Inferring assimilation and denitrification from high frequency nitrate measurements

Recent advances in automated sensor technology allow the measurement of nitrate (NO₃^-) at high frequencies over extended periods.  Here I address the use of such data to infer the magnitude of autotrophic assimilation and denitrification as mechanisms of nitrogen (N) removal in river systems.  In the spring-fed Ichetucknee River, FL, USA, the integrated diel variation of NO₃^- flux provides estimates of autotrophic uptake that agree closely with estimates based on the stoichiometry of gross primary production.  I use simulated data to evaluate the sensitivity of this approach to violation of various assumptions, and to demonstrate its efficacy even in systems where diel variation is relatively small.  In the Ichetucknee River, N mass balance indicates that denitrification is the dominant mechanism of N removal, but inter-day variation of NO₃^- maxima suggests that denitrification is fueled by very recent production.  Seasonal variation in finer-scale features of diel curves may reflect further inter-relationships between productivity and dissimilatory processes.  The high density and long duration of sensor records should eventually allow evaluation of more sophisticated models of coupled biogeochemical and hydrologic processes.