profondeur : 6 m; volume : 7000 m³). On a gardé une enceinte comme témoin; dans l'autre a été insufflé de l'oxygène pur. On a recueilli, à certains intervalles de temps, des échantillons d'eau et de plancton du lac et des deux enceintes. Les éléments nutritifs ont été analysés; la température, la transparence, la conductivité et le pH ont été mesurés sur place. L'effet le plus visible du traitement consistait dans une augmentation de la concentration de l'oxygène visant à produire des conditions normales pour la vie des poissons. Les effets de l'oxygénation sur les caractéristiques chimiques, physiques et biologiques du milieu sont discutés.

EN :

Lake Comabbio (Lombardy, Northern Itaty) is a shallow Lake (zmax = 8.0 m; z^-= 4.4 m; surface = 3.44 km²). Its naturally high trophic level has been increased by the huge nutrient loading from domestic effluents. Each year (except in winter, when commonly the lake is covered with ice) more phytoplankton blooms are observed and in the late summer oxygen depletion, with a consequent mass mortality of fish, occurs.

At present, to reduce the nutrient charge, the effluents are collected in a channel and diverted from the lake watershed. Because of the significant nutrient release from the sediment ("internal eutrophication") the reduction of the external loading may slow the eutrophication rate, but it is not sufficient to restore the lake to an acceptable mesotrophic state within a reasonable time.

To prevent the periodic mass mortality of fish and possibly to accelerate the restoration process of the ecosystem, the Lake Comabbio Protection Committee decided to oxygenate the hypolimnetic layer artificially. In order to avoid undesirable effects of aeration (e.g. nitrogen enrichment of lake water), dissolved oxygen injection was preferred. As there is no general agreement on the ecological effects of the oxygen (or air) addition it was decided, before applying oxygenation to the whole lake, to carry out experiments using the "enclosure" method. One experiment was carried out from 26th September to 9th December 1986 and this was replicated in 1987 from 22nd April to 30th November. This paper is concerned with the first experiment, as data treatment of the second is still in progress. Two cylindrical "enclosures" of PVC (diameter = 40.0 m; height = 6.0 m; volume = 7000 m³) were settled in Lake Comabbio to isolate a water column with its sediments. One enclosure was treated with oxygen from 15th November to 6th December, the other was kept as control. The water aspirated from the enclosure was oxygenated (20.3 mg O₂/l) and injected in the same enclosure at 4 m depth at a rate of 4 m³/h. During the enclosure settlement the fish escaped from the enclosures. To simulate a complete ecosystem, the plan was to add to each enclosure 60 kg of the most abundant fish species (Scardinius erythrophtalmus). By mistake, fish were added to the enclosure that was to be oxygenated but not to the control. Water and zooplankton were sampled simultaneously from both the enclosures as well as from the Lake. Zooplankton was collected by vertical hauls from the bottom to the water surface. The following parameters were measured : temperature, transparency, pH, electrical conductivity, dissolved oxygen, alkalinity, phosphorus and nitrogen compounds, chlorophyll-a and pheopigments. The injection of water rich in oxygen did not resuspend the sediments and the absence of bubbles showed that the oxygen had completely dissolved.

Soon after oxygenation the difference in oxygen concentration in the treated enclosure and in the control was progressively reduced until 26th November, when both the enclosures had the same oxygen concentration. At the end of the experiment (9th December) the oxygen concentration in the control was about 4 mg/l, whereas in the treated enclosure it attained a concentration of 6 mg/l, although in the latter the oxygen consumption was greater than in the control, owing to fish respiration and the increased activity of aerobic microorganisms. The higher concentrations of total phosphorus (TP), soluble reactive phosphorus (SRP) and ammonium (N-NH₄) measured in the lake were probably due to the external loading of nutrients. The nutrient concentrations in the treated enclosure were similar to those of the control. The greater concentration of nutrients was probably the cause of more abundant phytoplankton in the lake, which was also demonstrated by the lower values of water transparency, when compared with those measured in both the enclosures. The pH values of the control were similar to those of the treated enclosure. The pattern of abundance and species composition of the zooplankton in the treated enclosure did not differ from that of the control. The Entomostraca populations decreased in the lake as well as in both the enclosures from the beginning to the end of the experiment.

From this experiment, carried out when oxygen depletion affected the whole water column, we may conclude that oxygenation can prevent fish mortality, without significantly influencing the structure and biomass of the planktonic community. The variations in the plankton and chemical and physical characteristics of the water (except oxygen concentration) were essentially due to the season and mot to the artificial oxygenation. In addition, oxygenation, by abolishing the periodical mortality of fish, may also prevent the release of a huge amount of organic substances from the dead fish into the water.

FR :

Ce travail présente tes premières données sur la faune aquatique souterraine du Sud de l'Anti-Atlas marocain. Dans cette zone au climat semi-continental de type saharien à hiver tempéré, plus de 50 stations ont été prospectées, puits, sources et milieux hyporhéiques des oueds. 7 puits et 2 sources de la région de Guelmim, choisis en raison de leur intérêt stygobiologique, font l'objet d'une étude suivie pendant 2 ans, d'un point de vue faunistique et écologique. Ces stations sont brièvement décrites (localisation géographique et topographique, hydrogéologie, physico-chimie de l'eau), de même que leurs biocénoses. 59 taxons, dont 11 sont considérés comme des espèces stygobies, ont été récoltés dans les eaux souterraines, interstitiettes ou karstiques. L'étude porté plus particulièrement sur les Crustacés Péracarides : Thermosbénacés, Amphiphodes Métacrangonyctidés, Isopodes Cirolanidés et Stenaseltidés. Des remarques sur l'écologie de ces espèces ainsi que des hypothèses patéobiogéographiques sont présentées.

EN :

The first data recorded on the aquatic underground fauna of the South Moroccan Anti-Atlas are reported. More than 50 biotopes, wells, springs and river underflows, have been prospected in this Saharian province with a semi-continental climate. The fauna and ecology of seven wells and two springs, stygobiologically interesting, have been regularly investigated for two years. These nine stations are described briefly (geographic and topographic location, hydrogeology and physicochemical characteristics of the water) as well as their zoocenoses. The fauna of interstitial and karstic waters includes 59 taxa, 11 of which are stygobiontic. The study is focused mainly on the Peracarid Crustacea : Thermosbaenacea, Amphipoda Metacrangonyctidae, Isopoda Cirolanidae and Stenasellidae. Some remarks are added on the ecology of these species and a paleobiogeographic hypothesis is proposed.