Iron, nutrients, and humic‐type fluorescent dissolved organic matter in the northern Bering Sea shelf, Bering Strait, and Chukchi Sea

Abstract

The concentrations of dissolved Fe ([DFe]), total dissolvable Fe, humic‐type fluorescence intensity (humic F intensity) as humic‐type fluorescent dissolved organic matter (humic‐type FDOM), and nutrients were determined vertically in the northern Bering Sea shelf (Yukon River estuary region and St. Lawrence Island polynya region), the eastern Chukchi Sea shelf (Chukchi shelf polynya region), and the central Bering Strait during 30 July to 12 August 2007 and 30 June to 12 July 2008. In early July 2008, with a nearly peak river water discharge period, remarkably high humic F intensity and high [DFe] were found with a decrease in salinity (S = 26–30) in the surface water of the Yukon River estuarine region. These results suggest that riverine humic‐type FDOM is responsible for [DFe] in coastal waters, with natural organic ligand complexation of Fe(III) apparently playing an important role in iron transport to the northern Bering Sea shelf. In the St. Lawrence Island and Chukchi shelf polynya regions, cold dense waters (T = −1.5 to −1.7°C and σθ = 26.2 to 26.7) were found in deep and bottom waters. The cold waters are characterized by high nutrient and iron concentrations and high humic F intensity, probably resulting from two main processes: brine rejection during sea ice formation and transport across the sediment‐water interface during early diagenesis. In the central Bering Strait, the waters are vertically uniform with high salinity (S ≥ 32.5) and high nutrient and iron concentrations. These waters appear to originate from the outer shelf of the Bering Sea and are not affected by denitrification occurring in bottom sediments of the Bering shelf.