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The impacts of anchoring and mooring in seagrass, Studland Bay, Dorset, UK

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Studland Bay, Dorset, on the central south coast of England is shallow and well protected from the prevailing south-west winds, making it an ideal habitat for a dense seagrass bed of Zostera marina. The shelter and proximity to the port of Poole make it a popular anchorage. Bare patches in the seagrass habitat associated with boat anchoring and mooring are described. Shear vane stress of the seabed was measured in situ by SCUBA divers. When comparing the undisturbed seagrass sediment with the bare, impacted areas, the latter sediments are less cohesive, contain less organic material and have a lower silt fraction, infaunal organism number and taxa. A mechanism for the progression of an anchor scar is suggested, involving storm wave induced mobilisation and dispersion of the impacted sediments exposing the underlying rhizome mat, which is further undermined by crabs. Results from this work and studies on other seagrass species suggest that the recovery is far from straightforward. It may take many years, leading to the decline of the Studland Bay seagrass habitat and associated species.
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doi:10.3723/ut.(to be allocated) International Journal of the Society for Underwater Technology, Vol 29, No 3, pp 1–7, 2010
Technical Paper
The impacts of anchoring and mooring in seagrass,
Studland Bay, Dorset, UK
KJ Collins, AM Suonpää and JJ Mallinson
School of Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, UK
Abstract
Studland Bay, Dorset, on the central south coast
of England is shallow and well protected from the
prevailing south-west winds, making it an ideal habitat
for a dense seagrass bed of Zostera marina. The
shelter and proximity to the port of Poole make it
a popular anchorage. Bare patches in the seagrass
habitat associated with boat anchoring and mooring
are described. Shear vane stress of the seabed
was measured in situ while by SCUBA divers. When
comparing the undisturbed seagrass sediment with the
bare, impacted areas, the latter sediments are less
cohesive, contain less organic material and have a
lower silt fraction, infaunal organism number and taxa.
A mechanism for the progression of an anchor scar is
suggested, involving storm wave induced mobilisation
and dispersion of the impacted sediments exposing the
underlying rhizome mat, which is further undermined
by crabs. Results from this work and studies on other
seagrass species suggest that the recovery is far from
straightforward. It may take many years, leading to
the decline of the Studland Bay seagrass habitat and
associated species.
Keywords: Zostera marina, SCUBA diving, shear vane
stress, erosion, sediment infauna
1. Introduction
Seagrasses form important coastal habitats in sub-
littoral and intertidal zones for numerous reasons.
They are the only true marine angiosperms and
are therefore unique in the marine environment,
providing nurseries, refuges and foraging areas for
a variety of organisms (Hemminga and Duarte,
2000), as well as reducing coastal erosion and
improving water quality (Larkum et al., 2006).
They also deflect currents and dissipate the kinetic
energy of waves, which makes them suitable envi-
ronments for sediment deposition and retention,
thus binding the seabed together and providing a
natural coastal defence role (Widdows et al., 2008;
Bos et al., 2007).
The root and rhizome web of seagrasses sta-
bilises the sediment by binding it which reduces
re-suspension. Seagrasses cover 0.1–0.2% of global
seafloor (Erftemeijer and Lewis, 2006) and play
an important role in the nutrient and carbon
recycling in coastal habitats. The high biomass
production of seagrass areas makes them one of
the most productive of their kind in the world
(Short and Wyllie-Echeverria, 1996). They also
enrich biodiversity of coastal areas; the major food
chains in seagrass habitat are based on the detritus
and the microbes feeding on it (Thayer et al.,
1975). The leaf and root rhizome mass provides
a complex structural habitat with special niches,
making seagrass meadows a diverse habitat that can
provide shelter from predators (Virnstein, 1979).
In the UK the principal sub-littoral seagrass
species is Zostera marina. The European Union’s
‘Council Directive 92/43/EEC on the conservation
of natural habitats and of wild fauna and flora’
(the Habitats Directive) requires the maintenance
and/or restoration of natural habitats and species
of European interest. To help implement this, the
UK Marine Special Areas of Conservation (SAC) the
Financial Instrument for the Environment (LIFE)
Project identified a series of sub-features of the
Annex I marine habitats, including Zostera habitats
(Davison and Hughes, 1998).
Internationally, a declining trend in seagrass
meadows has been observed since the 1970s.
Natural causes for decline are storms, hurricanes,
earthquakes, deceases and grazing by herbivores
(Short and Wyllie-Echeverria, 1996). Many reports
show, however, that to great extent seagrasses are
lost because of human induced disturbances, such
as eutrophication, toxic chemicals, alien species,
coastal development, fisheries and boat activities
(Deis, 2000). Two species of seahorse are found in
the British Isles: the Spiny (Hippocampus guttulatus,
Cuvier, 1829) and the Short Snouted (Hippocampus
hippocampus, Linnaeus, 1758), both often associated
with seagrass habitats (Garrick-Maidment, 2007).
H. hippocampus is on the OSPAR priority list of
threatened and endangered species (International
Council for the Exploration of the Sea, 2003). A
study in parallel to the one described here has
tagged Studland Bay seagrass bed seahorses in
1
Collins et al. The impacts of anchoring and mooring in seagrass, Studland Bay, Dorset, UK
50°40N
50°39N
1°55W
1°58W
5m depth
contour
Poole Harbour
Studland
Studland
Bay
Seagrass
Fig 1: Location of the seagrass study in southern
Studland Bay, Dorset
situ to increase overall understanding of individual
seahorse behaviour, fidelity of breeding pairs,
frequency of pregnancy, habitat and seasonal
movements (Garrick-Maidment et al., 2010).
Whilst there is extensive literature on boating
impacts on range of seagrasses around the world,
there is very little information available on the
effects they have on Zostera marina. For Mediter-
ranean species of seagrass, Posidonia oceanica, the
anchoring impacts have been intensively studied
(Montefalcone et al., 2008), such as the severe
consequences of swinging chain moorings, which
have been shown to produce circular scars on
the seagrass meadows (Walker et al., 1989). The
impacts of anchoring and mooring are discussed
later, along with consequent habitat fragmentation
and sediment erosion.
