Collaborative Research: How do interactions of transport and stoichiometry maximize stream nutrient retention?

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There is considerable exchange of water and solutes among four compartments of streams: the fast moving part of the stream channel, in-channel storage zones, and shallow and deep sediments zones. Exchanges of water between these compartments promote dissolved nutrient reactions and removal. The team hypothesizes that nutrient reaction in streams is controlled by not only hydrologic transport, but also stoichiometry of nutrients (ratio of C:N:P). The team will test their hypotheses by conducting field data collection and stream solute injections at three contrasting Critical Zone Observatory (CZO) sites (Boulder Creek, a rocky mountain setting in Colorado; Catalina-Jemez, a low nutrient setting in New Mexico; and an agricultural landscape in Iowa). Across these three sites there is substantial variability in geology, hydrology, and background nutrient concentrations (and therefore nutrient limitations). The project will engage multiple graduate students with an emphasis on diversity. The team will also organize a workshop to promote and stimulate interaction and exchange of ideas and knowledge among the principal players in the stream restoration field, particularly young scientists and practitioners, stream restoration companies and local environmental agencies.The exchange of water and solutes among the river and its hyporheic zones result in a net reaction and removal on nutrients from the stream. Nutrient reaction and removal (carbon, nitrogen, and phosphorous) in streams is limited by not only the biomass available to take up nutrients, but is also stoichiometrically limited by the specific reaction. This project will work with the hypothesis that: (1) nutrient retention in streams is controlled by not only hydrologic transport, but also stoichiometry of nutrients (ratio of C:N:P); (2) each compartment of a stream (main channel, surface storage, shallow/deep hyporheic) has a different optimal stoichiometric need (i.e., C:N:P); and (3) depletion of dissolved oxygen from aerobic metabolism is a first-order control that causes a threshold change in the stoichiometric demand of C, N, and P as a compartment becomes anoxic and biogeochemical processes change. The concepts will be tested by conducting field data collection and stream solute injections at three contrasting Critical Zone Observatory (CZO) sites across variable hydrologic conditions at each site to test over a range of hydrologic transport conditions. The team will deploy a suite of methods including electrical resistivity imaging, nutrient tracer injections based on stoichiometric tradeoffs, the Tracer Additions for Spiraling Curve Characterization (TASCC method), application of the "smart" tracer resazurin in streams and use of shallow (MINIPOINT samplers) and deep (wells) hyporheic flow paths. Nutrient tracer injections are designed to specifically decipher stoichiometric controls on nutrient retention in each of the four compartments of the streams. The broader impacts will be communicated in a workshop setting that identifies reciprocal needs from academic research and restoration programs seeking to approach stream restoration projects more holistically.