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1 Introduction

The use of wind energy in marine areas only started a couple of decades ago. Offshore windfarms intrude upon an environment already heavily affected by human activities such as shipping and fishing. As they are large and complex installations and given the large numbers planned – at least in European waters – offshore windfarms can be expected to have significant impacts on marine ecosystems with long-lasting effects from both construction and operation.

Environmental impacts of offshore windfarms include destruction of the sea bottom and benthic communities, disruption of migrating species such as birds via barrier effects, and disturbance of sound-sensitive marine species through increased underwater noise. The various impacts affect species both concurrently and sequentially at different life stages and should therefore be assessed from a cumulative perspective.

Providing an environmentally suitable location has been chosen, a single windfarm does not necessarily have much more than a localised impact on the marine environment. Highly mobile species like marine fish, birds and mammals, however, will encounter offshore windfarms repeatedly in the course of a year. Multiple offshore windfarms can therefore have cumulative negative impacts on the conservation status of such species. For this reason we will focus on seabirds and migrating birds, and on underwater noise and its impact on marine mammals.

2 Seabirds

Seabirds show a range of different behaviours when confronted with offshore wind turbines. Some species do not hesitate to fly into windfarms to forage (some gull and tern species) or even to use the structures for resting (cormorant) (Dierschke & Garthe 2006). Birds of these species generally face a risk of collision with turbines (Everaert & Stienen 2007). Other species avoid windfarms either partly, leading to reduced bird densities (e.g. long-tailed duck) (Petersen et al. 2011), or almost entirely (e.g. red-throated diver and black-throated diver; gannet) (Dierschke & Garthe 2006), resulting in habitat loss.

A number of species are displaced by moving ships (Schwemmer et al. 2011), with service boats on the way to and from windfarms causing disturbance and temporary habitat loss. Indirect impacts can arise during windfarm construction, when pile driving displaces fish and thus reduces the food supply for fish-eating seabirds (Perrow et al. 2011). On the other hand, food supply may increase in the operational phase, because fisheries are excluded from windfarms and epibenthic organisms settle on the introduced structures.

In the case of lethal collisions, the effects of multiple windfarms can simply be added up, because the effects of one windfarm do not influence those of others. When it comes to habitat loss from avoidance and disturbance, too, the effects of multiple windfarms likewise at first appear simply to add up, with each additional windfarm increasing the impact on the various seabird populations. As avoidance includes the effect on birds flying through windfarms (Aumüller et al. 2013) and thus detours (Aumüller et al. 2013, Krijgsveld et al. 2010), fragmentation of the available habitat also has to be considered, resulting in greater habitat loss as modelled by Busch et al. (2013). Hence in a cumulative assessment of offshore windfarms the effects may act synergistically – all the more so since potential density effects in alternative marine habitats have not been investigated.

Future offshore windfarms should therefore be planned away from important seabird habitats to avoid high rates of collisions and habitat loss. Consideration should also be given to leaving corridors free of windfarms between seabird habitats so that birds can safely switch between sites.

3 Migrating birds

For various species of migrating birds, offshore windfarms act as barriers in daytime while lethal collisions predominately take place at night. The species spectra affected, however, differ fundamentally depending on whether species migrate diurnally or nocturnally.

Many species give windfarms a wide berth, including ducks, geese, swans, waders and auks plus certain other species such as fulmar, gannet, little gull, kittiwake and sandwich tern. Large gulls of the genus Larus, black-headed gull and common gull, in contrast, do not seem to be affected by windfarms, whereas it is possible that passerine species migrating during the day may even be attracted.

Reactions towards windfarms clearly differ between species. Severe consequences resulting from deviations in route could arise from future additions to the number of windfarms. Because of the additional energy demands, longer migration routes can affect both immediate survival and subsequent breeding success.

The literature suggests that none of the barrier effects identified so far have significant impacts on populations. However, there are circumstances where a barrier effect might lead indirectly to population-level impacts, for example where a windfarm effectively blocks a regularly used flight line between nesting and foraging areas, or where several windfarms cumulatively interact to create an extensive barrier that could result in diversions of many tens of kilometres (Drewitt et al. 2006).

Nocturnal migration is dominated by passerine species, in particular thrushes. Avian casualties at research platforms reflect the nocturnal species spectrum (Hüppop et al. 2012). Collisions with anthropogenic structures occur most of all when good weather conditions for migration worsen (e.g. clear sky changing to fog and drizzle, tailwinds turning into headwinds). In such conditions, birds tend to head for light sources such as those found on platforms and wind turbines.

Although birds generally migrate at lower heights over sea than over land, good weather conditions allow for migration at heights that do not pose risks for collision and where light sources are widely ignored (Hüppop et al 2006).