This study was carried out in Studland Bay,
Dorset, on the central south coast of England
(Figure 1). The sandy bay is an ideal habitat for
a dense seagrass bed of Zostera marina, covering
some 50ha, because it is shallow (dropping to 5m
depth 2km from the shore) and well protected from
the prevailing south-west winds. The shelter and
proximity to the port of Poole also make it a popular
anchorage. The impact to the seagrass habitat by
boat anchoring and mooring has led to concerns
over potential decline in the habitat and associated
species.
2. Methods
This study concentrated on direct comparison of
the sediment properties (grain size and shear vane
stress) and infauna of scar sites, each with a control
site of 1–2m within the adjacent seagrass meadow.
The density and extent of seagrass, Zostera marina,
in Studland Bay (Figure 1) was determined by
seabed towed video sledge, which recorded a track
that was 50cm wide (details are given in Lefebvre
et al., 2009). For broader surveys of anchoring and
mooring impacts, sidescan sonar with integrated
GPS (Humminbird 997c Combo) was used in
single-beam mode at 800kHz with a swath width of
30m.
The direct impact of anchoring activity was
examined by divers who photographed embedded
anchors of moored vessels and then returned to
the seabed immediately after the vessels had raised
their anchors. Surface photography was used to
record seagrass shoots and rhizomes caught up by
boat anchors. To investigate the bare patch (scar)
sizes and ground-truth of the sidescan images,
divers measured their dimensions with a tape
measure along known compass bearings. The scar
perimeters were also marked using metal rods to
establish the colonisation rate in the scars for
subsequent work.
The bed shear stress is an important sediment
property, as it describes the effect of currents and
waves acting through the friction they exert in the
seabed (Soulsby, 1997). To determine the bed shear
stress in mixed sediment, a sample can be taken and
analysed in the laboratory, or measurements can be
done in situ. Although the former is an established
method, removing sediment destroys the fine-scale
structure (Tolhurst et al., 2000), thus measuring the
strength in situ has an advantage as it enables an
undisturbed sampling (Dill and Moore, 1965).
In this study, bed shear stress was measured
in situ using a simple SCUBA diver operated,
hand-held shear vane (blade 4.9 ×5.7cm, Figure 2).
The spike was pushed vertically into the seabed until
the vane was level with the seabed surface. The
tangential force to rotate the vane was measured
with a spring balance (10kg) with a sliding marker
to record the maximum reading, which occurred at
the point of initial rotation. If a hard objected was
encountered during insertion of the vane, a new
position for sampling was selected. Ten replicate
measurements from each scar site were made, along
with ten replicate control measurements from 1–2m
within the adjacent seagrass bed.
Divers collected five replicate sediment cores
(11.5cm diameter ×6.5cm height, and volume of
0.7dm3) from each scar site, and similarly five
replicates from 1–2m within the adjacent seagrass
bed as controls. In the laboratory, samples were
rinsed over a 200µm sieve mesh to separate the
organisms from the sediment. The large parts of
seagrass – shoots and leaves – were removed by
hand. The sieved organisms were fixed in diluted
10% Formalin with Rose Bengal to stain infauna
2
Vol 29, No 3, 2010
2
1
(a) (b)
Fig 2: Shear vane stress instrument (a) showing the vane (1) which is rotated in the sediment and the
spring balance (2) which records the rotational force, and (b) the device being operated by a diver
tissue, and were subsequently picked out and
preserved in 80% alcohol. The individuals were
identified under a low power microscope to family
and species level where possible.
One sediment sample from each anchor and
adjacent control sampling site plus two replicates
from a mooring and adjacent control sampling
site were processed for sediment analysis. They
were dried at 100C for a minimum of 48hr then
shaken through a stack of sieves (0.5, 0.25, 0.125
and 0.063mm mesh). Each grade was weighted
and the percentage fractions, including the grain
size parameters (graphic mean, sorting, skewness)
and qualitative description for each sample, were
given using the GRADISTAT 6.0 grain size analysis
program.
3. Results
There are some 30 chain moorings in the south-west
corner of Studland Bay. Water depth (2–3m CD)
and tidal range (2m on springs) are both low
but the riser lengths are typically in excess of
10m, creating wide areas (30m2) kept clear of
seagrass as the boat and buoy swing around with
wind and tide. During the summer, up to 200
sailing and motor vessels (10–30m in length) are
anchored in the bay (Hatcher, 2009). The location
and impact of anchors has been directly observed
by SCUBA divers, who have confirmed that bare
patches (typically 1–4m2) are caused by anchoring.
Boat based surveys using towed video and
sidescan were carried out in 2008 and 2009, using
the results to determine locations for the detailed
SCUBA diving studies in summer 2009. Figure 3
shows sidescan images of area of seagrass meadow
impacted by mooring and anchoring. One feature
of the anchor scars was a distinct step down (10–
20cm) from the seagrass bed along at least one
edge, leaving the seagrass rhizome mat exposed and
often undercut. There were several occurrences of
shore crabs, Carcinus maenas, occupying burrows
beneath the seagrass rhizomes.
Seagrass meadows trap sediments, especially silts.
It was apparent to the divers that the sediment
in bare patches (associated with anchoring and
mooring chains) was less cohesive and more
mobile, hence the construction and use of the
shear vane stress instrument. Sediment strength was
measured by the shear vane in five anchor scar
sites and one mooring scar site (Figure 4), along
with measurement from the adjacent segrass bed at
each site. In all sites, the average shear vane stress
was significantly higher in the seagrass than in the
scars (t-test on paired scar/control, P<0.01).
Two-way analyses of variance (ANOVA) of the shear
vane stress from five anchor scars sites and five
adjacent control sites, each having ten replicates,
show that there is a statistically significant difference
(P=<0.001) between the scar sites and the
seagrass controls.
Paired sediment analyses (Table 1) show that
both the silt fraction and organic content was
less in the scars than the adjacent seagrass. The
grain size distribution in the samples with pair-
wise Student’s t-test showed significant difference
(degrees of freedom equalling 3, P<0.05) for
fine sediments (diameter 63µm) between anchor
scars and seagrass in all samples. Skewness (Sk) and
kurtosis (K) were similar in all samples, and mean
value for the sites showed little variation from log
normal distribution.
