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An approach to measurement of particle flux and sediment retention within seagrass (Posidonia oceanica) meadows
Aquatic Botany 65 (1999) 255–268
An approach to measurement of particle flux and sediment retention within seagrass (Posidonia oceanica) meadows E. Gacia ∗ , T.C. Granata, C.M. Duarte Centre d’Estudis Avançats de Blanes, CSIC, Ctra. Santa Bàrbara s/n, 17300 Blanes, Spain
Abstract Seagrass beds have traditionally been considered to act as sinks for particles due to the reduction of flow velocities by the plant canopy. Yet, there is a paucity of measurements to confirm this role. In this work we illustrate changes in flow in the presence and absence of Posidonia oceanica using an ADV, and provide direct measures of particle trapping by the use of sediment traps. We also describe a model to estimate sediment resuspension after measuring particle flux at different distances from the bottom. Measurements of particle flux are conducted parallel to the study of structural parameters of the Posidonia meadow potentially involved in both particle trapping and retention. Data obtained on velocity profiles confirm previous findings that seagrass canopies slow down current velocities with intensities proportional to the canopy height of the plants. The projected surface area of the plants (LAI) significantly correlated with the total amount of particles trapped within the Posidonia meadow, thus indicating seagrass canopy slightly increased particle trapping in the absence of resuspension. The trapping capacity of the canopy was not linearly correlated to LAI but significantly decreased at LAI above four, thus suggesting that other factors such as bending of the leaves and particle attachment to the surface may interfere with particle free sinking within the canopy at high projected surface area. The model proposed to estimate resuspension allowed to measure the retention capacity of the P. oceanica meadow, this being up to 15 times higher compared to a barren bottom during situations of high energy (large eddies reaching the bottom). The results obtained provide direct quantitative support to seagrass beds promoting sediment accretion and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Resuspension model; Sedimentation; Posidonia oceanica
∗ Corresponding author. Tel.: +34-972-33-61-01; fax: +34-972-33-78-06 E-mail address: [email protected] (E. Gacia)
0304-3770/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 4 4 - 3
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1. Introduction Seagrass beds have traditionally been considered to act as traps of particulate matter by enhancing the sedimentation and retention of particles. This belief is based on historical accounts of changes in sedimentation patterns in the presence of seagrass meadows (Wilson, 1949; Christiansen et al., 1981) supported by physical models describing changes in horizontal water flow when encountering the plant canopy (e.g. Fonseca et al., 1982, 1983; Koch, 1996), and to the existence of a rhizome and root system (Ginsburg and Lowenstam, 1958; Scoffin, 1970; Orth, 1977; Dauby et al., 1995) where sedimented material stays trapped. Seagrass shoot densities (Gambi et al., 1990) and architecture of the plants (Fonseca and Fisher, 1986) have been shown to be important factors strongly influencing the plant-water flow interaction. The area of influence of the substratum on the water flow is known as the benthic boundary layer (BBL), whose properties depend on both the flow velocity and the nature of the substratum. In the absence of vegetation, flow velocity (u) decreases logarithmically over depth (z), and the energy dissipation, defined as vertical shear (u* = du/dz), is maximal near the bottom. In vegetated sediments, a peak of energy dissipation occurs further from the sediment due to the interference of the canopy with the water flow (Gambi et al., 1990), thus resulting in a general retardation of the flow across the canopy, potentially enhancing sedimentation. The height above the bottom where vertical shear equals zero is known as the roughness height of the substratum (z0 ) and is elevated over the bottom in the presence of vegetation (Fonseca et al., 1982). While the effect of plant canopies on water flow provides the basis to explain the capacity of seagrass meadows to act as a particle sink, there is a paucity of direct measurements to confirm this. Moreover, the net sedimentation rate depends both on the settling flux of materials and the possible losses due to resuspension, and the effect of seagrasses on both these processes needs to be considered. Here we describe and demonstrate an approach to examine the effects of plant canopies on water flow and the associated effect on net sediment deposition and resuspension. We do so by measuring particle transport within Posidonia oceanica beds compared to adjacent areas bare of vegetation, by two different approaches: (1) inferences derived from velocity profiles obtained with an acoustic Doppler velocimeter (ADV; short time measurements within a scale of minutes) and (2) direct measurements of particle flux (both sedimentary and resuspended) using sediment traps to integrate this process over a time period of days. We then apply a model (Valeur, 1994) that allows the estimation of resuspension rates from measurements of particle flux at different heights above the sea floor. Finally, we test the relationship between different structural parameters of the meadow and particle flux within the canopy. We do this through the comparison of estimates of sedimentation and canopy structure across a depth gradient, and also through the experimental manipulation of the structural properties.
2. Methods The study was conducted in three P. oceanica meadows: Fanals (15 m depth), Giverola (12 m depth), and Jonquet (2 to 14 m depth), all situated in the NE-Mediterranean Spanish
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Fig. 1. Location of the Posidonia oceanica meadows studied along the NW-Mediterranean Spanish coast.
