Organic matter processing in tropical streams

Abstract

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, but other items, including fruits, flowers, wood and twigs, and terrestrial insects, are also important. Timing of allochthonous inputs can vary markedly due to the phenology of the riparian vegetation, retention mechanisms in the aquatic-terrestrial transition zone, and local climate (especially the incidence of high-rainfall events), but seasonality of litter inputs is different, and often much less marked, than is typical of streams in temperate latitudes. As in such streams, litter decomposition rates depend on the interaction of physical factors (flow, temperature), water chemistry (dissolved nutrients), and biological agents (micro-organisms and detritivores especially shredding invertebrates). Because vascular plant biodiversity in the tropics is high, varied leaf characteristics (hardness, phenolic content, and other aspects of leaf chemistry) contribute to great variability in breakdown rate: fast-decomposing leaves persist for a few days only, whereas highly recalcitrant species take well over a year to decompose. In all the above cases, the decomposition process includes an initial rapid leaching phase when water-soluble compounds are lost, followed by colonization by micro-organisms (fungi and bacteria), and subsequent mechanical breakdown of the leaf structure by invertebrate shredder and hydraulic forces. Undecomposed leaves are sometimes exported downstream during flood events, and thence deposited in water-logged riparian zones or, in some cases, forming dense accumulations of peat that are important as carbon sinks and as habitat for specialized biota. Recent research indicates that the role of invertebrate shredders in processing organic matter in tropical streams is less than in temperate latitudes, and there may be a higher proportion of material that is recalcitrant and/or exported from streams (or stored as peat) before it is decomposed completely. Autochthonous energy sources may be particularly important to consumers in tropical streams, and there is some evidence of a lesser reliance on allochthonous organic matter than in temperate streams.