Phytoplancton du lac municipal de Yaoundé (Cameroun) : Succession écologique et structure des peuplements
N. Kemka, T. Njine, S. H. Zébazé Togouet, D. Niyitegeka, M. Nola, A. Monkiedje, J. Demannou and S. Foto Menbohan
L'évolution spatio-temporelle, qualitative et quantitative du peuplement phytoplanctonique a été suivie au lac municipal de Yaoundé. Pour cela des prélèvements hebdomadaires ont été effectués de novembre 1996 à décembre 1997, au niveau d'une station de la zone centrale du lac.Trois descripteurs (indice de diversité spécifique, régularité et diagrammes rangs-fréquences) ont été utilisés pour l'analyse de la dynamique de la structure du peuplement. L'indice de diversité spécifique obtenu à partir des biomasses spécifiques, par la formule dérivée de celle de SHANNON et WEAVER (1949), est compris entre 0,68 et 4,64 bits/µg. La régularité (PIELOU, 1966) varie de 0,14 à 0,84; les faibles valeurs correspondant dans l'ensemble à la présence des espèces fortement dominantes. Les profils des diagrammes rangs-fréquences, établis en coordonnées log-log, associés aux faibles valeurs de la diversité, sont essentiellement caractéristiques des stades pionniers (1 et 1') de l'évolution d'un lac tel que décrits par MARGALEF (1967) et FRONTIER (1976). Le stade 2 est rare alors que le stade 3 est absent.
Une analyse simultanée basée sur les variations spatio-temporelles de la densité cellulaire et de l'indice de diversité spécifique, sur les valeurs de la régularité, et sur les profils des diagrammes rangs-fréquences révèle le caractère immature permanent des populations phytoplanctoniques inféodées à ce biotope, et caractérise ainsi un milieu eutrophe à hypereutrophe où aucun stade d'équilibre n'est atteint. La richesse du milieu en éléments biogènes et sa faible profondeur (Zmax=4,3 m) sont à l'origine de sa vulnérabilité. L'événement susceptible d'interrompre la succession des populations s'est avéré être le brassage des eaux, provoqué aussi bien par les pluies que les vents dont l'énergie cinétique est fréquemment suffisante pour entraîner un brassage complet d'une colonne d'eau de hauteur aussi faible.
The Yaounde municipal lake (3°52'N, 11°32'E) is a shallow hypertrophic water body (Z max=4.3 m), situated downstream from Yaounde. It is an artificial lake resulting from a dam constructed in 1951 across a small river named Mingoa. Like many other water bodies in urbanised areas, this ecosystem switched rapidly from being mesotrophic to hypertrophic during the 1980's, due to the expansion of human activity and an inefficient treatment of wastewater in its catchment area. This study was carried out with the objective to analyse the ecological succession and the spatio-temporal fluctuations of the phytoplankton population structure in this shallow equatorial lake. Samples were collected at weekly intervals from November 1996 to December 1997, at a station near its central zone (depth=3.5 m), using an opaque horizontal PVC Van Dorn sampler. Sampling operations were conducted at the following levels: surface; -0.5 m; -1 m; -1.5 m and -2.5 m.
Individual phytoplankton from 1% iodine acid preserved subsamples (BOURRELLY, 1990) were enumerated with an inverted microscope (Olympus CK2) at 200X magnification, using a Sedgwick-Rafter counting cell, according to UTHERMÖHL (1958). At least 400 individuals (colonies, bundles or trichomes, depending on existing life forms) were counted on each subsample. Specific biomass was then obtained using the biovolume method. To evaluate the population structure dynamics, the SHANNON and WEAVER (1949) specific diversity index method and the evenness (PIELOU, 1966) method were computerized using specific biomass, the evenness being the ratio between the real and the maximal diversity (log2 S, where S is the number of species). Also, rank-frequency diagrams were carried out in logarithmic co-ordinates according to FRONTIER (1969), using integrated samples obtained from grab samples collected over the entire water column.
