Biogeochemical Cycling of Mn and Fe in an Area Affected by Eutrophication: The Adriatic Sea

Measurements of dissolved Mn and Fe and total particulate Mn, Fe and Al were made on seawater samples collected in the Northern Adriatic Sea in different seasons (May 1994, June-July 1994, September 1994 and February 1995), and in the Central Adriatic Sea in June-July 1994. Porewaters were also analysed for dissolved Mn and Fe in February 1994, June 1994, September 1994 and February 1995. Seasonal observations of dissolved Mn and Fe and particulate excess Mn and Fe in the Northern Adriatic Sea have demonstrated pronounced temporal and spatial variability of their distribution. This variability was mainly due to: (a) storm events which broke down stratification of the water column and enhanced resuspension and advective transport of particulate forms, and (b) fluctuations of oxygen concentrations in the water column and sediment-water interface, related to the biodegradation of the organic matter produced during bloom periods. Large inputs of dissolved Mn and Fe were attributed to reductive solubilization and diffusion from the sediment-water interface. Particulate excess Mn and Fe concentrations were also increased by oxidation in the presence of oxygen of dissolved Mn and Fe, which had diffused from the sediment-water interface.