Scaling aquatic ecosystem processes like nutrient removal is critical for assessing the importance of streams and rivers to watershed nutrient export. We used pulse NH4+ enrichment experiments and measured net NH4+ uptake in 7 streams throughout a mountainous tropical river network in Puerto Rico to assess spatial variability in NH4+ uptake and to infer the physical, chemical, and biological characteristics that most influence its variation. Across 14 experiments, NH4+ uptake velocity (vf) ranged from 0.3 to 8.5 (mean = 2.7) mm/min and was positively related to algal biomass standing stock, measured as chlorophyll a. On average, 49% of experimentally added NH4+ was immediately transformed to NO3−, suggesting that nitrification can rival microbial and algal assimilation as a fate of streamwater NH4+. We considered the implications of our empirical results at the river-network scale based on a simple mass-balance model parameterized for the Río Mameyes watershed. Most catchment NH4+ inputs are delivered to 1st-order streams. Therefore, model results indicated that high NH4+ uptake rates in headwater streams limit NH4+ inputs to downstream reaches, thereby decreasing the role of larger streams in NH4+ removal at the river-network scale. In-stream nitrification resulted in additional NO3− inputs, which were more likely than NH4+ to be transported downstream because of lower biological demand for NO3− relative to NH4+. Given our estimates of catchment N loading to streams and rivers, we estimated that 39% of modeled watershed NO3− export was produced within the river network by nitrification. Together, these results suggest that streams and rivers can significantly transform the N load from their catchments.
Earth Systems Research Center
University of Chicago Press
Digital Object Identifier (DOI)
Koenig L.E., C. Song, W.M. Wollheim, J. Ruegg, W.H. McDowell. 2017. Nitrification increases nitrogen export from a tropical river network. Freshwater Science. 2017. 36(4):DOI: 10.1086/694906.
© 2017 by The Society for Freshwater Science.