CONICET | Buscador de Institutos y Recursos Humanos

Diatom assemblage composition and distribution from surface sediment samples of the Colorado River, Patagonia were analysed inrelation to environmental variables using multivariate techniques. The aim of this study was to determine the ecological preferences ofthe taxa and provide analogues for paleoenvironmental reconstructions in estuarine and ﬂuvial environments of southern South America.Cluster analysis identiﬁed two main diatom assemblages, one grouping sites with a marine inﬂuence, and another group inﬂuenced bytypical riverine conditions. Canonical correspondence analysis indicated that major ion concentrations contribute signiﬁcantly to explainthe variation in the composition of diatom assemblages. A total of 208 taxa belonging to 56 genera were recorded. In estuarine sites,the assemblages were characterized by marine-brackish tychoplankton, such as Cymatosira belgica, Rhaphoneis amphiceros, Delphineisminutissima, Paralia sulcata and Paralia sulcata var. coronota. Diatom communities in riverine sites were dominated by freshwater taxasuch as Staurosira venter, Pseudostaurosira brevistriata, Punctastriata glubokoensis and Punctastriata lancettula. Two Punctastriataspecies that were diﬃcult to distinguish with LM were examined in detail with SEM, and the value of detailed LM and SEM analysisfor distinguishing Punctastriata spp. and other small fragilarioids is discussed. Based on their autoecological aﬃnities, it is likely thatthe dominance of small fragilarioids is related to the increase in ionic concentration in the basin, as a consequence of both precipitationdecrease, and an increase in urban, industrial and agricultural activities. The construction of a ﬂood-control structure in the northernbranch of the delta modiﬁed the diatom community so that marine taxa were replaced by brackish-freshwater taxa in response to changesin salinity, substrata and water depth.

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