In Europe, an increasing number of offshore wind farms (OWFs) is being installed to combat climate change, resulting in local shifts in structural aspects (species, density, biomass) of biodiversity. However, there is no information available on functional changes in the marine ecosystem, hampering the assessment of the cumulative effect of multiple OWFs in larger geographical areas. The installation of wind turbines comes with a loss of soft sediment and associated ecological functioning (mineralisation of organic matter). Here, we estimated the loss and gain of ecosystem functioning related to the loss of permeable soft sediment and the increase in hard substrate due to the installation of a single wind turbine in the Belgian part of the North Sea. Closed core sediment incubations show that ammonium effluxes from the permeable sediments reach 0.44 mmol m-2 day-1, corresponding to a loss of ammonium production of 1064 mmol per day per turbine. Subsequent experiments, with the dominant fouling organism Mytilus edulis result in estimates of 1360 mmol ammonium produced per turbine per day, which is a net 28% increase. This ammonium excretion by the fauna is fuelled by their filtering activity to sustain their metabolism. Based on clearance rate experiments with dominant fouling organisms on offshore wind turbines (M. edulis, Jassa herdmani and Metridium senile), we estimate that the fouling fauna of a single turbine filters about 760000l water per day. The removed pelagic organic matter is released as faecal pellets causing an increase in organic matter and a fining of the sediments in the immediate vicinity of the turbine, with a feedback loop to decreased mineralisation efficiency due to a decreased penetration depth of advective water currents and a reduced trapping of diatoms. As such, it is clear that the installation of OWF generates functional changes to be taken into account when assessing the effect of OWFs.