Cell densities were highest in the uppermost meter of the water column at the beginning of the rainy season, due to the exponential development of species such as Planktothrix mougeotii, Chlamydomonas sp., Eudorina elegans, Euglena gracilis, Gonyostomum semen, Euglena texta, Phacus helicoïdes, Closterium limneticum, Cryptomonas ovata, Peridinium cf. gutwinskii, Trachelomonas hispida var. crenulatocollis and Cyclotella meneghiniana. The specific diversity index varied 0.68 and 4.64 bits/µg, and values gradually decreased from the first uppermost meter to the bottom of the water column. Evenness varied from 0.14 to 0.84 with a somewhat similar spatio-temporal variation as the diversity index. Low values of these descriptors generally correspond to the presence of a highly dominant species. The rank-frequency diagrams were mainly S-shaped, indicating the predominance of one species which in this study was either Chlamydomonas sp or Planktothrix mougeotii.
A simultaneous analysis of the ecological succession and the spatio-temporal variations of the phytoplanktonic structure revealed that the functioning of this ecosystem was essentially allogenic in nature and was strongly influenced by the supply of decayed organic matter from the Mingoa stream, and effluents from ministerial buildings and hotels located near the lake. This permanent supply of abundant biogenic elements, coupled with the low depth of the ecosystem, makes it vulnerable. Ecological succession, frequently limited to the pioneer stages (1 and 1'), reflected the briefness of the different phases of phytoplankton growth, and the consequences of disturbances regularly undergone by the phytoplankton community. The second stage was scarcely reached while the third was absent. These successions were mainly controlled by rainfall, as well as wind, which is responsible for the mixing of such a shallow water column. These results can be considered important for the Yaounde municipal lake and other water bodies close to densely populated areas in Cameroon for which there is no management plan. Urgent actions need to be carried out to rehabilitate this ecosystem that fluctuates between the eutrophic and hypertrophic status.
Étude de l'absorption des phosphates, azote, chrome et cadmium par trois algues vertes isolées à partir d'effluents urbains
R. A. Zerhouni, D. Bouya, C. Ronneau and J. Cara
La capacité de déphosphatation et de dénitrification ainsi que la sensibilité et l'accumulation de deux métaux lourds (Cr et Cd) de Scenedesmus quadricauda, Chlorella pyrenoidosa et Chlamydomonas reinhardtii isolées des eaux usées de la ville de Fès, ont été étudiées dans les conditions de laboratoire. Les résultats ont montré que S. quadricauda présente les meilleurs taux de réduction des phosphates (83%), et d'ammonium (75%). L'étude de la sensibilité vis à vis du chrome et du cadmium des trois espèces, a montré qu'au delà d'une concentration de 1 ppm, la croissance se trouve réduite et que S. quadricauda présente une sensibilité plus faible que les deux autres espèces. Les résultats des expériences d'absorption conduites avec du chrome VI marqué au51 Cr et du cadmium marqué au109 Cd ont montré que les quantités de chrome et du cadmium prélevées par S. quadricauda dépassent celles des deux autres espèces. L'utilisation potentielle des trois espèces dans le traitement des effluents urbains est discutée à la lumière des résultats de ces expériences.
Wastewater discharge is an environmental problem, especially in developing countries. Biological wastewater treatment seems to be an alternative for these countries due to its low cost. Algae are used in such wastewater treatment systems in many parts of the world. In addition to preventing environmental damage, the reuse of treated wastewater can be an alternative water supply in arid regions and during drought periods.
In this paper, three algal (Chlamydomonas reinhardtii, Chlorella pyrenoïdosa and Scenedesmus quadricauda) species were isolated from an urban effluent collected from a Mediterranean city (Fez, Morocco, situated between 4° to 7° W and 33° to 35° N) and their ability to remove phosphate and nitrogen was tested under laboratory conditions. The toxicity and uptake of chromium and cadmium were also studied.
Fez city wastewater was sampled monthly during one year (2000) at three main stations located along the Boukhrareb stream which crosses the city. The following parameters were measured according to methods described by RODIER J. (1996): Chemical oxygen demand, COD; Biological oxygen demand, BOD; NO3 -; NH4 +; PO43 -; electrical conductivity (EC) and organic matter. Heavy metals (Cr, Fe and Zn) were measured by INAA (Instrumental Thermal Neutron Activation Analysis) in wastewater samples dried on Teflon sheets at 70°C.
