Hydraulic and biological controls of biofilm nitrogen uptake in gravel‐bed streams

Abstract

Epibenthic biofilms are important in regulating nitrogen (N) fluxes in stream ecosystems. The efficiency of the regulation is controlled by hydraulic and biological processes and their interactions. However, knowledge on the underlying physical and biological processes, their controlling parameters, and interactions in stream ecosystems is still limited. To analyze the relative importance of hydraulic and biological controls on biofilm N uptake, we measured turbulence, biofilm N uptake using a stable isotope tracer, and biofilm biomass in two gravel-bed streams with contrasting nutrient concentrations for two seasons. We found high within-stream variability in biofilm areal N uptake and uptake velocity, which exceeded variability between streams and seasons by 60% and 30%, respectively. Sixty-four percent of the within-stream variability in uptake velocity was explained by hydraulic mass transfer and biofilm characteristics, which were described in terms of the turbulent dissipation rate and the biofilm biomass, respectively. We show that surface renewal theory based on scales of the smallest turbulent eddies can be used to estimate transfer velocities at the sediment-water interface and can be extrapolated to larger scales by spatial averaging. Our results improved the mechanistic understanding of the processes regulating biofilm N uptake at small scale which contributes to the understanding of ecosystem functioning in low-order streams and supports upscaling to larger spatiotemporal scales along stream networks.

Disentangling In‐Stream Nitrate Uptake Pathways Based on Two‐Station High‐Frequency Monitoring in High‐Order Streams

“…Flow pathways from different sub‐ecosystem compartments converge along the river network. The varying residence times, contacting volumes, and stream substrates also create spatial and temporal variations in biogeochemical reactions (Anlanger et al., 2021; Harvey et al., 2019). Net NO 3 − ‐N uptake ($${U}_{NET}$$) during the 2019‐05 WE_2 campaign was among the highest, potential because this was a more natural stream reach and occurred right after the annual wet season (Figure 1; Table 1).…”

Fifteen Years of Integrated Terrestrial Environmental Observatories (TERENO) in Germany: Functions, Services, and Lessons Learned

“…Its compact and modular design also allows the MOBICOS infrastructure to easily be transferred between sites or operated at multiple sites simultaneously. Between‐site replication of the same experimental design under different initial environmental conditions improved the understanding of causal relationships between natural environmental oscillations of aquatic ecological states and water quality (Anlanger et al., 2021; Graeber et al., 2021), anthropogenic stressors (Sunjidmaa et al., 2022), and their combined ecological impacts on these aquatic ecosystems (Iannino et al., 2021; Weitere et al., 2021).…”

Multiple stressors effects on nitrate uptake vary across benthic and hyporheic compartments

“…Previous research has demonstrated that elevated near-bed turbulence enhances the downward transport of dispersal cells through the water column to the benthic compartment (Besemer et al 2009) and increases nutrient uxes (Anlanger et al 2021;Risse-Buhl et al 2017). Thus, we suggest that the low turbulence above the ne sediment substrate might be the underlying cause for the observed lower NO3 uptake in the benthic compartment of the ne sediment treatment compared to the control.…”