Ticket for one trip around the nutrient spiral: The resazurin "smart" tracer of metabolically-active transient storage

A ‘‘smart’’ tracer is a tracer that provides, directly or through measurement of its concentration or in combination with another compound, at least one ‘‘bit’’ more of information about the environment through which it travels than a conservative tracer. We propose the experimental use of resazurin as a smart tracer for the investigation of metabolically active transient storage (MATS) in streams.  Resazurin is a weakly fluorescent redox-sensitive dye that undergoes an irreversible reduction to strongly fluorescent resorufin under mildly reducing conditions, most commonly in the presence of living microorganisms.  Resazurin reduction to resorufin proceeds very slowly (100s of hours) in surface water but very quickly (10s of minutes) in benthic sediments – i.e., resazurin reduces to resorufin during the first “loop” around the nutrient spiral.  The reduction rate is correlated to metabolic activity, specifically respiration.  Therefore, we can use the reduction reaction as a tracer of MATS in streams.  The use of metabolic parameters of transient storage obtained with resazurin may increase our understanding of the relevance of transient storage on stream biogeochemical processes at whole-reach scale, in particular nutrient spiraling.  We present evidence of these claims from two field studies in heterotrophic streams using resazurin to trace MATS.