Anthropogenic alterations to the global nitrogen (N) cycle have doubled reactive N flux into the biosphere and altered aquatic ecosystem function. Streams modify N loads carried to coastal ecosystems by converting N to organic forms or removing it as gaseous N. Understanding how streams transform N can offer insight to ecologists and land managers about how stream ecosystems function under elevated N loads. I researched how anthropogenic and landscape factors affect N transformations by studying streams in two distinct biomes and at multiple spatial scales.
At the landscape scale, I studied N concentrations in streams draining the Teton Range (Wyoming, USA), a sub-alpine and alpine ecosystem with variable lithology. Streams draining crystalline geology had higher N compared to streams draining carbonate geology, which had more vegetation, suggesting that lithology mediated patterns in vegetation and terrestrial N retention. At the reach scale, I studied how land use influenced N uptake and transformation in Midwestern streams (Michigan, USA) and found that dissimilatory N transformation rates (i.e., nitrification and denitrification) within streams were not affected by riparian zones, which are commonly used to mitigate water quality degradation. Dissimilatory N transformation rates were always < 10% of whole-stream N uptake and nitrification rates balanced denitrification rates, implying that denitrification did not represent net N loss from the water column. At the substratum scale, sediment organic carbon content correlated with denitrification, but only when nitrate concentration exceeded a threshold. Finally, I returned to the Tetons and found that grazing activity by invasive snails can increase periphyton N fixation rates in a stream with low N concentrations.
A synthesis of my findings from high N streams in the Midwest suggests that land-use practices have increased temporary N removal at the expense of permanent N removal. In the low N streams of the Tetons, observations from different spatial scales suggest that landscape factors that lower stream N concentrations and high rates of grazing together can influence the importance of N fixation in streams. My dissertation highlights multiple constraints on N processing in streams and emphasizes that basic ecological research can yield important results for management agencies.