This review of published and unpublished information demonstrates that offshore wind
farms (OWFs) have major effects on the benthos; that is, the seabed flora and fauna. By
adding artificial hard substrata to the marine ecosystem, OWFs create new habitat for
colonising benthic species, allowing attachment and attraction of hard-substratum species,
in ‘the artificial reef effect’. The general exclusion of fisheries further creates flourishing
soft-sediment benthic communities. Although wind farms hardly extend the distribution
range of hard-substratum species, they may be stepping stones for non-indigenous
and nuisance species. Such an increase in benthic diversity, however, is countered by
the loss of, disturbance to and/or alteration of the natural seabed. Despite this, it may
be concluded that OWFs create local hotspots of benthic diversity, directly influencing
the local marine food web. During construction, the biomass of forage species decreases,
affecting predatory and scavenging species negatively and positively, respectively. Mobile
predatory species tend to leave the area during construction. Once installed, the flourishing
benthic communities greatly increase in benthic foraging species and attract predators.
The surrounding natural sediments are affected by the deposition of organic matter from
the epibionts on the turbine monopoles and scour protection and by the altered predator
community. Given that a new ecological equilibrium in the benthic system will develop
over 20–30 years, it is arguable whether a return to the pre-construction state following
full decommissioning would be feasible or desirable. In contrast, a ‘renewables-to-reefs’
decommissioning scheme involving only partial removal of the wind farm could ensure
protection for ecologically valuable sites. While many data already exist, it is difficult to
detect significant effects because these are proportional to the degree of change and the
changes may take place at different spatial scales. This should be taken into account in
OWF monitoring.
Benthic communities are of significant ecological and socio-economic importance at a
global level. This includes acting as habitat for numerous species at all life-cycle stages,
and as a feeding ground for a range of predators. From a socio-economic perspective, these
predators can include species of commercial importance. Benthic communities operate
both directly and indirectly as food resources for such species. Therefore, the study of the
benthic environment around any activity in the marine environment, including offshore
renewable energy, is vital to identify potential effects and their significance.
All human activities in the marine environment have the potential, by their very
nature, to affect its natural structure and functioning. Because of both the direct effects on
the seabed and the intimate links between the water column and benthos (Gray & Elliott
2009), the seabed will always be directly or indirectly affected. The effects of offshore
wind farms (OWFs) on seabed communities comprise one of the most important elements
when considering the potential impacts of such developments, due to the inevitability of
effects arising, especially from monopiles bored into the substratum or gravity supported
on the seabed, their surrounding erosion protection layer and the installation of cable
routes (Wilson et al. 2010). Even developments in floating wind technology still require
anchor points and the connection of associated infrastructure, such as inter-array and
export cables (e.g. Butterfield et al. 2005; Statoil ASA 2017; see Chapter 1 in this volume).
Therefore, an understanding of the ways in which seabed communities are affected by
OWFs is vital, in part, so that appropriate mitigation measures can be identified and
deployed.
A set of key hypotheses have been generated for this chapter:
•Changes in seabed ecology as a result of installing a wind farm in the marine
environment are viewed as neither positive or negative in ecological terms. but just
different.
• The inherent variability of the seabed biota and hydrodynamic conditions may
prevent the subtle effects of OWFs being detected, in particular in current wind-farm
locations around the North Sea.
• Hard structures associated with OWFs are available as colonisation sites and
‘stepping stones’ for non-indigenous species.
• A focus on the structure of the benthos rather than its ecological functioning does not
satisfactorily assess impact.
• The effect of a wind-farm structure on the seabed is mirrored by an effect of the
seabed and its biota on the structure.
• Given the many human activities and pressures, there are in-combination synergistic
and antagonistic effects of all aspects of the same development, and cumulative
effects of different developments in the same area, which need to be disentangled.
• Location provides opportunities (for habitat creation) as well as threats (to the local
biota and habitats), and both need to be considered together.
• Climate change will increase the variability of an already highly variable system,
making it increasingly difficult to detect the effects of the wind farm and its structures