Loss of sea ice alters light spectra for aquatic photosynthesis

Domain: earth_science
Published: 2025-04-30
Editorial outcome: major_revision

Abstract
The dramatic loss of sea ice due to global warming is changing light conditions for marine primary production, but exactly how is not well understood. Previous studies revealed that small peaks in the absorption spectrum of liquid water, due to molecular vibrations of H2O, delineate a series of spectral niches for aquatic photosynthesis. Ice, however, has a smoother absorption spectrum and scatters light much more strongly than liquid water. Here, we show with a radiative transfer model that the loss of sea ice causes a pronounced blue shift, narrowing light spectra in the euphotic zone to shorter wavelengths. Furthermore, ice cover yields a smooth continuum of light spectra, whereas open water creates distinct spectral niches selecting for phytoplankton species with different photosynthetic pigments. These results indicate that the loss of sea ice will cause major changes in both the pigment and species composition of primary producers in polar ecosystems.

Introduction
Although sea ice still covers almost 10% of the Earth’s surface, in many areas the ice cover rapidly diminishes. The Arctic Ocean’s ice cover has decreased dramatically, and predictions suggest an ice-free Arctic Ocean in summer within the next few decades1,2. The Antarctic sea ice extent increased slightly from the 1980s to 2015, but this trend reversed to a major decline since 20163. Both sea ice and open ocean waters are inhabited by numerous photosynthetic microorganisms, including ice algae and phytoplankton, whose primary production provides the basis for the marine food web4–6. The high light intensities in the surface layer to which these primary producers will be exposed when the ice melts can have major effects on photoacclimation and community composition, and the implication of increased irradiance have therefore been extensively investigated7–9. However, laboratory experiments, field studies, and models have shown that the spectral composition of light is also a major selective factor among aquatic primary producers10–16. Hence, to what extent will the loss of sea ice alter the spectral composition of light available for aquatic photosynthesis in these new ice-free regions of the oceans?

Ice has a much higher albedo than liquid water, reflecting 20–90% of the incident solar radiation, depending on the presence of e.g., melt ponds or snow cover at the ice surface17–19. Furthermore, scattering of light in liquid water is mainly governed by H2O molecules and small suspended particles, is highest in the blue part of the spectrum, and declines at longer wavelengths. In contrast, scattering by sea ice is dominated by air bubbles and brine inclusions embedded in the ice20, which strongly scatter light in an essentially wavelength-independent manner21. Hence, although the amount of scattering varies among ice types17, sea ice generally has a much higher scattering coefficient and its scattering spectrum tends to be more “white” than that of liquid water (Fig. 1a).

At first glance, light absorption by ice and liquid water appears quite similar, with low absorption in the blue and high absorption in the red and infrared part of the spectrum (Fig. 1b). In liquid water, H2O molecules are constantly moving, while the covalent bonds between their oxygen and hydrogen atoms display three types of vibrations: symmetric stretching, antisymmetric stretching and bending. The overtones (harmonics) of these vibrational modes create a series of small peaks in the visible and near-infrared range of the absorption spectrum of liquid water22–25. Specifically, the small peaks at 401, 449, 514, 605 and 742 nm correspond with the 8th, 7th, 6th, 5th and 4th harmonics of the symmetric and antisymmetric O-H stretching mode23,26,27 (Fig. 1b). Furthermore, the subtle peak at 662 nm corresponds with a combination of O-H stretching and bending modes known as the 4.1 sub-harmonic. It has been shown11,15 that these small peaks in the absorption spectrum create large gaps in underwater light spectra, because light is attenuated exponentially with depth. These gaps delineate a series of distinct ‘spectral niches’ in aquatic ecosystems11,15,28. For example, light absorption by the 6th harmonics at 514 nm separates the blue niche (449–514 nm) of clear ocean waters from the green niche (514–605 nm) of many coastal waters. Likewise, light absorption by the 5th harmonics at 605 nm separates the green niche of coastal waters from the orange (605–662 nm) and red niches (662–742 nm) of humic lakes, and so on. The different spectral niches are exploited by different photosynthetic pigments of phytoplankton11,15.

In the crystal lattice of ice, however, H2O molecules are more strongly hydrogen-bonded than in liquid water, resulting in a broadening and weakening of the overtone bands29,30. Hence, in ice, the peaks of the 8th, 7th, 6th, 5th and 4.1 vibrational harmonics are flattened out, resulting in a much smoother absorption spectrum across the visible range (Fig. 1b). Moreover, the peak due to the 4th harmonics is smaller and shifted to longer wavelengths in ice as compared to liquid water20.

In view of these differences in scattering and absorption properties of ice and liquid water, it is likely that the disappearance of sea ice will change not only the intensity but also the spectral quality of light. Yet, exactly how light spectra for aquatic photosynthesis are affected by the loss of ice cover is still unclear.

In this study, we implement the optical properties of sea ice17,20,31 and liquid water23,25,32 into a state-of-the-art radiative transfer model33 to investigate how irradiance spectra below sea ice differ from irradiance spectra in open water (Fig. 1c, d). We focus our analysis on the euphotic zone (defined as the water layer extending from the surface to the depth at which light has been attenuated to 1% of the surface irradiance), where most photosynthetic activity takes place. Our results reveal that the loss of sea ice will cause marked changes in the spectral composition of light, which may alter the photosynthetic pigments favored by natural selection.

Results
General outline and model validation
We consider bare sea ice, representative of late spring and summer when the snow has melted. After snow melt, often a porous granular structure develops in the top layer of melting sea ice known as the surface scattering layer19. The transition zone below the surface scattering layer will be described as the drained layer, and the underlying columnar ice as the interior layer19,31. Our aim is to model the transmission of irradiance through sea ice to obtain the irradiance spectrum at the bottom of the ice, where dense assemblages of ice algae and under-ice phytoplankton blooms may develop (Fig. 1c). Irradiance spectra in sea ice and water were modeled with the radiative transfer model Hydrolight-Ecolight, version 6.0, which solves the radiative transfer equation numerically32,33. In our application, the model combines known optical properties of seawater, sea ice, chlorophyll a, colored dissolved organic matter (CDOM) and non-algal particles (NAP) with measurements of the incident solar irradiance spectrum, albedo, ice thickness and concentrations of chlorophyll a, CDOM and NAP (Supplementary Fig. S1 and Supplementary Table S1).

Although Hydrolight has been extensively validated for marine waters32, less data are available on the performance of Hydrolight in areas covered with sea ice. To validate the model predictions, we therefore first compared light spectra under sea ice predicted by the radiative transfer model with measured spectra2,34 under sea ice without snow cover at three different locations across the globe (Fig. 2).

In sea ice of the Central Arctic Ocean2,35, light absorption by CDOM and NAP was minor (aCDOM(440)=0.10 m−1, aNAP(440)=0.01 m−1), and the chlorophyll a concentration of ice algae was very low (~0.01 mg m−2). The light spectrum underneath an ice cover of ~1.5 m spanned a broad range of wavelengths, from violet to orange light (Fig. 2a).

In the Baltic Sea34, the sea ice was much thinner (19 cm), and hence irradiance levels under the ice cover of the Baltic Sea were much higher than in the Arctic Ocean. However, sea ice of the Baltic Sea contained more CDOM and NAP (aCDOM(440)=0.39 m−1, aNAP(440)=0.05 m−1) and a higher chlorophyll a concentration (1.0 mg m−2) than the Arctic Ocean. This implies that UV and violet light were more strongly absorbed than other wavelengths, and the irradiance spectrum under sea ice of the Baltic Sea was shifted to longer wavelengths (Fig. 2b).