The collected samples were incubated under controlled conditions and species of alga were identified, isolated, and then transferred to fresh culture medium. Chlamydomonas reinhardtii, Chlorella pyrenoïdosa and Scenedesmus quadricauda were chosen for these experiments because of their abundance in wastewater over the year and for their capacities for biomass production. The removal of nutrients from aqueous media was studied by using monospecific cultures. Algae were in the exponential growth phase when the experiments were started and BG11 culture medium was used under standard growth conditions. Initial concentrations of phosphate and nitrogen were 2.24 mg×L-1 (PO43 -), 12.84 mg×L-1 (NO3 -) and 0.26 mg×L-1 (N-NH4 +). The concentrations decreased during the experimental period, and degrees of removal ranged from 83 to 75% for phosphate, from 85 to 77% for nitrogen and from 52 to 75% for ammonium.
To evaluate the sensitivity of the algal cells to heavy metals, the algal strains were exposed for one week to chromium as K2Cr2O7 and cadmium as CdCl2 at concentrations of 0.1, 0.5, 1, 1.5 and 2 ppm (mg×L-1) of Cr or Cd. Culture growth was monitored by microscopic counting of algal cells to determine quantitative biomass changes. Results demonstrated that growth inhibition occurred at 1 ppm Cr and Cd, whereas levels of 1.5 and 2 ppm of both metals were lethal to the cultures, but no effect on growth was seen in algae exposed to 0.1 ppm of Cd or 0.1 ppm of Cr. Scenedesmus quadricauda was more resistant than Chlamydomonas reinhardtii and Chlorella pyrenoïdosa to chromium and cadmium.
Radiotracers (51Cr and 109Cd) were used to trace the biosorption of chromium and cadmium by the three algal strains. Cr(VI) as K2 Cr2O7 and Cd as CdCl2 were added to the algal suspension culture to give concentrations of 1, 1.5 and 2 ppm Cd and Cr respectively. At various stages during growth of the unialgal cultures, samples were removed, a pulse of51 Cr or109 Cd was added, and the uptake rates of Cr(VI) and Cd were determined. To evaluate the amount of heavy metals removed by the algal cells, microscopic cell counting was carried out. All the experiments were conducted under the above-mentioned conditions for each concentration studied, the measurements were repeated three times and the mean values from the three replicates were calculated. The results showed that chromium and cadmium removal by algae was dependent on the metal concentration in the culture medium. The quantity of metal removed also varied with algal species, where 20 µg, 23 µg and 35 µg of Cd were accumulated by 106 cells of Chlorella, Chlamydomonas and Scenedesmus respectively at a metal concentration of 2 ppm. About 25 µg of Cr were accumulated by 106 cells of Chlorella and Chlamydomonas whereas, 106 cells of Scenedesmus accumulated 32 µg of Cr at a concentration of 2 ppm.
The experimental data presented here demonstrated that there was not a great difference in the uptake capabilities between the three algal strains, and reaffirmed the potential of algae in the purification of metal-contaminated wastewater. This wastewater treatment system has many advantages and it is an efficient way to significantly reduce the release of nutrients (e.g., phosphorus, nitrogen, ammonia) and heavy metals (e.g., chromium and cadmium).
Bilan du phosphore dans un bassin versant du lac Léman : Conséquences pour la détermination de l'origine des flux exportés
J. M. Dorioz, P. Quetin, J. A. Lazzarotto, J. P. Bosse and J. P. Moille
Cet article s'intéresse à la dynamique du transfert du phosphore dans un bassin rural de la région lémanique, le bassin versant du Foron (51,5 km2). Nous montrons qu'en établissant, pour les périodes de tarissement, la relation concentration moyenne en phosphore - débit moyen à l'exutoire, il est possible d'évaluer globalement:
1) la rétention du P dans le réseau hydrographique lors de ces périodes;
2) la contribution moyenne des sources ponctuelles de P aux exportations du bassin.
La rétention est un phénomène temporaire, mais c'est une étape clé de la dynamique du phosphore. Elle crée d'importants stocks de P dans le réseau hydrographique; ces stocks se situent pour l'essentiel au niveau des sédiments comme l'ont montré diverses études antérieures et sont exportés lors des crues. Les quantités stockées à l'issue de longs étiages dépassent souvent largement les exportations enregistrées pendant les plus fortes crues. La rétention s'accompagne d'une transformation partiellement irréversible de la spéciation du phosphore transféré.
Ces phénomènes sont pris en compte pour établir des bilans annuels de pollution, et notamment la balance diffus - ponctuel.
