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



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 on
Sediment Retention by a Mediterranean Posidonia
oceanica Meadow: The Balance between Deposition
and Resuspension
E. Gacia
and C. M. Duarte
Centre d’Estudis Avanc¸ats de Blanes (CSIC), Ctra. Santa Ba`rbara s/n, 17300 Blanes, Spain
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
, 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
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
(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
Corresponding author. E-mail:
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.
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
under moderate wind conditions
(average of 7·923·15ms
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
, 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,
) and resuspended sedimentary ux
(Fr, g DW m
) 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
) 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
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:
where Ft is the total depositional ux (units in
) 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
: b=0), which, with the experimental
design used here, allowed resolution of resuspended
loads as low as 0·4gDWm
, 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
). 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
) combined with the measured sedi-
ment densities (g FW ml
) 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
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
quadrat in
which the number of shoots were counted. The above
ground biomass (units in g DW m
) and leaf area
index (LAI, m
leaf m
ground) of the seagrass bed
H (cm from the bottom)
100 150
Dt – Dp = Dr
g DW m
Ft measured with sediment traps:
Ft = a × e
–b H
Ft = Fp + Fr
Simulated fluxes:
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
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
leaves shoot
, and (3) above ground
biomass which ranged between 0·32·5gDW
. 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
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
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
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.
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
(mm day
Wind max speed
(m s
direction Site
(g DW m
(g DW m
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
19·66 963 <0·002 8 0·92
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
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
1·93 18·1<0·0001 15 0·99
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
19·56 999·7<0·0001 12 0·98
17·3 318·7<0·005 12 0·86
25/23/3/98 0·47·49 Q 4 sand
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
46·27 503 <0·0001 6 0·98
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
) were recorded during
spring, and occasionally during the rest of the year
(Table 1). Rainfall integrated between the sampling
events was strongly correlated (R
=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
Shoots m2
Above ground biomass
(g DW shoot1)
cm2 leaves shoot1
90 (a)
Canopy height (cm)
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).
100 cm
Depositional flux (g DW m
20 cm
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
g DW m2 d1
cm from the sea floor
Bottom 15
Depth (m)
5100 510
100 17/10/97
2Bottom 15
100 713/8/97
0Bottom 15
369 36
100 1117/7/97
2Bottom 15
100 28/59/6/97
0Bottom 15
200 400 600
100 28/42/5/97
2Bottom 15
91212 0
0 50 100800
Sand P. oceanica
0 15
g DW m2 d1
cm from the sea floor
Bottom 15
Depth (m)
5100 510
100 58/5/98
2Bottom 15
100 2026/3/98
0Bottom 15
50 100 150 50 100
100 25/23/3/98
0Bottom 15
5100 510
100 30/15/2/98
0Bottom 15
100 200 300
100 2227/12/97
0Bottom 15
150 200200 0
0 100 200 300
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
0·04 SE) compared to the areas
over bare sand (1·83gFWml
0·05 SE). The po-
tential sediment erosion associated with resuspension
events averaged 0·35 cm day
on bare sand and was
signicantly lower (0·10 cm day
; 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).
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.,
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
Erosional depth (cm d1)
Erosional depth (cm d1)
Posidonia oceanica
0 60
Resuspended flux/primary flux
Resuspended flux/primary flux
Posidonia oceanica
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
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
inside the meadow, and
4070 g DW m
on bare sand. These rates, in
the absence of advection, represent a sediment
accretion rate of 2 mm m
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
inside the meadow
and 76 146 g DW m
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.
