Influence of hydraulics on the uptake of ammonium by two freshwater plants

1. Macrophytes are important in the biogeochemistry of flowing rivers, although most information so
far has relied on measurements of nutrients in plant tissues. This yields only indirect information on
the nutrient uptake fluxes by roots and shoots and about nutrient translocation between roots and
shoots. Here, we studied nitrogen uptake through experiments with enriched 15N stable isotopes.
2. Two macrophytes (Potamogeton natans and Ranunculus fluitans) were grown in a closed race trackshaped
flume, allowing us to control the hydraulic conditions in and around the plants. Overall
ammonium uptake rates (micromol/g dry mass/h) were higher for R. fluitans than P. natans.
3. In addition to differences between the species, the spatial position of individuals within the plant
patch and water flow were also important in explaining ammonium uptake. Thus, ammonium
uptake was high at the leading edge of the patch and increased with velocity.
4. Plant characteristic, such as the angle at which the plants bent in the flow, was also correlated
with ammonium uptake. Differences in nutrient uptake associated with hydrodynamic parameters
raised the question of how the two are related. For both species, uptake was not correlated with
Reynolds stress, indicating the poor effect of turbulent mixing in determining ammonium uptake