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Microbial mat and surface sediment communities from a shallow oxbow lake in the Colorado River floodplain, Argentina

Abstract The epi-sedimentary microbial community of a shallow oxbow lake in the Colorado River floodplain was studied along a transect running from its concave margin to the center. Physico-chemical parameters characterized the lake as hypersaline (S = 38.8 ± 0.7), alkaline (pH = 8.0 ± 0.2), and stagnant. Core samples of the topmost sediment were taken at three equidistant points for a diatom analysis; furthermore, the easily-detachable microbial mat was sampled at the center of the lake. Diatoms were dominated by biraphid taxa with benthic or epiphytic habits, while planktonic taxa were scarce. The microbial mat consisted in a millimeter-layered bio-sedimentary structure, presenting high organic matter (36.0 ± 0.2%) and chlorophyll a (144.1 ± 8.1 µg Chl a g−1 DW sediment) contents. Filamentous cyanobacteria dominated the mat biomass in surface layers (0–6 mm), while motile benthic diatoms were the second-most abundant phototrophic group. Navicula salinicola was the dominant diatom. Heterotrophic components of community (proto- and metazoans) were also identified. Resistance biogenic structures such as chrysophyte stomatocysts and planktonic diatom resting spores, and abundant gypsum precipitates were found throughout, pointing to intrinsic hydro-ecological stress conditions (desiccation and salinity fluctuations) of an inland evaporitic system. This is the first report of microbial mats colonizing surface-sediments of an infilled oxbow lake.