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The present paper describes the morphology, taxonomy, and distribution of species belonging to the diatom genera Rhizosolenia s. str., Neocalyptrella, Pseudosolenia and Proboscia, found in the marine coastal waters of Buenos Aires Province, Argentina. The phytoplankton samples were collected from October 1994 to September 2000 at seven locations and were analyzed by light and scanning electron microscopy. Rhizosolenia antennata f. antennata, R. bergonii, R. hyalina, R. imbricata, R. setigera f. setigera, R. setigera f. pungens, Neocalyptrella robusta, Pseudosolenia calcar-avis, Proboscia alata and P. indica were studied and compared with allied taxa. Morphology of the frustule and of the resting spore of R. setigera, which is considered by some authors incerta sedis, shows that this species might be included into the genus Rhizosolenia s. str. Rhizosolenia hyalina is reported for the first time from Argentinean coastal waters. R. setigera f. setigera, R. setigera f. pungens, P. alata, and P. calcar-avis, cited as harmful non-toxigenic taxa in other geographical areas, were never associated with harmful events in the study area. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]