Benthic Communities at Horns Rev Before, During and After Con
Benthic Communities at Horns Rev Before, During and After Con
Benthic Communities at Horns Rev Before, During and After Con
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<strong>Horns</strong> <strong>Rev</strong>. <strong>Benthic</strong> communities Page 72<br />
colonis<strong>at</strong>ion of Cancer pagurus to the introduced hard substr<strong>at</strong>es was interpreted negligible<br />
impact from the piling activities on the popul<strong>at</strong>ion of Cancer pagurus. No investig<strong>at</strong>ions<br />
were made on the impacts from pile driving activities or from cable laying activities<br />
to the benthic communities.<br />
4.2.1.2. Introduction of hard bottom substr<strong>at</strong>es<br />
The most significant effect from the construction of the offshore wind farm was the loss of<br />
habit<strong>at</strong>s <strong>and</strong> the introduction of hard substr<strong>at</strong>e habit<strong>at</strong>s into a community exclusively<br />
domin<strong>at</strong>ed by infauna in s<strong>and</strong>y sediments.<br />
The fauna communities on the introduced hard substr<strong>at</strong>es are completely different from<br />
the infauna community as existed <strong>at</strong> the wind turbine sites prior to the erection of the wind<br />
turbines <strong>and</strong> the establishment of the scour protections. At the turbine sites, new habit<strong>at</strong>s<br />
were introduced th<strong>at</strong> changed the substr<strong>at</strong>es from pure s<strong>and</strong> to found<strong>at</strong>ions of steel, gravel<br />
<strong>and</strong> stones. A typical epifaunal community has replaced the n<strong>at</strong>ive infaunal community<br />
th<strong>at</strong> is characteristic of s<strong>and</strong>banks in the North Sea.<br />
<strong>During</strong> the 2003 to 2005 surveys <strong>at</strong> the introduced hard substr<strong>at</strong>es, all of the larger mobile<br />
epifaunal species previously observed were found. Except for the brown shrimp (Crangon<br />
crangon), the larger species were generally found more frequently <strong>and</strong> in higher numbers.<br />
More members found in the infauna Goniadella-Spisula community, typical for the infauna<br />
community <strong>at</strong> the <strong>Horns</strong> <strong>Rev</strong> area, were registered <strong>at</strong> the hard bottom substr<strong>at</strong>e <strong>at</strong> the<br />
turbine found<strong>at</strong>ions.<br />
The hard substr<strong>at</strong>es after construction of the wind farm were used as h<strong>at</strong>chery or nursery<br />
grounds for several species, which was an especially successful nursery for the edible crab<br />
(Cancer pagurus). The number <strong>and</strong> biomass of Cancer pagurus juveniles <strong>at</strong> the turbine<br />
sites has increased markedly from 2003 to 2005. Juveniles of the edible crab were especially<br />
found in large numbers on the monopiles in autumn with larger individuals often<br />
observed in caves <strong>and</strong> crevices among stones on the scour protection. More size classes of<br />
Cancer pagurus were found in March 2005. The growth of Cancer pagurus individuals<br />
were demonstr<strong>at</strong>ed from September 2003 to September 2005 indic<strong>at</strong>ing th<strong>at</strong> juvenile edible<br />
crabs utilise the turbine found<strong>at</strong>ions as nursery grounds. It is likely th<strong>at</strong> Cancer pagurus<br />
larvae <strong>and</strong> juveniles rapidly invade the hard substr<strong>at</strong>es from the breeding areas. Cancer<br />
pagurus females normally breed buried in s<strong>and</strong> in deeper areas of the North Sea <strong>and</strong> can<br />
be buried for months after m<strong>at</strong>ing. Juveniles settle in l<strong>at</strong>e summer/ early autumn <strong>and</strong> remain<br />
there until they reach a carapace width of 6-7 cm, which takes about 3 years, before<br />
they move to subtidal areas (Neal & Wilson, 2005). It is possible th<strong>at</strong> the juveniles in size<br />
class 2+ with carapace widths of approxim<strong>at</strong>ely 4 cm were about 2 years old <strong>at</strong> <strong>Horns</strong> <strong>Rev</strong><br />
. The average body weight of the juveniles found in the initial popul<strong>at</strong>ion in March 2003<br />
were higher than the average body weight of the juveniles found in March 2004 indic<strong>at</strong>ing<br />
th<strong>at</strong> the hard bottom substr<strong>at</strong>es were initially colonized by <strong>at</strong> least some young crabs <strong>and</strong><br />
not only by larvae. Further m<strong>at</strong>ure specimens were initially found <strong>and</strong> studies off the<br />
Dutch coast have also shown th<strong>at</strong> m<strong>at</strong>ure individuals of Cancer pagurus quickly invaded<br />
newly established artificial reefs (Leewis & Hallie, 2000).<br />
Doc. No. 2572-03-005 rev. 4