MEPS 118:283-294 (1995)  -  doi:10.3354/meps118283

The utilization of dissolved organic carbon (DOC) by bacteria in grazer-free cultures can be used to measure the concentration of labile DOC (DOC_L). A database of 126 measurements was used to test whether or not the concentration of DOC_L is positively related to total DOC. A positive and significant correlation between DOC_L and DOC was found for the entire database as well as for lakes, rivers and seawater. An average response of about 0.17 umol DOC_L umol^-1 DOC could be calculated and 60% of the variance was explained by DOC. The DOC_L concentration averaged 14, 19, and 19% of the total DOC pool in lakes, rivers and seawater, respectively; however, the median value of 25% for rivers was about twice the values for lakes and seawater. The high relative amount of DOC_L in half the rivers was explained by anthropogenic influence. The control of DOC_L concentrations was analyzed according to models of bacterial and substrate interactions applying Michaelis-Menten-Monod kinetics. It is suggested that the higher concentrations of DOC_L in eutrophic as opposed to oligotrophic systems may be explained by a higher 'theoretical community half-saturation constant' (K_s) in eutrophic systems. The consequence of a higher K_s will be a higher steady-state concentration of labile substrate, as was found. Other possible explanations were analyzed, but rejected as major causes for the cross-system difference.

Labile DOC . Cross-system differences . Control of labile DOC