INTRODUCTION During the past three decades diatoms have been used worldwide to determine the ecological quality of freshwaters (see e.g., WHITTON et al. 1991, WHITTON & ROTT 1996, PRYGIEL et al. 1999). Diatoms fulfill the requirements of good indicator organisms in aquatic ecosystems. They occur in a wide variety of environments and show a broad range of tolerance along several gradients of abiotic factors, while individual species have specific water chemistry requirements (ROUND 1991). Finally, sampling and processing are rela- tively simple and cost-effective. Standardized phytobenthos-based monitoring networks are one of the targets of the Water Frame- work Directive (WFD, EUROPEAN P ARLIAMENT & EUROPEAN COUNCIL 2000), a legislative set that aims at assessing and restoring the ecological status of inland water bodies throughout EU countries. This ecological status is defined as a deviation measurement between the characteristic structure of aquatic biota and the reference conditions of the same parameter. The reference conditions corre- spond to a water system with no or minor anthro- pogenic impacts (ECTOR & RIMET 2005). The use of benthic diatoms for biomonitor- ing purposes usually implies the application of DIATOM ASSEMBLAGES AND WATER QUALITYASSESSMENT IN THE DUERO BASIN (NW SPAIN) Belg. J. Bot. 141 (1) : 39-50 (2008) © 2008 Royal Botanical Society of Belgium S. BLANCO 1,2,* , L. ECTOR 2 , V. HUCK 2 , O. MONNIER 2 , H.M. CAUCHIE 2 , L. HOFFMANN 2 and E. BÉCARES 1 1 Área de Ecología, Universidad de León, E-24071 León, Spain 2 Department of Environment and Agro-biotechnologies, Public Research Centre - Gabriel Lippmann, 41, rue du Brill, L-4422 Belvaux, Grand-Duchy of Luxembourg ( * Author for correspondence; e-mail: sblal@unileon.es) Received 17 May 2007; accepted 9 October 2007. ABSTRACT. — In order to determine water quality status in the Duero basin (NW Spain), epilithic diatom samples were collected and analyzed in 137 stations in August 2004 following normalized standard protocols. The floristic particularities of the diatom assemblages in the basin are discussed. A total of 429 diatom taxa were identified in the basin and 90 taxa (21%) were new for the Iberian Peninsula. Especially noticeable was the presence of Achnanthidium rivulare, only known until now from North America. Ordination revealed the existence of five species assemblages related to the following environmental factors: 5-Day Biochemical Oxygen Demand (BOD 5 ), [PO 4 3- ], and conductivity. Light and scanning electron microscopy micrographs are provided for some common, infrequent or exotic diatom species present in the Duero basin. Three diatom indices (Specific Pollution Index SPI, European Index CEC and Biological Diatom Index BDI) were applied. All of them correlated significantly with water physical and chemical variables, but SPI achieved the best correlations, and is therefore recommended as the reference diatom index for this Mediterranean basin. KEY WORDS. — Bacillariophyta, bioindicators, diatom indices, Mediterranean Basin, Pollution Sensivity Index, Water Framework Directive.