At McMurdo Sound, in Antarctica2, the ice was ~2 m thick, light absorption by both CDOM and NAP was low (aCDOM(440)=0.06 m−1; aNAP(440)=0.01 m−1), whereas the chlorophyll a concentration of ice algae was very high (15.2 mg m−2). Hence, both violet-blue and red wavelengths were largely absorbed by chlorophyll, and the light spectrum underneath the ice cover was dominated by green and yellow wavelengths (Fig. 2c).

In general, the modeled spectra show good agreement with the measured spectra (R2 ranged from 0.93 to 0.99), confirming the robustness of the model predictions. Probably, the model predictions could be even further improved by fine-tuning the parameter values (e.g., by small changes in CDOM or chlorophyll concentration), but we preferred to base our model predictions as much as possible on measured parameter values (Supplementary Table S1).

Comparison of light spectra in ice-covered versus open water
Now that the model has been successfully validated, we use the radiative transfer model to compare irradiance spectra in ice-covered water and in open water without sea ice (Figs. 3 and 4). In our first example, we assume an ice thickness of 1 m, with a surface scattering layer19,31 but without snow cover. The model parameters for ice-covered and open water are otherwise identical, with the same incident solar irradiance (Supplementary Fig. S2) and very low CDOM and NAP concentrations representative of the clear oligotrophic ocean (aCDOM(440)=0.01 m−1; aNAP(440)=0.001 m−1). Concentrations of ice algae and phytoplankton are set to zero to predict irradiance spectra generated by the abiotic environment (i.e., by sea ice and seawater). In other words, we calculate spectra available for aquatic photosynthesis when new populations of ice algae or phytoplankton start to grow.

The model predicts that, in the first few centimetres below the surface, scalar irradiance (i.e., irradiance from all directions) is much higher in sea ice than in open water due to intense backscattering by ice (Fig. 3a, b). However, light penetrates less deep in sea ice than in open water. For example, integrated over the spectrum, only 20% of the total surface irradiance penetrates to a depth of 0.97 m in sea ice, whereas the same irradiance reaches 18.4 m depth in open ocean water (Fig. 3c, d). The more rapid light attenuation by sea ice is due to its high albedo and strong scattering in comparison to liquid water (Fig. 1; see also Supplementary Table S3). The model predictions also suggest differences in spectral composition of ice-covered versus open ocean water, although this is difficult to assess from Fig. 3a, b in view of the concomitant differences in total irradiance.

Most aquatic photosynthesis takes place in the euphotic zone, where light levels are sufficient to support primary production. We therefore investigated differences in spectral composition of the euphotic zone by comparing spectra of ice-covered versus open water at the same optical depth, i.e., at the same total irradiance integrated over the spectrum (350–900 nm). In this way, the shape of the spectra can be compared without potentially confounding effects of differences in total irradiance. The spectra are presented for five optical depths, at which the total scalar irradiance is attenuated to 20%, 10%, 5%, 2%, and 1% of the surface irradiance. The results reveal marked differences in spectral shape between ice-covered and open water (Fig. 3c, d). Light transmitted through ~1 m of sea ice spans essentially the entire photosynthetically active range from 400 to 700 nm (see the 20% optical depth in Fig. 3c). At 10% and 5% optical depth, water below sea ice still has a relatively broad irradiance spectrum that covers violet, blue and green wavelengths (400–600 nm; Fig. 3c), whereas open ocean water at the same optical depth is characterized by a more pronounced peak largely confined to the violet and blue part of the spectrum (400–500 nm; Fig. 3d). Further down in the euphotic zone, at 2% and 1% optical depth, the spectra in ice-covered and open ocean water become more similar, as the spectrum in the water column below sea ice gradually narrows down to violet-blue wavelengths as well. These results imply that, at least in this example, ice algae attached to the bottom of the ice and phytoplankton communities in the first several meters of water underneath the ice will be exposed to a broader range of wavelengths than phytoplankton communities at the same optical depths in open ocean water.

Ecosystems with different CDOM concentrations
CDOM absorbs strongly in the violet and blue part of the spectrum, and is a major determinant of the light conditions in aquatic ecosystems11,32. We therefore investigated the spectral composition of light for three different marine ecosystems that span a wider range of CDOM concentrations, from oligotrophic ocean waters with very low CDOM concentrations to coastal waters with high CDOM concentrations (Fig. 4). For each of these marine ecosystems, we calculated irradiance spectra directly below sea ice and in open water, at several optical depths within the euphotic zone. Without sea ice, the clear oligotrophic ocean is dominated by violet-blue colors, whereas the underwater spectrum is shifted to green and yellow wavelengths in coastal waters with high CDOM concentrations (Fig. 4d–f).

For all three ecosystems, the results show that irradiance spectra directly below sea ice span a wider range of wavelengths than irradiance spectra at the same optical depth in open water without sea ice (Fig. 4). For example, at an optical depth of 25% of the surface irradiance, the full width at half maximum (FWHM) of the irradiance spectrum below an ice layer in the oligotrophic ocean is 280 nm, whereas the FWHM in open ocean water is only 140 nm (Fig. 4a, d and Table 1). Similarly, in coastal water, the FWHM is 285 nm below ice, but only 190 nm in open water without sea ice (Fig. 4c, f and Table 1). Hence, the loss of sea ice generally results in a narrowing of the irradiance spectrum in the euphotic zone, irrespective of the CDOM concentration in the ecosystem.

Furthermore, at an optical depth of 25%, the mean wavelength of the irradiance spectrum in the oligotrophic ocean is shifted from the green part of the spectrum (550 nm) below sea ice to the blue part of the spectrum (472 nm) in open ocean water (Fig. 4a, d, and Table 1). This is a blue shift of almost 80 nm. Conversely, in coastal water, the mean wavelength is shifted from 605 nm below sea ice to 582 nm in open water, which is a shift of less than 25 nm (Fig. 4c, f and Table 1). Hence, the blue shift to shorter wavelengths due to the loss of sea ice is more pronounced in oligotrophic ocean waters with low CDOM concentrations than in coastal waters with high CDOM concentrations.

Variation in optical properties of sea ice
The albedo and scattering coefficient of sea ice vary widely among different ice types, depending on the presence of a surface scattering layer, melt ponds, the number and size distribution of air bubbles and brine inclusions, as well as ice temperature20,31,36. To investigate the sensitivity of our model predictions, we therefore compared irradiance spectra below sea ice predicted for different surface albedos (Supplementary Fig. S3) and different scattering coefficients (Supplementary Fig. S4). The results show that sea ice with a high albedo and high scattering coefficients will generate slightly broader irradiance spectra than sea ice with a low albedo and low scattering coefficients.

Furthermore, in early spring, sea ice is often covered by a layer of snow6,19. Snow consists of ice crystals and air. Repeated scattering at numerous air-ice interfaces gives snow a very high albedo, and its wavelength-dependent light attenuation can strongly diminish light levels37–39. Our calculations revealed that even a thin layer of only 3 cm of snow can remove almost 90% of the incident irradiance, in line with field observations21,39. Yet, low light levels below snow-covered ice can already provide sufficient irradiance for ice algae to grow40,41. We therefore also compared irradiance spectra below ice predicted for different degrees of snow cover, and extended the comparison to lower light levels of only 0.1% of the surface irradiance. The results show that sea ice covered by snow will generate even broader irradiance spectra than bare sea ice without snow cover (Supplementary Fig. S5).

