A cross-system analysis of labile dissolved organic carbon

Abstract

The utilization of dissolved organic carbon (DOC) by bacteria in grazer-free cultures can be used to measure the concentration of labile DOC (DOCL). A database of 126 measurements was used to test whether or not the concentration of DOCL is positively related to total DOC. A positive and significant correlation between DOCL and DOC was found for the entire database as well as for lakes, rivers and seawater. An average response of about 0.17 pm01 DOCL pmol-' DOC could be calculated and 60% of the variance was explained by DOC. The DOCL concentration averaged 14, 19, and 19% of the total DOC pool in lakes, rlvers 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 DOCL in half the rivers was explained by anthropogenic influence. The control of DOCL concentrations was analyzed according to models of bacterial and substrate interactions applying Michaelis-Menten-Monod kinetics. It is suggested that the higher concentrations of DOCL in eutrophic as opposed to oligotrophic systems may be explained by a higher 'theoretical community half-saturation constant' (K,) in eutrophic systems. The consequence of a higher K, 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 crosssystem difference.