A comparison of sediment, water column, and open-channel denitrification measured using membrane-inlet mass spectrometry in Midwestern rivers
Biogeochemical processes such as denitrification and respiration are understudied in rivers relative to headwater streams, at least partially due to methodological challenges and increased habitat complexity in rivers. Although benthic processes are assumed to drive biogeochemical processes in headwater streams, riverine ecosystem function may reflect a combination of benthic and water column dynamics. Accurate quantification of in situ denitrification rates remains a methodological challenge, and rates may also be highly variable both spatially and temporally. Commonly applied methods for measuring denitrification in streams either manipulate substrate availability or require isotopic tracer additions, and have not frequently been executed in rivers. We quantified both sediment and water column denitrification and oxygen demand (i.e., respiration) in five Midwestern rivers spanning a land use gradient using a sacrificial microcosm approach. Using membrane-inlet mass spectrometry (MIMS), we quantified dissolved nitrogen (N2) and oxygen (O2) fluxes from microcosms. We compared habitat-specific estimates across rivers, and also compared these to reach-scale denitrification and metabolism measured in one Midwestern river. For reach-scale estimates, we measured dissolved N2, O2, and argon (Ar) over 24 h and estimated denitrification and metabolism using an open-channel inverse modeling approach based on changes in O2:Ar and N2:Ar ratios.
Denitrification was measureable in the water column of two rivers, and in the sediment of four rivers. Sediment and water column denitrification ranged from 0-1.8 from 0-4.9 mg N m-2 h-1, respectively. One site exhibited net N-fixation in the water column. Our reach-scale estimate of denitrification was 8.2 mg N m-2 h-1, higher than the microcosm estimate from the same river (4.4 mg N m-2 h-1). The water column accounted for 0-219% of total denitrification, with the overestimate due to N-fixation at one river. Sediment and water column respiration were similar, ranging from -0.7 to -12.3 and from -1.9 to -10.1 mg O2 m-2 h-1 in the sediment and water column, respectively. The reach-scale metabolism model using gas ratios provided gross primary production and ecosystem respiration estimates similar to a traditional, open-channel O2 exchange model. Both the sediment and water column contribute to reach-scale processes in rivers, and biogeochemical transformations in the water column may be an important and overlooked component of riverine function. Our open-channel approach for estimating denitrification provides an integrative representation of reach-scale denitrification. This combination of habitat-specific and reach-scale denitrification estimates shows that rivers may be remove N at similar or higher rates than headwater streams.