Dredging and disposal: Settlement
of suspended sediments
Sediments dispersed during maintenance dredging
and disposal may resettle over the seabed and the
animals and plants that live on and within it. This
blanketing or smothering of benthic animals and
plants, may cause stress, reduced rates of growth
or reproduction and in the worse cases the effects
may be fatal (Bray, Bates & Land 1997).
Generally sediments settle within the vicinity of
the dredged area, where they are likely to have
little effect on the recently disturbed communities,
particularly in areas where dredging is a well-established
activity. However, in some cases sediments are distributed
more widely within the estuary or coastal area and
may settle over adjacent subtidal or intertidal
habitats possibly some distance from the dredged
The sensitivity of marine animals and plants to
siltation varies greatly and discussed briefly below.
In areas with high natural loads of suspended sediments,
the relatively small increases in siltation away
from the immediate dredging area are generally considered
unlikely to have adverse effects on benthic populations.
Assessment of the effects of siltation from capital
dredging in Morecambe Bay concluded that some smothering
of benthic animals was inevitable. It was suggested
that given that the area is subjected to regular
maintenance dredging of navigation channels and
berths and that the adjacent subtidal and intertidal
areas appear to be productive, it is unlikely that
effects from the proposed dredging programme will
have anything more than temporary and fairly localised
impacts (ABP Research R707 1997). Post-dredge surveys
of the deepened navigation channel to the Port of
Londonderry, Lough Foyle, which is in close proximity
to important commercial shell fisheries, indicated
that with appropriate care, substantial dredging
works can be undertaken with no adverse effects
on shell or other fisheries (Bates 1996).
Examples of the varying sensitivity of marine animals
and plants to siltation
- Animals with delicate feeding or breathing apparatus,
such as shellfish can be intolerant to increased
siltation, resulting in reduced growth or fatality
(ABP Research R707 1997).
- Maerl beds (calcified seaweed) are reported
to be sensitive to smothering due to channel dredging
(Birkett et al 1998).
- In important spawning or nursery areas for fish
and other marine animals, dredging can result
in smothering eggs and larvae. Shellfish are particularly
susceptible during spring when spatfall occurs.
- When smothering of intertidal areas occurs,
there may be subsequent effects on the availability
of animals and plants in bird/fish feeding areas.