Geoscience Reference
In-Depth Information
disturbance by fishing may cause changes in
nutrient cycling in some marine ecosystems
(Duplisea et al. 2001), which may be important
in avoiding eutrophication or anoxia in or near
the sea-bed.
4
Biological disturbance, e.g. introduction of
non-native species, although of the 53 non-
native species recorded in British waters, most
are causing concern in estuaries and coastal
environments, rather than on the open shelf.
5
Climate change, whereby long-term changes in
environmental conditions (temperature, salinity,
etc.) may result in changes in the distribution
and nature of some marine species assemblages.
Although not necessarily negative, a changed
sea-bed flora or fauna may lead to changes in sea-
bed stability or biogeochemical cycling. Where
biogenic sediments are significant contributors
to present sediment budgets, or the degree and
nature of bioturbation are changed, there is the
potential for some change to sedimentary condi-
tions at the sea bed.
In terms of inputs of sedimentary material to
the shelf system, issues fall into three categories:
1
volume - i.e. the presence of material that
may cause a problem to navigation or which
may tend to cause bathymetric changes deemed
undesirable;
2
composition - especially contamination, where
the sediment chemistry or biology may harm
the receiving environment (including sediment
organic content);
3
texture - which may be sufficiently different
from the naturally occurring material to be
deemed undesirable.
physical covering of disposed material with other
(cleaner or coarser) material in order to limit its
release and its exchange with the wider marine
environment. There is an ongoing debate about
the long-term effectiveness of capping, particu-
larly in deep water and in high-energy environ-
ments. Capping can be viewed as changing the
problem rather than solving it, but can fulfil
short- and medium-term needs.
3
Chemical treatment of the sediment.
Amongst others, remediation covers the fol-
lowing management options.
1
Non-intervention, whereby natural processes
are allowed to proceed without human interven-
tion (as opposed to, for example, the exclusion
of activities such as trawling from occurring
nearby).
2
'Restoration', whereby the ecosystem is re-
turned to the condition or state that would
exist had no dredging occurred. In practice, this
is rarely achievable. Athough it might be con-
sidered relatively easy to define a general goal
such as 'restoration', the complexity and dynamic
nature of marine systems mean that there
are genuine difficulties in establishing specific
objectives that will be scientifically credible and
measurable.
3
Rehabilitation, where some of the original eco-
logical features are replaced by different ones.
4
Habitat enhancement or creation, where the
original ecosystem is replaced by another, either
at the site of impact or elsewhere.
The need for remediation may depend upon
Government policy, or be assessed on a case-by-
case basis. Clearly, the sedimentary dynamics of
the receiving environment must be well under-
stood, so allowing an assessment to be made of
the technology used for material emplacement
and the likely and acceptable levels of natural
disturbance and dispersal. In terms of sediment
dynamics, receiving environments tend to fall
between two end members:
1
low-energy environments, quiescent and with
relative long-term stability;
2
high-energy environments, dynamic and poten-
tially dispersive.
It is not always clear whether natural retention
or dispersal is the best environmental outcome.
10.4.3 Environmental remediation
A working definition of remediation is '
the
action taken at a site subjected to anthropogenic
disturbance to restore or enhance its ecological
value
', of which the main categories are:
1
Physical removal of material (dredging), which,
as noted elsewhere, is mostly an issue in coastal
and estuarine environments.
2
Physical isolation of the material (contain-
ment), which may include capping of material
in situ
(Palermo et al. 1998). Capping is the