The effective management of phosphorus in watersheds requires knowledge of the origin and magnitude of phosphorus inputs. However, for large and complex watersheds, it is impossible to determine the phosphorus mass balances by measuring the flow from each source and the rate of transfer of phosphorus originating from upstream. Because of these difficulties, we have developed a strategy based on the measurement of phosphorus mass balances during base flow periods (with no storm-flow, which means with a constant or decreasing flow). This approach was applied to the Foron River watershed (drainage area 51.5 km2, annual average water discharge 0.5 m3 /s).
The Foron River, a hillside stream with a torrential flow regime, is a tributary of Lac Léman (a large meso-eutrophic lake also known as Lake Geneva). Land use within the Foron watershed is diverse, with 50% of the watershed area being in a naturally forested area (on steep upland slopes), 36% in agricultural land lying on a gentle relief of moraine deposits (70% permanent pasture and hay meadows, 30% cereal grain crops) and 3% in marshes. The population is scattered amongst four small villages, with an overall density of about 100 inhabitants per km2. Stream flow in the river was continuously monitored and automatically sampled at the outlet from 1990 to 1993. Additional grab samples were taken at various locations, springs, agricultural sub-watersheds and brooks in the naturally forested area. Total suspended solids, soluble total phosphorus, orthophosphate and total phosphorus were determined according to standard methods. In addition, sampling was carried out to determine phosphorus stored in river sediments in places that are known to be temporary sinks for sediments.
To validate the efficiency of the approach, we carried out a detailed study of the point sources and the sewage systems in the watershed. Only two-thirds of domestic wastewaters were processed in treatment plants. The minimum load of point source inputs was established to be about 60 kg total P per week and many point sources (that is to say discharged at discrete locations) remained unidentified (i.e., not all households were connected to sewers, farmsteads contributed for unknown quantities of total P).
During base flow periods, phosphorus was predominantly soluble and orthophosphate was found throughout the hydrographic network. Particulate phosphorus and suspended matter concentrations were negligible (respectively < 20 µg/l and <30 mg/l), were not related to flow discharge and originated mainly from point source discharges. The inputs from surface runoff on the watershed were also negligible, as reflected in the low concentrations of total phosphorus in the agricultural or forested sub-watersheds (less than 15 µg/L).
Total inputs of phosphorus into the river included not only this background phosphorus (evaluated to be less than 2 kg total-P per week), but also P mainly originating from point sources. Total phosphorus exported from the outlet varied from 20 to 110 kg total-P per week but was often much lower (up to 80%) than the minimum inputs due to point sources. During these periods phosphorus can accumulate in the hydrographic network, probably in the riverbed sediments and periphyton (macrophytes were not present).
The detailed pattern of phosphorus exports during these low flow periods showed that the amount of total phosphorus exported (90% soluble P) tended to increase with the weekly mean water flow, up to a water discharge rate of 0.8 m3 /s. Above this value, phosphorus export reached a plateau corresponding to a mean constant value (about 80 kg per week). Since weekly average inputs from point sources remained nearly constant, independent of stream flow, this increase in P export indicated an increase in the rate of transfer of total phosphorus from point sources discharged into the river. When the water discharge was > 0.8 m3 /s, a constant P export regime was reached as all the inputs were transported to the outlet. Consequently, output at the outlet was equal to input into the river. This approach permits the calculation of the total point sources. Knowing the inputs from point sources and the total P export at the outlet, we can calculate the diffuse sources.
Significant relationships between weekly mean total-P, soluble-P or orthophosphate concentrations at the outlet and weekly mean discharge were established to validate these observations. The best fit of these measured values didn't follow a standard law of dilution because the dilution effect was attenuated by the antagonist effect due to the increase in P transport inputs from sewage with increasing discharge. ([soluble P]=0.02+ 0.148 Q-1/2 ; n=82; r2 =0.8).
During base flow when Q < 0.8 m3 /s, a portion of phosphorus inputs accumulated in the river biota and sediments through deposition, biological uptake and sorption. This accumulation may explain the higher levels of total-P and the easily exchangeable P in the sediments downstream of point source discharges. The efficiency of these storage processes depends not only on water discharge, but also (secondarily) on some characteristics of the antecedent hydrological conditions. We found that the amount stored increased significantly in the weeks following large storm-flows. We assumed that this indicated the effect of an intense exchange of the P loaded sediments with unsaturated sediments that were generated by erosion of natural upstream areas. As in-stream processes alter phosphorus speciation, a part of soluble-P becomes particulate. During storm-flows following base flow periods with a discharge lower than the limit discharge, the stored phosphorus was released from both particulate and soluble forms and exported. We calculated an annual mass balance of soluble phosphorus to evaluate the amount of particulate phosphorus exported at the outlet due to the transformation of phosphorus from soluble to particulate forms. The result obtained suggested that ¼ of particulate-P (1 t per year) resulted from this speciation change.
