Benthic Communities and Habitats
A literature review of ecological research on offshore wind farms with regard to benthic habitats and communities is presented in this article. Information was gathered from reports compiled in the course of wind farm developments, research project reports (practical and theoretical approaches), and the scientific literature. Noise and vibration, heat emission, electromagnetic fields and disturbance have been identified as potential impact sources. Available information related to the different impact sources are reviewed in separate chapters. A summary of contents is given below.
NOISE AND VIBRATION Various source levels of offshore wind farm related noise are available. Studies linking both noise and marine wildlife observations are scarce. Response to noise could be observed under experimental conditions. Under field conditions, avoidance might be the most common response to underwater noise. Colonization of wind turbines is taken as an indication that noise and vibration do not have detrimental effects on the attached fauna. Further studies are required to significantly add knowledge about the effects of noise and vibration on marine invertebrates.
TEMPERATURE Temperature rise in the sediment is predicted based on different theoretical models. Results of such calculations are cited. Most studies predict the sediment temperature rise not to exceed 2 K at 20 cm sediment depth if the cable burial depth is 1 m. Models also predict much higher sediment temperatures in close vicinity to the cables. Heat emission is discussed in the context of changes in the physico-chemical conditions of sedimentary substrates, which could cause changes in the distribution of species. In the scientific literature, the change of benthic community composition is mainly discussed in connection with thermal discharges from power plants. Also, an assessment of likely effects of seawater warming for particular species is presented.
ELECTROMAGNETIC FIELDS The occurrence of electric and magnetic fields dependent on power cable type is explained. Data available on both anticipated and measured field strength are reviewed. In conclusion, none of the studies performed to date to assess the impact of undersea cables on migratory fish (e.g. salmon and eels) and all the relatively immobile fauna inhabiting the sea floor (e.g. molluscs), has found any substantial behavioural or biological impact. In regard to benthic marine invertebrates, such effects may only occur in close vicinity to the cables.
DISTURBANCE Disturbance has been identified as one of the major impact factors in the course of offshore wind farm development. The term “disturbance” includes a number of different impact factors. It is necessary to distinguish between indirect and direct effects. Indirect effects primarily affect the marine environment, and thus secondarily affect the benthic community. Direct effects on organisms include physical disturbance, damage, displacement and removal.
Effects on the marine environment include all changes in biotope characteristics. Such changes discussed include changes of current and wave regime, disturbance of the seabed, and habitat destruction. None of these changes is reported to affect the marine environment on a large scale.
Physical disturbance and damage to benthic organisms is widely discussed in the scientific literature. Contradictory results are presented regarding the effects on benthic communities due to disturbance, but based on the results of the majority of studies, changes in zoobenthic species composition, abundance or biomass are very likely to occur. Species regarded sensitive towards disturbance include e.g. the sea urchin Echinocardium cordatum and the bivalves Phaxas pellucidus and Mactra corallina, the brittle stars Ophiotrix fragilis and Ophiopholis aculeata, the hydroids Lafoe dumosa, Sertularella spp. and Campanulariidae. Other species are considered to possess either high mechanical resistance, high mobility or a high potential for regeneration, which enable them to tolerate disturbance. Recovery of disturbed communities is expected to take several years.
Introduction of artificial hard bottom along with the so-called “reef effect” are believed to have the greatest impact at the ecosystem level. Succession in the colonization of artificial hard-bottoms has been investigated in quite a number of studies. Calculation of production rates of epifouling communities has also been undertaken. The “reef effect” is expected to be confined to the close vicinity of reefs or structure. One example from San Diego Bay in southern California was found where the elimination of a seapen species could be documented within 200 m distance from the artificial reef, probably due to foraging reef fish. In connection with artificial reefs, the benefit for certain species is also discussed.
