During the research period from April 7 to April 26, 2018, ice and water samples were collected from the landfast sea ice and the underlying water near Cambridge Bay in the Kitikmeot Sea of the Canadian Arctic ( Figure 1 ). The sampling locations were chosen based on a transect that crossed a narrow-constricted waterway, following the findings of Dalman et al. (2019), who observed enhanced nutrient supply from the ocean to the sea ice towards the center of the Finlayson Islands. Four stations, labeled as st.1, st.2, st.3, and st.4, were selected for this study. Due to logistical issues, st.1 and st.2 were visited twice during this program, while st.3 and st.4 were visited only once. To examine the vertical distribution of biological components, sea ice samples were categorized into two sections: the bottom section and the upper section (consisting of the top, mid-1, mid-2, and mid-3 sections).

Before conducting the ice coring work, snow depths were measured at five different locations around the sampling sites. Using a Mark II Kovacs core barrel with an inner diameter of 9 cm, sea ice samples were extracted from each ice core hole, and their thickness was measured. For the analysis of the vertical distribution of various biological parameters, 10 cm segments of ice were obtained from the bottom and upper sections of the sea ice. The ice samples were transported to the laboratory in a dark insulated box to minimize light exposure. In the field laboratory, the ice samples were used for the analysis of pigments (Chl-a), macro-nutrients, biochemical components, and particulate organic carbon (POC) and particulate organic nitrogen (PON). Two ice segments from similar locations of ice columns obtained from each station were melted together without the addition of filtered seawater. The melting process was conducted in the dark at room temperature overnight.

Underlying water samples were collected using a submersible pump through the auger hole and stored in the LDPE water collection bottles. All the bottles containing water samples were placed in a cooler to maintain optimal conditions of low light and temperature. Subsequently, they were transported to the laboratory for a comprehensive analysis of both chemical and biological components.

The salinity of the melted ice and seawater samples was measured using a YSI model 30 salinity meter (YSI, Yellow Springs, Ohio), calibrated with a conductivity standard (50 mS cm⁻¹, YSI Catalog# 3169) with an accuracy of ± 0.1. Macro-nutrient concentrations, including ammonium, nitrite + nitrate, phosphate, and silicate, were measured by filtering the melted ice and water samples through a 0.7 μm pore-sized 47 mm GF/F filter after homogenization. The filtrates were collected in 50 ml conical tubes and stored in a freezer at −20°C until analysis. Macro-nutrient concentrations were determined using a QuAAtro Auto analyzer (Bran + Luebbe, Germany) at the National Institute of Fisheries Science in South Korea.

To measure the total Chlorophyll a (Chl-a) concentrations of ice algae and phytoplankton, melted ice and water samples were filtered onto 25 mm GF/F filters. For size-fractionated Chl-a analysis, the samples underwent sequential filtration through membrane filters of varying pore sizes. First, the samples were filtered through 20 μm pore-sized Nucleopore filters (47 mm), followed by 2 μm pore-sized Nucleopore filters (47 mm), and finally through 0.7 μm pore-sized Whatman GF/F filters (47 mm). This approach allowed us to discern the contributions of different size classes to overall Chl-a content.

After filtration, the membrane filters containing the retained particles were placed in 15 ml conical tubes and stored in a freezer at −80°C until analysis. The analysis of Chl-a was performed using a 10-AU fluorometer (Turner Design) calibrated with commercially purified Chl-a standard. Prior to analysis, membrane filters were extracted in 90% acetone at 4°C for 24 hours, following the protocol described by Parsons et al. (1984).

To measure the total carbohydrate (CHO), protein (PRT), and lipid (LIP) concentrations of particulate organic matter (POM) in the sea ice and underlying water, melted ice and seawater samples were filtered onto 47 mm GF/F filters. The filters containing the retained POM were then placed in petri dishes (50 mm diameter) and stored in a freezer (−80°C) until spectrophotometric analysis at Pusan National University in South Korea.

