Iron: Studies on the speciation and bioavailavility in the Baltic Sea

• Applicant: Dr. Christa Pohl (†)
• Subject Area: Atmospheric Science
• Term: from 2007 to 2012
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 47755900

The Baltic Sea is affected by anthropogenic and natural trace metals via atmospheric- and riverine input. The marine biogeochemical cycles and food webs respond to the addition of micronutrients to the marine environment. Entering the surface waters, the transport of trace metals is closely linked to the seasonal variation of the biological production and modification processes. In seawater Fe (Iron) belongs to the most important micronutrients. It mainly exists as organically complexed species, as insoluble Fe (III) species (oxy / hydroxyl) and to a lesser extent as dissolved Fe (II) species in ocean surface waters These Fe complexes are mostly removed from the euphotic zone by adsorption on or incorporation into particles and the transport through the water column. They are known to accumulated at horizontal interfaces / boundaries such as the pycnocline, the OMZ (oxygen minimum zone) or the redox boundary because of slow sinking rates. Redox interfaces with stagnant deep water bodies are typical for the central Baltic. Fe belongs to the redox-sensitive elements, which means that under anoxic conditions insoluble Fe(III) species are reduced to dissolved Fe(II) species. Together with phosphorus the Fe (II) species built up a soluble compound which is kinetically convenient. This is the reason for the increasing dissolved Fe(II) concentrations in the Baltic below the redoxcline. In the deeper anoxic layers dissolved Fe concentrations are decreasing because of the formation of the thermodynamically more stable compound Fe (II) sulphide, which equilibrates the soluble species. Parallel to the enrichment of insoluble FeS in the bottom sediments, there is a diffusive transport flux of dissolved Fe(II) spezies across the redoxcline in 150m depth. Fe catalyses the uptake of nitrogen in nitrogen fixing cyanobacteria. In this context it has been assumed, that dissolved Fe(II) is the more bioavailable species for the uptake during cyanobacteria growth. This leads to the main question: “Is Iron a limiting or a promoting factor of cyanobacteria blooms in the Baltic Sea?” Therefore this project should combine how far the formation of stagnant deepwater up to anoxic conditions, the resulting change of Fe Speciation (reduction to Fe(II)) and the diffusive Fe(II) transport to the euphotic zone are responsible for the fertilisation and the increase of cyanobacteria blooms?

• DFG Programme: Research Grants