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Impacts of Marine Aggregates and Maërl Extraction
The principal environmental concerns regarding marine aggregates
(sand and gravel) and maërl extraction activities arise from
the physical impacts on the seabed and the resulting biological
responses.
Dredging operations result in considerable short-term impacts
on benthic (seafloor) communities, i.e. reduction in density,
species numbers and diversity. The time for recovery following
cessation of dredging depends upon the nature, magnitude and duration
of the operation, the nature of the new sediment that is exposed
or subsequently accumulates at the extraction site, the larval
and adult pool of potential new colonisers, and the nature and
extent of the stresses that the community normally withstands
(Marine Institute 1999).
Impacts of Aggregate Extraction
Dredging for marine aggregates causes physical disturbance that
may have an impact on marine life in the vicinity of the extraction
area. The most obvious biological effect of sand and gravel extraction
is the removal and destruction of marine life in the area of extraction.
Also marine life living in the vicinity, particularly downstream,
of the extraction area may be impacted by the fine sediment particles
that are suspended by the action of the dredger. Close to the
extraction area, bottom-living organisms may be blanketed by stirred
up or spilled sediments. Further away, the increase in fine particles
in suspension may harm filter-feeding organisms, such as mussels,
owing to the abrasive effects of sediment passing over their feeding
and respiratory structures. These effects may cause a change in
the types of organisms living in areas subject to commercial sand
and gravel extraction, favouring opportunistic organisms that
can reestablish themselves more quickly after such physical disturbance
and changes to the seabed.
ICES Environmental Status of the European Seas 2003
Extraction methods
Commercial marine aggregate and maërl extraction commonly
involves the use of suction dredgers, which physically alter the
seabed leaving long, shallow tracks or large, deep depressions.
United Kingdom
Typically, in UK waters, trailer suction-hopper dredgers are used
for sand and gravel extraction. These employ a single, rear-facing
pipe. Dredging is carried out whilst the vessel is underway leading
to the production of shallow linear furrows approximately 1-3m
wide and generally 0.2-0.3m deep.
According to the UK Centre for Environment, Fisheries and Aquaculture
Science (CEFAS), "repeated dredging by trailer dredgers can
result in substantial lowering of the seabed across a wide area
and this will be related to the frequency of dredging and the
level of dredging intensity" (CEFAS 2001).
A number of UK vessels are also able to dredge by anchoring or
remaining stationary over the deposit. Static dredging is employed
in areas where the deposit is spatially restricted or locally
thick (e.g. in the Bristol Channel). In this case, dredging usually
results in saucer shaped depressions up to 8-10m deep and 200m
in diameter.
County Cork
The Celtic Sea Minerals Ltd maërl extraction site at Lonehort
Point off Bere Island in Bantry Bay consists of a mud/dead maërl
matrix, which lies adjacent to a live maërl bed. The company
uses a suction-hopper dredger vessel to take an average 120 tonnes
of maërl from the seabed at a time.
The extraction activity usually lasts for about 3 hours at a
time, is carried out 3 or 4 times per week, and takes place all
year round.
The material is lifted hydraulically by suction through a drag-arm
and pumped into the hopper bin aboard the vessel, which then transports
the material to Dinish Island, Castletownbere Harbour, where the
processing plant is located. The processing activity includes
the following phases: drying, sieving, grinding, blending and
packaging. Celtic Sea Minerals has developed a number of products
based on "calcified seaweed" for the Irish market, including
Shamrock Sea Cal fertiliser for use on grassland (De Grave
et al. 2000).
Physical effects
The most obvious impact due to dredging is seabed alteration,
with habitat modification resulting from (localised) changes in
sediment composition. The length of time that trailer-dredged
furrows or depressions created by static dredging will remain
as distinctive features on the seabed depends on the ability of
tidal currents or wave action to mobilise and move finer sediments
into them. Where exposure is moderate, furrows have been observed
to last for 3-7 years.