Migrations encountering anthropogenic structures at sea can lead to hundreds of casualties in single night, as seen for the research platform FINO1 (Hüppop et al. 2006, for causes see also Aumüller et al. 2011). Even though they seem impressive, however, the numbers remain flawed: It has not yet been possible to quantify the mortality rate in relation to actual migration intensity. The increase in mortality is probably well within the capacity of a population to compensate for additional losses (to regenerate) and hence has no effect on overall population levels. However, the cumulative increase in mortality resulting from a number of windfarms may exceed a population’s capacity for regeneration.

Even state-of-art methodologies are unable to quantify nocturnal migration at species level. Impact assessments at population level are therefore limited.

To protect migratory birds, it is recommended to provide lighting for offshore wind energy turbines, in line with demand. In nights of high migration when the weather is bad and visibility is poor, the approval authority (BSH) reserves the right to have the turbines temporarily switched off after having evaluated the situation (▶ Information box The Incidental Provision 21, see Chap. 12). At the spatial planning stage, it is crucial to avoid dead-end corridors between windfarms. The effects of corridor widths on migrating birds also need to be evaluated.

4 Underwater noise and marine mammals

Underwater sound naturally plays an important role for a number of aquatic or marine animals. Marine mammals such as whales in particular use natural and self-generated sounds to navigate, to detect food or predators, and to communicate with each other (Richardson et al. 1995). Anthropogenic noise entering the marine environment has the potential to impair these biologically important functions.

Underwater noise has become an issue of major concern in recent years with regard to human impacts on the marine environment in general (e.g. OSPAR 2009). In some marine areas, for example, the level of ambient noise has doubled every decade for the last 35 years (McDonald et al. 2006, Andrew et al. 2011). Noise from offshore windfarms and from their installation thus enters an already noisy environment and has to be assessed cumulatively.

Ambient noise demonstrates the fundamental cumulative character of underwater noise (◘ Fig. 17.1). Beside natural sounds generated by wind, waves and rain as well as various biological sounds, shipping contributes the most to ambient noise. In addition, there are localised but extremely loud anthropogenic sound events like seismic surveys, explosions and pile driving.

Fig. 17.1
figure 1

Underwater noise of different sources. The frequency range and noise level of natural and anthropogenic underwater sounds are shown (Coates 2002, modified).

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Regarding noise emissions from offshore windfarms, major concerns have been raised about the impact of pile driving especially on marine mammals. Steel tubes measuring several meters in diameter are driven by impact hammering into the sea bottom to form or fix the foundations of offshore wind turbines. Impulsive noise from pile driving has the potential to disturb or even injure marine mammals (e.g. Tougaard et al. 2006, OSPAR 2006, 2009).

The most common whale species in German waters is the harbour porpoise. Like other cetaceans, the harbour porpoise strongly depends on its sense of hearing for navigation, feeding and communication. Injuries in terms of hearing impairments such as a temporal threshold shift (TTS) occur at sound levels well below the source level emitted by a driven pile (Lucke et al. 2009, OSPAR 2009). In addition, as observed during construction of the Danish Horns Rev 1 windfarm, noise-induced disturbance of harbour porpoises can reach distances of more than 25 km from the pile driving site, thus covering an area of about 2,000 km2 (Tougaard et al. 2006). It should be mentioned that in the German Exclusive Economic Zone (EEZ), noise mitigation measures have to be applied when pile driving and emissions may not exceed a certain level to ensure that no harbour porpoises suffer TTS. A number of efficient noise mitigation measures and even low-noise foundations are available or under development (Koschinski & Luedemann 2013, ▶ see Chap. 16).

Neglecting other anthropogenic sound sources and considering only offshore windfarms due to the large number of projects, cumulative impacts occur on both a spatial and a temporal scale. In the German North Sea, more than 100 projects with up to 80 turbines each have been applied for, of which 30 have already been approved or are under construction or operational (◘ Fig. 17.2).

Fig. 17.2
figure 2

State of development of offshore windfarms in the German North Sea (as of September 2013).

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It can be projected that a number of projects will enter the construction phase each year for the next 20 to 30 years, emitting a large amount of anthropogenic impulsive noise into the marine environment. While compliance with the German noise threshold will substantially reduce the disturbance radius, the area affected will still add up to some hundreds of square kilometres, and the level of disturbance will be sustained year in, year out. Disturbance in the biological sense includes changes in behaviour, lost feeding time, or expenditure of extra energy to escape the area.

If multiple windfarms are built at the same time with pile driving done in alternation, animals escaping one disturbed area may enter another where pile driving then starts up, thus again suffering noise-induced disturbance. Such energy-related aspects are of special relevance during the most sensitive time of the reproduction phase when harbour porpoises give birth to and nurse calves.

On the other hand, cumulative impacts can even occur at the level of a single windfarm. Driving a pile into the sea bottom takes hundreds or thousands of hammer strokes. The cumulative energy impact on the ear of a receiving animal can induce TTS even if the sound level emitted by a single stroke does not have the capacity to do so (Southall et al. 2007).

In conclusion, windfarms will have cumulative impacts at various scales and affecting various features of marine life. Such impacts can be reduced or avoided by careful siting and by coordinating windfarm construction in both time and space.