Volume 4, numéro 3, 1991

FR :

Les peuplements d'Ephémèroptères, de Plécoptères et de Trichoptères de quatorze cours d'eau du massif vosgien granitique, ont été échantillonnés de mars 1988 à mars 1990. Les stations, au nombre de seize, ont été choisies en amont de toute agglomération et en dehors de zones d'activité agricole.

Les mesures de pH, conductivité et aluminium total, ont établi que sept ruisseaux sont acides (pH moyens : 4,67-5,62), très faiblement minéralisés (conductivités moyennes : 17-21,5 µS/cm) et présentent des concentrations moyennes en aluminium total, comprises entre 221 et 387 µg/l.

Les sept autres cours d'eau sont faiblement acides à neutres (pH moyens : 6,80-6,98), peu minéralisés (conductivité moyenne : 36-90 µS/cm) et montrent des concentrations moyennes eut aluminium variant de 41 à 78 µg/l.

Seules 29 espèces ont été récoltées dans les ruisseaux acides alors que 93 l'ont été dans les cours d'eau non acides.

Les Ephémèroptères se révèlent les plus sensibles aux conditions acides et disparaissent totalement. Parmi les Trichoptères, seuls les Polycentropodidae, les Rhyacophilidae et les Limnephilidae sont encore présents dans les ruisseaux acides. Alors que les Plécoptères Filipalpes sont bien représentés, certaines espèces étant même très abondantes, les Plécoptères Sétipalpes ne sont plus récoltés dans les cours d'eau acidifiés, à l'exception de Siphonoperla torrentium.

EN :

Acidification of freshwaters has become a serious problem in certain parts of the Vosges Mountains (Northeastern France). Aquatic organisms at all major levels are affected by decreased pH. As the Ephemeroptera, Plecoptera and Trichoplera are a significant par of the macrobenthos in mountain streams, the objective of this preliminary study was to determine how the physico-chemical environment in acidified running waters affects the qualitative composition of mayflies, stoneflies and caddis-flies communities compared with non acidified streams and to evaluate the reliability and possible use of the taxonomic groups as indicators of the extent of acidification.

A biological survey of fourteen streams was conducted in the Vosges mountains from March 1988 to March 1990. The areas investigated lie on granitic bed-rock and soils in the process of podzolisation. The sampling sites (sixteen), at altitudes of 600 to 1020 meters a.s.l. were located above built-in and agricultural areas thus avoiding organic pollution. The streams drain forested catchments regarded as sensitive to acid inputs and affected by forest decline. Silver fir (Abies alba), Norway spruce (Pecea abies) and European beech (Fagus sylvatica) are dominant.

Larvae and pupae of Ephemeroptera, Plecoptera and Trichoptera were collected by band picking and Surber sampler (mesh aperture 350 µm). Adults were collected with an insert net by beating bushes and trees or sweeping low herbage near streams. A long-handled net was required for high-flying Ephemeroptera and Trichoptera. Organisms were preserved in 5 % formalin or 70 % ethanol.

The pH was measured in the field using a specific glass electrode for low ionic solutions, compensated for temperature. Water samples were acidified for total aluminium analysis. Determination of aluminium was performed using an atomic absorption spectrophotometer equipped with a graphite furnace. Conductivity was measured in the laboratory al 20 °C.

Determination of pH revealed that acidification occurred et seven sites (sampling sites n° 8 to 16). Six were strongly acidic (mean pH : 4.7-5.2) and one moderately acidic (mean pH : 5,6). In such streams mean total aluminium ranged from 221 µg/1 to 387 µg/1 and mean conductivity from 17,0 µS/cm to 21.5 µS/cm. Minimum pH and maximum aluminium values were recorded during snowmelt or rein event. In all these streams the fish population (Salmo trutta fario) has completely disappeared.

Others streams (sampling sites n° 1 to 7) went from slightly acidic to neutral with a range pH between 6.8-7.0 and were characterized by average total aluminium varying from 41 µg/1 to 78 µg/l and average conductivity from 36 µS/cm to 90 µS/cm. These streams have been considered as reference for non acidified running waters.

The number of species occurring in acidified streams contrasted markedly with those of the non acidified streams. Specific richness of Ephemeroptera, Plecoptera and Trichoptera are reduced in acidic waters. Only twenty-nine species were found in acidic streams, whereas a total of ninety-three species were collected in non acidic streams.

Ephemeroptera

In the reference streams, the Ephemeroptera fauna was rich and diverse with twenty taxa. Some species, for example Epeorus sylvicola, Baetis alpinus and B. rhodani were very common and widespread. In contrast, in acidic water only one species, Baetis vernus, was actually present (3 larvae).

Trichoptera

Thirty-eight species of caddis flies were found in non acidic streams. Hydropsychidae (hydropsyche sp) Philopotamidae (Philopotamus ludificatus, P. montanus, Wormaldia sp) and Glossosomatidae (Glossosoma conformis, Agapetus sp) were the organisms most commonly present.

In acidic streams, eleven species were food. Rhyacopyhilidae (Rhyacophlla obliterata, R. potamoides, R. praemorsa), Polycentropodidae (Plectrocnemia sp, P. conspersa) and Limnephilidae (Drusus annulatus, D. discolor, Caetopterygopsis maclachlani, Chaelopteryx villosa, Anitella sp) were among the most abundant taxa.