coast (Fig. 1; 41◦ 410 N, 41◦ 550 N, 42◦ 180 N, respectively). Fanals is a rather exposed site, while the meadow at Giverola is somewhat more sheltered, and that at Jonquet is a very sheltered bay. Downward particle flux at different distances from the bottom inside the P. oceanica meadow and over the adjacent (15 m apart) barren bottom, was measured with sediment traps on 27 to 31/1, 22 to 26/2 and 27/3 to 2/4 1997 in Giverola. Parallel estimates of meadow structure and measures of downward particle flux across a depth transect (stations at 2.9, 5.1, 8.9, 11.6 m) and ADV profiles were conducted in Jonquet (15 to 22/5/1997). The clipping experiment took place in Fanals (7 to 13/8/1997). 2.1. Acoustic Doppler velocimeter (ADV) measurements An acoustic Doppler velocimeter (ADV, model Oceans, SonTek, San Diego, CA, USA) was used to obtain velocity data. This probe measures horizontal (u and v) and vertical (w) velocity components sampled at 25 Hz (25 times per second). The ADV probe was mounted on a cylindrical, stainless steel frame, in a down looking position, and oriented so that the u velocity component measured the dominant wave motion. A pointed tip on the central shaft of the frame was fixed into the sediments and stabilized with a tripod. The vertical
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position of the sampling volume on the frame could be adjusted to discrete heights above the bottom. For time series measurements, the sampling volume was located at nominal heights of 20 and 100 cm above the bottom. Velocity time series of 5–10 min were collected at these heights. For rapid measurements of the vertical structure of the horizontal velocity field, the frame was also used as a profiler where the central shaft functioned as a piston inside the cylindrical frame. Profiling was done by lifting the frame to the top of the shaft, and filling the cylinder with water through a special valve. This valve was then closed and the frame, with the probe, descended along the shaft at a near constant speed. The speed of the cylinder was regulated by a separate valve for the outflow. Travel times were between 30 and 55 s. The total distance traveled (95 cm) divided by the total travel time gave the mean fall velocity of the probe. This was verified using measurements of the w velocity component for a near zero mean flow. Using the mean fall velocity and the time of the sample, u components from six profiles were binned into 15 cm heights with more than 600 points per bin. Thus, rapid profiles are quite different from time series profiles, but they show the same tendencies in the magnitude and shape of u over the boundary layer as the time series profiles. Velocity profiles were made over sandy bottoms located at least 5 m outside of meadows, and above and inside the canopies, at a distance of at least 2 m from the leading edge of the meadows. Inside the plant canopy leaves were removed, when necessary, in the area just below the probe so they would not cross through the measurement volume of the ADV and interfere with velocity measurement. Velocity data were edited for spurious velocities, which we identified by low correlations (<70%) and signal strengths (<5 db) between the three transducer channels. The root mean square (rms) velocity of the u component was used to calculate the shear velocity, u∗ , and the roughness height, z0 , using the equation, u = (u∗ /κ)ln(z/z0 ),where k, the van Karman constant, was assumed to be equal 0.4. The 95% confidence intervals for u∗ , and z0 were according to Wilkinson (1984). Semilogarithmic plots of u vs. height above the bottom were used to calculate roughness height and velocity, by fitting the regression equation: u (cm s−1 ) = a − b ln height (cm)
(1)
where b (units cm s-1 ) provides an estimation of shear velocity, and the intercept on the ordinate (i.e., when u = 0) provide an estimate of roughness height (in cm). 2.2. Sediment traps Small sediment traps were designed to measure particle deposition, minimizing interference with the water flow and the plant canopy. We chose 20.5 ml cylindrical glass centrifugation tubes with an aspect ratio of 5 (16 mm diameter) following recommendations of Hardgrave and Burns (1979) and Blomquist and Hakanson (1989). The tubes were attached by groups of 5 to 30 cm long stainless bars, and were mounted with a separation of 4 cm from each other. Triplicate sediment traps for estimates of total depositional fluxes were fixed at 20 cm above the bottom. Traps were deployed by SCUBA divers, simultaneously within the Posidonia canopy and to adjacent unvegetated bottom for a total period between four and eight days. In order to allow the estimation of resuspension loads, sediment traps
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Fig. 2. Illustration of the approach used to deconvolute the depositional flux profile (from Valeur, 1994). Ft = total particle flux, Fp = flux of primary sediments, Fr = Flux of resuspended material, Dp = primary deposition, Dr = resuspended deposition. Units of fluxes in g DW m−2 day−1 .
fixed to a central pole 1.5 m tall, were placed upright over the sediment, at heights of 20, 40, 60 and 80 cm above the bottom, thereby allowing the examination of the relationship between downward particle flux and height above the bottom. Bars were arranged forming a downward spiral so that upper bars were never situated immediately above a lower bar. At the laboratory the content of the tubes was filtered through 25 mm GF/F filters and its dry weight (DW) was assessed after drying the filters for 24 h at 60◦ C. The description of sedimentation processes follows the terminology of Pejrup et al. (1996) to discriminate between the flux of primary settling matter (Fp, g DW m−2 day−1 ) and the flux of resuspended sediment (Fr, g DW m−2 day−1 ). The primary settling matter is defined as sediment particles, including autochtonous particulate matter and terrigenous matter, deposited for the first time at the bottom of the measuring site. Resuspended sediment consist of the same components, but which have been previously deposited at the measuring site. The primary and resuspended sediment flux are derived by deconvulting the total downward flux (Ft; units in g DW m−2 day−1 ), which is the sum of Fp and Fr, through the analysis of the vertical particle flux as a function of the height above the sea-bed (Fig. 2; Valeur, 1994). Total deposition (Dt; units in g DW m−2 day−1 ) is defined as Ft at the sediment surface. Since resuspension causes a diffusive supply of particles, increasing the particle load exponentially towards the sea-bed (Ichie, 1966), the changes in Ft with height above the bottom (in cm) should fit, in the presence of significant resuspension, to a negative exponential function of the form: Ft = a ∗ e−b
∗H
(2)
Fp would then correspond to the asymptotic values of Ft at distances over the sediment (in our case the measurements at 80 cm; see also Fig. 2) and the intercept of this primary flux profile with the sea-bed (i.e., Ft at H = 0) provides an estimate of the rate of primary
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deposition (Dp; units g DW m-2 day−1 ). The difference between Dt and Dp, represents the downward flux of resuspended sediments Dr (Dr = Dt − Dp). 2.3. Meadow structure and experimental manipulation Parallel to the sediment trap deployment, biomass, canopy height and leaf surface area of 10 randomly selected shoots of P. oceanica were measured. Dry weight was estimated after drying for 24 h at 60◦ C. Shoot densities were determined by randomly placing a 48 cm × 48 cm quadrat and counting the number of shoots within the total area. The above ground biomass (units in g DW m−2 day−1 ) and leaf area index (LAI, m−2 leaf m−2 ground) of the bed was extrapolated from individual shoot data using the density measurements. To test the effect of changes in meadow structure on water flow and sediment flux, we clipped the shoots of P. oceanica at lengths equivalent to 50% and 10% of the total canopy height (canopy height = 68 ± 11.5 cm). The manipulated area consisted of two 0.3 m wide by 2 m long quadrates oriented parallel to the dominant current. Leaf debris was allowed to be washed away from the study area for two days. Then ADV velocity profiles and sediment trap measurements of a sand plot, a full canopy length, and clipped plots with 50 and 10% of the canopy height were conducted as described above.