Overall, however, the patterns predicted by the model are robust. That is, irradiance spectra in the euphotic zone are narrowed down to shorter wavelengths in open water than below sea ice, across the full range of albedos, scattering coefficients, and snow cover investigated (Supplementary Figs. S3–5).

Spectral niches for aquatic photosynthesis
Next, we expand our comparison between the spectral composition of sea ice and water from only three marine ecosystems (Fig. 4) to a large number of marine ecosystems (Fig. 5). First, we calculated 85 total absorption spectra, as the sum of the absorption spectra of either sea ice or pure seawater, with a small amount of NAP, and 85 different CDOM concentrations. These CDOM concentrations cover the full range from the clearest ocean waters with almost no CDOM to humic-rich plumes of Arctic rivers with very high CDOM concentrations obtained from the vast amounts of organic carbon stored in the Arctic tundra42,43 (Fig. 5a, b). Similarly, we also calculated total scattering spectra for sea ice and liquid water (note that light is not scattered by CDOM). Subsequently, these absorption and scattering spectra were implemented in the radiative transfer model to predict the corresponding irradiance spectra at the euphotic depth (i.e., the depth at which irradiance over the entire spectrum equals 1% of surface irradiance) (Fig. 5c, d).

An overlay of all 85 irradiance spectra below sea ice reveals a nearly flat landscape without major valleys from 450 to 750 nm (Fig. 5c). This flat landscape is due to the smooth absorption spectrum of ice across the visible range and into the near-infrared (Fig. 5a). Hence, sea ice offers a continuum of irradiance spectra, in which the harmonics of the vibrational modes of H2O do not play a role.

In contrast, overlaying the 85 irradiance spectra of open water without ice cover reveals a rugged landscape of peaks and valleys (Fig. 5d). The deep valleys are induced by the harmonics of the vibrational modes of liquid water11,15 (Fig. 5b). They delineate a series of distinct spectral niches. Clear ocean waters have very low CDOM concentrations, and their underwater spectra show a major peak in the violet niche between the 8th and 7th harmonics (401–449 nm) and another major peak in the blue niche between the 7th and 6th harmonics (449–514 nm) (Fig. 5d). Coastal waters with intermediate to high CDOM concentrations have underwater spectra with a major peak in the green niche between the 6th and 5th harmonics (514–605 nm). River plumes with very high CDOM concentrations have underwater spectra in the orange and red niche between the 5th harmonics and 4.1 sub-harmonic (605–662 nm) and the 4.1 sub-harmonic and 4th harmonics (662–742 nm), respectively.

Methods
Radiative transfer model
Irradiance spectra in ice-covered water (Fig. 1c) and open water (Fig. 1d) were modeled with Hydrolight-Ecolight version HE 6.0, a state-of-the-art radiative transfer model that solves the radiative transfer equation numerically32,33. The radiative transfer equation describes how the radiance changes with depth in the sea ice or water column, depending on the inherent optical properties (IOP) of different components32,68. Depth-dependent scattering by sea ice is incorporated by assuming three distinct layers in the sea ice model19,69: a surface scattering layer (SSL) with high scattering, a drained layer (DL) with intermediate scattering, and interior ice (INT) with relatively low scattering (Supplementary Fig. S6).

For a horizontally homogeneous water body or ice layer with incident light from above, and negligible fluorescence and bottom reflectance, the change in spectral radiance L (W m−2 sr−1 nm−1) with depth z at wavelength λ is described by:

μ dL(λ,z,θ,φ) / dz = −c(λ,z)L(λ,z,θ,φ) + g(λ,z,θ,φ) (1)

where θ and φ are the zenith angle and azimuth angle, respectively, and μ = cos(θ) is the vertical projection of radiance L. The term c(λ,z) is the beam attenuation coefficient (m−1) describing the radiance lost due to absorption and scattering when passing through the water column or ice layer, and g(λ,z,θ,φ) is the radiance gained with depth due to scattering from adjacent paths.

The beam attenuation coefficient is defined as the sum of the total absorption coefficient a(λ,z) (m−1) and total scattering coefficient b(λ,z) (m−1):

c(λ,z) = a(λ,z) + b(λ,z) (2)

We defined five optically significant components in the model: seawater (sw), sea ice (ice), CDOM, NAP, and photosynthetic organisms, including ice algae and phytoplankton (algae). The total absorption and scattering coefficient are calculated from the absorption and scattering properties of these optical components:

a(λ,z) = a_sw(λ,z) + a_ice(λ,z) + a_CDOM(λ,z) + a_NAP(λ,z) + a_algae(λ,z) (3)

and

b(λ,z) = b_sw(λ,z) + b_ice(λ,z) + b_NAP(λ,z) + b_algae(λ,z) (4)

The contribution of CDOM to scattering is assumed negligible32.

The model calculates both planar and scalar irradiance spectra. Downwelling planar irradiance Ed(λ,z) (in W m−2 nm−1) is often measured by field studies, including the studies we used for our model validation (see below). It is defined as the total radiance, at a given wavelength and depth, from the upper hemisphere with respect to solid angle ω:

Ed(λ,z) = ∫ω=0^2π L(λ,z,ω) cos(θ) dω (5)

For randomly oriented photosynthetic cells, however, photons from all directions can be used with equal efficiency. Scalar irradiance Eo(λ,z) (W m−2 nm−1) is therefore a better all-round measure of the availability of light for photosynthesis68. It is defined as the total radiance, at a given wavelength and depth, from all directions:

Eo(λ,z) = ∫ω=0^4π L(λ,z,ω) dω (6)

Since photosynthesis is based on the absorption of photons, the irradiance units were converted from energy flux (W m−2 nm−1) into photon flux (photons m−2 s−1 nm−1). For our model analysis, we calculated scalar irradiance spectra at four optical depths, receiving 25%, 15%, 5%, and 1% of the incident solar irradiance (expressed as photon flux, integrated over 350–900 nm), respectively. The 1% optical depth is also known as the euphotic depth.

Inherent optical properties used in the model
Seawater. Absorption by pure water, a_w(λ), is lowest at 344 nm and increases rapidly toward longer wavelengths, reaching the highest value in the near-infrared (NIR) range22–25. We used the absorption coefficients of Pope and Fry23 for wavelengths above 550 nm and from ref. 25 for wavelengths shorter than 550 nm (Fig. 1b), assuming that salinity has a negligible effect on the absorption spectrum of water25. Elastic scattering by water molecules is inversely proportional to the wavelength to the power of 4. Specifically, we computed scattering coefficients of pure water according to Morel22:

b_w(λ) = 16.06 * (λ_0 / λ)^4.32 * β_w(90°; λ_0) (7)

with λ_0 = 550 and β_w(90°; 550) = 0.93 * 10−4. The scattering coefficient of pure water was adjusted for salinity S to calculate the scattering coefficient of seawater22,70:

b_sw(λ) = (1 + 0.3 * S / 37) * b_w(λ) (8)

where we assumed S = 32 g kg−1 as default value and a low salinity of S = 3.1 g kg−1 for the Bothnian Bay in the Baltic Sea34.

Sea ice. Absorption coefficients of sea ice in the visible and NIR part of the spectrum were obtained from Warren & Brandt71.

