Zooplankton is a key group in North Atlantic and Arctic food chains, and assessment and minimization of adverse effects from petroleum activities to this resource are important. The potential direct effects of produced water discharges on the biomass of Calanus finmarchicus were evaluated using a fully coupled, high resolution 3D hydrodynamic-ecological model system (SINMOD). Several scenarios with varying effects of produced water concentrations were considered. In order to reduce numerical dilution of the produced water effluents, a "sub grid" model component of higher resolution (80 m horizontal resolution vs 800 m for the main model grid) was developed and implemented. The results show that dilution and dispersion of produced water varies between locations. In general, realistically simulated concentrations of produced water were too low to have significant effects on the C. finmarchicus biomass and reproduction according to the toxicity-dilution profiles used, even when the toxicity of the produced water was increased 10-fold. The decrease in C. finmarchicus biomass was partially compensated by a slight increase in production.