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Sediment Retention by a Mediterranean Posidonia oceanica Meadow: The Balance between Deposition and Resuspension

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Abstract

The role of Posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at Fanals Point (NW Mediterranean, Spain) by comparing particle deposition within the meadow and adjacent bare sediment. Small sediment traps were used to measure deposition within and above the meadow and over bare sand. A model, based on measurements of particle deposition at increasing distances from the bottom, was used to partition the total depositional flux between primary (sediment particles deposited for the first time at the measuring site) and resuspended deposition (sediment particles that have been previously deposited at the measuring site). Measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition, and to form a balance of particle transport at the annual time scale. Significant differences in total deposition were found over time, ranging from 1·5 to 500 g DW m−2 d−1, including those between bare and vegetated sediments. The effect of P. oceanica in increasing primary deposition at an annual scale was modest, however, P. oceanica significantly buffered sediment resuspension, which was reduced more than three fold compared to the unvegetated bottom. The annual flux of deposition was dominated by settling of resuspended materials, which represented 85% of the total flux within the meadow, but 95% of the total deposition on bare sand. Thus, seagrass meadows reduce resuspension in the NW Mediterranean littoral, thereby contributing to increased sediment retention and, therefore, reducing erosion in the coastal zone.
Estuarine, Coastal and Shelf Science (2001) 52, 505–514
doi:10.1006/ecss.2000.0753, available online at http://www.idealibrary.com on
Sediment Retention by a Mediterranean Posidonia
oceanica Meadow: The Balance between Deposition
and Resuspension
E. Gacia
a,c
and C. M. Duarte
b
a
Centre d’Estudis Avanc¸ats de Blanes (CSIC), Ctra. Santa Ba`rbara s/n, 17300 Blanes, Spain
b
Institut Mediterrani d’Estudis Avanc¸ats (CSIC-UIB), C/Miquel Marque`s 21, 07190 Esporles, Mallorca
(Illes Balears), Spain
Received 11 June 2000 and accepted in revised form 10 November 2000
The role of Posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at Fanals
Point (NW Mediterranean, Spain) by comparing particle deposition within the meadow and adjacent bare sediment.
Small sediment traps were used to measure deposition within and above the meadow and over bare sand. A model, based
on measurements of particle deposition at increasing distances from the bottom, was used to partition the total
depositional flux between primary (sediment particles deposited for the first time at the measuring site) and resuspended
deposition (sediment particles that have been previously deposited at the measuring site). Measurements were conducted
monthly over a year to establish the magnitude and seasonality of deposition, and to form a balance of particle transport
at the annual time scale. Significant dierences in total deposition were found over time, ranging from 1·5 to
500gDWm
2
d
1
, including those between bare and vegetated sediments. The eect of P. oceanica in increasing
primary deposition at an annual scale was modest, however, P. oceanica significantly buered sediment resuspension,
which was reduced more than three fold compared to the unvegetated bottom. The annual flux of deposition was
dominated by settling of resuspended materials, which represented 85% of the total flux within the meadow, but 95% of
the total deposition on bare sand. Thus, seagrass meadows reduce resuspension in the NW Mediterranean littoral,
thereby contributing to increased sediment retention and, therefore, reducing erosion in the coastal zone.
2001 Academic Press
Keywords: Posidonia oceanica; deposition; resuspension; sediment; Mediterranean
Introduction
Seagrass beds are known to promote deposition of
particles, and loss of seagrass meadows is often
followed by sediment erosion (Wilson, 1949;
Christiansen et al., 1981;Hine et al., 1987). The eect
of seagrass beds on sediment deposition results from
the reduction of water flow (Scon, 1970;Fonseca
et al., 1983), and the protection of sediments from
resuspension due to energy dissipation by the plant
canopies (Ward et al., 1984;Fonseca & Fisher, 1986;
Eckman et al., 1989;Fonseca & Cahalan, 1992;
Koch, 1996). While the eect of seagrass canopies on
water flow and turbulence dissipation has been
studied in some detail (Fonseca et al., 1982;Fonseca
& Fisher, 1986;Fonseca, 1989;Eckman et al., 1989;
Gambi et al., 1990;Ackerman & Okubo, 1993;Koch,
1994;Koch & Gust, 1999), quantitative direct esti-
mates of the eect of seagrass canopies on particle
deposition (Almansi et al., 1987;Gacia et al., 1999)
and, particularly, resuspension (Dauby et al., 1995;
Gacia et al., 1999;Koch, 1999) are very few. The
paucity of information on sediment deposition and
resuspension within seagrass beds may be partially
attributable to diculties in the quantification of
those processes. Progress in our understanding of
the nature of sediment resuspension (Ichiye, 1966;
Ha˚kanson et al., 1989) has led to the development of
new approaches to estimate the primary and resus-
pended sediment fluxes in shallow coastal sediments
(Valeur, 1994;Pejrup et al., 1996). These approaches
can be used to address the eects of seagrass canopies
on particle deposition and resuspension, as demon-
strated by Gacia et al. (1999).
