Unravelling the sources and bioavailability of dissolved organic matter in lotic aquatic ecosystems

Dissolved organic matter (DOM) constitutes a large proportion of the organic carbon in all aquatic ecosystems and is often a significant carbon resource for heterotrophic microorganisms. Diverse sources, compositional complexity and multiple transformation pathways have made it difficult to generalize about the bioavailability of this material. Recently, there has been dramatic progress in identifying sources of DOM, describing its composition and 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 to a large river.