Plecoptera

In acidic streams, taxonomic richness (17 species) was about 50% lower than those obtained in the non acidic streams. The common species Perla marginata, Perlodes microcephala and Isoperla oxylepis were not listed when pH was below 5.6. At the opposit, some species were dominant and very abundant. For example, Brachyptera seticornis, was dominant in April, Leuctra nigra in May, Siphonoperla torrentium in June and Leuctra cingulala in July.

The sensitivity of organisms towards to acidification appeared to be different among these groupe of aquatic insects. Three types of responses can be distinguished in relation with pH and/or associated factors :

- High sensitivity of Ephemeroptera, which have completely disappeared

- High sensitivity of most Trichoptera families except the following : Polycentropodidae, Rhyacophilidae and Limnephilidae.

- Tolerance of Plecoptera filipalpia which were sometimes very abundant, but disappearance of Plecoptera Setipalpia except for Siphonoperla torrentium.

This analysis showed that many species of mayflies, stoneflies and caddis flies do not tolerate low pH (and/or) associated with low conductivity and elevated aluminium concentrations. Because the biological response is the result of present and post environmental situations, we suggest the use of these taxa as indicators of acidification in running waters, particularly when a restricted number of physico-chemical measurements is insufficient to appreciate the acidification level, as during low flow periods or during periodes of no acid stress.

Nevertheless, to be considered as a useful indicator of pollution in general and acidification in particular, a taxa (species, genera or family) should be widespread, relatively abundant, present all the year round, very sensitive to the studied factor and easy or relatively easy to identity.

For example, in unpolluted streams in the Vosges Mountains, Epeorus sylvicola, Perlodes microcephala, Hydropsyche sp, Philopotamus sp seem to fit these criteria.

However, in evaluating aquatic organisms as indicators of pollutional conditions, great caution is necessary because knowledge of the ecological requirements of the species is essential, several ecologic conditions other than the presence of a pollutant may limit the distribution of certain species. For this reason, the benthic macrofauna of a more significant number of sites of different chemical and physical characteristics should be collected.

FR :

Si de nombreux travaux ont porté sur l'influence des piscicultures intensives sur les rivières, on s'est peu intéressé aux conséquences des nombreuses créations de plan d'eau sur le milieu. Le but de cet article est de présenter en s'appuyant sur des exemples précis les différentes modifications apportées sur le milieu par une surface en eau d'élevage extensif.

Les aspects hydrologiques sont peu importants. Les changements physico-chimiques ont par contre de nombreuses conséquences, directes et indirectes :

- la température : le réchauffement estival agit sur les réactions chimiques, l'équilibre des biocénoses, la pathologie des poissons,

- le potentiel hydrogène, très variable en cas de faible dureté,

- l'oxygénation, liée à la température dont le déficit agit directement sur la faune pisciaire,

- la fertilisation par l'azote et les phosphates,

- les matières en suspension : action surtout manifeste lors des vidanges, directe sur le colmatage des frayères, indirecte sur la structure des communautés floristique et faunistique.

Le repeuplement des rivières par les poissons d'étang est fréquent. Les conséquences vont du déséquilibre faunistique à l'introduction d'espèces indésirables ou nuisibles.

Dans l'ensemble, on note un impact négatif sur les eaux courantes, particulièrement sur les rivières à salmonidés (1^ère catégorie). Sur les cours d'eau de 2^e catégorie, l'impact est toujours plus faible et on peut noter une action bénéfique due au lagunage.Une hiérarchisation des influences est donnée en tenant compte du type de plan d'eau et de son mode de gestion.

EN :

Although many studies relate to the influence of fish farming on rivers, little interest has been given to the environmental impact of artificial ponds. The aim of this article is to show through precise examples the various alterations in the environmental system caused by extensive fish breeding farms.

Hydrological aspects

Even in the absence of any breeding, the presence of a water surface brings about variations in the local water balance.

- losses due to infiltration may increase through the drilling of an impermeable substratum when digging the pond, but new springs are often observed, resulting in an inscreased stream flow.

- fosses due to evaporation vary according to local climate and environment.

Pond dams sometimes act as buffers against floods.

Influences can only be significant under particular conditions : regional temperature and humidity and the nature of the substratum.

Physico-chemical aspects

During the filling period, one of the greatest risks is the rise in temperature. Even though the latter only occasionally reaches 10 % of its initial value, the effects are numerous :

- reduction of dissolved oxygen content and acceleration of microbial processes of aerobic decomposition.

- effect on biocenosis : a 3 to 4 °C rise in average maximal temperature may result in a typological change in the streams normally hosting Salmonidae. The disappearance of benthic Invertebrata may be observec as well as their replacement by groups whose development is stimulated by temperature : e.g. Mollusca and Worms.

- effect on pollution : a 1 °C rise in temperature, from pH to basic values results in a growing concentration in ammoniacal nitrogen which turns out to be toxic. On the other band, a possible favourable affect upon second class fish breeding streams must be noted. Eutrophization and enhancement of the environment also occur in addition to an accelerated gametogenesis in some species. In the late summer, this results gametogenesis in some species. In the late summer, this results in larger fish an, consequently, increased chances of survival lacer. An EEC rule has set the tolerance limits of temperature rise according to the type of river (table 4).

If It remains sustined, the discharge of suspended matter may also have various influences. Except for accidents, the risk is slight, measured values are generally inferior to EEC standards. However, it is essential to consider that discharges minerals and organic compounds such as some phytosanitary products (Lindane, Atrazine) likely to cause immediate or delayed toxic effects.