Posidonia oceanica, the dominant seagrass species
in the Mediterranean, where it covers 50 000 km
2
(Bethoux & Copin-Monte´gut, 1986), is known to
be a reef-building organism (Pe´re`s & Picard, 1964)
capable of long-term sediment retention (Mateo et al.,
1997). Posidonia oceanica is however experiencing a
widespread decline throughout the Mediterranean Sea
c
Corresponding author. E-mail: gacia@ceab.csic.es
0272–7714/01/040505+ 10 $35.00/0 2001 Academic Press
(Marba` et al., 1996), which may reduce sediment
retention and increase beach erosion in the coastal
zone (Sestini, 1989). The present knowledge on the
capacity of P. oceanica, as well as other seagrass
species, to retain sediments is largely qualitative and,
therefore, insucient to be used to produce forecasts
of sediment dynamics in the coastal zone. The only
evaluation of sediment deposition in a P. oceanica
meadow at the annual scale available to date (Dauby
et al., 1995) did not assess the eect of seagrass, for
the traps used were deployed above the canopy, and
the assessment of the eect of the plants canopy was
not possible. There is, therefore, a need to test quan-
titatively the eect of P. oceanica on sediment depo-
sition and retention to expand our understanding of
the functions of P. oceanica meadows to their role in
sediment dynamics, thereby extending the basis for
conserving these threatened ecosystems.
We examine here, on an annual time scale, the
eect of the Mediterranean seagrass P. oceanica on
particle deposition and resuspension. In particular, we
test the hypotheses that sediment deposition is higher
under the seagrass canopy compared to unvegetated
sediments, and that sediment resuspension is more
signicant in the absence of vegetation. We do so by
comparing the magnitude and patterns of particle
deposition and resuspension within a P. oceanica
meadow and on bare sand. The necessary measure-
ments were conducted at monthly intervals over a year
to establish the balance between sediment deposition
and resuspension on an annual time scale, during
which the seagrass canopies experience considerable
seasonal variability.
Methods
The study was conducted in a P. oceanica meadow and
adjacent unvegetated sandy bottom at 15 m depth at
Fanals Point (4041N, 252E; Spain). Both areas
were situated parallel to shore and separated 20 m
from each other. The sandy area corresponded to a
transitional zone between the meadow and the rocks
from the clis. Fanals Point is an open area of the NE
Spanish Mediterranean littoral, with negligible tidal
ranges, exposed to waves and occasionally strong
long-shore currents. Near bottom velocities range
from 2 to 10 cm s
1
under moderate wind conditions
(average of 7·923·15ms
1
during the study
period, Granata unpubl. data) and the dominant
forcing is provided by wind and swell waves with
periods from 3 s to 15 s (Granata et al., unpubl. data).
The biomass of the P. oceanica meadow at Fanals
is amongst the highest recorded at depths >10 m
(Cebria´net al., 1997;Romero et al., 1998). The
interaction between the top of the plant canopy and
the water ow in this meadow has been reported to
produce frictional velocities of about 0·5cms
1
, rais-
ing the eective bottom boundary layer by about
10 cm compared to unvegetated sediments (Granata
et al., unpubl. data).
