Roy Haggerty, Alba Argerich, David D. Myrold, Eugènia Martà & Nancy B. Grimm Transient storage has become one of the most important concepts in hydrology in the past 25 years, helping to predict solute and heat transport in streams, and it is one of the most frequently quantified processes in stream ecology. However, recent research shows that transient storage has minimal explanatory power for nutrient retention in streams. This is a paradox because nutrient retention is closely tied to aerobic metabolism, which in turn is very strongly tied to hyporheic exchange, a major component of transient storage. Further, other processes that retain nutrients (sorption of phosphorus, and anaerobic microbial processes) also should happen preferentially in transient storage zones. Consequently, it is surprising that transient storage has not been a better predictor for nutrient retention. We hypothesize that nutrient retention is mostly associated with transient storage that is metabolically active. Metabolically active transient storage (MATS) is defined as water with velocity much slower than the mean in which there is significant primary production or respiration. We predict that MATS will include much of the hyporheic zone and some benthic areas of pools. Given known mechanistic relationships between nutrient transformations and metabolism, we predict that MATS zones will also be hotspots for nutrient retention. We have done preliminary work to develop a MATS-sensitive tracer, resazurin. Resazurin quickly reduces to resorufin in the presence of aerobic respiration. Our tests indicate that it is feasible to use resazurin in streams to measure MATS, and that a standardized methodology to measure MATS can be adopted similar to what is commonly done with a conservative tracer (e.g., NaCl) to measure transient storage. We will conduct a series of column and mesocosm experiments to relate nutrient retention to metabolism (focusing on respiration). We will also test the relationship between resazurin reduction and respiration in different substrates and multiple sediment samples, and further characterize resazurin sorption, photodegradation, and degradation. We will then conduct field experiments, adding nutrients and resazurin in streams. These will primarily be conducted in heterotrophic, lownutrient streams of Oregon, but a few experiments will also be conducted in Arizona and Spain to add a gradient in metabolism and dissolved inorganic nitrogen to phosphorus (DIN:P) ratio. Injections will also be carried out at different times of the year encompassing a metabolism gradient within the Oregon streams, so that our interpretations are not limited to inter-stream variability. Streams will be surveyed and geomorphically characterized to help control for physical differences. A preliminary model of MATS is presented, and will be further developed during the study. A computer code will be updated and released that can model and do parameter estimation for reactive transport in streams in the presence of MATS, where the timescales of exchange may be the conventional exponential distribution or another distribution. Broader impact: Correlation of nutrient retention with MATS would improve our understanding of the hydrologic component of nutrient retention and help us to predict transport of nutrients in streams. Resazurin reduction could prove a valuable tool for measuring MATS, and open the way for a tracerbased quantification of respiration. Lastly, we will work with an elementary school to gauge a stream and collect stream nutrient data as part of their science curriculum. The data are important to the City of Corvallis because of a proposal to use a large quantity of treated wastewater for irrigation in the watershed that the stream drains, and there is no gauge.
