Understanding the process involved in the production of organic carbon and the biological regulation of carbon fluxes has been, for a long time, one of the main objectives of theoretical and applied ecosystem studies. These studies have considered the analysis of the energy flux, expressed as organic matter or carbon through the ecosystem's structure. There has been a growing interests, in the last decade, toward a quantification of intra- and inter-systemic organic carbon fluxes. The main goal is to study the organization and development of ecosystems through time.

Within this theoretical framework, this thesis analyzes the effect of changes in the relative supply of allochtonous and autochtonous organic matter, measured as organic carbon, on the organization of lotic ecosystems. I propose a conceptual model of the flux of organic carbon, which corresponds to a modification of the model by Cummins et al (1973). The approximation used in this thesis, however, corresponds to the process-functional approach, and it differs from that of the original model. The model developed here represents the functional structure of the lotic ecosystems regarding the fluxes of both allocthonous and autochthonous organic carbon.

The study site, Rio Clarillo (33°41' S, 70°24' W) was characterized analyzing climatic and physico-chemical variables bimonthly during 1995 and 1996. The biomass and production of the ecosystem components was also determined. The following interactions between components were studied experimentally: 1) effect of the availability of allocthonous organic matter on the heterotrophic productivity of decomposers, 2) effect of the availability of inorganic nutrients on the primary productivity, 3) effect of the herbivory on primary producers, 4) interaction mechanisms between primary producers and decomposers. Intrasystemic flows of organic carbon were evaluated using the software ECOPATH II™.

The results show that the components of the Rio Clarillo ecosystem use both sources of carbon in the generation of their biomasses. Most of that carbon is recycled through the ecosystem. The allochtonous organic carbon is the main resource in the Rio Clarillo ecosystem. Nevertheless, the autochtonous carbon explained up to a 28% of the total flux. The ecosystem structure exhibited significant temporal changes, regarding the number, size and interactions among components. Those changes affected the total flux of organic carbon, especially through modifications in the relative importance of heterotrophic degradation of allocthonous organic carbon and autotrophic fixation of autochtonous organic carbon. During low-water periods, the importance of carbon fixation increases, producing an increment in ascendancy, which can be translated to an increase in the organization of the ecosystem. Finnally, a theoretical model is proposed to explain the temporal changes in heterotrophy and autotrophy in the metabolism of lotic ecosystems. The model is based on switching behavior of both type of metabolism as a response to meso and macroscale exogenous perturbations.