In Europe and beyond, there are ambitious plans to install marine renewable energy devices, or MREDs, ie wind, wave and tidal power plants. The construction and operation of MREDs will lead to, among other things, the emission of electromagnetic fields (EMF), subsea sound, and vibrations into the marine environment. Migratory fishes that respond to natural environmental cues, such as the Earth’s geomagnetic field move through the same waters that the MRED occupy, thereby raising the question of whether there are any effects of MRED on migratory and other fish species. Yet, the exact EMF emissions of cables from MREDs are not known and studies on potential impacts are in its infancy (see Gill et al. 2012).

Underwater sound impacts from MREDs have become a particularly important environmental issue. This is because water is an excellent medium for sound transmission. Sound travels more than four times faster underwater than in the air and absorbs less compared with air. On the other hand, vision, touch, smell and taste are limited in range and/or the speed of signal transmission. As a consequence, many forms of marine life use sound as their primary mode of communication, to locate a mate, to search for prey, to avoid predators and hazards, and for short- and long-range navigation. Activities generating underwater sound can affect these functions and, since sound can be far ranging, the spatial scale of impacts can be quite large as well.

Effects of sound on marine life

"Some species such as the harbour porpoise are very sensitive to disturbance due to wind farm construction."

The effects of sound can vary depending on a number of internal and external factors and can broadly be divided into masking of biological important sounds (own sounds or signals from others), behavioural disturbance, hearing loss (temporary or permanent) and injury. In extreme cases, and at very high received sound pressure levels that are usually close to the source, very intense sounds might also lead to the death of marine life. Pile-driving sound during construction of offshore wind farm monopiles and other foundations (e.g. jacket or tripod) is of particular concern as it reaches comparably high sound pressure levels (source sound pressure level > 250 dB re 1 µPa; see information box) and could have significant effects on marine mammal and fish species common in northern European waters.

Research has shown that some species such as the harbour porpoise are very sensitive to disturbance due to wind farm construction. It is also possible that construction sound could lead to temporary or even permanent hearing loss in marine mammals and fish, depending on the overall sound energy (the ‘acoustic dose’) that is received over time, for example from the many strikes that are needed to drive a pile into the seabed. Yet, there are many open questions with regards to impacts of MRED related sound on marine life. These information gaps pose challenges to the implementation of MREDs (see Thomsen 2010).

Studying the impacts: EU project

In a project for the European Union (EU) Commission, Directorate-General for Research and Innovation, DHI is undertaking a study of the environmental impacts of noise, vibrations and electromagnetic emissions from MREDs (Marine Renewables, Vibrations, Electromagnetics and Noise – MaRVEN).

"DHI is undertaking a study of the environmental impacts of noise, vibrations and electromagnetic emissions from MREDs"

MaRVEN will critically review the available scientific evidence and significance of those impacts and then make recommendations on solutions to mitigate or cancel the identified negative impacts. The investigation comprises several tasks including:

  • Provision of a historical review of the publications related to environmental impacts of marine renewable energy devices.
  • An in-depth analysis of studies on the environmental impacts of noise and vibrations during installation and operation of marine renewable energy devices.
  • An in-depth analysis of studies on the environmental impacts of electromagnetic emissions during the operation of marine renewable energy devices.
  • An in-depth analysis of the current norms and standards related to noise, vibrations and EMF for marine renewable energy systems.
  • Performance of relevant on-site measurements and field experiments to validate and build on the results obtained in above studies.
  • Preparation of a programme for further R&D with justified priorities.

In order to undertake the MARVEN project, DHI has partnered up with Cranfield University UK, who are the leading authorities on EMF impacts from MREDs. DHI and Cranfield University have assembled a team with seven other institutions:

  • Centre for Environment, Fisheries and Aquaculture Science (Cefas), United Kingdom.
  • Totalförsvarets forskningsinstitut (FOI), Sweden.
  • Scottish Association of Marine Science (SAMS), United Kingdom.
  • Deutsches Wind Energie Institut (DEWI), Germany.
  • Management Unit of the Mathematical Model of the North Sea (MUMM), Belgium.
  • Universitat Politècnica de Catalunya (UPC) / Laboratori d’Aplicacions Bioacústiques, Spain.
  • Quiet Oceans (QO), France.

The team members are from seven EU countries leading the implementation of renewable energy. Their expertise encompasses the key topics required for the study. It also brings together six members of the EU Task Study Group on Underwater Noise and other Forms of Energy (TSGN) that are directly involved in advice and recommendations with regards to the implementation of underwater noise regulation across the EU (Marine Strategy Framework Directive). There are also four members of Working Groups (including co-chairs) within the International Council for the Exploration of the Sea (ICES) associated with marine renewable energy.

