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Journal of Experimental Marine Biology and Ecology 243 2000 45–53
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Experimental evidence of reduced particle resuspension within a seagrass Posidonia oceanica L. meadow
Jorge Terrados , Carlos M. Duarte
´ Instituto Mediterraneo de Estudios Avanzados
CSIC-UIB, Edificio Mateu Orfila, Campus Universitario UIB
, Carretera de Valldemossa, km 7.5, 07071 Palma de Mallorca, Spain Received 15 March 1999; received in revised form 20 July 1999; accepted 23 July 1999
Abstract
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 , 47.7 of initial particle mass losses were recorded at P
. oceanica stations. The loss of tracer particles was lower P , 0.01 within the P
. oceanica meadow than over barren sand, which supports the hypothesis that seagrass leaf canopies are able to reduce particle resuspension.
2000 Elsevier Science B.V.
All rights reserved.
Keywords : Particle resuspension; Seagrasses; Posidonia oceanica; Mediterranean Sea
1. Introduction
Seagrass meadows are believed to stabilize the sediments and influence sediment deposition and resuspension Short and Short, 1984; Walker et al., 1996. Flume
measurements indicate that water flow is deflected by seagrass meadows resulting in an increase of current velocity above the leaf canopy but a decrease within it Fonseca et
al., 1982; Fonseca and Kenworthy, 1987; Gambi et al., 1990. In situ measurements confirm that water flow is reduced within the leaf canopy of seagrass meadows
Eckman, 1987; Ackerman and Okubo, 1993; Worcester, 1995; Koch, 1996; Komatsu,
Corresponding author. Fax: 134-971-173-248. E-mail address
: ieajtmclust.uib.es J. Terrados 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
P I I : S 0 0 2 2 - 0 9 8 1 9 9 0 0 1 1 0 - 0
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
