Offshore wind turbines constitute benthic secondary production hotspots on and around constructions

In response to climate change, the expansion of renewable energies leads to an increasing number of offshore wind farms in the North Sea. This comes along with an increase in (artificial) hard substrates in a mainly soft-bottom dominated marine area with so far largely unknown consequences for the underlying ecosystem functioning. We used a large combined dataset (both hard- and soft-substrate data) to model the secondary production of fouling communities on turbine foundations and of soft-bottom fauna inside and outside offshore wind farms (OWF) in the southern North Sea (Belgium, the Netherlands, Germany). We demonstrate that (1) a large amount of energy is channelled through fouling fauna on turbines (i.e., secondary production of fouling communities was on average 80 times higher than of soft-substrate communities), (2) 71 % of fouling production on turbines is released to the surrounding sediment (annual release: -221 ± 825 gC m^-2 y^-1 (SD)), and that (3) local production of soft-bottom communities is elevated up to a distance of 150-250 m from turbines. Production impacted area (PIA) was determined from hard- and soft-substrate data independently: mechanistic modelling of hard-substrate production export showed a production increase of 5 % up to 150 m from the turbine and generalised additive mixed models (GAMMs) based on soft-bottom fauna data suggested an elevated production up to 250 m from turbines. Accordingly, on the scale of an OWF (distance between turbines ∼1000 m), the local production "halo" effect around turbines affects about 11 % of an OWF area (dependent on OWF configuration). The observed changes in benthic energy flow may lead to so far unknown changes at the ecosystem level from plankton communities to apex predators.