## 4 Metals
For example, concentration ranges of 0.42-84.75 mg/kg for Pb [85] and of 0.02-0.690 mg/kg for Hg [110] were reported in the Danube river, and maximum values of 47.45 mg/kg for Pb and 0.99 mg/kg for Hg were reported by [PERSON] et al. in the Sf. Gheorghe Branch of the Danube Delta [121], while a maximum value of 263 mg/kg Pb was measured in the Begej canal [84] and 1.7 mg/kg Hg in the Olt river reservoirs [115]. Comparable maximum values were reported in the Danube River and Danube Delta for Cd (1.33 mg/kg [85] and 1.34 mg/kg respectively [121], and Zn (217 mg/kg [85] and 209.8 mg/kg [123]), whereas, maximum values of 3.26 mg/kg Cd and 975 mg/kg Zn were measured in the Begej Canal [84]. For As, a concentration range of 1.06-16.96 was reported in the Danube river, a maximum concentration of 20.55 mg/kg was measured in the Sf. Gheorghe Branch of the Danube Delta [121], and 43 mg/kg in the Begej canal [84]. Four multi-annual studies were identified concerning the presence of metals in water, sediment, and fish [85; 111; 123; 130]. A descendant trend in the concentrations of Pb and Zn in water samples from the Somova-Parches aquatic complex (Danube delta) between 2007-2012 was reported by [PERSON] et al. [123] and attributed to \"reducing emissions from the surrounding industrial activity\". In the lower section of the Danube River (km 375-km 175), mostly homogeneous evolution in time of metals concentrations in sediments was reported by [PERSON] et al. in a six-year study [85]. There were seasonal and age-dependent dynamics of Cu and Zn in different freshwater fish in Driester and Prut for the period between 2005 and 2010 [130]. The concentrations of heavy metals in sediments revealed seasonal variation and significant differences between the sampling sites [107; 113; 123]. The bioaccumulation capacity of these pollutants was studied by determining metals in microalgae [127] aquatic plants [128], plankton [122], mollusks [129], and fish [117; 137]. However, the diversity of the species studied, differences in the expression of the results (dry weight or wet weight), or different target analytes led to a difficult comparative analysis of the results. Nevertheless, the studies showed correlations between the heavy metals concentrations in water, sediments and the biota, especially for As, Cd, Pb, Cu, Ni, Cr, Hg, Co, and Zn, with various bio-concentration factors (BCF) depending on the biota species [109; 123; 125; 128; 129]. Exceeding of the maximum allowable concentration of heavy metals in surface waters according to the Directive 2013/39/EU [35] and Romanian legislation (Order 161/2006) [138] concerning environmental quality standards (EQS) for priority substances in the field of water policy were frequently reported. Cd concentrations exceeded the maximum EQS of 1.5 ug/L in the tributaries West Morava (reported value of 50 ug/L) [109] and Yantra (reported value 14.9 ug/L) [111], in the Danube river near Galati (km 150) and Tulcea (km 71) town (reported values of 15.7 50 ug/L and 18.4 50 ug/L respectively) [117], and in the Biosphere Reservation of Danube Delta (reported range of 3.5-10.5 ug/L) [123]. Pb concentrations exceeded the EQS of 14 ug/L in the Danube river near Galati and Tulcea towns (reported value of 21.6 ug/L and 14.6 ug/L respectively) [117] and in the Danube Delta Biosphere Reserve (maximum value of 48.06 ug/L) [122]. For Hg, concentrations exceeding the EQS of 0.07 ug/L were reported in the tributary Yantra River (reported value 68.15 ug/L) [111], in the tributary Olt river (maximum 1.5 ug/L) [107], and in the Danube river (reported value of 0.117 ug/L) [112]. Concentrations of Ni exceeding the EQS of 34 \(\mathrm{\SIUnitSymbolMicro g}\)/L were reported in the Danube Delta by [PERSON] et al., (maximum concentration of 76.4 \(\mathrm{\SIUnitSymbolMicro g}\)/L) [123] and [PERSON] et al., (maximum concentration of 79.1 \(\mathrm{\SIUnitSymbolMicro g}\)/L) [124].