Uptake of ammonium and soluble reactive phosphorus in forested streams: influence of dissolved organic matter composition

Abstract

Many microbes responsible for inorganic nutrient uptake and transformation utilize dissolved organic matter (DOM) as a nutrient or energy source, but little is known about whether DOM composition is an important driver of nutrient uptake in streams. Our goal was to determine whether incorporating DOM composition metrics with other more commonly considered biological, physical, and chemical variables improved our ability to explain patterns of ammonium (${\text{NH}}_{4}^{ + }$–N) and soluble reactive phosphorus (SRP) uptake across 11 Lake Superior tributaries. Nutrient uptake velocities (V_f) ranged from undetectable to 14.6 mm min⁻¹ for ${\text{NH}}_{4}^{ + }$–N and undetectable to 7.2 mm min⁻¹ for SRP. Logistic regressions suggested that DOM composition was a useful predictor of where SRP uptake occurred (4/11 sites) and ${\text{NH}}_{4}^{ + }$–N concentration was a useful predictor of where ${\text{NH}}_{4}^{ + }$–N uptake occurred (9/11 sites). Multiple regression analysis revealed that the best models included temperature, specific discharge, and canopy cover, and DOM composition as significant predictors of ${\text{NH}}_{4}^{ + }$–N V_f. Partial least squares revealed fluorescence index (describing the source of aquatic fulvic acids), specific ultraviolet absorbance at 254 nm (an indicator of DOM aromaticity), temperature, and conductivity were highly influential predictors of ${\text{NH}}_{4}^{ + }$–N V_f. Therefore, streams with higher temperatures, lower solute concentrations, more terrestrial DOM signal and greater aromaticity had greater ${\text{NH}}_{4}^{ + }$–N V_f. Our results suggest that DOM composition may be an important, yet often overlooked, predictor of ${\text{NH}}_{4}^{ + }$–N and SRP uptake in deciduous forest streams that should be considered along with commonly measured predictors.