Nutrient distributions associated with snow and sediment-laden layers in sea ice of the southern Sea of Okhotsk

Abstract

Samples of first-year sea ice, snow and under-ice water were collected in the southern Sea of Okhotsk in mid-February 2007 and 2008 to elucidate the processes controlling nutrient concentrations in sea ice. Temperature, salinity, oxygen isotopic ratio ( δ 18O) and inorganic nutrient concentrations (NO 3, NH 4, NO 2, PO 4 and SiO 2) were measured. Sea ice was categorized into four types; snow–ice, frazil ice, columnar ice and a mixture of granular and columnar ice, based on δ 18O composition and ice texture. Frazil ice dominated the total ice thickness (52.8%), and columnar ice was sandwiched between frazil ice layers, indicating dynamic ice-growth processes such as rafting and ridging. Furthermore, the ice was banded by layers of particulate materials (sediment layers), which were frequently encountered during cruises. High NO 3 and NH 4 concentrations were found in snow and snow–ice implying that these were supplied from the atmosphere with snowfall and incorporated into the sea ice through snow–ice formation. In the sediment-laden layers, which were categorized as frazil ice, NO 2, PO 4 and SiO 2 concentrations were highest of all the ice types and considerably enriched compared to parent seawater, suggesting the remineralization of the particulate organic matter. On the other hand, NO 3 concentrations in sediment layers were low (depleted), leading to extremely low N (NO 3 + NH 4 + NO 2): P ratios in sediment layers, from 0.2 to 0.8, with respect to that of under-ice water or Redfield ratio. These results suggest that in part of sediment-laden layers fixed-nitrogen was removed partially as molecular nitrogen (N 2) from the sea ice environment by anaerobic nitrate reduction processes (denitrification) by denitrifying bacteria while adding phosphate from associated remineralization of organic phosphorus. The effect of melting of snow and sea ice is dilution for salinity, NO 3 and SiO 2, no change in NO 2 and PO 4, and a minor enrichment for NH 4 in the mixed layer in spring and early summer. This suggests that snow/ice meltwater with different nutrient ratios than in under-ice water/Redfield ratio is supplied to under-ice water during melt season in April/May in southern Sea of Okhotsk. However, the impact of sediment-laden sea ice cannot be assessed at this point.