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
(g DW m
) and brackets encompass extreme events. Traps not
standing at the bottom but suspended from the water column are also indicated
(g DW m
(g DW m
(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
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514 E. Gacia and C. M. Duarte
... Similar to the effects initiated by water depth, the increased turbidity resulting from terrestrial inputs in estuarine environments was expected to affect the C org stocks in P. oceanica seagrass sediments within the study area. Light attenuation has been confirmed to decrease J o u r n a l P r e -p r o o f meadow productivity and canopy complexity (Alcoverro et al., 2001;Ruiz and Romero, 2001;Collier et al., 2009) contributing to lower sequestration of autochthonous C org (Keil and Hedges, 1993;Gacia and Duarte, 2001;Burdige, 2007). However, no significant difference was established between the sedimentary C org stocks found within sediments of the two depositional environments. ...
... In coastal temperate areas, seagrass meadows have been recognized for their high trapping and retention capacity of sediment particles (Jeudy de Grissac and Boudouresque, 1985;Gacia and Duarte, 2001). Sediments accumulated in the P. oceanica meadows may also be composed of a high percentage of biogenic carbonate particles resulting from calcifying biota associated with seagrass meadows (Boudouresque and Jeudy de Grissac, 1983;De Falco et al., 2000;Serrano et al., 2012). ...
... density, cover and biomass) contributed generally to the reduction of hydrodynamic energy resulting in higher trapping and retention of fine-grained sediment particles (i.e. silt and clay) from the water column, and consequently leading to higher sediment accretion rates (SAR)(Gacia and Duarte, 2001;Hendriks et al., 2008; Samper-Villareal et al., 2016; Serrano et al., 2016b; ...
In the last decades, the increasing necessity to reduce atmospheric carbon dioxide (CO2) concentrations has intensified interest in quantifying the capacity of coastal ecosystems to sequester carbon, referred to commonly as ‘Blue Carbon’ (BC). Among coastal habitats, seagrass meadows are considered as natural carbon sinks due to their capacity to store large amounts of carbon in their sediments over long periods of time. However, the spatial heterogeneity of carbon stocks in seagrass sediments needs to be better understood to improve the accuracy of BC assessments, particularly where there is high environmental variability. In the Mediterranean, Posidonia oceanica (L.) Delile constitutes extensive meadows considered as long-term carbon sinks due to the development of an exceptional structure known as ‘matte’, reaching several meters in height, which can be preserved over millennia. In order to specify the role of P. oceanica meadows in climate change mitigation, an estimate of carbon stocks has been conducted along the eastern coast of Corsica (NW Mediterranean). The approach is mainly based on the biogeochemical analysis of 39 sediment cores. Organic carbon (Corg; 327 ± 150 t ha⁻¹, mean ± SE) and inorganic carbon stocks (Cinorg; 245 ± 45 t ha⁻¹) show a high variability related to water depth, matrix (sandy vs rocky substrate) or the depositional environment (coastal vs estuary). The isotopic signature (δ¹³C) revealed a substantial contribution of allochthonous inputs of organic matter (macroalgae and sestonic sources) mainly in estuarine environment and shallow areas. The carbon stocks in the first 250 cm of matte (average thickness) were estimated at 5.6–14.0 million t Corg (study site) and 14.6–36.9 million t Corg (Corsica), corresponding to 11.6–29.2 and 30.4–76.8 years of CO2 emissions from the population of Corsica.
... The surrogate was constituted by 2 pairs of leaves of 60 cm and 0 cm long each, all of them of 8 mm width mimicking an adult P Oceanica seagrass. These values are in the range of natural P Oceanica (Gacia and uarte, 2001;Ondiviela et al., 201 ;Ruiz et al., 201 ) who reported leaf lengths of up to 1 m and widths of up to 1 cm, grouped in clumps of up to 6 or leaves. This configuration is also in accordance with the experiments made by Manca et al., (2012). ...
... The values obtained in this work confirm what various authors ( uarte et al., 2013;Fonseca et al., 1982;Gacia and uarte, 2001;Koch et al., 2006;Ondiviela et al., 201 ) observed regarding seagrass beds and coastal protection. Carrying out tests with a low seagrass density (less than 300 stem m 2 ) in a common submergence ratio for this type of ecosystem (0.32), attenuation of wave heights can be generated, which could have a substantial impact on sedimentary dynamics, producing less erosion on the shoreline when higher waves are present. ...