Scattering of light in sea ice is strongly affected by air bubbles, brine inclusions, and cracks in the ice. The number and size distribution of air bubbles and brine inclusions varies among different ice types, and the number of bubbles in sea ice often decreases with depth. We used a spectrally flat scattering coefficient of natural sea ice, with different values of b_ice for different layers, ranging from b_ice = 1 m−1 to b_ice = 1000 m−1 depending on the depth and location (exact values in Supplementary Tables S1 and S2)21,31,69. To describe the angular distribution of the scattered light, we used the Petzold average-particle phase function, which corresponds to a backscattering ratio of 0.018 (ratio of backscattering to total scattering, B = b_b / b) and asymmetry parameter of 0.9232,72.

CDOM. Absorption by CDOM decreases exponentially with increasing wavelengths:

a_CDOM(λ) = a_CDOM(λ_0) * exp(−S_CDOM * (λ − λ_0)) (9)

where a_CDOM(λ_0) is the absorption by CDOM at reference wavelength λ_0. The exponential slope S_CDOM varies slightly for different waters73,74, with values ranging from 0.004 to 0.036 nm−1. We used λ_0 = 440 nm as the reference wavelength and S_CDOM = 0.014 nm−1 for the slope. The value of a_CDOM(440) depends on the CDOM concentration. Our calculations were made for a wide range of CDOM concentrations, from a_CDOM(440) = 0.0025 m−1 in very clear ocean waters with extremely low CDOM concentrations75 to a_CDOM(440) = 5 m−1 in the river plumes of Arctic rivers with very high CDOM concentrations42,43.

NAP. NAP includes both organic particles (e.g., detritus) and inorganic particles (e.g., clay minerals) suspended in the water column or ice layer. Similar to CDOM, absorption by NAP decreases exponentially with wavelength:

a_NAP(λ) = a_NAP(λ_0) * exp(−S_NAP * (λ − λ_0)) (10)

where λ_0 is again the reference wavelength at 440 nm. The spectral slope S_NAP ranges from 0.006 to 0.017 nm−1, depending on the composition of the NAP74,76; we used S_NAP = 0.007 nm−1. The value of a_NAP(440) increases with the NAP concentration, as described by the empirical relation a_NAP(440) = 0.031 * NAP; it ranges from 0.001 m−1 in very clear ocean waters to sometimes more than 1 m−1 in turbid coastal waters74,76.

While CDOM has negligible scattering, NAP can be an important scattering component depending on the concentration, composition, and size of the suspended particles77,78. Here, we assume a mass-specific scattering function based on the average of the mass-specific scattering spectra of calcareous sand, red clay, yellow clay, and brown earth33. We used again the Petzold average-particle phase function to describe the angular distribution of the scattered light32,72.

Phytoplankton and ice algae. Due to their photosynthetic pigments, phytoplankton and ice algae can strongly affect the absorption and scattering properties of aquatic systems. We included measured chlorophyll concentrations in the model for our model validation (Fig. 2), using the chlorophyll-specific spectral absorption and scattering properties described in the literature79,80. However, in the remainder of this study, we are interested in the light field created by the abiotic environment, which provides a template for the initial development of new populations of phytoplankton and ice algae. Therefore, concentrations of phytoplankton and ice algae, and hence their absorption and scattering, were set to zero in our model calculations for Figs. 3–5.

Snow cover
For snow-covered ice in Supplementary Fig. S5, we assume a two-layer system with a snow layer of thickness z_S on top of an interior layer of ice (INT)19. Planar irradiance E_d(λ, z_S) transmitted through the snow layer was calculated by a simple semi-empirical model37:

E_d(λ, z_S) = (1 − α) * E_d(λ, 0) * exp(−κ(λ) * z_S) (11)

where α is the albedo of snow, E_d(λ, 0) is the incident solar irradiance, and κ(λ) is the wavelength-dependent extinction coefficient of snow. We used the incident solar irradiance of Supplementary Fig. S2 and the albedo and wavelength-dependent extinction coefficient of snow measured by Perovich37, taking into account the thickness of the snow layer. The planar irradiance transmitted through the snow served as input for the ice underneath.

Model validation with field data
To validate the predictions of the radiative transfer model for ice-covered waters, we compare the model predictions with measured planar irradiance spectra below sea ice (Fig. 2). For this purpose, we used published field measurements from three locations with different optical properties: Amundsen Basin (station 231) in the Central Arctic Ocean2,35, the Bothnian Bay (station D3) in the Baltic Sea34 and McMurdo Sound in Antarctica (Figure 6.15 in ref. 2). At each location, planar irradiance spectra, E_d(λ), in the visible and NIR range were measured both above the snow-free ice surface and below sea ice with hyperspectral TriOS RAMSES radiometers. Field measurements2,34,35 of planar irradiance spectra above the sea ice (Supplementary Fig. S1), albedo of the snow-free ice surface, ice thickness, and concentrations of chlorophyll a, CDOM, and NAP served as input for the model.

NAP concentrations at the Central Arctic Ocean and McMurdo Sound were not reported in Lund-Hansen et al.2,35. Instead, we used a_NAP(440) = 0.01 m−1 measured in Arctic sea ice by ref. 76. Furthermore, the CDOM concentration at McMurdo Sound was not measured by ref. 2, and scattering coefficients of sea ice were not measured at any of the three locations. Hence, a_CDOM(440) at McMurdo Sound and the scattering coefficients of sea ice were estimated by least-squares fits of the model to the measured spectra. Estimates of the scattering coefficients at the Central Arctic Ocean and McMurdo Sound were constrained by the range of values reported for sea ice by ref. 31, while we allowed lower scattering coefficients for the Baltic Sea because of its low salinity34.

For each location, the parameter values and functions used in the radiative transfer model are specified in Supplementary Table S1.

