FIGURE 6.8 Example of a river.As the river flows,organic matter is accumulated.Much of this or- ganic matter comes from surrounding terrestrial environments.It can be seen from the photograph that there is considerable organic matter just in the form of plant life.This level of organic matter leads to increases in the ratio of heterotrophic populations,which more often than not predominate in these en- vironments.(From Corel Gallery www.corel.com)