The concentrations of each biochemical component were quantified following the method described by Bhavya et al. (2019). For CHO, filtered samples were extracted using the method described by Dubois et al. (1956), and a glucose solution (1 mg mL⁻¹, SIGMA) was used as a standard for calibration. PRT was extracted using the method from Lowry et al. (1951), and a protein standard (Bovine Serum Albumin; 2 mg mL⁻¹, SIGMA) was used to create a calibration curve. LIP was measured using the methods of Bligh and Dyer (1959) and Marsh and Weinstein (1966). A tripalmitin solution (SIGMA) was used to create a calibration curve, and the concentration of LIP was estimated using the equation derived from the curve. The quantification of each biochemical component was performed using a HITACHI UH5300 spectrophotometer. The measurement uncertainties were ± 4.3, ± 8.5, and ± 5.6% for carbohydrate, protein, and lipid concentrations, respectively.

The food material (FM) was defined as the sum of CHO, PRT, and LIP concentrations, following the approach described by Danovaro et al. (2000). The biopolymeric carbon (BPC) was calculated using the equation described by Fichez (1991a); Fichez (1991b) and Danovaro et al. (2000).

To measure the particulate organic carbon (POC) and nitrogen (PON) concentrations and stable carbon isotope composition, the melted ice and water samples were filtered onto 25 mm GF/F filters. These filters were then stored in the freezer at −20°C until they were prepared for mass spectrometric analysis at the stable isotope laboratory of the University of Alaska Fairbanks (UAF). The uncertainties for the measurements are ± 3.7% for POC and ± 4.5% for PON, respectively. The stable carbon and nitrogen isotope ratios of the filtered samples were expressed in parts per thousand (‰), providing information on the isotopic composition of carbon and nitrogen in the samples. Additionally, the C/N ratio (mol/mol) was calculated using the POC and PON concentration results.

The average snow depth at the sampling sites was 11.1 ± 4.5 cm, while the average sea ice thickness was 146.8 ± 17.0 cm, respectively ( Table 1 ). The average sea ice thickness at st.1 was significantly thinner compared to other sampling sites in this study (t-test, p < 0.01). The average bulk salinity of the bottom sections was 6.0 ± 0.5, while the upper ice sections had an average of 5.2 ± 1.3. The highest salinity of 9.5 was observed in the upper section of the sea ice. However, no significant differences were found in salinity among the different vertical sections of the sea ice (one-way ANOVA, p > 0.05). In comparison, the underlying water showed higher salinity values with an average of 28.5 (S.D. = ± 1.4), and there was a significant difference in salinity between melted ice samples and the underlying water (one-way ANOVA, p < 0.05).

Figure 2 and Table 2 present the vertical distributions of major nutrients (ammonium, nitrite + nitrate, phosphate, and silicate) in the sea ice and underlying water. The average concentrations of these nutrients in the upper ice section were 1.87 ± 1.50 µM for ammonium, 0.88 ± 0. 38 µM for nitrite + nitrate, 0.09 ± 0.06 µM for phosphate, and 0.91 ± 0.45 µM for silicate. The average concentrations for each nutrient in the bottom ice section were 2.48 ± 1.00, 1.31 ± 0.48, 0.38 ± 0.27, and 1.05 ± 0.48 µM for ammonium, nitrite + nitrate, and silicate, respectively. The average phosphate concentration in the bottom ice section was higher than that in the upper ice sections (one-way ANOVA, p < 0.05). However, no significant differences were observed in the average concentrations of ammonium, nitrite + nitrate, and silicate among the upper ice sections (one-way ANOVA, p > 0.05). Regarding the underlying water, there were no significant differences in the average concentrations of ammonium and nitrite + nitrate compared between each ice section (one-way ANOVA, p > 0.05). However, the average phosphate and silicate concentrations were higher in the underlying water than in the entire ice sections, except for the phosphate concentration in the bottom ice section (one-way ANOVA, p < 0.05).

Figure 3 and Table 3 illustrate the vertical distributions of total and size-fractionated Chl-a concentrations in the sea ice and underlying water. The total Chl-a concentration in the entire sea ice column ranged from 0.03 to 53.43 μg L⁻¹, with an average of 6.37 ± 14.52 μg L⁻¹. The upper ice section had an average Chl-a concentration of 0.16 ± 0.21 μg L⁻¹, whereas higher concentrations were observed in the bottom ice section, averaging 31.23 μg L⁻¹ (S.D. = ± 17.19 μg L⁻¹) (one-way ANOVA, p < 0.05). The majority of Chl-a was concentrated in the bottom ice section, with a high proportion of micro-sized cells comprising an average of 70 ± 4%, followed by nano- (27 ± 3%) and pico-sized cells (3 ± 2%). In contrast, significantly lower concentrations (ranging from 0.03 to 0.72 μg L⁻¹, mean ± S.D. = 0.16 ± 0.21 μg L⁻¹) were observed in the upper ice section, with nano-sized cells dominating at an average of 48 ± 12%, followed by micro-sized (mean ± S.D. = 32 ± 13%) and pico-sized cells (mean ± S.D. = 19 ± 9%). No significant differences were found in total and size-fractionated Chl-a concentrations within the upper ice sections (one-way ANOVA, p > 0.05). A negative relationship between the column-integrated Chl-a and the snow depth was observed (y = −0.42x + 11.22, r = − 0.65, p < 0.01, where x is the snow depth and y is the column-integrated Chl-a) during the study period.