Full-text available
New flume experiments with surrogate seagrass meadows are presented. The experiments included the creation of a full-scale and realistic Posidonia Oceanica model to evaluate the effect over wave attenuation, sediment transport and shoreline erosion. A hydrodynamic and morphodynamic comparison between cases with seagrass and without seagrass for two wave energy conditions was performed. Meadow density and submergence ratio were constant for tests under irregular waves. The wave height reduction, bar crest location, total sediment transport and shoreline position were used to evaluate the coastal protection efficiency of the meadow. The measured wave heights suffered a reduction due to the presence of the seagrass. These reductions were persistent in the area located between the onshore edge of the meadow and the depth of closure, being of greater magnitude in the more energetic case. All tests showed the development of a bar and the migration of the crest offshore over time. However, the dissipation of the incoming wave energy on the meadow made the bar crest stay closer to the shoreline and consequently generated a lower freeboard. In addition, the bar migration rate was reduced by the simulated meadow effect, in particular in the lower energetic case. A sediment volume significantly smaller was transported offshore when the seagrass meadow was present in both wave conditions. Additionally, in the higher energy case, a smaller shoreline retreat was observed when the meadow was present.
... Salt marshes, mangroves, and seagrass beds can uptake 16.5 million tons of CO2 per year and are indeed the world's most efficient carbon sinks. Seagrass beds alone account for 15% of the total carbon fixed in global waters [8] and provide a number of important ecosystem services, such as water filtration [9], sediment entrapment [10], serving as a food source [11,12], functioning as a habitat [13][14][15][16][17][18], and stabilizing coastlines [19]. Given their role as carbon sinks [20], seagrass beds have been proposed to be a natural means of controlling the rate of global climate change (GCC) [21]. ...
Seagrass beds serve as important carbon sinks, and it is thought that increasing the quantity and quality of such sinks could help to slow the rate of global climate change. Therefore, it will be important to (1) gain a better understanding of seagrass bed metabolism and (2) document how these high-productivity ecosystems are impacted by climate change-associated factors, such as ocean acidification (OA) and ocean warming (OW). A mesocosm-based approach was taken herein in which a tropical, Western Pacific seagrass species Thalassia hemprichii was cultured under either control or OA-simulating conditions; the temperature was gradually increased from 25 to 31 °C for both CO2 enrichment treatments, and it was hypothesized that this species would respond positively to OA and elevated temperature. After 12 weeks of exposure, OA (~1200 ppm) led to (1) increases in underground biomass and root C:N ratios and (2) decreases in root nitrogen content. Rising temperatures (25 to 31 °C) increased the maximum quantum yield of photosystem II (Fv:Fm), productivity, leaf growth rate, decomposition rate, and carbon sequestration, but decreased the rate of shoot density increase and the carbon content of the leaves; this indicates that warming alone does not increase the short-term carbon sink capacity of this seagrass species. Under high CO2 and the highest temperature employed (31 °C), this seagrass demonstrated its highest productivity, Fv:Fm, leaf growth rate, and carbon sequestration. Collectively, then, it appears that high CO2 levels offset the negative effects of high temperature on this seagrass species. Whether this pattern is maintained at temperatures that actually induce marked seagrass stress (likely beginning at 33–34 °C in Southern Taiwan) should be the focus of future research.
... Posidonia and other seagrasses are known to enhance deposition of particles, and baffle sediment between their rhizomes and leaves (e.g. Gacia and Duarte 2001;Hendriks et al. 2009). This is achieved by the reduction of hydrodynamic energy, hindering resuspension, which may lead to the deposition of finer particles than would normally be the case for such shallow settings (Duarte and Chiscano 1999;Duarte et al. 2005;Duffy 2006). ...