Total and resuspended sedimentary ux was moni-
tored inside the meadow and on bare sand using small
sediment traps from May 1997 until June 1998. Traps
consisted of 20·5 ml cylindrical glass centrifuge tubes
with a height vs. diameter ratio (aspect ratio) of 5
(16 mm diameter), within the aspect ratios recom-
mended by Blomqvist and Ha˚kanson (1981). The
tubes were attached in groups of ve, about 4 cm
apart from each other, to 30 cm long horizontal stain-
less steel rods. Previous studies showed that at the
same distances from the bottom, the variance between
units mounted on the same frame was not signicantly
dierent to that between units mounted on dierent
frames (ANOVA, P>0·5). Thus, all sedimentation
tubes were used as replicate units when deployed at
the same depth, whether mounted in the same or
dierent frames, to estimate particle ux.
Discrimination between primary (Fp,
gDWm
2
d
1
) and resuspended sedimentary ux
(Fr, g DW m
2
d
1
) was based on the terminology
of Pejrup et al. (1996), where primary settling material
is dened as sedimenting particles, including autoch-
tonous particulate matter and advected materials, that
have not yet been deposited at the bottom of the
measuring site. The resuspended ux consists of the
same components but those that were deposited pre-
viously at the measuring site. The total depositional
ux (Ft, g DW m
2
d
1
) is the sum of Fp and Fr.
Based on this denition, the primary and resuspended
sediment ux can be derived by deconvulting the total
downward ux (Ft; units in g DW m
2
d
1
),
through the analysis of the vertical particle ux as a
function of the height above the sea-bed (Figure 1;
Ha˚kanson et al., 1989;Valeur, 1994). The approach is
based on the principle that the resuspended load
declines exponentially with increasing height above
the sediment source (i.e. sediment surface; Ichiye,
1966;Ha˚kanson et al., 1989;Valeur, 1994;Perjup
et al., 1996). The changes in Ft with height above the
bottom should t, in the presence of signicant resus-
pension, to a negative exponential function of the form:
Ft=ae
b*H
where Ft is the total depositional ux (units in
gDWm
2
d
1
) and H (cm) is the distance from the
sediment surfaces. The presence of resuspension is,
thus, derived from the statistical signicance of the
506 E. Gacia and C. M. Duarte
model (i.e. h
o
: b=0), which, with the experimental
design used here, allowed resolution of resuspended
loads as low as 0·4gDWm
2
d
1
, well below the
values inferred for the Mediterranean littoral (Dauby
et al., 1995). Ft at the sediment surface corresponds to
the total depositional ux (Dt). Fp would then corre-
spond to the asymptotic values of the exponentially
declining Ft with increasing distance from the sea oor
(see also Valeur, 1994;Gacia et al., 1999), which
provides an estimate of the rate of primary deposition
(Dp; units g DW m
2
d
1
). The downward ux of
resuspended sediments, Dr, is then estimated by the
dierence between Dt and Dp. This method to separ-
ate deposition uxes is based on the conrmed as-
sumption source (cf. Ichiye, 1966;Ha˚kanson et al.,
1989;Valeur, 1994;Pejrup et al., 1996) that the initial
condition is a uniform distribution of primary sedi-
menting material within the water column, and that
strong particle gradients will occur in the presence of
sediment resuspension in the area of study. The
method, validated experimentally by Perjup et al.
(1996), has been shown to provide good estimates of
primary and resuspended sediment uxes in systems
where resuspended sediments were not homogenized
across the entire water column (Valeur, 1992;Perjup
et al., 1996). The depth of the station studied, about
15 m, together with the coarse nature of the sedi-
ments, ensured that sediment resuspension did not
reach far above the sediment surface (heights where
resuspended sediments were detected 0 chd1 m),
thereby ensuring the applicability of the method.
The total depositional ux (Ft) was measured with
sediment traps situated at 20 cm from the bottom,
except at 12 cm in December, when the canopy level
was at the lowest, thereby always remaining within the
canopy when deployed inside the meadow. Resuspen-
sion loads were estimated with sediment traps xed in
groups of ve to a central 1·5-m tall vertical pole, at
heights of 20 (12 in December), 40, 60, 80 and
100 cm above the bottom, thereby allowing the ex-
amination of the relationship between the downward
depositional ux and the height of the sedimentation
tubes above the bottom. Three replicated sediment
traps (ve units each) for the estimation of total
deposition at 20 cm height (i.e. a total of 15 replicated
sedimentation tubes) and one structure with ve units
of ve sediment traps each positioned at 20, 40, 60,
80 and 100 cm above the bottom, were assembled by
SCUBA divers over bare and vegetated sediments.
