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Introduction Although biological monitoring is considered a fundamental tool in Europe (European Uníon, 2000) and in other developed countries´ water legislations, in Argentina it only concerns research and there are no standardized methodologiesto be applied in water management activities. Rivers have been managed from a political and an engineering point of view and the associated specific environmental problems (e.g. floods, pollution) have been solved with local measures leading to the collapse of living riverine systems. In the last decades a new paradigm in the evaluation of surface waters quality was identified and concepts like biotic integrity or ecological status were defined to adequately managewater resources. The Río Quequén Grande watershed has a surface of about 9.940 km2and is located in the southeast of Buenos Aires province, Argentina. It outflows into the Atlantic Ocean. Data and investigation methodology The aim of this ongoing research is to test methodologies for the assessment of ecological status and to recognize natural and anthropogenic spatial heterogeneity of the Río Quequén Grande watershed. A multidisciplinary approach is being developed. The watershed has been characterized, using an integrated system of the geological, geomorphological, sedimentological, hydrological, geochemical and biologícal information and a Geographic Information System (GIS) has been developed. Topographic and bathymetric profiles, textural bed sediments analysis and chemical water characterization of the main tributaries and the main course are also available. A set of aerial photographs scale 1:20.000 of Río Quequén Grande were also analyzed. From the rectified aerial photos the active channel of the Río Quequén Grande was digítized and all the caliche outcrops that crossed the channel were mapped. These data together with fluvial cross sections were used for the hydraulic analysis trough the River Analysis System of the Hydrologic Engineering Center (US ARMYCORPSOFENGINEERS2,002). Landuse mapwas integratedand analyzed together GIS information. Bio-monitoring activities As a first approach, a biological monitoring campaign has been defined for integrating ecological information with preexistent data. In March 2007, nine sites were sampled. At each site water temperature, conductivity, pH and redox potential were measured in situ and samples for principal nutrients analyses were collected. General habitat quality was also evaluated and benthic algal communities - epipelon, epiphyton and epilithon, macroalgae - aquatic and riparian vegetation were sampled. The biological sampling was carried out following the European Standard EN 13946 (2003) norm, and Kelly et al. (1998). The general habitat assessment was implemented considering the characteristics of the channels and banks following U.S. EPA recommendations, modified in Toja et al. (2006). The specific composition of riparian vegetation was established on the basis of qualitative samples. Migration probes and estimations of percentage of living cells were held to evaluate the health of the epipelic community. The relative abundance of all algal groups was evaluated. For diatoms counts part of the samples was treated with peroxide to eliminate organic matter following European Standard EN 13946 (2003). Results and Discussion Pronounced differences in chemical parameters were detected, especially in relation to conductivíty that reached values of 13,000 ¡JS/cm(Table 1). At the proximity of the mouth of the Río Quequén Grande, the maximum conductivity was registered. Algae typical from brackish waters as the genus Entomoneís were found. Only in one of the studied sites (Arroyo Las Mostazas), a punctual source of pollution associated to drastic changes of the redox potential was identified. Diatoms dominated the periphytic communities of all the sampling sites. Only at the Arroyo (A) Pecado Castigado both blue-green and ,green alqae reached a relative abundance of near 20%. The analyses of the algal groups was also useful to evaluate the effect of an organic polluted dumping at the A Las Mostazas. The comparison of the algal specific composition showed clear differences in the relative abundance of the blue-green algae that increased to 25%. These differences were also reflected in the specific composition of diatom communities. Upstream the dumping, Nitzschía palea, a species characteristic of critical water quality, dominates, while downstream Nítzschíaumbonata, characteristic of bad quality water, is the most diffused species. The comparison of the results obtained from the different analyzed communities evidenced that the epilithon was the preferred indicator to monitor water quality of this watershed. The epipelon, proposed as water quality indicator in the Pampean region (Gómez and Licursi, 2001), was rejected due to the high percentage of died diatoms registered in the most part of the sites. Epiphyton varied in the different sectors of the basin in relation to the distribution of the macrophytes that support these communities. Although these are preliminary results, there are evidences that future monitoring based on algae must be focused on epilithic diatoms. The habitat assessment field data showed differences among the sites in a wide range of situations. The quality of the sites was generally insufficient with exception of the A Quequén Grande, that has a very good quality water. This situation can be related to a high flow and the presence of high bank that unable the cattle to access the river bed and the broad band of riparian native vegetation. This first experience in the application of the cIassifications proposed by the US EPA (Barbour et al., 1999) evidenced the necessity to create a cIassiftcation that takes into account the vegetation of the region characterized by the dominance of herbaceous species. The riparian vegetation showed a higher degree of native vegetation components at the A Pecado Castigado and a lower extent at the A. El Chancho that has a lower percentage of exotic species (Table 2). The first site is also characterized by a higher percentage of grasses and a lower percentage of dicotyledonous species. The presence of weed, exotic species and the increase of dicotyledonous species evidence disturbance, while richness anq increase of grasses show a major degree of uncontaminated conditions. Final considerations In general the preliminary biological results indicate that the monitored rivers have a insufficient water quality possibly related to the diffused pollution due to intensive agricultural activities. The research requires further investigations, bio-monitoring should be continued and it is foreseen the increasing of monitoring sites. Also, these first results indicate that in the future monitoring can be based on algae. Particularly the analyses should be focused to epilithic diatoms. Macrophytes could also give good information.

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