The creation of a pond is clearly a cause of instability in the composition of water immediately downstream, especially regarding the pH and, indirectly, NH3 content. This risk is all the greater since restituted water is the result of overflow. As for oxygen, the problem may be assessed differently, according to the mode of restitution; overflow may improve oxygenation whereas overflow caused by an emptying device may cause deoxygenation. Whatever the mode of restitution, downstream water is enriched with soluble and particulate organic matter which can promote bacterial development and oxygen intake.

Finally, it is worth noting that pend phytoplankton is likely to culture the downstream river, all the more intensely since the sluggishness of water flow will be stopped up.

The impact will depend upon the kind of pond involved : a « ballast tank » unconnected with the stream will have a lesser influence than a derivated pond. Since maximal impact will occur in the discharging channel before reaching the stream's downstream zone. The influence will be much more harmful to Salmonidae streams which are more sensitive to a reduction in dissolved oxygen. The imbalance will be heightened by fish losses which are likely to happen, since all species must compete with the surrounding fauna and will be favored by environ mental imbalance.

During the emptying operation, besides the circulation of elements trapped by sediment, the addition of suspended matter will involve e high risk for fishbreeding downstream from the discharge. If, during the draining operation, discharge volume is lower than EEC standards (25 mg/l), the emptying operation causes content to reach more than 100 mg/l, with immediate or delayed side affects.

The most direct harm done is asphyxiation of most fish under certain circumstances. This is heightened is a real risk of bringing about a rise in temperature : passing from a Salmonidae to a Cyprinidae environment both through disruption of the inverterbrata fauna, and sealing of the spawining places dug in the streambed gravel at emptying periods. The risk of a reduction in fauna varies and invertebrata biomass should also be brought up.

Whenever fish are discharged, repopulation of the river usually results. In this case again, the affects will be negative for Salmonidae streams or intermediate-type water in which the replacement and introduction of new species can occur. Everything will depend upon the breeding carried out in the pond.

In 2nd-category fish-breeding rivers, only a reinforcement of population and possibly a diversification of species will be observed.

As a conclusion, all artificial ponds have a negative impact upon freshwater especially in 1st-category fish farming rivers : e.g. changes in physico-chemical characteristics, introduction of species. This impact occurs during the filling period of the pond (variation in temperature) and the emptying operation (various discharges). In some cases, favourable consequences can be observed (table 1) : natural creation of a lagoon with trapping of elements in solution (phosphates, nitrogen). Moreover, the presence of alcalinity is stronger because of water sediment transfers, as is pH, showing a better fish farming potential.

The impact on freswater depends upon the type of pond and the method used for emptying.

FR :

A partir de références bibliographiques encore réduites, les auteurs ont mis au point un protocole expérimental d'évaluation de la toxicité des sédiments de rivière, après extraction au dichlorométhane et mesure de la toxicité par le lest de Bioluminescence Bactérienne.

Ce protocole simple appliqué à quarante-trois prélèvements correspondant à des situations de pollution diversifiées, a permis d'établir et de proposer une typologie des sédiments en fonction de leur niveau de toxicité.

Correspondant à une approche nouvelle (tests biologiques de toxicité sur sédiments), la méthodologie apparaît particulièrement intéressante pour la gestion patrimoniale des cours d'eau (réseaux de surveillance, mise en évidence de situations de pollutions), et devrait être intégrée dans les opérations de mesure de l'impact des pollutions toxiques sur les écosystèmes aquatiques.

EN :

As toxic substances in river water are not easy to detect and to measure, sediments able to precipitate them have been userd, successfully, to quaintly metals and a limited number of organics. Actually, analytical determinations, very expansive, time consuming and never exhaustive, are not an adequate means for a general estimation of the presence of toxic organic substances.

Another developing approach consists in determining effects not the substances themselves (contained in the sediments) but their toxic effects on biotests applied directly on all the sediments or on the extracts. This approach has been used by relatively few teams, principally in North-America and in The Netherlands, in limited areas, to evaluate the local impacts of industrial effluents on sea and-or river waters. On the contrary, the object of this work is to measure the interest of this kind of approach, at a large Water Basin, first to establish the toxic profiles of rivers with areas of pollution, and second to constitute a memorization, susceptible of being compared at regular intervals for an estimation of general depollution policies .Based on data taken from the literature, an organic extraction of sediments with dichloromethane as solvent (without pretreatment) and photobacterium phosphoreum luminescence inhibition test were chosen. Actually, according to the literature, dichloromethane is the must convenient solvant and toxic activities of extracts are well correlated with organic toxic contents in the few cases where, exceptionally, the two approaches have been carried out simultaneously. Similarly, the bacterial luminescence test, Microtox (commercial name), quick and inexpensive, is well correlated with results from other biotests when, occasionally, both were utilised. Moreover, Microtox was well tested in the Agence laboratory.

Different conditions of extraction were carefully tested, before the adoption of the definite protocol : whole sediments (10-g) are mixed (slow agitation in rolling flasks) for 6 hours with dichloromethane (100-mL) and sodium sulphate (50 g). Dichloromethane is then eliminated and the remaining solid materials are washed twice with fresh dichloromethane (2 x 20 mi). Dichloromethane extract and washings are collected, dried over sodium sulphate and concentrated (below 40 °C) to 1 mL, further adjusted to 3 mL. A portion measured accurately of this concentrate is added to ethanol and concentrated to remove dichloromethane.

