# Sea-ice arches structure Arctic primary production hotspots at the Last Ice Area gateway

Foucaut Tachon, Karen Nieto, Philippe Massicotte, Philippe Archambault, Marie Latil, Simon Bélanger, Marcel Babin, Karl Attard, Mathieu Ardyna

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Foucaut Tachon, Karen Nieto, Philippe Massicotte, Philippe Archambault, Marie Latil, et al. Sea-ice arches structure Arctic primary production hotspots at the Last Ice Area gateway. 2025. hal-05388281

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# Sea-ice arches structure Arctic primary production hotspots at the Last Ice Area gateway

Foucaut Tachon

foucaut.tachon@gmail.com

Universite Laval https://orcid.org/0009-0004-9356-3802

Karen Nieto

Universite Laval

Philippe Massicotte

Universite Laval

Philippe Archambault

Université Laval https://orcid.org/0000-0001-5986-6149

Marie Latil

Universite Laval

Simon Bélanger

Marcel Babin

Laval University

Karl Attard

University of Southern Denmark

Mathieu Ardyna

Takuvik Joint International Laboratory, Laval University https://orcid.org/0000-0002-4703-6655

## Article

### Keywords:

**Posted Date:** October 28th, 2025

**DOI:** https://doi.org/10.21203/rs.3.rs-7827911/v1

**License:** © This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License

**Additional Declarations:** There is **NO** Competing Interest.
**Abstract**

Nares Strait, located at the southern edge of the Last Ice Area, is a significant ecological zone where seasonal sea-ice arches influence regional ice dynamics, air-sea exchange and ecosystem productivity. By modulating ice export, stratification, and light penetration, these arches determine the timing, location, and intensity of phytoplankton blooms. Based on two decades (2003–2023) of satellite sea-ice observations and depth-resolved primary production estimates, we found a pronounced increase in production, particularly in northern regions where earlier ice retreat and longer open-water seasons promoted phytoplankton growth. Two recurrent productivity hotspots were identified. The first is a variable northern hotspot in Kennedy Channel, which only emerged under specific ice-arch configurations and bathymetric conditions, highlighting the structural role of arches in regulating bloom dynamics. In contrast, the second hotspot, located in the North Water Polynya (*Pikialasorsuaq*), sustained consistently high production across all sea ice conditions. This stability, in contrast to previously reported declines, highlights the North Water Polynya as an ecologically and culturally significant marine area. Our findings identify sea-ice arches as ecosystem constitutive features within the Last Ice Area and emphasize the critical role of the North Water Polynya for biodiversity conservation and subsistence of Indigenous under continued Arctic warming.

**Key Points**

* Seasonal sea-ice arches act as fundamental ecosystem constitutive features, shaping bloom dynamics at the southern gateway of the Last Ice Area (LIA).

* Two recurrent productive hotspots are identified in Nares Strait (NS), with contrasting sensitivity to ice-arch regimes and topography.

* Primary production (PP) in the North Water Polynya (NOW, *Pikialasorsuaq*) remains high and stable despite shifting sea-ice dynamics.

* Earlier ice retreat has increased light availability, driving a long-term rise in Arctic PP.

**Introduction**

Arctic sea ice is undergoing rapid decline in both thickness and extent, driven by accelerated polar warming (Stroeve et al., 2018; Ardyna & Arrigo, 2020; Heuzé et al., 2024). These changes fundamentally alter the balance between light availability and nutrient supply, two key drivers of primary production (PP) in polar marine ecosystems. Thinner and more mobile ice favors earlier phytoplankton blooms, sometimes even beneath the ice cover (Arrigo et al., 2014; Ardyna & Arrigo, 2020). Conversely, enhanced stratification resulting from freshwater inputs and surface warming inhibits vertical nutrient replenishment, ultimately limiting bloom magnitude (Tremblay & Gagnon, 2009; Slagstad et al., 2015). Whether Arctic ecosystems will transition toward higher or lower productivity under continued warming remains an open question and is likely to vary regionally. This uncertainty is particularly pronounced in
poorly studied outflow shelf systems of the Arctic Ocean, where changes in sea-ice dynamics propagate downstream and influence local biogeochemical conditions (Carmack & Wassmann, 2006).

Nares Strait (NS), a narrow (~30 km) channel between Canada's Ellesmere Island and Greenland, is one of the five major gateways through which sea ice is exported from the Arctic Ocean (Kwok, 2005, see Fig. 1a). Unlike the broader Fram Strait, ice export through NS is dominated by multi-year ice (MYI) (Kwok et al., 2010), and is strongly regulated by the seasonal formation of two ice arches in Robeson Channel and Smith Sound (Vincent, 2019; Kirillov et al., 2021; Moore et al., 2023). These ice arches are large consolidated packs of ice laterally anchored on the coast of narrow passages where the circulation (here southward) stabilizes them. These arches can block ice transport for extended periods, effectively shielding the Last Ice Area (LIA), identified as the region expected to lose its perennial ice cover last, from the seasonally ice-free waters of northern Baffin Bay. It has been hypothesized that ice arches act as ecosystem constitutive features, influencing phytoplankton bloom dynamics at the southern boundary of the LIA by modulating ice export, stratification, and light penetration (Marchese et al., 2017; Moore et al., 2021). However, this ecological role has yet to be demonstrated directly, leaving a critical gap in our understanding of how physical processes shape marine productivity in this region.