Data availability
The spectral data generated in this study have been deposited in Figshare: https://figshare.com/s/abe54f20cd0165596170.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports a detailed study of the differences in expected spectral light properties in the ocean beneath an ice cover and without an ice cover. Noteworthy results include the predicted spectral narrowing of the light field available for photosynthesis in open ocean with no ice. This narrowing includes a spectral signature with shorter wavelengths and more pronounced spectral niches. The results suggest that reduction of sea ice may likely lead to changes in pigment and taxonomic composition of primary producers in the polar ecosystem. The manuscript is clear, organized, very well written, and contains clear figures. The conclusions are well supported and the analysis and methodology is clear and straightforward. I recommend publication as is, after attention to the following very minor technical corrections.\nreplace “ice”, with “sea ice” in many instances throughout the text\n45: brine “inclusions” (more general than “channels”), several instances throughout manuscript. Also, not sure what is meant by “snow pockets”?\n85: equation (singular), not equations\n123: “(without snow cover and ice algae)”, also without a surface scattering layer? (as defined in Reference 19)\n165: “Superimposition” I think “superposition” instead\n411: Add references to Fig 1c caption", "reviewer_2": "The manuscript titled, “Loss of sea ice alters light spectra for aquatic photosynthesis” by Soja‐Woźniak et al., presents the idea that as climate change impacts sea ice cover in polar regions, there will be a loss of a light spectra-specific niches that could impact primary producer biodiversity. I note that my background in radiative transfer is not strong enough to comment on this aspect of the manuscript. Instead, I focus on the biological interpretation of the results. The concept is intriguing; however, the arguments and examples to explain how biodiversity could change are limited. I have provided some examples below relative to this statement.\n(1) The comparison of change between under an ice cover to light available in the open ocean is really only comparable at the same intensity levels (i.e., all things being held equal, what is the change?). In the paragraph starting on line 265, Beeler Sooho (1987) is cited who compared P. antarctica between open water (ice edge) and under the ice. As noted in Beeler Soohoo et al. (1987), these sites were under completely different light intensities (open water vs ice covered) and spectra (due to absorption associated with congelation and platelet algal communities above the under-ice advected bloom) that led to their conclusions relative to changes in chl-specific absorption coefficients. In the submitted manuscript, the change in absorption near 500 nm from Beeler Soohoo et al. (1987) seems to be interpreted more relative to the physical radiative transfer differences that sea ice causes (i.e., smooths and shifts spectra relative to open water) rather than the impacts of overlying algal pigments and total irradiance on photoacclimation of an algal community between the open water and under-ice sites.\n(2) In paragraph starting on Line 224, it is argued that the smoothed and shifted spectra in the ice and ice-covered environment helps explain why diatoms and Phaeocystis dominate these environments. This statement was then refuted on line 258-259, “… diatoms and prymnesiophytes are not restricted to ice-covered waters…”. In fact, they often dominate open water blooms when nutrients are replete. Similar to point (1) above, the interpretation seems to miss the impact of intra- (package effect) and inter- (influence of high algal concentrations in the ice environment) cellular self-shading that can cause photoacclimation and production of different pigments. Also, as rightfully pointed out in the discussion of the same paragraph, nutrients can greatly impact species composition, particularly in reference to diatom versus Phaeocystis dominance; however, the role of nutrients is largely glossed over in this paragraph.\nAs these arguments carry a large portion of the discussion, I believe this manuscript requires a bit of re-focus before being considered for publication.", "reviewer_3": "This study investigates the potential ramifications of the possible disappearance of summer sea ice in the Arctic and Antarctic on the light environment experienced by marine microalgae. The study places particular emphasis on the significance of \"peaks\" present in the absorption spectrum of pure seawater, which are notably absent in the absorption spectrum of pure sea ice. Through a series of radiative transfer simulations conducted in both sea ice and open ocean conditions, disparities are observed in the irradiance spectrum at the base of the euphotic zone.\nWhile the abstract appeared promising, I found myself encountering several points of concern:\nFirstly, sea ice in polar oceans typically exhibits low levels of CDOM. Simulations utilizing an absorption coefficient for CDOM reaching 20 m-1 are notably unrealistic. Even in the Baltic Sea, which serves as an imperfect proxy for the Arctic or Antarctic Ocean, reported in-situ data suggests aCDOM barely exceeds 1 m-1. Yet, a considerable portion of the conclusions drawn from the simulations hinge on substantial variations in aCDOM, surpassing the influence of differences in absorption between water and sea ice.\nSecondly, the simulations depicted in figures 3 and 4 assume sea ice thicknesses of up to 30 m. However, the average thickness in the Arctic is approximately 1.5 m, and even less in the Antarctic. The rationale behind simulating such thick sea ice to compare equivalent optical depths appears questionable.\nThirdly, considering that nearly two-thirds of the Arctic Ocean is covered by first-year sea ice that melts annually, it prompts inquiry into whether significant distinctions exist between regions with perennial ice cover and those ice-free in summer. This warrants a thorough examination of existing data to discern any notable differences, rather than delving into discussions about cyanobacteria absent from polar oceans. Furthermore, it's worth noting that cryptophytes, which also possess phycobilins, are found at high latitudes.\nMoreover, sea ice behaves somewhat akin to clouds, with its albedo primarily governing photon loss in a largely neutral manner within the visible spectrum. Consequently, the irradiance spectrum under sea ice broadly resembles the incident solar spectrum, save for the absorption of red light by the ice. Removing sea ice would elevate PAR at the surface without substantially altering the irradiance spectrum. This resemblance is likely more pronounced few meters below the surface.\nHence, it appears that the primary change in surface irradiance resulting from sea ice removal is a general decrease, which warrants further consideration, at least for contextualizing the results.\nOverall, I believe this study may overemphasize the impact of differences in the absorption spectrum of ice and seawater, exaggerate the influence of CDOM in ice through the use of excessively high values, simulate sea ice thicknesses that are unrealistic, and engage in speculative discourse regarding the potential impact of summer sea ice disappearance on spectral niches and microalgae selection in polar oceans, without sufficiently grounding its arguments in existing literature. Therefore, I would cautiously advise against publication at this time."}, "author_response": "Response to the reviewer’s comments\nResponse to Reviewer 1:\nThis manuscript reports a detailed study of the differences in expected spectral light properties in\nthe ocean beneath an ice cover and without an ice cover. Noteworthy results include the predicted\nspectral narrowing of the light field available for photosynthesis in open ocean with no ice. This\nnarrowing includes a spectral signature with shorter wavelengths and more pronounced spectral\nniches. The results suggest that reduction of sea ice may likely lead to changes in pigment and\ntaxonomic composition of primary producers in the polar ecosystem. The manuscript is clear,\norganized, very well written, and contains clear figures. The conclusions are well supported and the\nanalysis and methodology is clear and straightforward. I recommend publication as is, after\nattention to the following very minor technical corrections.\nResponse: We thank Reviewer #1 for the positive feedback and kind appreciation of our manuscript.\nreplace “ice”, with “sea ice” in many instances throughout the text\nResponse: We have replaced “ice” by “sea ice” at more than 50 places in the manuscript.\n45: brine “inclusions” (more general than “channels”), several instances throughout manuscript.