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Changes in dissolved nutrients in the lower Ebro river: Causes and consequences

Abstract:

Water quality time series data regarding nutrient concentration and chlorophyll in the lower Ebro River and its estuary were analyzed over an 18 year period (1987-2004). Causes and consequences of changes in nutrient concentration are discussed and some hypotheses about it are established. A very significant decrease in the concentration of the dissolved nutrients, especially phosphorus, was observed after the mid 90s. This decrease coincides with the improvement of urban sewage treatment in the most important cities of the Ebro basin. There was also a significant positive correlation between the concentration of dissolved phosphorus and the concentration of total chlorophyll. Data suggest that low flow conditions together with the lower dissolved phosphorous and, therefore, less phytoplankton were likely the main factors causing the increase of water transparency. These conditions propitiated the spread of macrophytes and the massive appearance of the black fly. In the lower river and its estuary, the water undewent a decrease in phytoplankton, which improved its eutrophy condition. Results of this study suggest that the decrease in phosphorus is the main cause of the recent changes in the lower Ebro ecosystem, but other factors can also play a role by causing a further increase in water transparency and macrophyte spreading (positive feed-back). © Asociación Ibérica de Limnología.