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Les lacs Tanganyika et Malawi sont, de par leur volume, les deux plus grands lacsafricains. Ces réservoirs semblent pour l'instant épargnés par la pollution en éléments entrace. Il est toutefois crucial, en raison de leurs caractéristiques hydrologiques, de poserla question du temps de réponse de ces systèmes à une pollution chronique potentiellevéhiculée par les affluents. Cet article simule ainsi cette réponse dans la fractiondissoute suite à l'introduction pendant 50 ans de polluant par tous les affluents. Cettedémarche s'appuie sur un modèle hydrologique intégrant les trois compartiments des colonnesd'eau (épi-, méta- et hypolimnion) et sur la prise en compte de la réactivité des élémentsdissous dans ces compartiments par l'intermédiaire du taux de rétention élémentaire. Ainsiquatre types d'éléments sont considérés, (i) le type Cl, non réactif, (ii) le type Si,réactif-nutritif, (iii) le type Mn et (iv) le type V tous deux réactifs sensibles auxconditions d'oxydo-réduction. La réactivité de l'élément, l'efficacité du mélange verticalainsi que la position de l'oxycline dans la colonne d'eau conditionnent l'amplitude et lacinétique de réponse des systèmes ainsi que le temps de retour à la situation initiale aprèsl'arrêt des apports polluants. Ces caractéristiques propres à l'élément et au lac influentsur le risque potentiel encouru par l'écosystème et l'homme. Ainsi la pollution affecteprincipalement les eaux de surface (types Cl et V), les réseaux trophiques (type Si), leseaux profondes (types Si et Mn) et le compartiment sédimentaire (types Mn et V).Lakes Tanganyika and Malawi are the largest African lakes as measured by volume. Theyconstitute essential water and protein resources for the surrounding populations. Theseaquatic systems have become stressed due to high human population density, growth andassociated activities. While eutrophication was apparent locally and organic pollutants weredetected in fish and water, concentrations of several dissolved trace elements of potentialconcern corresponded to uncontaminated systems. However, due to their hydrological features,it was important to characterise the lake response time to chronic contamination loaded bythe tributaries. This paper presents two simulations of this response, in the dissolvedfraction, following 50 years of pollutant input by the tributaries. The first simulationcorresponded to an annual pollutant input that was the same for both lakes, resulting inmean river input concentrations of 5.0 U L-1 and 3.7 U L 1 (where U is a weight or molarunit), respectively, for lakes Tanganyika and the Malawi. The second simulation correspondedto an annual input proportional to the lake volume, with mean river input concentrations of5.0 U L 1 and 1.5 U L 1, respectively, for lakes Tanganyika and the Malawi. The pollutedinput was loaded by the dissolved fraction with the exception of Mn-type elements, whichwere carried by the particulate fraction. This approach was based on an annual hydrologicalmodel of three water column compartments (epi-, meta- and hypolimnion) of these meromicticlakes. In addition, the reactivity of dissolved elements in the water column was taken intoconsideration. The reactivity was characterised by the elemental retention rate thatquantifies dissolved-particulate interactions linked to biological and physico-chemicalprocesses. The reactivity of trace elements was assessed through their concentrationdistribution profile in the water column. Four element types were considered: thenon-reactive elements characterised by homogenous concentrations in the water column(Cl-like); the micronutrient-type elements (Si-like) characterised by a strong positiveconcentration gradient below the thermocline; redox-sensitive elements (Mn-like)characterised by a strong positive concentration gradient below the oxycline and otherredox-sensitive elements (V-like) characterised by a strong negative concentration gradientbelow the oxycline. Trace elements (F, Al, Fe, Mn, V, Ba, Sr, Mo, Cr, Ni, Co, Cu and Pb) inboth lakes were associated with these element types but they did not necessarily belong tothe same type in both lakes. Other elemental types likely occurred (e.g., carbonate type andFe types) but they were not clearly identified. After 50 years, surface concentrationsranged from 0 to 1.15 U L 1 in Lake Tanganyika and from 0 to 2.40 U L 1 in Lake