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 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.