In the underlying water, the Chl-a concentration was 0.15 μg L⁻¹ (S.D. = ± 0.14 μg L⁻¹), which was significantly lower than the concentrations in the bottom ice sections but not statistically different from those of the upper ice sections (one-way ANOVA, p > 0.05). The phytoplankton community in the underlying water was dominated by pico-sized cells, accounting for 42% (S.D. = ± 16%), followed by nano-sized (mean ± S.D. = 32 ± 4%) and micro-sized cells (mean ± S.D. = 25 ± 17%).

Figure 4 and Table 3 summarize the average concentrations of CHO, PRT, LIP, and FM in the POM within the sea ice column. The average concentrations of CHO, PRT, LIP, and FM in the sea ice column were 0.98 ± 0.64, 0.10 ± 0.27, 0.67 ± 0.55, and 1.75 ± 1.34 mg L⁻¹, respectively. The average concentrations of CHO, PRT, and LIP in the bottom ice section were significantly higher than those in the upper ice sections (one-way ANOVA, p < 0.05). PRT was rarely detected in the upper ice sections, while CHO and LIP were present with average concentrations of 0.72 mg L⁻¹ (S.D. = ± 0.28 mg L⁻¹) and 0.44 mg L⁻¹ (S.D. = ± 0.14 mg L⁻¹), respectively. No significant differences were observed in CHO, PRT, LIP, and FM concentrations among the upper ice sections (one-way ANOVA, p > 0.05). In the underlying water, the average concentrations of CHO, PRT, LIP, and FM were 0.20 ± 0.09, 0.01 ± 0.01, 0.08 ± 0.02, and 0.29 ± 0.11 mg L⁻¹, respectively. The concentrations in the underlying water were much lower than those of the entire ice sections, no statistical difference was found in the average PRT concentrations between the upper ice sections and underlying water (one-way ANOVA, p > 0.05).

The major biochemical component of POM in the sea ice column was CHO, accounting for 59% (S.D. = ± 8%), followed by LIP (mean ± S.D. = 38 ± 4%) and PRT (mean ± S.D. = 2 ± 6%). There were no significant vertical differences in the contribution of each component to FM within the entire ice sections, except for PRT, which had a higher proportion in the bottom ice section compared to the upper ice sections (one-way ANOVA, p > 0.05). The POM in the underlying water mainly consisted of CHO (mean ± S.D. = 68 ± 4%) and LIP (mean ± S.D. = 30 ± 5%), with a smaller proportion of PRT (mean ± S.D. = 2 ± 2%). The average contributions of CHO and PRT in the underlying water were statistically different from those in the bottom ice sections, while no statistical differences were found in the average contributions of CHO and PRT between the underlying water and the upper ice sections (one-way ANOVA, p > 0.05). The average contributions of LIP were not statistically different between the underlying water and the entire ice sections (one-way ANOVA, p > 0.05).

Figure 5 and Table 3 provide information on the vertical distributions of POC, PON, and BPC concentrations in the sea ice and underlying water. The ranges of POC, PON, and BPC in the ice column were 0.57-3.73 mg L⁻¹ (mean ± S.D. = 1.22 ± 0.83 mg L⁻¹), 0.02-0.40 mg L⁻¹ (mean ± S.D. = 0.08 ± 0.10 mg L⁻¹), and 0.45-2.95 mg L⁻¹ (mean ± S.D. = 0.94 ± 0.74 mg L⁻¹), respectively. There were no significant differences in the average concentrations of these components among the upper ice sections (one-way ANOVA, p > 0.05), but the concentrations in the bottom ice section were significantly higher than those in the upper ice sections (one-way ANOVA, p < 0.05).

