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Abstract
Abstract. To indentify sources and transport mechanisms of iron in a coastal marine environment, we conducted measurements of the physiochemical speciation of Fe in the euphotic zone at three different locations in the Baltic Sea. In addition to sampling across a salinity gradient, we conducted this study over the spring and summer season. Moving from the riverine input characterized low salinity Bothnian Sea, via the Landsort Deep near Stockholm, towards the Gotland Deep in the Baltic Proper, total Fe concentrations averaged 114, 44, and 15 nM, respectively. At all three locations, a decrease in total Fe of 80–90% from early spring to summer was observed. Particulate Fe (PFe) was the dominating phase at all stations and accounted for 75–85% of the total Fe pool on average. The Fe isotope composition (δ 56Fe) of the PFe showed constant positive values in the Bothnian Sea surface waters (+0.08 to +0.20‰). Enrichment of heavy Fe in the Bothnian Sea PFe is possibly associated to input of aggregated land derived Fe-oxyhydroxides and oxidation of dissolved Fe(II). At the Landsort Deep the isotopic fractionation of PFe changed between −0.08‰ to +0.28‰ over the sampling period. The negative values in early spring indicate transport of PFe from the oxic-anoxic boundary at ∼80 m depth. The average colloidal iron fraction (CFe) showed decreasing concentrations along the salinity gradient; Bothnian Sea 15 nM; Landsort Deep 1 nM, and Gotland Deep 0.5 nM. Field Flow Fractionation data indicate that the main colloidal carrier phase for Fe in the Baltic Sea is a carbon-rich fulvic acid associated compound, likely of riverine origin. A strong positive correlation between PFe and chl-a indicates that cycling of suspended Fe is at least partially controlled by primary production. However, this relationship may not be dominated by active uptake of Fe into phytoplankton, but instead may reflect scavenging and removal of PFe during phytoplankton sedimentation.
Highlights
Iron (Fe), the fourth most abundant element of the Earth’s crust, has a dynamic and fundamental role in all earth surface systems
At the Bothnian Sea station, a weak stratification was developed in May, which became stronger towards the end of sampling period (Fig. 3)
The Bothnian Sea water is strongly terrestrial influenced, which is reflected in the general pattern of positive δ56Fe
Summary
Iron (Fe), the fourth most abundant element of the Earth’s crust, has a dynamic and fundamental role in all earth surface systems. The role of iron as a regulating element for phytoplankton in large regions of the world ocean, referred to as the High Nutrient, Low Chlorophyll regions, is well established (de Baar et al, 2005; Boyd et al, 2007). Low Fe open ocean systems are of interest for the understanding of Fe biogeochemistry, and knowledge about Fe cycling in high-Fe regimes (Boyd et al, 2007).
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