An important stepping stone for marine renewables

"Environmental impact assessment is getting more and more important in the consenting procedures"

MaRVEN will be an important stepping stone in the further implementation of marine renewable energy devices in the EU, in which the environmental impact assessment is getting more and more important in the consenting procedures.

A variety of countries have installed or begun to build offshore wind farms, and the first marine wave and tidal plants are being developed as well. Some reviews have covered impacts of noise (see, for example Thomsen et al. 2006) and electromagnetic emissions (see, for example Gill et al. 2012) and to a lesser extent vibrations on marine life but most of these works are generic and do not cover all topics or all regions of the EU.

Both the review and field work undertaken in MaRVEN will be used to propose technical mitigation measures to reduce or cancel the identified negative effects. Finally, the programme for further R&D with justified priorities should inform researchers working in the field, stakeholders (for example planners and NGO’s) but also regulators in the EU and beyond.

The study commenced in December 2013 and will continue until June 2015.

Underwater sound (taken from WODA 2013)

Sound pressure: Sound in water is a travelling wave in which particles of the medium are alternately forced together and then apart. The sound can be measured as a change in pressure within the medium, which acts in all directions, described as the sound pressure. The unit for sound pressure is Pascal (Newton per metre squared).

Each sound wave has a pressure component (in Pascals) and a particle motion component indicating the displacement (metres), the velocity (metres per second) and the acceleration (metres per second squared) of the medium in the sound wave. Depending on the receptor mechanisms, marine life is sensitive to either pressure or particle motion or both. The pressure can be measured
with a pressure-sensitive device such as a hydrophone (an underwater microphone).

Due to the wide range of pressures and intensities and taking the hearing of aquatic organisms into account, it is customary to describe these using a logarithmic scale. The most generally used logarithmic scale for describing sound is the decibel scale (dB).
The sound pressure level (SPL) of a sound is given in decibels (dB) by:

SPL (in dB) = 10 log10 (P2/P02)

where P is the root mean square sound pressure and P0 is the reference pressure. The reference pressure in underwater acoustics is defined as 1 micropascal (µPa). As the dB value is given on a logarithmic scale, doubling the pressure of a sound leads to a 6 dB increase in sound pressure level. As the reference pressure for measurements in air is 20 µPa, and water and air differ acoustically, the dB levels for sound in water and in air cannot be compared directly.

Particle motion
Most terrestrial animals are sensitive to sound pressure. However, fish and many invertebrates are also sensitive to particle motion. Particle motion sensitivity has been shown to be important for fish responding to sounds from different directions.

Sound or noise?
The terms noise and sound are not clearly distinguished. Commonly, sound is a very broad term including all acoustic waves, whereas noise refers to sound that is unwanted. But as we do not really know what marine organisms perceive as ‘unwanted’, this article uses the more neutral term ‘sound’. The only exceptions are scientifically established terms such as ambient noise or when reference is made to work using that term.

References

  • Gill AB, Bartlett M, Thomsen F (2012) Potential interactions between diadromous teleosts of UK conservation importance and electromagnetic fields and subsea noise from marine renewable energy developments. Journal of Fish Biology 81:664-695
  • Richardson WJ, Malme CI, Green Jr CR, Thomson DH (1995) Marine mammals and noise, Vol 1. Academic Press, San Diego, California, USA
  • Thomsen F (2010) Sound impacts. In: Huddleston J (ed) Cowrie – Understanding the environmental impacts of offshore wind farms, Information Press, Oxford, p 32-43
  • Thomsen F, Lüdemann K, Kafemann R, Piper W (2006) Effects of offshore wind farm noise on marine mammals and fish, biola, Hamburg, Germany on behalf of cowrie ltd, Newbury, UK
  • WODA (2013) Technical guidance on: Underwater sound in relation to dredging World Organisation of Dredging Associations Delft

 

Author notes

Frank Thomsen, Senior Marine Scientist and Business Development Manager, DHI, Agern Alle 5, DK-2970 Hørsholm;

DHI is a not-for-profit consulting and research organisation specifically focused on water environments. DHI’s expertise spans a range of activities, from groundwater (freshwater in rivers and lakes), to industrial, drinking and storm water to coastal and oceanic marine waters. DHI has undertaken 50 years of dedicated research in more than 140 countries. It employs more than 1200 opeople in 30 countries who are involved in consulting, policy issues, R&D and software sales and support.www.dhigroup.com