Pliocene (Zanclean to mid Piacenzian) marine deposits from Cape Vigli on southwestern Rhodes (Mediterranean Sea, Dodecanes, Greece) are described, and assigned to the newly defined Cape Vigli Formation. Highly fossiliferous sandy to silty mudstones contain a diverse parautochthonous mollusc assemblage dominated by articulated specimens of the bivalves Pinna nobilis and Megaxinus ellipticus, and by abundant Persististrombus coronatus gastropods. These taxa formed part of a seagrass community. Combined with sedimentological evidence and microfossil assemblages (foraminifera, calcareous nannofossils), they indicate a very shallow (<5 m deep) depositional environment in a sheltered marine embayment close to the coast. The fossil community is characterised by the occurrence of several Late Miocene to Pliocene warm-water mollusc taxa, which had their acme in the Zanclean and have become extinct in the Mediterranean in the early Piacenzian. Consequently, these molluscs testify to a Zanclean to mid Piacenzian age. This assignment is corroborated by the calcareous nannofossil assemblages of the deposits, which support a potential latest Miocene (latest Messinian) to Early Pleistocene (early Gelasian) age (calcareous nannofossil zones NN12 to NN17). The occurrence of marine Pliocene deposits on Rhodes indicates that the depositional history of the island is more intricate than previously assumed. The shallow marine nature of the studied strata provides new evidence to reconstruct the position of the palaeo-coastline during Pliocene times, when Rhodes was still connected to Anatolia. So far, fossiliferous shallow marine strata of Zanclean to mid Piacenzian age in the eastern Mediterranean had been documented from Hatay Province (southern Turkey) and Agia Triada (southwestern Peloponnese, Greece). The newly discovered locality of Cape Vigli thus provides a significant, novel dataset, which helps to evaluate the palaeobiogeography of Pliocene biota.
... The erect colonies of C. pyriformis are typical for calm water conditions and are often associated with macro-algae [45]. High abundances of erect and branching colonies (e.g., C. pyriformis, Scrupocellaria sp., Crisia sp.) on P. crispa and P. oceanica shoots are likely related to water current gradients inside P. crispa mats [46], as demonstrated before for P. oceanica meadows [47][48][49]. This is further supported by an experimental study that has confirmed negative phototaxis for the larvae of some erect bryozoan species [50], since strong light gradients are present in both habitats [46]. ...
Full-text available
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). ...
Full-text available
To assess the effect of sea level rise (SLR) on a 3,800-ha eelgrass meadow in Padilla Bay National Estuarine Research Reserve in Puget Sound, Washington, USA, we coupled the Marsh Equilibrium Model (MEM) with the Relative Elevation Model (REM), combining their respective strengths in simulating aboveground and belowground processes. We then modified the hybrid model to reflect an empirical relationship between eelgrass stem density and sediment accretion, making it the first model of its kind to do so. We used field data to initialize and calibrate the model, then simulated surface elevation change under various SLR and suspended sediment scenarios and tested it against a 12-year surface elevation table dataset from the site. 100-year simulations projected relative elevation loss along at least half of the elevation gradient for all SLR scenarios, and along the entire gradient for three SLR scenarios, with greater loss at higher elevations. The current suspended sediment concentration is thus insufficient for the entire eelgrass meadow to keep pace with SLR, with up to a four-fold increase required, however this presents a management conundrum in that the required sediment load may prevent eelgrass from meeting its light requirements. The main contributions of this study thus include: the novel combination of MEM and REM models, the inclusion of stem density as a factor controlling accretion, the use of a long-term data record for model initialization, calibration, and validation, and the finding that increasing sediment inputs to maintain the elevation of the habitat in the long term may be detrimental to eelgrass health in the short term.