This report comprises a review of the literature on the effects of offshore wind farms on fish. It documents the present state of knowledge by summarising the available results on offshore activities and their impacts on marine fish. Reports of field investigations, conclusions by analogy (fundamental research), and specific research are evaluated. The focus of the literature review is on the North and Baltic Seas, but where necessary also other marine waters are included. Up to now, only few direct investigations on existing wind farms are available. Therefore, other offshore utilisations are also discussed. These are mainly oil and gas exploitation, and sand and gravel extraction. To a certain extent, conclusions by analogy are possible. Four impact complexes are regarded as relevant for fish fauna: (1.) sound and vibration, (2.) sediment disturbance, (3.) introduction of hard substrates, and (4.) electromagnetic fields.
With the prospect of many offshore wind farms planned in sea areas of north-western Europe, there is an increasing demand for information about their impact on the marine environment. Along with marine mammals and migrating birds, seabirds are in the focus of interest for scientists as well as for the public. In order to provide a comprehensive basis for the assessment of possible impacts from wind farms at sea, this report summarises the results of seabird studies conducted at already existing offshore wind farms (mainly Utgrunden and Yttre Stengrund in Sweden and Tunø Knob, Horns Rev and Nysted in Denmark) and discusses the extent and quality of the studies. Relevant results from coastal wind farms and other technical activities at sea are taken into account as well. The three main effects possibly affecting seabirds are: habitat loss due to disturbance, barrier effects, and fatal collisions.
According to recent studies, six out of the 35 seabird species regularly living in German waters strongly avoid offshore wind farms (Red-throated Diver, Black-throated Diver, Gannet, Common Scoter, Guillemot, Razorbill), and one other species (Long-tailed Duck) was recorded which showed much lower numbers in wind farm areas after construction than before. Seven species occur within wind farms which do not show any obvious effects, and three gull species even increased in numbers compared to the preconstruction period. For 18 seabird species, it is not known how and whether the wind farms affect their habitat use. Those species which do not occur in wind farm areas suffer habitat loss greater than the wind farm area itself, due to the distance they keep from the turbines. Physical habitat loss due to the introduction of a hard bottom fauna on foundations and scour protections seems to be of minor importance, but it is also not known whether, and if so to what extent, seabirds will make use of this new food supply, and also of attracted fish.
Information about flying seabirds is mostly restricted to migrating birds, which may behave differently to seabirds during local movements, such as foraging flights or flights to and from roosts. It appears that eight species (the same as those mentioned in the context of habitat loss, and also the Velvet Scoter and the Black Guillemot) commonly fly detours instead of crossing offshore wind farms. Detours were also noted for another four species, but it is not clear whether this happens regularly. A total of 15 species (mostly gulls and terns, but also staging Long-tailed Ducks and Red-breasted Mergansers) were found to fly through wind farms commonly; no information is available for eight species. Detours, especially if flown regularly, increase the energy consumption of seabirds, and it is even possible that the habitat fragmentation caused by the technical barriers will lead to their giving up certain sea areas.
Although one collision of Eiders was witnessed at a Swedish offshore turbine, no other information about mortality from collisions at offshore wind farms is available. As 13 seabird species belonging to different systematic groups were found as casualties at coastal wind farms, seabirds must fundamentally be regarded as vulnerable to collisions. However, collision rates, and hence estimates of additive mortality, remain to be investigated in future.
In addition to direct mortality, possibly occurring due to collisions, indirect effects may impact the population sizes of those seabird species which avoid offshore wind farms. If density-dependent effects lead to lower energy intake rates in replacement habitats after displacements from wind farm areas, the mortality rate should increase. In addition, carry-over effects may have negative impacts on the reproduction rate because of a possible connection between poor body condition on arrival and subsequent breeding success.
Proposed methodologies for the impact assessment at offshore wind farms are reviewed briefly and evaluated with respect to the recent results concerning seabirds at operating turbines. In general, assessment procedures can be improved by concentrating on those species which avoid wind farms. In addition, avoidance distances and thus the necessary sizes of buffer zones are now better known. However, as collision rates, effects of increased seabird densities at sea and possible habituation effects (most studies so far cover only one or two years of the operational period) are largely unknown, no methodologies yet exist which might help to fully assess these effects.
