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ous nature may, however, only arise in cases where
cables are drawn across habitats that include
threatened or habitat forming (e.g. coral, seagrass)
species (e.g. Duarte et al. 1997, Marba et al. 1996).
Cables could increase the temperature of the surrounding
water and seabed. The only study that
to the authors´ knowledge dealt in depth with this
issue, estimated the increase in temperature of the
sediment above a buried cable to be insignificant;
approximately 0.2 C°, and the increase in seawater
temperature would only be 0.000006 C° (BERR
2008). However, cumulative effects of substantial
numbers of turbines, and localised effects of much
greater elevation of temperature in sediments surrounding
cables need to be evaluated.
Conclusions
Although re-suspension of particulate material can
increase mortality of fish, larvae and eggs, impacts
should be local and short-term, and should in most
cases cause small impacts on whole fish assemblages.
Research on sensitivity of fish, including larvae
and eggs, to sediment loads and sediment dispersal
is relatively advanced. Certainty: 4.
Impacts on invertebrates are less studied; Certainty:
3. Careless siting of turbines could affect threatened
species with narrow distribution ranges, but
generally impacts on benthic species assemblages
in the area as a whole would be local, short-term
and small.
5 Impacts on hydrodynamics and
changed nutrient transports
Wind power structures will affect water flow, and
this will be critical to marine organisms since it
influences larval recruitment, sedimentation, the
availability of food and oxygen and the removal of
waste (Breitburg et al. 1995; Snelgrove & Butman
1994, Zettler & Pollehne 2006). Recent results from
analytical modelling suggested that wind wakes created
by large wind farms could generate significant
up-welling or down-welling velocities in the vicinity
of farms even at quite moderate wind speeds
(Broström 2008). This could affect nutrient transport
and the local ecosystem as a whole. However,
no field observations confirming the model have
yet been reported.
The operational phase is likely to have chronic
effects on the nature of subtidal sediments. Most
wind-turbine developments are in shallow water
with predominantly mobile seabeds. There may
be localised erosion of unconsolidated material
due to scour around the toe of the structure; in
some locations this can be extensive resulting in
depressions several meters deep around the base
and influencing sediments up to 25 m from the
structure itself (Wallingford 2005). These changes
in sediment characteristics influence the associated
infaunal and benthic communities (Martin et
al. 2005, Schröder et al. 2006) and nutrient regeneration
(Danovaro et al. 1999, Maar et al. 2009)
around the structures. The extent of erosion by
scour can be reduced by the introduction of rock
42 GREENING BLUE ENERGY - Identifying and managing biodiversity risks and opportunities of offshore renewable energy
armour or anchored polypropylene fronds (1–1.5
m in length) to stabilise sediment, although these
additions will also have effects on the marine life
(see below).
The influence of sheer stress on the transport of
sediment and the subsequent effects on sediment
characteristics and the associated benthic community,
especially organisms living within the sediment,
are well described (Ong & Krishnan 1995, Joschko
et al. 2004, Schröder et al. 2006). Also, where water
movement is slowed there will be increased deposition
of suspended material. The entrapment and
deposition of organic matter, including material
that originates from fish and sessile organisms on
and around an artificial reef, can provide a source
of food for the benthic community up to 40 m away
and cause localised changes in composition and
production of macro-invertebrate assemblages as
well as chemico-physical parameters adjacent (up
to ~1m) to the structure (e.g. Bray et al. 1981, Kellison
& Sedberry 1998, Schröder et al. 2006, Wilding
2006, Maar et al. 2009). Around a research platform
aiming to mimic conditions around wind turbines,
effects on the benthic species assemblages
were noted 15 m from the structure (Schröder et
al. 2006). However, in the monitoring program at
Horns Rev in Denmark no distance related effects
on sediment dwelling animals (infauna) were discerned
within the wind farm area as a whole (Dong
Energy et al. 2006). Modelling in the same program
suggested that the changes in current velocity
within 5 m from the foundation would be less than
15%, and 1.5-2% between foundations.