A total of 1473 organisms were identified in this
study. The total fauna seagrass to scar ratio was
1134:339, indicating much higher abundance in
the seagrass compared to the anchor and mooring
scars. The fauna was identified mainly to family
3
Collins et al. The impacts of anchoring and mooring in seagrass, Studland Bay, Dorset, UK
(a) (b)
Fig 3: Sidescan sonar images (each 20m across) of seagrass bed showing scars due to (a) mooring
chain and (b) anchoring
Table 1: Sediment properties of anchor and mooring scar compared with adjacent seagrass control sites,
showing graphical mean grain size (D50 8), kurtosis (K), shorting (6), skewness (Sk), silt fraction
(63µm%) and organic content (org DW g1)
Seagrass Scar
Anchor A B C D A B C D
D50 (8)1.5 1.48 1.38 1.51 1.48 1.47 1.44 1.44
K1.04 0.9 1.14 0.93 0.8 0.82 0.98 1.10
60.42 0.39 0.6 0.39 0.34 0.35 0.41 0.45
Sk 0.06 0.09 0.21 0.1 0.04 0.05 0.12 0.17
63µm% 6.70 4.70 5.80 6.60 2.00 1.70 2.60 3.80
org DWg13.10 7.95 5.48 5.19 6.86 1.54 1.99 2.24
Seagrass Scar
Mooring M1 M2 M1 M2
D50 (8)1.31 1.23 1.00 1.10
K0.91 0.82 0.82 0.77
60.57 0.59 0.59 0.61
Sk 0.16 0.17 0.15 0.1
63µm% 5.20 1.60 4.50 1.50
org DWg14.56 2.49
Site
Shear vane stress kg cm–1
Scar
Seagrass
A
B
C
D
E
M
ABCDEM
Fig 4: Shear vane stress measurements in anchor
(A–E) and mooring (M) scars with adjacent
seagrass control sites; the average value (n=10)
±1 standard deviation is shown
level, except for the most common bivalve, Loripes,
and skeleton shrimps (Caprellidae). Overall, 54
families/species from eight taxonomic groups were
found.
The diversity of taxa also was higher in seagrass
compared to the scar areas, with 50 and 38 fam-
ilies/species, respectively, found in their samples.
More amphipod families were found in Zostera
marina, while in the latter samples only were
the skeleton shrimps (Caprellidae) and the rarer
Loripes,L. lucinalis present. Overall, the infauna was
dominated by polychaetes, oligochaetes, bivalves
and amphipods. Oligochaete tubificid worms were
the most common individual fauna found in the
samples. The diversity was highest among the
polychaetes, with 18 families present in the samples.
A pair-wise Student’s t-test was used to compare
means of the samples collected from sediment
within the seagrass with that from the anchor
and mooring scars. This showed a significant
difference in the total mean number of organisms
per sampling site (P<0.01, Figure 5). Of the
taxonomic groups, bivalves, amphipods, annelids
and skeleton shrimps (Caprellidae) differed sig-
nificantly between the samples from scars and
seagrass sites (t-test, P<0.05). Two-way ANOVA
4
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Site
12345
Number of individuals/0.7dm3
Scar
Number of taxa/0.7dm3
Scar
**
**
**
**
Seagrass
Seagrass
0
20
40
60
80
ABCDM
0
5
10
15
20
Fig 5: Sediment core (0.7dm3) infauna analysis
from anchor (A–D) and mooring (M) scars with
adjacent seagrass control sites; number of
individuals (above) and number of taxa (below).
The average value (n=5) ±1 standard deviation is
shown and ** denotes significant difference
(P<0.01)
of the number of organisms from four anchor scar
sites and four control seagrass sites, each with five
replicates, show a significant difference between
scar and control (P<0.001).
4. Discussion
4.1. Anchoring
Anchoring can negatively affect seagrass in during
its three stages: dropping the anchor, dragging and
recovering the anchor chain. When an anchoring
system is deployed the seagrass are bent, which
disrupts the growth and reproduction processes
(Ceccherelli et al., 2007; Montefalcone et al., 2008).
The free chain on the seafloor then cuts the
leaves and pulls the rhizomes from the seabed
as it is dragged over the bottom following the
movement of the boat, as has been directly observed
in Studland Bay in the current study. Anchor fall
(Francour et al., 1999) and recovery (Milazzo et al.,
2004) both cause damage. It has also been shown
that anchor type has an impact to the degree of the
disturbance, with larger anchors having a greater
impact on Posidonia oceanica (Milazzo et al., 2004).
Montefalcone et al. (2008) studied seagrass beds
a few days and then four months after anchoring.
They showed that a shoot and rhizome density
declined directly after anchoring and in deeper
areas four months later. Shallow areas showed no
difference between control and disturbed areas
after four months of anchoring. Posidonia oceanica
is a slow growing species and therefore the recovery
after disturbance is shown to be slow. Ceccherelli
et al. (2007) found that the leaf length of the
Posidonia oceanica was still reduced after one year of
disturbance.
For fast growing Brazilian Halodule wrightii, Creed
and Filho (1999) found that simulated anchoring
reduced shoot density by half four days after the
disturbance. Nine months later, seagrass rhizome
and biomass densities had recovered to control
levels. Creed and Filho (1999) argued that the fast
recovery should be considered with caution, as it
may be attributable to warm latitude enhancing the
growth, or the small size (0.25m2) of the simulated
anchor scars.
4.2. Moorings
Swinging chain moorings have been shown to
produce circular scars on the seagrass meadows,
as in this study (shown earlier in Figure 3). In
Western Australia, seagrass loss caused by moorings
was estimated at <2% (Walker et al., 1989). Even
though the impact area was small, Walter et al.
argued that the effects were greater because the
damage was caused in the middle of the seagrass
canopy.
