Abstract. The precipitation of ikaite (CaCO 3 ·6H 2 O) in polar sea ice is critical to the efficiency of the sea ice-driven carbon pump and potentially important to the global carbon cycle, yet the spatial and temporal occurrence of ikaite within the ice is poorly known. We report unique observations of ikaite in unmelted ice and vertical profiles of ikaite abundance and concentration in sea ice for the crucial season of winter. Ice was examined from two locations: a 1 m thick land-fast ice site and a 0.3 m thick polynya site, both in the Young Sound area (74 • N, 20 • W) of NE Greenland. Ikaite crystals, ranging in size from a few µm to 700 µm, were observed to concentrate in the interstices between the ice platelets in both granular and columnar sea ice. In vertical sea ice profiles from both locations, ikaite concentration determined from image analysis, decreased with depth from surface-ice values of 700-900 µmol kg −1 ice (∼25 × 10 6 crystals kg −1 ) to values of 100-200 µmol kg −1 ice (1-7 × 10 6 crystals kg −1 ) near the sea ice-water interface, all of which are much higher (4-10 times) than those reported in the few previous studies. Direct measurements of total alkalinity (TA) in surface layers fell within the same range as ikaite concentration, whereas TA concentrations in the lower half of the sea ice were twice as high. This depth-related discrepancy suggests interior ice processes where ikaite crystals form in surface sea ice layers and partly dissolve in layers below. Melting of sea ice and dissolution of observed concentrations of ikaite would result in meltwater with a pCO 2 of <15 µatm. This value is far below atmospheric values of 390 µatm and surface water concentrations of 315 µatm. Hence, the meltwater increases the potential for seawater uptake of CO 2 .

We present the first year-round estimates of benthic primary production at four contrasting shallow (3-22 m depth) benthic habitats in a southwest Greenland fjord. In situ measurements were performed using the noninvasive aquatic eddy-correlation (EC) oxygen (O 2 ) flux method. A series of high-quality multiple-day EC data sets document the presence of a year-round productive benthic phototrophic community. The shallow-water sites were on average autotrophic during the spring and summer months, up to 43.6 mmol O 2 m 22 d 21 , and heterotrophic or close to metabolic balance during the autumn and winter. Substantial benthic gross primary production (GPP) was measured year-round. The highest GPP rates were measured during the spring, up to 5.7 mmol O 2 m 22 h 21 (136.8 mmol O 2 m 22 d 21 ), and even at low light levels (, 80 mmol quanta m 22 s 21 ) during late autumn and winter we measured rates of up to 1.8 mmol O 2 m 22 h 21 (43.2 mmol O 2 m 22 d 21 ) during peak irradiance. The benthic phototrophic communities responded seasonally to ambient light levels and exhibited year-round high photosynthetic efficiency. In situ downwelling irradiances as low as , 2 mmol quanta m 22 s 21 induced an autotrophic response and light saturation indices (I k ) were as low as 11 mmol quanta m 22 s 21 in the winter. On an annual timescale, the average areal rate of benthic GPP was 11.5 mol O 2 m 22 yr 21 , which is , 1.4 times higher than the integrated gross pelagic primary production of the , 30-50 m deep photic zone of the fjord. These results document the importance of benthic photosynthesis on an ecosystem level and indicate that the benthic phototrophic compartment should be accounted for when assessing carbon and nutrient budgets as well as responses of coastal Arctic ecosystems to climate change.

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