Loss of seabed areas

10.5. Loss of seabed areas

Newell et al. (1998) state that removal of the topmost 0.5 m of sediment is likely to eliminate benthos from the affected area. Likewise, denial of an area through place-ment of OWF-infrastructure on that area, will eliminate the species and communities within the affected areas. Intolerance to such loss will invariably be very high, but for most invertebrate species, the recoverability will also be high, if infrastructure is re-moved in the future.

10.6. Introduction of hard substrate

Introduction of hard structures may locally effect water flow rates. Some species are sensitive to changes in flow rate, if the change is sufficiently large to effect feeding strategy, oxygen uptake etc. As the seafloor in much of Horns Reef is dynamic, most species in the project area will be tolerant of changes in the water flow regime.

The sensitivities of the investigated species to the secondary ‘reef effect’ of introduc-ing hard substrates will be variable. The sub-surface sections of turbine towers and scour protections will introduce new types of sub-littoral habitats and increase the heterogeneity in areas previously consisting only of relatively uniform sand.

Some local species are expected to be sensitive to increased predation pressures, while many will not be prey items for the species connected with the hard substrate. It should also be noted, that the benthic communities in the Horns Reef area show natu-ral variations in spatial and temponatu-ral distribution (Leonhard & Birklund, 2006), and that most of the species investigated are already common prey items for other inverte-brates, fish and birds in the area.

10.7. Electromagnetic fields and heat 10.7.1 EMF

Electro-sensitive organisms are known to be able to detect two types of electric field:

localised polar and larger scale uniform electric fields (Gill et al., 2005).

The sensitivity to electric and magnetic fields by marine organisms is quite variable.

The lowest known electrical field detectable by elasmobranchs (sharks, rays and skates) is 0.5 μV/m, and sharks have been shown to react to magnetic fields of 25-100 μT (Meyer et al., 2004).

Strong electrical fields, such as used for electrofishing have a pronounced effect on bony fishes and elasmobranchs, but can also affect invertebrates:

 Razor clams Ensis sp. have been observed to emerge from the seabed at minimum electrical field strengths of ~40-50 V/m (30 Hz pulsed ) DC (Breen et al., 2011).

 In a study on side effects in benthic invertebrates when using a commercial electrofishing trawl system*, it was found that rag worms Alitta virens, green crabs Carcinus maenas and American razor clams Ensis directus, suffered a 3-7 % increase in mortality when subjected to simulated in situ exposure at

HR3-TR-024 v3 87 / 121 distances of 10-40 cm from the pulsed beam trawl electrodes (van Marlen et

al., 2009). In the same study, no significant effect was found to occur in the species common prawn Palaemon serratus, cut trough shell Spisula subtrun-cata and common starfish Asterias rubens. (* The precise electrical field strength and specifics were not provided, as they are considered trade se-crets. It is however assessed that they are comparable, or slightly less, to the field strengths reported by Breen et al., 2011).

Current knowledge on the impacts from power cables on electro-sensitive or magneto-sensitive invertebrate species is generally lacking and demonstrated sensitivities are quite variable, making informed assessments difficult. An overview in Normandeau et al., 2011 of conducted studies can be found in Appendix 4.

Weaker fields, such as those expected around the subsea cables of Horns Rev 3 OWF, are generally not believed to elicit strong effects in invertebrates. It is, however, possible that magnetic fields generated from submarine power cables may have an effect on some magneto-sensitive species like migratory crustaceans, which are thought to be sensitive to the Earth’s magnetic fields (Gill et al., 2005).

Studies have investigated responses to lesser electric or magnetic fields in at least three marine invertebrate phyla: Echinodermata, Mollusca and Arthropoda:

 Exposure to 60 Hz magnetic fields (3.4-8.8 mT) and magnetic fields over the range DC-600 kHz (2.5-6.5 mT) can alter the timing of early embryonic devel-opment in embryos of the purple sea urchin Strongylocentrotus purpuratus (Levin & Ernst, 1995).

 No significant effects were found in survival rate and fitness of, amongst other species, brown shrimp Crangon crangon and blue mussels Mytilus edulis, in response to exposure to static magnetic fields of 3.7 mT for several weeks under laboratory conditions (Bochert & Zettler, 2004).

 Brown shrimp have been recorded as being attracted to magnetic fields asso-ciated with a wind farm cable (ICES, 2003)

 No significant effects were found on Dungeness crabs Metacarcinus magister in food detection when exposed to EMF as well as EMF-detection and avoid-ance/attraction (3 mT DC) (Woodruff et al., 2012).

Magnetic field emissions, of the orders above (mT) can potentially cause interactions from the cellular through to the behavioural level in coastal organisms (Gill et al., 2005). However, most invertebrates are not considered sensitive to EMF levels likely to be associated with the Horns Rev 3 OWF.

10.7.2 Heat

Benthic communities can be sensitive to increases of temperature (Hiscock et al., 2004) and warming of coastal water can increase the oxygen thresholds for hypoxia-driven mortality of benthic organisms (Vaquer-Sunyer & Duarte, 2001).

Most of the benthic organisms in Table 10.1, are not expected to be sensitive to po-tential increases of sea bottom temperature in the Horns Rev 3 project area. All but

HR3-TR-024 v3 88 / 121 two of the species have large geographical distribution ranges, which extend to either

South West European, Mediterranean, subtropical or tropical waters. Only the species Ophelia borealis and Ophiura ophiura, are restricted to more northern ranges. Howev-er, both species are found in The Limfjord, where bottom temperatures in the summer can become significantly higher than at Horns Reef. The bottom temperature varia-tions in the Horns Reef area are also quite high, see Figure 10.2, and the organisms present are expected to be adapted to such variations.

Figure 10.2 A) Mean summer and B) mean winter bottom temperatures (°C) from 1998 to 2007. (From Neumann et al., 2009)

Hermit crab

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