4.3. Habitat fragmentation
Montefalcone et al. (2008) argued that even though
the impact of anchoring may be only short term,
anchors are rarely deployed in the same place
subsequently. Figure 3 graphically illustrates this for
Studland Bay. Different deployments of the anchor
and chain can cause greater consequences from
which Posidonia oceanica may not recover in one
growing season. The repeated action of ripping out
the shoots by anchor and chain may have a long
term effect. Francour et al. (1999) similarly found
that repeated anchoring reduced Posidonia oceanica
shoot density and plant cover. Similarly, Hastings
et al. (1995) found that the seagrass meadows
experienced large amount of fragmentation caused
by moorings in Western Australia. Here the seagrass
meadows mainly consisted of Posidonia sp. and
Amphibolis sp. These species are known to have
slow rhizome growth rate and therefore a slow
recovery rate. Posidonia sp. rhizome growth rate was
estimated to be 10–20cm yr1and Amphibolis sp.
20–50cm yr1.
Many studies have compared seagrass habitat to
unvegetated, bare sand showing a higher species
5
Collins et al. The impacts of anchoring and mooring in seagrass, Studland Bay, Dorset, UK
abundance and diversity in seagrass meadows
(Edgar et al., 1994; Boström and Bonsdorff, 1997;
Mattila et al., 1999). Similarly in this current
study, lower numbers of organisms and taxa have
been found in the unvegetated scars compared
to the adjacent seagrass. The disturbance which
increases the fragmentation of the seagrass bed
has a consequent effect on species abundance and
diversity (Bowden et al., 2001).
4.4. Sediment mobility
Locally the impact of moorings has been studied
in an estuary on south coast of England (Herbert
et al., 2009). Herbert et al. observed a change in
sediment composition in mooring locations, which
they suggested was caused by the swinging mooring
chain scraping the bottom, leading to a coarser
sediment structure. Habitat fragmentation hinders
seagrass meadow recovery (Ceccherelli et al., 2007),
especially in soft sediments.
Seagrass cover loss can also increase sedi-
ment mobility, which in turn may decrease sea-
grass growth rate. In Western Australia (Hastings
et al., 1995), similar to this study, moorings were
associated with bare sand patches. These patches
were exposed to wave action, and in places where
seagrass rhizome growth rate could not cope with
erosion rate, seagrass cover was reduced and some
of the areas did not recover at all. Hastings et al.
(1995) suggested that recovery might be affected by
high erosion in the area.
This study has shown that the sediment in both
mooring and anchor scars is less cohesive, contains
less organic material and has a lower silt fraction,
lower infaunal species number and taxa. All these
are connected. The deepening of the scar, exposing
the underlying rhizome mat, suggests a mechanism
for the progression of anchor scar damage (see
Figure 6), which is described in the following
paragraph.
When an anchor is pulled up it cuts into the
seagrass rhizome mat, tearing a hole in its fabric.
Storm wave action, particularly during the winter,
mobilises the unprotected sediment winnowing out
the silt fraction, reducing its cohesion. Further
storm wave action disperses the sediment into the
surrounding seagrass where it is trapped, leaving a
depression in the seabed. Crabs and further storm
action continue to undermine the edge of the
surviving seagrass.
A simplistic approach would assume that the
seagrass can simply re-grow and re-colonise the
damaged areas. However, damage to the rhizome
structure and its undermining severely impede
recovery. Sidescan images of the same scars during
this study before and after winter 2008–2009
indicate expansion of the scars but no evidence
Fig 6: Photograph of the edge of a seagrass scar,
showing the eroded edge exposing the rhizome
mate, and suggested mechanism for sediment
export
of re-colonisation during summer 2009. The edges
of these scars have been marked with metal pins
hammered into the seabed and will form the
baselines for future long-term studies.
If anchor and mooring damage were repaired
through natural re-growth from one year to the
next, then it could be argued that these are
sustainable activities. Results from this study and
from those with other seagrass species suggest
that the recovery is far from straightforward, and
the impacts of anchoring and mooring could
potentially lead to the decline of the Studland Bay
seagrass habitat and its associated species.
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... The process of anchor drop, set and retrieval all act to mobilise and displace sediments creating anchor pits on the seabed (Backhurst and Cole, 2000;Collins et al., 2010) or long furrows associated with anchor or chain drag (Ganteaume et al., 2005;Luger and Harkes, 2013). The depth to which anchors penetrate sediments is a function of anchor weight, its velocity on contacting the seabed and the sediment type (Allan, 1998;DNV, 2010;Luger and Harkes, 2013). ...
... An experimental examination of pits produced by recreational anchoring and anchor retrieval reveal penetration to a depth of 9 cm from a 20 kg anchor (Backhurst and Cole, 2000). This may represent an underestimate, as pits to twice this depth were recorded in a seagrassdominated anchorage (Collins et al., 2010). Penetration of the anchors of merchant vessels are on a much larger scale; routinely exceeding a metre, particularly in fine-grained unconsolidated sediment (Fig. 6b). ...
... Francour et al., 1999;Okudan et al., 2011). Chains directly abrade (Williams, 1988;Hastings et al., 1995) or completely uproot seagrass shoots (Milazzo et al., 2004), reducing seagrass cover (Colomer et al., 2017) and creating anchor pits (Collins et al., 2010). Damage to seagrass tissue can, in turn, increase their susceptibility to disease or reduce the ability of a stand to produce chlorophyll (the state of being achloritic) (Williams, 1988), leading to further seagrass loss. ...
Article
Millions of recreational boats and ~ 65,000 ocean-going merchant ships anchor routinely. Anchor and chain scour associated with these vessels mechanically disturb the seabed having implications for marine environments globally. Our review summarises the scientific literature that examines the response of biota to anchor scour across five habitats; unvegetated sediments; seagrass; rhodolith beds; coral and rocky reefs. Forty-one studies met our criteria with > 85% of articles targeting recreational-based disturbances, mostly focussed on seagrass. Investigations of anchor scour from ships comes almost exclusively from cruise ships anchoring on coral reef. All research examined reported biota responding negatively to anchor scour, either directly or indirectly. Effects to biota were dependent on the spatio-temporal scale of the perturbation or the life-histories of the organisms impacted. We highlight several key knowledge gaps requiring urgent investigation and suggest a range of management strategies to work towards sustainable anchoring practices and the preservation of valuable seabed environments.
