Environmental Engineering Reference
In-Depth Information
Displacement of Benthic Organisms by Installation of the Project
Bottom disturbances will result from the temporary anchoring of construction ves-
sels, digging and refilling the trenches for power cables, and installation of permanent
anchors, pilings, or other mooring devices. Motile organisms will be displaced and
sessile organism destroyed in the limited areas affected by these activities. Displaced
organisms may be able to relocate if similar habitats exist nearby and those habitats
are not already at carrying capacity. That is, each population has an upper limit on
size, called the
carrying capacity
. Carrying capacity can be defined as being the
optimum number of individuals of a species that can survive in a specific area over
time. A particular pond may be able to support only a dozen frogs based on the food
resources for the frogs in the pond. If 30 frogs took up residency in the same pond, at
least half of them would probably die because the pond environment would not have
enough food for all of them to live. Carrying capacity, symbolized as
K
, is based on
the quantity of food supplies, the physical space available, the degree of predation,
and several other environmental factors.
Species with benthic-associated spawning or whose offspring settle into and
inhabit benthic habitats are likely to be most vulnerable to disruption during project
installation. Temporary increases in suspended sediments and sedimentation down
current from the construction area can be expected. The potential effects of sus-
pended sediments and sedimentation on aquatic organism are periodically reviewed
(Newcombe and Jensen, 1996; Wilber and Clarke, 2001; Wilber et al., 2005; Wood
and Armitage, 1997). When construction is completed, disturbed areas are likely to
be recolonized by these same organisms, assuming that the substrate and habitats
are restored to a similar state. For example, Lewis et al. (2003) found that numbers
of clams and burrowing polychaetes (worms) fully recovered within a year after con-
struction of an estuarine pipeline, although fewer wading birds returned to forage on
these invertebrates during the same time period.
Alteration of Habitats for Benthic Organism during Operation
Installation of the project will alter benthic habitats over the longer term if the trenches
containing electrical cables are backfilled with sediments of different size or compo-
sition than the previous substrate. Permanent structures on the bottom (ranging in size
from anchoring systems to seabed-mounted generators or turbine rotors) will sup-
plant the existing habitats. These new structures would replace natural hard substrates
or, in the case of previously sandy areas, add to the amount of hard bottom habitat
available to benthic algae, invertebrates, and fish. This could attract a community of
rocky reef fish and invertebrate species (including biofouling organisms) that would
not normally exist at that site. Depending on the situation, the newly created habitat
could increase biodiversity or have negative effects by enabling introduced (exotic)
benthic species to spread. Marine fouling communities developed on monopiles for
offshore wind power plants are significantly different from the benthic communities
on adjacent hard substrates (Wihelmsson and Maim, 2008; Wilhelmsson et al., 2006).
Changes in water velocities and sediment transport, erosion, and deposition caused
by the presence of new structures will alter benthic habitats, at least on a local scale.
This impact may be more extensive and long lasting than the effects of anchor and cable
installation. Deposition of sand may impact seagrass beds by increasing mortality and