3. Results and discussion 3.1. Water flow across P. oceanica canopies Velocity data from the Posidonia meadow in Jonquet, showed that velocity profiles over sand decreased smoothly (Fig. 3a) and exponentially toward the bottom (Fig. 3b). Velocities through the canopy also decayed exponential toward the bottom, except near the base of the stem where there was a slight increase in velocity (Fig. 3a). Current velocity was reduced in the upper portion of the canopy (60 cm above the sea-floor), where the exposed leaf area was greatest, enhancing viscous drag. The increased velocity near the base of the plant stems was a result of a reduced leaf area and thus a lower resistance to the flow associated with the narrow stem. In the upper portion of the canopy, velocities were more variable and had a steeper velocity gradient associated with the plants’ ability to more effectively slow the flow. This same trend was observed in the Fanals clipping experiment, where less flow restriction and lower vertical velocity gradients were found with reduced leaf height (Fig. 4a,b). For both these meadows, the velocity in the upper portion of the canopy followed a logarithmic scaling, which allowed the use of shear velocity and roughness height to characterize the boundary layer. Although confidence intervals were broad, both shear velocity and roughness height were greater for the meadow (u∗ = 1.33 ± 0.78 cm s−1 and z0 = 30 ± 93 cm), compared to the sand bottom (u∗ = 0.68 ± 0.24 cm s−1 and z0 = 5.0 ± 7.4 cm), which indicates an increase in flow deformation produced by plant friction. In Fanals, the manipulation experiment confirmed the observed effects of changes in plant canopy structure on flow characteristics (Fig. 5). The shear velocity and roughness height were greatest for the full canopy height and were progressively reduced for 50 and 10% canopy height (see Table 1).
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Fig. 3. Time averaged velocity profiles over sand (open) and over a meadow (filled) in Jonquet, Spain (a); and the corresponding log-velocity distribution of the profile (b). Solid lines represent fitted regression lines (Table 1). Grey bars illustrate P. oceanica canopy mean height ± SE.
Thus, increased friction by the plant leaves yields increased shear velocity and roughness height, while reducing the magnitude of the velocities compared to those over the unvegetated bottom (i.e. sandy substrates). A decrease in flow velocity should lead to enhanced particle deposition within the Posidonia meadow compared to those over sandy bottoms. Similarly, higher energy dissipation by the plants (which implies reduction of both, vertical and horizontal velocities) should buffer resuspension processes below the plant canopies.
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Fig. 4. Time averaged, velocity profiles in a meadow in Fanals, Spain (a); and the corresponding log-velocity distribution of the profile (b). Filled symbols represent the full height of the canopy (100%), open circles represent meadows clipped at 50% height, and open diamonds represent meadows clipped at 10% of the full canopy height. Grey bars illustrate P. oceanica mean canopy height ± SE.
3.2. Sedimentation rates The total deposition fluxes in our sites oscillated between 5 and 35 g DW m−2 day−1 , in agreement with the ranges derived from seasonal studies in nearby areas of the WMediterranean: 0.2–45 g DW m−2 day−1 within a Posidonia bed at 36 m depth in Corsica
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Fig. 5. Changes in benthic boundary layer properties: u∗ = shear velocity and z0 = roughness height, after in situ manipulation of canopy height in Fanals.
Table 1 Shear velocities and roughness heights estimated from semilogarithmic plots of velocity (u) vs. height profiles (see Eq. (1)) of the different treatments: full canopy length, 50 and 10% canopy lengths, of the clipping experiment. Values are expressed ±95% c.l. Canopy length
u∗ (cm s−1 )
z0 (cm)
P
N
R2
Full 50% 10%
0.48 ± 0.22 0.30 ± 0.17 0.13 ± 0.03
10 ± 2.20 2.3 ± 0.13 0.04 ± 0.01
<0.05 <0.05 <0.05
7 6 6
87% 72% 98%
(Dauby et al., 1995), and 0.6–107 and 0.6–317 g DW m−2 day−1 on a barren bottom at 18 m in Banyuls, France (Charles et al., 1995; Grémare et al., 1997, respectively). These fluxes, collected at 20 cm above the bottom, did not show statistically significant greater deposition rates within the Posidonia canopy compared to that over bare sand (Fig. 6). Instead, the total downward flux in February was significantly higher (p < 0.001) over bare sand compared to that inside the seagrass bed. The downward flux of particles fitted the resuspension model in January over sand, and both in bare and vegetated bottoms in March (Fig. 7; Table 2). The model (Eq. (2)) allowed the estimation of the primary and resuspended sedimentation rates, which varied significantly among samples (see March data in Table 2). In January the contribution of resuspended material was one-fifth of the total deposition over sandy bottom, while in March resuspended materials comprised half of the total material trapped inside the bed, and 90% of the total deposition over bare sand (Table 2). These results indicate that the very high sedimentation rates measured over bare sand in March are attributable to a much greater sediment resuspension there compared to
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Fig. 6. Comparison of the particle flux trapped at the 20 cm sediment traps deployed simultaneously within the P. oceanica meadow and the unvegetated sandy bottom in Giverola. Solid line represents the 1 : 1 line.