... Anchors used by the largest commercial vessels may weigh in excess of 25 tonnes (Davis et al. 2016) and are generally stockless anchors (a heavy set of flukes connected by a pivot or ball and socket joint to a shank). There are three stages of anchoring: dropping the anchor, dragging to lay out anchor chain and setting the anchor, then recovering the anchor and chain (Collins et al. 2010). While the anchor itself can impact the seabed, the most significant impacts result from the action of anchor chains dragging across the seabed as the anchored vessel swings around and moves in response to currents and wind, potentially affecting a large area of the benthos within the swing radius (Collins et al. 2010;Panigada et al. 2008) (Figure 7). ...
... There are three stages of anchoring: dropping the anchor, dragging to lay out anchor chain and setting the anchor, then recovering the anchor and chain (Collins et al. 2010). While the anchor itself can impact the seabed, the most significant impacts result from the action of anchor chains dragging across the seabed as the anchored vessel swings around and moves in response to currents and wind, potentially affecting a large area of the benthos within the swing radius (Collins et al. 2010;Panigada et al. 2008) (Figure 7). The area impacted and magnitude of the impact is a function of the frequency of anchoring, dimensions and type of anchor used, anchor chain length (dependent on the size of the vessel, but usually 3 to 5 times water depth), currents and weather conditions, water depth, seabed type and the character of the biota present Milazzo et al. 2004;Montefalcone et al. 2006). ...
... Vessel positioning, dropping and retrieving of the anchor, and the movement of anchor/mooring chains, can disturb seafloor substrate which can lead to sediment becoming resuspended, and subsequent re-sedimentation (Collins et al. 2010 ...
Technical Report
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Vessels involved in commercial marine shipping in Canada engage in the movement of goods or people by sea on the Arctic, Atlantic, and Pacific oceans. To explore the ways that the activities associated with commercial shipping can impact the marine environment, a suite of activity-based Pathways of Effects (PoE) conceptual models were developed. PoE conceptual models describe the pathways (linkages) between human activities, associated stressors, and their effects on endpoints, based on current knowledge. A visual representation of each PoE model is supported by text describing each pathway linkage based on scientific literature or expert opinion. Indigenous and local knowledge were not used in the current work. PoE models are useful tools for the scoping phase of a variety of environmental assessment, such as ecological risk assessment, environmental impact assessment, and cumulative effect assessments as they clearly outline activities and stressors and clarify connections between human activities and potential effects on ecological endpoints, and provide a science-based foundation for decision-making. The objective of these models and their supporting evidence is to provide a systematic review of the effects of shipping-associated activities on marine ecosystems. PoE models have been developed for five activities associated with commercial marine shipping in Canada: 1) anchoring and mooring, 2) vessel at rest, 3) grounding and sinking, 4) movement underway, and 5) discharge (divided into two PoE models: ‘debris’ and ‘other’). The PoEs were developed to be broad enough to be adapted for application in a range of environments and locations and detail the potential stressors and effects that could be considered in an assessment. The activity-based PoE models contain fourteen stressors (e.g., substrate disturbance, vessel strikes) and are related to three effects (change in fitness, mortality, and change in habitat) on ten generic endpoints (e.g., marine mammals, physical habitat). The models only include activities related to the commercial movement of goods and people by vessels, not included in this document are other vessel activities such as fishing, seismic surveying, dredging, port operations (e.g., when at-berth and while berthing). Non-commercial vessels (e.g., recreational vessels) are also not specifically included in these models. Though endpoints have been identified for illustrative purposes here, ultimately the assessor is responsible for comprehensively scoping the specific endpoints (e.g., valued components) and stressors to be considered in any assessment. PoE models do not include any evaluation of the relative or absolute impact from these activities on specific endpoints; this would occur in a subsequent assessment step, such as risk assessment.
... shell fishing, boat anchoring and mooring in some areas of the bay; Barragan- Muñoz and Andrés-García, 2020). This type of pressures modifies sediment stability with consequences for vegetation survival (Collins et al., 2010) and, indirectly, for light availability. From low intertidal to shallow subtidal elevations, the benthos of Cadiz Bay is covered by dense populations of Cymodocea nodosa (aprox. ...
... The resilience of C. nodosa could improve with strategies to regulate activities directly modifying sediment stability and water transparency (e.g. shell fishing or boat anchoring and mooring; Collins et al., 2010), or by regulating activities indirectly affecting water transparency as nutrient loading (i.e. eutrophication; Paar et al., 2021). ...
Article
Full-text available
Cadiz Bay is a shallow mesotidal lagoon with extensive populations of the seagrass Cymodocea nodosa at intertidal and shallow subtidal elevations. This work aims to understand the mechanisms behind the resilience of this species to gradual sea level rise by studying its acclimation capacity to depth along the shallow littoral, and therefore, to gradual variations in the light environment. To address this objective, these populations have been monitored seasonally over a 10 year period, representing the longest seasonal database available in the literature for this species. The monitoring included populations at 0.4, − 0.08 and − 0.5 m LAT. The results show that C. nodosa has a strong seasonality for demographic and shoot dynamic properties-with longer shoots and larger growth in summer (high temperature) than in winter (low temperature), but also some losses. Moreover, shoots have different leaf morphometry depending on depth, with small and dense shoots in the intertidal areas (0.4 m) and sparse large shoots in the subtidal ones (− 0.08 and 0.5 m). These differences in morphometry and shoot dynamic properties, combined with the differences in shoot density, explain the lack of differences in meadow production balance (i.e. meadow growth-meadow losses) between the intertidal (0.4 m) and the deepest population (− 0.5 m), supporting the long term resilience of Cymodocea nodosa in Cadiz Bay. This study contributes to the understanding of the mechanisms behind seagrass stability and resilience, which is particularly important towards predicting the effects of climate change on these key coastal ecosystems, and also highlights the value of continuous long-term monitoring efforts to evaluate seagrass trajectories.
... In Mexico, the health of seagrass and coral reef ecosystems play a crucial role to coastal tourism [ 19 , 39 , 55 ], which in Quintana Roo alone, it is worth $10 billion per year [54] In Mexico, seagrass meadows cover an area of 400,000 ha of coastal waters and the standing stock of stored carbon biomass in seagrass is estimated at 48 million tonnes [24] . This biomass supports fish populations and higher organisms in adjacent ecosystems [16] . Each year global seagrass loss continues at rates estimated at 7% [71] . ...