Bathymetric changes resulting from the significant lowering of
the seabed have the potential to cause a drop in current strength
resulting in the deposition of finer sediments, which may contribute
to a localised depletion of oxygen, and lead to a change in benthic
community structure. On the other hand, where dredging exposes
deeper gravelly deposits, seabed sediments may be coarsened, again
affecting benthic community structure (CEFAS 2001).
CEFAS (2001) points out that particular dredging practices can
also contribute to the fining or coarsening of sediments over
time. For example, the aggregate industry carries out screening
activities in order to meet specific sand/gravel requirements
of the construction industry.
"Typically the construction industry requires marine aggregate
to be supplied with a gravel content of greater than 50% …
Where the in-situ gravel content of the dredged resource is lower
than this, dredgers employ on-board screening to increase the
gravel content of cargoes. Vessels use either static screen boxes
or screening towers to alter the composition of the dredged aggregate,
by passing the water/aggregate mix over a mesh screen. Assuming
that the intention is to increase the gravel content, a proportion
of the finer material and water will pass through the screen,
and be returned to the sea by means of a reject chute. This process
can be reversed if the intention is to produce a sand cargo, with
the coarse fraction of the dredged aggregate being rejected"
(CEFAS 2001).
Over time, this screening activity has the potential to significantly
change the composition of sediments within a dredged area.
Dredging can also produce plumes of re-suspended sediments as
a result of mechanical disturbance of the seabed, the outwash
of material from spillways from the vessel hopper, and from the
rejection of unwanted sediment fractions by screening activities.
The extent of the sediment plume and fall-out, and the increased
turbidity of the water column will depend on the particle size,
total quantity of material suspended and the local hydrodynamics.
The significance of re-sedimentation from plume fall-out on the
benthic fauna and flora and its effect on the rate of re-colonisation
of the affected seabed area is unknown (CEFAS 2001). However,
impacts are likely to include the smothering of sedentary benthic
organisms, behavioural changes and gill impairment of fish, possible
changes in the chemical environment, liberation of organic materials
that affect oxygen levels, and liberation of harmful seabed gases
such as methane and hydrogen sulphide (see Figure
1 [pdf 7k]).
[In UK waters] Aggregate extraction, in contrast
to fishing activity, is restricted to smaller and strictly defined
areas. However, in some places within the licensed dredged areas,
the impact on the seabed can be greater per unit area than bottom
fishing as both the substrata and fauna are removed, which prolongs
the recovery of the habitat and benthic community. Such major
impacts can be limited, however, as some areas within a licensed
area are commercially unattractive because the aggregate resource
is too thin. Once an area has been dredged and aggregate removed,
the operator generally moves on and recovery begins. Areas that
are heavily fished, however, may never fully recover because the
seabed is disturbed before recovery has taken place.
UK Habitat Action Plan: Sublittoral sands and gravels
Removal of marine aggregates from the large sand and gravel banks
that lie parallel with the coast in the western Irish Sea may
expose the east coast to increased wave action and higher rates
of erosion — thus exacerbating the risk posed to Ireland's
east coast by climate change.
Biological effects
The most significant consequence of marine aggregate extraction
is the direct removal of the substrate and the associated benthic
fauna and flora. Alteration of the seabed, together with bathymetric
changes and increased turbidity, can lead to short- or long-term
changes in the composition and abundance of species in both benthic
and fish communities, with knock-on impacts on bird and fish populations
that normally feed on these resources. If spawning fish, which
require a stable seabed substrate and environmental conditions,
use an area then disruption of egg-laying can occur.
De Grave and Whitaker's (1999) study of the environmental impact
of dredging for maërl at the Celtic Sea Minerals extraction
site at Lonehort Point concluded that changes in benthic community
structure had occurred along the lines of other types of dredging
activity. Most noticeable was a shift in the dominance from omnivorous
crustaceans and polychaetes to suspension-feeding bivalves. This
is linked to the relative instability of the sediment at the dredged
site, mobilisation of food resources and the increased turbidity.