Table 2 Components of the downward particle flux profiles derived from the fitted resuspension model (Eq. (2), Fig. 2). Estimates of primary deposition (Dp) were obtained from sediment trap measurements at 80 cm from the bottom, and resuspended deposition (Dr) was estimated from the fitted model parameters. R2 is the coefficient of determination of the fitted model (Eq. (2)), and P is the probability that the resuspended deposition (Dr) equals 0 Date
Site
Dp (g DW m−2 day−1 )
Dr (g DW m−2 day−1 )
P
N
R2
27 to 31/1/1997
Sand Posidonia Sand Posidonia Sand Posidonia
33.8 35.5 4.7 3.6 5.5 5.5
6.41 0 0 0 47.5 3.48
<0.05 0.48 0.41 0.09 <0.005 <0.0001
10 13 8 8 10 10
45% 5% 11% 40% 72% 83%
22 to 26/2/1997 27/3 to 2/4/1997
that inside the bed, as expected from the effect of the canopy in buffering energy penetration to the sediments. Analysis of the structural parameters of the Posidonia bed in Jonquet (Table 3) showed no pattern of canopy particle trapping as function of shoot density, biomass or canopy height (Fig. 8), but the projected surface area of the plants (LAI) was significantly and strongly correlated (p < 0.005) with the total deposition rate within the bed (Fig. 8d). These data contrast with the comparative results (sand vs. Posidonia; Fig. 6) obtained with the sediment traps in Giverola, where no significant differences in sedimentation were found in
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Fig. 7. Vertical particle flux profiles measured simultaneously within the P. oceanica meadow and the sand bottom in Giverola from January to March 1997. See Table 2 for flux estimates. Table 3 Mean and standard error of structural parameters of the Posidonia oceanica bed in Jonquet across a depth gradient. Mean values and standard error are provided Meadow
Depth (m)
Density (shoots m−2 )
Biomass (g DW m−2 )
Canopy height (cm)
LAI (m2 m−2 )
Giverola
12 12 12
247 ± 40 340 ± 44 444 ± 60
190 316 469
28 ± 4.4 35 ± 3.8 46 ± 5.3
3.4 ± 0.67 5.1 ± 1.89 7.5 ± 2.36
Jonquet
2.9 5.1 8.9 11.6
198 ± 29.60 259 ± 53.70 224 ± 43.60 173 ± 61.58
118 176 219 177
28.8 ± 1.34 33.1 ± 1.46 55.8 ± 1.21 56.9 ± 3.00
1.85 ± 0.15 2.89 ± 0.25 3.99 ± 0.19 2.79 ± 0.21
Fanals
15 15 15
291 ± 133 291 ± 133 291 ± 133
560 303 62
68 ± 11.52 35 ± 9.46 7.2 ± 1.76
4.6 ± 0.81 2.5 ± 0.27 0.4 ± 0.12
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Fig. 8. The relationship between sedimentation rates across a depth transect in Jonquet and the structural parameters of the P. oceanica meadow: (a) leaf biomass, (b) shoot density, (c) canopy height, and (d) Leaf area index (LAI).
the presence of vegetation. In contrast, the results obtained on sheltered Jonquet meadow, where resuspension processes could not be detected, support the notion that seagrasses act as particle traps at increasing structure of the meadow (see Fonseca and Cahalan, 1992). Our results suggest that the capacity of seagrasses to increase particle deposition is more significant at low particle concentration in the water (low sedimentation) and in the absence of resuspension, as suggested by Duarte et al., 1998. Our field measurements show seagrass architecture, particularly LAI, to be more important than seagrass shoot density in determining the amount of energy dissipation by the plants, in agreement with comparative interespecific studies (Fonseca and Fisher, 1986) where canopy friction was more related to the amount of water column occupied by a plant, than to the meadow density itself. However, the results of the experimental manipulation of canopy height in Fanals suggests this relationship to be nonlinear, with a significantly reduced sedimentary flux at high leaf area index (LAI > 4 m2 m−2 ; Fig. 9). This suggests that other factors, such as bending of the leaves and particle attachment to their surfaces, may interfere with the deposition of free sinking particles at high canopy densities.
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Fig. 9. The relationship between total depositional fluxes and the leaf area index of the manipulated meadow at Fanals (LAI = 4.6 m2 m−2 ) and that of plots clipped at 50% (LAI = 2.5 m2 m−2 ) and 10% (LAI = 0.4 m2 m−2 ) of the full canopy height.
4. Conclusion In summary P. oceanica canopies increased shear velocity, energy dissipation, and raised the roughness height of the bottom, compared to the unvegetated sediments. P. oceanica canopies influenced sedimentation processes in two ways, (1) by buffering resuspension rates due to the dissipation of energy and rescaling the turbulence at the canopy; and (2) by increasing the amount of primary deposition with moderate increases in LAI under sheltered conditions in particle-poor waters. While the effect of canopies on the increased primary flux of sedimentary materials is modest, their effect on resuspension rates (at least 10-fold lower) is highly significant. These effects provide direct quantitative support to notion that seagrass beds promoting sediment accretion, and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes, which has proven elusive in the past.
Acknowledgements This research was funded by project PhaSE (contract MAS3-CT96-0053) of the ELOISE programme of the European Commission. We thank Lars K. Nielsen for advises on measurements of sedimentation and resuspension, and to Josep Vilaseca for valuable help on the design of the sediment traps.
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Charles, F., Amouroux, J.M., Grémare, A., Baudart, J., 1995. A bioassay approach to temporal variation in the nutritional value of sediment trap material. J. Exp. Mar. Biol. Ecol. 191, 65–81. Christiansen, C., Christoffersen, H., Dalsgaard, J., Norberg, R., 1981. Coastal and nearshore changes correlated with die-back in eelgrass (Zostera marina L.). Sedimentary Geol. 28, 163–173. Dauby, P., Bale, A.J., Bloomer, N., Canon, C., Ling, R.D., Norro, A., Robertson, J.E., Simon, A., Théate, J.M., Watson, A.J., Frankignoulle, M., 1995. Particle fluxes over a Mediterranean seagrass bed: a one year case study. Mar. Ecol. Prog. Ser. 126, 233–246. Duarte, C.M., Benavent, E., Sanchez., M.C., 1998. The microcosm of particles within seagrass (Posidonia oceanica) canopies, Mar. Ecol. Prog. Ser., submitted for publication Fonseca, M.S., Fisher, J.S., Zieman, J.C., Thayer, G.W., 1982. Influence of the seagrass, Zostera marina L. on current flow. Estuar. Coast. Shelf Science 15, 351–364. Fonseca, M.S., Zieman, J.C., Thayer, G.W., Fisher, J.S., 1983. The role of current velocity in structuring eelgrass (Zostera marina L.) Meadows. Estuar. Coast. Shelf Science 17, 367–380. Fonseca, M.S., Cahalan, J.A., 1992. A preliminary evaluation of wave attenuation by four species of seagrass. Estuar. Coast. Shelf Science 35, 565–576. Gambi, M.C., Nowell, A.R., Jumars, P.A., 1990. Flume observations on flow dynamics in Zostera marina (eelgrass) beds. Mar. Ecol. Progr. Ser. 61, 159–169. Ginsburg, R.N., Lowenstam, H.A., 1958. The influence of marine bottom communities on depositional environment of sediments. J. Geol. 66, 310–318. Grémare, A., Amouroux, J.M., Charles, F., Dinet, A., Riaux-Gobin, C., Baudart, J., Medernach, L., Bodiou, J.Y., Vetion, G., Colomines, J.C., Albert, P., 1997. Temporal changes in the biochemical composition and nutritional value of the particulate organic matter available to surface deposit-feeders: a two year study. Mar. Ecol. Prog. Ser. 150, 195–206. Hardgrave, B.T., Burns, N.M., 1979. Assessment of sediment trap collection efficiency. Limnol. Oceanogr. 