Environmental conflicts of interest are important to account for when environmental policies are designed. This paper explores the quantitative connection between urban waste water treatment, coastal eutrophication, and fish biomass in the mesotrophic Gulf of Riga (northern Europe). The probable effect on the water quality from one clearly defined abatement measure, improved urban sewage treatment has been studied. Furthermore, the implementation cost and the likely effect on total fish biomass

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... have also been assessed. Computer simulations using the previously published model CoastMab suggested that good water quality according to the EU Marine Strategy Framework Directive could be achieved if urban sewage treatment would be upgraded to Nordic and German standards, and not only around the Gulf of Riga but in the whole Baltic Sea drainage basin. The Secchi depth would double according to these simulations while total phosphorus and summer chlorophyll concentrations would decrease by 54% and 53%, respectively. The total fish biomass should be expected to decrease by about 42% if "good" water quality (as defined in European Union directives) should be achieved. However, changes in total fish biomass could also be offset by changes in other important determinants such as climate related variables or fishing pressure. The study estimated that it could take about 20-40 years after abatement action for the trophic state in the Gulf to stabilise again. Upgrading urban sewage treatment to this extent would cost 468-1,118 million euros per year. Treatment could have substantial positive effects on the water quality of the Gulf but could also have adverse side effects on the total fish biomass.