\nAlso, not sure what is meant by “snow pockets”?\nResponse: We changed “brine channels” to “brine inclusions” throughout the manuscript.\nFurthermore, we removed the “snow pockets” (line 47).\n85: equation (singular), not equations\nResponse: We corrected “equations” to “equation” (singular), both on former line 85 (now line 88)\nand in the Methods section (line 334).\n123: “(without snow cover and ice algae)”, also without a surface scattering layer? (as defined in\nReference 19)\nResponse: Good point; actually this was a very important comment. In our previous manuscript\nversion, we had incorporated a depth-dependent scattering coefficient with a surface scattering layer\nin the model predictions for the central Arctic Ocean, Baltic Sea and McMurdo Sound Antarctica in\nFigure 2. However, we had ignored the surface scattering layer in our model calculations for Figures 3\nand 4. Instead we had assumed a uniform scattering coefficient with a low value representative of\nthe interior ice layer in Figures 3 and 4. (Our reasoning was that we wanted to keep the model as\nsimple as possible). However, this assumption resulted in sea ice thicknesses of up to 30 m, which is\nunrealistic as Reviewer #3 correctly pointed out.\nWe have now included a surface scattering layer (SSL) in the model calculations for Figure 3\nand 4, using the same approach as in Reference 19 (Smith et al. 2022; see also Briegleb & Light\n1\n2007). The values of the depth-dependent scattering coefficient in Figures 3 and 4 are based on\nFigure 8 of Light et al. (2015) and the values we applied in our model validation for the central Arctic\nOcean, Baltic Sea and McMurdo Sound Antarctica (our Figure 2). The higher scattering coefficients in\nthe top layers result in a much faster light attenuation. Hence, the ice thicknesses at the 5% and 1%\nlight levels are now reached within a few meters, thus resolving the comment of Reviewer #3.\nThe addition of a surface scattering layer has led to the following changes in the manuscript:\nWe have added a sentence in the Methods section (lines 337-340) and we added the new Extended\nData Figure 4, to explain how depth-dependent scattering has been incorporated in our model. We\nprovide the new parameter values for the depth-dependent scattering coefficient in Extended Data\nTable 2. All model calculations have been redone based on this new model structure, and all graphs\nin Figure 3, Figure 4 and Extended Data Figure 3 have been redrawn. We have updated the new\nvalues obtained from the model calculations in Table 1 and Extended Data Table 3. We have adjusted\nour descriptions of these figures and tables throughout the main text, in accordance with the new\nresults. Finally, returning to the reviewer’s comment, we now mention on former line 123 (now line\n126) that the model includes a surface scattering layer.\n165: “Superimposition” I think “superposition” instead\nResponse: We replaced “superimposition” by “overlay” (line 170 and line 175).\n411: Add references to Fig 1c caption\nResponse: In the caption of Figure 1c we have now written “The scattering coefficient of sea ice\nvaries depending on the amount of air bubbles and brine inclusions. Here we show a value of b = 20\nice\nm-1, representative for the interior layer of Arctic sea ice”, and added a reference to the paper of\nBonnie Light et al. (2015; see their Figure 8).\nLight, B., Perovich, D. K., Webster, M. A., Polashenski, C. & Dadic, R. Optical properties of melting\nfirst-year Arctic sea ice. Journal of Geophysical Research: Oceans 120, 7657–7675 (2015).\nResponse to Reviewer 2:\nThe manuscript titled, “Loss of sea ice alters light spectra for aquatic photosynthesis” by Soja-\nWoźniak et al., presents the idea that as climate change impacts sea ice cover in polar regions, there\nwill be a loss of a light spectra-specific niches that could impact primary producer biodiversity. I note\nthat my background in radiative transfer is not strong enough to comment on this aspect of the\nmanuscript. Instead, I focus on the biological interpretation of the results. The concept is intriguing;\nhowever, the arguments and examples to explain how biodiversity could change are limited. I have\nprovided some examples below relative to this statement.\nResponse: We thank Reviewer #2 for the kind and constructive comments on the biological\ninterpretation of our results.\n(1) The comparison of change between under an ice cover to light available in the open ocean is\nreally only comparable at the same intensity levels (i.e., all things being held equal, what is the\nchange?). In the paragraph starting on line 265, Beeler Sooho (1987) is cited who compared P.\n2\nantarctica between open water (ice edge) and under the ice. As noted in Beeler Soohoo et al. (1987),\nthese sites were under completely different light intensities (open water vs ice covered) and spectra\n(due to absorption associated with congelation and platelet algal communities above the under-ice\nadvected bloom) that led to their conclusions relative to changes in chl-specific absorption\ncoefficients. In the submitted manuscript, the change in absorption near 500 nm from Beeler\nSoohoo et al. (1987) seems to be interpreted more relative to the physical radiative transfer\ndifferences that sea ice causes (i.e., smooths and shifts spectra relative to open water) rather than\nthe impacts of overlying algal pigments and total irradiance on photoacclimation of an algal\ncommunity between the open water and under-ice sites.\nResponse: This was an important comment for our revisions. Although it is always a struggle to keep\nNature manuscripts short and to the point, we admit that our original description in this paragraph\nwas too concise. We fully agree with the reviewer that the study of Beeler Soohoo et al. compared\nsites that differ both in light intensity and spectral composition. However, we could not find any field\nexamples in the literature where absorption spectra of the same algal species were compared\nbetween open water and ice-covered water at the same light intensity (even though we fully agree\nwith the reviewer that that would have been the ideal case to compare our model predictions with).\nTherefore, although their study does not provide the perfect comparison, we do think that the\nfindings of Beeler Soohoo et al. (1987) are interesting, because of their detailed description of the\nirradiance spectra in and below sea ice and because of the observed difference in pigmentation\nbetween Phaeocystis below sea ice and in open water.\nTherefore, we made the following changes in the manuscript:\n- In the first paragraph of the Introduction, we added a sentence to explain from the start that\nprimary producers will be exposed to high light intensities when the ice melts, which can\nhave major effects on photoacclimation and community composition (lines 34-37).\n- In the Discussion, we now explain that - in the study of Beeler Soohoo et al. - the irradiance\nlevels below sea ice were low and the spectrum was dominated by green wavelengths due to\nlight attenuation by sea ice and self-shading by high densities of ice algae (lines 305-307).\n- Furthermore, we now added in the Discussion that comparison of their findings with our\nmodel predictions is not straightforward. We have discussed the caveats as follows: “Yet,\ninterpretation of these findings is not straightforward. Algae exposed to high irradiances\nnear the water surface tend to have low contents of accessory photosynthetic pigments\n(including fucoxanthin) and high contents of photoprotective pigments to avoid\nphotodamage (Robinson et al. 1997 ; Galindo et al. 2017; Lund-Hansen et al. 2020).\nEnhanced fucoxanthin contents are therefore often interpreted as a photoacclimative\nresponse to low irradiances, which may explain the high fucoxanthin contents under ice\nreported by Beeler Soohoo et al. (1987). At these low irradiances, however, acclimation to\nthe prevailing light spectrum below sea ice likely also plays a role, as enhanced fucoxanthin\ncontents clearly extend light harvesting for photosynthesis to longer wavelengths.” (lines\n315-323).\nRobinson, D. H., Kolber, Z., & Sullivan, C. W. (1997). Photophysiology and photoacclimation in surface\nsea ice algae from McMurdo Sound, Antarctica. Marine Ecology Progress Series 147: 243-256.\nGalindo, V., Gosselin, M., Lavaud, J., Mundy, C. J., Else, B., Ehn, J., ... & Rysgaard, S. (2017). Pigment\ncomposition and photoprotection of Arctic sea ice algae during spring. Marine Ecology Progress\nSeries 585: 49-69.\nquality, and pigment composition of Arctic sea ice algae. Marine Ecology Progress Series 648:95–110.\n(2) In paragraph starting on Line 224, it is argued that the smoothed and shifted spectra in the ice\nand ice-covered environment helps explain why diatoms and Phaeocystis dominate these\nenvironments. This statement was then refuted on line 258-259, “… diatoms and prymnesiophytes\nare not restricted to ice-covered waters…”. In fact, they often dominate open water blooms when\nnutrients are replete.