Modern diatoms from a temperate river in South America: the Colorado River (North Patagonia, Argentina)

Diatom assemblage composition and distribution from surface sediment samples of the Colorado River, Patagonia were analysed in relation to environmental variables using multivariate techniques. The aim of this study was to determine the ecological preferences of the 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 by typical riverine conditions. Canonical correspondence analysis indicated that major ion concentrations contribute significantly to explain the 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, Delphineis minutissima, Paralia sulcata and Paralia sulcata var. coronota. Diatom communities in riverine sites were dominated by freshwater taxa such as Staurosira venter, Pseudostaurosira brevistriata, Punctastriata glubokoensis and Punctastriata lancettula. Two Punctastriata species that were difficult to distinguish with LM were examined in detail with SEM, and the value of detailed LM and SEM analysis for distinguishing Punctastriata spp. and other small fragilarioids is discussed. Based on their autoecological affinities, it is likely that the dominance of small fragilarioids is related to the increase in ionic concentration in the basin, as a consequence of both precipitation decrease, and an increase in urban, industrial and agricultural activities. The construction of a flood-control structure in the northern branch of the delta modified the diatom community so that marine taxa were replaced by brackish-freshwater taxa in response to changes in salinity, substrata and water depth.