Typically, dredging causes an initial reduction in the abundance,
species diversity and biomass of the benthic community. A controlled
study of the impacts of marine gravel extraction on the macrobenthos
(i.e. excluding benthic micro-organisms) at a site off the east
coast of England showed that significant reductions had occurred
in numbers of species (62%), abundance (94%) and the biomass (90%)
following the removal of 52,000 tonnes of material by a trailer
suction dredger (Kenny and Rees 1994).
Restoration
Dredging clearly has an impact on benthic communities
but the nature of this impact varies widely according to the intensity
of dredging and pre-existing environmental conditions — in
particular the inherent stability or mobility of the seabed sediments.
Recovery tends to be more rapid in unstable dynamic environments
such as shallow water mobile sands typically ranging from a few
months to between 2-4 years. Conversely in deep water stable gravels
recovery of some long-lived species can take in excess of 15 years.
While broad generalisations can be made on recovery in relation
to sediment types, the considerable variation in recovery within
and between habitat types dictate that meaningful assessments
of recovery can only be undertaken on a site-specific basis, incorporating
local environmental factors.
BMAPA, The Crown Estate and English Nature 2004
The factors influencing the restoration of local populations
include the types of organisms that remain in the vicinity following
sediment extraction, the life histories and mechanisms of dispersal
in different fauna and flora, the patchiness of the environment,
the spatial and temporal variability of dredging disturbance,
the effects of existing or new residents on the substratum, and
the potential interaction between dredging disturbance and other
perturbations such as storm disturbance (Boyd and Rees 2001).
Upon cessation of dredging activities, benthic fauna and flora
begin to re-colonise the affected area. Despite a general lack
of baseline information for commercial marine aggregate extraction
sites, Boyd and Rees (2001) suggest that a general pattern of
re-colonisation is emerging. The first phase involves the settlement
of a few opportunistic species that are able to take advantage
of the dredged and sometimes unstable sediments. Re-colonization
can either be by adults or larvae from the surrounding area if
the disturbed area is similar to the original substrate, or by
larvae from more distant sources if the sediment is markedly different.
A second phase is characterised by a reduced community biomass,
which was observed to persist for three years in North Norfolk.
This may be caused by increased sediment (mainly sand) in transport,
which scours the epibenthos (i.e. benthic organisms living primarily
on, or closely associated with, the seabed). Over time, the sediment
transport approaches the pre-dredging equilibrium, allowing the
restoration of community biomass. However, re-establishment of
a benthic community similar to that which existed prior to dredging
can only be attained if the topography and original sediment composition
are restored (Boyd and Rees 2001). In cases where sediments are
regularly disturbed by tidal currents, the benthic community may
remain at an early successional stage (Kenny and Rees 1994).
The estimated time required for the restoration of benthic communities
following marine aggregate extraction vary. Available evidence,
largely obtained from experimental studies, suggests that substantial
progress towards "recovery" could be expected within
2-3 years of cessation of dredging in sandy gravel habitats exposed
to moderate wave exposure and tidal currents. However, preliminary
observations from a recent study of a historic commercial extraction
site off Harwich, East Anglia, indicate that the "recovery"
period may be more prolonged (i.e. greater than 4years), especially
for sites dredged repeatedly (Boyd et al. 2001).
With regard to maërl, De Grave et al. (2000) state that
given the slow growth rate of maërl, it can be assumed that
once extraction commences on any given maërl bed this will
inevitably result in the partial or complete obliteration of the
bed and its associated fauna and flora. In addition, sedimentation,
where it occurs following re-suspension of material as a result
of dredging, will impede re-colonization and re-growth. Once a
maërl bed is extracted to a level at which it is no longer
economically viable, the operation will have to move to a new
location.
