Nitrogen Stable Isotopes in Primary Uptake Compartments Across Streams Differing in Nutrient Availability

Abstract

High variability in the natural abundance of nitrogen stable isotopes (δ15N) has been reported for primary uptake compartments (PUCs; e.g., epilithon, filamentous algae, bryophytes, macrophytes) in human-impacted aquatic ecosystems, but the origin of this variability is not yet well understood. We examined how δ15N of different PUC types relate to δ15N of dissolved inorganic nitrogen (DIN) species (nitrate and ammonium) and to the stream nutrient concentrations in which they grow. We selected 25 reaches located across the fluvial network of La Tordera catchment (NE Spain, 868.5 km2), encompassing a gradient of human pressures from headwaters to the river valley. δ15N-PUC variability was mostly explained by location within the fluvial network and was strongly related to the δ15N of DIN species, especially of ammonium. Models were stronger for PUCs growing within the stream channel and thus using streamwater as their main source of nutrients. Regression models including nutrient concentrations improved the prediction power for δ15N-PUCs, suggesting that nutrient concentrations and stoichiometry cannot be ignored in explaining the natural abundance of nitrogen isotopes in PUCs. These results provide insights into what controls variability in δ15N of PUCs within a stream network, with implications for the application of stables isotopes as an ecological tool.