Traps were previously lled with subsurface seawater
and covered with caps that were removed after a few
minutes of deployment to avoid the collection of
sediments possibly resuspended during the manipula-
tions. A total of 12 measurements were conducted
separated at monthly intervals integrating sampling
periods between 3 and 10 days (Table 1).
Sediment samples were collected in October 1997
using a 20 cm diameter corer to measure sediment
densities inside and outside the meadow. Three rep-
licate cores, containing the top 5 cm of sediment, were
collected from both areas and the relationship be-
tween fresh weigh (FW) per volume was measured.
The potential sediment erosion was estimated from
measurements of maximum resuspension rates
(g DW m
2
d
1
) combined with the measured sedi-
ment densities (g FW ml
1
) and the water content of
the sediments (relationship between dry weight and
fresh weight) from inside and outside the meadow.
In the laboratory, the traps were inspected for active
swimmers (i.e. zooplankton) that were removed if
present (Michaels et al., 1990). The contents of the
tubes were ltered through 25 mm pre-weighted
GF/F lters and were dried to constant weight at
60 C (minimum of 24 h) before weighing, thereby
also allowing subsequent nutrient analyses of the
samples.
Parallel to the time of sediment trap deployment, 10
randomly-selected shoots of P. oceanica were collected
to measure canopy height, leaf surface area and above
ground biomass. Plants were dried at 105 C for a
minimum of 24 h. Shoot densities were determined by
haphazard placements of a 5050 cm
2
quadrat in
which the number of shoots were counted. The above
ground biomass (units in g DW m
2
) and leaf area
index (LAI, m
2
leaf m
2
ground) of the seagrass bed
0200
100
Dp
Fp
H (cm from the bottom)
50
80
60
40
20
100 150
Dt
Dt – Dp = Dr
g DW m
–2
d
–1
Ft measured with sediment traps:
Ft = a × e
–b H
Ft = Fp + Fr
Simulated fluxes:
Ft
F 1. Illustration of the ux prole separation method
(from Valeur, 1994). Ft=total deposition, Fp = ux of
primary deposition, Fr= ux of resuspended sediments,
Dp= primary deposition, Dr = resuspended deposition.
Sediment retention by Posidonia oceanica 507
were calculated from measurements of the individual
shoots using the density measurements. Meteorologi-
cal data (i.e. precipitation, wind speed and direction)
during the sampling period were recorded at a
meteorological station about 3 km south of the
location of the sediment traps.
Temporal changes in meadow characteristics (bio-
mass, canopy height, leaf surface area and shoot
densities) were analysed using ANOVA for repeated
measurements. Data on total deposition were log
(natural)-transformed prior to statistical analysis to
fulll the requirements of homogeneity of the vari-
ance. Non-parametric comparison of the signicance
of dierences in deposition rates between bare and
vegetated sediments were based on Wilcoxon sign
ranked test (Sokal & Rohlf, 1981). Average values are
reportedSE.
Results
Posidonia oceanica showed strong seasonality in (1)
canopy height, which ranged between 15 and 70 cm;
(2) leaf area index which ranged between 68
397 cm
2
leaves shoot
1
, and (3) above ground
biomass which ranged between 0·32·5gDW
shoot
1
. The maximum values were observed in July
and August, they declined in October, and reached a
minimum between November and December, to re-
cover slowly from January to March, when signicant
growth was again recorded (Figure 2). No dierences
in shoot density (Figure 2,P>0·45) were found over
the sampling period, which is consistent with the long
life span of P. oceanica shoots and indicates that all
the reported seasonality was due to changes in leaf
development.
The total depositional ux varied over two orders of
magnitude within the P. oceanica meadow and over
two and half orders of magnitude on bare sand from
May 1997 to June 1998. Signicant dierences in
total deposition were found over time and between
bare and vegetated sediments (Figure 3; Wilcoxon
signed rank test, P<0·05). Deposition rates at 100 cm
over the sediment surface were highly correlated in
the presence and absence of vegetation (R
2
=0·99,
T 1. Seasonal variation in the downward particle ux at the sandy and a vegetated station in Fanals Point (Spain).
