Introduction

The impact of climate change through global warming has been a concern for
some time now. As a result of this concern, the United Nations Convention on
Climate Change was developed at the Rio Convention on Climate Change and biodiversity
in 1992. Following the Rio Convention, targets were set for ratifying countries
to reduce their CO2 emissions, as defined by the Kyoto Protocol (1997).

In order for countries to achieve this reduction in CO2 emissions, there must
be a move to the production of electricity from renewable sources other than
fossil fuel combustion. Of these sources, perhaps the most realistic and economic
is wind power. With this in mind, the United Kingdom has one of the largest
offshore wind resources in Europe. The use of this “renewable” resource to generate
electricity, through the development of offshore windfarms, is recognized as
a key element in meeting the United Kingdom’s commitment to reducing greenhouse
gases, and is supported by a number of global conservation organizations, including
Greenpeace, the World Wide Fund for Nature and Friends of the Earth. However,
a number of areas of environ mental concern remain in the development and utilization
of this technology.

Some of these concerns, such as the possibility of birds striking the rotating
blades of offshore wind turbines, have already been addressed extensively, while
others have not. This paper draws upon research from a number of sources to
consider the possible impacts of the noise and vibration generated by wind farms
on the marine habitat: organisms ranging from whales to lobsters. In addition
to this, we consider the possible, positive impact of wind turbines to act as
an artificial habitat (the “reef effect”) and so increase the diversity of species
and biological productivity of windfarm locations.

Other than the issue of “bird strikes,” the noise and vibration generated by
constructing and decommissioning, and possibly through the operation, of offshore
wind turbines, is likely to be one of the issues concerning the environmental
impacts of offshore wind on marine wildlife. This issue is particularly important,
as many marine organisms are known to use sound in a range of applications,
from communication to sensing their surroundings. Thus, interference with the
sounds marine organisms produce could disrupt their communication abilities,
or the ability of an individual animal to sense its surrounding environment.
Furthermore, there is the potential for noise and vibration generated by windfarms
to have more adverse impacts, such as damaging the noise-sensing organs of marine
life.

This paper will consider the existing noise sources present in the sea, the
likely noise and vibration generated by offshore wind farms, the sensitivity
of marine organisms to sound and finally, the possible impact of noise and vibration
from offshore wind farms on the denizens of the sea.

Existing Sources of Noise and Vibrations in the Marine Environment

Physical (or natural) noise and vibration in the ocean can come from many different
sources. These include geological disturbances, wave interactions, breaking
waves and spray, wind, rain and hail. The range of frequencies associated with
these natural processes can often be very broad. For example, noise produced
by wind and rain can range from 1Hz to 25 kilohertz (kHz) with source levels
of up to 100 decibels (dB). Conversely, some processes can produce narrow ranges
of high-energy noise and vibration, such as earthquake events where frequencies
are commonly between five and 15Hz with source levels as powerful as 240dB.
Thus, the sea can be a very noisy environment even without human intervention.

The sound pressure levels given in Figure 1 are peak sound intensities in the
sound frequency spectrum produced by several anthropogenic noise sources. As
we can see, the sound produced by the Svante Offshore Wind Farm is relatively
quiet when compared to some sources of noise in the marine environment.

Fig. 1 A comparison of selected underwater anthropogenic noise sources
(peak intensities taken from Richardson et al., 1995 and Westerberg, 1999)

Noise
and vibration from human activities (anthropogenic sources) are generally of
mid- to low frequency (between 10 and 1000Hz) and include: shipping and transportation,
dredging, construction, hydrocarbon and mineral extraction, geophysical survey,
sonar, explosions and ocean science studies, but could also have very high source
levels, as described in Figure 1. For example, noise associated with geophysical
and seismic surveying regularly produces sound levels above 200dB and low-frequency
sonar may produce sound intensities that far exceed 200dB. These sounds are
categorized as “transient” if their duration is brief, such as explosions of
the pulse of a seismic airgun; or “continuous” if they persist for long periods,
such as the noise generated by an oil-drilling platform or the anticipated sound
of an operating offshore wind farm.

