Nutrient dynamics in an oligotrophic arctic stream monitored in situ by wet chemistry methods

Diurnal trends in hydrochemical components of stream and river water, especially nutrients, is growing in interest as instrumentation capable of measuring at fine time scales becomes increasingly available. In this growing body of work, there are few studies that simultaneously report the dynamics of the major nutrients nitrate, phosphate, and ammonium through time. We used an in situ nutrient autoanalyzer to simultaneously measure nitrate, phosphate, and ammonium concentrations with wet chemistry methods in an arctic headwater stream. We operated the analyzer under two sampling regimes: (1) time interval (hourly) sampling to examine fine time scale nutrient dynamics and (2) continuous sampling (1 s data) to evaluate nutrient uptake from a pulse solute addition experiment. Hourly sampling showed inverse diurnal oscillating trends of nitrate and ammonium concentrations for several days during base flow conditions. We propose that this trend is a result of in-stream nutrient processing (autotrophic demand and nitrification) combined with increased lateral inputs of water from the active (thawed) soil layer at night, after evapotranspiration (ET) has ceased. Pulse additions of ammonium resulted in rapid increases in nitrate concentration, confirming potential magnitude of nitrification in this system. Phosphate concentrations were usually at or below detection limits, consistent with results from previous manual sampling of this stream. We conclude that as studies examining fine time scale nutrient trends in streams and rivers increase, the ability to examine the behavior of multiple nutrients simultaneously will be pertinent to assess the underlying mechanisms driving those trends. Key Points Unique diel trends of nitrate with ammonium were observed Nitrogen trends driven by combined biological processing and hydrology This work has the potential to advance our knowledge of stream nutrient cycling