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From mechanical to chemical impact of anchoring in seagrasses: The premises of anthropogenic patch generation in Posidonia oceanica meadows

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Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are used to study human impacts on the structure of seagrass meadows but they are not able to assess the indirect and long term implication of mechanical destruction. We chose to investigate the possible changes in the substrate chemistry given contrasted boat impacts. Our observations show that hydrogen sulfide concentrations remain high at 15 and 20 m depth (42.6 µM and 18.8 µM) several months after the highest period of anchoring during the summer. Moreover, our multidisciplinary study reveals that anchoring impacts of large boats at 15 and 20 m depth can potentially change the seascape structure. By taking into account both structural and chemical assessments, different managing strategies must be applied for coastal areas under anthropogenic pressures.
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Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
From mechanical to chemical impact of anchoring in seagrasses: The
premises of anthropogenic patch generation in
Posidonia oceanica
meadows
Arnaud Abadie a,d,c ,Pierre Lejeune a, Gérard Pergentc, Sylvie Gobert
a
Station de Recherches Sous-marines et Oceanographiques (STARESO), Pointe Revellata, BP 33,20260
Calvi, France b Laboratory of Oceanology. MARE Centre, University of Liege. B6C, 4000 LIEGE, Sart
Tilman, Belgium
c
EqEL-FRES 3041, UMR CNRS SPE 6134, University of Corsica, 20250 Corte, France
ABSTRACT
Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This
anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several
tools are used to study human impacts on the structure of seagrass meadows but they are not able to
assess the indirect and long term implication of mechanical destruction. We chose to investigate the
possible changes in the substrate chemistry given contrasted boat impacts. Our observations show
that hydrogen sulfide concentrations remain high at 15 and 20 m depth (42.6 µM and 18.8 µM) several
months after the highest period of anchoring during the summer. Moreover, our multidisciplinary
study reveals that anchoring impacts of large boats at 15 and 20 m depth can potentially change the
seascape structure. By taking into account both structural and chemical assessments, different
managing strategies must be applied for coastal areas under anthropogenic pressures.
Keywords:
Anchoring; Conservation; Seagrass; Seascape; Patch
1. Introduction
Over the last decades, marine ecosystems all around the world have been facing impacts of human
activities at various extents (Halpern et al. 2008; Jorda et al. 2012). This statement is particularly
observed in the Mediterranean Sea at the level of the coastal habitat formed by seagrass meadows
(Grech et al. 2012; Giakoumi et al. 2013). Seagrasses play a major ecological and economical role at the
level of the global ocean, covering an area reaching up to 500,000 km2 (Costanza et al. 1997; Short et
al. 2007; Cullen-Unsworth and Unsworth 2013). Thus, they constitute a nursery (Beck et al. 2001), a
large carbon sink (Fourqurean et al. 2012), as well as a protection against coastal erosion by
attenuating waves and currents (Ondiviela et al. 2014). Among Mediterranean seagrasses,
Posidonia
oceanica
(L.) Delile is the most studied due to its major ecological and economical role (Ruiz et al.
2009; Vassallo et al. 2013). The meadows it forms are observed from the surface to 40 m depth and are
subject to the impact of human activities like coastal development, eutrophication, trawling, fish farms
and anchoring (Boudouresque et al. 2009; Giakoumi et al. 2015b).
Along the French Mediterranean coasts, the main substrate affected by boat anchoring appear to be
Posidonia oceanica
(Holon et al. 2015). Anchoring inside
P
.
oceanica
meadows seems to have various
degrees of impact according to its density, frequency, the type of anchor and the depth as well as the
size of boats (Boudouresque et al. 2012). Thus, repeated anchoring of cruise ships, at depths greater
than 15 m, causes large-scale degradations of the meadows (Ganteaume et al. 2005b; Abadie et al.
2015). In the same way small units, less than 10 m long, can have an important impact at a local scale
(Francour et al. 1999; Milazzo et al. 2004; Ceccherelli et al. 2007).
At the present day, studies mainly targeted the degradation of small boats at shallow depths i.e. less
than 10 m. Few works treat the effects of larger pleasure ships anchoring which can measure more
than 80 m long and have an important impact in confined areas (Abadie 2012). In order to assess their
impact, several parameters are classically measured: the meadow density, the mat structure and the
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
bottom cover (Boudouresque et al. 1995; Francour et al. 1999; Pergent-Martini et al. 2005). However,
some of these metrics seem not relevant enough to assess the damages observed on P.
oceanica
meadows. More specifically, classical indicators can indicate a good state of conservation of the
meadow with no anthropogenic impact when tracks of bare mat (Fig. 1a) are clearly observed (Milazzo
et al. 2004; Ganteaume et al. 2005a).
Intensive anchoring can lead to modifications of substrate qualities, passing from meadows to large
bare mat areas in which anchoring tracks are visible (Fig. 1b). This phenomenon also induces a change
in sediments nature going from carbonate sediments possibly oxygenized by the living plant to fine
particles filling crevices inside decomposing organic tissues forming an anoxic bare mat (Mateo and
Romero 1997). Such evolution of the substrate qualities can lead to the hydrogen sulfide (H2S)
intrusion in healthy meadows of the area, limiting the plant development (Holmer et al. 2003; Marbà et
al. 2006). Thus, it has been observed that in carbonate sediments H2S concentrations higher than 10
µM can cause a limitation of P.
oceanica
growth (Calleja et al. 2007).
This study aims to trigger a new way to approach the study of the anchoring impact on seagrass
meadows by (1) testing the relevance of the classical structural tools (e.g. meadow density and cover,
mat compactness); (2) exploring the relevance of chemical properties of the sediment as a new tool;
and by extension; (3) assessing the impact of large leisure ships in a confined area; and lastly, (4)
investigating the possible consequences for management and conservation of the areas concerned
correlated with anchoring pressure.
Fig. 1. a) Anchoring track inside a
P. oceanica
meadow at 30 m depth in CalviBay (Corsica, France); b)
Bare mat
of Posidonia oceanica
generated by intensive anchoring at 18 m depth in Calvi Bay (Corsica,
France) with furrows dug by large ships (photos: Arnaud Abadie).
2. Material and methods
This study was conducted in Alga Bay (8°43'52" E; 42°34'20" N), an area of 1 km2 of intensive anchoring
in Calvi Bay (Corsica, France), colonized by a
P. oceanica
meadow covering 0.78 km2 (Fig. 2). This site
encompasses a particular structure called "return river", a large sand patch where no seagrass meadow
can grow, possibly due to strong bottom currents deriving from the surface ones reflected by the coast
as described by Boudouresque and Meinesz (1982).
Six stations on two different sites in Calvi Bay were studied at three different depths, i.e. 10 m, 15 m
and 20 m. Three stations were chosen as control in a continuous meadow with no traces of impact
from human activities near the research facility of STARESO (C10, C15 and C20). Three stations were
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
sampled at Alga Bay in areas of intensive anchoring (A10, A15 and A20) where it can generate
anthropogenic patches.
Fig. 2. Map of the study site. Green polygons in Calvi represent the mapping
of Posidonia oceamica
meadows realized with data of 2010 (Abadie 2012).
2.1. Anchoring pressure assessment
A boat counting in Alga Bay was daily performed in the afternoon from May to October 2014 (the
touristic period in Corsica) where anchoring frequency is the higher. Ships sizes were classified in three
categories according to their length: <10 m; 10-20 m; and >20 m. In parallel, the substrate of
anchoring (meadow, rock or sand) was assessed. Moreover, the spatial distribution of boats anchored
in the area was investigated by using AIS positioning system of leisure boats (www.marinetraffic.com)
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
from 2012 to 2014 as well as direct catches obtained between 2012 and 2014. These observations were
inserted in the G1S software ArcGis® 10 coupled with a map of P.
oceanica
meadows in the area from
previous studies (Michel et al. 2012; Jousseaume et al. 2013; Richir et al. 2015).
