Impact of current speed on mass flux to a model flexible seagrass blade

Seagrass and other freshwater macrophytes can acquire nutrients from surrounding water through their blades. This flux may depend on the current speed ( U ), which can influence both the posture of flexible blades (reconfiguration) and the thickness of the flux‐limiting diffusive layer. The impact of current speed ( U ) on mass flux to flexible blades of model seagrass was studied through a combination of laboratory flume experiments, numerical modeling and theory. Model seagrass blades were constructed from low‐density polyethylene (LDPE), and 1, 2‐dichlorobenzene was used as a tracer chemical. The tracer mass accumulation in the blades was measured at different unidirectional current speeds. A numerical model was used to estimate the transfer velocity ( K ) by fitting the measured mass uptake to a one‐dimensional diffusion model. The measured transfer velocity was compared to predictions based on laminar and turbulent boundary layers developing over a flat plate parallel to flow, for which K ∝U 0.5and ∝ U , respectively. The degree of blade reconfiguration depended on the dimensionless Cauchy number, Ca , which is a function of both the blade stiffness and flow velocity. For large Ca , the majority of the blade was parallel to the flow, and the measured transfer velocity agreed with laminar boundary layer theory,   K ∝U 0.5. For small Ca , the model blades remained upright, and the flux to the blade was diminished relative to the flat‐plate model. A meadow‐scale analysis suggests that the mass exchange at the blade scale may control the uptake at the meadow scale.