Dominant wind direction is expressed as compass quadrats, Q 1= North to East, Q 2=East to South, Q 3 = South to West and
Q 4= West to North. Fp = downward ux of primary deposition, Fr = downward ux of resuspended sediments. R
2
is the
coecient of determination of the model used to partition the total ux into these components (Equation 1), Pis the
probability that the resuspended ux (Fr) equals 0.
a
Indicates signicance of the model at (P<0·05). NIs the number of
observations, which is variable due to dierential success in recovering intact sediment traps
Date
Rainfall
(mm day
1
)
Wind max speed
(m s
1
)
Wind
direction Site
F
p
(g DW m
2
d
1
)
F
r
(g DW m
2
d
1
)PNR
2
24/42/5/97 0·310·33 Q 3 sand 6·34 0 0·92 9 0·02
Posidonia 5·99 0 0·05 12 0·48
28/57/6/97 41·115·59 Q 2 sand
a
19·66 963 <0·002 8 0·92
Posidonia
a
25·15 170 <0·0001 12 0·94
29/7/97 0 3·59 Q 4 sand 4·20 0 0·98 15 0·33
Posidonia 3·19 0 0·08 15 0·54
713/8/97 0 10·42 Q 4 sand 6·96 0 0·27 10 0·45
Posidonia 8·72 0 0·22 10 0·51
17/9/97 5·27·74 Q 4 sand
a
4·41 0·48 <0·02 15 0·68
Posidonia 5·40 0 0·42 14 0·03
2124/11/97 2·39·81 Q 4 sand
a
1·93 18·1<0·0001 15 0·99
Posidonia
a
2·19 18·1<0·0001 14 0·98
2227/12/97 6 8·70 Q 4 sand 8·31 0 0·27 13 0·43
Posidonia 7·08 0 0·06 13 0·65
30/15/2/98 27·58·79 Q 4 sand
a
19·56 999·7<0·0001 12 0·98
Posidonia
a
17·3 318·7<0·005 12 0·86
25/23/3/98 0·47·49 Q 4 sand
a
4·74 4·07 <0·02 15 0·61
Posidonia 4·96 0 0·34 15 0·33
20/326/3/98 17·29·90 Q 4 sand
a
46·27 503 <0·0001 6 0·98
Posidonia
a
38·3 284 <0·0005 15 0·98
58/5/98 0 13·10 Q 2 sand 5·72 0 0·47 15 0·02
Posidonia 6·30 0 0·87 13 0·13
25/6/98 6·411·70 Q 1 sand 8·79 0 0·33 10 0·54
Posidonia 8·50 0 0·17 11 0·60
508 E. Gacia and C. M. Duarte
P<0·0001) and did not dier (Wilcoxon signed rank
test, P>0·34), indicating that the dierences in bulk
deposition at 20 cm reect the eect of the plants.
Meteorological conditions during the study period
were characteristic of the northwestern Mediterranean
with rainfall in the autumn and winter, and occasional
heavy storms in late spring and mid summer (Table
1). Strong winds (>10 m s
1
) were recorded during
spring, and occasionally during the rest of the year
(Table 1). Rainfall integrated between the sampling
events was strongly correlated (R
2
=0·90, P<0·0005,
N=11) to the total deposition measured on bare sand,
but was not correlated with the deposition under the
P. oceanica canopy (P>0·6, N=11), suggesting that
the plants alter the depositional patterns.
The vertical proles of bulk sedimentary ux (Fig-
ure 4) showed three dierent patterns: (1) the pres-
ence of resuspension, characterized by an exponential
increase in sedimentary ux towards the sediment
surface (e.g. November, 1997), (2) an increased depo-
sition within the P. oceanica canopy indicated by a
sudden increase in depositional ux at the canopy
(e.g., July 1997), and (3) the absence of any evidence
of resuspension and canopy eects (e.g. December
1997). Statistically signicant resuspension events
(pattern 1) were observed in 50% of the measure-
ments over bare sand and 40% of the measurements
inside the meadow (Table 1), increased sedimentary
ux within the canopy (i.e. pattern 2) was detected in
16% of the events inside the meadow, and the depo-
sitional ux was homogenous with height from the sea
0J
500
1997
(d)
Shoots m2
M
400
300
200
100
J J A S O ND J FMAM
1998
0.0
3.0
(c)
Above ground biomass
(g DW shoot1)
2.0
1.0
0
500
(b)
cm2 leaves shoot1
400
300
200
100
10
90 (a)
Canopy height (cm)
70
50
30
F 2. Temporal variation in the structure of the P.
oceanica meadow; (a) canopy height, (b) leaf surface area,
(c) the aboveground biomass , and (d) the shoot density.
