| Summary: | This thesis studies the microbial communities inhabiting freshwater ecosystems of Byers Peninsula (Livingston Island, Antarctica). This is an ice-free area of around 60 km2 that holds numerous lakes, ponds, rivers and wetlands. As a deglaciated region, the functioning of aquatic ecosystems is very closely linked with the surrounding land. So, when snow melting occurs during summer, interactions with the catchment become more intense, and coincide with enhanced biological activity. We conducted a systematic survey in the most representative lakes during some consecutive austral summers between 2002 and 2006. Most of these lakes are located on a plateau, where the landform facilitates water retention and they show well-defined surface outlets. According to their nutrient concentrations and the phytoplankton biomass, these lakes range merely from ultra-oligotrophic to oligotrophic. Our surveys have revealed the existence of a large standing stock of mosses (Drepanocladus longifolius) at the bottom of some of them, thus suggesting that primary production could mainly be allocated in the benthic compartment. On the other hand, the shallow lagoons located in coastal areas usually show bigger catchments, which are largely covered by mosses cushions and plants. These coastal lagoons have a higher nutrient content due to the inputs of elephant seals (Mirounga leonina) dwelling in their vicinities, and somewhat greater salinity due to more exposure to sea spray. Bacterial abundances vary broadly among lakes (0.5 to 6.5 x 106 cell mL-1), thus also underlying this inland-coastal gradient of productivity. Still, these bacterial abundances are greater than is expected for these nutrient-poor lakes. We also performed a multivariate analysis that reveals other factors such as the lakes’ morphometry to explain the more subtle differences in their trophic status. In general, all the lakes hold a few species and apparently exhibit a simple food web dominated by microbial communities. The only relevant metazoan species we found are ...
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