Johnson, L. T. 2008. The influence of land use on the role of dissolved organic carbon and nitrogen in stream nutrient processing. Ph.D. Dissertation, University of Notre Dame, South Bend, Indiana, USA.
Headwater streams not only dominate river networks, but also supply resources, provide habitat, and improve water quality, and are therefore integral to the health of entire river basins. Yet headwater streams are also highly susceptible to degradation via anthropogenic activities because of their small size. Agricultural and urban land use activities often increase stream inorganic nitrogen (N) and phosphorus (P) concentrations and reduce riparian canopy cover. Previous research has found that agricultural and urban land use can saturate biotic inorganic N and P demand, but less is known about the impacts of human land use on the cycling of organic C and N. By examining each component of nutrient spiraling in streams, I examined how anthropogenic land use may alter stream dissolved organic carbon (DOC) and nitrogen (DON) dynamics.
I first examined nutrient limitation of benthic biofilms in 72 streams across North America draining native vegetation, agriculture, or urbanization. I found that human land use differentially influenced heterotrophic versus autotrophic nutrient limitation and these metabolic pathways need to be investigated separately to fully understand how stream biota may respond to nutrient enrichment. I also measured labile DOC and DON uptake in 18 low-gradient streams in southwest Michigan and in 6 highly autotrophic streams in northwest Wyoming. Results from these two studies indicated that labile DON and DOC uptake can be equivalent to, or even higher than ammonium uptake, a solute characterized as being highly bioreactive. I concluded that interactions between C and N, as well between organic and inorganic forms of N, should be considered when examining the effect of human land use on streams. Finally, I quantified autochthonous DON production as a source of N to stream biota and related DON production to other transformations of nitrate in numerous streams across North America as part of the LINXII project. I found that autochthonous DON production becomes an increasingly important part of N cycling in streams draining anthropogenic land use, due to increases in primary production associated with open riparian canopies. By examining DOC and DON, relatively understudied solutes, my dissertation research has shed light on the complex effects of human land use on streams, the mechanisms behind stream N saturation, and the interactions between the biogeochemistry of multiple stream nutrients.
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