Error bars representSE (N= 10).
0J
60
1997
100 cm
Depositional flux (g DW m
2
d
1
)
M
40
20
J J A S O ND J FMAM
1998
0
300
20 cm
200
100
494
F 3. Depositional ux at 20 cm and 100 cm from the
bottom within and above a P. oceanica meadow, respectively
(solid bars) and unvegetated oor (empty bars) at Fanals
Point (NE Spain). Error bars representSE (N= 3).
Sediment retention by Posidonia oceanica 509
bed (i.e. pattern 3) in 16% of the measurements over
bare sand and 8% of the events inside the meadow
(Table 1).
The resuspended deposition was higher in the ab-
sence of vegetation than within the P. oceanica (Figure
5). Resuspension within the meadow, when present,
0 15
100
g DW m2 d1
2124/11/97
cm from the sea floor
Bottom 15
14
Depth (m)
14.2
14.4
14.6
14.8
80
60
40
20
5100 510
8
100 17/10/97
2Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
2
0
46246
100 713/8/97
0Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
369 36
8
100 1117/7/97
2Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
46246
150
100 28/59/6/97
0Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
200 400 600
8
100 28/42/5/97
2Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
46
91212 0
0 50 100800
246
Sand P. oceanica
*
0 15
100
g DW m2 d1
25/6/98
cm from the sea floor
Bottom 15
14
Depth (m)
14.2
14.4
14.6
14.8
80
60
40
20
5100 510
8
100 58/5/98
2Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
2
0
46246
100 2026/3/98
0Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
50 100 150 50 100
15
100 25/23/3/98
0Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
5100 510
400
100 30/15/2/98
0Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
100 200 300
15
100 2227/12/97
0Bottom 15
14
14.2
14.4
14.6
14.8
80
60
40
20
36912
150 200200 0
0 100 200 300
036912
Sand P. oceanica
F 4. Box plots showing the distribution of vertical particle ux measured at increasing heights from the sediments
within the P. oceanica meadow and adjacent unvegetated station from April 1997 to June 1998 at Fanals Point (NE Spain).
Boxes encompass 50% of the values, the line represents the median value, and the bars extend to the 95% condence limits.
The dashed lines indicate the height of the canopy at the time of sampling, as indicated by the drawing of the plant contour.
510 E. Gacia and C. M. Duarte
was most pronounced during the winter when canopy
height was low (e.g. November 1997, January and
March 1998; Table 1,Figure 2). Sediment densities
did not dier inside and outside the meadow (t-test,
P>0·2), although there was a tendency towards
sediments with higher densities inside the meadow
(1·95gFWml
1
0·04 SE) compared to the areas
over bare sand (1·83gFWml
1
0·05 SE). The po-
tential sediment erosion associated with resuspension
events averaged 0·35 cm day
1
on bare sand and was
signicantly lower (0·10 cm day
1
; Wilcoxon signed
rank test, P<0·05) inside the meadow. In the absence
of resuspension, the ux of primary settling matter did
not dier within plant canopies and bare sediments
(Wilcoxon signed rank test, P>0·69).
Discussion
The summer and early autumn were characterized by
low deposition rates, consistent with the low particle
concentration in the water column (Duarte et al.,
1998) and the low frequency of storms inducing
turbulence at the bottom. Yet, episodic events involv-
ing rainfall and rough seas resulted in high depo-
sitional uxes in spring, late autumn and winter,
combining resuspension (Mo´naco et al., 1990;
Bavestrello et al., 1995) and the input of suspended
particles from the nearby Tordera River (Vaque´et al.,
1997).
The signicantly lower depositional rates within the
plant canopy compared to the unvegetated oor are
probably attributable to the reduced turbulence inside
the canopy (e.g. Eckman et al., 1989;Gambi et al.,
1990), which buers resuspension (Gacia et al., 1999;
Terrados & Duarte, 1999). Comparative data from a
P. oceanica meadow and a nearby unvegetated area in
Junquet (NE Spanish Mediterranean) indicate that
the presence of this seagrass reduced near-bottom
turbulence by 2·5 fold (Granata et al., unpubl. data).
