Shade and flow effects on ammonia retention in
macrophyte-rich streams: implications for water quality
Controlled releases of NH4-N and conservative tracers (Br and Cl ) to five reaches of four streams with contrasting
macrophyte communities have shown differing retentions, largely as a result of the way plants interact with stream flow and velocity.
First-order constants (k) were 1.0e4.8 d 1 and retention of NH4-N was 6e71% of amounts added to each reach. Distance travelled
before a 50% reduction in concentration was achieved were 40e450 m in three streams under low-flow conditions, and
2400e3800 m at higher flows. Retention (%) of NH4-N can be approximated by a simple function of travel time and k, highlighting
the importance of the relationship between macrophytes and stream velocity on nutrient processing. This finding has significant
management implications, particularly with respect to restoration of riparian shade. Small streams with predominantly marginal
emergent plants are likely to have improved retention of NH4-N as a result of shading or other means of reducing plant biomass.
Streams dominated by submerged macrophytes will have impaired NH4-N retention if plant biomass is reduced because of reduced
contact times between NH4-N molecules and reactive sites. In these conditions water resource managers should utilise riparian
shading in concert with unshaded vegetated reaches to achieve a balance between enhanced in-stream habitat and nutrient
processing capacity.