... Within the benthic zonation of the Mediterranean Sea (Pérès and Picard, 1964), P. oceanica meadows constitute the climax stage of the softbottom ecosystem succession of the infralittoral zone. Posidonia oceanica is an ecosystem engineer that stabilizes the seafloor with its dense rhizome-meshwork, while the leaves favour local sedimentation by baffling floating particles (Boudouresque and Grissac, 1983;Moriarty and Boon, 1989;De Falco et al., 2000;Gacia and Duarte, 2001;Sanchez-Vidal et al., 2021). By creating a complex three-dimensional structure with separate foliar and rhizomatic strata, it provides the microhabitat for a wide variety of organisms, many of them with epiphytic living modes, such as encrusting red algae, bryozoans, molluscs, hydrozoans, and foraminifera (Chimenz et al., 1989;Langer, 1993;Murray, 2006;Pardi et al., 2006;Frezza et al., 2011;Mateu-Vicens et al., 2014). ...
Well-preserved remains of a Posidonia oceanica meadow have been found in a lower Pleistocene (Calabrian) succession cropping out near Fauglia (Tuscany, Italy). This paper analyses and describes the benthic foraminiferal community associated with this well-preserved Pleistocene P. oceanica meadow, with the purpose of testing the usefulness of foraminifera as an Indirect Palaeo-Seagrass Indicator (IPSI), providing both qualitative and quantitative parameters to recognize other meadow-related environments where fossil remains of seagrasses are not preserved. Despite being influenced by some diagenetic processes that might have affected the foraminiferal assemblage, the Fauglia succession represents a suitable setting for testing benthic foraminifera as IPSI in a temperate environment. Considering this limitation, together with other ecological constraints such as seawater temperature, this study provides evidence of the high potential of benthic foraminifera as IPSI. Several parameters such as the modified Foraminifera Index (FI’), the “long vs. short life-span index” (ILS), and the presence of permanently attached, encrusting foraminifera were tested. New indexes were also developed, namely the K/R ratio, consisting of the ratio between keeled Elphidium and the sum of rounded elphidiids (e.g., Elphidium translucens, Cribroelphidium, Porosononion) and Haynesina, and the K/REXT ratio, consisting of the ratio between keeled Elphidium and the sum of rounded elphidiids, Haynesina and other related genera that display a rounded periphery (i.e., Astrononion, Melonis, Nonion, Nonionella, Valvulineria). All these indexes were examined, suggesting their potential use for recognizing seagrass-related palaeoenvironments. The K/REXT ratio (and K/R) in association with the presence of permanently attached foraminifera revealed to be the most reliable palaeo-seagrass indicators, suggesting that this combination could be very useful also in other case studies where diagenesis altered the foraminiferal association.
Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change.
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Many coastal ecosystems, such as coral reefs and seagrass meadows, currently experience overgrowth by fleshy algae due to the interplay of local and global stressors. This is usually accompanied by strong decreases in habitat complexity and biodiversity. Recently, persistent, mat-forming fleshy red algae, previously described for the Black Sea and several Atlantic locations, have also been observed in the Mediterranean. These several centimetre high mats may displace seagrass meadows and invertebrate communities, potentially causing a substantial loss of associated biodiversity. We show that the sessile invertebrate biodiversity in these red algae mats is high and exceeds that of neighbouring seagrass meadows. Comparative biodiversity indices were similar to or higher than those recently described for calcifying green algae habitats and biodiversity hotspots like coral reefs or mangrove forests. Our findings suggest that fleshy red algae mats can act as alternative habitats and temporary sessile invertebrate biodiversity reservoirs in times of environmental change.