"Although possibly some remnants of the maërl bed may
persist, it is highly unlikely that these will be able to re-colonise
and re-establish the former maërl bed, at least on a human
time scale" (De Grave et al. 2000).
A 2004 study by Emu Ltd on behalf of the British Marine Aggregate
Producers Association (BMAPA), The Crown Estate and English Nature
produced Guiding Principles for Remediation
of marine aggregate extraction sites.
Management
It is UK Government policy to adopt a precautionary approach
when considering applications for marine aggregate extraction,
particularly in areas that are important for fish spawning, migration
routes, or as nursery and over-wintering grounds (DETR 2001).
Where there is evidence of a decline in fish population size,
in parallel with the continued extraction of marine aggregate
in UK waters, one management strategy has been to restrict aggregate
extraction on the assumption that aggregate removal may be impeding
the onshore migration of fish.
Rogers and Nicholson (2001) point out, however, that for such
a strategy to be complete, it must be accompanied by clear objectives
and a monitoring programme, which can evaluate the success, or
otherwise, of the management action.
The Marine Institute (1999) has said that the environmental implications
of offshore aggregate extraction such as alteration of benthic
habitats, coastal erosion, conflicts with fisheries, and so forth,
"should be weighed against the economic benefits before any
major extraction is authorised."
The Foreshore Acts 1933 to 1998 require that aggregate and maërl
extraction activities taking place on State-owned foreshore (i.e.
the land and seabed between the high water mark and the 12 nautical
mile limit [1]) require a Foreshore licence to
be granted by the Minister for Communications, Marine and Natural
Resources [2] prior to the commencement of any
works [3].
Certain developments are subject to the European Communities
(Environmental Impact Assessment) (Amendment) Regulations, 1999
[S.I. No.93 of 1999]. An application for any development above
the relevant threshold in the Regulations (an extraction area
greater than 5 hectares) must include an Environmental Impact
Statement (EIS). For proposed developments below the 5 hectare
threshold, an appraisal of the environmental effects of development
must be submitted by the applicant to allow the Minister to decide
whether it is likely to have significant effects on the environment.
Where the decision is "yes" an EIS is mandatory [4].
The Local Government (Planning and Development) Acts and Regulations
require that, before undertaking any development that is not exempted
from planning control, the applicant must also seek and obtain
permission from the local planning authority (i.e. County Council,
Corporation, etc.).
In November 2001 the ICES working group on the effects of extraction
of marine sediments on the marine ecosystem (WGEXT) reported to
the OSPAR Biodiversity Committee that, in Ireland, approaches
and guidelines to Environmental Impact Assessment "are currently
under review and will be developed in line with future policy
regarding the issue of licences for aggregate extraction"
(OSPAR 2001).
With regard to maërl, De Grave et al. (2000) state that
should a proposal be put to the regulatory authorities to extract
a live maërl bed or a maërl debris deposit, "the
application should be carefully scrutinised, both for economic
feasibility and from a biodiversity point of view. Certainly,
extensive site investigations should be carried out to calculate
the potential longevity of the extraction period, especially as
it is assumed that most maërl beds in Irish waters are only
in the order of 1-2 m in thickness … Secondly, an extensive
study into the local biodiversity and community structure should
be carried out."
Given the listing of both the main maërl bed-forming species
(Lithothamnion corallioides and Phymatolithon calcareum)
in Annex V of the Habitats Directive, the authors add that "any
application to extract live or dead maërl should be required
to submit an Environmental Impact Statement. Although no specific
guidelines in relation to information to be contained in such
a document are in existence for maërl beds, generic guidelines
on both a national and international level are in existence as
are international guidelines dealing with the extraction of marine
gravel deposits. The latter have a direct bearing on the information
to be supplied, in view of the similarities between both habitats.
Although special cognisance should be taken of the perhaps unique
features of the maërl habitat vs. gravel habitats, these
would primarily relate to the living nature of the resource itself"
(De Grave et al. 2000).
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