The δ ¹³C value of POM in the sea ice column ranged from −28.3 to −24.0‰, with an average of −26.9‰ (S.D. = ± 1.0‰) ( Figure 5D ). The highest δ ¹³C value was observed in the bottom ice sections, and the mean δ ¹³C value of the upper ice sections was significantly lower than that of the bottom ice sections (one-way ANOVA, p < 0.05). Furthermore, a strong linear relationship was found between δ ¹³C and POC in the bottom ice sections (y = 1.48*x-29.27, r = 0.97, p < 0.01, where x is POC, y is δ ¹³C). The C/N ratio in the sea ice column ranged from 10.0 to 45.9, with an average of 24.6 (S.D. = ± 9.8‰) ( Figure 5D ). The C/N ratio was higher in the upper ice sections compared to the bottom ice sections (one-way ANOVA, p < 0.05).

In the underlying water, the average concentrations of POC, PON, and BPC were 0.18 ± 0.10, 0.01 ± 0.01, and 0.14 ± 0.05 mg L⁻¹, respectively. The average δ ¹³C value of POM and C/N ratio in the underlying water were −26.0 ± 0.3‰ and 16.6 ± 3.2, respectively. The concentrations of POC, PON, and BPC in the underlying water were significantly lower than those in the entire ice sections (one-way ANOVA, p < 0.05). The average δ ¹³C value and C/N ratio in the underlying water were statistically different from those in each ice section, except for the bottom ice sections (one-way ANOVA, p < 0.05).

BPC accounted for 51 to 100% (mean ± S.D. = 74 ± 16%) and 58 to 100% (mean ± S.D. = 83 ± 16%) of the POC in the ice column and underlying water, respectively ( Figure 6 ). The highest BPC contribution was observed in the underlying water, but there were no significant differences among the entire ice sections and the underlying water (one-way ANOVA, p < 0.05).

The column-integrated Chl-a concentration in the sea ice ranged from 2.59 to 8.07 mg m⁻², with an average of 5.05 mg m⁻² (S.D. = ± 2.26 mg m⁻²). The integrated concentration of FM, BPC, POC, and PON ranged from 1.68 to 2.51 g m⁻² (mean ± S.D. = 2.05 ± 0.39 g m⁻²), 0.90 to 1.32 g m⁻² (mean ± S.D. = 1.10 ± 0.20 g m⁻²), 1.16 to 1.76 g m⁻² (mean ± S.D. = 1.47 ± 0.25 g m⁻²), and 0.06 to 0.14 g m⁻² (mean ± S.D. = 0.09 ± 0.03 g m⁻²), respectively ( Table 4 ). The contribution of each parameter in the bottom ice sections to the entire sea ice columns varied from 11 to 66% ( Table 5 ). In the upper ice sections, the average Chl-a concentrations were significantly lower compared to those in the bottom ice sections (one-way ANOVA, p < 0.05) ( Figure 7A ). The Chl-a was highly concentrated in the bottom ice sections, accounting for 59% (S.D. = ± 10%) of the column-integrated concentrations. On the other hand, the average contributions of the other parameters (FM, BPC, POC, and PON) in the bottom ice sections to the column-integrated concentrations were substantially lower (20 ± 6, 21 ± 7, 19 ± 5, and 28 ± 7%, respectively) compared to Chl-a ( Figure 7B ).

A linear relationship was observed between the Chl-a concentration in the bottom ice section and the column-integrated Chl-a during this study period (y = 0.13*x + 0.96, r = 0.99, p < 0.01, where x is the Chl-a concentration in the bottom ice section and y is the integrated Chl-a) ( Figure 7C ). However, such relationships were not observed for the other parameters between the bottom ice section and the column-integrated values in this study ( Figure 7D ).

The study area near Cambridge Bay in 2018 exhibited diverse environmental conditions that influenced the sea ice ecosystem. One notable aspect was the marked variation in sea ice thickness across the sampling sites, with site 1 exhibiting considerably thinner ice in comparison to other locations. This consistent pattern of sea ice thickness distribution corresponds with earlier studies conducted by Dalman et al. (2019) and Kim et al. (2020), suggesting a recurring trend. It is worth noting that st.1 is characterized by a shallow sill generating currents exceeding 20 cm s⁻¹ beneath the sea ice (Dalman et al., 2019). This hydrodynamic feature significantly influences sea ice thickness by transferring additional momentum and heat flux from the ocean to the sea ice (Widell et al., 2006; Melling et al., 2015; Dalman et al., 2019). These flow dynamics can also lead to the transport of deeper saline water to shallower depths (Gosselin et al., 1985; Cota et al., 1987; Vancoppenolle et al., 2013), further illustrating the intricate interplay between hydrodynamics and ice thickness.

