Modelling the impacts of emergent macrophytes on nitrogen fate and transport in small streams

Abstract

The work presented in this thesis combines numerical modelling with field experimentation to investigate the impacts of emergent macrophyte stands (watercress, Nasturtium officinale) on the fate and transport of nitrogen in small streams. A numerical model is developed that simulates both the hydraulic and biokinetic impacts of these plants. The transport model utilises the one-dimensional transient storage zone (TSZ) approach in a Lagrangian framework, and achieves numerical and mass conservation improvements over traditional Eulerian-based techniques for many types of systems. A complementary water quality sub-model is also developed that simulates macrophyte biokinetics and nitrogen speciation. The models are supported by two sets of field releases of nitrogen and a conservative salt into a watercress-dominated reach. Mesocosm experiments are also performed that isolate the plants while nitrogen uptake and leaf photosynthesis are simultaneously measured. A number of follow·up releases of a conservative salt into the same reach are performed, together with modelling, to further investigate the hydraulic relationships between macrophyte presence and key transient storage ZODe parameters. Finally. investigations using the developed numerical tools illuminate the relationships between nitrogen uptake lengths (Sw) (a popular metric for describing nitrogen retention in streams) and key TSZ parameters, which in tum are related to watercress presence. The transient storage zone model is shown to be well suited to simulating solute transport in streams with emergent macrophyte stands. Macrophytes are shown to increase the size of the transient storage zones (As) and decrease the net exchange rate coefficient between the TSZs and the main channel (0). Measured watercress uptake rates are positively correlated with water column nitrogen concentration but uncorrelated with plant photosynthesis, with nighttime uptake rates statistically equivalent to daytime rates. Seasonality is also shown to be important. as mass specific uptake is significantly higher during early summer experiments compared to autumn measurements, Numerical investigations reveal that, for systems with uniform kinetics, Sw is linearly related to As (with a slope <= 0), but is independent of a. However, for systems with nonuniform kinetics, Sw decreases rapidly with a for a given As. An important implication of these findings is that, for systems with kinetics dominated by marginal macrophytes, the removal of macrophytes will generally promote longer Sw values but can, for certain conditions, decrease Sw. This implication is confirmed with numerical model simulations. The tools and concepts developed during this research can aid in the interpretation of future experimental results and serve as important decision support tools for stream management.