Stream nitrogen uptake dynamics from ambient to saturation across development gradients, stream network position, and seasons
McNamara, Rebecca Anne
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The balance between stream nitrogen (N) loading and retention determines stream network nutrient export dynamics. Nutrient retention can be altered due to changes in hydrology, nutrient loading, and biological community response to increased nutrient availability. We quantified physical and biological contributions to total nutrient retention and determined biological uptake kinetics from ambient to saturation across six stream reaches across the West Fork Gallatin Watershed (a 1st to 4th order, headwater, 240 km² watershed) which has experienced rapid exurban, resort development leading to increased watershed nutrient loading over the last four decades. We conducted 17 stream tracer experiments (constant-rate and instantaneous additions) using both conservative (chloride (Cl)) and biologically active (nitrate (NO 3-N)) tracers across a range of watershed areas (WA), stream discharges, seasons, development intensities, and ambient NO 3-N concentrations, reflecting varying degrees of development and upland nutrient loading (i.e., wastewater effluent). Ambient uptake (U amb) was calculated and Michaelis-Menten kinetic models were used to quantify maximum areal uptake rates (U max) and half-saturation constants (K m) for each experimental reach. In the West Fork Gallatin Watershed, the majority of added NO 3-N was physically retained within stream reaches of smaller WA with decreasing physical retention as WA increased. However, as WA increased, biological retention of added NO 3-N became increasingly important and exceeded physical retention in the two largest watersheds. Further, U amb and U max values increased with greater WA, and U max was greatest in the summer and lowest in the winter. Our results demonstrated that nutrient uptake variability between stream reaches was related to WA, discharge, ambient NO 3-N concentration, and season. Although some streams in the watershed no longer appear to be functioning at pre-development levels, none demonstrated saturation with respect to NO 3-N, yet with continued development and increased loading, nutrient saturation could occur. We suggest that quantifying physical and biological contributions to total retention and determining uptake kinetics from ambient to saturation over space, time, and development intensities can yield new insight into the capacity of stream networks to buffer nutrient loading.