The snow cover on the sea ice introduces distinct physical characteristics, including a high albedo and light scattering capacity upon exposure to incident light (Petrich and Eicken, 2010). Particularly, a thick snow layer exceeding 30 cm can significantly hinder light penetration to the bottom ice algae during the spring growing season (Rysgaard et al., 2001; Sturm and Massom, 2009; Campbell et al., 2015). Although the average snow depth in our study (11.7 ± 5.0 cm) appears to pose relatively minor limitations on light penetration, the observed distinct negative relationship between snow depth and column-integrated chlorophyll-a (Chl-a) concentrations points to the significant influence of snow cover on the light conditions experienced by the ice algal community. These findings indicate that the presence of snow cover did indeed influence the light conditions experienced by the ice algal community during the study period (Campbell et al., 2015; Leu et al., 2015).

Given that a significant proportion of ice algae inhabit brine channels (Arrigo et al., 2014), the nutrient content within these channels becomes pivotal in governing their growth dynamics. In this regard, analyzing bulk nutrient concentrations serve as a reliable indicator of the prevalent nutrient conditions within the sea ice. While the bulk nitrite + nitrate concentration within the entire sea ice column in our study falls within the reported ranges (Campbell et al., 2016; Kim et al., 2020), the concentration of phosphate and silicate were notably lower than previously documented levels at Cambridge Bay during the spring (Campbell et al., 2016; Dalman et al., 2019; Kim et al., 2020). This contrast extends the nutrient concentrations in the underlying water, which remain consistent with prior studies conducted in the region (Campbell et al., 2016; Dalman et al., 2019; Kim et al., 2020; Back et al., 2021). These distinct nutrient patterns reflect Cambridge Bay’s unique environmental context characterized by low salinity and N-limited environmental condition due to the limited water exchange with the neighboring water bodies (McLaughlin et al., 2004; Campbell et al., 2016). This is further substantiated by the lower N:P ratios in the underlying water and the bottom ice section relative to the Redfield ratio (Redfield et al., 1963), suggesting a potential N-limited environmental condition during the study’s observation period. Conversely, the notably elevated average N:P ratio in the upper ice section ( Figure 2E ) indicates a potential P-limited condition for ice algal cells within the ice interior (Meiners et al., 2002).

While a potential for correlation between inorganic nutrients and biological parameters was explored, no statistically significant correlation emerged. Possible explanations for this lack of correlation could include the utilization of non-chronological data or a limited of sample size. Notably, Campbell et al. (2016) identified a temporal correlation between POC and Chl-a concentrations in the bottom ice section (5 cm) at Cambridge Bay, underscoring the importance of seasonality. In this study, the constrained data coverage might have hindered the establishment of correlations between nutrients and other biological factors.

Identifying the source of POM within sea ice is imperative due to its capacity to incorporate substances from various origins during sea ice formation (Garrison et al., 1990; Giannelli et al., 2001; Amon, 2004; Riedel et al., 2007; Stedmon et al., 2007; Dieckmann and Hellmer, 2010; Bachy et al., 2011). To discern this origin, the carbon isotopic signature of POC (δ ¹³C), the C/N ratio, and the contribution of BPC to POC (%) have been widely employed across various oceans (Danovaro et al., 2000; Lobbes et al., 2000; Lee and Whitledge, 2005; Kim et al., 2016; Kim et al., 2021).

Our investigation revealed that the observed δ ¹³C values in both the sea ice column and the underlying water consistently fall within a previously reported range in polar waters dominated by marine autotrophs (Schubert and Calvert, 2001; Kennedy et al., 2002). Furthermore, the strong correlation between POC and δ ¹³C observed in this study supports findings from the Arctic and Antarctic Oceans (Fry, 1996; Gradinger, 2009; Thomas et al., 2010).

