Dynamics of plant-flow interactions for the seagrass Amphibolis antarctica: Field observations and model simulations

Seagrass canopies influence water flow partly as a consequence of their mor phology. Amphibolis antarctica (Labill.) Sender et Aschers. ex Aschers, an Australian endemic, is different morphologically from more-commonly studied blade-like seagrasses such as Zostera and Thalassia. Field measurements an d model predictions were used to characterize water flow within and above a n A. antarctica meadow. A series of high resolution three-dimensional velocity measurements were ob tained within, above and adjacent to A. antarctica meadows at different hei ghts above the seabed. Field observations on the effect of seagrass canopy on flow show an overall damping effect. Power spectra of the velocity data revealed a reduction in energy from 500 (cm s(-1))(2) s(-1) to 10 (cm s(-1) )(2) s(-1) within the canopy. Profiles of kinetic energy were calculated fr om in situ velocity measurements at 5 cm increments from 10 cm to 80 cm abo ve the seabed, within and above the seagrass canopy. There was an intensifi cation of flow where the canopy structure was densest (approximately 40 cm above the seabed) and slightly above it. The baffling effect of the canopy was most effective 25 cm above the seabed: here the flow was reduced from 5 0 cm s(-1) at free surface to 2-5 cm s(-1). A slight increase in flow withi n the canopy was seen 10 cm above the sediment due to reduced friction exer ted by the lower leafless stems of the plants. A high resolution three-dimensional hydrodynamic model was coupled to a ten -layer canopy model for shallow coastal site dimensions. By applying differ ent friction factors to various parts of the plant, mimicking its architect ure, water flow was shown to be altered by the plant canopy according to it s morphology. The derived computational results were in good agreement with the observed in situ velocity and kinetic energy changes. As a result of t his study it is now possible to accurately predict plant-flow interactions determining pollen and particles distribution and dispersal. (C) 2000 Acade mic Press.