Abstract
From 2005 onwards, a baseline monitoring programme was carried out to assess whether the construction of offshore wind farms induced changes in the soft‐sediment endobenthos in the Belgian Part of the North Sea. The natural, temporal variability of the endobenthos was detected without any apparent effects from the wind farms. To understand underlying processes and detect future effects, a targeted (small‐scale) sampling strategy was initiated in 2010, alongside the baseline monitoring activities. The sandy offshore sediments near one gravity based foundation (GBF) can be altered due to (1) previously created pits, (2) changing hydrodynamics, (3) organic enrichment and (4) prohibition of beam trawl fisheries. Benthic samples were therefore taken at 1, 7, 15, 25, 50, 100 and 200 meters from the scour protection system and along four gradients, in order to stress the importance of currents around the GBF. In 2010, high mean total densities and macrobenthic biomass were detected at one and seven meters from the GBF. The natural occurring Nephtys cirrosa community shifted to a community dominated by Monocorophium acherusicum, Lanice conchilega and Spiophanes bombyx. One year later, higher macrofaunal densities were also observed at 15 and 25 meters from the GBF and this primarily on the Southwest gradient. Furthermore, a decline in median grain size was observed between 2010 and 2011, suggesting changing hydrodynamics around the GBF. These results illustrate the importance of dedicated small scale and short term investigations in national monitoring strategies for offshore wind farms as they permit a better understanding of the large scale observations.