To summarize, analyses of phosphorus mass balances during base flow could lead to a new procedure to evaluate the mean total mass of P originating from point sources, without referring to field surveys. In addition, this procedure allowed the quantification of both total-P storage as well as changes in speciation that occur during transfer of phosphorus in the hydrographic network.
A. Siala, S. Boufi, R. B. Amar and A. B. Salah
La présente étude porte sur le traitement physico-chimique d'un effluent industriel chargé d'encre de seiche issu de l'industrie de conditionnement des produits de la pêche. Différentes combinaisons de coagulant -floculant ont été étudiées. Le sulfate d'aluminium (SA), divers polyélectrolytes et certains adjuvants pulvérulents ont été utilisés. L'efficacité du traitement par coagulation-floculation a été analysée en suivant l'évolution de la turbidité et de la DCO résiduelle du surnageant.
Les résultats de cette étude ont révélé qu'à pH 6-7, l'addition de SA à une concentration supérieure à 1000 mg.l-1 entraîne la coagulation des particules d'encre de seiche. L'addition d'un polyélectrolyte en association avec le SA permet de réduire la concentration requise en SA à environ 300 mg.l-1, tout en améliorant l'efficacité du processus de coagulation-floculation. Le recours à un adjuvant pulvérulent en combinaison avec le SA et un polyélectrolyte accroît notablement les performances du traitement et permet d'obtenir un effluent ayant une turbidité résiduelle de 7 NTU.
The aim of this work was to compare the efficiency of a coagulation-flocculation process using different systems individually or in combination. The waste water used was an industrial cuttlefish effluent principally composed of colloidal particles from the cuttlefish. The initial turbidity and chemical oxygen demand were approximately 700 NTU and 22,000 mg×L-1 respectively. Two parameters were used to assess the process efficiency: the residual turbidity and the chemical oxygen demand (COD) of the supernatant.
Three systems were investigated: aluminium sulphate (SA), polyelectrolytes and a ternary combination of SA-polyelectrolyte and colloidal microparticle adjuvant. Results demonstrated that at pH 6-7, SA caused some coagulation of the suspension, but the dosage required (about 1000 mg×L-1) far exceeded those conventionally used in water treatment. As a consequence, an appreciable amount of SA, which was above the authorised limits, remained in the supernatant. Indeed, adsorption isotherm measurements revealed that at pH 6.5-7 the residual proportion of SA attained was about 20%. The addition of polyelectrolytes in combination with SA allowed reduction of the SA dosage to 150 mg×L-1. Amongst the polymers tested, the cationic polymer was the most efficient. The ability to bring about flocculation was positively related to the molecular weight, which is in agreement with a bridging flocculation mechanism. Starch, a natural, low-cost polymer was also an efficient flocculent even though it is a non-charged polymer. The ability of starch to bring about a flocculation action was explained by its relatively low solubility, which leads to some aggregation of macromolecular chains that act as anchoring sites for the coagulated ink particles. With this system, a double treatment in the presence of 150 mg×L-1 SA and 25 mg×L-1 polydiallyldimethylammonium chloride (PDMAC) or starch lead to a supernatant with a turbidity and COD of 150 NTU and 5500 mg×L-1 respectively.
The third part of our study was concerned with the effect of the addition of an insoluble microparticle adjuvant in combination with SA and polyelectrolytes. Different components were investigated, but among them only colloidal silica and MgO bring about a significant enhancement of the flocculation process. This effect was explained by a bridging mechanism in which the adjuvant microparticles act as sites that favour the anchoring of the coagulated ink particles. Using a triple treatment in the presence of 150 mg×L-1 SA, 25 mg×L-1 PDMAC and 0.75 g×L-1 MgO removed about 95 % of the ink particles. The supernatant obtained was characterised by a low turbidity value (about 7 NTU) and a COD value about 2000 mg×L-1.