At Fanals Point, P. oceanica reduced resuspension
rates by, on average, three fold compared to that in
the adjacent unvegetated oor. Hence, P. oceanica
signicantly reduced sediment erosion by restricting
resuspension to the upper mm of the sediment instead
of 3 mm as observed in unvegetated sediments. Re-
suspension within the meadow was greatest at the
time of minimum canopy development, as expected
from the nding of strong eects of the projected
surface area of the plants on the capacity of seagrass to
dissipate energy (Gacia et al., 1999) and, therefore,
prevent sediment from erosion. Moreover, the modest
increase in primary deposition observed in July 1997,
when canopy development was maximal, is consistent
with the reports of increased deposition with increased
canopy development (Eckman et al., 1989;Gacia
et al., 1999).
Posidonia oceanica slows current velocities and in-
creases the roughness height of the boundary layer
(Gacia et al., 1999), thus potentially enhancing par-
ticle trapping. In the meadow at Fanals Point, a net
increase of primary deposition at the annual basis was
not detected. Evidence of increased primary depo-
sition within the P. oceanica canopy was found in two
out of the 12 samplings, and even in those events, the
eect was modest (<30% increase). The eect of
the canopy on the annual primary deposition was
0.00 0.06
0.06
Erosional depth (cm d1)
Sand
1:1
Erosional depth (cm d1)
Posidonia oceanica
0.040.02
0.04
0.02
0 60
60
Resuspended flux/primary flux
Sand
1:1
Resuspended flux/primary flux
Posidonia oceanica
4020
40
20
50
30
10
10 5030
F 5. The relationship between the ratio of resus-
pended vs. primary deposition and the potential erosion of
vegetated (P. oceanica) and unvegetated sediments at Fanals
Point (NE Spain).
Sediment retention by Posidonia oceanica 511
low. This conclusion is in contrast with the general
opinion that seagrasses enhance sediment deposition
(Ginsberg & Lowenstam, 1958;Scon, 1970;Thayer
et al., 1975;Orth, 1977;Short & Short, 1984;Walker
et al., 1996) and suggest that the mechanisms of
particle trapping within seagrass meadows are com-
plex and may vary within systems. To illustrate this
point, recent studies on a shallow Thalassia testudinum
bed in Florida (Koch, 1999) have shown that resus-
pension of particles can be enhanced via ow intensi-
cation near the bottom in shallow seagrass stands
exposed to intense tidal currents. However, in the
Mediterranean littoral, our data clearly show that the
presence of P. oceanica enhances sediment stability by
preventing resuspension, despite a marginal eect of
the vegetation on the primary sediment deposition.
These results thus support recent conclusions on the
role of seagrasses in near-shore sedimentary processes
(Fonseca, 1996) indicating that much of the seagrass
erosion prevention is due to sediment retention.
The range of total deposition captured at Fanals
Point varied between 1·5 and 500 g DW m
2
d
1
,
which is comparable to the range previously reported
from the NW Mediterranean (Table 2). On an
annual basis, primary deposition accounted for
4190 g DW m
2
y
1
inside the meadow, and
4070 g DW m
2
y
1
on bare sand. These rates, in
the absence of advection, represent a sediment
accretion rate of 2 mm m
2
y
1
inside the meadow,
and t within the range of vertical rhizome elongation
(a plant response to sediment accretion) measured for
P. oceanica in other Spanish Mediterranean meadows
(Marba` & Duarte, 1997). Resuspension events mobi-
lized 24 360 g DW m
2
y
1
inside the meadow
and 76 146 g DW m
2
y
1
on bare sand, thus
representing 85% of the total deposition inside the
meadow in Fanals, and 95% of the total deposition on
bare sand. These values are even higher than the 70%
resuspension reported by Dauby et al. (1995) in a
36 m deep P. oceanica meadow in Corsica. Our re-
sults, therefore, conrm the importance of resuspen-
sion for the dynamics of sediments in the NW
Mediterranean littoral, and point out the importance
of the vegetation coverage to modulate the turbulence
at the bottom responsible of sediment resuspension.
