Agriculture and municipal wastewater are the principal sources of eutrophying nutrients in many water ecosystems. We develop a model which considers the characteristics of agricultural and municipal nutrient abatement. The model explicitly accounts for the investment needed to set up wastewater treatment facilities, and makes it possible to determine the optimal timing of investment as well as the optimal agricultural and municipal abatement levels. We apply the model to the Finnish coastal waters of the Gulf of Finland. Our results indicate that substantial savings in abatement costs and the damage associated with eutrophication could be obtained by constructing the facilities needed to process all the wastewaters entering the coastal ecosystem. The optimal timing of investment is shown to hinge on both the economic and ecological characteristics of the ecosystem.