While C/N ratios within the sea ice column during our study were notably higher than the general ratio of phytoplankton (6-10) (Redfield et al., 1963; Miller and Wheeler, 2012; Kim et al., 2015), it is noteworthy that ice algal POM can exhibit a wide range of C/N ratios, ranging from 3 to 50, depending on light and nutrient conditions (Gosselin et al., 1990; Gradinger, 2009; Niemi and Michel, 2015; Campbell et al., 2016). In our case, the P-limited conditions in the upper ice section likely contributed to the elevated C/N ratios for POM. Conversely, the ratios within the bottom ice section remained consistent with the reported range for the Canadian Arctic Archipelago (6.4-17.0) (Niemi and Michel, 2015). Moreover, the pronounced contributions of BPC to POC in both sea ice and underlying water provided additional support for the notion that the POM in this study predominantly originated from marine autotrophs (Pusceddu et al., 1996; Danovaro and Fabiano, 1997; Danovaro et al., 2000). Overall, the convergence of δ ¹³C values, C/N ratios, and BPC contribution offers insights into the origin and composition of POM within the sea ice, affirming marine autotrophs as the primary contributors to the POM in this study.

Our investigation revealed the average Chl-a concentrations for under-ice phytoplankton that exceeded reported values during the Arctic polar night period (0.01-0.06 µg L⁻¹; see Berge et al., 2015 references therein). This likely results from re-illumination following the dark winter period in our study region. Previous studies underscore that many sea ice diatoms, having endured long darkness, can rapidly resume photosynthesis upon re-illumination (Wulff et al., 2008; McMinn et al., 2010; Nymark et al., 2013; Kennedy et al., 2019). The higher Chl-a concentration in the bottom ice section implies that available light supported photosynthesis for autotrophs within the upper ice sections. However, the prevailing cold air temperatures during our study (ranging from −31.3 to −7.2°C; https://www.canada.ca/en.html) are likely to have restrained the increase in temperature within the upper sea ice section, consequently impeding brine channel connectivity (Olsen et al., 2017). Therefore, the low contribution of micro-sized cells to total Chl-a within the upper sea ice could be due to a low biological activity caused by cold ice temperature during this study. In contrast to the upper ice section, the bottom ice section could have experienced higher temperatures as a result of its proximity to the underlying seawater, which could have been at the freezing point of seawater (−1.8°C). This potentially leads to increased porosity in the lower ice, facilitating algal activity and enabling exchanges between the bottom ice section, the ice-water interface, and the underlying water (Petrich and Eicken, 2010). For these reasons, the bottom ice section, known as an ideal habitat for ice algae to thrive (Kaartokallio et al., 2007; Arrigo et al., 2010), could have displayed a notably heightened Chl-a concentration contrary to the upper ice section within this investigation. Moreover, this heightened concentration was concomitant with a significant proportion of micro-sized cells, consistent with polar ocean studies (Kaartokallio et al., 2007; Arrigo et al., 2010; Arrigo, 2014; Leu et al., 2015).

Contrasting with a micro-sized cell dominance in the phytoplankton community during the pelagic bloom in Cambridge Bay in July 2016 (Marshall, 2018), our study interval was characterized by a prevailing abundance of pico-sized cells, indicating the non-occurrence of a phytoplankton bloom. Notable under-ice phytoplankton blooms dominated by diatoms have been reported in the Chukchi Sea when melt ponds constituted a significant portion of sea ice cover (30-40%; Arrigo et al., 2014; Laney and Sosik, 2014). The melt ponds can transmit a substantial portion of incident light (approximately 60%) to the underlying water, fostering massive phytoplankton blooms beneath the sea ice (Arrigo et al., 2014; Laney and Sosik, 2014). However, in this study, no visible melt ponds were observed around the sampling sites. Moreover, Campbell et al. (2016) reported that less than 5% of the incident light reached the underlying water under thin snow cover (< 5cm) at Cambridge Bay during the spring of 2014. Consequently, this constrained light availability might have limited phytoplankton growth during our study’s observation period.

In summation, our findings illuminate that while Chl-a concentrations were notably elevated within the bottom ice section, re-illumination likely contributed to photosynthesis within the upper ice sections. Yet, environmental factors, notably the limited alight availability could have acted as constraints, impeding phytoplankton bloom development in the underlying water.

