| Land-use has significantly altered nutrient ratios in surface waters worldwide. But there has been little research examining the interaction between dissolved organic carbon (DOC) with nutrients (N and P), or with functional processes in streams. This dissertation investigates the effects of human land-use in relation to stream processes: nutrient uptake and limitation (N, P, DOC) and leaf decomposition across the Portneuf River watershed, southeastern Idaho, USA. Additionally, we tested a possible control on nutrient uptake rates by examining response of stream biofilms to nutrient concentrations. Human land-use ranged from 4-60% cover and was associated positively with stream water DOC and marginally with NO3-, but not with PO4-3. I found no effect of land-use on nutrient retention, nor any strong relationship between retention and stream nutrient concentrations. Based on the current heavily impacted condition of this watershed, it appears that biofilm communities are at or over nutrient saturation thresholds, and have no additional ability to absorb nutrients. Leaf decomposition has been thought to be a powerful indicator of ecosystem response to changes caused by land-use. We examined leaf decay rates across a range of land-use in mainstem and tributary locations. Leaf decay rates slowed with increased concentrations of DOC and with sediment, which both increased with land-use. Leaf decay did not vary with stream water concentrations of NO3- or PO4-3. We conclude that leaf decomposition is a sensitive metric of land-use impact when location is accounted for, and that sediment and DOC concentration override the influence of elevated nutrient concentrations. A possible mechanism affecting the storage, processing and export of nutrients is the response of biofilm community structure. In heavily impacted areas, biotic demand for DOC slowed, perhaps as a result of saturation caused by additional carbon (e.g., algal exudates) or due to autotrophs outcompeting heterotrophs. In replicated laboratory mesocosms we found autotrophs increased with NO3- addition while fungi decreased with DOC addition, suggesting competitive dominance by N-limited algal components. This research shows that nutrient availability, depending on the stoichiometic ratio of nutrients, can mediate biofilm community structure, and in turn feedback to affect uptake and export in stream ecosystems. |