2.2. Meadow structure
The impact of anchoring on the
P. oceanica
meadows was assessed using six metrics commonly used
in the study of its impact on seagrass meadows, i.e. the density, the proportion of
orthotropic/plagiotropic rhizomes, the mat compactness, and the rhizomes baring. Ten replicates of
the meadow density were randomly counted using quadrats of 25 cm x 40 cm and classified according
to the grid of UNEP-MAP-RAC/ SPA (2011). Assessment of the proportion of orthotropic/plagiotropic
rhizomes was performed during density measures and interpreted thanks to the classes made by
Charbonnel et al. (2000) (Table 1). Mat compactness was investigated given the method and
classification of Francour et al. (1999) using a 1 m long rod and a 5 kg weight, repeating ten times the
measure for each station (Table 1). Twenty replicates per station of the rhizomes baring, i.e. the
distance between rhizome and substrate, were measured and classified according the protocol of
Boudouresque et al. (1980). Meadow cover was measured using a 30 cm x 30 cm quadrat hold at arm-
length 3 m above the meadow (Gravez et al. 1995). Thirty replicates were performed for this measure
and results were interpreted given the scale of Charbonnel et al. (2000) (Table 1). This measure was
standardized by keeping the same observer for all measures and placing a depth gauge on the
quadrat in order to avoid a distance variation from the vegetation. Finally, 20 longest standing leaves
per station (corresponding to the canopy height) were measured in September after the touristic
period).
2.3. Conservation Index (CI)
The Conservation Index (CI) was used as a reflection of damages in
P. oceanica
meadows visually
observed by scuba diving. Triplicate transects were made to calculate the CI for each station according
to the process of Moreno et al. (2001) :
CI = L/(L + D)
where L (%) corresponds to the proportion of living
P. oceanica
and D (%) the percentage of bare mat.
Four intervals were calculated to assess the meadow's state of conservation on each station:
1. CI<(xmean-1/2s)
2. CI from (xmean-1/2s) to xmean
3. CI from xmean to (xmean+1/2s)
4. CI>(xmean+1/2s)
where mean (xmean) and standard deviation (s) were calculated from all Cl values of the study.
2.4. Sediment chemistry and nutrients
Sediment chemistry was studied by sampling pore water in the control (C) and anchoring (Am)
meadow, as well as in anchoring bare mat patches' sediments (Ap) given the method of Gobert et al.
(2006). Collection was performed in September after the warmest period of the year and in November
when seawater temperature starts to decrease.
Thus, the concentration of several essential components was studied in the substrate: dissolved
dioxygen (O2), free hydrogen sulfides (H2S) and nutrients.
The pore water sampling for O2 measurements was made within the oxygenic layer at a maximum
depth of 1 cm in the sediments. O2 concentration was obtained using a iodine titration with thiosulfate
according to the method of Winkler (1888) with an automatized system for small sampling volumes
(Carpenter 1965; Strickland and Parsons 1972) adapted by R. Biondo, (Laboratory of Oceanology-
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
University of Liège).
The sample collection for H2S and nutrient analysis was performed in triplicates inside the layer
encompassing the plant living parts at 10 cm depth in the substrate. H2S concentration was measured
with a silver/ sulfide ISM-146 FTH 25-XS electrode, coupled with a Sulfide
Anti-
Oxydant Buffer (SAOB)
solution given the protocol of Brooks (2001). For detailed protocols of the measure of O2 and H2S, see
Appendix S1 and S2 in supplementary material. Ammonium (NH4+) and Nitrite (NO2-)/ Nitrate (NO3-)
concentrations were measured by using a SKALAR auto-analyzer following the method of Aminot and
Kérouel (2007) adapted for oligotrophic samples (Laboratory of Oceanology-University of Liège).
Table 1: Meadow structure parameter interpretation according to their value.
Parameter
Unit
Range
Interpretation
Classification reference
Meadow density
Shoots.m-2
Depends of depth
High
Good
Normal
Moderate
Bad
UNEP-MAP-RAC/SPA 2011
Proportion of
orthotrophic/plagiotropic
rhizomes
%
<30%
30 to 70%
>70%
Stable meadow
Slight trend to progress
Net trend to progress
Charbonnel et al. 2000
Mat compactness
cm
<50cm
50 to 100 cm
>100 cm
Strong compactness
Medium compactness
Weak compactness
Francour et al. 1999
Rhizomes baring
cm
<5cm
5 to 15 cm
>15cm
Low baring
Medium baring
High baring
Boudouresque et al. 1980
Meadow cover
%
>80%
60-80%
40-60%
20-40%
<20%
Very high covering
High covering
Medium covering
Low covering
Very low covering
Charbonnel et al. 2000
2.5. Statistical analyses
Statistical analyses were performed under the R 3.0.2 software using the FactoMineR package.
Normality of structural parameters values was checked using a Shapiro-Wilk test. Stations were then
statically tested two by two (control vs anchoring) for each depth (10 m, 15 m and 20 m) with an
unpaired t-test (after checking their homoscedasticity with a Fisher test) for Gaussian data, and with a
Mann Whitney test for non-parametric ones.
T
-tests were followed by a Tukey post-hoc test and Mann
Whitney tests by a Dunns test.
Relations between the structural (i.e. meadow density, mat compactness, rhizomes baring, meadow
cover, plagiotropic/orthotropic rhizomes proportion, Conservation Index and canopy height) and
chemical (i.e. O2, H2S and NH4+ concentrations) parameters were investigated with a Pearson matrix of
correlation.
Finally, a cluster analysis was performed using the Ward method of aggregation and Euclidean
distances between individuals for testing dissimilarity. Clusters were thus defined by minimizing the
loss of inertia (i.e. the Euclidean distance) between several individuals (i.e. stations) when grouping
them. Prior to the analyses, data were standardized to take into account the difference of units. First,
structural parameters alone were considered. Then only chemical features were used. Finally, both
structural and chemical parameters were computed in order to study the impact of chemical
parameters on the evaluation of the meadows' state of conservation.
3. Results
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
3.1. Ships: spatial distribution and frequenting
The spatial ship distribution in Alga Bay follows a bathymetrical zo-nation according to data from 2012
to 2014 (Fig. 3). Small boats (length < 10 m) appear to anchor at shallow depths (< 5 m, Fig. 3). The
majority of ships measuring between 10 and 20 m prefers to anchor outside the meadow in the main
part of the return river at depths shallower than 15 m, when bigger ships choose to lay their anchors in
the meadow from 10 to 30 m depth (Fig. 3).
Anchoring substrates vary widely according to ship sizes in Alga Bay (Fig. 3). Sand appears to be the
preferred anchoring substrate for small boats (53%, Fig. 3) while those of medium and large size chose
P. oceanica
meadows (respectively 55% and 84%, Fig. 3). In general, ships anchor equally in sand (45%)
and meadow (47%), few anchoring on rock (8%, Fig. 3).
A total of 1768 ships anchoring in Alga Bay were observed from May to October 2014, encompassing
43% (754 ships) of small boats (length < 10 m), 53% (935 ships) of medium size (length 10-20 m) and
7% (79 ships) of large size (length > 20 m) (Fig. 4). Period of most intense anchoring occurs from mid -
July to mid-August, reaching a frequenting peak of 74 ships the 7th of August (Fig. 4).
