DECOMPOSITION IN ESTUARINE ECOSYSTEMS

SUMMARY

The varied sources of estuarine plant detritus and the processes and organisms involved in its decomposition are discussed in this review. In the case of emergent and peripheral vegetation, microbial decomposition commences in the phylloplane, with fairly rapid leaching of DOM occurring soon after immersion. Residual POM, largely cellulose, is decomposed more slowly by cellulolytic micro-organisms. Phytoplankton start contributing to the DOM pool before senescence by excreting soluble substances during normal metabolism and interesting regulatory mechanisms are Involved in the bacterial utilization of this material. Bacteria play a more important role than fungi in decomposition and the latter appear to be prominent only in the breakdown of mangrove litter. Predaceous microflagellates and other protists occur in association with the bacteria and feed on them, enhancing decomposition by maintaining optimal bacterial growth rates. They appear to play a more important role in mineralization than do the bacteria, which are more effective in the conservation of nutrients upon which their efficiency of detritus incorporation Is dependent. Aerobic decomposition tends to terminate in complete mineralization, whereas under anaerobic conditions incomplete oxidation yields organic end products such as volatile fatty acids. A comparison of this process with the functioning of the herbivore rumen leads to the conclusion that the latter is more cost-effective in energy terms. Contrary to earlier published statements that estuaries are major exporters of energy in the form of detritus, many export very little of this material, or are net importers. Finally, the methods for studying decomposition in estuaries must be carefully chosen because of the environmental diversity of these systems. This is particularly true of radio-labelled substrates used for uptake and turnover studies; there appears to be no universal substrate or approach.