Concentrations of dissolved organic matter (DOM) in lakes are often an order of magnitude greater than concentrations of particulate organic matter; nevertheless, the biogeochemical analysis of DOM is described in only a few textbooks on limnology (most thoroughly by Wetzel). The orgins of dissolved organic substances are largely photosynthetic; DOM is either autochthonously synthesized by littoral and pelagic flora through secretions and autolysis of cellular contents, or allochthonously generated in terrestrial systems of the drainage basin, composing largely of humic substances refractory to rapid microbial degradation. The role of DOM in lacustrine ecosystems, as energy source and system regulator, however, is still poorly known.
The aim of this book is: (1) to present state-of-the-art reviews of the role of dissolved autochthonous and allochthonous organic matter in pelagial and littoral zones; and (2) to focus attention on poorly understood but critical topics and hence to provide direction for future research activity.

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Origin and nature of DOM in lakes.- Natural and man-caused factors affecting the abundance and cycling of dissolved organic substances in precambrian shield lakes.- Sources, concentrations and characteristics of organic matter in softwater lakes and streams of the Swedish forest region.- Leaching from vegetation of soluble polyphenolic compounds, and their abundance in temporary ponds of the Doñana National Park (SW Spain).- Will an increased greenhouse impact in Fennoscandia give rise to more humic and coloured lakes?.- Impacts of environmental changes on the biogeochemistry of aquatic humic substances.- DOM as an energy source.- The influence of humic substances on lacustrine planktonic food chains.- Extracellular dissolved organic carbon released from phytoplankton as a source of carbon for heterotrophic bacteria in lakes of different humic content.- Allochthonous dissolved organic matter as an energy source for pelagic bacteria and the concept of the microbial loop.- Dissolved organic carbon in a humic lake: effects on bacterial production and respiration.- Planktonic food chains of a highly humic lake. I. A mesocosm experiment during the spring primary production maximum.- Planktonic food chains of a highly humic lake. II. A mesocosm experiment in summer during dominance of heterotrophic processes.- Bacterioplankton in a small polyhumic lake with an anoxic hypolimnion.- Microbial control of dissolved organic carbon in lakes: research for the future.- DOM as a system regulator.- Gradient-dominated ecosystems: sources and regulatory functions of dissolved organic matter in freshwater ecosystems.- Littoral-pelagial interchange and the decomposition of dissolved organic matter in a polyhumic lake.- Extracellular enzymes in a polyhumic lake: important regulators in detritus processing.- Effects of different molecular weight fractions of dissolved organic matter on the growth of bacteria, algae and protozoa from a highly humic lake.- Vertical distributions of bacteria and algae in a steeply stratified humic lake under high grazing pressure from Daphnia longispina.- Migrations of haemoglobin-rich Daphnia longispina in a small, steeply stratified, humic lake with an anoxic hypolimnion.- Concluding remarks.- Concluding remarks.