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The application of remote sensing technology to water quality monitoring has special significance for lake management at regional scale. Water clarity expressed in terms of Secchi disk transparency (SDT) is a highly useful indicator of trophic status and ecosystem health. In this study, we related Landsat TM and ETM+ data with ground observations to develop a model for the estimation of SDT which can be used as a standardized procedure for regional-scale lake clarity assessment in the central region of Argentina. Samples were taken from two reservoirs of the region. Pearson correlation coefficients and step-wise multiple regression analysis were used to evaluate correlation between Landsat bands and measured SDT. Results suggested that Landsat band 3 plus the ratio 1/3 was a consistent and reliable predictor of SDT (R2 = 0.80). The algorithm was validated (R2 = 0.81) and applied to the November 10, 2010 ETM+ image obtaining a map that characterized water clarity of reservoirs within the study area. The procedure presented here could become a low cost measurement tool for water management authorities and decision-makers, obtaining simpler and practical results for regional water clarity monitoring.

The aim of this study was to evaluate the co-occurrence of some frequent fecal indicator bacteria and cyanobacteria species at San Roque reservoir (SRr), highlighting the relationship between their variability and environmental factors. This study was carried out at SRr (31 22 0 S, 64 27 0 O) located west of the city of Cordoba, Argentina. Physico-chemical measurements and sampling for microbiological and nutrient determinations were done at three recreational sites, during seasons of major touristic activity (springsummer), with the aim of identifying temporal patterns of microorganism variability. In addition, we carried out laboratory experiments with local isolated bacteria (Escherichia coli and Enterococcus spp.) and axenic Microcystis aeruginosa cultures to assess an association among these microorganisms and evaluate whether it was positive or negative. Our results showed an inverse relationship between cyanobacteria and thermotolerant coliform bacteria (TtC) at SRr, which was only observed when high cyanobacteria concentration occurred (!3.4 Â 10 4 to 1.2 Â 10 6 cell.mL À1). Experimental results also showed this type of relationship: E. coli and Enterococcus spp. decreased their growth in the presence of M. aeruginosa. In addition, it was observed that the variability of main phytoplanktonic species and TtC at SRr was mainly explained by temporal patterns, that is, the seasonal changes and the hydrological year. In summary, our data indicated that cyanobacteria high abundance during bloom periods could be an inhibiting factor for bacteria growth at SRr. Moreover, we demonstrated that nutrients such us P and N are not the main factors determining the variability of studied microorganisms in this eutrophic reservoir, but other factors, such as climatic conditions (temperature and precipitations), have a major influence. This study would provide a more comprehensive view on the dynamics of microorganism populations with sanitary relevance in eutrophic reservoirs.

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