In situ rapid measures of total respiration rate capture the super labile DOC bacterial substrates of freshwater

Abstract

Super labile dissolved organic carbon (DOC) in freshwater supports high rates of bacterial respiration. 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 the DOC 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. The surface water mean bacterial respiration rate was 0.58 ± 0.4 mol C m−3 d−1, whereas below 6 m, it was 4.8 ± 1.8 mol C m−3 d−1. Globally these are amongst 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 the atmosphere from freshwater are being underestimated using existing methods because of the time lost with sample manipulation and long incubation times.