46 J
. Terrados, C.M. Duarte J. Exp. Mar. Biol. Ecol. 243 2000 45 –53
1996; Gacia et al., 1999, but also suggest that seagrass leaf canopies generate turbulence Ackerman and Okubo, 1993; Koch, 1996, particularly at the top of the leaf
canopy Grizzle et al., 1996. The reduction of water flow inside seagrass meadows might enhance sedimentation and reduce resuspension. Indeed, seagrass leaf canopies
can trap suspended materials Short and Short, 1984, and reduce wave energy Fonseca and Cahalan, 1992 and sediment movement Fonseca and Fisher, 1986; Fonseca, 1989.
While in situ measurements of suspended matter Bulthuis et al., 1984; Ward et al., 1984; Duarte et al., 1999 are consistent with the hypothesis that seagrass meadows
enhance sedimentation and reduce resuspension, no direct measurements of particle resuspension in seagrass meadows are yet available. Measurements of downward particle
flux inside and outside meadows of the Mediterranean seagrass Posidonia oceanica have recently allowed the calculation of resuspended fluxes Gacia et al., 1999. The
modelled estimates indicate that the leaf canopy of P
. oceanica meadows can reduce particle resuspension by as much as 5-fold under high-energy conditions. These
modelled results must be, however, verified through independent, direct observations of sediment resuspension. In particular, the model approach used by Gacia et al. 1999
cannot resolve resuspension events of low intensity. Furthermore, the effects of seagrass leaf canopies on turbulence seem to differ between medium–high- and low-flow regimes
Worcester, 1995.
The goal of this study is to test experimentally if seagrass leaf canopies are able to reduce particle resuspension under low-energy conditions. We used labelled fragments of
dried P . oceanica L. leaves deposited at the level of the sediment surface as tracers to
evaluate particle loss through resuspension within a 15-m deep meadow of the Mediterranean seagrass P
. oceanica and on an adjacent sandy bottom devoid of vegetation.
2. Material and methods
The study was performed at Punta de Fanals Girona, NE Spain; 41841.539 N, 2850.509 E in a 15-m deep P
. oceanica L. meadow growing on coarse sand. Three permanent stations were selected within the P
. oceanica bed and three over the adjacent sandy bottom, which was devoid of vegetation. The minimum distance between any two
stations was 3 m. Five experiments were performed during the summer 29 June, 22 July, 5 August, 26 August, and 17 September 1998, when mean wave height is lowest
´ Cebrian et al., 1996. Shoot density of P
. oceanica in the experimental area varied
22
between 204 and 350 shoots m and shows no clear seasonality Gacia and Duarte,
personal communication, while the average shoot length varied from 6662 SE cm to 9262 SE cm between the P
. oceanica stations. The effect of the P
. oceanica leaf canopy on particle resuspension was evaluated using dried fragments of P
. oceanica leaves as tracer particles. Leaf fragments were chosen because they represent an important fraction of the particle pool in P
. oceanica meadows Dauby et al., 1995; Duarte et al., 1999. The length of the fragments of P
. oceanica leaves varied between 1 and 18 mm median, 3.5 mm; n 5 766, and the width between
J . Terrados, C.M. Duarte J. Exp. Mar. Biol. Ecol. 243 2000 45 –53
47
0.5 and 3 mm median, 1 mm; n 5 635. Fragments were labelled red TITAN E spray
paint, Barcelona, Spain to distinguish them from the ‘natural’ pool of P . oceanica leaf
particles and, therefore, to distinguish between resuspension and net flux of particles during the experiments. Prior to the experiments, known amounts 0.4–0.5 g DW of
tracer particles were placed each in 20-ml screw-capped plastic scintillation vials and soaked in 4 formalin solution in seawater for 2–3 days to prevent the decomposition
of the tracer particles during the experiments by microorganisms and to deter detritivores. Infiltration of the lacunar system of the tracer particles by the formalin
solution was facilitated by placing the vials opened at 2 80 kPa vacuum until no bubbles were released by the particles about 10 min.
The tracer particles were not deployed directly on the bottom surface, but inside lower halves of plastic Petri dishes diameter, 87 mm; wall height, 13 mm to standardize the
deployment surface among stations and experiments, and facilitate the complete collection of the extant tracer particles at the retrieval of the experiments. The half Petri
dishes were placed levelled on the surface of the bottom at each of the stations by SCUBA divers Fig. 1. The surface of the bottom at the sand stations was flat and had
no obvious relief, making the placement of the half Petri dishes straightforward. The half Petri dishes were within 1–5 cm of and completely surrounded by seagrass shoots at the
P
. oceanica stations, and sometimes it was necessary to displace small stones, shells or seagrass leaves to create a levelled surface where the half Petri dishes could be placed.
The half Petri dishes had a nut glued at the center of the outer face of the bottom to maintain them in place during the experiments Fig. 1.