Malawi. Thedifference between the lakes was linked to the greater volume of Lake Tanganyika, mainly inits hypolimnion, and to the longer vertical water exchange time in Lake Tanganyika. ForCl-type elements the concentration response decreased for both lakes from the epi- to thehypolimnion with similar kinetics for the epi- and metalimnion and a delay for thehypolimnion. For Si-type elements the response decreased in Lake Malawi from the hypo- tothe epilimnion and for Lake Tanganyika the maximal concentration was calculated in themetalimnion. The concentration range was higher in Lake Malawi than in Lake Tanganyika. Forthe Mn-type elements, the maximum concentration was calculated in the hypolimnion with ahigher response in Lake Malawi. The metalimnetic water concentration of Lake Tanganyikaincreased slightly and epilimnetic and metalimnetic waters of Lake Malawi did not react. ForV-type elements the epilimnetic waters were more sensitive to the increase, with a higherresponse for Lake Malawi. In Lake Malawi concentrations also increased in the metalimnion.Concentrations in the hypolimnetic zone of both lakes and metalimnetic zone in LakeTanganyika remained zero. Depending on the element type and on the lake, the time requiredto return to initial conditions, when contaminant inputs stopped, varied from 30 to 7 300years. In the epilimnetic zone of both lakes the intensity of reaction and the pollutionpersistence were higher for Cl-type elements. For Si-type elements, mainly in Lake Malawi,the vertical input from deep waters was sufficient to sustain productivity even after theinput of pollutants was stopped. For these elements the dissolved contamination was mainlystored in deep waters. For Mn-type elements the contamination was also stored in deep waterswith a relatively slow net transfer to the sedimentary compartment. V-type pollutants weretransferred from the dissolved to the particulate phase in deep waters leading to arelatively rapid net transfer to the sediment. Once the pollutant was in the system anduntil its evacuation to the outlet or to sediment, the risk for the ecosystem and for thepopulation was associated with its presence in the dissolved phase of the surface water. Therisk was then higher for Cl- and V-type elements as well as for the Si-type elements thatwere introduced into the web food. For the Si- and Mn-type elements that were mainly storedin deep waters, the associated risk was linked to a breaking of the thermo-halinestratification or to a reinforcement of vertical mixing. For the V-type elements and alsofor the sedimentary fraction of the Mn-type elements, the risk was also associated withpossible remobilization from the sediments due to physico-chemical changes at thewater-sediment interface.Element reactivity, efficiency of the vertical mixing and the depth of the oxyclinecontrol the importance and the kinetic response. They also controlled the time to attaininitial conditions once contaminant inputs were stopped. These features, relative to theelement and to the lake, were key parameters in the assessment of the potential risk forboth the ecosystem and people that rely on these lakes. Even if the elemental typology wasthe same for both lakes, elements can be considered a different type from one lake toanother. Contamination from the same pollutant would then have different consequences, forinstance regarding the associated risk. Computed hydrochemical budgets were simple butrealistic, illustrating the behaviour of elements in the water column. Computation of thisbudget requires the knowledge of global water column fluxes, which have to be improvedmainly for Lake Tanganyika. The element's reactivity was mainly linked to liquid-solidreactions. It would be interesting in future studies to characterise particulate phases andtheir reactivity and to introduce such processes in hydro-geochemical models. Computationsof chronic contamination response indicate that for both lakes, due to the inertia of thehydrochemical system, the lack of lake water contamination does not imply a systematic lackof pollution in the tributaries. Once pollution is detected, it will be persistent. A globalwatershed monitoring program should be organised in the near future. Monitored parametersshould be relevant to metallic and organic pollutants, as well as eutrophication

Sensibilité à la pollution métallique de deux grands lacs africains (Tanganyika et Malawi)