Year-round measurements reveal seasonal drivers of nutrient uptake in a snowmelt-driven headwater stream

Climate change may affect export of essential nutrients to downstream ecosystems in regions that host substantial winter snowpack, yet nutrient uptake is rarely quantified in snow- and ice-covered streams and rivers. To evaluate how nutrient uptake varied year-round in relation to commonly identified drivers of uptake (nutrient concentrations, biological activity, organic matter supply and composition), we measured uptake of ammonium (NH4) and soluble reactive phosphorus (SRP) at 2 to 4 week intervals for 3 years and dissolved organic carbon (DOC) for 2 years in a forested headwater stream in Michigan's Upper Peninsula. Uptake velocities (V-f) did not vary significantly among seasons for NH4 or SRP, but a much greater range in V(f )was observed for NH4 in spring and fall. DOC V-f was only detectable in summer and fall and never in winter or spring. Key biological and chemical drivers of uptake also exhibited seasonal differences: benthic algal biomass was greater in winter than all other seasons, dissolved organic matter was more aromatic in spring than fall or winter, and DOC concentrations were higher in spring than winter. NH4 V-f was nonlinearly related to temperature, with the greatest uptake observed at moderate temperatures in spring and fall, which coincided with seasonal transitions that provide optimal light conditions for autotrophs or organic matter for heterotrophs. Our results suggest that alteration to the timing of key environmental variables will affect the magnitude of nutrient uptake, and that in-stream processing during winter cannot be ignored.