Article
Sargassum mats in Mexican bays reduce the biodiversity of coral and seagrass nursery habitats. Three bays in Quintana Roo, Mexico were chosen to determine the environmental stress caused by Sargassum natans and S. fluitans on coral, seagrass and fish populations. For both control sites, Yal Ku Lagoon and Half Moon Bay with little to zero Sargassum cover, benthic communities and the physico chemical characteristics of the waters were not impacted. In Soliman Bay, Sargassum mats cover large areas in the shallows and shore and smother the seagrass and corals. Under the Sargassum mats light and dissolved oxygen levels were significantly lower. Anoxic conditions were found, with levels as low as 0.5mg/L for oxygen and a 73 % decrease in light. Water temperature was 5.2 ± 0.1˚C (mean ± SE) warmer under the Sargassum mats. By determination of weight (grams per day) and growth (mm per day), the stress caused by Sargassum mats in Soliman Bay caused a seven-fold decrease in productivity of T. testudinum compared to other sites. Taxonomic diversity was also reduced with lower biomass and an altered species distribution. To improve these ecosystems, pre-emptive conservation management and protection must be priority for future ecosystem health and biodiversity.
... Grain size, wave exposure and current velocities will all influence the mobility of the sediment at a particular location (Soulsby, 1997). In a study by Collins et al. (2010), the sediment in unvegetated areas linked to anchoring and mooring disturbance were less cohesive, contained less organic material and had a lower silt fraction than surrounding habitats where seagrass was present. ...
Article
Full-text available
As the artificial defenses often required for urban and industrial development, such as seawalls, breakwaters, and bund walls, directly replace natural habitats, they may produce population fragmentation and a disruption of ecological connectivity, compromising the delivery of ecosystem services. Such problems have increasingly been addressed through “Working with Nature” (WwN) techniques, wherein natural features such as species and habitats are included as additional functional components within the design of built infrastructure. There now exists a convincing body of empirical evidence that WwN techniques can enhance the structural integrity of coastal works, and at the same time promote biodiversity and ecosystem services. While these benefits have often been achieved through modification of the hard surfaces of the coastal defense structures themselves, the desired ecological and engineering goals may often demand the creation of new soft substrates from sediment. Here we discuss the design considerations for creating new sediment habitats in the intertidal zone within new coastal infrastructure works. We focus on the sediment control structures required to satisfy the physiological and ecological requirements of seagrass and mangroves – two keystone intertidal species that are common candidates for restoration – and illustrate the concepts by discussing the case study of soft habitat creation within a major multi-commodity port.
... The first one is due to the commercial activities developed inside (accidental spills, hull cleaning, painting, etc.,) and, secondly, because they are receptors or industrial and urban activities (e.g., wastewater emissions). The environmental impacts of marinas have long been a topic to many scholars and researchers [100,155,156], specially related to recreational boats which are responsible for significant environmental challenges for waterways and seagrass [95,[157][158][159]. ...
Article
Full-text available
Marinas are known to be features related to nautical tourism. Nevertheless, the responsibility of managers does not lie solely on providing accurate services to boats, seafarers and visitors. Thus, an effective management should include other factors, because marinas are embodied in a singular space, with links to diverse nature. Landscape, known as the relationship between people and their environment, represents a set of these links. In this paper, we attempt to delve into the marina management and landscape. Firstly, a targeted exploration of the main issues related to the management of marinas was accomplished. Secondly, based on the previous items, a screening was carried out from a landscape viewpoint with the aim to stablish which elements of marina’s management are significant when tackling landscape. The results indicated that there is a concern with environmental aspects, specifically, on issues related to marine pollution and water quality. However, the determination of the main management-related issues, valued from a landscape perspective, may provide the main issues that need to be addressed in decision-making processes, incorporating the landscape dimension. Thus, we have attempted to understand and discuss how the landscape should be considered in marina management as a potential competitive advantage.
... While chain moorings are commonly used worldwide, they have a range of undesirable impacts. Most obviously, these include destruction of the biodiversity of the impacted benthic communities (Herbert et al. 2009;Serrano et al. 2016), which include plant communities such as seagrass meadows (Collins et al. 2010;Glasby and West 2018;Unsworth et al. 2017). These habitats have been identified as nursey grounds for species important for both commercial and recreational fisheries (Nordlund et al. 2018). ...
Technical Report
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We have completed and analysed performance assessment surveys at nine local population sites for spotted handfish in the Derwent estuary from 2015-2019. To this time series we have also incorporated historic data for individual sites back to 1998. Local populations generally show stability of occurrence but with some difference in abundance (as measured by estimates of fish densities per habitat) by years. At one site, Ralphs Bay, while there has been a large reduction in population size over the last 15 years, animals continue to persist in low numbers. At another site with low densities, Howrah Beach, no fish were sighted in 2019. At both these sites the densities of animals may be at levels that are currently difficult to detect with our current survey effort (<3 per hectare). Outside of the Derwent Estuary several more potential local populations were identified. In additional to a population discovered in the D’ Entrecasteaux Channel in 2016, the National Handfish Recovery Team (NHRT) received notification from Huon Aquaculture (HAC) and Tassal that they had both observed spotted handfish during Remotely Operated Vehicle (ROV) surveys undertaken as part of environmental assessments in Storm Bay. HAC also reported an additional site in the Huon estuary, adding to the near-by site reported in 2016. This brings the total known extant locations for spotted handfish to 13. In 2018 we tested a new type of ceramic artificial spawning habitat (ASH) for spotted handfish. Fish showed a preference for ceramic ASH but this habitat type had a lower survival rate than plastic. We redesigned the ASH and found it had a much-improved survivorship compared to the old design, though at one site high losses of all ASH types still occurred. As we found that ASH was only used by handfish when densities of natural spawning substrates (ascidians both native and introduced) were below a critical level, in the 2019 surveys we counted ascidians as well as handfish which enabled improved targeting of our ASH plantings Juvenile red and spotted handfish have continued to survive and grow in captivity, though in two cases groups of 10 and 8 spotted handfish were lost to disease. There were no captive breeding events by the adult spotted handfish in the 2018-2019 season but both the lone captive red handfish and a spotted handfish did lay infertile eggs. We hypothesis, based on their observed natural behaviours that fish may need to be kept in isolation to become gravid. Two more red handfish egg masses were collected and ~50 juveniles have since hatched. Successful captive breeding will require a method to determine sex of handfish with minimal disturbance as well as an improved understanding of breeding biology and behaviours. Genetic analysis of spotted handfish was conducted on fin clips collected between 1998-2008 across multiple sites in Derwent estuary. Local populations, while genetically diverse, appear to be isolated from each other, with the exception of their closest neighbouring population. This suggests that local populations were not subject to in-breeding effects when these samples were taken. Local populations, or at best groups of nearest neighbours’ local populations, should be considered independently for conservation management for if they collapse then it is unlikely they will re-establish naturally from outside recruitment. From opportunistically sourced genetic material, eDNA markers for red handfish have also been developed. This may provide a method to detect and narrow the search for unknown populations of red and possibly other species of handfish. Conservation of Handfish Four environmentally sensitive (ES) moorings were deployed and one ‘before’ and two ‘after’ rounds of assessments conducted. At one site spotted handfish were observed in the recovering scar from the removed chain mooring. Modelling of various mooring designs suggests that ES mooring designs reduce maximum loads in various vessels by between 39% to 57% in extreme weather conditions, when compared to traditional chain moorings. A website handfish.org and a fundraising campaign to “name a handfish” were also launched as part of the Handfish Conservation Project.
