Direct measurement of the d13C signature of carbon respired by bacteria in lakes: Linkages to potential carbon sources, ecosystem baseline metabolism, and CO2 fluxes
Limnol. Oceanogr., 53(4), 2008, 1204-1216 | DOI: 10.4319/lo.2008.53.4.1204
ABSTRACT: Using a novel method to measure the isotopic signature (d13C) of respiratory CO2 produced by bacterioplankton, we determined the proportion of terrigenous vs. algal-derived organic carbon (OC) respired by bacteria in a series of eight lakes in southern Que´bec (Canada). The lakes are located within the same general basin but span a large range in trophic status, morphometry, and dissolved OC (DOC) concentrations. Isotopic d13C values of respired CO2 ranged from -28.4‰ to -32.5‰ across a gradient of lakes and streams. These values were compared with those of potential OC sources within the lakes (terrigenous and algal) using a mass balance model. The proportion of terrigenous OC respired varied from 3% to >70% and was strongly negatively correlated to lake chlorophyll a (Chl a) concentrations and weakly positively correlated to DOC: Chl a concentrations. While both total plankton and bacterial respiration (BR) increase with lake Chl a concentration, the component of BR that is supported by terrigenous OC, which ranges from 0.7 to 1.7 mg C L-1 h-1, stays essentially constant along the trophic gradient, increasing only slightly with DOC concentration. There is a relatively constant baseline BR supported by terrigenous OC, which becomes diluted by the BR of algal OC as the lakes become more productive. The estimated production of CO2 through BR of terrigenous OC in the epilimnion explains on average 60% of the estimated air-water CO2 flux calculated for these lakes, suggesting that the processing of allochthonous OC by bacteria is a major component of this flux.