The role of phytoplanktonic size fractions in the microbial food webs in two north Patagonian lakes (Argentina)

A current concept concerning organic matter dynamics within pelagic food webs consists of two processes: the traditional food web (nutrients – phytoplankton – zooplankton – fish), which considers that the phytoplankton is the dominant ‘producer’, and the microbial loop (AZAM et al. 1983), which implies that a portion of matter and energy flows through unicellular organisms, such as bacteria, to mixotrophic and heterotrophic protists, bearing in mind that several phytoplanktonic organisms can also be consumers. Based on these two concepts, the small-sized fractions of phytoplanktonic communities play fundamental roles in the pelagic trophic interactions. In particular, in oligotrophic waters the autotrophic picoplankton can dominate the phytoplanktonic biomass and production (STOCKNER & ANTIA 1986, STOCKNER 1988), and at the same time, together with bacteria, constitute the prey for the protistan assemblage. The nanoplanktonic size fraction, to which most of the mixotrophic protists belong, also represents the food for herbivorous zooplanktonic organisms.