As the population sizes of seabirds are the comparative basis for the assessment of impacts, possible effects of offshore wind farms must be addressed in a cumulative approach, which cannot be restricted to other wind farms alone, but which must also consider such factors as disturbance and displacement by ship traffic and habitat loss due to sand and gravel extraction.
Open questions remain as to the behaviour of seabird species not covered by the recent studies and to seabird behaviour during adverse weather conditions (e.g. storms), when visibility and maneuverability may be negatively affected. In general, it appears that more direct observations (e.g. ship-based) should be undertaken in order to study avoidance and feeding behaviour of seabirds within wind farms. Furthermore, monitoring of prey species would help to get a better understanding of the distribution of seabirds in and around wind farms. However, in order to learn more about the impact of displacement and barrier effects on population sizes and population dynamics, fundamental studies of density effects in overwintering seabirds are essential.
Offshore wind farm development has kept up its momentum in recent years. In Germany all projects – except a few which have been rejected by the authorising agency – have been continued, while internationally, an increasing number of wind farms are planned, and a few have even been already installed. Progress has also been made with regard to the studies on the potential impact of offshore wind turbines on the marine environment in general and marine mammals in particular. This report summarises the available information on the latter, and discusses the extent and quality of the data to the extent possible.
The main focus has been placed on the acoustic effects of wind turbine related sound emissions on the three marine mammal species abundant in the German waters, harbour porpoises, harbour seals and grey seals. This includes on the one hand data on sound emissions from wind turbines and on the other, information on its biological impact on the animals. Since the potential impact of acoustic emissions on marine mammals is a complex issue in any case, and since relevant knowledge in many areas is insufficient or non-existent, a useful way to proceed is to build theoretical models of the effects. As a first step, a compilation of the non-acoustical parameters which must be known for such a model is included in this report.
The results of the Danish studies clearly indicate that the construction of wind turbines has an immediate negative effect on the abundance of harbour porpoises and their habitat use in the wind farm areas. It remains unclear which factors would explain for the differences found between the effects documented at the two large Danish wind farms and for how long these effects will last. Given the yet-inconclusive results, any transfer of conclusions on the effects of the construction – except for the immediate effects of the pile driving – from one wind farm site to another must be treated with caution. Currently, no significantly new data can be drawn and transferred from studies on comparable activities.
The sound measurements of operational sound emissions indicate that there will be practically no acoustic overlap between adjacent wind farms. Moreover, it is assumed that the impact range of operational noise emissions on marine mammals is relatively small. However, the fact that the available data have been collected on small and medium-size wind turbines must also be considered. Any transfer of the resulting assessment on the potential impact of large-scale turbines must be treated with caution, and may even be inadequate.
No data exist so far on the potential impact of methods applied to investigate the bottom structure at construction sites. However, comparable studies indicate a potential threat to marine mammals, especially with regard to seismic studies.
The use of explosives to decommission the turbines would generate intense acoustic impulses that would be audible over great distances and would be very likely to cause severe injury or impairment to marine mammals at close range if done with no precautionary measures. No other available techniques can be assessed in this regard as yet, and must be studied in greater detail with regard to safety zones and potential mitigation measures. In addition, alternative methods should be developed and tested.
Measures for attenuating the wind turbine related sound emissions represent the best and most realistic strategy to reduce potential negative effects on the marine environment. The most comprehensive information is currently available on sound attenuation during the construction phase. Even though no detailed information has been published to date, an air-bubble curtain has been successfully used in study to reduce the emitted sound during pile driving activities. However, this technique, as well as alternative methods, must be tested for their effectiveness and applicability to the construction of wind turbines in the North and Baltic Seas.