3.2. Meadow structure and conservation
Meadow densities are classified as "normal" according to their depths for two control (C) stations (C10
and C20) and all anchoring (A) stations A10, A15 and A20 (Fig. 5a). The control station at 15 m depth
appears as "good" (437 ± 112 shoots.m-1). Mean values of meadow density obtained at 15 m depth are
significantly different (t-test:
t
= 892;
p
= 0.0097; df = 18). The same statement is made concerning the
proportion of orthotropic/plagiotropic rhizomes which correspond to a stable meadow for all stations
(Fig. 5e) except for the station A20 which is considered to have a "slight trend to progress" (70/30 ±
25%). Mat compactness is characterized as "strong" for all stations sampled (Fig. 5b) with the
exception of A20 with a "medium" mat compactness revealed by a relatively high penetration length of
the rode (51 ± 9 cm). A significant difference (
t
-test:
t
= 6.172;
p
< 0.0001; df = 18) of the compactness
was observed at 20 m between the control station and the anchoring one. Rhizomes baring is
"medium" for all stations except for C10 (3.6 ± 1.9 cm) and A20 (2.0 ± 1.1 cm) where it is "weak" (Fig.
5c), these differences at 10 m (
t
-test:
t =
4.493;
p =
0.0001; df = 28) and 20 m (Mann Whitney test:
p =
0.0002; U = 24) being confirmed by the statistical analysis. Meadow cover varies from "very high" (C10,
C15, and A10) to "high" (C20, A15 and A20) (Fig. 5d), thus highlighting a difference of the mean
meadow cover between the control station and the anchoring one at 15 m depth (Mann Whitney test:
p <
0.0001; U = 182.5). At last, canopy height shows no significant differences between control and
anchoring stations, its value decreasing from June to September at all depths (Fig. 5f).
The state of conservation of each control station, expressed by the Conservation index (CI), is the
higher for C10 and C15 (1 ± 0.00 for each station), and decreases (0.98 ± 0.03) for C20 (Table 2). At
Alga Bay, the anchoring site, the station at 10 m depth (A10) appears to have a state of conservation
(CI = 0.97 ± 0.03) similar to C20 while the deeper station stations (A15 and A20) have a lowest one
(with a Cl of respectively 0.85 ± 0.11 and 0.79 ± 0.15; Table 2).
Table 2 : Interpretation scale of the Conservation Index (CI) and its mean value SE). C: control; A:
anchoring. 10, 15 and 20: depth.
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
Fig. 3. Ship positioning using AIS and direct catches from 2012 to 2014 at Alga Bay coupled with a
map of marine habitats and proportion of anchoring on the three different substrates in 2014 for ships
with a length lower than 10 m; between 10 and 20 m, upper than 20 m and for all classes.
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
Fig. 4. Daily boat counting and size classes from June to September 2014 in Alga Bay.
3.3. Patch and meadow chemistry
In September, lowest concentrations of O2 were found inside the patch at -10 m (Ap10: 102.7 µM)
while a decrease with the depth in control meadows is observed (Table 3). The same pattern is
observed in November with concentrations similar in both control (C) and anchoring (Am) meadows.
Low concentrations in O2 are related to high concentration in H2S inside the patches at the three
depths in September (20.5 µM at -10 m; 9.9 µM at 15 m; 12.8 µM at 20 m). High sulfide concentrations
are also found in control meadows in September, except at 15 m (Table 3). In November, H2S
concentrations are relatively low at -10 m for all stations when they remain high at 15 and 20 m
(except for C15).
NO2- and NO3- show very low concentrations in both September and November. NH4+ shows in
September an increase of its concentration along with the depth (Table 3), as well as higher values
inside anchoring patches (except for Ap15). The same pattern is observed in November with the
exception that lower values are found in anchoring patches at 10 m instead of 15 m (Table 3).
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
Fig. 5. Mean value (± SE) at 10,15 and 20 m depth of a) meadow density (green: good; yellow: normal)
; b) penetration length (green: high compactness; yellow: medium compactness); c) rhizomes baring
(green: weak; yellow: medium); d) meadow cover (blue: very high covering; green: high covering); e)
orthotropic/plagiotropic rhizomes proportion, colored bar: orthotropic, white bar: plagiotropic (green:
stable meadow; yellow: slight trend to progress); f) mean canopy height" above a pair of bars indicates
a significant difference between the two mean values. C = control; A = anchoring; j = June; s =
September.
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
Table 3 : Concentration of dissolved oxygen (O2) within the first centimeter of substrate and mean
concentration of free hydrogen sulfides (H2S), nitrite (NO2-), nitrate (NO3-) and ammonium (NH4+) in
the first ten centimeters of substrates SE) in September (Sep.) and November (Nov.) at control (C),
anchoring meadow (Am) and anchoring patch (Ap) stations.
O2(µM)
H2S (µM)
NO2- (µM)
NO3- (µM)
NH4+
(µM)
Station
Sep.
Nov.
Sep.
Nov.
Sep.
Nov.
Sep.
Nov.
Sep.
Nov.
C10
206.0
226.8
39.1 (±37.7)
4.0 (±1.1)
0.06 (±0.00)
0.20 (±0.12)
0.22 (±0.09)
1.92 (±1.54)
4.68 (±2.44)
4.40 (±1.56)
Am10
232.7
213.4
12.0 (±12.4)
4.2 (±0.8)
0.08 (±0.03)
0.06 (±0.00)
0.22 (±0.09)
0.30 (±0.21)
4.74 (±1.78)
4.44 (±0.87)
Ap10
102.7
143.6
20.5 (±30.2)
9.3 (±4.7)
0.06 (±0.00)
0.06 (±0.00)
0.14 (±0.03)
0.24 (±0.00)
10.18 (±8.13)
2.76 (±0.22)
C15
195.1
199.6
0.4 (±0.5)
8.0 (±5.3)
0.06 (±0.00)
0.08 (±0.03)
0.42 (±0.31)
0.38 (±0.18)
10.34 (±6.21)
6.32 (±3.81)
Am15
145.2
189.3
8.1 (±8.4)
33.4 (±44.7)
0.12 (±0.00)
0.08 (±0.03)
0.76 (±0.70)
0.42 (±0.33)
10.76 (±5.38)
12.38 (±8.65)
Ap15
116.9
122.0
9.9 (±7.2)
42.6 (±56.2)
0.08 (±0.03)
0.06 (±0.00)
0.16 (±0.03)
0.22 (±0.12)
6.70 (±3.47)
16.82 (±7.69)
C20
191.8
210.8
16.5 (±21.9)
20.9 (±19.4)
0.08 (±0.03)
0.08 (±0.03)
0.30 (±0.16)
0.34 (±0.19)
14.38 (±7.99)
7.44 (±3.31)
Am20
177.7
211.5
0.9 (±0.8)
13.2 (±16.1)
0.08 (±0.03)
0.08 (±0.03)
0.40 (±0.34)
0.26 (±0.09)
13.82 (±9.51)
7.70 (±5.77)
Ap20
155.5
183.6
12.8 (±21.0)
18.8 (±8.3)
0.06 (±0.00)
0.10 (±0.07)
0.16 (±0.07)
0.50 (±0.45)
20.74 (±26.40)
13.14 (±15.02)
Table 4 : Pearson's matrix of correlation comparing both structural and chemical parameters of the
control and anchoring meadow at a depth of 10,15 and 20 m. Density: meadow density; Compact: mat
compactness; Rhiz. Bar.: rhizome baring; Cover: meadow cover; Ortho. prop.: Orthotropic rhizomes
proportion; CI: conservation index; ch: canopy height; o2: oxygen; h2 s: hydrogen sulfide; nh4:
ammonium; j: June; s: September; n: November.
