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This report summarises the Monash University research outcomes for Project B2.2/2.3—Protection and restoration of urban freshwater ecosystems: informing management and planning.

Nitrogen management is imperative. Synthetically derived nitrogen and an increase in impervious surfaces have rapidly increased the amount of nitrogen in rivers and coastal waters. These greater nitrogen loads can lead to economic and environmental losses, so their careful management is imperative.

As urban development increases, constructed wetlands are becoming more common as tools for reducing nitrogen and other pollutant loads, but their effectiveness is variable and limited. In particular, information about temporal changes and the important processes involved in removal is limited, which makes effectively managing nitrogen loads difficult.

Urban wetlands show promising potential as a management tool.

We used two different approaches to investigate the effectiveness of constructed wetlands at removing nitrogen:

  • We examined whether natural abundance stable isotopes of nitrogen (δ15N) and oxygen (δ18O) could be used as a functional indicator of nitrogen processing in constructed wetlands. We also examined the effectiveness of natural abundance stable isotopes as a potential monitoring tool (part 1).
  • We examined the isotope pairing technique using two competing nitrogen pathways: denitrification and dissimilatory nitrate reduction to ammonium (DNRA) (part 2). Isotope pairing uses labelled NO3- to measure actual rates of nitrogen pathways (denitrification and DNRA) under in situ conditions, to determine the effectiveness of denitrification as a removal process across temperature and carbon conditions.

Our study has some important implications for nitrogen management:

  • Nitrogen processing is dynamic on a temporal scale (seasonal and diurnal), which should be considered when developing monitoring programs.
  • The nitrogen processing capacity of the Melbourne wetlands we studied was variable, and as such, more site-specific management strategies may be appropriate.
  • Denitrification is an important nitrogen removal pathway. To better understand the importance of assimilation, ammonium uptake should be considered because primary producers prefer NH4+ over NO3-.
  • This is one of the first studies to simultaneously measure denitrification and DNRA with a suite of co- predictors (temperature, organic carbon, porewater iron and sulfide, nitrate, and chlorophyll-a), and it emphasises that DNRA can be favoured under certain conditions that promote nitrogen recycling.
  • Groundwater-fed constructed wetlands give rise to new complexities for management and monitoring because of significant variability on both a temporal and spatial scale (within the wetland).
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