... Scars showed evidence of intensive sediment mixing, which lead to the OC stocks being significantly lower than sediments under undisturbed seagrass [47]. In the UK, moorings, which are also present at Studland Bay, have also been shown to negatively impact seagrass cover with one mooring chain potentially responsible for the loss of up to 122m 2 of local seagrass [48]. Studland Bay is one of the most highly contested seagrass sites in the UK, with forceful opinion on either side as to whether it should be designated as an MCZ. ...
Conference Paper
The requirements of nations to respond to the Paris Climate Agreement by outlining National Determined Contributions (NDC’s) to reduce their emissions is placing an increased global focus on the spatial extent, loss and restoration of seagrass meadows. Despite such interest, local carbon storage trends and the spatial extent of seagrass remains poorly mapped globally, and knowledge of historical loss is limited. In the British Isles this information is largely absent. The primary aim of this work was to provide a foundation of knowledge on seagrass Blue Carbon and the status of seagrass in the British Isles, to 1) better inform local conservation and management, and 2) further advance the field’s understanding of trends in sediment carbon storage. The work raised questions about the globally accepted standards for Blue Carbon research, particularly in extrapolating estimates from short (<40cm) to long (>100cm) cores. This underestimated carbon stocks by >40% in one site. Across 13 studied seagrass meadows, seagrass carbon stocks were similar, apart from at one anomalous site, and differences could not be explained by sediment silt content or aboveground biomass. Despite local similarities, on a European scale the average recorded carbon stocks were high, representing the second most carbon dense sediment per hectare of any documented European country. I found that seagrass sediments disturbed by anchoring and mooring activates had significantly less sediment carbon than undisturbed seagrass sediment. Finally, I documented 8,493 ha of recently mapped seagrass in the British Isles. With high certainty, 41% of British seagrass has been lost since 1936, and historic seagrass losses could be as high as 92%. The results are discussed in terms of conservation and management of seagrass, particularly pertaining to Blue Carbon provisions.
... The loss of seagrass meadows due to boat anchors has been documented in India for various seagrass species of Palk bay and Gulf of Mannar region 9 . Outside India, it has been reported for species like Zostera marina (Linnaeus) of San Francisco Bay, USA 10 and Studland Bay, UK 11,12 Posidonia oceanica [(Linnaeus) Delile] in the Mediterranean Sea 13,14 and mixed seagrass species of Rottnest Island, Australia 15 . Loss of seagrass meadows eventually resulted in the loss of valuable ecosystem services, such as release of stored carbon of 4.2 kg Corg m -2 (ref. ...
Preprint
Full-text available
Seagrass ecosystems are lost due to habitat disturbance, coastal development and human pressure. We assessed the impact of boat anchors from traditional fishing and recreational activities on the seagrass Halophila ovalis from the Andaman and Nicobar Isalnds of India. The plant density, biomass, morphometrics, canopy height and percentage cover were estimated from two sites of Govind Nagar beach of Andaman and Nicobar Islands. The shoot density of H. ovalis was reduced by physical damage caused by boat anchors. The morphometrics of H. ovalis, such as number of leaves per ramet, leaf length, width and horizontal rhizome length were significantly reduced when impacted by boat anchors. Seagrass canopy height and percentage cover were reduced by 41% and 47% respectively. Though the impact of boat anchors reported here is on small-scale, it may impact feeding grounds of locally endangered dugongs. Therefore, proper management and preventive measures should be implemented to prevent the loss of dugong grass habitats from tourism, recreational and fishing activities.
Thesis
The archipelago of the northern Baltic Sea contains shallow, submerged soft sediments that are colonised by diverse aquatic plant communities. Such diverse communities are valuable assets for investigating the relationships between species traits and ecosystem processes, to understand the ecology of submerged aquatic plants. This thesis constitutes three experiments conducted in situ using SCUBA in the northern Baltic Sea. The purpose of these experiments was to investigate how plant biomass production is related to plant functional traits, growth strategies, and functional diversity, as well as the role of infauna to plant functional trait-productivity relationships. Overall, results showed that plant functional diversity can be related to productivity likely by selecting for light capture traits, that the finite sediment nutrient source was likely affected by plant biomass-driven demands, and finally infauna can affect plant functional trait-productivity relationships. Overall, by using plant trait and functional diversity investigations, this thesis has improved the collective understanding of submerged aquatic plant functioning. Web link to thesis summary here: http://urn.fi/URN:ISBN:978-951-51-6087-4
Article
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The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40 % reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (tau(0)). Despite this increase in tau(0) there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [tau(e)] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z marina having little effect on sediment erodability (bare: tau(e) = 0.05 Pa; Z. marina: tau(e) = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment.