Variables
Density
Compact
Rhiz. Bar.
Cover
Ortho.
prop.
CI
ch_j
ch_s
o2_s
o2_n
h2s_s
h2s_n
nh4_s
nh4_n
Density
1
-0.556
0.463
0.876
0.703
0.740
0.720
0.696
0.660
0.282
0.410
-0.635
-0.900
-0.618
Compact.
1
-0.478
-0.369
-0.669
-0.787
-0.570
-0.462
-0.259
-0.146
-0.667
-0.004
0.452
0.180
Rhiz. Bar.
1
0.159
0.367
0.354
0.204
-0.096
0.360
-0.251
-0.063
0.052
-0.405
-0.041
Cover
1
0.681
0.652
0.896
0.881
0.812
0.689
0.588
-0.872
-0.889
-0.864
Ortho. prop.
1
0.972
0.807
0.492
0.794
0.464
0.430
-0.626
-0.470
-0.760
CI
1
0.763
0.530
0.658
0.364
0.496
-0.509
-0.492
-0.639
ch_j
1
0.848
0.852
0.826
0.767
-0.761
-0.744
-0.875
ch_s
1
0.533
0.752
0.857
-0.648
-0.814
-0.668
o2_s
1
0.687
0.329
-0.871
-0.612
-0.941
o2_n
1
0.710
-0.731
-0.438
-0.818
h2s_s
1
-0.289
-0.569
-0.427
h2s_n
1
0.594
0.961
nh4_s
1
0.574
nh4_n
1
3.4. Computation of structural and chemical parameters
Among the structural parameters, rhizome baring (Rhiz. Bar.) appears to have weak correlation with all
the other one (Table 4). Mat compactness (Compact.) appears to be less correlated with meadow cover
(Cover) and the canopy height in June (ch_j), this last parameter having also few links with the
proportion of orthotropic/plagiotropic rhizomes (Ortho. Prop.; Table 4). Between chemical parameters,
H2S concentrations in November (h2s_n) show a strong correlation with O2 (o2_n) and NH4+ (nh4_n)
ones at the same period (Table 4). This link is not found in September where only H2S and NH4+
concentrations are correlated. Through the two different periods, oxygen in September (o2_s) is
strongly correlated with the hydrogen sulfide in November (h2s_n) and ammonium (nh4_n). Looking at
both structural and chemical parameters, all chemical parameters appear correlated with meadow
cover (Cover) and canopy height (ch_j and ch_s) and to a lesser extent with meadow density (Density;
Table 4). In contrary, both mat compactness (Compact.) and rhizome baring (Rhiz. Bar.) are not linked.
These observations are more contrasted concerning the proportion of orthotropic/plagiotropic
rhizomes (Ortho. Prop.) and the Conservation Index (CI) where correlations are only found in
November for H2S and NH4.
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
The cluster analysis computing the structural parameters alone shows three classes linking anchoring
stations at 15 and 20 depths (A15 and A20), when the two control meadows corresponding (C15 and
C20) are grouped with the anchoring station at 10 m (A10), leaving the control one (C10) within a
single class (Fig. 6a). When using only chemical features the cluster result changes, aggregating the
stations A10 and C10 together and grouping C15, C20 and A20, leaving A15 alone (Fig. 6b). Adding the
chemical parameters to the structural ones, three classes, encompassing each two stations, are found
linking A15 and A20 but aggregating C10 with A10 and C15 with C20 (Fig. 6c).
Few variables being too highly correlated, i.e. with a correlation greater than 0.900 (Table 4), they are
not overrepresented in the clustering analysis.
Fig. 6. Cluster analysis of the stations described by a) structural parameters alone; b) chemical
parameters alone and c) both structural and chemical parameters of the control (C) and anchoring (A)
meadow at a depth of 10,15 and 20 m. The dotted line materializes classes' separation according to
their dissimilarity.
4. Discussion
By studying both structural and chemical parameters of two seagrass meadows, one facing intensive
anchoring and the other being under no known human pressure, this study highlights the influence of
large boats anchoring on the chemistry of
Posidonia oceanica
meadows' substrate and thus, on the
seascape structure too.
4.1. An intensive anchoring for a small area?
The first step in a study of anchoring impact on seagrasses should be the analysis and characterization
of its frequency according to the size and bathymetry of the area. In the present work, the anchoring
pressure at Alga Bay, reaching 0.8 boats.ha-1.d-1 during the peak period, appears moderate compared
to previous works in Corsica (Jousseaume et al. 2013) or in Port-Cros, France witnessing up to 8.8
boats.ha-1.d-1 (Ganteaume et al. 2005a). However, anchoring pressure cannot be described by boats
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
density alone but requires in complement the proportion of boats size and their favorite substrate. In
this case, although being less numerous than small and medium boats, big ships (length > 20 m)
largely favors meadows for anchoring (84%) to sandy and rocky bottoms, leading to more important
mechanical damages at higher depths (Ganteaume et al. 2005b). It is mainly due to the fact that these
ships need deep water to anchor and that
P. oceanica
meadows are more present at these depths.
Conversely, small boats anchor on shallow sites where rocky and sandy substrates are dominant. Thus,
taking into account all these aspects, anchoring appears to be intensive in Alga Bay with a high
probability of an impact visible on the meadow structure.
4.2. From structural to chemical impact of anchoring
In this study, the analysis of the classical parameters referring to the structure of the
Posidonia
oceanica
meadow are not clearly able to depict the direct observation made by scuba diving (i.e. large
patches of bare mat crossed by anchoring tracks, Fig. 1). Here rhizomes baring and mat compactness
appear not relevant for the study of anchoring, anchors impacting the superficial part of the mat while
mat compactness investigate the whole mat thickness. This statement has already been made for mat
compactness by Milazzo et al. (2004) and Ganteaume et al. (2005a) for meadow cover and density.
Unlike these works, this case encompasses the study of big boats damages and not only the one of
small to medium ships. Thus, large ships anchoring will directly pull out whole portions of the meadow
and lead to the creation of anthropogenic patches (Fig. 7), revealed by the Conservation Index, the
meadow density, and the proportion of orthotropic/plagiotropic rhizomes. In contrast rhizomes will
not be partially uprooted and no effect will be witnessed by the rhizomes baring and mat
compactness. Thus, large ships' anchoring causes a mechanical destruction similar to the trawling one
(Boudouresque et al. 2009; Kiparissis et al. 2011 ; Pergent etal. 2013).
Another limit of the structural parameters is their incapacity to assess the impact of the substrate
change (from leafy to bare mat).
Seagrasses are known to be able to release oxygen in the sediments through their roots to create a
small oxic zone (Pedersen et al. 1998; Greve et al. 2003), this function being suppressed by the
destruction of the canopy by anchoring and thus of the photosynthesis process. These modifications
are particularly observable by studying the impact of fish farms, their action leading to large areas of
bare mats where an increase of the organic matter in decomposition leads to a decrease of the oxygen
available and the intrusion of hydrogen sulfide (Pergent-Martini et al. 2006; Holmer and Frederiksen
2007; Apostolaki et al. 2010). Although no input of organic matter - at least not in the same range as
fish farms - are involved in anchoring, the same process is observed here with a decrease of oxygen
concentration at the surface of the sediments at all depths and an intrusion of hydrogen sulfide (H2S)
inside anchoring patches. High temperatures enhancing high concentrations of H2S in
Posidonia
oceanica
meadows (Garcia et al. 2012), a decrease along with the temperature should be observed in
this case in November. In contrary, an increase in both meadows and patches is observed at the
stations where the biggest boats anchor (A15 and A20), no thermocline being observed during the two
sampling periods with an homogeneous water column of a temperature of 23.5 °C in September and
20.8 °C in November (Richir et al. 2015).
