Ecological effects caused by submerged aquatic vegetation not only depend on the plants and their morphology but also on the flow and transport patterns of dissolved and suspended constituents near the canopy. Canopy height is a major variable in any quantitative analysis of plant biomass and constituent transport in its vicinity. Height of eelgrass Zostera marina canopies changes due to bending of the blades under varying current regimes. In this paper, I mathematically modeled the coupling between eelgrass blade bending and water flow. Based on the balance of forces of drag, lift, friction, weight and buoyancy on a single blade, the model defined the bending of blades (i.e. height of canopy) and the flow response within and above the canopy. This coupling was tested using laboratory data and indicated that the model performed adequately. Both model results and laboratory data confirmed that the bending of blades, and hence canopy height, was very sensitive to current magnitude and directly influenced current profile. Identifying canopy height is a major factor in defining spatial distribution of grass biomass from optical or acoustic remote sensing devices. The model has direct implications for biological issues related to the plants themselves and to their associated organisms, such as the vertical distribution of photosynthesis within the canopy and the effect of current shear on recruitment of organisms on the blades. It can also be used to study how eelgrass canopies affect horizontal transport of constituents, such as dissolved oxygen, nutrients and organic carbon, and particulate material such as pollen, larvae, plankton and detritus.

Ecological effects of the interaction between submerged aquatic vegetation and currents depend on the plants and their associated organisms as well as the large-scale transport of dissolved and suspended constituents near the canopy. Mathematical models for airflow within plant canopies were adapted to describe water flow through and above meadows of aquatic eelgrass Zostera marina. The resulting model provided the vertical distribution of velocity and shear in a water column within the meadow, and it was developed to automatically conserve flow within the canopy. It was tested and calibrated with data from the laboratory and the field, and it performed adequately. The flow profile was nearly exponential within the canopy and logarithmic above it. The model was used to study how the eelgrass canopy affected the horizontal transport of conservative constituents. The most important finding was that the vertical distribution of a constituent determines whether the canopy will reduce or enhance its transport through the water column. This effect has direct implications for transport of nonconservative constituents such as dissolved oxygen, nutrients, organic carbon, and particulate pollen, larvae, plankton, and detritus. It also has direct implications for biological issues such as vertical distributions of photosynthesis and of recruitment of organisms on blades of grass while they are exposed to varying degrees of currents and shear.

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