Seasonal variation in nutrient uptake in a 1st-order tributary of Lake Superior and implications for climate change

In-stream biogeochemical cycling can control the timing and form of nutrients exported from watersheds to downstream ecosystems, and seasonal changes in light availability, discharge, temperature, or nutrient inputs may affect nutrient transformation and retention. Without an understanding of how in-stream biogeochemical cycling varies seasonally in snow-dominated regions it is uncertain how climate change will affect nutrient export to downstream ecosystems. Further compounding this uncertainty, few studies have examined in-stream nutrient processing during winter. Long-term monitoring (30 years) of climate and snowpack at Calumet watershed, a first order tributary of Lake Superior, has documented trends of increasing winter temperatures and greater snowmelt contributions to early season runoff. Identifying environmental variables that drive nutrient uptake is important because these observed trends may shift the timing of nutrient pulses relative to water temperatures and light availability. We hypothesized that ammonium (NH4) uptake velocity, a measure of nutrient uptake efficiency, would be greater in spring and fall due to increased light availability and nutrient pulses contributed by snowmelt in spring and leaf litter in fall. To test this hypothesis, we measured nutrient uptake velocity of ammonium (NH4) at 2-4 week intervals for one year in Calumet watershed by releasing inorganic nutrients (NH4Cl, KH2PO4) and a conservative tracer (rhodamine WT) into the stream and quantifying changes in nutrient and tracer concentrations along the stream reach. Canopy cover, ambient NH4 concentrations, stream water temperature, periphyton biomass, and discharge were also measured to identify which environmental covariates affected NH4 uptake velocities. The lowest NH4 uptake velocities were observed in winter (2.33 mm min-1) and summer months (2.03-2.08 mm min-1). Spring NH4 uptake velocities were variable: the greatest uptake velocities were observed following snowmelt (4.93-6.56 mm min-1), and they declined in late April (1.9 mm min-1) and early May (2.07 mm min-1) but increased again in late May (5.68 mm min-1). In late fall, the NH4 uptake velocity (4.63 mm min-1) was comparable to the peak velocities observed in spring. Linear regression showed that stream temperature, ambient nutrient concentration, and canopy cover were not significant predictors of uptake velocity, but we are continuing to build our time series of measurements. Our results support our hypothesis that stream organisms take up nutrients more quickly in spring and fall relative to winter and summer seasons but we cannot yet identify which environmental conditions drive these patterns. Therefore, it is important to incorporate temporal variation when estimating in-stream nutrient cycling, particularly in snow-dominated ecosystems where climate change is expected to increase temperatures, which may alter snowpack dynamics