Bacterial bioluminescence tests, in duplicate, are performed on ethanol extracts, diluted (1 %) in salified (sodium chloride 2 %) distilled water. Estimates of the EC 50 (concentration causing a 50 % reduction in bioluminescence) obtained using linear regression analysis are converted into Toxic Units and referred to the net organic weight, measured separately, at 550 °C. This expression of results is particularly relevant when sediments with different organic net weights are compared.

The protocol has been applied to sediments collected at 43 sites in the principal rivers of Rhône Méditerranée Corse Basin and in special distant from significant domestic and industrial activities.

The toxic amplitude observed (ratio of l/200 between lowest and highest toxic units obtained with the panel) and the good reproductibility of the whole process (10 % to 25 %) were consistent with a trial subdivision in 5 classes as follow :

Class 1 : Out of pollution - Class 2 : Moderate toxicity - Class 3 : Important toxicity - Class 4 : Very important toxicity - Class 5 : Exceptional toxicity.

The relevancy of this classification was estimated when sediments were classed according to their toxic content. The classification proves to be correct in must cases. Exceptionally toxic sediments were collected in sites affected by large industries and agglomerations as (CHASSE and SAINT-VALLIER) on River Rhone near Lyon and important chemical plants, or GIVORS, on the small River GIER, markedly affected by many various industries. On the contrary sediments of moderate toxicity corresponded to sites relatively far from sources of important pollutions, e.g., ARLES on the River RHONE near the sea and a number of sites on the River SAONE. Moreover, when repeated sampling was performed on the same sites, the results were consistent, belonging to the same class of toxicity.

The general object of this work was not to determine a definite classification of sediment toxicities, but to assess the relevancy of this approach. In our opinion, it was proved that bioassays on sediment extracts, and specially the bacterial bioluminescence assay, are a valuable tool, before the material impossibility to determine the toxic substances content. Moreover this approach could be used at the Basin. More investigations are necessary to define more accurately the number and level of toxic classes and e.g. to establish correlations with perturbations of benthic communities living in the sediments. It is also a contribution to the general knowledge and action based on biotests performed on effluents and different compartments of rivers.

FR :

Afin de tenter de déterminer les parts respectives des activités bactérienne et algale, nous avons testé d'une part, l'action d'une substance antibactérienne (la gentamycine), et d'autre part, l'action d'un inhibiteur métabolique des cellules eucaryotes (la cycloheximide) sur des cultures d'algues et de bactéries et sur des échantillons provenant du milieu naturel et soumis à des filtrations différentielles. Les effets de ces inhibiteurs ont été testés au niveau des activités hétérotrophe et photosynthétique globales, mais également au niveau de l'incorporation des marqueurs radioactifs lors de la synthèse des macromolécules et des composés de faible poids moléculaire.

Les résultats obtenus laissent apparaître que l'inhibition de l'activité bactérienne par la gentamyclne est significative mais non complète (pourcentage d'inhibition moyen = 67 %). De plus, l'efficacité de la gentamycine augmente avec la durée d'incubation. Par ailleurs, les effets secondaires de la gentamycine sur les activités hétérotrophe et photosynthétique d'une culture de Melosira italica subsp. subarctica sont acceptables seulement pour des incubations de courte durée (< 4 heures).

En revanche, l'emploi de la cycloheximide s'est révélé sans aucun effet significatif sur les activités photosynthétique et hérérotrophe de la culture de Melosira, même après 24 h d'incubation.

A partir des échantillons prélevés en milieu naturel, l'emploi de la gentamycine a permis de réduire l'interférence bactérienne dans les mesures d'activité hétérotrophe algale. Enfin, nous avons pu constater que la gentamycine modifie l'allocation des marqueurs radioactifs dans les macromolécules.

EN :

The ability of many planktonic algae to use particulate and/or dissolved organic carbon directly by phagotrophy or osmotrophy in laboratory cultures is well documented (DROOP, 1974; NEILSON and LEWIN, 1974; HELLEBUST and LEWIN, 1977; BIRD and KALFF, 1986). In axenic cultures, numerous microalgae grow in the dark with micromolar concentrations of diverse organic nutrients as their sole sources of carbon and energy (RIVKIN and PUTT, 1987). However, to demonstrate algal heterotrophy in the field, it is necessary to differentiate between bacterial and algal activities. In the course of this study, we tested the effect of an antibacterial substance (gentamycin) and of a metabolic inhibitor of eukaryotic cells (cycloheximide) on algal and bacterial cultures, and also on lake water samples submitted to differential filtration. The effect of these inhibitors was tested both at the overall heterotrophic and photosynthetic activities level and the level of the incorporation of radiolabeled tracers in macromolecules and low molecular weight compounds.

Gentamycin was tested on bacteria and on an axenic culture of the diatom Melosira italica subsp. subarctica, the dominant species of the spring phyto-planktonic bloom of many temperate lakes. Bacterial culture was obtained by filtration of a senescent culture of Melosira through a 0.45 µm pore-size membrane. During exponential growth, gentamycin (40 µg.ml^-1) was added to different flasks containing 100 ml of culture, 30 mn after gentamycin addition, Na H¹⁴CO₃ (12µCi/100 ml) was introduced into the flasks. In each case, two replicates were incubated in the light and two in the dark for 0.5, 1, 2, 4, 6, 8, 12 and 24 hours. After incubation, aliquots were collected on a filtration membrane (0.45 µm) and the radioactivity was measured using an LKB liquid scintillation counter to estimate photosynthetic activity. Incorporation of inorganic carbon into macromolecules was measured using the procedure described by LANCELOT and MATHOT (1985), which allows the separation of lipids, polysaccharides, proteins and low molecular weight compounds (e.g. amino acids, organic acids and monosaccharides) by virtue of their relative solubilities in different extraction solvents :

