Drivers of nitrogen transfer in stream food webs across continents

Abstract

Studies of trophic-level material and energy transfers are central to ecology. The use of isotopic tracers has now made it possible to measure trophic transfer efficiencies of important nutrients and to better understand how these materials move through food webs. We analyzed data from thirteen N-ammonium tracer addition experiments to quantify N transfer from basal resources to animals in headwater streams with varying physical, chemical, and biological features. N transfer efficiencies from primary uptake compartments (PUCs; heterotrophic microorganisms and primary producers) to primary consumers was lower (mean 11.5%, range<1% to 43%) than N transfer efficiencies from primary consumers to predators (mean 80%, range 5% to >100%). Total N transferred (as a rate) was greater in streams with open compared to closed canopies and overall N transfer efficiency generally followed a similar pattern, although was not statistically significant. We used principal component analysis to condense a suite of site characteristics into two environmental components. Total N uptake rates among trophic levels were best predicted by the component that was correlated with latitude, DIN:SRP, GPP:ER, and percent canopy cover. N transfer efficiency did not respond consistently to environmental variables. Our results suggest that canopy cover influences N movement through stream food webs because light availability and primary production facilitate N transfer to higher trophic levels.

Partitioning assimilatory nitrogen uptake in streams: an analysis of stable isotope tracer additions across continents

“…Although not included directly in this analysis, reach‐scale N storage by macroinvertebrate consumers was also variable, and averaged 9.0% across sites (range = 0.3–25%; Norman et al. ).…”

“…Understanding the impact of biota on temporal patterns in stream N concentrations is dependent on N assimilation, release, and trophic transfer, and these remain poorly linked in stream ecosystems (but see Norman et al. ).…”

“…). Thus examining the magnitude and controls on assimilatory NH 4 + ‐N uptake and storage, as described in this manuscript, as well as its transfer to stream consumers (Norman et al., in press ) is critical for understanding how streams regulate downstream N transport. This synthesis from 17 ecosystem‐scale 15 N tracer additions quantifies the contribution of autotrophic and heterotrophic uptake compartments to assimilatory N uptake in headwater streams and shows that autotrophic compartments account for a larger proportion of N uptake than predicted based on standing stocks alone.…”

The freshwater biome gradient framework: predicting macroscale properties based on latitude, altitude, and precipitation

“…Norman et al. () reviewed isotopic experiments from streams across several continents to describe the efficiency of nitrogen transfer from primary uptake compartments, such as epilithon and detritus, to animals. They found nitrogen transfer rates were greater with open canopy.…”

“…They found no significant global pattern of any metric of complexity across the latitudinal gradient, but they did observe complexity driven largely by photoperiod and seasonality, which should relate to latitude. Norman et al (2017) reviewed isotopic experiments from streams across several continents to describe the efficiency of nitrogen transfer from primary uptake compartments, such as epilithon and detritus, to animals. They found nitrogen transfer rates were greater with open canopy.…”