Spatial and temporal variability of the water and sediments quality in the Alqueva reservoir (Guadiana Basin; southern Portugal)

The purpose of this work was to evaluate the dynamic of the water quality from the Alqueva reservoir (Guadiana River Basin, Portugal) and identify the most important parameters that influence its ecological and chemical status. The results could indicate preventive and/or remediation actions that are necessary to improve its quality and status. Water and sediment samples were collected between 2011 and 2012, at five sampling stations, and analyzed for: (i) water - pH, temperature, dissolved oxygen, electrical conductivity, chloride, total phosphorus, total nitrogen, ammonium, nitrate, nitrite, biochemical and chemical oxygen demand, total Fe, Mn, and As; and (ii) sediments - pH, electrical conductivity, organic matter, total nitrogen, total phosphorus, major and trace elements. The results from the water column showed that the organic descriptors exceeded the Portuguese guideline values for water quality for multiple uses at most of the sampling stations. As for nutrients, Ajuda is the station where the concentrations of the total nitrogen and total phosphorus exceeded the guideline values in most months. Ammonium achieved concentrations above the allowed, during the study, in all locations. Trace elements were more abundant in the sediments, surpassing the maximum levels for the protection of aquatic life for As, Cd and Pb, at Alcarrache, Lucefécit and Álamos, respectively. The use of multivariate analysis showed that the major parameters that explained the water quality variability were the nutrients in the water column, and trace elements in the sediments. Comparing the results from this study with results obtained since 2006, we can observe an obvious increment of the organic descriptors and nutrients in the water body. Further, several parameters and observations indicate an increase of the eutrophication process. So, it is urgent to develop preventive actions and remediation processes to stop the degradation so as to improve the quality of the water in this reservoir.