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.