Hydrological pulses and burning of dissolved organic carbon by stream respiration

Stream metabolism plays a significant role in the global carbon cycle. Storm events can lower stream metabolic activities by removing standing biomass and river bed stock of organic matter. However, hydrological events could also stimulate stream ecosystem respiration (ER) by providing dissolved organic carbon (DOC) derived from soils. Here, I show how hydrological connectivity between land and water affects fluxes of DOC and daily whole stream bacterial respiration over an annual cycle in streams rich in DOC in north-west Europe. The novelty of the approach resides in combining continuous whole stream metabolism with hydrological flow paths and water chemistry to quantify the in situ fate of DOC at ecosystem scale, with an estimation of all major stream carbon fluxes (land-derived CO2, in-stream biotic CO2, HCO3, and DOC) at catchment scale. An average 23% +/- 11% of the annual DOC inputs from the land was respired away by benthic microbial metabolism within about an hour of transit time in small watersheds (about 1 km(2)). Stream ER was highly related to discharge and was stimulated for as long as the hydrological connectivity between land and water remained, as indicated by soil moisture continuous monitoring. In-stream heterotrophic respiration represented 16% +/- 7% of the annual total carbon fluxes (also including HCO3, land-derived CO2, and DOC) at the catchment outlet under stable flows. This study suggests that DOC supply (soil carbon loss) will increase with rainfall, stimulating aquatic respiration, and CO2 emissions in streams.