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
Fig. 7. Hypothesis on the succession of processes from mechanical damages to the expansion of
anthropogenic patches; 1 ) destruction of the canopy by anchoring, 2) fine particles deposit leading to
an increase of the organic matter and its degradation; 3 ) settlement of the alien species
Caulerpa
cylindracea;
and 4) expansion of the anchoring patch with intrusion of hydrogen sulfide (H2S).
The higher concentrations of pore water inorganic nitrate found in Alga Bay at 15 and 20 m depth
inside anchoring patches and the surrounding meadow, are another consequence of the change in
substrate. Here, the higher concentrations in ammonium (NH4+) could be the result of the particulate
organic nitrate's ammonification (Romero et al. 2006). These values constitute a sufficient nutrient
enrichment for
P. oceanica
development although recolonization does not occur (Lopez et al. 1998;
Gobert et al. 2002).
These unsuitable conditions for a recolonization by the meadow (Marbà et al. 2006), coupled with the
continuation of the high anchoring rate in the area, will thus favor the expansion of anthropogenic
patches and modify the whole sediment chemistry (Pergent-Martini et al. 2006). In this way, a new
arrangement of anthropogenic patches, possibly combined with natural ones, leads to a new seascape
(Abadie et al. 2015). The new areas of bare mat thus created are a suitable substrate for the settlement
of the alien species
Caulerpa cylindracea
Sonder (Katsanevakis et al. 2010; Kiparissis et al. 2011) which
is able to release H2S inside sediments (Garcias-Bonet et al. 2008) (Fig. 7). The so new-generated small
patches may lead to a long-term larger fragmentation of the meadow through aggregation process.
Such phenomenon has been studied in
Zostera noltii
Hornemann meadows on the Portuguese coast
by Cunha et al. (2005). In a wider viewpoint, vegetation systems, and thus seagrass seascapes, are
theoretically subject to aggregation models (Irvine et al. 2016).
4.3. The contribution of chemical parameters in seagrass meadows conservation's assessment
This study highlights the fact that using structural tools alone to assess anchoring in seagrass
meadows can lead to a misevaluation of their state of conservation. In the present work, control
stations at 15 and 20 m, meaning meadows with no traces of impact from human activities, form a
cluster, i.e. they have the same characteristics than the anchoring station at 10 m depth. It reflects no
significant anchoring impact on the meadow at this depth. Similarly, chemical features alone aggregate
a station visibly impacted by anchoring (A20) with meadows (C15 and C20) under no anthropogenic
pressures. When adding chemical parameters to the structural ones, this station is classified in the
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
same group than the control site at the same depth, reveling that no impact is observable. In the same
way stations Al5 and A20 are grouped, stating the impact of anchoring at these depths. Considering
the long term process induced by a change in sediments chemistry, the measure of several chemical
parameters can provide information about the possible recovery of a meadow under an intensive
human pressure (Holmer et al. 2008). We suspect that it will be difficult for the meadow in Alga Bay at
15 and 20 m depth to recover given the continuation of anchoring while the meadow at 10 m seems
to have the same state of conservation than the control one.
Observation of chemical modifications within seagrass meadows linked with anthropogenic impacts
has already been highlighted by Jones and Unsworth (2016) in
Zostera marina
Linnaeus across the
British Coast. This study thus reveals an excess of nitrogen within
Z. marina
leaves, associating this
observation with the poor water quality and the disturbance of boat-based activities, of which
anchoring. Chemical response of the plant to anthropogenic pressures was also stated within
Cymodocea nodosa
(Ucria) Ascherson meadows in Greece by Papathanasiou et al. (2016). These
accounts indicate that physiological and chemical changes observed within the meadows are not
confined to
P. oceanica
and the Mediterranean Sea. They are also observed in other seagrass species
with contrasted morphology and seasonal response to environmental changes, supporting the
importance of multi-disciplinary approaches for conservation assessment.
Measurement of chemical features remains however time consuming and requires specific equipment
for each element studied. In this way, new simplified protocols should be developed. Nevertheless, like
structural parameters who have already proved their utility to assess seagrasses meadow state of
conservation (Montefalcone et al. 2006; Gobert et al. 2009; Lopez y Royo et al. 2010), chemical
measures are non-destructive, have a good capacity of replication and a great potential to provide a
deeper insight (Romero et al. 2007). Moreover, these tools can be integrated in the future conservation
indices based on an ecosystemic approach in the framework of the European Marine Strategy
Framework Directive (MSFD) (Personnic et al. 2014). They also can be used in the development of
descriptors specific to anchoring, built for an easy comprehension by stakeholders and managers.
5. Conclusion
Mechanical damages of intensive anchoring in
P. oceanica
meadows induce a change in the substrate
nature leading to the generation of bare mat areas (anthropogenic patches) at 15 and 20 m depth
where the bigger ships are observed. Modifications in chemical processes, and more precisely the
intrusion of hydrogen sulfide, decrease the possibility of a recolonization by the meadow leading to
the expansion of patches. The development of non-destructive chemical indicators easy to perform,
coupled with structural tools, will provide precious information for assisting decisions in conservation
issues about mechanical impacts in seagrass meadows which, in some cases, are stoppable when
effectively assessed (Giakoumi et al. 2015a).
Acknowledgment
The authors thank Willy Champenois and Alberto V. Borges from the Chemical Oceanography Unit for
their scientific advice and participation all along the analysis process of oxygen and hydrogen sulfides,
as well as Renzo Biondo from the Laboratory of Oceanology of the University of Liège for his support
during the nutrient measurements. This study is part of the STARE-CAPMED (STAtion of Reference and
rEsearch on Change of local and global Anthropogenic Pressures on Mediterranean Ecosystems Drifts)
program funded by the Territorial Collectivity of Corsica and by The French Water Agency (PACA-
Corsica). A. Abadie acknowledges a C1FRE Ph.D. grant (2013/0470) of the French ANRT (Association
Nationale Recherche Technologie). This article is the MARE publication No. 332.
Published in: Marine Pollution Bulletin (2016)
Status: Postprint (author’s version)
Appendix A. Supplementary data
Supplementary data to this article can be found online at
http://dx. doi.org/10.1016/j.marpolbul.2016.06.022.
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... The environmental impacts of anchoring have received increasing attention since the earliest identified scientific publication [1]. Most studies have focused on leisure boats [4][5][6][7][8][9][10][11] but there are also a growing number of studies on anchorage by commercial ships [2,[12][13][14][15]. Still, there are significant knowledge gaps relating e.g. to long term impacts on different kinds of benthic environments from anchoring of large ships [2,12]. ...
... Particularly Sensitive Sea Areas (PSSA) and marine spatial planning are the most promising area-based tools that could be used to regulate anchoring explicitly. Special areas and emission control areas provided in the 1973/1978 International Convention for the Prevention of Pollution from Ships, as amended (MARPOL), 4 do not include any other associated protective measure and therefore contribute only incidentally to reducing pollution in anchoring areas that are physically located within the established boundaries of such areas. ...
... In M/V Virginia G, ITLOS held that such a connection between fishing and the bunkering of foreign vessels fishing in the EEZ exists, since this enables (fishing vessels) to continue their activities without interruption at sea" [47]. 4 regulating ships at anchor. Since ships do not have an indefinite right to anchor, national legislation may well impose time-related limitations on ships at anchor [64]. ...
