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Concerns have been raised regarding the effects that
the entrapment and deposition of organic matter
may have in strongly stratified water bodies, where
anoxic conditions already prevail. Field observations
have confirmed that localized anerobic conditions
may occur around the feet of the turbines (M.H.
Andersson, personal communication 2009). Other
potential impacts include ammonium excretion by
the mussels, which could increase growth rates of
phytoplankton and filamentous algae (Kautsky &
Wallentinus 1980, Norling & Kautsky 2008, Maar et
al. 2009). This has also has been indicated in field
observations (Malm 2005, Maar et al. 2009). Moreover,
filtration by the large numbers of mussels on
the turbines could deplete phytoplankton and, as a
result, cause lower biomass of filtrating animals on
the seabed, including mussels, up to 20 m from a
turbine (Maar et al. 2009).
The biomass of filter feeding animals, such as blue
mussels and barnacles, is higher on the seabed
around turbines compared with reference areas,
while the biomass of macroalgae, particularly species
of red algae, is lower (Wilhelmsson & Malm
2008; Maar, et al. 2009). As shown around oilrigs, at
larger scales (e.g. Love et al. 1999), new mussel beds
can form around wind turbines, as a consequence of
dislodgement of mussels from the structures. These
beds create hot spots of biological activity, and can
fundamentally alter the natural soft bottom assemblage
(Norling & Kautsky 2008, Maar et al. 2009). So
far these changes in macrofauna and flora composition
have only been observed within a few meters
from each turbine (Wilhelmsson et al. 2006).
Boat traffic around offshore wind farm. Photo: E.On Climate & Renewables
Many species of fish and crustaceans use artificial
reefs primarily as a refuge from predators and
water movements, and forage mainly in the neighbouring
habitats (Ambrose & Anderson 1990, Kurz
1995, Einbinder et al. 2006). Densities of benthic
prey items have, in some of studies, been shown
to decrease with proximity to these artificial reefs
due to increased predation (Davis et al. 1982, Kurz
1995, Jordan et al. 2005). The suggested radii of
influence on biomass of prey and macroalgae
around an artificial reef range between 15 m and
100 m. This probably depends on visibility and the
levels of risk for the fish as they move away from
the shelter of the artificial reef (Davis et al. 1982,
Kurz 1995, Einbinder et al. 2006). Low biomass of
common prey species has been recorded on the
seabed around wind turbines, which could be a
result of increased predation pressure from fish
and crustaceans associated with the turbines (Maar
et al. 2009). In many areas, enhanced biomass production
of prey species on and around the turbines,
may be cancelled out by this increase in predation
pressure. For example, it has been estimated that,
unless blue mussels and other prey were produced
on and around the turbines, shore crabs, shrimps
and fish associated with turbines in the Nystedt
wind farm would have needed food resources
equivalent to what is provided in a larger area than
Identifying and managing biodiversity risks and opportunities of offshore renewable energy - GREENING BLUE ENERGY 43