24, 1124–1136. Ichie, T., 1966. Turbulent diffusion of suspended particles near the ocean bottom. Deep-Sea Res. 13, 679–685. Koch, E.W., 1996. Hydrodynamics of a shallow Thalassia testudinum Bed in Florida, USA. In: Kuo, J.K., Phillips, R.C., Walker, D.I., Kirkman, H. (Eds.), Proceedings Internat. Seagrass Biol. Workshop, Rottnest Island, pp. 105–110. Orth, R.J., 1977. The importance of sediment stability in seagrass communities. In: Coull, B.C. (Ed.), Ecology of Marine Benthos, University of South Carolina Press, Columbia, pp. 281–300. Pejrup, M., Valeur, J., Jensen, A., 1996. Vertical fluxes of particulate matter in Aarhus Bight, Denmark. Cont. Shelf Res. 16 (8), 1047–1064. Valeur, J.R., 1994. Resuspension mechanisms and measuring methods. In: Floderus, S., Heiskanen, A., Olesen, M., Wassmann, P. (Eds.), Sediment Trap Studies in the Nordic Countries. Marine Biological Laboratory, Helsingor, pp. 185–203. Scoffin, T.P., 1970. The trapping and binding of subtidal carbonate sediments by marine vegetation in Bimini Lagoon, Bahamas. J. Sedim. Petrol. 40, 249–273. Wilkinson, R.H., 1984. A method for evaluating statistical error associated with logarithmic velocity profiles. Geo-Marine Letters 3, 49–52. Wilson, D.P., 1949. The decline of Zostera marina L. at Salcombe and its effects on the shore. J. mar. biol. Ass. UK 28, 395–412.
An approach to measurement of particle flux and sediment retention within seagrass (Posidonia oceanica) meadows E. Gacia ∗ , T.C. Granata, C.M. Duarte Centre d’Estudis Avançats de Blanes, CSIC, Ctra. Santa Bàrbara s/n, 17300 Blanes, Spain
Abstract Seagrass beds have traditionally been considered to act as sinks for particles due to the reduction of flow velocities by the plant canopy. Yet, there is a paucity of measurements to confirm this role. In this work we illustrate changes in flow in the presence and absence of Posidonia oceanica using an ADV, and provide direct measures of particle trapping by the use of sediment traps. We also describe a model to estimate sediment resuspension after measuring particle flux at different distances from the bottom. Measurements of particle flux are conducted parallel to the study of structural parameters of the Posidonia meadow potentially involved in both particle trapping and retention. Data obtained on velocity profiles confirm previous findings that seagrass canopies slow down current velocities with intensities proportional to the canopy height of the plants. The projected surface area of the plants (LAI) significantly correlated with the total amount of particles trapped within the Posidonia meadow, thus indicating seagrass canopy slightly increased particle trapping in the absence of resuspension. The trapping capacity of the canopy was not linearly correlated to LAI but significantly decreased at LAI above four, thus suggesting that other factors such as bending of the leaves and particle attachment to the surface may interfere with particle free sinking within the canopy at high projected surface area. The model proposed to estimate resuspension allowed to measure the retention capacity of the P. oceanica meadow, this being up to 15 times higher compared to a barren bottom during situations of high energy (large eddies reaching the bottom). The results obtained provide direct quantitative support to seagrass beds promoting sediment accretion and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Resuspension model; Sedimentation; Posidonia oceanica
∗ Corresponding author. Tel.: +34-972-33-61-01; fax: +34-972-33-78-06 E-mail address: [email protected] (E. Gacia)
0304-3770/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 4 4 - 3
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1. Introduction Seagrass beds have traditionally been considered to act as traps of particulate matter by enhancing the sedimentation and retention of particles. This belief is based on historical accounts of changes in sedimentation patterns in the presence of seagrass meadows (Wilson, 1949; Christiansen et al., 1981) supported by physical models describing changes in horizontal water flow when encountering the plant canopy (e.g. Fonseca et al., 1982, 1983; Koch, 1996), and to the existence of a rhizome and root system (Ginsburg and Lowenstam, 1958; Scoffin, 1970; Orth, 1977; Dauby et al., 1995) where sedimented material stays trapped. Seagrass shoot densities (Gambi et al., 1990) and architecture of the plants (Fonseca and Fisher, 1986) have been shown to be important factors strongly influencing the plant-water flow interaction. The area of influence of the substratum on the water flow is known as the benthic boundary layer (BBL), whose properties depend on both the flow velocity and the nature of the substratum. In the absence of vegetation, flow velocity (u) decreases logarithmically over depth (z), and the energy dissipation, defined as vertical shear (u* = du/dz), is maximal near the bottom. In vegetated sediments, a peak of energy dissipation occurs further from the sediment due to the interference of the canopy with the water flow (Gambi et al., 1990), thus resulting in a general retardation of the flow across the canopy, potentially enhancing sedimentation. The height above the bottom where vertical shear equals zero is known as the roughness height of the substratum (z0 ) and is elevated over the bottom in the presence of vegetation (Fonseca et al., 1982). While the effect of plant canopies on water flow provides the basis to explain the capacity of seagrass meadows to act as a particle sink, there is a paucity of direct measurements to confirm this. Moreover, the net sedimentation rate depends both on the settling flux of materials and the possible losses due to resuspension, and the effect of seagrasses on both these processes needs to be considered. Here we describe and demonstrate an approach to examine the effects of plant canopies on water flow and the associated effect on net sediment deposition and resuspension. We do so by measuring particle transport within Posidonia oceanica beds compared to adjacent areas bare of vegetation, by two different approaches: (1) inferences derived from velocity profiles obtained with an acoustic Doppler velocimeter (ADV; short time measurements within a scale of minutes) and (2) direct measurements of particle flux (both sedimentary and resuspended) using sediment traps to integrate this process over a time period of days. We then apply a model (Valeur, 1994) that allows the estimation of resuspension rates from measurements of particle flux at different heights above the sea floor. Finally, we test the relationship between different structural parameters of the meadow and particle flux within the canopy. We do this through the comparison of estimates of sedimentation and canopy structure across a depth gradient, and also through the experimental manipulation of the structural properties.
