Groundwater controls on nutrient cycling in a Mojave desert stream

Abstract

1. Groundwater fluxes of nitrogen and dissolved organic carbon (DOC) were investigated in Grape Vine Canyon Stream in the Mojave Desert focusing on the rate of inputs and the fate of groundwater‐derived nutrients in the stream. Discharge rates from different ground waters were measured using an end‐member mixing model coupled with injections of a conservative solute tracer into the stream channel.2. In surface water, nitrate concentration averaged 1.13 mg N L–1 and DOC concentration averaged 1.82 mg C L–1.3. Groundwater discharge into Grape Vine Canyon Stream was derived from three sources. Nitrate concentration varied among the three groundwater sources with mean concentrations of 0.56, 0.94 and 0.08 mg N L–1. DOC, in contrast, did not vary among ground water sources, with an overall average concentration of 2.96 mg C L–1.4. In the surface stream, nitrate concentration was two‐fold greater than the concentration predicted from groundwater input, indicating that in‐stream processes generated nitrate. Stream DOC concentration was lower than predicted based upon groundwater input rate. The production of nitrate and loss of DOC suggest that DOC is lost through mineralisation of dissolved organic matter, possibly resulting in the mineralisation of dissolved organic nitrogen to ammonium and subsequent transformation to nitrate via nitrification. In further support of this hypothesised linkage, DOC loss explained 80–89% of the variance in nitrate production in Grape Vine Canyon Stream.