The Anticipating Noise Produced by an Operation Offshore Wind Farm

The probable noise produced above the water by operating offshore wind farms
is expected to be broadly similar to that produced by onshore turbines and is
of little environmental concern. However, there have been very few studies conducted
to characterize the underwater noise environment generated by offshore wind
turbines, although some data does exist. Measurements of the underwater acoustic
environment of the Svante Offshore Wind farm (a single 220kW turbine, 35m tall
on a tripod foundation) were taken in 1994 at a range of distances, depths and
wind speeds. Beyond frequencies of 100Hz, sound levels were below ambient sea
levels of around 80dB at 100m from the turbine (Westerberg 2001, pers. comm.).
Below frequencies of 100Hz, the noise ranged from approximately 80 to 100dB
in intensity with a peak of 103dB at 16Hz (Westerberg 1994, 1999).

While more modern turbines are expected to be much larger than the Svante turbine
and may therefore be more noisy, the intensity of underwater noise produced
by the Svante turbine is much lower than many other anthropogenic sources, as
shown in Figure 1.

Sensitivity of Marine Organisms to Noise and Vibration

Figure 2 describes the known sensitivity of several marine species to noise
and vibration. These sensitivity curves, termed “behavioral audiograms,” are
developed by subjecting an individual to noises of known frequencies and sensitivities.
The behavioral reactions to specific noises are noted.

Figure 2 – Underwater “behavioral audiograms” for selected marine wildlife
species (adapted from Vella et al., 2001).

Many species cannot hear the very low-frequency noise produced by the Svante
turbine. For example, the sensitivity of the bottlenose dolphin only extends
to approximately 100Hz at the low-frequency end of the spectrum. For a bottlenose
to hear the Svante , the turbine would have to produce sounds at 100Hz in an
intensity range greater than 130dB. Conversely, the sensitivity of the cod does
overlap the sound frequencies and intensities produced by the Svante Turbine.
Thus, we would expect the cod to be able to hear the turbine noise and possibly
display some sort of behavioral reaction such as attraction of repulsion.

Behavioral audiograms have not yet been developed for the larger, mysticete
whales such as the blue and grey whales. This is because the sensitivity of
these animals has not yet been determined under scientific conditions. However,
because they use very low-frequency noises for communication, it is very likely
that they will be sensitive to the noise generated by offshore wind farms.

Audible sensitivity in the majority of invertebrate species, such as the squid
and lobster, has also not been determined. This is because they do not hear
in the conventional manner. The majority of invertebrate species do not possess
organs developed to detect sound waves. However, many do possess structures
and organs sensitive to pressure. Therefore, some invertebrate species may be
able to detect the pressure component of a sound wave when they are very close
to the source of the sounds (within meters).

Finally, it must be noted that modern offshore wind farms may be far louder
in noise frequency and intensity than the Svante offshore wind farm.

Possible Reactions of Marine Species to Noise and Vibration Generated by an
Offshore Wind Farm

The possible impacts of noise and vibration on marine organisms are considered
for the following four marine wildlife groups: squid and lobster, fish, seals,
and whales and dolphins.

Squid and Lobster

Evidence of noise- and vibration-related effects on invertebrates largely consider
the effects of seismic investigations. Here, very high sound levels are produced
which affect invertebrates in a very localized area (suggested to be within
several meters of a very loud sound source) (Vella et al., 2001). Apart from
such seismic surveys, no adverse impacts are expected. Indeed, studies at the
Horns Rev offshore wind farm in Denmark show colonization of turbine foundations
by many marine species within five months of construction (Leonhard, 2000).
The likely effect of wind farm construction would therefore be locally increased
numbers of hard-substrate colonizing species. At present, no use of anti-fouling
substances has been proposed and thus, the extent of colonization will depend
upon the number and size of turbine foundations and any additional habitat provided
by foundation protection.