2. Methods The study was conducted in three P. oceanica meadows: Fanals (15 m depth), Giverola (12 m depth), and Jonquet (2 to 14 m depth), all situated in the NE-Mediterranean Spanish
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Fig. 1. Location of the Posidonia oceanica meadows studied along the NW-Mediterranean Spanish coast.
coast (Fig. 1; 41◦ 410 N, 41◦ 550 N, 42◦ 180 N, respectively). Fanals is a rather exposed site, while the meadow at Giverola is somewhat more sheltered, and that at Jonquet is a very sheltered bay. Downward particle flux at different distances from the bottom inside the P. oceanica meadow and over the adjacent (15 m apart) barren bottom, was measured with sediment traps on 27 to 31/1, 22 to 26/2 and 27/3 to 2/4 1997 in Giverola. Parallel estimates of meadow structure and measures of downward particle flux across a depth transect (stations at 2.9, 5.1, 8.9, 11.6 m) and ADV profiles were conducted in Jonquet (15 to 22/5/1997). The clipping experiment took place in Fanals (7 to 13/8/1997). 2.1. Acoustic Doppler velocimeter (ADV) measurements An acoustic Doppler velocimeter (ADV, model Oceans, SonTek, San Diego, CA, USA) was used to obtain velocity data. This probe measures horizontal (u and v) and vertical (w) velocity components sampled at 25 Hz (25 times per second). The ADV probe was mounted on a cylindrical, stainless steel frame, in a down looking position, and oriented so that the u velocity component measured the dominant wave motion. A pointed tip on the central shaft of the frame was fixed into the sediments and stabilized with a tripod. The vertical
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position of the sampling volume on the frame could be adjusted to discrete heights above the bottom. For time series measurements, the sampling volume was located at nominal heights of 20 and 100 cm above the bottom. Velocity time series of 5–10 min were collected at these heights. For rapid measurements of the vertical structure of the horizontal velocity field, the frame was also used as a profiler where the central shaft functioned as a piston inside the cylindrical frame. Profiling was done by lifting the frame to the top of the shaft, and filling the cylinder with water through a special valve. This valve was then closed and the frame, with the probe, descended along the shaft at a near constant speed. The speed of the cylinder was regulated by a separate valve for the outflow. Travel times were between 30 and 55 s. The total distance traveled (95 cm) divided by the total travel time gave the mean fall velocity of the probe. This was verified using measurements of the w velocity component for a near zero mean flow. Using the mean fall velocity and the time of the sample, u components from six profiles were binned into 15 cm heights with more than 600 points per bin. Thus, rapid profiles are quite different from time series profiles, but they show the same tendencies in the magnitude and shape of u over the boundary layer as the time series profiles. Velocity profiles were made over sandy bottoms located at least 5 m outside of meadows, and above and inside the canopies, at a distance of at least 2 m from the leading edge of the meadows. Inside the plant canopy leaves were removed, when necessary, in the area just below the probe so they would not cross through the measurement volume of the ADV and interfere with velocity measurement. Velocity data were edited for spurious velocities, which we identified by low correlations (<70%) and signal strengths (<5 db) between the three transducer channels. The root mean square (rms) velocity of the u component was used to calculate the shear velocity, u∗ , and the roughness height, z0 , using the equation, u = (u∗ /κ)ln(z/z0 ),where k, the van Karman constant, was assumed to be equal 0.4. The 95% confidence intervals for u∗ , and z0 were according to Wilkinson (1984). Semilogarithmic plots of u vs. height above the bottom were used to calculate roughness height and velocity, by fitting the regression equation: u (cm s−1 ) = a − b ln height (cm)
(1)
where b (units cm s-1 ) provides an estimation of shear velocity, and the intercept on the ordinate (i.e., when u = 0) provide an estimate of roughness height (in cm). 2.2. Sediment traps Small sediment traps were designed to measure particle deposition, minimizing interference with the water flow and the plant canopy. We chose 20.5 ml cylindrical glass centrifugation tubes with an aspect ratio of 5 (16 mm diameter) following recommendations of Hardgrave and Burns (1979) and Blomquist and Hakanson (1989). The tubes were attached by groups of 5 to 30 cm long stainless bars, and were mounted with a separation of 4 cm from each other. Triplicate sediment traps for estimates of total depositional fluxes were fixed at 20 cm above the bottom. Traps were deployed by SCUBA divers, simultaneously within the Posidonia canopy and to adjacent unvegetated bottom for a total period between four and eight days. In order to allow the estimation of resuspension loads, sediment traps
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Fig. 2. Illustration of the approach used to deconvolute the depositional flux profile (from Valeur, 1994). Ft = total particle flux, Fp = flux of primary sediments, Fr = Flux of resuspended material, Dp = primary deposition, Dr = resuspended deposition. Units of fluxes in g DW m−2 day−1 .