- lipids were extracted with a 2/1 (v/v) chloroform-methanol mixture;

- low molecular weight compounds were extracted with hot ethanol;

- proteins were precipated with TCA at elevated temperature; this also separated them from polysaccharides. Nucleic acids were recovered with the polysaccharides fraction. Results are given as a percentage of total radioactivity. The same method, as that described for photosynthetic incorporation, was used to measure incorporation of glucose ³H (19 nmoles.l^-1) and for the allocation of radioactive marker in macromolecules. To assess the effect of cycloheximide, the same procedure as that described for gentamycin was used. However, due to the lack of references, three cycloheximide concentrations were tested = 50, 100 and 150 µg.ml-1, and aliquots were collected on a 0.2 µm pore-size membrane.

Field samples were taken in Lake Pavin, an oligomesotrophic French lake, during the sedimentation of Melosira italica subsp. subarctica, the prevailing species of the spring bloom. The effects of gentamycin on photosynthetic and heterotrophic incorporations were tested with the came procedure as that described for cultures. However, radioactivity was measured for different size tractions : 0.2-0.45 µm, 0.45-5 µm and 5-160 µm, after incubations for 0.5, 1, 3, 6, 12 and 24 hours. Phytoplanktonic cells were counted on a Wild M40 inverted microscope and bacterial enumeration was realized in epifluorescence microscopy after staining with acridine orange (HOBBIE el al., 1977).

Results show that gentamycin used at 40 µg.ml-1 inhibits significantly but not completely the activity of the bacterial culture (mean inhibition percent = 67 %). Gentamycin efficiency increases with incubation time, the inhibition reaching 81 % after 24 h. At the same time, the secondary effects of gentamycin on heterotrophic and photosynthetic activities of Melosira italica subsp. subarctica in culture were only tolerable with short incubation times (< 4 h), when the percents of inhibition were respectively 13.6 and 12.2%. On the other band, cycloheximide produced no significant effect on photosynthetic and heterotrophic activities of Melosira italica in culture, the percent of inhibition always remaining below 6.5 %.

The use of gentamycin in natural samples reduced bacterial interference with algal heterotrophic activily measurements. The percent of inhibition caused by gentamycin was high (< 76 %) In the small-size fraction where the bacterial biomass predominate on the phytoplanktonic one.

Lastly, in all samples, we could demonstrate that gentamycin modifies the allocation of inorganic carbon and radioactivity in macromolecules. The incorporation in proteins is significantly reduced essentially to benefit of the incorporation in low molecular weight compounds.

FR :

Le Rio Desaguadero relie le lac Titicaca (alt. 3 810 m) au lac Poopo (alt. 3 670 m) dans le bassin endoréique de l'Altiplano, en Bolivie, (fig. 1). Le bassin versant est constitué de terrains sédimentaires et volcaniques, avec une végétation steppique et aucune source de pollution organique. Le climat très variable présente d'une alternance de saisons humide et sèche. Le Rio Mauri fournit 90 % du flux de MES estimé à 6,6.10⁶ t an^-1 (d'après GUYOT et al., 1990).

Des analyses de COT et d'autres paramètres chimiques ont été réalisées sur 15 stations en 3 campagnes (fig. 2 et tableau 1). Une ACP (fig. 3) révèle une corrélation entre COT et MES, correspondant à deux épisodes de crues (déc. 87 et fév. 88) (tableau 2). Des hypothèses concernant l'origine du carbone organique et les flux sont avancées (tableau 3). Un flux de base, principalement du COD provenant du lac Titicaca, représenterait 18 000 t an^-1. Mais le COP, quoique fortement corrélé aux MES, n'a pas la même origine que celles-ci (fig. 4); il proviendrait principalement des zones connexes au Rio Desaguadero riches en végétation aquatique.

La contribution de ces flux aux bilans du carbone organique paraît négligeable pour le lac Titicaca et peu importante pour le lac Poopo. Mais le maintien des interconnexions entre le Rio Desaguadero et ses zones humides doit être intégré dans les projets d'aménagement.

EN :

The Rio Desaguadero is the outlet of Lake Titicaca and flows into Lake Poopo in the endorheic watershed of the Altiplano in Bolivia (alt. 3 650 m); its main tributary is the Rio Mauri (fig. 1). The transport of dissolved and suspended solids has been recently evaluated by GUYOT et al. (1990). The data on Total Organic Carbon obtained for the first time in this hydrosystem allow to complement this investigation with an estimation of the organic carbon transport.

The Altiplano is a basin filed with tertiary and quaternary sediments. The Eastern cordillera is constituted of primary rocks; the Rio Mauri flow from a tertiary volcanic watershed. The climate is rather cold, semi-arid (700 to 400 mm yr^-1) and the vegetation is steppe-like with extensive pastures (sheep, lamas). The rainy season lasts from December to March. No direct pollution source can affect the TOC. The hydrology is very variable (see GUYOT et al., 1990). The base flow issues from Lake Titicaca and depends on the water level in the lake. The Total Suspended Solids (TSS) content is very high during floods. The annual flux of TSS is 6.6 10⁶ t yr^-1 at Ulloma, 90 % of transported sediment coming from the Rio Mauri.

TOC was measured during 3 sampling series at 15 different stations (table 1). The 25 ml samples were acidified (pH 1) with H3 PO4 and sent to Lyon (France) to be analysed with a Dohrmann DC 80. In February 1988, Dissolved Organic Carbon was measured on filtered samples (Whatman GF/C decarbonated at 550°C). The major elements were analysed in La Paz (Bolivia).

