Land use influences stream nitrate uptake in the Lake Taupo catchment

Abstract

We quantified nitrate (NO3 −) uptake during summer for 11 streams draining into oligotrophic Lake Taupo. Nitrate uptake lengths (S w ) ranged from 258 to 12035 m, uptake velocities (V f ) ranged from 0.2 to 6.8 m/day and removal efficiencies (P r ) ranged from 1% to 59%. Nitrate uptake velocities and areal uptake rates were, respectively, 9- and 26-fold higher for streams in pasture and pine forest land use than in native vegetation. However, average NO3 − export from these streams was also 8-fold higher. Our results suggested that NO3 − uptake velocity (V f ) was highest in streams with higher dissolved phosphorus (P) concentrations and flow velocities, and smaller benthic substrate size, which were characteristic of catchments dominated by pasture and pine forest land use. Uptake (as removal efficiency, P r ) accounted for an average 16% of the stream NO3 − export to Lake Taupo. However, since most uptake is assimilatory, removal from the water column is likely to only represent temporary sequestration of NO3 − and a conversion of inorganic to organic and/or particulate N.