Variation in nitrate uptake and denitrification rates across a salinity gradient in Mediterranean semiarid streams

Streams are significant locations for nitrate (NO3 (-)) processing within landscapes. This is especially important in dry climates given the limited water availability for biological processes elsewhere. In arid and semiarid regions, many streams are naturally saline. Elevated salinity can constrain the structure and function of aquatic organisms, which is expected to increase worldwide being associated to global warming. We investigated whole-reach NO3 (-) uptake and denitrification in nine semiarid streams of variable water salinity (i.e. from freshwater to hyposaline) to test if NO3 (-) processing would decrease with increasing salinity. We used pulse additions and Tracer Addition for Spiraling Curve Characterization (TASCC) to measure whole-reach uptake of added NO3 (-), and the acetylene block technique to measure sediment denitrification. TASCC results showed that only five of nine streams were able to retain added NO3 (-). Of these five retentive streams, four were saline; however, salinity did not control significantly the variation in whole-reach NO3 (-) uptake observed across streams. Other measured environmental variables such as streambed NH4 (+) and organic carbon availability were better at explaining this variation. Denitrification was detected in all streams except one and its variation across streams was also independent of salinity. Although denitrification rates tended to be high, their contribution to whole-reach NO3 (-) uptake was insignificant (a parts per thousand currency sign2.16 %). Alternative pathways, heterotrophic assimilation and/or dissimilatory NO3 (-) reduction to NH4 (+), were probably responsible for most whole-reach NO3 (-) uptake. Together, our results highlight that the function of streams in controlling external NO3 (-) inputs is highly variable and salinity does not apparently constrain this role.