fixed to a central pole 1.5 m tall, were placed upright over the sediment, at heights of 20, 40, 60 and 80 cm above the bottom, thereby allowing the examination of the relationship between downward particle flux and height above the bottom. Bars were arranged forming a downward spiral so that upper bars were never situated immediately above a lower bar. At the laboratory the content of the tubes was filtered through 25 mm GF/F filters and its dry weight (DW) was assessed after drying the filters for 24 h at 60◦ C. The description of sedimentation processes follows the terminology of Pejrup et al. (1996) to discriminate between the flux of primary settling matter (Fp, g DW m−2 day−1 ) and the flux of resuspended sediment (Fr, g DW m−2 day−1 ). The primary settling matter is defined as sediment particles, including autochtonous particulate matter and terrigenous matter, deposited for the first time at the bottom of the measuring site. Resuspended sediment consist of the same components, but which have been previously deposited at the measuring site. The primary and resuspended sediment flux are derived by deconvulting the total downward flux (Ft; units in g DW m−2 day−1 ), which is the sum of Fp and Fr, through the analysis of the vertical particle flux as a function of the height above the sea-bed (Fig. 2; Valeur, 1994). Total deposition (Dt; units in g DW m−2 day−1 ) is defined as Ft at the sediment surface. Since resuspension causes a diffusive supply of particles, increasing the particle load exponentially towards the sea-bed (Ichie, 1966), the changes in Ft with height above the bottom (in cm) should fit, in the presence of significant resuspension, to a negative exponential function of the form: Ft = a ∗ e−b
∗H
(2)
Fp would then correspond to the asymptotic values of Ft at distances over the sediment (in our case the measurements at 80 cm; see also Fig. 2) and the intercept of this primary flux profile with the sea-bed (i.e., Ft at H = 0) provides an estimate of the rate of primary
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deposition (Dp; units g DW m-2 day−1 ). The difference between Dt and Dp, represents the downward flux of resuspended sediments Dr (Dr = Dt − Dp). 2.3. Meadow structure and experimental manipulation Parallel to the sediment trap deployment, biomass, canopy height and leaf surface area of 10 randomly selected shoots of P. oceanica were measured. Dry weight was estimated after drying for 24 h at 60◦ C. Shoot densities were determined by randomly placing a 48 cm × 48 cm quadrat and counting the number of shoots within the total area. The above ground biomass (units in g DW m−2 day−1 ) and leaf area index (LAI, m−2 leaf m−2 ground) of the bed was extrapolated from individual shoot data using the density measurements. To test the effect of changes in meadow structure on water flow and sediment flux, we clipped the shoots of P. oceanica at lengths equivalent to 50% and 10% of the total canopy height (canopy height = 68 ± 11.5 cm). The manipulated area consisted of two 0.3 m wide by 2 m long quadrates oriented parallel to the dominant current. Leaf debris was allowed to be washed away from the study area for two days. Then ADV velocity profiles and sediment trap measurements of a sand plot, a full canopy length, and clipped plots with 50 and 10% of the canopy height were conducted as described above.
3. Results and discussion 3.1. Water flow across P. oceanica canopies Velocity data from the Posidonia meadow in Jonquet, showed that velocity profiles over sand decreased smoothly (Fig. 3a) and exponentially toward the bottom (Fig. 3b). Velocities through the canopy also decayed exponential toward the bottom, except near the base of the stem where there was a slight increase in velocity (Fig. 3a). Current velocity was reduced in the upper portion of the canopy (60 cm above the sea-floor), where the exposed leaf area was greatest, enhancing viscous drag. The increased velocity near the base of the plant stems was a result of a reduced leaf area and thus a lower resistance to the flow associated with the narrow stem. In the upper portion of the canopy, velocities were more variable and had a steeper velocity gradient associated with the plants’ ability to more effectively slow the flow. This same trend was observed in the Fanals clipping experiment, where less flow restriction and lower vertical velocity gradients were found with reduced leaf height (Fig. 4a,b). For both these meadows, the velocity in the upper portion of the canopy followed a logarithmic scaling, which allowed the use of shear velocity and roughness height to characterize the boundary layer. Although confidence intervals were broad, both shear velocity and roughness height were greater for the meadow (u∗ = 1.33 ± 0.78 cm s−1 and z0 = 30 ± 93 cm), compared to the sand bottom (u∗ = 0.68 ± 0.24 cm s−1 and z0 = 5.0 ± 7.4 cm), which indicates an increase in flow deformation produced by plant friction. In Fanals, the manipulation experiment confirmed the observed effects of changes in plant canopy structure on flow characteristics (Fig. 5). The shear velocity and roughness height were greatest for the full canopy height and were progressively reduced for 50 and 10% canopy height (see Table 1).
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Fig. 3. Time averaged velocity profiles over sand (open) and over a meadow (filled) in Jonquet, Spain (a); and the corresponding log-velocity distribution of the profile (b). Solid lines represent fitted regression lines (Table 1). Grey bars illustrate P. oceanica canopy mean height ± SE.
Thus, increased friction by the plant leaves yields increased shear velocity and roughness height, while reducing the magnitude of the velocities compared to those over the unvegetated bottom (i.e. sandy substrates). A decrease in flow velocity should lead to enhanced particle deposition within the Posidonia meadow compared to those over sandy bottoms. Similarly, higher energy dissipation by the plants (which implies reduction of both, vertical and horizontal velocities) should buffer resuspension processes below the plant canopies.
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Fig. 4. Time averaged, velocity profiles in a meadow in Fanals, Spain (a); and the corresponding log-velocity distribution of the profile (b). Filled symbols represent the full height of the canopy (100%), open circles represent meadows clipped at 50% height, and open diamonds represent meadows clipped at 10% of the full canopy height. Grey bars illustrate P. oceanica mean canopy height ± SE.
3.2. Sedimentation rates The total deposition fluxes in our sites oscillated between 5 and 35 g DW m−2 day−1 , in agreement with the ranges derived from seasonal studies in nearby areas of the WMediterranean: 0.2–45 g DW m−2 day−1 within a Posidonia bed at 36 m depth in Corsica
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Fig. 5. Changes in benthic boundary layer properties: u∗ = shear velocity and z0 = roughness height, after in situ manipulation of canopy height in Fanals.
Table 1 Shear velocities and roughness heights estimated from semilogarithmic plots of velocity (u) vs. height profiles (see Eq. (1)) of the different treatments: full canopy length, 50 and 10% canopy lengths, of the clipping experiment. Values are expressed ±95% c.l. Canopy length
u∗ (cm s−1 )
z0 (cm)
P
N
R2
Full 50% 10%
0.48 ± 0.22 0.30 ± 0.17 0.13 ± 0.03
10 ± 2.20 2.3 ± 0.13 0.04 ± 0.01
<0.05 <0.05 <0.05
7 6 6
87% 72% 98%
(Dauby et al., 1995), and 0.6–107 and 0.6–317 g DW m−2 day−1 on a barren bottom at 18 m in Banyuls, France (Charles et al., 1995; Grémare et al., 1997, respectively). These fluxes, collected at 20 cm above the bottom, did not show statistically significant greater deposition rates within the Posidonia canopy compared to that over bare sand (Fig. 6). Instead, the total downward flux in February was significantly higher (p < 0.001) over bare sand compared to that inside the seagrass bed. The downward flux of particles fitted the resuspension model in January over sand, and both in bare and vegetated bottoms in March (Fig. 7; Table 2). The model (Eq. (2)) allowed the estimation of the primary and resuspended sedimentation rates, which varied significantly among samples (see March data in Table 2). In January the contribution of resuspended material was one-fifth of the total deposition over sandy bottom, while in March resuspended materials comprised half of the total material trapped inside the bed, and 90% of the total deposition over bare sand (Table 2). These results indicate that the very high sedimentation rates measured over bare sand in March are attributable to a much greater sediment resuspension there compared to
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Fig. 6. Comparison of the particle flux trapped at the 20 cm sediment traps deployed simultaneously within the P. oceanica meadow and the unvegetated sandy bottom in Giverola. Solid line represents the 1 : 1 line.