The ecosystem formed by the marine flowering plant Posidonia oceanica is a biodiversity reservoir and provides many ecosystem services in coastal Mediterranean regions. Marine meiofauna is also a major component of that biodiversity, and its study can be useful in addressing both theoretical and applied questions in ecology, evolution, and conservation. We review the meiofaunal diversity in the meadow ecosystem of P. oceanica by combining a literature review and a case study. First, we gathered records of 672 species from 71 published studies, as well as unpublished sources, highlighting 4 species exclusive to this ecosystem. Eighteen of those studies quantified the spatial and temporal changes in species composition, highlighting habitat-specific assemblages that fluctuate following the annual changes experienced by these meadows. Hydrodynamics, habitat complexity, and food availability, all three inherently linked to the seagrass phenology, are recognized in the literature as the main factors shaping the complex distribution patterns of meiofauna in the meadows. These drivers have been identified mainly in studies of Copepoda and Nematoda, and their effect may depend ultimately on species-specific preferences. Second, we tested the generality of these observations using marine mites as a model group, showing that similar ecological preferences might be found in other less abundant meiofaunal groups. Overall, our study highlights the high diversity of meiofauna in meadows of P. oceanica compared with algae and sessile macrofauna associated with this seagrass and shows the complexity of the interactions and habitat use by meiofauna associated with the seagrass.
The annual sedimentation rates along a Portofino Promontory cliff were studied using small sediment traps fixed directly to the rocks at 15, 20 and 25 m depth. The highest quantity of coarse matter, related to the rainfall, was collected in the top trap. Fine sediments were due to the local sea conditions and increased from the shallow to the deepest trap. This suggests that resuspension of fine sediments from the sea floor may represent an important fraction of settling matter at lower levels of the submerged coastal cliff. -Authors
In the Indian River, Florida, traps were used to measure the rate of deposition in a Thalassia seagrass meadow and in an adjacent (sandy) grass-free area. Electromagnetic current-meter measurements showed that the mean flow velocity in the grass-free area is higher than in the seagrass area. Therefore, the increased depositional rate of mud within the seagrass is due to slowing of water currents by the grass blades. -from Authors
A review of research in the Mediterranean area, in order to evaluate the effects of the sea level rise, is presented. Various hypotheses on the rising causes are considered. -from Author
The interannual changes in leaf formation and vertical growth rates and their correlation to the records available of environmental change (rainfall, mean sea level, water temperature, and transparency) were examined in 15 Posidonia oceanica meadows growing along the Spanish Mediterranean coast between 1967 and 1992. P. oceanica leaf production fluctuated interannually, but it did not exhibit any steady trend toward decline, indicative of non human effects on changes in water quality in these areas. Conversely, the steady decline in vertical rhizome growth rate of P. oceanica observed in two sites suggests that shoreline erosion there could derive from human activities. In all meadows examined, interannual variability in vertical rhizome growth of P. oceanica showed clear oscillating trends, suggesting alternating episodes of sediment erosion and accretion every 7 yr and at least every 25 yr. Mean sea level and surface water temperature have been increasing for the last two decades, but water transparency has been declining. However, overall trends only accounted for 24-37% of the long-term climatic variance. Rainfall interannual changes were dominated by time scales of 8 and 28 yr, whereas water transparency, temperature, and sea level showed dominant time scales in the oscillations of 4 and 15 yr, 6 and 20 yr, and 11 and 27 yr, respectively. In addition, 33% of P. oceanica vertical growth variability in the southern Spanish Mediterranean coast derived from variability in rainfall, suggesting a rise of erosive coastal conditions during rainy years. The similarity in the interannual changes of seagrass growth over a wide spatial scale (1,000 km), together with the significant coupling between seagrass growth and climate variability, points out climate change, and not widespread deterioration derived from anthropogenic pressure, as the main source of the observed changes in the Mediterranean littoral zone.
Dried leaf fragments of the Mediterranean seagrass Posidonia oceanica were used as tracer particles to test if seagrass leaf canopies reduce particle resuspension. Half Petri dishes containing a known mass of tracer particles were deployed for 24 h, five times during the summer period inside a 15-m deep P. oceanica meadow and on an adjacent sandy bottom devoid of vegetation. The loss of tracer particles was consistently high (>62.9% of initial particle mass) at sand stations, while both high (>79.2% of initial particle mass) and low (