An approach to measurement of particle flux and sediment retention within seagrass (Posidonia oceanica) meadows

Seagrass beds have traditionally been considered to act as sinks for partic les due to the reduction of how velocities by the plant canopy, Yet, there is a paucity of measurements to confirm this role. In this work we illustra te 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 sedi ment traps. We also describe a model to estimate sediment resuspension afte r measuring particle flux at different distances from the bottom. Measureme nts of particle flux are conducted parallel to the study of structural para meters of the Posidonia meadow potentially involved in both particle trappi ng and retention. Data obtained on velocity profiles confirm previous findi ngs that seagrass canopies slow down current velocities with intensities pr oportional to the canopy height of the plants. The projected surface area o f the plants (LAI) significantly correlated with the total amount of partic les trapped within the Posidonia meadow, thus indicating seagrass canopy sl ightly increased particle trapping in the absence of resuspension, The trap ping capacity of the canopy was not linearly correlated to LAI but signific antly decreased at LAI above four, thus suggesting that other factors such as bending of the leaves and particle attachment to the surface may interfe re with particle free sinking within the canopy at high projected surface a rea. The model proposed to estimate resuspension allowed to measure the ret ention 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 sup port to seagrass beds promoting sediment accretion and demonstrate a promis ing avenue to provide the needed empirical support for the effect of seagra sses on depositional processes. (C) 1999 Elsevier Science B.V. All rights r eserved.