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A Water Quality Index (WQI) is a numeric expression used to evaluate the quality of a given water body and to be easily understood by managers. In this study, a modified nine-parameter Scottish WQI was used to assess the monthly water quality of the Douro River during a 10-year period (1992-2001), scaled from zero (lowest) to 100% (highest). The 98,000 km(2) of the Douro River international watershed is the largest in the Iberian Peninsula, split between upstream Spain (80%) and downstream Portugal (20%). Three locations were surveyed: at the Portuguese-Spanish border, 350 km from the river mouth; 180 km from the mouth, where the river becomes exclusively Portuguese; and 21 km from the mouth. The water received by Portugal from Spain showed the poorest quality (WQI 47.3 +/- 0.7%); quality increased steadily downstream, up to 61.7 +/- 0.7%. In general, the water quality at all three sites was medium to poor. Seasonally, water quality decreased from winter to summer, but no statistical relationship between quality and discharge rate could be established. Depending on the location, different parameters were responsible for the episodic decline of quality: high conductivity and low oxygen content in the uppermost reservoir, and fecal coliform contamination downstream. This study shows the need to enforce the existing international bilateral agreements and to implement the European Water Quality Directive in order to improve the water quantity and quality received by the downstream country of a shared watershed, especially because two million inhabitants use the water from the last river location as their only source of drinking water.

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In this study, the effects of increasing copper (Cu) concentrations on the denitrification pathway and on the diversity of a denitrifier community and gene expression were evaluated in intertidal sandy sediments of the Douro River estuary (north-west Portugal). The results demonstrated that sediment denitrification rates were highly affected by Cu; almost complete inhibition (85%) of the process was observed in sediments amended with 60 μg Cu g(-1) wet wt sediment. Moreover, the addition of Cu stimulated the accumulation of both N(2)O and NO(2)(-) and inhibited the rate of NO(3)(-) uptake. Further, the amendment with even the lowest Cu concentration (4 μg Cu g(-1) wet wt sediment) yielded a drastic decrease in the abundance of nirK, nirS and nosZ (between 79% and 81%) assessed by means of real-time PCR. In agreement, reverse transcription-PCR-denaturing gradient gel electrophoresis analysis of nirK, nirS and nosZ transcripts showed a progressive decrease in the diversity of the transcription products of these genes with an increase of the Cu concentration.

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The Douro River drains a large part of the Iberian Peninsula before it flows into the Atlantic Ocean in northern Portugal. Although heavily dammed throughout its course, the Douro forms a mesotidal estuary in the last 22 km. No data had ever been collected in this estuary prior to 1994. An analysis of salinity data measured once a month in 1994 and current data obtained in the fall of 1994 revealed a salt wedge estuary highly dependent on river discharge: strong vertical salinity stratification under conditions of low runoff, whereas during high river flows the whole estuary became a river. The tide in the fall of 1994 was very close to a standing wave throughout the estuary; the tidal range, as high as 2.8 m at spring tides, was not powerful enough to destroy the vertical stratification. Tidal straining prevented mixing within the salt wedge during ebb; however, the column became vertically mixed at the end of flood. The salt wedge became arrested at three favored positions determined by the bathymetry. The salt intrusion, more sensitive to freshwater inflow than to tidal action, penetrated as far as 2/3 of the estuary's length. Residence times were estimated with a box model and were dependent on river discharge; they varied from 8 hours with early spring (March) inflows to 16 days under summer runoff conditions. © 2000 Ifremer/CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS Résumé-L'estuaire du Douro (Portugal) : coin salé mésotidal. Le Douro est un fleuve dont le bassin versant est le plus important de la Péninsule ibérique car il draine 17 % de son territoire. Avec un débit liquide moyen de 480 m 3 s-1 , le Douro se jette dans l'océan Atlantique à Porto. Le long du cours principal du fleuve, long de 930 km, et de ses tributaires, environ soixante grands barrages hydroélectriques contrô lent le flux d'eau douce arrivant à l'estuaire. L'estuaire mésotidal est actuellement confiné aux derniers 22 km. L'analyse des données (i) de salinité, obtenues entre janvier et décembre 1994 en marée de vive eau, et (ii) de courants durant l'automne 1994, a démontré l'existence d'une intrusion saline liée au débit du fleuve. Une situation de faible débit conduit à une stratification verticale prononcée. Quand le débit augmente, l'eau douce domine dans l'estuaire. Durant l'automne 1994, la marée était proche d'une onde stationnaire avec une amplitude de 2,8 mètres. Cependant, elle n'était pas suffisamment puissante pour perturber la stratification verticale de la colonne d'eau. La friction provoquée par le courant est le mécanisme de mélange le plus important, très active en amont de l'intrusion saline. Trois points nodaux principaux d'arrêt des eaux de mer, conditionnés par la bathymétrie, se localisent à 2,6; 7,5 et 15 km de l'embouchure. L'intrusion saline est donc plus sensible au débit du fleuve qu'à l'action tidale. La pénétration d'eau de mer vers l'amont couvre environ deux tiers de l'estuaire. Le temps de résidence de la masse d'eau dans l'estuaire, estimé a partir d'un modèle de boîtes, dépend fortement d...