The biogeochemistry of some trace metals in some eutrophic areas : the Adriatic Sea and the north-western Black Sea
In the northern Adriatic and north western Black Sea, relatively high concentrations of nutrients inducing eutrophication, have been observed. The biodegradation of organic matter, which occurred below the thermocline or at the sediment-water interface, influence the reduction-oxidation potential in the water column. Fluctuation of the reduction-oxidation potential in the water column influence the distribution of metals which are redox sensitive elements (Mn, Fe and Co). Dissolved Mn and Fe concentrations were high in some cases mainly due to the reduction of Mn- and Fe- oxides which occurred in microenvironments in the dominantly oxic medium at the thermocline or in low O2 zones near the bottom. The distributions were also influenced by riverine inputs. Dissolved Mn, Fe and Co concentrations were generally higher in the north-western Black Sea than in the Adriatic Sea suggesting that diagenetic reactions involving the destruction of organic carbon are more intense in the north-western Black Sea than in the Adriatic Sea. Total particulate concentrations of Mn and Fe were high, due to reprecipitation of Mn2+ and Fe2+ in presence of oxygen, riverine inputs and advective transport. Trace metal distributions (Co, Pb, Cd, Zn, Cu, Ni) were influenced by the Mn and Fe cycling and by riverine inputs. Relatively high dissolved and leachable or total particulate concentrations were generally found in surface waters at stations directly influenced by the River Po and the River Danube. High dissolved metal concentrations were found at the thermocline or near the sediment-water interface where dissolution of Mn- and Fe- oxides, on which metals were adsorbed, occurs under suboxic or anoxic conditions. Dissolved concentrations of Pb, Cd, Cu, Ni and Zn are generally similar in the northern Adriatic Sea and in the north-western Black Sea. Particulate Fe, Pb, Ni, Cd and Zn concentrations were higher in the northern Adriatic Sea than in the north-western Black Sea reflecting the strong influence of riverine inputs on the shallow northern part of the Adriatic where the depth hardly ever exceeds 40 m.