Coastal upwelling areas are highly productive systems and were initially characterized by having a short food chain, being ecologically efficient in the trophic transfer. Large micro-phytoplankton, predominating under high availability of nutrients in the mixed layer, are grazed by large herbivorous zooplankton, and they, in turn, are consumed by planktivorous fishes. Under this scheme, little attention was paid to the role of micro-organisms in these areas. This thesis provides an assessment of the temporal variability in the structure of micro-organism assemblages and of the trophic interactions in microbial food webs in the Humboldt Current System (HCS) off Concepcion, central Chile, as a basis to understand the relevance of the carbon flow through the microbial food web in this coastal upwelling area