Article
Full-text available
Ships waiting at anchor are associated with considerable environmental pressures and impacts. Growing and congested anchoring areas are receiving increasing scholarly attention to understand the environmental effects caused by large sea-going ships anchoring in coastal waters. While there is a lack of studies addressing the entire spectrum of environmental pressures and impacts from ships at anchor, ranging from scouring of the seafloor, operational emissions and discharges and waste from maintenance carried out while at anchor, it is well established that anchoring is associated with such pressures. This article takes a problem-oriented approach since there are potential cumulative environmental impacts of ships waiting at anchor. From both a public and private law perspective, we examine the legal structures and challenges associated with the regulation of ships waiting at anchor. We also analyze the public and private law factors that may limit the ability to prevent harmful anchoring practices. Our examination shows that while coastal states have significant jurisdictional powers to regulate anchoring in coastal waters, the current international regulatory framework addresses anchoring incidentally and lacks mechanisms for considering the cumulative impacts of anchoring. Furthermore, the incentives for ships to spend a considerable amount of time at anchor appear to differ substantially across different types of charterparties. Improved regulation requires better scientific knowledge, substantial mapping of legal structures, and a stakeholder survey providing a basis for the exploration of potential contractual practices that may reduce market incentives for anchoring.
... The intense activity of pleasure boating can lead to a decrease in the structural complexity of these habitats and to the loss of specific richness (Venturini et al., 2016). The direct mechanical damage caused by anchoring on seagrass meadows is the most studied damage worldwide and in the Mediterranean (Francour et al., 1999;Milazzo et al., 2002Milazzo et al., , 2004Cancemi et al., 2008;Lloret et al., 2008;Abadie et al., 2016;Paoli et al., 2020;Rigo et al., 2020). The extent of the damage can be influenced by the type of habitats where the anchoring takes place, the number and the size of boats, the type of anchor, the weather conditions and the compactness/solidity of the seabed (Milazzo et al., 2002). ...
... P. oceanica provides essential habitats for many coastal species, being a reproduction and nursery area. Moreover, this seagrass is a source of oxygen, a trap for sediments favouring the water transparency, a reducer of hydrodynamism, defending coasts from erosion, also thanks to the creation of banquettes (Guillén et al., 2013;Vassallo et al., 2013;Abadie et al., 2016;Paoli et al., 2016;Traboni et al., 2018). ...
Article
Recreational boating is a worldwide leisure activity playing an important role in the context of coastal economy but also causing significant damage on the environment, above all in the absence of adequate management measures. Among the damages, the anchoring on sensitive habitats, with the consequent loss of structural complexity, density, cover and specific richness, is widely reckoned and its consequences are long lasting and evident. The aim of this work is to provide an environmentally focused accounting model (SAILOR) to evaluate the net impact of anchoring on sensitive habitats. In this research SAILOR model has been tested on Posidonia oceanica, one among the most vulnerable in Mediterranean. An innovative approach is proposed in order to develop sustainable management strategies for recreational boating in MPAs in the framework of a very strong sustainability. In particular, it starts from the assessment of the natural capital of P. oceanica with the purpose to keep it at least intact. The model procedure is defined considering factors that affect the anchoring damage: type of habitats, seabed compactness, number and size of boats, type of anchor. Considering these data, the net impact is evaluated as surfaces and natural capital (emergy analysis) balancing the anchoring damage and the regenerative capacity. Being recreational boating spatially distributed, the model outputs the results both as values and sustainability maps at different spatial level (overall MPA, protection zones and even smaller), according to management needs. In this way, MPA managers can quickly and easily understand the sustainability status of the MPA, identify suffering areas in order to establish ad hoc actions and develop alternative scenarios. The model, based on a generalized and iterable procedure applicable to any area, is tested on Portofino MPA (Italy): the annual average net impact is 279 m² (equivalent to 1.51E+11 sej and 1′577 em€ of natural capital), eroding the 0.089% of P. oceanica in the anchorable areas.
... If the oil mixes with the water column, this may result in a reduction in growth, respiratory and 101 cardiac alterations, fin erosion and reproduction impairment (National Ocean Service, 2018c). As shown in a study carried out by Francour et al. (1999) and Abadie et al. (2016), anchoring can cause damage to Posidonia oceanica and studies carried out by Tuck et al. (2011) also showed that anchoring can cause damage to Maerl bed benthic habitats. ...
... Posidonia oceanica. Another study carried out byAbadie et al. (2016) also showed that apart from mechanical changes, anchoring also brings about chemical changes to Posidonia oceanica. This study showed that the Posidonia oceanica substrate chemistry changed when anchoring activities occurred since the presence of hydrogen sulfide was observed numerous months after anchoring activities took place. ...
Thesis
By the end of 2020, Malta needs to manage its Marine Protected Areas, in accordance to the Natura 2000 framework and Marine Spatial Planning Directive. In view of the increase of marine uses throughout Maltese waters, especially in the Marine Protected Areas around the Maltese Islands, their proper management is required. This dissertation seeks to identify marine uses in the North-East Marine Protected Area and juxtapose these marine uses over protected benthic habitats through Geographic Information Systems, to assess which of these uses occur over protected benthic habitats in the North-East Marine Protected Area. It also seeks to quantify the scale of potential perceived threats of these uses between different user categories and assess the conflict of opinions of these different user categories on whether these marine uses may have a negative impact or not on the benthic environment, in the North-East Marine Protected Area. Several gaps in the literature were identified, since a very limited number of studies have been carried out on the North-East Marine Protected Area. Due to limited management, the North�East Marine Protected Area is at risk of losing its protected benthic habitats such as Posidonia oceanica and Maerl bed benthic habitats.The methodology was carried out through the utilisation of the Geographic Information System Mapping, which was used to plot maps by overlaying the marine use shapefiles on protected benthic environments shapefiles in North-East Marine Protected Area, to assess the location of these marine uses occurring over these protected environments. A face-to-face questionnaire was also formulated to assess the potential perceived negative impacts of these marine uses over the benthic environment according to the different user categories. An online questionnaire was later formulated, due to the COVID-19 social distancing regulations. The qualitative results were presented through maps, which showed that all the marine uses being analysed in this dissertation all occurred over protected benthic habitats in the North-East Marine Protected Areas. The quantitative results were obtained through the statistical analysis of the questionnaire. These results also showed that the user categories agreed that bottom longlines and anchoring activities may have a negative impact on the benthic environment and dive sites frequented and not frequented by tourists may have no negative impact on the benthic environment. The user categories seemed to have a conflict of perception on whether the other marine uses may have had a negative impact or not. In conclusion, the results showed that some marine uses may have a negative impact on the benthic environment. Therefore, the researcher came up with several propositions on how to manage the North-East Marine Protected Area, such as giving popular dive sites a sensitivity index and limiting the amount of divers on every dive, to also manage recreational fisheries through bag limits and to allow surface fishing over Posidonia oceanica and Maerl during particular periods throughout the year, to educate aquaculture workers on proper feeding practices and to carry out regular spot checks in local fish farms and install artificial reefs in anchoring and bunkering areas to promote marine life.
... Although this species is one of the main targets of conservation actions, the regression of P. oceanica meadows is well-documented over the whole Mediterranean basin (Boudouresque et al., 2009;Marbà et al., 2014;De Los Santos et al., 2019). As for other seagrasses worldwide (Collins et al., 2010;Sagerman et al., 2020), among the anthropogenic impacts undergone by P. oceanica meadows, mechanical impacts, particularly related to anchoring by large recreational vessels, are of increasing importance (Ceccherelli et al., 2007;Montefalcone et al., 2008;Abadie et al., 2016;Deter et al., 2017;Carreño and Lloret, 2021) due to the development of recreational boating over the past decades (Cappato et al., 2011;Carreño and Lloret, 2021). Despite the relatively small size of the Mediterranean Sea (<1% of the surface area of the world's seas and oceans), more than half of the world fleet of large recreational vessels (>24 m in length) frequent Mediterranean waters for at least eight months per year (Cappato et al., 2011;Carreño et al., 2019;Carreño and Lloret, 2021), mainly in the western Mediterranean basin (e.g. ...