A significant proportion of CHO and LIP within the sea ice during our study was likely derived from the inherent biological characteristics of POM that became entrapped during ice formation. The seasonal process of sea ice formation in the coastal area of Cambridge Bay typically begins in early September, reaching its maximum thickness by the following April (Shen et al., 2021). The frigid atmospheric conditions during the dark Arctic winter cause the sea ice to grow downward (Langhorne and Robinson, 1983; Petrich and Eicken, 2010). As the ice thickens, POM from the underlying water becomes incorporated into the sea ice.

Although macromolecular composition data of phytoplankton and ice algae POM during the polar night period in the Arctic is limited, studies from the Antarctic polar night provide valuable insights. Kim et al. (2021) noted that during the Antarctic polar night at Jang Bogo Station (JBS) in the Ross Sea in 2015, CHO constituted the predominant component (73.0%) of POM, followed by LIP (20.9%), and PRT (6.1%). A similar observation of elevated proportions of CHO and LIP in under-ice phytoplankton was made by Park et al. (2021) at JBS. While these investigations pertain to the Antarctic region, they suggest a prevalent composition of CHO and LIP in the polar region phytoplankton during the dark winter. Under adverse conditions, diatoms have been found to suppress metabolism and enhance the accumulation of energy storage compounds (CHO and LIP) within their cellular structure (Kuwata et al., 1993; Zhang et al., 1998; Oku and Kamatani, 1999). Given that diatoms constitute the predominant ice algal community in Cambridge Bay (Campbell et al., 2016; Dalman et al., 2019), the elevated compositions of CHO and LIP within sea ice could signify a physiological adaptation for survival under harsh conditions, coinciding with the diminished activity of ice algae. The low proportion of micro-sized cells relative to the total Chl-a within the sea ice further supports the inference of reduced biological activity in the upper ice section. Conversely, the heightened proportion of PRT, accompanied by a higher proportion of micro-sized cells to the total Chl-a in the bottom ice section, indicates a thriving ice algal bloom with relatively higher activity during our study period (Morris, 1981; Kim et al., 2020).

These insights enhance our understanding of the intricate relationship between POM composition and the distinct environmental conditions characterizing polar regions during periods of limited light availability.

Traditionally, research efforts in the Arctic have predominantly focused on investigating the bottom section of sea ice, primarily due to its hosting of the majority of primary producers, particularly ice algae. These microorganisms typically inhabit the lowermost centimeters of the ice layer, benefiting from optimal environmental conditions for efficient light and nutrient utilization (Lee et al., 2008; Arrigo et al., 2010; Leu et al., 2015; Kim et al., 2020). Previous investigations conducted in Cambridge Bay have primarily concentrated on the bottom ice section (Campbell et al., 2016; Dalman et al., 2019; Kim et al., 2020), and the reported Chl-a concentrations in these studies align with the spectrum of the entire ice column-integrated Chl-a concentrations observed in our study. However, it is important to note that Chl-a distribution can be distributed within the interior of sea ice, not solely confined to its the bottom section. Several studies have reported that the presence of Chl-a within the upper ice sections, emphasizing the need for a comprehensive assessment of the entire sea ice column, including the upper segment, to achieve precise assessments of integrated Chl-a concentrations (Gradinger and Zhang, 1997; Gradinger, 1999).

Our study demonstrates that while the biological parameters exhibited the highest concentrations in the bottom ice section, their contributions to the overall sea ice column remained below approximately 60%. This notable finding implies that a substantial proportion, approximately 40%, of POM might be sequestered in the upper section of the sea ice. This realization carries considerable implications: previous research predominantly centered on bottom ice algae might have overlooked this reservoir, which could potentially serve as a critical early nutrient source for upper trophic organisms during the subsequent growth seasons (Runge and Ingram, 1988; Legendre et al., 1992; Søreide et al., 2010; Vancoppenolle et al., 2013).

As sea ice melting proceeds, POM from both the upper and bottom sections is released into the surface water. This process holds the potential to exert a significant influence on marine biogeochemistry and food webs (Michel et al., 1996; Fortier et al., 2002; Constable et al., 2003; Michel et al., 2006; Arrigo, 2014; Olsen et al., 2017). Therefore, to attain a comprehensive understanding of the ecological role of organic matter within sea ice across ice-covered oceans, it is crucial to consider POM present in the upper sea ice section and its consequent contribution to the encompassing sea ice column.