3.4. Variations of nutrient concentrations in seawater-origin ice
Although nutrient concentrations decreased as the snow fraction
increased in the topmost sea ice, the distributions of nutrient
concentrations differed among the different nutrient components (Figure
5). Therefore, we here evaluate the effects of primary production,
organic matter remineralization, and denitrification on nutrient
concentrations in both first-year and multi-year ice. We first compared
salinity to nutrient concentrations (Figure 7). Sea-ice salinity is a
conservative component that only changes through physical processes such
as brine drainage or exchange with under-ice water. Therefore, we
investigated deviations from a dilution line based on the salinity and
nutrient concentrations of the under-ice water collected during JARE60
to understand the biological processes affecting the nutrient
distributions in the sea ice. Data plotting above the dilution line
indicate nutrient supply due to the remineralization of organic matter
by heterotrophic organisms and bacteria in the sea ice or from external
sources (e.g., the atmosphere); conversely, data points below the
dilution line indicate nutrient uptake by ice algae during primary
production (e.g., Thomas et al., 1995).
Si(OH)4 concentrations in first-year ice locate
significantly below the dilution line (Figure 7a), whereas those in
multi-year ice are closer to or above the dilution line (Figure 7b). The
distributions of chl.a concentrations in the sea ice (Figure 4d, h)
indicate the presence of ice algae, which would mainly be diatoms
(Fragilariopsis sp. and Pleurosigma sp. (photo in 4c, d)
based on the cell counting for ice algae community assemblage in this
study. Indeed, in first-year ice, primary production by diatoms consumes
a large amount of Si(OH)4 (Tréguer & De La Rocha,
2013), and the slow dissolution of diatom frustules should limit the
resupply of Si(OH)4. In contrast, in multi-year ice, the
more protracted dissolution of diatom frustules should resupply
Si(OH)4, eventually balancing the consumption of
Si(OH)4 by primary production. However, in core
JARE57_P31.5 (the deepest core studied), Si(OH)4concentrations plot well above the dilution line (Figure 7b), indicating
that the resupply of Si(OH)4 by dissolution of diatom
frustules exceeded consumption.
Consistent with a previous sampling at the same location (Nomura et al.,
2018), PO43− concentrations plot above
the dilution line in both first-year and multi-year ice (Figure 7c, d),
suggesting resupply by the remineralization of organic matter. The
difference in the resupply rates of
PO43− and Si(OH)4 is
due to the fact that the remineralization of
PO43− is faster than the dissolution
of Si(OH)4 (Fripiat et al., 2017).
NO3− concentrations in most first-year
and multi-year ice cores plot below the dilution line (Figure 7e, f).
This is expected for first-year ice, in which primary production
consumes NO3−, as with
Si(OH)4. In contrast, remineralization in multi-year ice
should resupply NO3−, although our
data do not reflect this. Therefore, we also measured
NO2− concentrations (Figure 7g, h),
which plot close to the dilution line for first-year ice and are shifted
upward for multi-year ice. From this result, we expect that
denitrification resupplied NO2− over
longer periods in the multi-year ice. Indeed, denitrifying bacterial
species has been confirmed in Antarctic ice (Staley & Gosink, 1999),
which consume NO3− and supply
NO2− (Rysgaard et al., 2008). To
better understand the effect of denitrification on the nutrient balance,
we calculated DIN:P, the ratio of dissolved inorganic nitrogen (DIN,
i.e., NO3− +
NO2−) to
PO43− (Figure 8). In most cores, DIN:P
values were lower than the Redfield ratio (16:1) and the diatom nutrient
uptake ratio (12:1; Takeda, 1998), which is thought to reflect the
transformation of NO3− and
NO2− to NO, N2O, and
N2 by denitrification. The value of N*, which has been
suggested as a proxy for denitrification, is calculated as follows
(Gruber & Sarmiento, 1997; Nishioka et al., 2021; Yoshikawa et al.,
2006):
N* = ([NO3−] − 16
[PO43−] + 2.9) × 0.87. (3)
Figure 9 shows N* profiles of multi-year ice cores JARE56_NMS and
JARE57_P31.5. In the lower parts of these cores, N* values decreased to
negative values, reaching a minimum of −12.3 μmol L−1in core JARE57_P31.5. Chl.a concentrations are also negatively
correlated with N*. Previous studies have reported that denitrification
occurred in oxygen-depleted and organic matter-rich parts of the ice
column (Rysgaard et al., 2008). It is therefore reasonable that
denitrification occurred at depths where high chl.a concentrations
indicate abundant organic matter. To our knowledge, because multi-year
ice formed a long time ago in this area (about 40 years ago) (Higashi et
al., 1982; Ushio, 2006), salinity is low (Figure 2c), and it is unlikely
that brine channels physically drain the ice. Therefore, the air and
brine pockets in multi-year ice has remained isolated and disconnected
from exchanges with the atmosphere and under-ice water.
To examine the possibility of oxygen exchange through the brine channel
network in the sea ice, we focused on the brine volume fraction, a proxy
of sea-ice permeability. Calculating brine volume fraction requires the
ice temperature and salinity; however, we could not obtain realistic ice
temperature data due to the difficulty of inserting a temperature sensor
in the sea ice, especially in low-salinity, snow-origin ice. The ice
temperature increased during coring due to friction, and the air
temperature was often positive (Nomura et al., 2011a), precluding
accurate measurement of the sea ice temperature. Therefore, for the
brine volume calculation, we assumed the temperature of the sea ice to
be −2 to 0 °C, a reasonable estimate given the positive air temperature.
We obtained brine volume fractions, estimated following Leppäranta and
Manninen (1988), of 2.4 ± 2.6% (mean and standard deviation) at all
depths in the multi-year ice. Typically, sea ice is considered permeable
when the brine volume fraction exceeds 5–7.5% (Golden et al., 1998;
Pringle et al., 2009; Zhou et al., 2013). Therefore, the multi-year ice
collected in this study was impermeable, precluding any exchanges
through brine channels. Therefore, gas components such as oxygen must
have been consumed in a closed system within the ice by heterotrophic
organisms and bacteria. Furthermore, the high density of the hard,
low-salinity superimposed ice suggests that gases could not be exchanged
with the atmosphere, consistent with previous studies that were unable
to detect gas fluxes over superimposed ice at the same location (Nomura
et al., 2012, 2013). Therefore, oxygen in the isolated interior sea ice
could only have been depleted by heterotrophic organisms and bacteria,
promoting denitrification in multi-year ice (Figure 9).
Based on our results, we developed a conceptual illustration for the
evolving nutrient concentrations in multi-year fast ice (Figure 10).
Immediately after ice formation, when nutrient concentrations have not
yet been affected by biological processes, the ratio of salinity to
nutrients in the brine discharged from the growing sea ice should plot
along the dilution line. Then, during the first year, nutrient uptake by
primary production of ice algae decreases nutrient concentrations in the
ice while the salinity is reduced by brine convection. Over multiple
years, remineralization by the degradation of organic matter resupplies
nutrients to the ice while salinity continues to decrease. Once ice
algae have exhausted the nutrient supply and decompose, and brine
exchange with seawater is established at the bottom of the ice column,
the nutrient concentrations surpass the dilution line (Figure 10). In
addition, denitrification decreases
NO3− concentrations and modifies the
speciation of nitrogen within the ice.