Vulnerability of mixotrophic algae to nutrient pulses and UVR in an oligotrophic Southern and Northern Hemisphere lake.

Nutrient inputs and ultraviolet radiation (UVR) are global factors affecting the structure and functioning of aquatic ecosystems, particularly clear-water ecosystems. We performed experiments in two model lakes highly exposed to UVR fluxes in order to test the effect that future increases in mineral nutrients transported by dust aerosol might exert on primary producers depending on the likelihood of atmospheric inputs. Lake La Caldera (Northern Hemisphere) has been receiving recurrent dust inputs from the Sahara Desert while lake Los Cántaros (Southern Hemisphere) has been less affected by dust aerosol. UVR × Nutrient synergistically stimulated primary production (PP), chlorophyll a (Chl a), with a smaller increase in phytoplanktonic biomass in La Caldera, but not in Los Cántaros, where nutrient addition unmasked the UVR inhibitory effect on phytoplankton. The proportional decrease of mixotrophic nanoflagellates (MNFs) after the nutrient pulse (in Los Cántaros) and the long-term decline of MNFs in La Caldera associated with the increase in aerosol-dust intrusions from the Sahara during the last 40 years suggest that a future scenario of intensified aerosol events from desert and desertified areas would not only reduce functional diversity with the decline of MNFs, but would ultimately alter the C flux towards the grazing chain in oligotrophic ecosystems.

Effects of Volcanic Pumice Inputs on Microbial Community Composition and Dissolved C/P Ratios in Lake Waters: an Experimental Approach.

Volcanic eruptions discharge massive amounts of ash and pumice that decrease light penetration in lakes and lead to concomitant increases in phosphorus (P) concentrations and shifts in soluble C/P ratios. The consequences of these sudden changes for bacteria community composition, metabolism, and enzymatic activity remain unclear, especially for the dynamic period immediately after pumice deposition. Thus, the main aim of our study was to determine how ambient bacterial communities respond to pumice inputs in lakes that differ in dissolved organic carbon (DOC) and P concentrations and to what extent these responses are moderated by substrate C/P stoichiometry. We performed an outdoor experiment with natural lake water from two lakes that differed in dissolved organic carbon (DOC) concentration. We measured nutrient concentrations, alkaline phosphatase activity (APA), and DOC consumption rates and assessed different components of bacterial community structure using next-generation sequencing of the 16S rRNA gene. Pumice inputs caused a decrease in the C/P ratio of dissolved resources, a decrease in APA, and an increase in DOC consumption, indicating reduced P limitation. These changes in bacteria metabolism were coupled with modifications in the assemblage composition and an increase in diversity, with increases in bacterial taxa associated with biofilm and sediments, in predatory bacteria, and in bacteria with gliding motility. Our results confirm that volcanic eruptions have the potential to alter nutrient partitioning and light penetration in receiving waterways which can have dramatic impacts on microbial community dynamics.

Planktonic ciliates from an oligotrophic south Andean lake, Morenito Lake (Patagonia, Argentina).

In this contribution we have studied the planktonic ciliates from Morenito Lake, an oligotrophic lake situated in the South Andes of Argentina. Six species that are new records for South America or Argentina are described and illustrated. Besides, population dynamics of the ciliate species were studied during a spring-summer period. Strobilidium lacustris and Balanion planctonicum were the most abundant species, showing a maximum in mid summer. Paradileptus elephantinus was present all over the period while Urotricha furcata was observed in late summer samples. Strobilidium lacustris and S. humile were found to be occasional species during the studied period. The recorded ciliate assemblage with oligotrichs and prostomates as dominant indicate the oligotrophic condition of the lake.

Planktonic ciliates from an oligotrophic South Andean lake, Morenito Lake (Patagonia, Argentina)

In this contribution we have studied the planktonic ciliates from Morenito Lake, an oligotrophic lake situated in the South Andes of Argentina. Six species that are new records for South America or Argentina are described and illustrated. Besides, population dynamics of the ciliate species were studied during a spring--summer period. Strobilidium lacustris and Balanion planctonicum were the most abundant species, showing a maximum in mid summer. Paradileptus elephantinus was present all over the period while Urotricha furcata was observed in late summer samples. Strobilidium lacustris and S. humile were found to be occasional species during the studied period. The recorded ciliate assemblage with oligotrichs and prostomates as dominant indicate the oligotrophic condition of the lake

Structure and dynamics of food webs in Andean Lakes

Se ha realizado un estudio de los sistemas de alimentos de los lagos oligotróficos en la región de Bariloche. Se nota diferencias comparados con los lagos del hemisferio norte. Se podrían crear cambios con la incorporación de salmonidas exóticas y distintas especies de árboles