Production and use of detritus in various freshwater, estuarine, and coastal marine ecosystems

In both freshwater and marine habitats, vascular marine plants arc little used by animals that graze directly on them, because they have a relatively high content of indigestible fiber and a low content of nitrogen. The chief emphasis of detritus research in the 1970s was to show how microorganisms progressively reduce the content of fiber and increase the content of nitrogren in vascular plant detritus, rendering it nutritious for animals. Algal (seaweed, diatom, etc.) detritus starts with a lower fiber content and a higher nitrogen content. Many animals can use it directly, and a very short period of microbial colonization renders it highly nutritious. As a result, a high proportion of the algal carbon originally produced passes into animals via detrital food webs, while a low proportion of vascular plant carbon does so. Much more of the latter simply supports microbial respiration. In the 1980s it was shown, particularly for freshwater habitats, that the dissolved organic matter (DOM) released by plants while living or in the early stages of decomposition readily precipitates on surfaces and forms amorphous particulate matter with a low content of refractory material. These particles are highly nutritious for animals and are used directly by freshwater fish such as Sarotherodon (= Tilapia ), which is commercially important, especially in Africa and South America. It is suggested that the DOM pathway may be ecologically more significant than the POM (particulate organic matter) pathway and that processes analogous to those shown for lakes and rivers probably occur in estuarine and coastal waters. There is much circumstantial evidence to suggest that planktonic food webs based on DOM are much more important than previously thought. The conversion of DOM to POM through the “microbial loop” and its utilization in higher trophic levels is an urgent topic for further study.