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Fluxes of dissolved forms of iron and manganese across the sediment-water interface were studied in situ in the Gulf of Finland and the Vistula Lagoon (Baltic Sea), and in the Golubaya Bay (Black Sea) from 2001 to 2005. Fluxes were measured using chamber incubations (Jch), and sediment cores were collected to assess the porewater and solid phase metal distribution at different depths to calculate diffusive flux (Jpw). Measured and calculated benthic fluxes of manganese and iron were directed out of sediment for all sites and were found to vary between 70-4450 and 5-1000 mole m-2 day-1 for manganese and iron, respectively.
Benthic fluxes of manganese were found to correlate with manganese concentration in the porewater of the top sediment layer positively. The manganese fluxes were not influenced by redox conditions in the near-bottom water. On a large timescale manganese fluxes depend on redox conditions in bottom water, because a change of redox conditions would lead to a change of dissolved manganese content of the porewater. However, on a timescale of a single chamber incubation manganese fluxes did not depend on oxygen concentration (redox conditions), because for dissolved manganese the rate of oxidation is much lower than the rate of release from sediment. Diffusive flux of manganese constitutes 25-70% of the measured flux determined with the chamber incubation (on average Jch/Jpw=3). This indicates that another processes such as bacterial dissolution or bio-irrigation also play an important role in manganese benthic flux formation.
Our results showed the importance of bottom water redox conditions for benthic iron fluxes. We measured no fluxes at oxic conditions, intermediate fluxes at anoxic conditions and high fluxes at suboxic conditions. Oxidation of released dissolved iron occurred very rapidly under oxic conditions right after its release from the sediment. Under suboxic and anoxic conditions in the bottom water, the iron flux was dependent both on its concentration in porewater of the surface sediment and on the content of iron in the solid phase of sediment. The chamber measured and diffusive fluxes of total iron differed much more than for manganese, 0.01Jch/Jpw50. Thus, bio-irrigation, sediment resuspension and chemical processes at the interface are much more important for iron fluxes than for manganese fluxes.
The flux of iron was dominated by iron(II) under the studied conditions. Contribution of iron(III) to the total iron flux did not exceed 20%. In the Vistula Lagoon metal-organic complexes contributed up to 20% of the total manganese flux, and up to 70% of the total iron flux. Organically-complexed manganese and iron at the other stations were mostly not involved in the benthic exchange.
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