Les plans d’eau, du «bas delta» du fleuve Sénégal sont de plus en plus confrontés aux problèmes d’efflorescences à cyanobactéries qui menacent la ressource en eau des États riverains. Une étude, couplant approches in situ et expérimentales, a été menée pour analyser les principaux facteurs biologiques qui interviennent dans l’apparition et le contrôle de ces phénomènes. L’étude in situ a permis d’analyser la distribution spatio-temporelle du phyto et du zooplancton et les relations entre ces communautés dans 13 sites représentatifs à différentes saisons. L’importance croissante, des cyanobactéries et notamment d’espèces à potentiel toxique comme Cylindrospermopsis raciborskii, Anabaena solitaria, Anabaena flos-aquae et Microcystis aeruginosa a été mise en évidence. Certains aspects des communautés zooplanctoniques (avantage des cyclopoïdes sur les calanoïdes, présence-abondance d’espèces indicatrices, faibles biomasses par rapport au phytoplancton) indiquent une tendance à l’eutrophisation des plans d’eau. Une forte association entre communautés phyto et zooplanctoniques a été mise en évidence par des analyses multivariables. Une étude expérimentale sur le broutage a montré que les cyanobactéries filamenteuses sont consommées et/ou fractionnées avec une efficacité variable selon les brouteurs et que le zooplancton est un facteur important de contrôle descendant des cyanobactéries filamenteuses (jusqu’à 50% du stock par jour). Des expériences de toxicité ont clairement mis en évidence des effets négatifs des cyanobactéries sur la survie et la croissance du zooplancton, mais avec une sensibilité différente des espèces aux toxines. Des dégradations musculaires et intestinales ainsi que des modifications de l’activité de l’acétylcholinestérase ont été mises en évidence chez des copépodes mais plusieurs éléments suggèrent que les effets ne seraient pas liés aux cyanotoxines classiques, mais plutôt liés à la présence d’autres métabolites émis soit par les cyanobactéries, soit par la flore associée puisque les cultures utilisées n’étaient pas axéniques. / The lower delta of the Senegal river faced with problems linked to cyanobacterial bloom which threaten the water resource of The riverine countries. A study, coupling in situ and experimental approaches, was carried out to analyze the biologicalfactors which intervene in the appearance and the control of these phenomena. The in situ study made analyzed the space time distribution of the phyto- and the zooplankton communities and their relationships in 13 representative sites at various seasons. The increasing importance, of the cyanobacteria and in particular of species with toxic potential like Cylindrospermopsis raciborskii, Anabaena solitaria, Anabaena flos-aquae and Microcystis aeruginosa was highlighted.Certain aspects of the zooplanktonic communities (importance of cyclopoïds compared to calanoïds, presence-abundance ofindicating species, low zooplankton biomasse compared to the phytoplankton) indicate a trend towards eutrophication. Astrong association between phyto- and zooplankton communities was highlighted by multivariable analyzes. Grazing experiments showed that the filamentous cyanobacteria are consumed and/or split with a variable efficiency according to the grazers and that the zooplankton exert a strong top-down control of the filamentous cyanobacteria (up to 50% of stock perday). Toxicity experiments clearly highlighted negative effects of the cyanobacteries on zooplankton survival and growth, butwith a variable sensitivity according to the species. Ultrastructural deformations and muscular and intestinal degradations aswell as modifications of the acétylcholinestérase activity were highlighted for copepods, but several elements suggest thatthe effects would not be related to the traditional cyanotoxins, but rather related to the presence of other metabolites emittedeither by the cyanobacteria, or by the associated flora since the cultures used were not axenic.

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O arsênio está amplamente distribuído no ambiente, sendo encontrado sob formas orgânicas e inorgânicas. É classificado como cancerígeno humano, com base em estudos epidemiológicos. Sua presença na água potável é relacionada com efeitos à saúde como hipertensão, diabetes, câncer de pele e de órgãos internos. Devido aos efeitos adversos observados em populações expostas ao arsênio é interessante dispor de uma revisão atualizada sobre os aspectos toxicológicos deste elemento. Para a revisão foram utilizadas 308 publicações do período 1920 a 2003, considerando quatro temas: presença de arsênio no ambiente, toxicologia do arsênio, efeitos na saúde humana e biomarcadores para arsênio. As informações compiladas mostram que são necessários mais estudos sobre as formas químicas e biodisponibilidade do arsênio, temas identificados como pontos críticos para um melhor entendimento da toxicologia do arsênio. Também é sugerido o desenvolvimento de biomarcadores validados e específicos para sua utilização no processo de avaliação de risco do arsênio para humanos. / Arsenic is widely distributed in the environment in the form of organic and inorganic compounds. Based on epidemiological studies, arsenic is classified as a carcinogenic substance. Several disorders (e.g., hypertension, diabetes, skin cancer and internal cancer) have been related to arsenic in drinking water. Thus the adverse effects of arsenic on exposed populations supports the idea of preparing a review on its toxicology and 308 publications from 1920 to 2003 were revised for this purpose. The review is focused on four selected subjects: arsenic in the environment, toxicological aspects, effect on human health and biomarkers for arsenic. From this study, speciation and bioavailability were identified as critical topics requiring further research for improving our understanding on arsenic toxicology. The development of validated and specific biomarkers for using in arsenic risk assessment on humans beings is also an important suggestion.

O arsênio está amplamente distribuído no ambiente, sendo encontrado sob formas orgânicas e inorgânicas. É classificado como cancerígeno humano, com base em estudos epidemiológicos. Sua presença na água potável é relacionada com efeitos à saúde como hipertensão, diabetes, câncer de pele e de órgãos internos. Devido aos efeitos adversos observados em populações expostas ao arsênio é interessante dispor de uma revisão atualizada sobre os aspectos toxicológicos deste elemento. Para a revisão foram utilizadas 308 publicações do período 1920 a 2003, considerando quatro temas: presença de arsênio no ambiente, toxicologia do arsênio, efeitos na saúde humana e biomarcadores para arsênio. As informações compiladas mostram que são necessários mais estudos sobre as formas químicas e biodisponibilidade do arsênio, temas identificados como pontos críticos para um melhor entendimento da toxicologia do arsênio. Também é sugerido o desenvolvimento de biomarcadores validados e específicos para sua utilização no processo de avaliação de risco do arsênio para humanos. / Arsenic is widely distributed in the environment in the form of organic and inorganic compounds. Based on epidemiological studies, arsenic is classified as a carcinogenic substance. Several disorders (e.g., hypertension, diabetes, skin cancer and internal cancer) have been related to arsenic in drinking water. Thus the adverse effects of arsenic on exposed populations supports the idea of preparing a review on its toxicology and 308 publications from 1920 to 2003 were revised for this purpose. The review is focused on four selected subjects: arsenic in the environment, toxicological aspects, effect on human health and biomarkers for arsenic. From this study, speciation and bioavailability were identified as critical topics requiring further research for improving our understanding on arsenic toxicology. The development of validated and specific biomarkers for using in arsenic risk assessment on humans beings is also an important suggestion.