The North Water Polynya (NOW, *Pikialasorsuaq*) has long been recognized as one of the most productive polynyas in the Arctic, supporting large phytoplankton blooms and sustaining high biomass (Tremblay et al., 2002). However, recent studies indicate ecological shifts, including a decline in PP (Bélanger et al., 2013) and a reduced contribution of diatoms to the phytoplankton community (Blais et al., 2017). Several mechanisms have been proposed to explain these trends: increased stratification limiting vertical nutrient replenishment, enhanced ice export reducing light availability, and upstream nutrient drawdown associated with earlier ice retreat and longer open-water periods in NS and farther north (Carmack et al., 2006; Carmack and Wassman, 2006; Tremblay and Gagnon, 2009). These changes have significant implications for regional ecosystems and the ecosystem services they support, from maintaining one of the Arctic's richest food webs to sustaining the cultural and subsistence practices of Inuit communities. These concerns have generated strong interest in better understanding the future trajectory of NOW, both in terms of Arctic marine ecosystems functioning and the livelihoods of the communities that depend on them.

Contrary to previous observations of decline, our results show that the annual PP in the NOW has remained relatively stable over the past two decades, despite notable changes in sea-ice dynamics. To reconcile this apparent paradox, we integrate two decades (2003–2023) of satellite-derived sea-ice data with depth-resolved PP estimates. This approach enables us to identify persistent productivity hotspots, assess their contrasting sensitivities to ice-arch dynamics and bathymetric settings, and, for the first time, demonstrate the role of sea-ice arches as ecosystem constitutive features at the southern gateway of the Last Ice Area.

# Data and Methods
## 2.1. Satellite data

The Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART; Ricchiazi et al., 1998) was used following Bélanger et al. (2013) for the generation of look-up tables (LUTs, courtesy of Simon Bélanger) in order to retrieve the incident spectral downwelling irradiance $Ed$ ($0^+$, $\lambda$, $t$) at the sea surface ($0^+$), with a 5 nm spectral resolution ($\lambda$) at 3 h timesteps ($t$). The model inputs were solar zenith angle ($\theta_s$) and values of, cloud fraction (CF) and total ozone concentration ($O_3$) from MODIS (MYD08, https://ladsweb.modaps.eosdis.nasa.gov). Surface chlorophyll a concentration (Chl, $mg m^{-3}$) was derived from remote sensing reflectance using the Arctic-optimized version of the semi-analytical Garver-Siegel-Maritorena algorithm (GSM) from Li et al. (2023).

We used the Level-3 binned daily reflectance (Rrs) from the Ocean Colour Climate Change Initiative sinusoidal version 6 product (OC-CCI v6.0, https://climate.esa.int/en/projects/ocean-colour/; Sathyendranath et al., 2023). As this study is based on Bélanger et al.'s (2013) PP model, which in turn was based on MODIS spectral bands, the OC-CCI v6.0 Rrs (MERIS bands) have been adjusted in this upgraded version. This involved shifting the 490, 560, and 665 nm Rrs to 488, 555, and 667 nm, respectively, without significant impacts on inherent optical property (IOP) results (Lee et al., 2009). A linear interpolation between 510 and 560 nm bands have been applied to generate Rrs(531).

## 2.2. TAKUVIK-UQAR primary production Model

Primary Production (PP) was estimated with a depth- and wavelength-resolved model upgraded from Bélanger et al. (2013), as applied in Li (2024) and maintained by Takuvik & UQAR (2025). The upgraded version incorporates improved spatial resolution of atmospheric inputs, and a resolution of the vertical Chl profile following Ardyna et al. (2013), which captures the typical subsurface chlorophyll maximum (SCM) found in Arctic, and of IOP profiles. The carbon fixation rate was calculated daily with the photosynthesis-irradiance model of Platt et al. (1980):

where $Chl(z)$ is chlorophyll a concentration at depth $z$ ($mg m^{-3}$), $P_B^{max}$ is the light-saturated Chl-normalized carbon fixation rate ($mgC.mgChl-1.h-1$) set to 2 based on field measurements in Arctic waters (Harrison and Platt, 1986; Huot et al., 2013), $Ek(z)$ is the saturation irradiance modelled here as a function of PUR ($z,t$) (derived from the LUTs; in $\mu mol photon m^{-2}s^{-1}$), and $PUR(z)$ is the photosynthetically usable radiation ($\mu mol photon m^{-2}s^{-1}$) is estimated as a function of PUR following Huot et al. (2013) calculated for each optical depth at a 3 h time interval following Arrigo et al. (1998). The calculated PP, integrated to a maximum depth of 100 m constrained by local bathymetry, was linearly interpolated in time with a maximum 8-day sliding window. Pixels with out-of-range values for the phytoplankton Chl-specific absorption coefficient, spectral decay constant for coloured detrital material absorption (aCDM) and power-law exponent for the particulate backscattering spectra from GSM were classified invalid and filtered out following Li (2023). Years 2004 and 2014 were excluded due to cloudiness covering more than 50% of the ice-free pixels during both spring and summer (Fig. S1 in the online resource).
## 2.3 Sea-ice concentration and arches

Daily sea-ice concentration (SIC) at 12.5 km resolution for the period 2003–2023 was obtained from SSM/I data (CERSAT/Ifremer; Ezraty et al., 2007) using the ARTIST algorithm (Kaleschke et al., 2001). Values outside the physical range (0–100%) were removed (Andersen et al., 2007), and a 15% SIC threshold was applied to minimize contamination in ocean-colour retrievals near the ice edge (Bélanger et al., 2007; Marchese et al., 2017). Freeze-up and break-up were defined as periods in autumn and summer, of at least 21 consecutive days with SIC rising above or decreasing below 35%, respectively (Marchese et al., 2017, see S6). Ice-arch configurations were classified following Vincent (2019) and Moore et al. (2023) into three scenarios: a southern arch, when the arch consolidated in Smith Sound; a northern arch, when it formed in Robeson Channel; and no arch, when neither structure developed during winter. These scenarios capture the primary modes of interannual variability in ice dynamics that modulate sea-ice export through Nares Strait.

