Selective incorporation of dissolved organic matter (DOM) during sea ice formation

Abstract

This study investigated the incorporation of DOM from seawater into b2 day-old sea ice in tanks filled with seawater alone or amended with DOM extracted from the microalga, Chlorella vulgaris. Optical properties, including chromophoric DOM (CDOM) absorption and fluorescence, as well as concentrations of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved carbohydrates (dCHOs) and dissolved uronic acids (dUAs) were measured. Enrichment factors (EFs), calculated from salinity-normalized concentrations of DOM in bulk ice, brine and frost flowers relative to under-ice water, were generally N1. The enrichment factors varied for different DOM fractions: EFs were the lowest for humic-like DOM (1.0-1.39) and highest for amino acid-like DOM (1.10-3.94). Enrichment was generally highest in frost flowers with there being less enrichment in bulk ice and brine. Size exclusion chromatography indicated that there was a shift towards smaller molecules in the molecular size distribution of DOM in the samples collected from newly formed ice compared to seawater. Spectral slope coefficients did not reveal any consistent differences between seawater and ice samples. We conclude that DOM is incorporated to sea ice relatively more than inorganic solutes during initial formation of sea ice and the degree of the enrichment depends on the chemical composition of DOM.

Physical and bacterial controls on inorganic nutrients and dissolved organic carbon during a sea ice growth and decay experiment

“…We therefore conclude that the addition of riverine DOC, being half of the total DOC, notably changed the composition compared to the prevailing marine (mainly phytoplankton-derived) DOC in the seawater. Thus, the SWR mesocosms contained a higher proportion of refractory DOM than SW. Our data agree with the report that the more labile forms of DOC are better retained in sea ice than the refractory forms (e.g., humic acids) (Jørgensen et al, submitted for publication;Müller et al, 2013), and that the DOC_n concentrations in ice may be even lower than in the under-ice water when the water contains higher concentrations of soil-derived DOC (Granskog et al, 2005;Hagström et al, 2001). Furthermore, Dittmar and Kattner (2003b) referred to the intramolecular contraction and coiling of humic acids with increasing salinity to explain differences of their behavior in size-exclusion chromatography.…”

“…This explanation is at least true for fluorescent DOM, since optical measurements performed during this experiment showed a selective incorporation of different fluorescent DOM fractions in sea ice (i.e., amino-acid-like and humic-like fluorescent DOM) (Jørgensen et al, submitted for publication). Our range of EF for DOC is consistent with the one previously presented for artificially produced DOM (1.0-2.7) under similar ice growth conditions (Müller et al, 2013).…”

“…One explanation is that the direct incorporation favors the accumulation of dissolved compounds in sea ice, although this has only been shown for DOC Müller et al, 2013) and NH 4 + . This explanation is at least true for fluorescent DOM, since optical measurements performed during this experiment showed a selective incorporation of different fluorescent DOM fractions in sea ice (i.e., amino-acid-like and humic-like fluorescent DOM) (Jørgensen et al, submitted for publication).…”

“…Previous studies have indicated selective incorporation of DOM during sea ice formation (Aslam et al, 2012;Giannelli et al, 2001;Müller et al, 2013), raising the question as to whether or not there is a segregation among dissolved compounds during the incorporation phase, and in particular, whether the incorporation is comparable between Arctic and Antarctic sea ice because of the different compositions of DOM in the parent waters. Various physical mechanisms induce changes in the nutrient pools in ice after the initial incorporation.…”

Broad-scale predictability of carbohydrates and exopolymers in Antarctic and Arctic sea ice

“…Normalization of dCHO concentrations to seawater salinity [to estimate the degree of cryo-concentration of material, or losses during brine drainage (25,34)] revealed significant enhancement ( Fig. S1) relative to average seawater concentrations (20 μmol C L −1 at all locations).…”