Article
Full-text available
Studland Bay contains an extensive seagrass (Zostera marina) bed and two species of seahorse: spiny (Hippocampus guttulatus) and short snouted (H. hippocampus). All are protected under UK and international legislation. Five H. guttulatus have been tagged and all re-sighted several times within the seagrass bed. Home ranges of 30–400m2 were found. The three tagged males have all been observed to be pregnant throughout the summer months suggesting up to five broods per year. On one occasion the courtship display was recorded. This study has demonstrated the value of volunteer divers in monitoring which would not have been so successful without them. This project arose from concerns over negative impacts of boat anchoring and mooring on the seagrass habitat. It is hoped that the insights gained into site fidelity, territory size and breeding ecology of the seahorses will contribute to management plans for this unique site.
Article
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Many natural and human-induced events create disturbances in seagrasses throughout the world, but quantifying losses of habitat is only beginning. Over the last decade, 90000 ha of seagrass loss have been documented although the actual area lost is certainly greater. Seagrasses, an assemblage of marine flowering plant species, are valuable structural and functional components of coastal ecosystems and are currently experiencing worldwide decline. This group of plants is known to support a complex trophic food web and a detritus-based food chain, as well as to provide sediment and nutrient filtration, sediment stabilization, and breeding and nursery areas for finfish and shellfish. We define disturbance, natural or human-induced, as any event that measurably alters resources available to seagrasses so that a plant response is induced that results in degradation or loss. Applying this definition, we find a common thread in many seemingly unrelated seagrass investigations. We review reports of seagrass loss from both published and ‘grey’ literature and evaluate the types of disturbances that have caused seagrass decline and disappearance. Almost certainly more seagrass has been lost globally than has been documented or even observed, but the lack of comprehensive monitoring and seagrass. mapping makes an assessment of true loss of this resource impossible to determine. Natural disturbances that are most commonly responsible for seagrass loss include hurricanes, earthquakes, disease, and grazing by herbivores. Human activities most affecting seagrasses are those which alter water quality or clarity: nutrient and sediment loading from runoff and sewage disposal, dredging and filling, pollution, upland development, and certain fishing practices. Seagrasses depend on an adequate degree of water clarity to sustain productivity in their submerged environment. Although natural events have been responsible for both large-scale and local losses of seagrass habitat, our evaluation suggests that human population expansion is now the most serious cause of seagrass habitat loss, and specifically that increasing anthropogenic inputs to the coastal oceans are primarily responsible for the world-wide decline in seagrasses.
Article
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1. A study was set up in the Port-Cros National Park in order to determine the effects of boat anchoring on Posidonia oceanica seagrass beds.2. Experiments on the effects of anchors on the seagrass meadows revealed that, on average, 34 shoots were destroyed during an anchoring cycle (lock-in and retrieval), especially when the seagrass mat compactness is weak and the extent of rhizome baring is high.3. Five parameters of the Posidonia oceanica beds (meadow cover, shoot density, extent of rhizome baring, proportion of plagiotropic rhizomes, degree of meadow fragmentation) were considered and it was shown that the extent of rhizome baring was not correlated with anchoring pressure. Meadow cover and mean shoot density were positively correlated with high anchoring pressure.4. The proportion of plagiotropic (i.e. horizontally growing) rhizomes and the degree of meadow fragmentation were positively correlated with moderate anchoring pressure. Copyright © 1999 John Wiley & Sons, Ltd.
Article
An inexpensive instrument, consisting of a small vane attached to the shaft of a calibrated torque screwdriver, has been developed for measuring the shear strength of sea-floor sediments. It can be used either in situ or on sediment samples removed to the ship or laboratory.
Article
Invertebrate faunas associated with vegetated and unvegetated habitats were sampled over a 15-month period in Western Port, Victoria, in order to identify consistent faunal differences between the two habitat types. Intertidal seagrass habitats in Western Port supported much higher numbers of macroinvertebrate species per 150 mm core () than intertidal unvegetated habitats (), with a mean of 17 species per core found in the deeper unvegetated channel habitats. Species within channel habitats were more patchily distributed than elsewhere; consequently, the total number of species collected from channel habitats over all sampling occasions was high (265 species) and close to that collected from seagrass habitats (300 species), with a lower number (185 species) collected from intertidal unvegetated habitats. Estimated annual epifaunal production was much higher in intertidal seagrass habitats (17.2 g·m−2·yr−1) than in unvegetated habitats (3.3 g·m−2·yr−1). A substantial decline in seagrass cover in Western Port over the past twenty years is inferred from these data to have reduced epifaunal production by an estimated ≈ 2500 tonnes ash-free dry weight (AFDW) per year within the bay. Annual infaunal production differed little between intertidal seagrass (62 g ·m−2·-yr−1) and unvegetated habitats (54g·m−2·yr−1), so total infaunal production has probably changed little over that time. However, infaunal production was significantly correlated with the amount of organic material in the sediment, so long-term declines in infaunal production may have occurred as a consequence of declining production levels of seagrass detritus. Two corers with different diameter were used to obtain data on faunal size-distribution patterns within the 125 μm to 16 mm animal size range. Small diameter (50 mm) corers were significantly more effective at sampling benthos <2 mm sieve size than large diameter (150 mm) corers. Faunal size-distribution patterns differed greatly between sampling locations, and meiofauna and small macrofauna were disproportionately abundant in the presence of seagrass. While faunal size-distribution plots were polymodal at most sites, production was rarely low at the macrofaunal/meiofaunal boundary, and size-related maxima and minima did not correspond with modes in sediment particle sizes, as was expected following Northern Hemisphere studies.
Article
Boat moorings have been found to produce circular scours in seagrass meadows, ranging from 3 to 300 m2. “Cyclone” moorings (which have three anchors and a swivel) are much less damaging to seagrass meadows than “swing” moorings (with a single anchor and chain).The total area of seagrass meadow lost due to moorings totals some 5.4 ha in the Rottnest Island, Warnbro Sound and Cockburn Sound regions of Western Australia, with most loss (3.14 ha) in the Rottnest region. While the relative area of seagrass meadow lost is small (<2%), there is considerable visual impact in some areas.The scours created by moorings in the seagrass canopy interfere with the physical integrity of the meadow. Though relatively small areas of seagrass are damaged by moorings, the effect is much greater than if an equivalent area was lost from the edge of a meadow.