Benthic Communities at Horns Rev Before, During and After Con

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