Open Water-Emergent Vegetation Interface Influences Hydro-biogeochemical Dynamics

Excess nitrogen (N) in aquatic environments is a global water quality issue. Emergent vegetation, as found in wetlands, floodplains, or ditches, can alter hydrology and biogeochemistry to favor N removal. Nevertheless, predicting N removal outside of well-studied locations is difficult due to the lack of mechanistic understanding linking hydro-biogeochemical processes. Here, we test the hypothesis that interfaces between open water flow and vegetated regions in aquatic environments are hot spots for N removal through enhanced mixing of water with complementary resource availability that facilitates higher denitrification rates. Using a series of lab experiments in a recirculating flume mesocosm, we assessed the impact of model rigid emergent vegetation on water chemistry and flow (mean velocity, turbulent intensity, and Reynolds stresses) gradients across the vegetation boundary under four streamwise velocities (U; 0-9 cm/s) and two canopy densities. Results show that water chemistry is sensitive to experimental variables, such that there are significant differences in concentrations between velocities and densities, and that concentration gradients persist across this interface. Specifically, nitrate and pH decreased into the canopy regardless of density, while dissolved oxygen only decreased into the dense canopy. Estimated system-wide N removal was greater for the dense canopy than the sparse. Preliminary results show that velocity consistently decreased from the open channel into the dense canopy. Reynold's stresses (Re stress) and turbulent intensity (TI %) were highest outside the canopy interface, shifting closer to the boundary with increasing U. For the same U, the region of heightened Re stress and TI% (i.e. mixing zone) shifts closer to the canopy boundary from upstream to downstream, indicating developing flow conditions along the canopy length. This work can help enhance our ability to predict and improve rates of N removal in natural aquatic ecosystems.