Biogeochemical Cycling in Lake Superior Tributaries: Seasonality, Quantity and Quality of Export

Seasonal and spatial variability in environmental factors may affect dissolved organic matter composition and nutrient transformation and retention in streams. The objective of this research was to quantify and describe seasonality, quantity, and quality of nutrient processing and export of ammonium (NH4), soluble reactive phosphate (SRP), and dissolved organic carbon (DOC) into Lake Superior through intensive study in a small 1st order watershed coupled with snapshot measurements across 12 tributaries that varied in size, location, and wetland coverage. Our results suggest biodegradable C is exported from a small headwater stream year-round and that DOC mineralization rates can be stimulated by additions of NH4 and labile C (Chapter 2). We found that biodegradable DOC varied synchronously among 3 rivers that varied in size by three orders of magnitude. Furthermore, these rivers exported 9 to 17% of annual DOC in biodegradable form, which may then fuel biological activity in nearshore zones of Lake Superior. Modeling of historical loads of DOC suggests that spring loads of DOC have increased and fall loads have decreased over a 26 year period, but annual loads have not (Chapter 3). Across eleven tributaries variability in NH4 uptake velocity was explained by watershed area, discharge, and fluorescence index of DOC (indicator of microbial and terrestrial sources; Chapter 4). Temporally detailed measurements (every 2 to 4 weeks for 3 years) of nutrient uptake in a small headwater stream indicated light availability, algal and periphyton biomass, solute concentrations, and pH were important predictors of NH4 uptake velocity (Chapter 5). We found a similar magnitude of NH4 uptake velocity during winter and summer measurements while SRP uptake velocity was greater in summer than winter (Chapter 5). Overall the greatest uptake velocities were observed in