Fish

Intermittent noise associated with activities during the construction of wind
farms (vessel movements, seismic survey, piling etc.) is well within the range
of the behavioral audiograms of fish (Figure 1 and Figure 2). This is supported
by observations of their reactions, which have commonly demonstrated changes
in behavior, such as alarm and startle responses (Vella et al., 2001). Such
responses may be of particular significance if a wind farm is in close proximity
to spawning or nursery ground areas, and particularly if construction is prolonged.
Of the fish species included in Figure 2, only the audiogram of cod falls within
the noise range of the Svante turbine, suggesting that some sort of behavioral
response would be expected. Investigations at the Svante wind farm have shown
that the number of cod in the local area of the operating turbine are greater
than in the surrounding area (Westerberg, 1999). This presumably reflects the
ability of animals to habituate to a continuous noise stimulus. Similar effects
have been observed around other “noisy” structures such as oil platforms (Valdemarsen,
1979).

Intermittent, loud noise may therefore have an adverse effect on local fish
populations, causing alarm responses and probable movement of fish away from
construction areas. This could be significant if construction affected spawning
or nursery areas. But when wind farms are operating normally, fish appear to
readily habituate and utilize wind farm sites at higher than normal densities,
taking advantage of the shelter provided and probably also the additional food
resources provided by colonizing animals (Vella et al., 2001). Furthermore,
a study of the effects of operational noise on migrating fish (Westerberg, 1999)
did not show a significant effect of the Svante wind farm on migrating eel direction.

Seals

In general, seals show avoidance reaction to anthropogenic noise and activity
when it is close, and is probably perceived to be a threat. However, this is
most probably a response to visual cues rather than noise. In general, both
harbor and grey seals (both found on U.K. coasts) seem to habituate to most
anthropogenic sounds and activities.

While on land, the most common reaction to construction noise and activity will
be alarm behavior. If disturbance is sufficient, seals will leave and re-enter
the water. In general though, this behavior is triggered by very close human
approach (tens to hundreds of meters, depending on frequency of exposure to
human activity).

Studies have shown that the most common reaction to construction activities
when seals are already in the water is avoidance, but again, this may be a reaction
to visual cues rather than any noise produced by the construction. This avoidance
behavior may result in seals being excluded from feeding grounds or areas of
importance. However, it is likely that this exclusion will only be during the
short construction period. Furthermore, there is some evidence to suggest that
seals will quickly habituate to construction activity and noise, as was observed
during construction and production at the Näsrevet Wind farm in Sweden (Westerberg,
1999).

The ability of seals to detect low-frequency sound (<1000Hz) has not been clearly demonstrated. It is therefore unlikely that seals will be able to hear the underwater noise produced by turbines. It should also be noted that seal audiograms have only been developed for a few individuals, and so may vary considerably. Also, seals are inquisitive in nature, and it is likely that they will investigate local wind farms and may use these areas as feeding grounds, particularly if fish population densities are higher around offshore wind turbines.

Whales and Dolphins

The cetacean species most likely to be affected by wind farm construction and
operation around the United Kingdom are the dolphins and porpoises. Other whale
species are much less likely to be affected, due to their absence in large numbers,
although some species are regularly observed in inshore waters, including the
minke whales found off the Northumberland Coast.

As we have discussed earlier, the noise generated during constructional activities
at sea is generally of low frequency (mostly under 1000Hz) and where very high
sound levels are produced (such as during seismic surveys), noise production
is intermittent. Dolphins do not appear to be sensitive to low-frequency sound
(Figure 2) and often approach vessels. The reaction of individuals to noise
may, however, vary with their activity and motivational state. For example,
when socializing, dolphins may approach vessels but avoid them during feeding
(Richardson et al., 1995). When exposed to sudden loud noises, dolphins are
therefore likely to show responses ranging from subtle changes in behavior to
avoidance reactions.

