María Eugenia Llames

Seasonal Variability of Optical Properties in a Highly Turbid Lake (Laguna Chascomús, Argentina)

We study the underwater light field seasonality in a turbid lake, Laguna Chascomús (Buenos Aires, Argentina). We report (1) relationships between optical properties (OPs) and optically active substances (OASs); (2) relationships between inherent (IOPs) and apparent (AOPs) optical properties; and (3) the seasonal variability in OASs and OPs. Light absorption was dominated by the particulate fraction. The contributions of phytoplankton pigments and unpigmented components were similar. The best predictors of total particulate absorption, unpigmented particulate absorption, turbidity and vertical attenuation coefficient were total suspended solids or their ash content. Many OASs and OPs varied seasonally. The concentrations of OASs were higher during spring and summer, resulting in lower transparency and higher turbidity. However, mass‐specific absorption coefficients displayed lower values during spring and summer. Thus, the higher light attenuation observed during spring and summer resulted from higher concentrations of relatively less absorptive OASs. Collectively, these results suggest that: (1) light extinction is enhanced during spring and summer; (2) the enhanced light extinction is due to changes in the particulate fraction; (3) the enhanced light extinction is mostly due to an increase in the amount of particulate material; and (4) the increase of particulate matter also enhanced light extinction through increased scattering.

Alternative states drive the patterns in the bacterioplankton composition in shallow Pampean lakes (Argentina)

We assessed the influence of environmental factors in shaping the free-living bacterial community structure in a set of shallow lakes characterized by contrasting stable state patterns (clear-vegetated, inorganic-turbid and phytoplankton-turbid). Six temperate shallow lakes from the Pampa Plain (Argentina) were sampled over an annual cycle, and two fingerprinting techniques were applied: a 16S rDNA analysis was performed using denaturing gradient gel electrophoresis (DGGE) profiles, and a 16S-23S internally transcribed spacer region analysis was conducted by means of automated ribosomal intergenic spacer analysis (ARISA) profiles. Our results show that the steady state that characterized the different shallow lakes played a major role in structuring the community: the composition of free-living bacteria differed significantly between clear-vegetated, inorganic-turbid and phytoplankton-turbid shallow lakes. The state of the system was more important in determining these patterns than seasonality, geographical location or degree of hydrological connectivity. Moreover, this strong environmental control was particularly evident in the pattern observed in one of the lakes, which shifted from a clear to a turbid state over the course of the study. This lake showed a directional selection of species from a typical clear-like to a turbid-like community. The combined DGGE/ARISA approach revealed not only broad patterns among different alternative steady states, but also more subtle differences within different regimes.

Microbial pelagic metabolism and CDOM characterization in a phytoplankton-dominated versus a macrophyte- dominated shallow lake

Dominant primary producer in macrophyte- or phytoplankton-dominated shallow lakes might imply differences in dissolved organic carbon (DOC) composition. We compared chromophoric dissolved organic matter (CDOM), plankton respiration (R), and bacterial (BP) and primary production (PP), in two contrasting shallow lakes. We hypothesized that DOC from the macrophyte-dominated lake would be qualitatively inferior, so that it can support a lower yield than DOC from the phytoplankton-dominated one. Macrophyte-dominated lake had more humic and aromatic CDOM, though molecular weight was similar in both lakes. A clear synchronism between lakes was observed in mean depth and several CDOM absorption coefficients, suggesting an external driver of the variation in DOC concentration and CDOM quality. The positive BP-PP and BP-Chl-a correlations in the macrophyte-dominated lake point out to a dependence of bacteria on phytoplankton for a supply of labile DOC. In turn, BP in the phytoplankton-dominated lake was balanced with grazing by HF (heterotrophic flagellates). The significantly higher HB:DOC and HF:DOC carbon ratios in the phytoplankton-dominated lake also suggest that better DOC quality would mean relatively more efficient C transfer to higher trophic levels. According to PP:BP and PP:R ratios both lakes should be considered autotrophic, although the macrophyte-dominated lake would be comparatively more heterotrophic.

Patterns of dissolved organic matter across the Patagonian landscape: a broad-scale survey of Chilean and Argentine lakes

Dissolved organic matter (DOM) is a complex mixture of carbon compounds from autochthonous and allochthonous sources. Dissolved organic carbon (DOC) concentrations and optical metrics of DOM provide clues as to the sources and processes affecting the DOM pool. Herein we provide the first broad-scale characterisation of DOM from Patagonian lakes across a strong west–east precipitation gradient. Fifty-eight lakes from Northern Patagonia (Argentina and Chile) plus six lakes from the Antarctic Peninsula were sampled during summer 2000–01. Six DOM metrics were evaluated: DOC absorbance at 254nm (a254) and 350nm (a350), DOC-specific absorbance at 254nm (a254/DOC) and 350nm (a350/DOC) and spectral slope between 275 and 295nm (S275–295). The DOM of Chilean maritime lakes and shallow ( 15m) Andean lakes, which is consistent with their longer water retention time. Steppe lakes, mostly from endorheic basins, made up the most heterogeneous group with regard to DOM characteristics.

Interplay between stochastic and deterministic processes in the maintenance of alternative community states in Verrucomicrobia-dominated shallow lakes

We analyzed the interplay between neutral and deterministic processes in maintaining contrasting alternative bacterioplankton communities through time in highly productive shallow lakes and evaluated the relevance of these processes when a regime shift from a clear to a turbid state occurred. We observed that local assembly is ruled primary deterministically, via local habitat filtering, with a secondary role of stochastic processes. We also found a hierarchy in the environmental sorting: while an unusual Verrucomicrobia dominance characterizes the three systems, local conditions limit within-bacterial community membership to closely phylogenetically related and ecologically similar taxa. These results indicate that bacterial abilities to establish in these lakes are strongly determined by their traits, and point toward special physiological adaptations to persist when these systems undergo a regime shift. Altogether, these results hint to a divergence in function among these alternative communities, mediated by major shifts in bacterial community trait structure, particularly regarding carbon use.

LOTUS spp: BIOTECHNOLOGICAL STRATEGIES TO IMPROVE THE BIOECONOMY OF LOWLANDS IN THE SALADO RIVER BASIN (ARGENTINA)

The Salado River Basin region is the most important livestock breeding area in Argentina, wherethe Lotus species has been traditionally cultivated as forages. Nearly 60% of their land surface is dominated by salt-affected soils with severe constraints for crop cultivation. In order to cope with that limitation, farmers have utilized species such as non-native L. tenuis (ex- Lotus glaber), which shows a very good adaptation. As a result, inter-seeding of L. tenuis has been proposed as a strategy of choice for improving forage production in marginal areas. The increase in soil quality by these means is achieved by an increment of the organic matter content, improvement of soil fertility as well as microbial biodiversity. Thus, the introduction of L. tenuis and/or other Lotus genotypes could have enormous benefits for similar constrained lands around the world. We are developing an integrated analysis of the changes that occur in soils under legume production. We will not only analyze the microbial diversity associated, but also soil physical and chemical characteristics and the impact of different legume-microbes association on mitigation of GHG emissions. In addition, we are identifying the main genetic determinants associated with interesting agronomic traits such as plant tolerance against biotic and abiotic stresses and the content of condensed tannins. Our future and present research will build a solid base for the improvement of agronomicallyimportant species and the development of better strategies for the management of constrained lands such as the lowlands in the Argentinean Pampas.