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

Restricted Item. Print thesis available in the University of Auckland Library or may be available through Inter-Library Loan.

Restricted Item. Print thesis available in the University of Auckland Library or may be available through Inter-Library Loan. The work presented in this thesis combines numerical modelling with field experimentationto investigate the impacts of emergent macrophyte stands (watercress, Nasturtiumofficinale) on the fate and transport of nitrogen in small streams. A numerical model isdeveloped that simulates both the hydraulic and biokinetic impacts of these plants. Thetransport model utilises the one-dimensional transient storage zone (TSZ) approach in aLagrangian framework, and achieves numerical and mass conservation improvements overtraditional Eulerian-based techniques for many types of systems. A complementary waterquality sub-model is also developed that simulates macrophyte biokinetics and nitrogenspeciation.The models are supported by two sets of field releases of nitrogen and a conservative saltinto a watercress-dominated reach. Mesocosm experiments are also performed that isolatethe plants while nitrogen uptake and leaf photosynthesis are simultaneously measured. Anumber of follow·up releases of a conservative salt into the same reach are performed,together with modelling, to further investigate the hydraulic relationships betweenmacrophyte presence and key transient storage ZODe parameters. Finally. investigationsusing the developed numerical tools illuminate the relationships between nitrogen uptakelengths (Sw) (a popular metric for describing nirrogen retention in streams) and key TSZparameters, which in tum are related to watercress presence.The transient storage zone model is shown to be well suited to simulating solute transportin streams with emergent macrophyte stands. Macrophytes are shown to increase the size ofthe transient storage zones (As) and decrease the net exchange rate coefficient between theTSZs and the main channel (0). Measured watercress uptake rates arc positively correlatedwith water column nitrogen concentration but uncorrelatcd with plant photosynthesis, withnighttime uptake rates statistically equivalent to daytime rates. Seasonality is also shown tobe important. as mass specific uptake is significantly higher during early summerexperiments compared to autumn measurements,Numerical investigations reveal that, for systems with uniform kinetics, Sw is linearlyrelated to As (with a slope <= 0), but is independent of a However, for systems with nonuniformkinetics. Sw decreases rapidly with a for a given As. An important implication ofthese findings is that, for systems with kinetics dominated by marginal macrophytes, theremoval of macrophytes will generally promote longer Sw values but can, for certainconditions, decrease Sw. This implication is confirmed with numerical model simulations.The tools and concepts developed during this research can aid in the interpretation of futureexperimental results and serve as important decision support tools for stream management.