Toxicité de l'ion fluorure envers les organismes d'eau douce et effets de la dureté - revue et nouvelle analyse de données existantes
A. Giguère and P. G.C. Campbell
Les plus importantes sources anthropiques de fluorures dans les systèmes d'eau douce comprennent les eaux usées municipales, les industries productrices de fertilisants et les alumineries. Plusieurs études montrent que la toxicité des fluorures est réduite lorsque le test toxicologique est réalisé en eau dure plutôt qu'en eau douce. Trois mécanismes peuvent être invoqués pour expliquer une telle tendance : (I) influence des ions de dureté (Ca2+ ; Mg2+) sur les organismes tests (soit au niveau de la barrière biologique séparant l'organisme de son milieu, soit au niveau de leur métabolisme interne); (II) complexation entre le fluorure et les ions de dureté dans le milieu d'exposition, menant à une réduction de la concentration en fluorure libre (F-); (III) précipitation de fluorite (CaF2) dans les milieux d'exposition, menant à une réduction de la concentration effective en fluorures. Pour identifier le ou les mécanisme(s) responsables de l'effet protecteur de la dureté, nous avons réalisé une revue de la littérature existante sur les poissons, les invertébrés et les insectes aquatiques d'eau douce. Parmi ces études, les plus complètes ont été sélectionnées et la spéciation des fluorures modélisée pour chaque cas. Les modélisations réalisées indiquent que la spéciation physique du fluorure (distinction entre les espèces dissoutes et particulaires) a beaucoup plus d'importance que sa spéciation chimique en solution dans les systèmes étudiés.
Important anthropogenic sources of fluoride to the aquatic environment include municipal waste waters and effluents from fertilizer producing plants and aluminum refineries. Many studies have demonstrated that fluoride toxicity to aquatic organisms is reduced when the toxicological test is performed in hard water rather than soft water. In principle at least three mechanisms could explain this trend: (I) a direct beneficial influence of the hardness cations (Ca2+ ; Mg2+) on the test organism (either externally, at an epithelial membrane, or internally); (II) complexation between fluoride and the hardness cations, leading to a reduction in free the fluoride concentration (F-) in the exposure media; (III) fluorite precipitation (CaF2) in the exposure media, leading to a reduction in the effective fluoride concentration. The present literature review was designed to identify which of these mechanisms might be responsible for the apparent protective effect of hardness on fluoride toxicity.
An inventory of the existing literature on the toxicity of fluoride to freshwater fish, invertebrates and aquatic insects was prepared. The most complete studies were selected and the chemical data needed to model cation and anion speciation in the exposure media were extracted from the papers. Speciation at equilibrium was then modelled using as input data the total concentrations of the key constituents (calcium, magnesium, fluoride and chloride), together with the temperature and the pH.
The initial speciation calculations revealed a particularity of the chemical systems studied: frequently precipitation of fluorite (CaF2) was predicted by the speciation model (MINEQL+), but the article from which the data had been extracted did not mention the appearance of a precipitate. Fluorite solubility, at a pH of 7 and at an ionic strength of 2.7 mM, is approximately 17 mg CaF2 /L (0.22 mM). When high fluoride concentrations are used in hard water, both fluoride and calcium concentrations are predicted to decrease markedly as a result of fluorite precipitation. This analysis of the published results thus suggests that the reported lower toxicity of fluoride in hard waters is likely due to the chemical precipitation of CaF2 and MgF2, resulting in a decrease in the effective fluoride concentration to which the test organisms are exposed. In other words, changes in the physical speciation of fluoride (i.e., its distribution among dissolved and particulate species) are much more important than changes in its chemical speciation in solution.
Given the low solubility of fluoride in hard waters, it would seem difficult if not impossible to carry out fluoride toxicity tests in hard water. However, in a few fluoride toxicity studies the researchers checked for precipitation by monitoring fluoride and calcium concentrations throughout the toxicity test. In some of these cases, even though speciation calculations predicted fluorite formation at the exposure concentrations used, the authors did not detect any precipitation; these systems were thus apparently in a metastable, over-saturated state, where the kinetics of precipitation were slow relative to the duration of the toxicity test. The chemical equilibrium software was used to simulate fluoride speciation in these systems, by allowing the over-saturated solid phases to remain in solution. In particular, we looked for evidence that under such circumstances the hardness cations exerted a beneficial effect. However, no clear picture emerged from this second analysis: two of the studies designed particularly to test the effect of calcium on fluoride toxicity showed a protective effect, whereas one study indicated the opposite effect, i.e. an increase in fluoride toxicity as the calcium concentration was raised. All fish studies for which calcium concentrations were available (N=20 studies; 58 toxicity tests) were pooled and tested for a possible calcium effect on fluoride toxicity. No relationship was observed between fluoride ion toxicity (LC50, expressed as calculated free [F-]) and calculated dissolved calcium concentrations for these pooled data (Fig. 2). Fluorite solubility was the most important factor influencing the data point distribution in the relationship. The same exercise was performed for all the invertebrate studies (N=11 studies; 22 toxicity tests) but again no relationship was found (Fig. 3).