Table 2 Components of the downward particle flux profiles derived from the fitted resuspension model (Eq. (2), Fig. 2). Estimates of primary deposition (Dp) were obtained from sediment trap measurements at 80 cm from the bottom, and resuspended deposition (Dr) was estimated from the fitted model parameters. R2 is the coefficient of determination of the fitted model (Eq. (2)), and P is the probability that the resuspended deposition (Dr) equals 0 Date
Site
Dp (g DW m−2 day−1 )
Dr (g DW m−2 day−1 )
P
N
R2
27 to 31/1/1997
Sand Posidonia Sand Posidonia Sand Posidonia
33.8 35.5 4.7 3.6 5.5 5.5
6.41 0 0 0 47.5 3.48
<0.05 0.48 0.41 0.09 <0.005 <0.0001
10 13 8 8 10 10
45% 5% 11% 40% 72% 83%
22 to 26/2/1997 27/3 to 2/4/1997
that inside the bed, as expected from the effect of the canopy in buffering energy penetration to the sediments. Analysis of the structural parameters of the Posidonia bed in Jonquet (Table 3) showed no pattern of canopy particle trapping as function of shoot density, biomass or canopy height (Fig. 8), but the projected surface area of the plants (LAI) was significantly and strongly correlated (p < 0.005) with the total deposition rate within the bed (Fig. 8d). These data contrast with the comparative results (sand vs. Posidonia; Fig. 6) obtained with the sediment traps in Giverola, where no significant differences in sedimentation were found in
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Fig. 7. Vertical particle flux profiles measured simultaneously within the P. oceanica meadow and the sand bottom in Giverola from January to March 1997. See Table 2 for flux estimates. Table 3 Mean and standard error of structural parameters of the Posidonia oceanica bed in Jonquet across a depth gradient. Mean values and standard error are provided Meadow
Depth (m)
Density (shoots m−2 )
Biomass (g DW m−2 )
Canopy height (cm)
LAI (m2 m−2 )
Giverola
12 12 12
247 ± 40 340 ± 44 444 ± 60
190 316 469
28 ± 4.4 35 ± 3.8 46 ± 5.3
3.4 ± 0.67 5.1 ± 1.89 7.5 ± 2.36
Jonquet
2.9 5.1 8.9 11.6
198 ± 29.60 259 ± 53.70 224 ± 43.60 173 ± 61.58
118 176 219 177
28.8 ± 1.34 33.1 ± 1.46 55.8 ± 1.21 56.9 ± 3.00
1.85 ± 0.15 2.89 ± 0.25 3.99 ± 0.19 2.79 ± 0.21
Fanals
15 15 15
291 ± 133 291 ± 133 291 ± 133
560 303 62
68 ± 11.52 35 ± 9.46 7.2 ± 1.76
4.6 ± 0.81 2.5 ± 0.27 0.4 ± 0.12
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Fig. 8. The relationship between sedimentation rates across a depth transect in Jonquet and the structural parameters of the P. oceanica meadow: (a) leaf biomass, (b) shoot density, (c) canopy height, and (d) Leaf area index (LAI).
the presence of vegetation. In contrast, the results obtained on sheltered Jonquet meadow, where resuspension processes could not be detected, support the notion that seagrasses act as particle traps at increasing structure of the meadow (see Fonseca and Cahalan, 1992). Our results suggest that the capacity of seagrasses to increase particle deposition is more significant at low particle concentration in the water (low sedimentation) and in the absence of resuspension, as suggested by Duarte et al., 1998. Our field measurements show seagrass architecture, particularly LAI, to be more important than seagrass shoot density in determining the amount of energy dissipation by the plants, in agreement with comparative interespecific studies (Fonseca and Fisher, 1986) where canopy friction was more related to the amount of water column occupied by a plant, than to the meadow density itself. However, the results of the experimental manipulation of canopy height in Fanals suggests this relationship to be nonlinear, with a significantly reduced sedimentary flux at high leaf area index (LAI > 4 m2 m−2 ; Fig. 9). This suggests that other factors, such as bending of the leaves and particle attachment to their surfaces, may interfere with the deposition of free sinking particles at high canopy densities.
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Fig. 9. The relationship between total depositional fluxes and the leaf area index of the manipulated meadow at Fanals (LAI = 4.6 m2 m−2 ) and that of plots clipped at 50% (LAI = 2.5 m2 m−2 ) and 10% (LAI = 0.4 m2 m−2 ) of the full canopy height.
4. Conclusion In summary P. oceanica canopies increased shear velocity, energy dissipation, and raised the roughness height of the bottom, compared to the unvegetated sediments. P. oceanica canopies influenced sedimentation processes in two ways, (1) by buffering resuspension rates due to the dissipation of energy and rescaling the turbulence at the canopy; and (2) by increasing the amount of primary deposition with moderate increases in LAI under sheltered conditions in particle-poor waters. While the effect of canopies on the increased primary flux of sedimentary materials is modest, their effect on resuspension rates (at least 10-fold lower) is highly significant. These effects provide direct quantitative support to notion that seagrass beds promoting sediment accretion, and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes, which has proven elusive in the past.
Acknowledgements This research was funded by project PhaSE (contract MAS3-CT96-0053) of the ELOISE programme of the European Commission. We thank Lars K. Nielsen for advises on measurements of sedimentation and resuspension, and to Josep Vilaseca for valuable help on the design of the sediment traps.
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