-(Effects of nutrient impoverishment on phytoplankton biomass: a mesocosms experimental approach in a shallow eutrophic reservoir (Garças Pond), São Paulo, southeast Brazil). Nutrient impoverishment in mesocosms was carried out in a shallow eutrophic reservoir aiming to evaluate the nutrient removal technique as a method for eutrophication reduction. Garças Pond is located in the Parque Estadual das Fontes do Ipiranga Biological Reserve situated in the southeast region of the municipality of São Paulo. Three different treatments were designed, each consisting of two enclosures containing 360 liters of water each. Mesocosms were made of polyethylene bags and PVC pipes, and were attached to the lake bottom. Treatment dilutions followed Carlson's trophic state index modified by Toledo and collaborators, constituting the oligotrophic, mesotrophic, and eutrophic treatments. Ten abiotic and 9 biological samplings were carried out simultaneously. Trophic states previously calculated for the treatments were kept unaltered during the entire experiment period, except for the mesotrophic mesocosms in which TP reached oligotrophic concentrations on the 31 st day of the experiment. In all three treatments a reduction of DO was observed during the study period. At the same time, NH 4 + and free CO 2 rose, indicating decomposition within the enclosures. Nutrient impoverishment caused P limitation in all three treatments during most of the experiment period. Reduction of algal density, chlorophyll a, and phaeophytin was observed in all treatments. Competition for nutrients led to changes in phytoplankton composition. Once isolated and diluted, the mesocosms' trophic state did not change. This led to the conclusion that isolation of the allochthonous sources of nutrients is the first step for the recovery of the Garças Pond.Key words -biomass, impoverishment, mesocosms, phosphorus, phytoplankton RESUMO -(Efeitos do empobrecimento de nutrientes sobre a biomassa fitoplanctônica: estudo experimental (mesocosmos) em reservatório eutrófico raso (Lago das Garças), São Paulo, Brasil sudeste). Empobrecimento de nutrientes foi realizado em mesocosmos localizados em reservatório eutrófico raso visando testar a técnica de remoção de nutrientes como método para diminuir o teor de eutrofização. O Lago das Garças situa-se na Reserva Biológica do Parque Estadual das Fontes do Ipiranga, região sudeste do Município de São Paulo. Foram delineados três tipos de tratamentos, cada qual composto por dois mesocosmos contendo 360 litros de água. Os mesocosmos foram construídos com sacos de polietileno transparente e canos de PVC e fixados próximos à região litoral do reservatório. As diluições dos tratamentos seguiram o Índice de Estado Trófico de Carlson modificado por Toledo e colaboradores, constituindo os tratamentos oligotrófico, mesotrófico e eutrófico. Foram realizadas, simultaneamente, 10 amostragens abióticas e nove bióticas. As trofias previamente estabelelecidas mantiveram-se imutáveis nos mesocosmos durante a maior parte do período...