Attenuation of water flow inside seagrass canopies of differing structure

An understanding of how habitat structure influences physical environmental processes that are important to organisms utilizing the habitat is a necessary basis for predicting biological responses to habitat variation. Seagrass meadows represent an important coastal nursery habitat that modifies the local flow environment. We used basic fluid-dynamic balances to construct a simple model of the effects of seagrass habitat structure on mean flow within and above the canopy, and tested quantitative predictions of the model against published flume observations and our own field measurements. In the field, flow reduction was detected in 10 of 13 cases inside the canopies of 5 seagrass beds varying in vegetation density (11 to 52 m2 m–3) and upstream flow (5 to 14 cm s–1). The field data demonstrated greater flow reductions inside the canopy with increasing vegetation density. Flume data further confirmed a quantitative prediction of our model that the vertically integrated flow velocity inside the canopy would vary inversely with the square root of vegetation density. The model also predicted that the width of the ‘seagrass-edge zone’, in which flow decelerates, is a declining function of vegetation density, indicating that ‘edge effects’ (and by inference variation among patches of differing sizes) change predictably with seagrass bed structure. Empirical observations and simplified theory relating mean flow reduction to seagrass vegetation density can now be used to generate predictions of dependent biological responses such as variation in gamete fertilization, larval and spore settlement, and growth rates of organisms responsive to fluxes.