The tracer particles were taken to the bottom inside capped plastic vials, and were deployed into the half Petri dishes by carefully placing the vials upside down 5 mm
above them, opening the vial caps and letting the particles sediment slowly into the half Petri dishes. One vial was used for each half Petri dish. Three half Petri dishes
containing tracer particles were placed randomly at each of the stations, for each experiment, and retrieved after 24 h. Three additional Petri dishes were placed, tracer
particles deployed into them, and retrieved immediately for each experiment to account for tracer losses as a result of handling. Recovered tracer particles present in the Petri
dishes were separated from other materials, dried at 608C for 24 h and weighed. The loss
Fig. 1. Schematic representation of one of the half Petri dishes used in the experiments and of how were they placed on the sediment surface.
48 J
. Terrados, C.M. Duarte J. Exp. Mar. Biol. Ecol. 243 2000 45 –53
of tracer particles expressed as a percentage of the initial mass of tracers was calculated for each Petri dish.
The hydrodynamic conditions prevailing during the experiments were calm, charac- ´
teristic of summer conditions in the area wave height, 0.25 m, Cebrian et al., 1996, and the reported near-bottom current velocities at the experimental site during this time of
21 21
the year are low both inside 1–2 cm s and outside 2.4–4.3 cm s
the P . oceanica
meadow Gacia et al., 1999. Doppler measurements of current velocity above 100 cm above sediment surface and inside 10–20 cm above sediment surface the same P
. oceanica bed performed 3 weeks before the initiation of the experiments and under
similar wave conditions significant wave height, 30 cm indicate that current velocity at
21
the experimental stations is low , 5 cm s , and that total kinetic energy is reduced
by 95 from above the leaf canopy to near the bottom, but only by 50 for the same vertical distance at the sand stations Granata et al., unpublished data.
A relative estimate of integrated water motion during the experiments was obtained from the dissolution rate of plaster blocks Muus, 1968; Gambi et al., 1989; Jokiel and
Morrissey, 1993; Thompson and Glenn, 1994; Komatsu, 1996. Blocks were made by pouring a mixture of 550 g of calcium sulfate hemihydrate and 394 ml of water into a
flexible ice-cube tray. A 10-cm galvanized iron wire was inserted in each block while hardening to facilitate the placement of the blocks in the bottom. Blocks were dried at
558C for 5 days time to reach constant weight and weighed. Three plaster blocks were placed at each of the stations and retrieved at the same time as the Petri dishes. Weight
loss of each block was measured after drying at 558C during 5 days. A characterization of the dissolution of each batch of plaster blocks was performed by measuring the
dissolution of blocks in still seawater in the laboratory. To this end, two to three blocks were placed inside each of two portable coolers filled with 16 l of seawater collected at
the experimental site. Water inside the coolers was maintained at the same temperature as that in the experimental site 238C, constant during all the experiments by placing the
coolers inside an incubation chamber. After 24 h the blocks were dried at 558C for 5 days and their weight loss estimated. The formula
Weight loss Exposure time
field field
]]]]]] ]]]]]]]
3 Weight loss
Exposure time
still seawater still seawater
provides a dimensionless index of the dissolution of the blocks during the experiments relative to block dissolution in still seawater and, therefore, the index is proportional to
the increase in diffusivity due to water motion Jokiel and Morrissey, 1993; Thompson and Glenn, 1994.
Particle loss from the Petri dishes as a proxy of particle resuspension and water motion were compared between P
. oceanica and sand stations in each of the experiments using a t-test Sokal and Rohlf, 1981. We performed individual tests for each of the
experiments because hydrodynamic conditions, as indicated by the water motion index, were different between experiments Table 1. The overall significance of the effect of
the P . oceanica leaf canopy on the loss of tracer particles was tested by combining the
probabilities P of all the t-tests performed, and calculating the number 22 o ln P,
2
which has a x distribution Sokal and Rohlf, 1981.
J . Terrados, C.M. Duarte J. Exp. Mar. Biol. Ecol. 243 2000 45 –53
49 Table 1
Loss of tracer particles as of the weight of particles at the onset of the experiment and water motion a dimensional index, see Section 2 in a P
. oceanica meadow in Punta de Fanals NE Spain Experiment
Particle loss Water motion
mean6SD, n 53 mean6SD, n 53
29 June 1998 P
. oceanica 79.2616.4
Not determined Sand
62.9618.9 Statistical comparison
t 51.11, P 50.3303 22 July 1998
P . oceanica
97.861.7 2.3960.04
Sand 99.360.5
2.3860.04 Statistical comparison
t 51.36, P 50.2440 t 50.48, P 50.6534
5 August 1998 P
. oceanica 78.9613.4
1.9260.17 Sand
98.460.9 1.9460.05
Statistical comparison t 52.51, P 50.0659
t 50.21, P 50.8442 26 August 1998
P . oceanica
47.7619.9 2.1760.10
Sand 88.966.1
2.2260.09 Statistical comparison
t 53.42, P 50.0269 t 50.59, P 50.5891
17 September 1998 P
. oceanica 38.0619.2
2.4160.09 Sand
98.061.8 2.5260.05
Statistical comparison t 55.38, P 50.0058
t 51.89, P 50.1319
3. Results