Article
The anchoring of large recreational craft constitutes one of the main threats in shallow marine habitats. In the Mediterranean, this activity has seen constant development during the last decades, causing major physical disturbances in Posidonia oceanica meadows and associated ecosystem services. In this context, the main aims of the present study are to estimate the impact of this anchoring on P. oceanica meadows surface areas and carbon fixation and sequestration capacities, in a particularly highly-frequented area (Sant'Amanza bay, Corsica Island). Accurate benthic marine habitats mapping was performed yearly (2019–2021) using a drone coupled with ground-truthing data. The evaluation of carbon fixation and sequestration by the plant was measured at 12 stations within the bay (−5 m to −30 m). The maps of the marine habitats reveal an extensive regression of P. oceanica meadows (7.5 ha) between 2019 and 2020. This destruction represents a 9% decline in the total carbon fixation and sequestration performed each year by P. oceanica meadows within the bay. The related ecosystem services loss is estimated at 4.72 million € yr⁻¹. Although an overall decline of boat anchoring in the seagrass meadow has been observed (e.g. recent enforcement of anchoring regulations), other solutions should be experimented to manage this major carbon sink.
... The specific geomorphology of the dead matte area with mild hydrodynamics (ICRAM, 2008) and moderate depth, where the wave-orbital motions potentially causing sediment resuspension and erosion are expected to be limited (Fonseca and Bell, 1998;Samper-Villarreal et al., 2016), and the establishment of the breakwaters, which further attenuated water movement (Sprovieri et al., 2011), provided protection from physical disturbance and significant erosion. The absence of mechanical damage such as anchoring, that can lead to extensive erosion of dead matte patches (Abadie et al., 2016), also provided a degree of physical protection. Although we have not measured the mineralization rate of sedimentary organic pools, available data from the area show microbial degradation of organic matter (Oliveri et al., 2016). ...
Article
Full-text available
We assessed the potential of dead seagrass Posidonia oceanica matte to act as a biogeochemical sink and provide a coherent archive of environmental change in a degraded area of the Mediterranean Sea (Augusta Bay, Italy). Change in sediment properties (dry bulk density, grain size), concentration of elements (Corg, Cinorg, N, Hg) and stable isotope ratios (δ13C, δ 15N) with sediment depth were measured in dead P. oceanica matte and unvegetated (bare) sediments in the polluted area, and an adjacent P. oceanica meadow. Principal Component Analysis (PCA) revealed a clear clustering by habitat, which explained 72% of variability in our samples and was driven mainly by the accumulation of N and Hg in finer sediments of the dead matte. Assessment of the temporal trends of Corg, N and Hg concentrations in the dead matte revealed changes in the accumulation of these elements over the last 120 years, with an increase following the onset of industrial activities 65 y BP (i.e., yr. 1950) that was sustained even after seagrass loss around 35 y BP. Despite a decrease in Hg concentrations in the early 1980s following the onset of pollution abatement, overall Hg levels were 2-fold higher in the local post-industrial period, with a Hg enrichment factor of 3.5 in the dead matte. Mean stocks of Corg, N and Hg in 25 cm thick sediment deposits (4.08 ± 2.10 kg Corg m-2, 0.14 ± 0.04 kg N m-2, 0.19 ± 0.04 g Hg m-2) and accumulation in the last 120 yr (35.3 ± 19.6 g Corg m-2 y-1, 1.2 ± 0.4 g N m-2 y-1, 0.0017 ± 0.0004 g Hg m-2 y-1) were higher in the dead matte than bare sediment or adjacent P. oceanica meadow. Our results indicate that dead P. oceanica matte maintained its potential as a biogeochemical sink and, like its living counterpart, dead matte can serve as an effective archive to allow for reconstructing environmental change in coastal areas of the Mediterranean where severe perturbations have led to P. oceanica loss. Appropriate management for contaminated areas should be prioritized to prevent release of pollutants and carbon from dead mattes.
... Among the main drivers causing seagrass bare areas by local scale physical disturbances are boat anchoring and propeller scars (Dawes et al., 1997;Francour et al., 1999;Abadie et al., 2016), and harvesting, raking and dredging activities (Peterson et al., 1987;Boese, 2002;Orth et al., 2002;Alexandre et al., 2005;Cabaço et al., 2005;Neckles et al., 2005;Boese et al., 2009;Barañano et al., 2017Barañano et al., , 2018Garmendia et al., 2021). In Northern Spain, seagrasses coexist with several marine activities that include small-scale fisheries (e.g., cuttlefish, eel and sardines) and shellfishery (e.g., clams and cockles) as well as recreational activities such as windsurfing, kite surfing, bathing, canoeing and recreational fishing (Bas Ventín et al., 2015), with related threats due to the effects of navigation, anchoring and fishing practices. ...
Article
Full-text available
This investigation illustrates the spatial and temporal dynamics of a Zostera marina seagrass meadow affected by clam harvesting. Photointerpretation of satellite imagery corresponding to years 2007, 2013, 2017 and 2018, combined with field monitoring in 2019 allowed assessing the spatial coverage, population dynamics and genetic characterization of the Z. marina population in areas impacted and non-impacted by the shellfishing activity. The impacted meadow displayed a highly fragmented and discontinuous seagrass matrix anthropogenically induced by the periodical disturbance associated with bottom raking. A continuous colonization process characterized the seagrass landscape, where the area occupied by the meadow varied by a two-fold factor, with changes even exceeding 86% in some years. Only 740 m² (ca. 15%) of the seagrass matrix remained vegetated in the four years monitored in this investigation. The number of patches showed a large interannual variability, exceeding 100% in the four years studied, ranging from 58 to 199, while the border effect perimeter/area indicator showed a two-fold variation ranging between 1 and 2. Clearly differentiated patterns were observed in shoot density, biomass, and flowering density between shellfishing-induced patches of different sizes and the long-term non-impacted areas. A significant pattern of genetic differentiation among impacted and control populations were also found. Our results showed that population dynamics varied as a function of Z. marina patch-sizes, thus reinforcing the need for a combined approach involving seascape structure and patch dynamics with population dynamics and genetic structure to assess the impact of disturbances on seagrass ecosystems.
... No se han encontrado datos previos de índice de conservación para las Islas Baleares. En un estudio realizado en Córcega se considera que la pradera está en un mal estado de conservación si el índice de conservación es menor a 0.88, como es el caso de la zona media en Cala Vedella (Abadie et al., 2016). ...