## 2.4 Identification of productivity hotspots

The *k*-means clustering method (Hartigan & Wong, 1979), which has been previously applied in the Amundsen Sea (Feng et al., 2022), was applied on the maximum primary production value per pixel subset. This unsupervised classification algorithm partitions pixels into groups based on the similarity of their feature values, here defined as the maximum annual primary production across the time series. Because pixels are assigned according to their distance from mean-based centroids, outliers can strongly influence the clustering. To avoid abrupt pixel-to-pixel variations, we first applied a 7 × 7 median filter to the climatological PP maps to smooth the data and remove outliers before clustering.

We used the Sentinels Application Platform (SNAP) *k*-means cluster analysis algorithm and the number of clusters from 2 to 11. The default parameters for the random seed (*rs* = 31415) and number of iterations (*i* = 30) were used to group pixels with similar production. Based on our observations, we selected *k* = 6 for the clustering, as this configuration clearly highlights the two highly productive areas consistently observed across the time series, with a cluster mean primary production of 61.2 ± 5.0 gC m⁻² y⁻¹. Using *k* = 5 produces a very similar outcome, while increasing the number of clusters beyond 6 does not alter the representation of the two productive regions and offers little additional insight. A more exhaustive analysis of the clustering procedure, including sensitivity tests for *k*, is provided in the Supplementary Information.

# Results and Discussion

## 3.1 Ice-arch dynamics drives primary production variability

Recent pan-Arctic studies report a 30–50% increase in net primary production (PP) over recent decades, primarily attributed to longer open-water seasons and improved light availability as sea ice retreats (Arrigo & van Dijken, 2015; Ardyna & Arrigo, 2020; Lewis et al., 2020). Yet this apparent coherence conceals strong regional contrasts, where multiple environmental drivers shape PP. Nutrient
replenishment from Pacific and Atlantic inflows (Oziel et al., 2019; Tremblay et al., 2015), lateral exchanges across shelves (Carmack & Wassmann, 2006), and upwelling along shelf breaks (Ardyna et al., 2017; Michel et al., 2015) all contribute to the spatial mosaic of productivity across the Arctic. Outflow shelves such as NS remain especially underexplored, despite their central role in exporting sea ice, freshwater, and biogeochemical properties from the Last Ice Area (LIA) into Baffin Bay.

Our analysis of the NS-Baffin Bay system between 2003 and 2023 highlights a marked spatial heterogeneity in annual PP, spanning more than two orders of magnitude (Fig. 1b). In the LIA area, values remain below $0.1 \text{ gC m}^{-2} \text{ y}^{-1}$, whereas recurrently ice-free waters in the NOW sustain rates exceeding 60 $\text{gC m}^{-2} \text{ y}^{-1}$ (Fig. 1a, 2a). This variability underscores the tight coupling between PP, sea-ice retreat (see S6), and season length, consistent with pan-Arctic trends (Arrigo & van Dijken, 2015; Ardyna & Arrigo, 2020; Lewis et al., 2020), but here accentuated by local constraints (Fig. 2a). The juxtaposition of perennial ice cover to the north with the highly productive NOW downstream amplifies environmental gradients, locking northern sectors into light-limited regimes while southern regions transition rapidly toward nutrient limitation. Beyond ice dynamics, lateral forcing from outlet glaciers and the complex bathymetry of Kane Basin and Kennedy Channel further modulate circulation and stratification, shaping nutrient availability and reinforcing sharp contrasts in PP across this outflow shelf system (Michel et al., 2006; Münchow et al., 2007).

Both the climatological mean map (Fig. 1b) and the Hovmöller diagrams (Fig. 2a) highlight two recurrent hotspots of elevated PP. The Southern Hotspot (SH), located near Smith Sound at the entrance of the NOW, sustains the highest mean productivity ($61.7 \pm 0.2 \text{ gC m}^{-2} \text{ y}^{-1}$; see Fig. 1a, b). This persistence suggests favorable local conditions, including recurring thin-ice zones and stratification that reliably trigger bloom initiation (Barber et al., 2001). In contrast, the Northern Hotspot (NH) in Kennedy Channel near Cape Jackson exhibits lower average productivity ($38.5 \pm 0.2 \text{ gC m}^{-2} \text{ y}^{-1}$; see Fig. 1a, 1b) but pronounced interannual variability. This northern feature is tightly linked to local bathymetric constraints and the recurrent formation of heat polynyas (Kirillov et al., 2022), which intermittently generate windows of favorable light and nutrient conditions for bloom development. Together, these two regions exemplify how the interplay of sea ice, stratification, and bathymetry structure PP at the gateway of the LIA.

Temporal Hovmöller diagrams (Fig. 2a) further reveal the regulatory role of ice arches on hotspot dynamics. When the northern arch consolidates in Robeson Channel, downstream ice flux is suppressed, leading to prolonged open-water conditions in Kennedy Channel and promoting the development of the NH, as observed in 2009, 2010, 2017, and 2019. Conversely, the collapse of the southern arch in Smith Sound triggers earlier and more extensive openings in Kane Basin, producing anomalously warm conditions such as the + 5°C sea-surface temperature (SST) anomaly documented in 2009 (Vincent, 2013). In years with a stable southern arch, the NH is absent or strongly suppressed due to persistent sea ice cover, while the SH maintains relatively stable PP. These patterns confirm that ice arches act not only as mechanical barriers to ice floes but also regulate bloom timing, intensity, and spatial distribution across the system. Annual PP correlates significantly with the number of open-water pixels during the
productive season but not with the timing of break-up, here defined as opening day (April–October; Kendall's τ = 0.54, p = 0.004; Fig. 2b). Yet the absence of a long-term trend in PP (Kendall's τ = 0.135, p = 0.441, see S4) may suggest that productivity trajectories at the LIA gateway are not determined by monotonic sea-ice retreat alone, but by the shifting interplay of ice-arch stability, regional hydrography, and bathymetric setting.

