The characteristics of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) in Antarctic sea ice

Abstract

a b s t r a c tAn investigation of coloured dissolved organic matter (CDOM) and its relationships to physical and biogeochemical parameters in Antarctic sea ice and oceanic water have indicated that ice melt may both alter the spectral characteristics of CDOM in Antarctic surface waters and serve as a likely source of fresh autochthonous CDOM and labile DOC. Samples were collected from melted bulk sea ice, sea ice brines, surface gap layer waters, and seawater during three expeditions: one during the spring to summer and two during the winter to spring transition period. Variability in both physical (temperature and salinity) and biogeochemical parameters (dissolved and particulate organic carbon and nitrogen, as well as chlorophyll a) was observed during and between studies, but CDOM absorption coefficients measured at 375 nm (a 375 ) did not differ significantly. Distinct peaked absorption spectra were consistently observed for bulk ice, brine, and gap water, but were absent in the seawater samples. Correlation with the measured physical and biogeochemical parameters could not resolve the source of these peaks, but the shoulders and peaks observed between 260 and 280 nm and between 320 to 330 nm respectively, particularly in the samples taken from high light-exposed gap layer environment, suggest a possible link to aromatic and mycosporine-like amino acids. Sea ice CDOM susceptibility to photo-bleaching was demonstrated in an in situ 120 hour exposure, during which we observed a loss in CDOM absorption of 53% at 280 nm, 58% at 330 nm, and 30% at 375 nm. No overall coincidental loss of DOC or DON was measured during the experimental period. A relationship between the spectral slope (S) and carbon-specific absorption (a n 375 ) indicated that the characteristics of CDOM can be described by the mixing of two broad end-members; and aged material, present in brine and seawater samples characterised by high S values and low a n 375 ; and a fresh material, due to elevated in situ production, present in the bulk ice samples characterised by low S and high a n 375 . The DOC data reported here have been used to estimate that approximately 8 Tg C yr À 1 ( $ 11% of annual sea ice algae primary production) may be exported to the surface ocean during seasonal sea ice melt in the form of DOC.

Role of sea ice in global biogeochemical cycles: emerging views and challenges

“…As DOC is further concentrated within sea ice brines, organisms living in brine are exposed to DOC concentrations that are up to 3 orders of magnitude higher than in seawater. Coloured Dissolved Organic Matter (CDOM) constitutes a significant fraction of the sea ice DOM pool and can significantly contribute to the attenuation of sunlight (particularly in the UV wavelength ranges) and serve as substrate for photochemical reactions that can remineralize the CDOM breaking it down into more labile compunds [Belzile et al 2000;Norman et al, 2011].…”

Chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic

“…Studies in the strongly fluvially impacted subarctic Gulf of Finland have demonstrated the importance of CDOM as a solar UV absorber within the sea ice (Ehn et al, 2004;Uusikivi et al, 2010), the influence of riverine input on the source composition of sea ice CDOM (Granskog et al, 2006), and the distinct optical properties of CDOM in sea ice as compared to that in under-ice seawater (Ehn et al, 2004;Granskog et al, 2005). More recently, surveys have been made on the distribution, photoreactivity (Norman et al, 2011), and fluorescent characteristics (Stedmon et al, 2011b) of DOM and CDOM in Antarctic sea ice in the Weddell Sea and the South Indian Ocean off Wiles Land. The CDOM in Antarctic sea ice contains mycosporine-like amino acids (MAAs) (Norman et al, 2011) and amino-acid-like fluorescent materials that are unique to sea ice brine (Stedmon et al, 2011b).…”

“…More recently, surveys have been made on the distribution, photoreactivity (Norman et al, 2011), and fluorescent characteristics (Stedmon et al, 2011b) of DOM and CDOM in Antarctic sea ice in the Weddell Sea and the South Indian Ocean off Wiles Land. The CDOM in Antarctic sea ice contains mycosporine-like amino acids (MAAs) (Norman et al, 2011) and amino-acid-like fluorescent materials that are unique to sea ice brine (Stedmon et al, 2011b).…”

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

“…Microbial EPS exist in a dynamic equilibrium from dissolved polysaccharides (dEPS <0.2 μm) to complex particulate EPS that can form gels on the millimeter to centimeter scale (8). Here we focus on the biologically relevant dissolved carbohydrates (dCHO) that constitute a substantial fraction of the DOC in sea ice (9)(10)(11)(12)(13) (Fig. 1).…”

“…1). dCHO are concentrated from sea ice DOC by dialysis (>8 kDa), with subsequent treatment allowing the definition of four subcomponents of the total dCHO pool: (i) dissolved uronic acids (dUA), produced by ice diatoms and ice bacteria (14)(15)(16), that confer strong cross-linkages between polymer chains (8), forming low solubility EPS complexes within brine channels (8,14,17); (ii) dEPS, produced by sea ice algae (9,12,18,19) and isolated from dCHO by 70% (vol/vol) alcohol precipitation; (iii) a low solubility fraction of dEPS obtained by 30% (vol/vol) alcohol precipitation, containing complex EPS molecules (dEPS complex ), often produced by algae with reduced biological activity or when under physiological stress (9,13,19); and (iv) a fraction of highly soluble carbohydrates that are not considered EPS (dCHO non-EPS ), do not precipitate in alcohol, and are produced by many actively growing ice algae (9,14).…”