Quantification of metabolically active transient storage (MATS) in two reaches with contrasting transient storage and ecosystem respiration

Water transient storage zones are hotspots for metabolic activity in streams although the contribution of different types of transient storage zones to the whole-reach metabolic activity is difficult to quantify. In this study we present a method to measure the fraction of the transient storage that is metabolically active (MATS) in two consecutive reaches with contrasting hydrological and biological characteristics. We used combined additions of resazurin (Raz) and Cl in a reach scoured to bedrock and in a reach containing a deep alluvial deposit. The MATS zones measured 0.002 m(2) in the bedrock reach (37% of transient storage) and 0.291 m(2) in the alluvial reach (100% of transient storage). The effective rate coefficient of Raz transformation in the MATS of the bedrock reach was approximately 16 times that of the alluvial reach. However, when we take into account the contribution of the MATS zone to overall metabolic activity, Raz transformation in the MATS zone was 2.2 times slower in the bedrock reach than in the alluvial reach. The difference was similar to the difference in ecosystem respiration, which was 1.8 times lower in the bedrock reach than in the alluvial reach, suggesting that the MATS zones were important contributors to ecosystem respiration. Results indicate that the quantification of MATS can improve our understanding of the role that transient storage zones play on stream metabolic processes and demonstrate the utility of Raz as a "smart" tracer that provides new information on metabolic activity at a whole-reach and at smaller scale.