Iron and macro-nutrient concentrations in sea ice and their impact on the nutritional status of surface waters in the southern Okhotsk Sea

Abstract

To elucidate the roles of sea ice in biogeochemical cycles in the Sea of Okhotsk, the concentrations of macro-nutrients (NO3+NO2, PO4, SiO2, and NH4) and trace elements (Fe, Al) were measured in samples of sea ice, overlying snow, and seawater. The oxygen isotope ratio (δ18O) in the sea ice was used to distinguish between snow ice and seawater-origin ice. Except for NH4, the macro-nutrient concentrations were lower in sea ice than in surface water in the ice-covered area. A linear relationship between salinity and concentrations of NO3+NO2, PO4, and SiO2 in the sea ice indicated that these macro-nutrients originated mainly from seawater.The Fe concentrations in sea ice were variable and several orders of magnitude higher than those in surface water in the ice-covered area. The Fe concentrations in the sea ice were positively correlated with Al concentrations, the suggestion being that the Fe contained in the sea ice originated mainly from lithogenic mineral particles. The annual Fe flux into the surface water from sea ice melting in the southern Sea of Okhotsk was estimated to be ∼740μmolFem−2yr−1. This flux is comparable to the reported annual atmospheric Fe flux (267–929μmolFem−2yr−1) in the western North Pacific. In spring, sea ice melting may slightly dilute macro-nutrient concentrations but increase Fe concentrations in surface water. These results suggest that sea ice may contribute to phytoplankton growth by release of Fe into the water column and have a large impact on biogeochemical cycles in the Sea of Okhotsk.