Temporal variation of N and P uptake in 2 New Zealand streams

Abstract

Temporal variation of nutrient uptake in streams may be large because nutrient uptake is driven by many factors that vary substantially over time. Although many studies have compared nutrient uptake among streams, the range and variation of nutrient uptake within streams is known only for a few streams and a few nutrients. We investigated the monthly variation of NH4+, NO3−, and PO43− uptake in 2 New Zealand streams over 1 y. To measure uptake, each nutrient was added individually along with a conservative tracer (Cl−) into each stream on 3 successive days in each month. Ambient nutrient concentrations were low and nutrients were efficiently removed from the water column, with maximum uptake velocities (vf) of 71, 12, and 11 mm/min for NH4+, NO3−, and PO43−, respectively. Nutrient uptake varied considerably during the year (CV = 37–109%), with shortest nutrient uptake lengths (Sw) and highest vf generally in spring and summer months. The range of vf occurring within the streams spanned 25 to 89% of the range of vf among other streams. The range of uptake rates (U) within the streams was lower, accounting for 2 to 40% of the range among other streams. Variation in Sw was largely explained by changes in velocity and effective depth. Physical factors (temperature, transient storage) and chlorophyll a were generally poor predictors of vf and U. There was little correlation in uptake among nutrients, suggesting different factors were responsible for uptake of each nutrient. Our results show that the range and variation of nutrient uptake within some streams can be large. Within-stream variation should be considered when comparing among streams and may be useful for understanding what factors drive nutrient uptake in streams.