Experiment Findings
Full-text available
Experimental design: For the analysis of macroinvertebrates associated with the Posidonia oceanica meadow, the factor was depth. Three levels were established: an external zone (38.914oN, 1220oE), at a depth of 15.5 m, another in the intermediate zone, at 12.5 m (38.915oN, 1222oE) and a last one near the coast, at 4 m ( 38.913oN, 1.222oE) (Figure 1). The variable to be measured was the abundance of individuals per m2. A non-destructive method was used, based on visual censuses and with autonomous diving equipment. In each study area, 3 random transects of 10 meters long by 1 meter wide were made. In total, 9 random transects were carried out in Cala Vedella. Similarly, for the study of the epiphytes, 5 bundles of Posidonia oceanica were taken per zone. Their study was carried out by counting the animal and vegetable cover of the outer distal and proximal parts of the outer leaves of each bundle, following the method used by Eugene (1978). Statistic analysis: Once the data was obtained, an analysis was performed with PERMANOVA to see the existence of differences in the composition of macroinvertebrates between the 3 zones. In addition, a dendrogram and an nMDS were performed in order to observe the dispersion between zones. In both cases, the Bray Curtis distance was used (Clarke and Warwick, 2001). For the epiphytes, an ANOVA was used with the R Project software (R Core Team, 2017) to observe the differences in the total cover, of vegetables and of epiphytes depending on the season and the position on the leaf with the following model linear: Xijk=μ+Zonai+Hojaj+Zona*Hojaij+Ɛk(ij) For the total cover, the data were transformed for the homogeneity of variances and an alpha of 0.05 was used, while for the animal and vegetal cover, when homoscedasticity was not met, an alpha of 0.01 was used. To see the differences between the different factors, the Tukey test was used. In order to obtain a better characterization of the macroinvertebrate populations, different diversity indices were used, such as the Shannon-Weaver index (Shannon and Weaver, 1949) and the Pielou evenness (Pielou, 1966) with the PAST program (Hammer et al. al., 2001).
... No se han encontrado datos previos de índice de conservación para las Islas Baleares. En un estudio realizado en Córcega se considera que la pradera está en un mal estado de conservación si el índice de conservación es menor a 0.88, como es el caso de la zona media en Cala Vedella (Abadie et al., 2016). ...
Experiment Findings
Full-text available
Experimental design: For the analysis of macroinvertebrates associated with the Posidonia oceanica meadow, the factor was depth. Three levels were established: an external zone (38.914oN, 1220oE), at a depth of 15.5 m, another in the intermediate zone, at 12.5 m (38.915oN, 1222oE) and a last one near the coast, at 4 m ( 38.913oN, 1.222oE) (Figure 1). The variable to be measured was the abundance of individuals per m2. A non-destructive method was used, based on visual censuses and with autonomous diving equipment. In each study area, 3 random transects of 10 meters long by 1 meter wide were made. In total, 9 random transects were carried out in Cala Vedella. Similarly, for the study of the epiphytes, 5 bundles of Posidonia oceanica were taken per zone. Their study was carried out by counting the animal and vegetable cover of the outer distal and proximal parts of the outer leaves of each bundle, following the method used by Eugene (1978). Statistic analysis: Once the data was obtained, an analysis was performed with PERMANOVA to see the existence of differences in the composition of macroinvertebrates between the 3 zones. In addition, a dendrogram and an nMDS were performed in order to observe the dispersion between zones. In both cases, the Bray Curtis distance was used (Clarke and Warwick, 2001). For the epiphytes, an ANOVA was used with the R Project software (R Core Team, 2017) to observe the differences in the total cover, of vegetables and of epiphytes depending on the season and the position on the leaf with the following model linear: Xijk=μ+Zonai+Hojaj+Zona*Hojaij+Ɛk(ij) For the total cover, the data were transformed for the homogeneity of variances and an alpha of 0.05 was used, while for the animal and vegetal cover, when homoscedasticity was not met, an alpha of 0.01 was used. To see the differences between the different factors, the Tukey test was used. In order to obtain a better characterization of the macroinvertebrate populations, different diversity indices were used, such as the Shannon-Weaver index (Shannon and Weaver, 1949) and the Pielou evenness (Pielou, 1966) with the PAST program (Hammer et al. al., 2001).
Article
In Mediterranean, Posidonia oceanica develops a belowground complex structure ('matte') able to store large amounts of carbon over thousands of years. The inventory of blue carbon stocks requires the coupling of mapping techniques and in situ sediment sampling to assess the size and the variability of these stocks. This study aims to quantify the organic (Corg) and inorganic (Cinorg) carbon stocks in the P. oceanica matte of the Calvi Bay (Corsica) using sub-bottom profiler imagery and biogeochemical analysis of sediment cores. The matte thicknesses map (average ± SD: 2.2 m ± 0.4 m) coupled with marine benthic habitat cartography allows to estimate matte volume at 12 473 352 m3. The cumulative stocks were assessed at 20.2-50.3 kg Corg m-2 and 26.6-58.7 kg Cinorg m-2 within the first meter of depth on matte (3632 ± 486 cal yr BP). The data contributed to estimate the overall carbon stocks at 389 994 t Corg and 615 558 t Cinorg, offering a new insight of the heterogeneity of blue carbon stocks in seagrass meadows. Variability of carbon storage capacity of matte influenced by substrate is discussed.
Technical Report
Full-text available
La Liste Rouge des Ecosystèmes (LRE) de l’Union International pour la Conservation de la Nature (UICN) a été développée afin de pouvoir évaluer le risque d’effondrement des écosystèmes. Avant de pouvoir initier un nouveau chapitre national marin de LRE portant sur les herbiers de phanérogames marines de France métropolitaine, un travail préliminaire est essentiel pour s’assurer de la faisabilité d’une possible évaluation. Cette étude de faisabilité s’appuie sur un bilan de l’existant pour définir un cadre d’évaluation cohérent, proposer les types d’écosystème à évaluer et estimer la pertinence des données existantes pour l’application de la méthodologie d’évaluation UICN.
Article
Full-text available
Seagrass ecosystems face widespread threat from reduced water quality, coastal development and poor land use. In recent decades, their distribution has declined rapidly, and in the British Isles, this loss is thought to have been extensive. Given increasing knowledge of how these ecosystems support fisheries production, the understanding of their potential rapid loss, and the difficulty in restoring them, it is vital we develop an understanding of the risks they are under, so that management actions can be developed accordingly. Developing an understanding of their environmental status and condition is therefore critical to their long-term management. This study provided, to our knowledge, the first examination of the environmental health of seagrass meadows around the British Isles. This study used a bioindicator approach and involved collecting data on seagrass density and morphology alongside analysis of leaf biochemistry. Our study provides, to the best of our knowledge, the first strong quantitative evidence that seagrass meadows of the British Isles are mostly in poor condition in comparison with global averages, with tissue nitrogen levels 75% higher than global values. Such poor status places their long-term resilience in doubt. Elemental nutrient concentrations and morphological change suggest conditions of excess nitrogen and probable low light, placing many of the meadows sampled in a perilous state, although others, situated away from human populations were perceived to be healthy. Although some sites were of a high environmental health, all sites were considered at risk from anthropogenic impacts, particularly poor water quality and boating-based disturbances. The findings of this study provide a warning of the need to take action, with respect to water quality and disturbance, to prevent the further loss and degradation of these systems across the British Isles.
Article
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Vegetation patch-size distributions have been an intense area of study for theoreticians and applied ecologists alike in recent years. Of particular interest is the seemingly ubiquitous nature of power-law patch-size distributions emerging in a number of diverse ecosystems. The leading explanation of the emergence of these power-laws is due to local facilitative mechanisms. There is also a common transition from power law to exponential distribution when a system is under global pressure, such as grazing or lack of rainfall. These phenomena require a simple mechanistic explanation. Here, we study vegetation patches from a spatially implicit, patch dynamic viewpoint. We show that under minimal assumptions a power-law patch-size distribution appears as a natural consequence of aggregation. A linear death term also leads to an exponential term in the distribution for any non-zero death rate. This work shows the origin of the breakdown of the power-law under increasing pressure and shows that in general, we expect to observe a power law with an exponential cutoff (rather than pure power laws). The estimated parameters of this distribution also provide insight into the underlying ecological mechanisms of aggregation and death.