Raw TOC data are presented (fig. 2), with a Principal Components Analysis (PCA) for 9 parameters on all the samples (fig. 3). The first axis (52 % of total variance) is a scale of mineralization; it reveals a progressive concentration of dissolved elements in the Lake Poopo outlet (fig. 3B first cluster). The mineralization of these samples seems to be inversely correlated to the discharge flowing from Lake Titicaca for the corresponding series. The second axis (23 % of total variance) shows a correlation between the TOC and the TSS (fig. 3B, 4th cluster). This correlation is obvious for the samples of Dec. 87 and Feb. 88 (table 2), when TSS concentrations are high due to flash floods of some effluents. Equation (1) gives the hast regression of TOC vs TSS.

The organic Carbon transported is constituted of dissolved (DOC) or Particulate Organic Carbon (POC). Estimation presented for DOC and POC transport at the three gauging stations (table 3) are based on the following hypotheses :

In the Rio Desaguadero, DOC concentrations seam relatively stable (fig. 2D). Upstream of the Rio Mauri, the TOC transported at an average concentration of 11 mg. l^-1 is assumed to be mainly DOC flowing out from Lake Titicaca. DOC transport may constitute a relatively constant base flow of organic carbon.

Estimation of POC transport is much more difficult. During floods from tributaries, the rise in TOC concentrations correlated with an increase of TSS seems to be essentially due to the POC. Following a classical method (MEYBECK, 1982), a non-linear regression of percentages of POC In TSS vs TSS was calculed (equation 2), using the samples with high TSS concentrations (fig. 4A). Then, the POC fluxes were estimated using daily data of TSS for the gauging stations CA and UL. As most of the TSS come from the Rio Mauri results from this estimation that the flow rate of TOC from this stream would reach 7 t km^-2yr^-1. But this value according to MEYBECK (1982) is close to the export of a tropical rainforest. Thus, this hypothesis of a POC transport directly linked to the TSS flux must be rejected.

POC and TSS, although correlated, have distinct origins. This is shown by the tact that the percentages of POC in TSS in the samples used to calculate equation (2) are much higher than those given by MEYBECK (1982) for world rivers (fig. 4B).

A possible explanation for this phenomenon is that the POC could come from Rio Desaguadero Borders and wetlands, often rich in aquatic vegetation, and very extensive in some places (especially upstream of Nazacara and downstream of Eucaliptus). This particulate carbon might be flushed into the mainstream during rainy periods, while at the same time TSS flow down from tributaries. Hypotheses on POC transport (table 3) in the Rio Desaguadero are thus based on a POC/TSS correlation (equation (2)) valid only for the mainstream, the contribution of Rio Mauri to the total flux being estimated on the basis of e TOC export rate of 0.5 t km^-2 yr^-1.

Contribution of these fluxes to organic carbon budgets seems to be negligible for Lake Titicaca, whose phytoplanctonic primary production is estimated at 4 10⁶ t C yr^-1 (RICHERSON et al., 1977). When considering only the labile fraction of TOC (ITTEKOT, 1988), the readily matabolizable organic carbon input in Lake Poopo hydrosystem could be evaluated to 36 000 t yr^-1, corresponding to about 13 g C m^-2yr^-1; that might be unimportant for this presumably highly productive lake system.

In conclusion, these preliminary data lead to hypotheses which consider the Rio Desaguadero as a very particular hydrosystem in organic carbon transport. If most of the particulate carbon comes from margins, the necessity for maintaining interconnexions between the main Carmel and the wetlands fn future regulation projects must be emphasized.

FR :

Une série d'expériences a été réalisée pour évaluer les effets de l'agitation sur le bilan photosynthétique de cultures d'algues en masse en bassins extérieurs. La méthode utilisée compare la production d'échantillons marqués au C¹⁴ disposés (1) selon un profil vertical classique, (2) sur un système rotatif faisant transiter un flacon entre la surface et le tond du bassin. Des mesures ont été effectuées périodiquement en milieu de journée sur un cycle annuel, et ont été complétées par 8 incubations réalisées entre le lever et le coucher du soleil pour un jour donné. Dans toutes les expériences réalisées durant la période estivale, la photosynthèse est supérieure dans le flacon mobile. Les effets des variations lumineuses (par le biais des mouvements de l'eau) sur te bilan photosynthétique de la colonne d'eau sont particulièrement notables au lever et au coucher du soleil; ce bilan peut être jusqu'à 2.5 fois plus important pour une colonne d'eau en mouvement. Le calcul d'un bilan journalier par extrapolation des profils de photosynthèse instantanés permet d'estimer le gain de production obtenu par agitation du milieu. Par contre pendant la période hivernale, la photosynthèse est supérieure dans le flacon immobile. Ces résultats ont des implications évidentes sur le monitoring de cultures d'algues en masse en bassins extérieurs.

EN :

Experiments were carried out to define the effect of a particular mixing system (circular round a horizontal axis) upon the primary productivity of high-rate alga cultures. We know that mixing prevent algal cell sedimentation, gaseous and nutritional gradients, and variations in algae light reception. So, we wanted to quantity the effect of mixing on algal productivity with regard to flashing light (positive effect in comparison with continuous light).

