Bacterial production on humic and nonhumic components of dissolved organic carbon

Dissolved humic substances make up ∼50% or more of the dissolved organic carbon (DOC) in freshwater ecosystems, yet their trophodynamic roles remain unresolved. We separated DOC from two freshwater environments, a lake and a blackwater marsh, into two fractions, humic DOC and nonhumic DOC, using an XAD‐8 resin to selectively adsorb the humic substances. Results of microcosm studies revealed that the humic fraction of DOC was used by natural bacterial assemblages from the lake and marsh as a C and energy source, as indicated both by increases in bacterial biovolume and rates of bacterial incorporation of 3 Hthymidine. Humic substances supported fourfold less bacterial secondary production per unit of initial C, however, than did nonhumic substances from the same environment. Bacterial utilization of humic compounds accounted for a significant fraction of the total bacterial production on DOC, measured as increases in bacterial biovolume; humic substances supported an average of 22% of total growth on DOC from the lake and 53% of the total growth on DOC from the marsh. The relative bioavailability of both the humic and nonhumic fractions of DOC differed between the lake and blackwater marsh, with less bacterial production per unit of initial C occurring on marsh‐derived dissolved compounds.