3.2. Vertical profiles of sea-ice nutrient concentrations
Figure 4 presents vertical profiles of NO3, PO43−, and Si(OH)4concentrations in first-year and multi-year ice. NO3 concentrations in first-year ice varied widely with depth in core JARE60_KU1 (0–2.6 μmol L−1), but were in the range 0–1 μmol L−1 in other cores (Figure 4a). NO3 concentrations in multi-year ice were 0–0.5 μmol L−1 in the top half of the ice column, and the first maximum at those depths, peaking at ~1.0 μmol L−1, was observed at a depth of –200 cm in some ice cores (Figure 4e). At greater depths in the cores, NO3 concentrations were as low as 0–0.5 μmol L−1, and the maximum of 3.5 μmol L−1 was observed at the bottom of the sea ice. In first-year ice, PO43− concentrations varied between cores but were fairly constant at shallow to medial depths, and were greatest at the bottom of the ice column (Figure 4b). In multi-year ice, however, PO43−concentrations were extremely low from the ice–snow interface to a depth of –150 cm, and the maximum concentration of 1.6 μmol L−1 was observed at a depth of –360 cm in core JARE57_P31.5 (Figure 4f). Si(OH)4 concentrations varied little with depth in the first-year ice (Figure 5c), but mimicked the PO43− profile in multi-year ice.
Our results show clear differences between the vertical nutrient profiles in first-year and multi-year ice (Figure 4). First-year ice generally has the lowest nutrient concentrations in snow, and concentrations increase slightly in sea ice, but they remain relatively constant until peaking at the bottom of the ice column (Figure 4a–c). In contrast, in multi-year ice, the upper half of the ice column was nutrient poor, similar to the overlying snow, and nutrient concentrations increased from the middle of the column to maximum concentrations in the bottom half of the column (Figure 4e–g). Because nutrient concentrations in snow were consistently low (Figure 4e–g), the upper half of the multi-year ice columns, comprising snow-origin ice, also contained low nutrient concentrations.
Such vertical nutrient distributions are considered to be closely related to physical characteristics such as salinity, δ18O value, and ice structure (discussed in Section 3.3). Furthermore, the high concentrations of various nutrients at the bottom of both first-year and multi-year ice columns are most likely related to the high concentrations of chl.a in the bottommost ice column (Figure 4d, h), which undergoes both photosynthesis by the ice algae and remineralization by the degradation of organic matter (Roukaerts et al., 2021; Rysgaard & Glud, 2004; Rysgaard et al., 2008; Thomas et al., 1995) and receives nutrients supplied from under-ice water (Nomura et al., 2009; Vancoppenolle et al., 2010).
The mean value and standard deviation of each nutrient component and physical parameter are reported in Table 3 for snow and each sea-ice type. Nutrient and chl.a concentrations were low in snow and superimposed ice and particularly high in columnar ice. In contrast, snow-ice, which is affected by both snow and seawater, had intermediate nutrient and chl.a concentrations. Our results clearly indicate that a greater snow influence results in lower nutrient and chl.a concentrations in sea ice.