Although the sensitivity of mysticete whales has not been measured, they are
thought to be sensitive to low-frequency noise over considerable distances.
They will almost certainly, therefore, be sensitive to constructional noise
and will most probably show avoidance reaction or give construction sites a
wide berth. As with odontocetes, however, responses may be mixed and males in
search of mates, for example, may ignore or tolerate noise that females with
young may avoid.

The noise and vibration of an operating wind farm is only expected to exceed
ambient levels at very low frequencies, possibly under 100Hz. As described above,
there is little evidence that animals such as the bottlenose dolphin and harbor
porpoise can perceive sounds at these frequencies. After familiarization with
wind farms and habituation to any noise which is perceived, they are unlikely
to be adversely affected and indeed may exploit wind farm sites as feeding grounds.

It has been suggested that behavioral audiograms for mysticete whales such as
the minke may be centered in the vicinity of 100-200Hz (Potter and Delroy, 1998).
If this assumption is correct — and it is likely that it is — then
there is a possibility that communication or other behavior could be affected
in these majestic creatures.

The environmental implications of offshore wind are not all negative. One positive
aspect currently under debate is the issue of new habitat creation and the artificial
reef affect.

New Habitat Creation

Fish tend to aggregate around objects placed in the sea. This phenomenon has
been widely used in the development of Fish Aggregating Devices. However, the
attraction of fish to objects is poorly understood. It is postulated that fish
are attracted to submerged objects as they provide shelter from currents and
wave action and safety from predators.
Oil and drilling platforms in the North Sea have also been shown to provide
a hard, stable substrata for colonization by a diverse range of marine organisms
including seaweeds, mussels, barnacles, tubeworms, hydroids, sponges, soft corals
and other invertebrates. These organisms, which require a hard surface upon
which to attach, attract various free-living invertebrates and small fish. These
in turn attract larger organisms, thereby increasing species diversity, biomass
and general productivity. This may be particularly so if hard-substrate structures,
such as offshore wind farms, are placed in sandy environments.

Conclusions

On the basis of the available data, it is likely that seals and dolphins and
porpoises will show initial avoidance to wind farms, followed by habituation
and possibly attraction to these sites as feeding grounds. The reaction of the
blue, grey and minke whales is unknown in the absence of data regarding their
audible sensitivity. However, it is possible that they will show a behavioral
response to the low-frequency sound that wind farms are likely to produce. The
significance of this response will depend upon the proximity of wind farms to
areas of importance for whales and migratory routes.

From the information available for operating offshore wind farms and other “noisy”
offshore structures such as oil and gas platforms, it is expected that effects
on fish population dynamics will be determined by immigration/attraction of
fish to wind farms following construction. No adverse impacts on marine invertebrates
are expected by the noise and vibration generated by turbines.

However, the full effects of offshore wind farms on marine wildlife, particularly
mammals, fish and migratory fish behavior and ecology can only be usefully determined
through monitoring. Additional studies into the effects of offshore wind farms
on marine species are desperately required. To some extent, this data may soon
be available with the monitoring programs planned for various Scandinavian wind
farms. These include such programs as satellite tracking of seals and whales
and dolphins in the proximity of the Horns Rev Windfarm in Denmark. Data from
this program would identify whether or not marine mammals avoid wind farms or
whether they use them as a feeding ground.

Offshore wind is a good idea. Certainly, when considering the bigger picture
— the reduction of greenhouse gas emissions and subsequent reduction in
global warming — it is difficult to find any argument to impeded the progression
of this novel technology. However, we must take care not to overlook the smaller
picture — the possible impacts on the local environment and the organisms
that live there.

The Centre for Marine and Coastal Studies, University of Liverpool, intends
to maintain its position at the forefront of this new field in the utilities
industry, both in continuing its research into the environmental implications
of offshore wind and in providing services such as Environmental Impact Assessments.

References

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Those Who Live There. Acoustics & Vibration Asia 1998 Conference Proceedings
pp. 56-71.
3 Richardson, W.J.; Greene, C.R.; Malme, C.I.; Thompson, H.H (1995) Marine Mammals
and Noise. Academic Press, San Diego.
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