Rivers transport large amounts of nitrogen to coastal waters. This has a possible negative influence on coastal ecosystems. To control these problems, environmental policy plans increasingly focus on reducing the nitrogen load in rivers. However, to what extent nitrogen inputs need to be reduced is not easily indicated. Critical loads, reflecting the amount of nitrogen that can enter the coastal waters without negative impacts, may be used as a basis for environmental targets. The purpose of this study is to analyse the extent to which critical loads are exceeded in the Ebrié Lagoon (Ivory Coast). For this, we applied a mass balance approach. The results indicate that the nitrogen loads exceed the critical loads in almost every part of the lagoon. To reduce the nitrogen loading of the lagoon to the critical level, reductions in total nitrogen inputs of at least 21% may be needed compared to current levels. However, within the lagoon we identified regions where larger reductions (up to 80%) are needed to avoid critical concentrations being exceeded. The use of a mass balance approach could also be useful in other data-poor regions.