Several factors other than hardness affect fluoride toxicity to aquatic organisms. Fluoride toxicity to fish increased with exposure duration up to 200 h, where it reached a threshold LC50 level around 100 mg/L (5.3 mM) expressed as free fluoride (Fig. 4). Fish life stage (Fig. 5), the temperature of the exposure media (Fig. 6) and the chloride concentration also affected fluoride toxicity in fish. For invertebrates, fluoride toxicity increased with exposure duration but to a lesser extent than for fish.
In summary, water hardness clearly reduces fluoride toxicity to aquatic organisms by limiting the equilibrium solubility of the fluoride ion. However, in those cases where the precipitation of CaF2 (s) and MgF2 (s) is slow, and where the hardness cations and fluoride co-exist in the dissolved state in the exposure medium, the experimental evidence for a protective effect of hardness on fluoride toxicity is equivocal. To answer the question, new experiments should be performed under conditions that take into account the behaviour of calcium and fluoride in the natural environment. Metastable environments where fluoride concentrations exceed the solubility limit imposed by CaF2 or MgF2 could be reproduced in laboratory toxicity tests by using continuous flow systems. For tests below the solubility limit, toxicity tests with varying levels of Ca or Mg could be designed to stay within the solubility range of CaF2 or MgF2. In both cases, dissolved calcium, magnesium and fluoride concentrations should be monitored throughout the toxicity tests.
Modélisation de l'évolution du couvert nival, sur le sous-bassin LG4 de la rivière La Grande dans le nord du Québec, à l'aide du modèle français CROCUS
S. Savary, J. P. Fortin, M. Bernier and E. Martin
Au Québec, l'accumulation de la neige au sol prend une grande importance dans bon nombre d'activités humaines comme la production d'hydroélectricité. Face à cette importance de la neige, il devient fort intéressant de faire appel à des outils de modélisation dans le but de faire le suivi complet du manteau neigeux par la simulation des différents processus physiques agissant sur le couvert nival. En ce sens, la présente recherche vise à évaluer la capacité d'adaptation aux conditions environnementales du Québec, du modèle français multicouches CROCUS de suivi et de fonte du couvert nival, en vue de son utilisation ultérieure. Le choix du site d'étude s'accorde avec la présence de stations d'acquisition de données sur le couvert nival et la disponibilité d'un large éventail de données météorologiques essentielles aux modélisations.
Suite au prétraitement des données d'entrée, le modèle CROCUS a pu être étalonné dans le but d'évaluer ses aptitudes à simuler l'évolution de l'épaisseur et de l'équivalent en eau du couvert nival, de même que les profils de densité et de température de ce couvert. Compte tenu des données disponibles, les résultats des périodes d'étalonnage et de vérification peuvent être considérés comme très bons et très encourageants pour des applications ultérieures de CROCUS au Québec.
In Québec, snow accumulation on the ground is important for a large number of activities, including hydroelectric production. Snow being an important environmental aspect to look at for operations in cold regions, making use of adequate models able to simulate the various physical processes affecting the characteristics of the snowpack throughout the entire snow season becomes also quite important. So, the present project aims to evaluate the ability of the CROCUS French multilayer snow model to be adapted to Northern Quebec environmental conditions. The choice of the study area has been done on the basis of data availability on both the temporal evolution the snowpack at three automatic stations and the meteorological variables necessary as input to the model. Following preprocessing of input data, the CROCUS model has been calibrated in order to evaluate his capability to simulate the temporal evolution not only of the thickness and water equivalent of the snowpack, but also of the density and temperature profiles within the pack. Given the available data, the results for both the calibration and verification periods can be considered as good and very encouraging for future applications of the CROCUS model in Québec.
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