The structure, dynamic and trophic interactions of the microbial food web (MFW) were investigated in two lakes of different trophic state: the oligo-mesotrophic Piburger See (PIB) and the hypertrophic Lago Rodo (LR). Additionally the interaction with the classical food web was analyzed. Abundance and biomass of the microbial food web components as well as bacterial production (BP) and bacterivory were quantified at 5- to 15-day intervals during 1 year in the euphotic zone of both lakes. Main physical, chemical and biological parameters were also studied.

In PIB the planktonic food web was more diverse and complex with many possible trophic interactions. Several mixotrophic species (flagellates, ciliates) are numerically important components of the MFW. Mixotrophy seems to represent an advantage during low resources supply (light, food) periods like during the ice-cover. Autotrophic picoplankton was another conspicuous component of the MFW representing an important carbon pool (up to 43 % of total picoplankton biomass). Aplastidic flagellates were the main controllers of bacterial production. Ciliates had a very low impact (< 3.1 % of the water column cleared per day). The ciliate assemblage presented several figures different from previous studies in this lake like the presence of nanociliates (e.g. BALANION PLANCTONICUM) at the surface in abundances ca. 10 times higher (101,400 ind. per liter) than the maximum registered before. Nanociliates played a considerable role as grazers on phytoplankton and bacteria, making them an important link for higher trophic levels during periods of high abundance. The clear-water phase was the period of more effective energy transfer from the MFW to higher trophic levels.

The shallow LR presented a less diverse and complex food web structure. The lake sustained a high phytoplankton biomass of up to 390 grams per liter, which reduced the euphotic zone to the first 50 cm of the water column. The phytoplankton community was dominated by the cyanobacterium PLANKTOTHRIX AGARDHII which together with a high fish predation exerted by the small CNESTERODON DECEMMACULATUS imposed harsh conditions to the zooplankton community dominated by rotifers. The rotifer ANURAEOPSIS reached very high abundances of up to 120,000 ind. per liter in summer. Cladocerans were inexistent for most time of the year and therefore the carbon flow to higher trophic levels was mainly through the microbial components. Aplastidic flagellates consumed on average 91 and 76 % of the bacterial carbon produced during summer and winter, respectively. Metazooplankton, ciliates and flagellates abundances were lower compared to other eutrophic systems supporting the idea that in very productive systems the carrying capacity of prey populations are kept at lower values due to the strong predation pressure. I show for the first time the permanent presence in the plankton of grazing-resistant bacteria up to 210 micro-m long. This seems to be a consequence of the high predation pressure effected by the aplastidic flagellates and the absence of other large bacterivores. Bacterial turnover times (5 to 42 h) estimated using an empirical conversion factor for the calculation of the number of cells produced are the lowest registered until now for field conditions. My results support the previous idea that there is a decline in bacterial numbers related to chlorophyll concentration along the trophic gradient. Although past models relating bacterial abundance and chlorophyll to bacterial production underestimate the observed value in LR, my results indicate an increase with increasing trophy.

Bacterial production was positively correlated to bacterial abundance in both lakes but the relationship was significantly improved by the inclusion of temperature. In LR these two parameters explained 82 % of the variability in BP while in PIB explained 35 %. The substitution of the bacterial abundance by the abundance of active (INT-reducing) bacteria gave the same value for LR but increased by 50 % the explained variability in PIB, confirming the importance of considering the metabolic active fraction of the bacterial assemblage in this type of analysis.