3 - Organic Matter Processing in Tropical Streams

Abstract

This chapter focuses on the organic matter processing in tropical streams. Organic matter derived from many sources provides a basis for stream food webs. In terms of weight, leaves from the surrounding land constitute the largest allochthonous source of energy for stream consumers besides, fruits, flowers, wood and twigs, and terrestrial insects. Litter decomposition rates depend on the interaction of physical factors, water chemistry, and biological agents especially shredding invertebrates. The largest proportion of allochthonous organic matter entering most streams comprises leaves, bark, and wood— especially twigs. Fungi and bacteria growing on the leaf surface and inside the mesophyll produce enzymes that degrade structural polysaccharides, such as cellulose, resulting in a softening of leaf structure and an increase in food value for shredders. Processing of allochthonous detritus in streams can be affected by other aspects of stream hydrology. Foremost among these is retentiveness, which determines where decomposition of allochthonous organic material actually takes place once it has entered the stream. In seasonal tropical streams, strong rainstorm events at the onset of the wet season flush out much of the litter that might have accumulated during lower-flow periods. Frequent recurrence of spates of flood events may “reset” the system and carry away leaves before degradation can occur. Sediment transport during spates may also bury leaf litter. Microbes also contribute to the decomposition of organic matter in tropical streams. There is evidence that omnivorous fishes and decapod crustaceans play key roles in detrital processing in some tropical streams.