\nResponse: We fully agree with the reviewer that diatoms and prymnesiophytes dominate not only in\nice-covered seas, but also in several other habitats (e.g., deeply mixed waters, turbid coastal waters).\nIn our view, however, the statement that “diatoms dominate ice-covered environments” and the\nstatement that “diatoms can also dominate other habitats” are not contradictory. Both statements\nare true. That is, diatoms are quite versatile and occupy several different ecological niches. However,\ntriggered by a comment of Reviewer #3, that we should shift our focus to the phytoplankton of polar\nseas instead of cyanobacterial pigments, we have completely reorganized this part of the Discussion.\nTherefore, this paragraph has been largely rewritten (lines 285-301), and in doing so the statement\nthat ““… diatoms and prymnesiophytes are not restricted to ice-covered waters…” has been removed.\nSimilar to point (1) above, the interpretation seems to miss the impact of intra- (package effect) and\ninter- (influence of high algal concentrations in the ice environment) cellular self-shading that can\ncause photoacclimation and production of different pigments. Also, as rightfully pointed out in the\ndiscussion of the same paragraph, nutrients can greatly impact species composition, particularly in\nreference to diatom versus Phaeocystis dominance; however, the role of nutrients is largely glossed\nover in this paragraph.\nResponse: As outlined in our response to point (1) above, we now briefly discuss intercellular self-\nshading, photoacclimation and the production of different pigments in the context of the study of\nBeeler Soohoo et al. (1987) (lines 302-323). Furthermore, we are fully aware that nutrients can\ngreatly impact species composition (see previous publications by several members of our team).\nAlthough nutrients are not the main topic of our current manuscript, we now discuss the role of\nnutrient limitation in the context of the observed shift from diatoms to Micromonas in the Canada\nBasin of the Arctic Ocean (e.g. Li et al. Science 2009; Tremblay et al. 2015) (see the new text on lines\n286-291).\nAs these arguments carry a large portion of the discussion, I believe this manuscript requires a bit of\nre-focus before being considered for publication.\nResponse: We agree, and the above comments were very helpful to improve the ecological and\nphysiological context of our manuscript.\nResponse to Reviewer 3:\nThis study investigates the potential ramifications of the possible disappearance of summer sea ice\nin the Arctic and Antarctic on the light environment experienced by marine microalgae. The study\nplaces particular emphasis on the significance of \"peaks\" present in the absorption spectrum of pure\nseawater, which are notably absent in the absorption spectrum of pure sea ice. Through a series of\n4\nradiative transfer simulations conducted in both sea ice and open ocean conditions, disparities are\nobserved in the irradiance spectrum at the base of the euphotic zone.\nWhile the abstract appeared promising, I found myself encountering several points of concern:\nFirstly, sea ice in polar oceans typically exhibits low levels of CDOM. Simulations utilizing an\nabsorption coefficient for CDOM reaching 20 m-1 are notably unrealistic. Even in the Baltic Sea,\nwhich serves as an imperfect proxy for the Arctic or Antarctic Ocean, reported in-situ data suggests\naCDOM barely exceeds 1 m-1. Yet, a considerable portion of the conclusions drawn from the\nsimulations hinge on substantial variations in aCDOM, surpassing the influence of differences in\nabsorption between water and sea ice.\nResponse: We agree with the reviewer that sea ice in polar oceans usually exhibits low levels of\nCDOM, and that aCDOM = 20 m-1 is indeed an extremely high value that is unrealistic for marine\necosystems. Nevertheless, the large Arctic rivers of Canada, Alaska and Russia flowing into the Arctic\nOcean can easily reach aCDOM values of 1 to 5 m-1, because of the enormous amounts of organic\ncarbon stored in tundra peat layers. See for example the recent studies of Novak et al. (Frontiers in\nMarine Science 2022) and Juhls et al. (Remote Sensing of Environment 2022).\nOur aim is to systematically investigate differences in irradiance spectra predicted for water\nversus sea ice, across a wide range of CDOM concentrations representative of different marine\necosystems. Therefore, we have now lowered the maximum CDOM value accordingly. In Figure 3 and\nExtended Data Figure 3, we have removed the panels with highest aCDOM of 10 m-1, and now only\nshow results for aCDOM = 0.01, 0.1 and 1 m-1. Furthermore, we have adjusted the wide range of\naCDOM values in Figure 4 by lowering the maximum value to 5 m-1. We now mention in the legend of\nFigure 4, in the main text (lines 162-164) and in the Methods section (lines 404-405) that these high\naCDOM values of up to 5 m-1 are representative for the river plumes of Arctic rivers, citing the studies\nof Novak et al. (2022) and Juhls et al. (2022). Although these changes affected our figures (Figure 3,\nFigure 4, and Extended Data Figure 3), they did not alter our main conclusions.\nNovak MG, Mannino A, Clark JB, Hernes P, Tzortziou M, Spencer RGM, Kellerman AM and Grunert B\n(2022) Arctic biogeochemical and optical properties of dissolved organic matter across river to sea\ngradients. Front. Mar. Sci. 9: 949034. doi: 10.3389/fmars.2022.949034\nJuhls B, Matsuoka A, Lizotte M, Bécu G, Overduin PP, El Kassar J, Devred E, Doxaran D, Ferland J,\nForget MH, Hilborn A, Hieronymi M, Leymarie E, Maury J, Oziel L, Tisserand L, Anikina DOJ, Dillon M,\nBabin M (2022) Seasonal dynamics of dissolved organic matter in the Mackenzie Delta, Canadian\nArctic waters: Implications for ocean colour remote sensing. Remote Sensing of Environment 283:\n113327. doi: 10.1016/j.rse.2022.113327\nSecondly, the simulations depicted in figures 3 and 4 assume sea ice thicknesses of up to 30 m.\nHowever, the average thickness in the Arctic is approximately 1.5 m, and even less in the Antarctic.\nThe rationale behind simulating such thick sea ice to compare equivalent optical depths appears\nquestionable.\nResponse: This is an important comment that has led to major changes throughout our manuscript.\nIn Figures 3 and 4 (and also in Extended Data Figure 3), we had ignored the surface scattering layer.\nInstead we had assumed a uniform scattering coefficient with a low value representative of the\ninterior ice layer. (Our reasoning was that we wanted to keep the model as simple as possible).\nReviewer #3 correctly points out.\nIn line with a comment by Reviewer #1 (see above), we have now included a surface\nscattering layer (SSL) in the model calculations for Figure 3 and 4, using the same approach as in the\nmodels of e.g., Briegleb & Light (2007), Light et al. (2015) and Smith et al. (2022). The values of the\ndepth-dependent scattering coefficient in Figures 3 and 4 are based on Figure 8 of Light et al. (2015)\nand the values we had applied in our model validation for the central Arctic Ocean, Baltic Sea and\nMcMurdo Sound Antarctica (our Figure 2). The higher scattering coefficients in the top layers result in\na much faster light attenuation. As a consequence, the ice thicknesses at the 5% and 1% light levels\npredicted by the model are now only a few meters or less, and hence within the realistic range (see\nthe new versions of Figures 3 and 4).\nThe addition of a surface scattering layer has led to the following changes in the manuscript:\nWe have added a sentence in the Methods section (lines 337-340) and we added the new Extended\nData Figure 4, to explain how depth-dependent scattering has been incorporated in our model. We\nprovide the new parameter values for the depth-dependent scattering coefficient in Extended Data\nTable 2. All model calculations have been redone based on this new model structure, and all graphs\nin Figure 3, Figure 4 and Extended Data Figure 3 have been redrawn. We have updated the new\nvalues obtained from the model calculations in Table 1 and Extended Data Table 3. Finally, we have\nadjusted our descriptions of these figures and tables throughout the main text, in accordance with\nthe new results.\nBriegleb BP and Light B (2007) A delta-Eddington multiple scattering parameterization for solar\nradiation in the sea ice component of the Community Climate System Model. NCAR Tech. Note TN-\n4721STR, 100 pp, National Center for Atmospheric Research, Boulder, Colorado.\nLight B, Perovich DK, Webster MA, Polashenski C, Dadic R (2015). Optical properties of melting first-\nyear Arctic sea ice. Journal of Geophysical Research: Oceans 120: 7657-7675.