**3.2 Stability of the North Water Polynya productivity**

NOW has long been recognized as the most productive polynya of the Northern Hemisphere, sustaining intense phytoplankton blooms and supporting one of the richest Arctic food webs (Barber et al., 2001; Tremblay et al., 2002). Due to its ecological and cultural significance, it has been the focus of numerous studies examining phytoplankton dynamics and ecosystem responses (e.g., Bergeron & Tremblay, 2014; Blais et al., 2017; Marchese et al., 2017; Olivier et al., 2020; Moore et al., 2023). Several of these analyses reported either a decline in net community production (NCP) or primary production (PP) (Bergeron & Tremblay, 2014; Blais et al., 2017), or a reduction in bloom magnitude despite earlier and longer bloom seasons (Marchese et al., 2017).

Unlike studies relying solely on surface chlorophyll from satellite ocean colour, our approach integrates depth-resolved PP, explicitly accounting for the subsurface chlorophyll maximum (SCM), which recurrently contributes to total production in Arctic systems (Ardyna et al., 2013). This methodological refinement partly explains the contrast with earlier findings of decline. This discrepancy may reflect a sampling bias, particularly the underestimation of PP occurring deeper in the water column. Another study by Blais et al. (2017) based their conclusions on in situ PP measurements made during CCGS Amundsen expeditions between 2006 and 2011, but their dataset excluded the productive year 2009, when PP reached unusually high values (Fig. 2b, S3). Similarly, Bergeron & Tremblay (2014) used a nutrient drawdown time series (1997–2011) that pointed to decreasing NCP. When considering the longer 2003–2023 satellite-derived record, however, annual PP in the NOW shows stability, with a slight positive trend of about + 0.1 TgC per decade (Fig. S3). This stability is consistent with recent observations of rising nutrient concentrations in Arctic waters (Barut, in prep.), which may help sustain production despite ongoing sea-ice change.

Moore et al. (2023) further demonstrated that ice-arch scenarios (southern, northern, or none) exert little influence on ice opening and biomass within the NOW itself. However, their findings were based on surface chlorophyll averages and not on integrated PP. Our results confirm the broader stability of the system: PP remains consistently high, regardless of arch configuration, with average values of 62.1 ± 0.2 gC m⁻² y⁻¹, 63.8 ± 0.2, and 60.6 ± 0.2 for no-arch years, northern-arch, and southern-arch, respectively (Fig. 3a, 3b, 3c). This stability contrasts with the dependence of Kennedy Channel productivity to arch dynamics and highlights the NOW as a resilient system within the LIA.

Although the NOW as a whole generally shows stable PP over time (Fig. 3d, Table 1), spatial heterogeneity exists. An area of recurrent elevated production is observed in its northern sector, near Cape Alexander (Fig. 1a), consistent with local characteristics with a possible island effect features and
recurring thin-ice conditions. This localized signal complements the broader picture of system-wide stability and points to the coexistence of resilient background production with finer-scale variability in bloom timing and amplitude. Taken together, these findings emphasize that while the NOW is shaped by the larger-scale dynamics of Nares Strait, its capacity to sustain consistently high PP across shifting ice regimes makes it an ecological and cultural stronghold within the Arctic (Fig. 3d, S3).

<table><caption>Table 1
Annual PP of the Total Area, the Northern Hotspot and the Southern Hotspot (in TgC.y⁻¹) and their contributions to the total PP (in %).</caption><thead><tr><th>Year</th><th>Southern Hotspot<br>(TgC.y-1)</th><th>Northern Hotspot<br>(TgC.y-1)</th><th>Total Area<br>(TgC.y-1)</th></tr></thead><tbody><tr><td>2003</td><td>1.083(41%)</td><td>0.007(0%)</td><td>2.620</td></tr><tr><td>2005</td><td>0.958(47%)</td><td>0.062(3%)</td><td>2.047</td></tr><tr><td>2006</td><td>1.116(44%)</td><td>0.023(1%)</td><td>2.520</td></tr><tr><td>2007</td><td>0.921(35%)</td><td>0.230(9%)</td><td>2.624</td></tr><tr><td>2008</td><td>0.923(41%)</td><td>0.060(3%)</td><td>2.267</td></tr><tr><td>2009</td><td>1.432(30%)</td><td>0.428(9%)</td><td>4.752</td></tr><tr><td>2010</td><td>0.753(33%)</td><td>0.175(8%)</td><td>2.274</td></tr><tr><td>2011</td><td>0.910(42%)</td><td>0.101(5%)</td><td>2.158</td></tr><tr><td>2012</td><td>1.212(43%)</td><td>0.035(1%)</td><td>2.826</td></tr><tr><td>2013</td><td>1.238(47%)</td><td>0.011(0%)</td><td>2.646</td></tr><tr><td>2015</td><td>1.029(41%)</td><td>0.004(0%)</td><td>2.499</td></tr><tr><td>2016</td><td>1.492(37%)</td><td>0.090(2%)</td><td>4.033</td></tr><tr><td>2017</td><td>1.017(31%)</td><td>0.319(10%)</td><td>3.291</td></tr><tr><td>2018</td><td>1.043(43%)</td><td>0.006(0%)</td><td>2.413</td></tr><tr><td>2019</td><td>0.884(27%)</td><td>0.219(7%)</td><td>3.259</td></tr><tr><td>2020</td><td>1.361(36%)</td><td>0.163(4%)</td><td>3.746</td></tr><tr><td>2021</td><td>0.993(35%)</td><td>0.084(3%)</td><td>2.836</td></tr><tr><td>2022</td><td>0.799(35%)</td><td>0.102(5%)</td><td>2.265</td></tr><tr><td>2023</td><td>0.849(37%)</td><td>0.022(1%)</td><td>2.296</td></tr></tbody></table>

## 3.3 Ice-arch control of productivity hotspots and bloom phenology
While the NOW exhibits remarkable stability in its annual PP (Fig. 3d, S3), finer-scale patterns reveal
strong sensitivities to ice-arch dynamics farther upstream in Kane Basin and Kennedy Channel (see
Fig. 1a). Clustering analysis distinguished two contrasting regions: the SH, observed consistently across
all years with a stable PP, and the NH, which occurs intermittently depending on ice-arch configuration.

