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The rivers and streams of the Pampean plain are subjected to the impact of agriculture, cattle-raising and industrial activities. The largest urban center of Argentina is located here. The most important stresses on rivers and streams in the Pampean plain are organic enrichments (discharge of insufficiently treated sewage), nutrients, heavy metals, pathogenic agents, pesticides, herbicides and physical changes produced by dredging and canalisation. The epipelic community is suitable for biomonitoring purposes because it allows for comparing similar substrates along the rivers and streams. A total of 164 samples of epipelic diatoms were collected during 1995-1999 from Pampean rivers and streams. The analysis of these samples resulted in the development of a specific biotic index: the Pampean Diatom Index (IDP). The results were correlated with the main chemical water characteristics and with other biotic indices. This study suggests that the IDP is integrating organic pollution and eutrophication and can be applied for monitoring the biological quality of rivers and streams in the Pampean plain.

The purpose of this study was to explore the autecology of diatom species inhabiting the epipsammon and epiphyton in the freshwater-mixohaline zone of the Río de la Plata estuary. The diatoms are a conspicuous component of those communities. We discuss the optima and tolerance ranges of diatoms for the following environmental variables: conductivity, turbidity, pH, nutrients (phosphate, nitrate, nitrite, and ammonium), and dissolved oxygen levels as well as both the chemical and biological oxygen demand. The study was carried out on the Argentinean coastline between 34°27' S, 58°30' W and 35°23' S, 57°08' W. In total, 32 sampling sites influenced by different land uses were monitored along 168 km of shoreline. Epipsammic samples of the intertidal zone were taken at low tide during spring 2005, autumn and winter 2006, spring and summer 2007, and autumn 2008. Epiphytic samples were taken during summer, autumn, and spring 2000 and spring 2002. In total, 224 benthic species were identified in the 120 samples analyzed; 81 species had a frequency greater than 5% in the total sample dataset with more than 1% of relative abundance in at least 1 sample and were chosen for estimation of their optima and tolerances for selected water-quality characteristics. The physicochemical data analyzed indicated 2 gradients -increases in conductivity and turbidity along with decreases in the concentration of nutrients and organic matter -that generated different types of habitats for the species investigated.KEY WORDS: Diatoms · Optima · Tolerance range · Autecology · Epipsammon · Epiphyton · Coastal-plain estuary · Río de la Plata Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 418: [105][106][107][108][109][110][111][112][113][114][115][116][117] 2010 optima and tolerances for various aspects of water quality such as pH, salinity, temperature, nutrients, and light availability (i.e. turbidity;Cooper 1995). In a freshwater ecosystem, the dependence of benthic diatoms on nutrients has been so unequivocally clarified that microalgae have been classified according to trophic and saprobity classes and are thus used as sensitive indicators of water quality (Sláde

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