DECOMPOSITION IN FRESHWATER ECOSYSTEMS

SUMMARY

This review examines the sources and composition of organic matter and the decomposition of particulate and dissolved organic matter (POM and DOM) in freshwater ecosystems. The main points to emerge from the review are listed below.

1. Terrestrial plant material is an important source of allochthonous POM in lotlc systems.

2. In lentic systems important autochthonous sources of DOM are the algae and aquatic macrophytes.

3. Little Is known about dissolved organic carbon (DOM) compounds in freshwaters and they are not easily categorized chemically.

4. The decomposition of wood is largely a surface phenomonon carried out by fungi since gallery-forming insects are lacking.

5. Difficulties arise in describing decomposition of plant material in rivers because of methodological problems. The data generally show that leaching of DOM is followed by microbial breakdown and further processing by invertebrates. The microbes are mainly hyphomycete fungi but bacteria can play a significant role in later stages.

6. In lentic systems bacteria are the prime decomposers of plant material. This difference from lotic systems may be related to turbulence. Both aquatic macrophytes and algae show an initial loss of DOM, which can be attributed to lysis or autolysis. Questions remain about the factors causing death of aquatic plants and the distinction between senescence and death. The significance of invertebrates in the decomposition of plant litter remains uncertain in these systems.

7. The rate and sequence of events In decomposition of plant material in aquatic ecosystems is dependent on the species and components of the plants concerned.

8. A large volume of data exists on the assimilation and decomposition of DOM in aquatic ecosystems. The process is primarily bacterial but the significance of bacteria in decomposing POM and DOM in freshwaters remains unclear because of the lack of a universal substrate for estimating total bacterial production.

9. The data demonstrate that our understanding of decomposition in freshwater ecosystems is deficient; this can be mainly attributed to lack of suitable methodology.

10. Continuing developments in the use of radioisotope technology, exoenzymatic assays and aquatic chemistry permit an optimistic outlook for a better understanding of decomposition processes in fresh waters.

KEYWORDS:

• Rivers
• lakes
• bacteria
• fungi
• microbial processes
• DOM
• POM
• FPOM
• CPOM.