Human activities have increased concentrations of nutrients including nitrogen (N) and phosphorus (P) in freshwater ecosystems worldwide. Understanding how excess nutrients are processed is critical for public and ecological health. Headwater streams are important sites of nutrient transformation, but little is known about how temporal variation (e.g., seasonal change) and stream restoration strategies influence rates of nutrient cycling. My dissertation focuses on how seasonality and restoration, alone and in combination, control nutrient uptake rates at the whole-stream and substratum-specific scales in 3 headwater streams in the Upper Peninsula of Michigan.
By measuring whole-stream and substratum-specific rates of nutrient uptake across seasons, I found that although heterotrophic processes typically dominate in forested headwater streams, variation in nutrient uptake was also explained by autotrophic activity. My results suggested changes in streambed substrata composition strongly influences seasonal patterns of nutrient processing at the whole-stream scale. These conclusions are significant in the context of restoration, which often results in changes to the streambed â€œlandscapeâ€
I also used seasonal measurements of nutrient uptake to document the influence of two contrasting restoration strategies on stream ecosystem function. For a trout habitat enhancement, I found that physical changes in stream habitat translated into few biological effects on uptake rates and fish communities, and concluded it may not represent a sustainable method for increasing trout abundance. For a wood addition study, I found increased nutrient uptake following intermediate disturbance (e.g. storms), contrasting with significant decreases in uptake following large storms. Therefore, the effect of wood addition on ecosystem function was variable across a disturbance gradient, and should be considered in future restoration.
My dissertation research demonstrates that nutrient uptake in headwater streams is seasonally dynamic, with significant stream-specific variability, all of which is linked to differences in streambed substrata composition. Changes to stream ecosystems such as those resulting from restoration can influence seasonal patterns of nutrient processing, altering the timing of nutrient delivery to downstream ecosystems. Overall, nutrient uptake rates are sensitive metrics for integrating changes in stream biological activity, and represent a powerful tool for characterizing the influence of seasonal change and restoration on stream ecosystem function.