Vertical distribution of iron(II) and its relation to organic substances in Lake Biwa, Japan

Iron speciation in freshwater is largely regulated by pH, dissolved oxygen and characteristics of dissolved organic compounds that coexist in the water. Solubility of iron(III) is very low in neutral pH, but some organic compounds serve as chelators with iron(III) to form dissolved stable complexes. In Lake Biwa, Japan, dissolved iron concentration is very high (mean 84 nmol L -l, median 28 nmol L -l in the north basin; Mrro et al. 2004). However, when taking into account iron complexation, the labile iron concentration is very low and may serve as a co-limiting factor of primary production, as seen in the eutrophic Lake Kasumigaura, Japan (NAGAI et al. 2006). Iron(II) is a minor component in oxic circumneutral freshwater because it is immediately oxidized to iron(III) species., It has high potential bioavailability for primary production, however, because of its high solubility and low complexation compared with iron(III) that sometimes form a stable organic complex unavailable to phytoplankton (IMAI et al. 1999). According to SIVAN et al. (1998), iron(II) exists in the epilimnion ofthe oxic freshwater Lake Kinneret in the order oftens o f nmol L -• While iron(II) is oxidized by Oz, 0 2·and H20 2 in water, colored dissolved organic matter (CDOM), such as humic substances, serves as a mediator o f photoreduction o f iron(III) to iron(II) at the lake surface (EMMENEGGER et al. 200 l). We analyzed the vertical profiles of iron(II) in summer and autumn in Lake Biwa. Because the concentration of iron(II) is estimated to be low, solid phase extraction of iron(II)ferrozine complex was investigated using a modified method ofYI et al. (1992). To verify the photoreductive formation of iron(II) at the lake surface, the water of Lake Biwa was irradiated by sunlight.