We used water from waste stabilization ponds (secondary pond output), with the native algae population dominated by Euglena and Chlorella (in Aveyron, South of France). The method used compares algal production of a water column with ¹⁴C-labelled samples i) in ten flasks placed along a classical vertical profile, ii) in a rotating system with a flask moving between the top and the bottom of the pond (0.5 m). The production is here considered as the mean of a mixed vertical profile. At the same time, physico-chemical and biological parameters were recorded (high load for N and P).

Experiments were made periodically in the mid-afternoon over one year, and were completed by 8 incubations made between sunrise and sunset for on day (in August 1989). The aim was to evaluate the mixing affect with various environmental conditions. Physico-chemical parameters vary greatly over a day, and higher production values are measured in the mid-afternoon. Production observed for the stationary profile fitted the Steele model, and showed active photosynthetic from sunrise until sunset, with inhibition at high light intensities; maximal photosynthesis rates were measured between 4.00 and 5.00 PM.

Results are compared to moving flask values : in all the summer experiments photosynthesis was higher in the moving flask. Light variation effects (through water movements) on the water column photosynthetic budget are particularly notable at sunrise and sunset bouts : this budget may be over 2,5 times that of homogeneous nonturbulent water. During winter however, photosynthesis is higher for the stationary profile. We compared production recorded with the two systems, and calculated the mathematical relationship between the two systems over one day (exponential) and over the year (linear relationship).

By extrapolation of the instant photosynthesis profile to a 24-hour period, we can evaluate the production gain with culture mixing. We calculated the net production for various days over the year; around the end of April, the mixed system became more productive than the stationary one.

Considering our results in environmental conditions, it is obvious that they are not representative of all mixing systems, particularly the techniques in which algae cell position shift continually with respect to the photic zone : the positive effect of mixing in our ponds may be linked to the flashing light effect. In the literature, we can see that photosynthetic activity increases with flash time duration. Our production figures for various hours along the day (or various flash durations) are in agreement with these results.

During low photosynthetic activity periods, the flashing light effect can not act since light is available at all depths of pond. The most important factor acting, in our experiments, seems to be light, since the two systems are in similar conditions.

These results have clear implications for monitoring intensive outdoor algal cultures.

FR :

Dans ce travail, le cycle journalier de l'oxygène dissous en bassins de pisciculture semi-intensive a été étudié en relation avec des données météorologiques. Nous disposions pour cela de 22 semaines d'enregistrements (mesures horaires) de janvier 1988 à décembre 1989, dont une semaine de mesures sous glace. Chaque période de mesures a été traitée comme une série chronologique en pratiquant une décomposition saisonnière par la méthode des moyennes mobiles qui permet d'extraire à partir d'une série de mesures la tendance sur la période étudiée et la composante journalière.

Cette méthode, montre que le cycle nycthéméral de l'oxygène dissous garde son aspect et classique toute l'année, excepté sous la glace. Toutefois, l'amplitude journalière de la teneur en oxygène varie beaucoup selon la saison, et nous avons pu distinguer une période estivale où le métabolisme de l'étang semblait accéléré. L'analyse du cycle nycthéméral de l'oxygène dissous, dans lequel on distingue production et consommation, comparé à ceux de la température de l'eau et de l'insolation journalière moyenne, a permis d'étudier ce phénomène et de montrer l'importance des facteurs météorologiques dans la détermination du bilan de l'oxygène dissous en étang.

EN :

Dissolved oxygen is a very important factor in fish ponds, particularly under intensive conditions. Thus, the daily fluctuations of dissolved oxygen have been studied in different countries and under different conditions (lakes, ponds, rivers), but with data series generally limited to a few measurements per day. The automatic data recorder of the IRRA'S experimental station provides very long series of data concerning of dissolved oxygen, temperature and meteorological factors through hourly measurements which allow the use of mathematical analysis like the moving average method. This automatic data recorder bas been in use since September 1987 and the data obtained have been analysed in research on summer oxygen deficit prevention. After 3 years' measurements it was interesting to investigate the annual change in the daily fluctuations of the dissolved oxygen. For this, daily hourly measurements of dissolved oxygen, temperature and solar radiations were taken and examined during 22 weeks from January 1988 to December 1989 (one week under ice cover). Oxygen data were analysed using seasonal decomposition by the moving average method, and the results were compared with the temperature and solar radiation measurements.

The measurements were taken from 6 of the 16 IRRA'S experimental ponds (in the Dombes region, to the North-East of Lyon, France) and in a fish pond. All these ponds were semi-intensive fish-ponds stocked with Carp (Cyprinus carpio) Tench, (Tinca tinca), Grass Carp (Ctenopharingodon idella) and Silver Carp (Hypophthalmichthys molitrix).

The seasonal decomposition through the moving average method devides the time series data into a trend (long term fluctuations), seasonal indices (or daily component) and residuals, but only seasonal indices, representing the daily variations, are studied here. The daily component was next spin into oxygen production and consumption if respiration is taken to be constant over one day and can be evaluated through the night oxygen decrease (EDELINE et al., 1968; ROMAIRE and BOYD, 1978; BARTHELEMY and COUTURE, 1990).

Through this method, it appeared that the daily fluctuations of dissolved oxygen had the same classical aspect all the year round, except under ice cover, where it was impossible to observe a significant oxygen production. Such fluctuations, which are the result of the effect of oxygen production and consumption, have also been observed under different conditions (BOUD and LICHTKOPPLER, 1979; SIN and CHIU,1982; KEPENYES and VARADI, 1984; MARZOLF and SAUNDERS, 1984; GINOT, 1988; COUTURE, 1988). The aspect of the daily cycles was relatively constant but it was possible to distinguish 3 periods according to the amplitude of these cycles :

- a winter period with less than 2 mg/l daily variations,

- a summer period when these variations were greater titan 4 or 7 mg/l,

- an intermediate period in spring and autumn.

