The process of reservoir eutrophication has been recognised as a central problem in tropical reservoir environmental quality. Effects of eutrophication are complex interactions involving a decrease in water quality, especially loss of aquatic biodiversity, occurrence of undesired species such as cyanobacteria with its cyanotoxins, mass development of macrophytes such as Egeria densa with its mechanical impact on turbines, and an increase in greenhouse gas emissions, mainly of methane. The eutrophication process can be described by the OECD critical load concept or related models. The phosphorus use efficiency is given by the Chl a–P – relationship, indicating eutrophic conditions by only 10 μg L−1 P in Itaparica Reservoir, Brazil. Eutrophication of the reservoir is quantified for internal phosphorus sources (inflow, mineralisation of inundated soils and vegetation, net cage aquaculture) and external ones (agriculture, emissions of natural caatinga vegetation and rural communities) The actual internal P load is calculated to be 0.40 g m−2 a−1, and the critical P load is given with 1.20 g m−2 a−1. The external P load amounts about 1.16 g m−2 a−1 and thus exceeds the critical export rate of 7.1 kg km−2 a−1 by 50%, thus a bundling of measurements has to be considered when attempting to promote re-oligotrophication.