The presence or absence of arches influences the timing and intensity of NH blooms (Marchese et al., 2017). When the southern arch fails to consolidate, open-water conditions extend northward by early spring, leading to earlier ice breakup in Kane Basin and Kennedy Channel. This favors the initiation of the NH bloom as early as May, advancing the season by nearly two months compared to years when the southern arch persists (Vincent, 2019; Moore et al., 2023). A consolidated northern arch in Robeson Channel reduces the downstream flux of MYI and also contributes to prolonge open-water conditions downstream and further enhance NH productivity. In this configuration, annual PP in the NH can increase up to $74.4 \pm 0.2 \text{ gC m}^{-2} \text{ y}^{-1}$, nearly triple that observed in years without arches (Fig. 4a-b).

By contrast, the SH demonstrates greater robustness. Even in years when nutrient drawdown might be
expected upstream, local conditions such as recurring thin-ice zones, stable stratification, and
predictable light penetration (Barber et al., 2001) support relatively consistent bloom initiation and
magnitude. One might expect intensified and earlier NH blooms to trigger cascading effects
downstream, with nutrient depletion and advective transport reducing PP in the SH. Yet our results
indicate otherwise: despite high SIC variability upstream, SH productivity remains confined within a
narrow range across ice-arch scenarios, in sharp contrast to the high SIC interannual variability of the NH
(Fig. 4c-d). This stability suggests that local conditions damp the SH against upstream forcing,
decoupling its dynamics from northern variability.

Years of high productivity (i.e. 2007, 2009, 2016, 2017, 2019, 2020) were found to be linked to distinct
environmental forcing patterns. For instance, the 2009 season, characterized by a long-lived northern
arch and the early collapse of the southern arch, produced anomalously warm conditions (+ 5°C SST
anomaly; Vincent, 2013) and sustained open water that favored prolonged NH blooms. In contrast, the
2017 season combined thin ice and early collapse with unusual atmospheric forcing linked to the
breakdown of the Beaufort High (Moore et al., 2018), leading to exceptional bloom conditions in the NH.
Beyond affecting the magnitude of PP, ice-arch dynamics also alter the phenology of NH blooms by
shifting their timing, duration, and seasonal intensity, with potential consequences for food-web coupling
and carbon fluxes. Taken together, these results confirm that sea-ice arches act both as passive
mechanical barriers and ecosystem constitutive features that determine not only where hotspots
emerge, but also when and how long they persist. While the SH near the NOW remains consistently
productive, the NH in Kennedy Channel is more transient and opportunistic, with its presence, magnitude,
and phenology depending on the shifting interplay of ice-arch stability and episodic environmental
forcing.

**3.4 Ice-arch dynamics and ecosystem resilience at the Last Ice Area gateway**
Our findings reveal that sea-ice arches are more than passive physical barriers: they act as ecosystem constitutive features that regulate the timing, intensity, and spatial distribution of PP at the southern gateway of the LIA. By shaping the duration of open-water seasons and the advection of MYI, arch stability drives the emergence of contrasting hotspots. The NH is a dynamic feature, with its phenology and magnitude varying according to arch persistence and bathymetric setting. By contrast, the SH, embedded in the NOW, demonstrates remarkable stability, sustaining stable PP across ice-arch scenarios thanks to recurrent thin-ice zones and reliable stratification. These results illustrate how the interplay of ice dynamics structures the productivity of an underexplored Arctic outflow shelf system.

Despite earlier predictions of declining productivity in the NOW, our model suggests that annual PP has remained relatively stable, supporting one of the Arctic's richest food webs and sustaining a cultural way of life for Inuit communities. Yet this stability is not guaranteed: accelerating MYI loss and the potential destabilization of ice arches could undermine the system's long-term stability. Our results emphasize the need to recognize NOW not only as a productivity hotspot, but as a keystone ecosystem whose biodiversity and cultural importance make it central to the functioning of the rapidly changing Arctic and whose protection should be prioritized through marine protected area initiatives and Indigenous stewardship.

# Declarations

## 4. Acknowledgments

The primary production model used in this study was developed thanks to support from the Canada Excellence Research Chair on Remote Sensing of Canada's new Arctic Frontier and from the Sentinel North project. The Refuge-Arctic project was conducted using the Canadian research icebreaker CCGS Amundsen with the support of the Amundsen Science program funded by the Canada Foundation for Innovation (CFI) Major Science Initiatives (MSI) Fund. We wish to thank the officers and crew of the CCGS Amundsen and the entire TAKUVIK laboratory team for planning the field work, and all other scientists and technicians involved in the Refuge-Arctic campaigns for their contribution to field work and data collection. The project was conducted under the scientific coordination of Centre national de la recherche scientifique CNRS/Université Laval Takuvik Joint International Laboratory (IRL3376). We also thank Québec-Océan and the Polar Continental Shelf Program for their in-kind contribution in terms of polar logistics and scientific equipment. The Refuge-Arctic project is funded by the following French and Canadian programs and agencies: Amundsen Science, ArcticNet, BNP Paribas Foundation, Centre National d'études spatiales (CNES) and the CNES-TOSCA "Alg-O-Nord", Centre National de la Recherche Scientifique (CNRS), Crown-Indigenous Relations and Northern Affairs Canada (RCAANC), European Research Council (ERC), Flotte Océanographique française (FOF), Fonds de recherche du Québec - Nature et technologies (FRQNT), Institut Nordique du Québec (INQ), Institut français de recherche pour l'exploitation de la mer (IFREMER), Institut Polaire français Paul-Émile Victor (IPEV), Les Enveloppes Fluides et l'Environnement (LEFE), Mission pour les Initiatives Transverses et Interdisciplinaires (MITI), Natural Sciences and Engineering Research Council of Canada (NSERC), Fisheries and Oceans Canada
(DFO), Transforming Climate Action (TCA), Sentinelle Nord and Université Laval. We also wish to thank the Denmark's Independent Research Fund (Grant No. 2064-00021B) and the Polar Ocean Mitigation Potential project (Horizon Europe, Grant No. 101136875).

