Quantitative Relationship between Resazurin and Respiration in Stream Ecosystems

Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.After three decades of active research in hydrology and stream ecology, the connection between solute transport, stream metabolism and nutrient dynamics is still unresolved. This existing gap obscures the functionality of stream ecosystems and how they interact with other landscape processes. To date, determining rates of metabolism is accomplished with techniques that are not spatially representative, mainly because of the limited sample volume of the methods (e.g., benthic and hyporheic chambers) and the uncertainties associated with them (e.g., estimation of reaeration rates in the two-diel technique). On the other hand, correlations between solute transport and nutrient dynamics have shown weak or even contradictory results. Clearly, the finding of mechanistic relationships among solute transport, stream metabolism and nutrient retention is required to advance our understanding and predictive ability to assess the growing pressure that exists, worldwide, on stream ecosystems. \ud \ud We hypothesize that most metabolism and nutrient retention is associated with key active areas within transient storage zones, where high hydraulic and biogeochemical gradients stimulate processing by microorganisms and macrophytes. These zones are located in the near-subsurface of hyporheic zones and in the benthos of pools and eddies in surface transient storage zones; they are referred to as metabolically active transient storage (MATS) zones. To quantify MATS, we characterized the use of the bio-reactive tracer resazurin (Raz). In the presence of respiration, Raz undergoes an irreversible reduction to strongly fluorescent resorufin (Rru). Laboratory and mesocosm experiments have shown that the transformation of Raz to Rru is negligible in the water column, but rapid in colonized sediments; this is consistent with previous independent findings about biogeochemical hot spots. Results from field experiments strongly suggest that the Raz to Rru transformation is correlated with whole-reach respiration and can differentiate metabolic activity in reaches with contrasting hydrological and biological characteristics