Evidence for significant protein-like dissolved organic matter accumulation in Sea of Okhotsk sea ice

Mats A. Granskog,Susann Müller,Daiki Nomura,Takenobu Toyota,Hiroshi Hattori,Andreas Krell

doi:10.3189/2015aog69a002

Abstract

AbstractAbsorption and fluorescence of chromophoric dissolved organic matter (CDOM) in sea ice and surface waters in the southern Sea of Okhotsk was examined. Sea-water CDOM had featureless absorption increasing exponentially with shorter wavelengths. Sea ice showed distinct absorption peaks in the ultraviolet, especially in younger ice. Older first-year sea ice had relatively flat absorption spectra in the ultraviolet range. Parallel factor analysis (PARAFAC) identified five fluorescent CDOM components, two humic-like and three protein-like. Sea water was largely governed by humic-like fluorescence. In sea ice, protein-like fluorescence was found in considerable excess relative to sea water. The accumulation of protein-like CDOM fluorescence in sea ice is likely a result of biological activity within the ice. Nevertheless, sea ice does not contribute excess CDOM during melt, but the material released will be of different composition than that present in the underlying waters. Thus, at least transiently, the CDOM introduced during sea-ice melt might provide a more labile source of fresher protein-like DOM to surface waters in the southern Sea of Okhotsk.

Highlights

The Sea of Okhotsk belongs to the seasonal sea-ice zone and is at the lowest latitude at which sea ice is found, with maximum ice extent of 50–90% in late winter (Granskog and others, 2010)
We examine the composition of chromophoric dissolved organic matter (CDOM) in sea ice and under-ice waters in the southern Sea of Okhotsk and the adjacent Saroma-ko lagoon in late winter and spring
We present the first data on CDOM absorbance and fluorescence of sea ice in the region, as well as in underlying surface waters

Summary

Introduction

The Sea of Okhotsk belongs to the seasonal sea-ice zone and is at the lowest latitude at which sea ice is found (besides the Bohai Sea), with maximum ice extent of 50–90% in late winter (Granskog and others, 2010). The northern Sea of Okhotsk receives significant terrestrial inputs of dissolved organic matter (DOM) from the Amur River, which during ice formation are injected to deeper waters, together with brine, and transported in the subsurface into the southern Sea of Okhotsk and further to the Pacific (e.g. Nakatsuka and others, 2004). Water masses are composed of a mixture of waters entering the region, where the Soya current brings warmer waters from the Japan Sea, Pacific waters enter through the Kuril Straits, and the Sakhalin current transports waters from the northern shelf to the south. The water masses in the eastern part of Saroma-ko lagoon, located adjacent to the Sea of Okhotsk on the northern shore of Hokkaido, Japan, consist mainly of adjacent Okhotsk Sea water, the freshwater input from the Saromabetsu River and seasonally of ice melt (Shirasawa and Lepparanta, 2003; Nomura and others, 2009). Almost the entire surface of Saroma-ko lagoon is generally covered with sea ice from early January through early April, with large year-to year variability

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