**5. Data Availability**

The original datasets used to reproduce the findings of this study are publicly available on Zenodo for the primary production at https://doi.org/10.5281/zenodo.14715624. Sea-ice concentration data (12.5 km resolution grids of sea ice concentration from the 85 Ghz channel of SSM/I) are available from IFREMER FTP servers at ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/psi-concentration/data/arctic.

**6. Computer code**

The analysis of the model output was done using the open-source software Python. Post-processing scripts will be provided upon request to the corresponding author.

**References**

1. Andersen, S., Tonboe, R., Kaleschke, L., Heygster, G. & Pedersen, L. T. Intercomparison of passive microwave sea-ice concentration retrievals over the high-concentration Arctic sea ice. *J. Geophys. Res. Oceans* 112, C8 (2007).

2. Ardyna, M., Gosselin, M., Michel, C., Poulin, M. & Tremblay, J. É. Environmental forcing of phytoplankton community structure and function in the Canadian High Arctic: contrasting oligotrophic and eutrophic regions. *Mar. Ecol. Prog. Ser.* 442, 37–57 (2011).

3. Ardyna, M. *et al*. Parameterization of vertical chlorophyll *a* in the Arctic Ocean: impact of the subsurface chlorophyll maximum on regional, seasonal, and annual primary production estimates. *Biogeosciences* 10, 4383–4404 (2013).

4. Ardyna, M. *et al*. Recent Arctic Ocean sea-ice loss triggers novel fall phytoplankton blooms. *Geophys. Res. Lett.* 41, 6207–6212 (2014).

5. Ardyna, M., Babin, M., Devred, E., Forest, A., Gosselin, M., Raimbault, P., & Tremblay, J. É. (2017). Shelf-basin gradients shape ecological phytoplankton niches and community composition in the coastal Arctic Ocean (Beaufort Sea). *Limnology and oceanography, 62*(5), 2113–2132.

6. Ardyna, M. & Arrigo, K. R. Phytoplankton dynamics in a changing Arctic Ocean. *Nat. Clim. Change* 10, 892–903 (2020).

7. Arrigo, K. R., Worthen, D., Schnell, A., & Lizotte, M. P. (1998). Primary production in Southern Ocean waters. *Journal of Geophysical Research: Oceans, 103*(C8), 15587–15600.

8. Arrigo, K. R. *et al*. Phytoplankton blooms beneath the sea ice in the Chukchi Sea. *Deep-Sea Res. II* 105, 1–16 (2014).
9. Arrigo, K. R. & van Dijken, G. L. Continued increases in Arctic Ocean primary production. *Prog. Oceanogr.* 136, 60–70 (2015).

10. Barber, D. G., Hanesiak, J. M., Chan, W. & Piwowar, J. Sea-ice and meteorological conditions in northern Baffin Bay and the North Water polynya between 1979 and 1996. *Atmos.-Ocean* 39, 343–359 (2001).

11. Bélanger, S., Ehn, J. K. & Babin, M. Impact of sea ice on the retrieval of water-leaving reflectance, chlorophyll *a* concentration and inherent optical properties from satellite ocean colour data. *Remote Sens. Environ.* 111, 51–68 (2007).

12. Bélanger, S., Babin, M. & Tremblay, J. É. Increasing cloudiness in the Arctic damps the increase in phytoplankton primary production due to sea-ice receding. *Biogeosciences* 10, 4087–4101 (2013).

13. Bergeron, M. & Tremblay, J. É. Shifts in biological productivity inferred from nutrient drawdown in the southern Beaufort Sea (2003–2011) and northern Baffin Bay (1997–2011), Canadian Arctic. *Geophys. Res. Lett.* 41, 3979–3987 (2014).

14. Blais, M. *et al*. Contrasting interannual changes in phytoplankton productivity and community structure in the coastal Canadian Arctic Ocean. *Limnol. Oceanogr.* 62, 2480–2497 (2017).

15. Carmack, E., Barber, D., Christensen, J., Macdonald, R., Rudels, B., & Sakshaug, E. (2006). Climate variability and physical forcing of the food webs and the carbon budget on panarctic shelves. *Progress in Oceanography*, 71(2–4), 145–181.

16. Carmack, E. & Wassmann, P. Food webs and physical-biological coupling on pan-Arctic shelves: unifying concepts and comprehensive perspectives. *Prog. Oceanogr.* 71, 446–477 (2006).

17. Ezraty, R., Girard-Ardhuin, F., Piolle, J. F., Kaleschke, L. & Heygster, G. *Arctic & Antarctic sea ice concentration and Arctic sea ice drift estimated from Special Sensor Microwave data – User's manual V2.1.* (CERSAT/IFREMER, 2007).

18. Feng, J., Li, D., Zhang, J. & Zhao, L. Variations and environmental controls of primary productivity in the Amundsen Sea. *Front. Mar. Sci.* 9, 891–896 (2022).

