Assessment of nutrients and oxygen regime in the Lower Danube water

Abstract

The oxygen regime and nutrients are among the quality indicators covered by the Water Framework Directive 2000/60/EC (WFD) that must be monitored in order to establish the ecological status of the surface water. The WFD defines, for each indicator, five quality classes that are transposed in Romania by Ministerial Order (M.O.) 161/2006. The main aim of this study is to assess the oxygen and nutrient regime of the Lower Danube water according to M.O. 161/2006. 35 water samples were collected monthly from 7 sampling stations situated near the cities of Braila and Galati during the period of March-July 2019. Three oxygen regime indicators i.e. Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD₅), Chemical Oxygen Demand (COD), and five nutrients i.e. Nitrate nitrogen (N-NO₃^-), Nitrite-nitrogen (N-NO₂^-), Ammonium-nitrogen (N-NH₄⁺), Total nitrogen (TN) and Ortho-phosphates (P-PO₄^3-) were analyzed in this study. The results showed that DO and BOD₅ were parameters which recorded significant temporal variations (spring-summer) in all 7 sampling station. DO levels varied from values specific for the quality Class I in spring to values specific for the quality Class III during the summer, while BOD₅ values varied from Class I to Classes II-IV. Following the evaluation of the nutrient values, the Danube water quality was classified in Classes I, II, or III. Nutrient concentrations corresponding to a lower quality were recorded near the confluence areas of the Danube with the Siret and the Prut Rivers (S3 and S6 sampling stations).