As these 3 periods did not occur at the same time of the year for 1988 & 1989, it appeared that the meteorological conditions (temperature and solar radiations) were important factors for determining these periods in the year.

The study of oxygen production and consumption gave the same conclusions.

These results have been compared with meteorological data : mean water temperature over 7-day periods (168 measurements), plus mean cumulative daily solar radiations over 7 days.

There appeared significant correlations between the oxygen production and the consumption, with these meteorological factors, but these relations were different for the « summer » period (June to October) and the rest of the year. There was a delay between the period when temperature and solar radiations are best for primary production (June and July) and the period when the pond’s metabolism is maximum.

There have been no measurements on primary production or algal population dynamics to explain these results clearly, but considering the observations of GRYGIEREK and WASILEWSKA (1979) and ELORANTA (1985) it is possible to give an explanation. In May-June the ponds are characterized by clear water (a higher than 60 cm transparency was observed) with low density algal populations, thus the pond ecosystem is not ready to use thermic and light energy. This delay between meteorological conditions and oxygen dynamics have been observed by ELORANTA (1985) and BOYD (1985).

These results are useful in pond management, particularly when it is necessary to prevent oxygen depletion. The risk of oxygen deficits is greater in August-September, when the ecological conditions are very unstable owing to the important daily oxygen fluctuations. In this case, meteorological changes like temperature decreases or rain-fall can perturb the ecosystem and cause algal die-off with a great risk of oxygen depletion.

The aim of this study was to show the possibilities of seasonal decomposition by the moving average method in the analysis of oxygen measurements. When it is possible to get long serie data, with constant skip, this method is very valuable for studying long term and daily fluctuations. The decomposition of the daily component through the method of EDELINE et al. (1968) also gives indications on primary production and oxygen consumption.

FR :

II est bien connu que la DBO peut étre mesurée par des méthodes respiromédriques. Les manipulations sont ainsi rendues plus aisées mais la durée de la mesure reste inchangée. La demande biologique en oxygène à court terme (DBOACT) a été proposée pour évaluer rapidement la DBO. Habituellement, ces mesures étaient réalisées dans un respiromètre fermé avec une boue acclimatée et en état de respiration endogène. Récemment on a suggéré de travailler avec un respiromètre ouvert, aéré continuellement durant le test. Nous avons réalisé un tel appareil avec une acquisition et un traitement automatique des données, en utilisant un micro-ordinateur. De cette façon, il est facile de tester différentes méthodes (intégration de la courbe en sac, hauteur de pic, comparaison à des étalons, usage d'étalons internes) pour évaluer la DBO. Nous avons également examiné l'influence du type de substrat, de l'âge et des caractéristiques des boues. Notre conclusion est que aucune des méthodes testées ne fournit une détermination précise de la DBO₅ pour des substrats complexes. La méthode reste utile pour contrôler la variabilité de l'influent dans une station d'épuration, lorsque la biomasse est acclimatée au substrat et qu'on ne désire pas une mesure très précise.

EN :

The usual way of measuring BOD is the dilution method. The consumption of oxygen is measured in the dark at 20 °C during 5 days. Manometric methods were proposed already some decades ago. Their principle is that every molecule of oxygen consumed la transformed into CO₂ which is absorbed in a solution of KOH, creating a pressure variation in the bottle. From this pressure variation the BOD can be calculated. By this way experiments were made easier but the duration of the measure remained unchanged. Other respirometric methods were also proposed, especially Short Term Biological Oxygen Demand (STBOD) to evaluate BOD rapidly. Usually such determinations were done in closed respirometers with acclimated sludge under endogeneous respiration condition. Recently, it was suggested to proceed in an open respirometer aerated continuously during the test. We set up this type of respirometer with acquisition and automatic processing of the data, using a microcomputer. In this way it was easy to test different procedures. Among these procedures we tested :

- the method of integration of curve in sack-from,

- the method of peak level,

- the method of comparison to standards,

- the method of internal standards.

The principles of these methods are given in more detail in the text. We examined the influence of the type of substrate, of the type of biomass used and of the sludge retention time on the results. The substrates tested were :

- glucose,

- domestic waste water with and without detergent,

- diluted eggs,

- diluted beer,

- effluents from the paper industry.

Three different types of biomass and these sludge retention times (fresh, 5 days and 20 days) were used. Automatisation of the test and automatic processing of the data were carried out in basic language with an « Apple II » microcomputer.

The test lasted between 37' and 3 hours 10', depending on the conditions (substrate, concentration, type of sludge,...)

Analysis of the results were computerized by the Analysis of Variance (ANO-VA) method and results are lasted in 3 tables.

From this study we can conclude that STOD methods have some advantages :

- they are much faster than the classical method,

- they can be easily automatized,

- they are cheap,

- oxygen is really consumed by biological processes.

For simple substrates these methods can lead to good results but for complex substrates reproductibility and precision are rather poor. Among the methods tested the only acceptable one seems to be the integration of the curve in sack-form. Lack of reproductibility appears to be intrinsic to the method.

Some possible improvements are presented, such as washing the activated sludge between two experiments, but are not very practical.

Despite these limitations the method can be useful to control the variability of influent in a sewage treatment plant. In such a case, the biomass is acclimated to the substrate and variations of influent BOD can be measured.