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The productivity of Ria de Aveiro is sustained by the large quantities of dissolved inorganic nutrients, mainly from freshwater inputs (e.g. agricultural drainage) and organic-rich anthropogenic point sources (e.g. urban sewage). The concentration of silicate and nitrate showed a similar seasonal variation and conservative behaviour, since 63 to 96% of the total variance of the concentration of silicate and 88 to 96% of the total variance of the concentration of nitrate were explained by salinity. On the other hand, the concentration of nitrite, ammonium, and phosphate did not seem dependent on the freshwater inputs, showing a random distribution with salinity. Nutrient concentrations were always higher in the two inner areas than at the outer boundary of the system. Chlorophyll a showed higher concentrations throughout spring and summer, and also in the inner areas. The annual variation of N:P, N:Si and phytoplankton biomass suggests that there is a large excess of nitrate. The results suggest that within a system with low overall eutrophic condition (OEC) and low susceptibility (US-NEEA index), the quality status of different areas within the system can vary (Applying the EU-WFD criteria proposed for the trophic classification of the Baltic Sea). This work also suggests that in coastal lagoons such as Ria de Aveiro, where the quality status of certain specific areas may vary, these particular areas should be taken under consideration for specific monitoring programmes and management measurements, particularly with regard to anthropogenic point sources

Assessment of spatial environmental quality status in Ria de Aveiro (Portugal)