19. Hartigan, J. A. & Wong, M. A. Algorithm AS 136: A k-means clustering algorithm. *J. R. Stat. Soc. C Appl. Stat.* 28, 100–108 (1979).

20. Harrison, W. G. & Platt, T. Photosynthesis-irradiance relationships in polar and temperate phytoplankton populations. *Polar Biol.* 5, 153–164 (1986).

21. Hastrup, K., Mosbech, A. & Grønnow, B. Introducing the North Water: histories of exploration, ice dynamics, living resources, and human settlement in the Thule Region. *Ambio* 47 (Suppl 2), 162–174 (2018).

22. Heuzé, C. & Jahn, A. The first ice-free day in the Arctic Ocean could occur before 2030. *Nat. Commun.* 15, 1–10 (2024).

23. Huot, Y., Babin, M. & Bruyant, F. Photosynthetic parameters in the Beaufort Sea in relation to the phytoplankton community structure. *Biogeosciences* 10, 3445–3454 (2013).
24. Kahru, M., Lee, Z., Mitchell, B. G., & Nevison, C. D. (2016). Effects of sea ice cover on satellite-detected primary production in the Arctic Ocean. *Biology letters*, 12(11), 20160223.

25. Kaleschke, L. et al. SSM/I sea-ice remote sensing for mesoscale ocean-atmosphere interaction analysis. *Can. J. Remote Sens.* 27, 526-537 (2001).

26. Kirillov, S. et al. On the physical settings of ice bridge formation in Nares Strait. *J. Geophys. Res. Oceans* 126, e2021JC017331 (2021).

27. Kirillov, S. et al. The role of oceanic heat flux in reducing thermodynamic ice growth in Nares Strait and promoting earlier collapse of the ice bridge. *Ocean Sci.* 18, 1535-1557 (2022).

28. Kwok, R. Variability of Nares Strait ice flux. *Geophys. Res. Lett.* 32, L24502 (2005).

29. Kwok, R., Pedersen, L. T., Gudmandsen, P. & Pang, S. S. Large sea-ice outflow into the Nares Strait in 2007. *Geophys. Res. Lett.* 37, L03502 (2010).

30. Lee, Z., Lubac, B., Werdell, J. & Arnone, R. An update of the quasi-analytical algorithm (QAA_v5). *IOCCG Software Report* 1, 1-9 (2009).

31. Lewis, K. M., van Dijken, G. L. & Arrigo, K. R. Changes in phytoplankton concentration now drive increased Arctic Ocean primary production. *Science* 369, 198-202 (2020).
https://doi.org/10.1126/science.aay8380

32. Lewis, K. M. & Arrigo, K. R. Ocean colour algorithms for estimating chlorophyll a, CDOM absorption and particle backscattering in the Arctic Ocean. *J. Geophys. Res. Oceans* 125, e2019JC015706 (2020).

33. Li, J., Matsuoka, A., Hooker, S. B., Maritorena, S., Pang, X. & Babin, M. A tuned ocean colour algorithm for the Arctic Ocean: a solution for waters with high CDM content. *Opt. Express* 31, 38494-38512 (2023).

34. Li, J., Matsuoka, A., Pang, X., Massicotte, P. & Babin, M. Performance of algorithms for retrieving chlorophyll a concentrations in the Arctic Ocean: impact on primary production estimates. *Remote Sens.* 16, 892 (2024).

35. Marchese, C. et al. Changes in phytoplankton bloom phenology over the North Water (NOW) polynya: a response to changing environmental conditions. *Polar Biol.* 40, 1721-1737 (2017).

36. Mayot, N. et al. Assessing phytoplankton activities in the seasonal ice zone of the Greenland Sea over an annual cycle. *J. Geophys. Res. Oceans* 123, 8004-8025 (2018).

37. Michel, C., Ingram, R. G. & Harris, L. R. Variability in oceanographic and ecological processes in the Canadian Arctic Archipelago. *Prog. Oceanogr.* 71, 379-401 (2006).

38. Michel, C. et al. Arctic Ocean outflow shelves in the changing Arctic: a review and perspectives. *Prog. Oceanogr.* 139, 66-88 (2015).

39. Moore, G. W. K., Schweiger, A., Zhang, J. & Steele, M. Collapse of the 2017 winter Beaufort High: a response to thinning sea ice? *Geophys. Res. Lett.* 45, 2860-2869 (2018).

40. Moore, G. K., Howell, S. E. L., Brady, M., Xu, X., & McNeil, K. (2021). Anomalous collapses of Nares Strait ice arches leads to enhanced export of Arctic sea ice. *Nature communications*, 12(1), 1.
41. Moore, G. W. K., Howell, S. E. L. & Brady, M. Evolving relationship of Nares Strait ice arches on sea ice along the Strait and the North Water, the Arctic's most productive polynya. *Sci. Rep.* 13, 9809 (2023).

42. Münchow, A., Falkner, K. K. & Melling, H. Spatial continuity of measured seawater and tracer fluxes through Nares Strait, a dynamically wide channel bordering the Canadian Archipelago. *Deep-Sea Res. I* 54, 722–742 (2007).

43. Olivier F, Gaillard B, Thébault J, Meziane T, Tremblay R, Dumont D, Bélanger S, Gosselin M, Jolivet A, Chauvaud L, Martel AL, Rysgaard S, Olivier A-H, Pettré J, Mars J, Gerber S, Archambault P (2020) Shells of the bivalve Astarte moerchi give new evidence of a strong pelagic-benthic coupling's shift occurring since the late 70s in the NOW Polynya. Phil. Trans. R. Soc. A. 378: doi.org/10.1098/rsta.2019.0353

44. Oziel, L. et al. Environmental factors influencing the seasonal dynamics of spring algal blooms in and beneath sea ice in western Baffin Bay. *Elem. Sci. Anth.* 7, 34 (2019).

