Unravelling the sources and bioavailability of dissolved organic matter inlotic aquatic ecosystems

Dissolved organic matter (DOM) constitutes a large proportion of the organi c carbon in all aquatic ecosystems and is often a significant carbon resour ce for heterotrophic microorganisms. Diverse sources, compositional complex ity and multiple transformation pathways have made it difficult to generali ze about the bioavailability of this material. Recently, there has been dra matic progress in identifying sources of DOM, describing its composition an d tracing its movement through microbial food webs. A number of conceptual, mechanistic and empirical approaches have been proposed to synthesize this rapidly expanding body of knowledge. This paper reviews these approaches, and some general hypotheses for focusing future research are proposed. As a first step in synthesizing the available information, predictions are made about how variability in sources, hydrology and biotic removal interact to generate temporal and spatial patterns in DOM from a stream's headwaters t o a large river.