Dissolved organic matter in Antarctic sea ice David N. Thomas, 1 Gerhard Kattner, 2 Ralph Engbrodt, 2 Virginia Giannelli, 1 Hilary Kennedy, 1 Christian Haas, 2 Gerhard S. Dieckmann 2 1 School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey LL59 5EY,Wales 2 Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany ABSTRACT . It has been hypothesized that there are significant dissolved organic matter DOM) pools in sea-ice systems, although measurements of DOM in sea ice have only rarely been made.The significance of DOM for ice-based productivity and carbon turnover therefore remains highly speculative. DOM within sea ice from the Amundsen and Bellingshausen Seas, Antarctica, in1994 and theWeddell Sea, Antarctica, in1992 and 1997 was investigated. Measurements were made on melted sea-ice sections in 1994 and 1997 and in sea-ice brines in 1992. Dissolved organic carbon DOC) and dissolved organic nitrogen DON) concentrations in melted ice cores were up to 1.8 and 0.78 mM, respect- ively, or 30 and 8 times higher than those in surface water concentrations, respectively. However, when concentrations within the brine channel/pore space were calculated from estimated brine volumes, actual concentrations of DOC in brines were up to 23.3 mM and DON up to 2.2 mM, although mean values were 1.8 and 0.15 mM, respectively. There were higher concentrations of DOM in warm, porous summer second-year sea ice compared with colder autumn first-year ice, consistent with the different biological activity sup- ported within the various ice types. However, in general there was poor correlation between DOC and DON with algal biomass and numbers of bacteria within the ice. The mean DOC/DON ratio was 11, although again values were highly variable, ranging from 3 to highly carbon-enriched samples of 95. Measurements made on a limited dataset showed that carbohydrates constitute on average 35% of the DOC pool, with highly vari- able contributions of 1^99%. INTRODUCTION Sea ice provides several habitats that support at times rich and varied biological assemblages reviewed by Palmisano and Garrison, 1993; Ackley and Sullivan, 1994). Since the early 1980s, efforts to investigate the biology of sea ice have intensi- fied, and much of the work has focused on the composition, physiology and ecology of the algae that dominate the sea-ice assemblages. There is also a substantial literature on the het- erotrophic activity and composition of the microbial network within sea ice Helmke and Weyland, 1995; Grossmann and others,1996; and citations therein). Increasingly, studies have concentrated on the physical and chemical limits constraining the biology and how in turn the organisms influence the nature of the ice matrix in which they are contained. High rates of inorganic nutrient remineralization have been measured, or at least implied from measurements of inorganic nitrogen and phosphorus within closed ice. Several studies have endeav- oured to relate bacterial activity to algal standing stocks or more directly to primary production taking place within the ice. However, it is surprising that very few studies have been conducted into the link between these two components by measuring the production and fate of dissolved organic matter DOM). It has long been speculated that levels of DOM must be high within the ice Grossmann and Dieckmann, 1994; Grossmann and others, 1996; Gu«nther and others, 1999), although very few measurements have actually been made to qualify this Mel'nikov and Pavlov, 1978; Apollonio, 1980; Bunch and Harland, 1990; Thomas and others, 1995, 1998, 2001; Smith and others, 1997). By default even less work has been conducted into the characterization of the DOM, even in the broadest sense. However, recently Amon and others 2001) have shown a large contribution of neutral sugar and amino acid carbon to total dissolved organic carbon DOC) in DOM extracted from multi-year Arctic ice.They conclude that this material was freshly produced and of algal origin. Sources of DOM within sea ice are, as in the open water, excretion of organic matter from all of the biology and release of organic matter on organism death and cell lysis. DOM con- centrations can be enhanced by mechanical damage due to the dynamic nature of the brine channel system with changing temperatures Eicken, 1992; Weissenberger and others, 1992; and citations therein). Of course, during sea-ice formation DOM is also incorporated into the ice matrix from the sea water itself Haas and others,1999; Giannelli and others, 2001). C. Krembs and others unpublished information, 2000) have shown that sea-ice diatoms release substantial quanti- ties of exopolymeric substances EPSs) that can alter brine pore structure around diatoms and protect them from ice crystal damage during freezing. These substances can also interconnect pores and may significantly affect the hysteresis of brine pore space and the permeability for solutes and micro- organisms. Other ice-active organic substances released by ice diatoms that roughen ice surfaces are proposed to promote binding sites for attached species, or increase light scattering Raymond and others,1994). The magnitude and quality of DOM within the sea-ice matrix is not just of significance to the sea-ice microbial net- Annals of Glaciology 33 2001 # International Glaciological Society 297