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The anthropogenic alteration of the nitrogen cycle results in the modification of the whole food web. And yet, the impact caused on nitrogen dynamics in marine systems is still very uncertain. We propose a workflow to evaluate changes to coastal nitrification by modelling nitrite dynamics, the intermediary compound. Nitrite concentrations were estimated with a simple steady state nitrification model, which was calibrated in 9 NW Mediterranean coastal sites with different anthropogenic pressures, located within 250 km. The results obtained indicate that nitrite peaks are observed in winter and explained by nitrification response to temperature, but these dynamics are altered in impacted coastal waters. We found the second step of nitrification to be more sensitive to temperature, which entails a significant impact of climate change on the decoupling of the two steps of nitrification. The results could be extrapolated to numerous coastal regions of the Mediterranean Sea with similar characteristics.