Physical and stoichiometric controls on stream respiration in a headwater stream

Abstract. Many studies in ecohydrology focusing on hydrologic transport argue that longer residence times across a stream ecosystem should consistently result in higher biological uptake of carbon, nutrients, and oxygen. This consideration does not incorporate the potential for biologically mediated reactions to be limited by stoichiometric imbalances. Based on the relevance and co-dependences between hydrologic exchange, stoichiometry, and biological uptake and acknowledging the limited amount of field studies available to determine their net effects on the retention and export of resources, we quantified how microbial respiration is controlled by the interactions between and the supply of essential nutrients (C, N, and P) in a headwater stream in Colorado, USA. For this, we conducted two rounds of nutrient experiments, each consisting of four sets of continuous injections of Cl− as a conservative tracer, resazurin as a proxy for aerobic respiration, and one of the following nutrient treatments: (a) N, (b) N+C, (c) N+P, or (d) C+N+P. Nutrient treatments were considered to be known system modifications that alter metabolism, and statistical tests helped identify the relationships between reach-scale hydrologic transport and respiration metrics. We found that as discharge changed significantly between rounds and across stoichiometric treatments, (a) transient storage mainly occurred in pools lateral to the main channel and was proportional to discharge, and (b) microbial respiration remained similar between rounds and across stoichiometric treatments. Our results contradict the notion that hydrologic transport alone is a dominant control on biogeochemical processing and suggest that complex interactions between hydrology, resource supply, and biological community function are responsible for driving in-stream respiration.