1. Introduction
Multi-year landfast ice (fast ice) is abundant around Syowa Station,
Lützow-Holm Bay, East Antarctica (Fraser et al., 2012; 2021; Ushio,
2006). This region receives relatively abundant snowfall and high snow
accumulation rates over sea ice (Toyota et al., 2016), forming ice of
snow origin such as ‘snow ice’ and ‘superimposed ice’ (Kawamura et al.,
1997; Nomura et al., 2012, 2018). In general, snow ice forms under the
weight of overlying snow on the sea ice, which depresses the ice–snow
interface to below sea level; the subsequent flow of seawater into the
snow forms a flooded slush layer that then freezes (Maksym & Jeffries,
2000; Sturm & Massom, 2017). During summer, the snow surface melts in
the sunlight and warmer temperatures; the snowmelt then infiltrates the
slush layer, decreasing its salinity to well below that of seawater
(Nomura et al., 2012, 2018). However, in Lützow-Holm Bay, ice thickening
due to the formation of snow ice does not necessarily depress the sea
ice–snow interface to below sea level. In this case, when the
shallowest snow thaws, the snowmelt permeates the sea ice–snow
interface below the freezing point and refreezes, forming superimposed
ice (Kawamura et al., 1997; Nomura et al., 2018). The difference between
these two types of ice is that snow ice forms from a mixture of snow and
seawater, whereas superimposed ice forms only from snow. Due to the
abundance of snow around Syowa Station, it is considered that
superimposed ice forms only after sea ice stops sinking under the weight
of overlying snow.
Because sea ice forms from seawater, the biogeochemical components in
seawater are preserved in sea ice upon its formation. Various changes
affect these biogeochemical components, particularly nutrients, after
their entrapment in sea ice. For example,
NO3−,
NO2−,
NH4−, and
PO43− are involved in biological
activities such as photosynthesis and remineralization by microorganisms
such as bacteria and phytoplankton, notably ice algae, phytoplankton
inhabiting the bottommost layers of sea ice (Arrigo et al., 2010; Gleitz
et al., 1995). Diatoms, which are considered to be the main primary
producers in sea ice, also use Si(OH)4 to form their
frustules (Arrigo et al., 2010; Tréguer & De La Rocha, 2013). In
addition to the consumption of nutrients by ice algae, high-salinity and
high-nutrient brines are discharged beneath the sea ice over time
(Fripiat et al., 2017). Nutrients are resupplied by remineralization
during the degradation of organic matter within sea ice by heterotrophs
and bacteria (Roukaerts et al., 2021; Thomas et al., 1995) as well as by
convection through brine channels (Nomura et al., 2009; Vancoppenolle et
al., 2010).
It has also been reported that denitrification reactions occur within
sea ice (Kaartokallio, 2001; Nomura et al., 2010; Nomura et al., 2018;
Rysgaard & Glud, 2004; Rysgaard et al., 2008; Staley & Gosink, 1999).
Sea ice can locally become isolated from the atmosphere and under-ice
water, resulting in the depletion of oxygen in the ice and promoting
denitrification. Previous studies have reported denitrifying bacteria in
Antarctic sea ice (Staley & Gosink, 1999) and have suggested or
confirmed denitrification reactions in Arctic and sub-Arctic sea ice,
e.g., in the Baltic Sea (Kaartokallio, 2001), along the coast of
Greenland and Arctic Ocean (Rysgaard & Glud, 2004; Rysgaard et al.,
2008), and in the southern Sea of Okhotsk (Nomura et al., 2010).
Primary production is also active in the nutrient-rich, seawater-flooded
slush layer formed at the sea-ice surface during the formation of snow
ice (Ackley & Sullivan, 1994; Fritsen et al., 1994; Kattner et al.,
2004; Nomura et al., 2018; Schnack-Schiel et al., 2001). The snow cover
on sea ice therefore plays an important role in increasing the
productivity of the sea-ice surface (e.g., Saenz and Arrigo, 2014). When
the slush freezes, it becomes snow ice, preserving the biogeochemical
formations (Kawamura et al., 1997; Nomura et al., 2018). However, the
thick sea ice produced in areas receiving abundant snowfall prevents
seawater from flowing along the sea-ice surface, limiting productivity;
therefore, snow cover can only increase productivity in the slush layer
by so much. Furthermore, whereas snow-free sea ice grows downward due to
the freezing of seawater, snow-covered sea ice grows upward due to the
formation of snow ice and superimposed ice, such that melting progresses
from the bottom of the sea ice (Kawamura et al., 1997). Thus, snow cover
greatly affects biogeochemical distributions and circulations in sea
ice.
Fast ice plays a crucial role in the biogeochemical cycle and marine
ecosystem functions of the Antarctic coast because it hosts
well-established habitats (in terms of nutrients, temperature, salinity,
and light) for stable primary production (Arrigo, 2017; Wongpan et al.,
2018). Because primary production in sea ice promotes the proliferation
of zooplankton, i.e., secondary and higher producers, it is essential to
know how the biogeochemical composition of fast ice changes. However,
although many studies have explored nutrient changes in first-year fast
ice (e.g., Lim et al., 2019), research remains limited on biogeochemical
components in multi-year fast ice that has continued to grow
year-by-year. Furthermore, of the few studies that have focused on
nutrient dynamics in multi-year land-fast ice (Nomura et al., 2018),
their sea-ice samples provide only single-year observations. Therefore,
no study has rigorously investigated the processes of nutrient cycling
in multi-year sea ice. Because multi-year fast ice has remained fixed to
the coast for a long time (Fraser et al., 2012; 2021), it is ideal for
examining the temporal evolution and interannual variation of nutrient
concentrations and other biogeochemical parameters in a single location.
The Japanese Antarctic Research Expedition (JARE) has monitored the
multi-year fast ice near Syowa Station in Lützow-Holm Bay for many
years. In recent years, it has been reported that the multi-year fast
ice in Lützow-Holm Bay periodically breaks up and flows out to sea
(Aoki, 2017; Ushio, 2006). Because ice thickness reflects the strength
of sea ice, sea-ice monitoring efforts have recently collected annual
ice core samples from an easily accessible and stable area near Syowa
Station, ideal samples for tracking year-to-year changes. Additionally,
because new ice formed after the partial ice breaks up and outflows, it
is possible to track the cycling of biogeochemical components through
the ice by comparing early stages of sea-ice formation with the initial
multi-year ice.
In this study, we analyzed multi-year fast ice near Syowa Station to
evaluate the effects of snow and biological activity on the vertical
distribution of nutrient concentrations in sea ice. We investigated the
factors affecting nutrient concentrations in the multi-year fast ice by
collecting and comparing nearby first-year ice. Specifically, based on
annual sampling during the 56–60th JAREs (2015–2019), we analyzed: (1)
the structure and formation process of ice using thin section
photographs, salinity, and stable oxygen isotopic ratios
(δ18O); (2) the vertical distribution of nutrient
concentrations in the fast ice; (3) the influence of snow (based on snow
fraction) on nutrient distributions; and (4) the cycling of nutrient
concentrations due to biological activity within fast ice using sea-ice
salinity, nutrient concentrations, and nutrient ratios as proxies.