Defining benchmark values for nutrients under the Water Framework Directive: Application in twelve Portuguese estuaries

• Nutrient conditions for transitional waters were not established under the WFD.
• Spatial and temporal variability of dissolved nutrients was assessed in 12 estuaries.
• Benchmarks were calculated as the 90th percentile of nutrient concentrations for salinity intervals.
• Low scores for nutrients were identified in 6 water bodies of 5 estuaries.

The Water Framework Directive requires the assessment of physicochemical parameters supporting the biological quality elements. To achieve this evaluation, water samples were collected in 89 sites located at 42 water bodies (WB) from 12 transitional systems in Portugal. Nitrate, nitrite, phosphate, ammonium and silicate concentrations were measured in surface and near-bottom water samples collected at high- and low-tide. The influence of salinity, biological consumption, morphology and river basin inputs was examined. Results showed that nutrient variability is estuary and site specific. To overcome this inter-specificity complexity, benchmark values for nutrients were estimated based on Portuguese estuaries previously classified with good ecological status for the phytoplankton quality element. This methodology allowed classifying the chemical status for nutrients in the WBs of the surveyed systems. Water bodies from Cávado, Ave, Douro, Tagus and Sado showed “Low” status due to ammonium that was often above the benchmark value. Comparatively, nutrient concentrations in all WBs of Minho, Ria of Aveiro, Mira and Guadiana were lower than the benchmark values and therefore a classification of High was given.