In summary, our results demonstrate the existence
of relatively high deposition rates at 15 m depth in
Fanals Point in the presence and absence of vegetation
due to resuspension events. Posidonia oceanica how-
ever, proved to buer resuspension signicantly com-
pared to the unvegetated sediment, thus, potentially
protecting, by more than three fold, the sediment
from erosion. These results provide direct quantitative
information on the role of seagrasses in promoting
sediment stability, and extends the basis for conser-
vation eorts in those endangered ecosystems to a
proper management of Mediterranean coastal areas.
Acknowledgements
This research was funded by the project PhaSE (con-
tract MAS3-CT96-0053) of the ELOISE program of
the European Commission. We are grateful to T.
Granata, J. Terrados and E. Benavent for help in the
eld, and G. Carreras and L. Rubio for laboratory
assistance. We also thank E. W. Koch and an anony-
mous referee for useful comments that signicantly
improved the manuscript.
T 2. Sediment deposition in dierent areas of the Mediterranean littoral. Values are in grams
dry weight m
2
day
1
(g DW m
2
d
1
) and brackets encompass extreme events. Traps not
standing at the bottom but suspended from the water column are also indicated
Reference
Max
(g DW m
2
d
1
)
Min
(g DW m
2
d
1
)
Depth
(m) Bottom
Burns et al. (1985) 5·30·2 100 water column
Mo´naco et al. (1990) 181 2·7 10 water column
Buscail et al. (1990) 171 (2379) 8·3 25 water column
Bavestrello et al. (1995) 17·52·5 15 Rock
Bavestrello et al. (1995) 20 2·5 20 Rock
Bavestrello et al. (1995) 20 2·5 25 Rock
Charles et al. (1995) 107 0·6 18 Sand
Gre´maire et al. (1997) 318 0·6 18 Sand
This study 494 1·5 15 Sand
Dauby et al. (1995) 10 (40) 0·336Posidonia oceanica
This study 215 2 15 Posidonia oceanica
512 E. Gacia and C. M. Duarte
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514 E. Gacia and C. M. Duarte
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With its geographically isolated location and geological history, the Mediterranean Sea harbors well-known biodiversity hotspots, such as Posidonia oceanica seagrass meadows. Recently, long-living mats formed by the fleshy red alga Phyllophora crispa have been described to be associated with a high diversity of sessile invertebrates in the Tyrrhenian Sea. One of the key taxa among these sessile invertebrates are bryozoans: their abundance, diversity, and spatial distribution in P. crispa mats represent a gap in scientific knowledge. Thus, we conducted a pilot study on bryozoan assemblages associated with P. crispa mats around Giglio Island (Tuscan Archipelago, Italy) in 2018, followed by a comparative study on four sites distributed around the island in the subsequent year, 2019. We compared these findings to bryozoan abundance and diversity on P. oceanica shoots and leaves during the second expedition. The findings revealed more than 46 families, with a significantly higher number of taxa identified in P. crispa mats (33) than in P. oceanica meadows (29). The Shannon diversity index was similar between P. crispa and P. oceanica shoots, while Pielou’s evenness index was lower in P. crispa mats. The most abundant families reported across all habitats were Crisiidae, Aetidae, and Lichenoporidae; but the most abundant family on P. crispa was Chlidoniidae (Chlidonia pyriformis). The assemblages associated with P. crispa differed among sites, with higher abundances but lower diversity on the exposed southernmost site. The total bryozoan abundance was significantly higher on P. crispa (average 2.83 × 106 ± 1.99 × 106 colonies per m2 seafloor) compared to P. oceanica meadows (average 0.54 × 106 ± 0.34 × 106 colonies per m2 seafloor). Our results show a high diversity of bryozoans on P. crispa thalli compared to P. oceanica meadows, which was consistent throughout the study. These findings confirm the value of the red alga-generated habitat for associated bryozoans and may have implications for future biodiversity assessments and conservation measures. Keywords:
... Some of these models have also added the effect of vegetation on accretion rates (Marani et al., 2007;Morris et al., 2002;Mudd et al., 2009;Swanson et al., 2013), including an accretion component modeled as a function of plant biomass or productivity. Several studies have examined this plant-accretion relationship and the mechanisms behind it, finding that vegetation plays an important role in facilitating sediment accretion (Fonseca et al., 1982;Gacia et al., 1999;Gacia and Duarte, 2001;Baustian et al., 2012;Cahoon et al., 2020). ...
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