45. Platt, T. & Gallegos, C. L. Modelling primary production. In *Primary Productivity in the Sea* (eds. Falkowski, P. G. & Woodhead, A. D.) 339–362 (Springer, 1980).

46. Platt, T. et al. Diagnostic properties of phytoplankton time series from remote sensing. *Estuaries Coasts* 33, 428–439 (2010).

47. Qikiqtani Inuit Association (QIA). *Sarvarjuaq and Qikiqtait: Inuit stewardship and the blue economy in Nunavut's Qikiqtani region* (QIA, 2020). Available at: https://www.qia.ca/

48. Rasmussen, T. A., Kliem, N. & Kaas, E. Modelling the sea ice in the Nares Strait. *Ocean Model.* 35, 161–172 (2010).

49. Ricchiazzi, P., Yang, S., Gautier, C. & Sowle, D. SBDART: a research and teaching software tool for plane-parallel radiative transfer in the Earth's atmosphere. *Bull. Am. Meteorol. Soc.* 79, 2101–2114 (1998).

50. Rousseeuw, P. J. Silhouettes: a graphical aid to the interpretation and validation of cluster analysis. *J. Comput. Appl. Math.* 20, 53–65 (1987).

51. Sathyendranath, S. et al. An ocean-colour time series for use in climate studies: the experience of the ocean-colour climate change initiative (OC-CCI). *Sensors* 19, 4285 (2019).

52. Shokr, M. E., Wang, Z. & Liu, T. Sea-ice drift and arch evolution in the Robeson Channel using the daily coverage of Sentinel-1 SAR data for the 2016–2017 freezing season. *The Cryosphere* 14, 3611–3627 (2020).

53. Slagstad, D., Wassmann, P. F. & Ellingsen, I. Physical constraints and productivity in the future Arctic Ocean. *Front. Mar. Sci.* 2, 85 (2015).

54. Snow, W. *Ice and Permafrost in the Arctic*. AMAP Report to the Arctic Council (AMAP, 2017).

55. Stroeve, J. & Notz, D. Changing state of Arctic sea ice across all seasons. *Environ. Res. Lett.* 13, 103001 (2018).
56. Takuvik & UQAR. *Arctic Ocean Net Primary Production Model Code (v1.0)*. (Zenodo, 2025).
https://doi.org/10.5281/zenodo.14715624

57. Tremblay, J. É., Gratton, Y., Fauchot, J. & Price, N. M. Climatic and oceanic forcing of new, net and diatom production in the North Water. *Deep-Sea Res. II* 49, 4927–4946 (2002).

58. Tremblay, J. É. & Gagnon, J. The effects of irradiance and nutrient supply on the productivity of Arctic waters: a perspective on climate change. In *Influence of Climate Change on the Changing Arctic and Sub-Arctic Conditions* (eds. Nihoul, J. C. J. & Kostianoy, A. G.) 73–93 (Springer, 2009).

59. Tremblay, J. É. *et al*. Global and regional drivers of nutrient supply, primary production and CO₂ drawdown in the changing Arctic Ocean. *Prog. Oceanogr.* 139, 171–196 (2015).

60. Vincent, R. F. The 2009 North Water anomaly. *Remote Sens. Lett.* 4, 1057–1066 (2013).

61. Vincent, R. F. A study of the North Water polynya ice arch using four decades of satellite data. *Sci. Rep.* 9, 20278 (2019).

62. Vincent, R. F. An examination of the non-formation of the North Water Polynya ice arch. *Remote Sens.* 12, 2712 (2020).

63. Wang, M. & Overland, J. E. A sea-ice-free summer Arctic within 30 years? *Geophys. Res. Lett.* 36, L07502 (2009).

## Figures

<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
**Bathymetry of the Nares Strait – Baffin Bay system** (a). Abbreviations represent locations along the system: Lincoln Sea (LS); Robeson Channel (RC); Kennedy Channel (KC); Kane Basin (KB); Smith Sound (SS); Cape Alexander (CA); North Water Polynya (NOW); Baffin Bay (BB). **Annual climatology (2003–2023) of primary production across the Nares Strait system, extending from the Lincoln Sea to the North Water Polynya.** (b) Latitudinal grid used for the Hovmöller plots (see Figure 2).

<figure><img src="image_3.png" /><figcaption>**Figure 2**</figcaption></figure>

(a) Monthly means of net primary production (PP, top panel) and sea-ice concentration (SIC, bottom panel) along latitude grid (Fig. 1b). (b) Relationship between annual PP and the number of open-water pixels cumulated during the growing season (April–October), with marker colour indicating the Opening Day (OD) when the SIC < 0.15 defined using Kahru (2016) methodology.
<figure><img src="image_4.png" /><figcaption>Figure 3</figcaption></figure>

**Climatological annual PP maps for years with contrasting ice-arch scenarios.** (a) Open years with no ice-arch formation in the Nares Strait-Baffin Bay system (2007, 2022). (b) Years with a persistent northern arch in Robeson Channel (2009, 2010, 2017, 2019). (c) Closed years with a persistent southern arch in Smith Sound (2003, 2005, 2006, 2008, 2011, 2012, 2013, 2015, 2016, 2018, 2020, 2021, 2023). White dashed polygons delineate the northern and southern hotspots identified using k-means clustering. The contribution of each hotspot to annual PP is shown (light blue = NH; light pink = SH). Ice-arch scenarios are classified following Moore (2023) and Vincent (2019).
<figure><img src="image_5.png" /><figcaption>Figure 4</figcaption></figure>

Daily time series of PP (a, b) and SIC (c, d) in the northern hotspot (NH) and southern hotspot (SH).
Years with a consolidated southern ice arch are shown in blue, and years without are shown in red. Bold
lines represent the mean for each scenario.

Supplementary Files

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* TachonNatureCEEsm.docx