Environmental Engineering Reference
In-Depth Information
(Fig. 13.6b); this is semi-quantitative, as only data from
studies where recovery was positive could naturally
be included (Versteegh
et al.
2004). Affecting up to
100 km
2
of intertidal flats, the mechanical dredging
for cockles alone is predicted to require recovery times
of 20-30 years. We regard this as the most optimistic
assessment, as recovery times may be much longer
if mechanical dredging has moved the intertidal eco-
system to a different, and much less biodiverse and
productive, stable state (Scheffer
et al.
2001).
One way to push a damaged intertidal flat system,
devoid of eelgrass meadows, oyster beds or mussel
banks due to dredging and trawling, towards the
original, more biodiverse state, is to plant seagrass,
build oyster reefs or create mussel beds. Subtidal
oyster reefs (
C. virginica
) in Pamlico Sound, North
Carolina, USA, do not function well when they are
dredged and lose height. With increasing depth of the
overlying water there is greater incidence of anoxia
and there are lower water-flow speeds (Lenihan &
Peterson 1998, Lenihan 1999). By increasing the
height of such damaged oyster reefs, the performance
of the oysters in terms of growth and survival
increased considerably (Lenihan 1999).
In the Dutch Wadden Sea, the creation of artificial
mussel beds by dumping dredged mussel on barren
intertidal flats has met with some success and fail-
ures. A mussel bed of
c
.0.5 ha, created in June 1987
by dumping 20,000 kg (fresh mass) of blue mussels
(
M. edulis
) on intertidal flats (Ens & Alting 1996) has
survived several years and attracted many mussel
predators. Eventually, this mussel bed was removed
again by commercial dredging for seed mussels (J.B.
Hulscher, personal communication). Other attempts to
create mussel beds by dumping mussels on intertidal
flats have met with mixed success. The creation of five
experimental mussel beds in the Dutch Wadden Sea
in October-November 2001 was a failure in that only
two of the five sites still had mussels left after the
winter (A.C. Smaal, personal communication). How-
ever, the amount of mussels deposited may have been
too small during a time of food scarcity for shellfish-
eating shorebirds in the Wadden Sea. We think that
the artificial establishment of intertidal mussel beds
on barren flats by carefully planned and supervised
dumps of sufficient magnitude could help to reverse
the process of ecological erosion caused by intense and
large-scale dredging (see Box 13.2).
Box 13.2 A vicious circle
Removal of dominant stocks of filter-feeding and
faecal-pellet-producing bivalves in combination with
the reworking of originally rather stable sediments and
an increase in tidal prism can lead to a cascade of
effects, to a negative biodepository spiral, that can turn
silty intertidal mudflats into much more sandy hab-
itats (Fig. 13.7). According to this hypothesis, the
mechanical removal of the large filter-feeders kicks
off sedimentary changes that then automatically lead
to the disappearance of other filter-feeders that also
produce biodepositorily important (pseudo-)faecal
pellets,
Macoma balthica
being just one example
(Risk & Moffat 1977). In this scheme of things, win-
ter storms are not the causal agent but are simply the
executors of processes that have their beginnings in
human perturbations of the apparently fragile balance
of the intertidal sedimentary systems.
As the restoration of such eelgrass meadows hap-
pens to be one of the targets of Dutch environmental
policy (e.g. van Katwijk
et al.
2000), quite some
experience has now accumulated with the trans-
plantation of seagrass (
Z. marina
) meadows on the
intertidal flats of the Dutch Wadden Sea (van Katwijk
et al.
1998, van Katwijk & Hermus 2000). To reduce
the effects of water turbidity and sediment mobility,
exclosures were successfully applied at sites where light
availability (a function of depth and turbidity) were
adequate. To increase transplantation success sheltered
locations were recommended by van Katwijk and
Hermus (2000). At a local scale, a stable mussel bed
could provide such a shelter, but the provision of
temporal, biodegradable, dam-like structures would also
provide refugia from where a successful transplanta-
tion of
Z. marina
could expand.
Thus, it is clear that restoration of biodiverse
intertidal flats that were rich in biogenic structures
such as mussel beds and eelgrass meadows, but were
then turned into barren flats inhabited only by a few
small polychaete and crustacean species as a result
of large-scale mechanical dredging, probably requires
more than many years without disturbances (Fig. 13.6).
Using an integrated approach, the artificial re-
establishment of ecosystem-engineering species like
