In situ rapid measure of total respiration rate capture super labile dissolved organic carbon in freshwater

Abstract

Super labile dissolved organic carbon (DOC) in freshwater supports ultrahigh rates of bacterial respiration rates. However, this DOC fraction has been overlooked because of methodological and instrument limitations, until now. Here a simple inexpensive respiration chamber-valve system is described that can be attached to an optical dissolved oxygen probe to measure in situ bacterial respiration. The dissolved oxygen (DO) probe and chamber capture a 2 L water sample at depth and immediately begin measuring the DO loss over time at in situ pressure and temperature. DO measures over 5 min provided enough DO data to determine DO decay rate, and subsequently, the rate of bacterial respiration and carbon demand. The method was validated for a mixed bacterial community in a batch reactor and applied to the freshwater of boreal, sub-tropical and tropical climatic zones. Surface water mean bacterial respiration rate was 0.58 ᠰ.4 mole C . m-3. d-1 where below 6 m it was 4.8 ᠱ.8 mole C . m-3. d-1. These are amongst some of the highest respiration rates measured in freshwater. This in situ method captured the freshwater respiration of the super labile DOC and shows how most often this DOC fraction is missed because water samples are removed from the water body to measure respiration. Terrestrial DOC inputs into freshwater ecosystems and current global estimates of pCO2 outgassing into atmosphere from freshwater are being underestimated using existing methods that require sample manipulation and long incubation times.