Abstract
A combination of numerical modelling and field and mesocosm experiments were used to quantify the impacts of watercress stands on nitrogen dynamics in a small New Zealand stream. A series of tracer releases, supported by numerical modelling, were performed to a reach dominated by watercress and then to the same reach cleared of watercress. Results show watercress to be dominating in-stream nitrogen processing through both rapid uptake and hydraulic impacts. A 6–11% loss of N mass was measured over a 100-m reach with watercress present, whereas no loss of N mass was measured over the same reach cleared of watercress. Transient storage was approximately four times larger with watercress present compared with the same reach without the plants. Model parameterisation was supported by a series of ex situ mesocosm experiments that quantified biomass-specific uptake rates (1–2.5 mg-N/g/day) for two different growth stages. These experiments also revealed a lack of coupling of uptake rates with measured photosynthesis rates, contrary to many past modelling approaches.
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