\ndoi:10.1002/2015JC011163\nSmith MM, Light B, Macfarlane AR, Perovich DK, Holland MM, Shupe MD (2022) Sensitivity of the\nArctic sea ice cover to the summer surface scattering layer. Geophysical Research Letters 49:\ne2022GL098349. doi:10.1029/2022GL098349\nThirdly, considering that nearly two-thirds of the Arctic Ocean is covered by first-year sea ice that\nmelts annually, it prompts inquiry into whether significant distinctions exist between regions with\nperennial ice cover and those ice-free in summer.\nResponse: We briefly touch upon optical properties of different types of sea ice in the Discussion\n(lines 216-226). In particular, we now compare sea ice with a weak surface scattering layer (SSL) and\nlow scattering coefficients against sea ice with a strong SSL and high scattering coefficients. This\ncomparison is somewhat akin to a comparison between first-year ice and multi-year ice, although we\nwant to be cautious in our wording here as other differences between ice types (e.g., ponded ice\nversus snow-covered ice) have a stronger effect on the optical properties of ice than age per se. The\nresults are shown in the new version of Extended Data Figure 3. In the Discussion, we have now\nadded: “The results show that sea ice with a weak surface scattering layer and low scattering\ncoefficients will have higher light transmission but slightly narrower irradiance spectra than sea ice\nwith a strong surface scattering layer” (lines 221-224). We agree that a more extensive comparison\n6\nof different ice types would also be interesting, but that would require a novel study with new data\nand new model analyses, which is beyond the scope of our current manuscript.\nThis warrants a thorough examination of existing data to discern any notable differences, rather\nthan delving into discussions about cyanobacteria absent from polar oceans. Furthermore, it's worth\nnoting that cryptophytes, which also possess phycobilins, are found at high latitudes.\nResponse: This was a key comment. We agree that cyanobacteria are essentially absent from the\npolar oceans. Hence, we have removed the absorption spectra of cyanobacteria from Figure 5b, and\nreplaced them by absorption spectra of phytoplankton taxa common in polar seas (prasinophytes\nand cryptophytes). See the new version of Figure 5b. We have completely rewritten the\naccompanying texts in the Discussion, which span four paragraphs (lines 239-301).\nSpecifically, we now explain that the small prasinophyte Micromonas polaris is one of the\nmost abundant planktonic algae of the oligotrophic Arctic Ocean, that it has a narrower absorption\nspectrum than diatoms, that its main photosynthetic pigments chlorophyll a and chlorophyll b\ncapture the violet and blue niche, and that it has strongly increased in abundance at the cost of\ndiatoms in the Canada Basin (e.g., Li et al. Science 2009) where ice cover has diminished considerably\nin recent decades (e.g., Babb et al. 2022). We also briefly describe the diversity of phycobilins in\ncryptophytes and the wavelengths they absorb (Greenwold et al. 2019), and the increasing\nabundance of cryptophytes in coastal waters of the West Antarctic Peninsula (e.g., Mendes et al.\n2023). Furthermore, we explain that our model suggests that the loss of ice cover will cause a\ndecrease in the diversity of cryptophyte pigment types, which is in line with the observation that the\ndiversity of cryptophytes in these coastal waters is surprisingly low (Brown et al. 2021).\nOf course, we are fully aware that many other factors affect the species composition of\nphytoplankton communities as well. Phytoplankton in the polar seas have to adjust to many changes\noccurring simultaneously, including changes in light quality and quantity, nutrient conditions,\nstratification, and many other biotic and abiotic variables. Nevertheless, the overall correspondence\nbetween changes in irradiance spectra due to the loss of sea ice and changes in pigmentation of ice\nalgae versus open ocean phytoplankton is remarkable, indicating that light quality is an important\nselective factor in communities of photosynthetic organisms.\nWe thank the reviewer for raising this comment, and we believe that these major changes in\nthe Discussion have strongly increased the ecological relevance of our study.\nLi, W. K. W., McLaughlin, F. A., Lovejoy, C. & Carmack, E. C. (2009). Smallest algae thrive as the Arctic\nOcean freshens. Science 326: 539.\nBabb, D. G. et al. (2022). Increasing multiyear sea ice loss in the Beaufort Sea: a new export pathway\nfor the diminishing multiyear ice cover of the Arctic Ocean. Geophysical Research Letters 49,\ne2021GL097595.\nGreenwold, M. J., Cunningham, B. R., Lachenmyer, E. M., Pullman, J. M., Richardson, T. L. & Dudycha,\nJ. L. (2019). Diversification of light capture ability was accompanied by the evolution of\nphycobiliproteins in cryptophyte algae. Proc. R. Soc. London B 286: 20190655.\nMendes, C. R. B., Costa, R. R., Ferreira, A., Jesus, B., Tavano, V. M., Dotto, T. S., Leal, M. C., Kerr, R.,\nIslabão, C. A., Franco, A. O. R., Mata, M. M., Garcia, C. A. E. & Secchi, E. R. (2023). Cryptophytes: an\nemerging algal group in the rapidly changing Antarctic Peninsula marine environments. Global\nChange Biology 29: 1791-1808.\nPeninsula. Limnology and Oceanography 66, 2470–2480.\nMoreover, sea ice behaves somewhat akin to clouds, with its albedo primarily governing photon loss\nin a largely neutral manner within the visible spectrum. Consequently, the irradiance spectrum\nunder sea ice broadly resembles the incident solar spectrum, save for the absorption of red light by\nthe ice. Removing sea ice would elevate PAR at the surface without substantially altering the\nirradiance spectrum. This resemblance is likely more pronounced few meters below the surface.\nHence, it appears that the primary change in surface irradiance resulting from sea ice removal is a\ngeneral decrease, which warrants further consideration, at least for contextualizing the results.\nResponse: We agree that the high albedo of sea ice removes a large fraction of the incident solar\nradiation in a wavelength-independent manner. Furthermore, light attenuation in sea ice is\ndominated by scattering due to air bubbles and brine inclusions, which is also essentially wavelength\nindependent, while absorption by ice makes a much smaller contribution to light attenuation and is\nhighest in the red part of the spectrum. These aspects are all included in our model, illustrated in\nFigure 1, and also highlighted in the first paragraph of the Discussion to explain why irradiance\nspectra below ice span a wide range of wavelengths. We have now added to this paragraph that\nirradiance spectra below sea ice “broadly resemble the incident solar spectrum” (lines 196-197).\nWe also agree that ice algae and phytoplankton living in or below the ice will be exposed to high\nsurface irradiances when the ice cover disappears. Therefore, in the first paragraph of the\nIntroduction, we have now added a sentence to explain from the start that primary producers will be\nexposed to high surface irradiance when the ice melts. Specifically, we have now written: “The high\nlight intensities in the surface layer to which these primary producers will be exposed when the ice\nmelts can have major effects on photoacclimation and community composition, and the implications\nof increased irradiance have therefore been extensively investigated (Arrigo et al. 2008; Galindo et al.\n2017; Lund-Hansen et al. 2020) (lines 34-37).\nFurthermore, we come back to this point in the Discussion, where we explain that algae exposed\nto high irradiances near the water surface tend to have low contents of accessory photosynthetic\npigments (including fucoxanthin) and high contents of photoprotective pigments to avoid\nphotodamage (Robinson et al. 1997; Galindo et al. 2017; Lund-Hansen et al. 2020) (lines 316-318).\nRobinson, D. H., Kolber, Z., & Sullivan, C. W. (1997). Photophysiology and photoacclimation in surface\nsea ice algae from McMurdo Sound, Antarctica. Marine Ecology Progress Series 147: 243-256.\nArrigo, K. R., van Dijken, G. & Pabi, S. Impact of a shrinking Arctic ice cover on marine primary\nproduction. Geophysical Research Letters 35, 2008GL035028 (2008).\nGalindo, V., Gosselin, M., Lavaud, J., Mundy, C. J., Else, B., Ehn, J., ... & Rysgaard, S. (2017). Pigment\ncomposition and photoprotection of Arctic sea ice algae during spring. Marine Ecology Progress\nSeries 585: 49-69.\nLund-Hansen, L. C. et al. (2020). Effects of increased irradiance on biomass, photobiology, nutritional\nquality, and pigment composition of Arctic sea ice algae. Marine Ecology Progress Series 648:95–110.\nof ice and seawater, exaggerate the influence of CDOM in ice through the use of excessively high\nvalues, simulate sea ice thicknesses that are unrealistic, and engage in speculative discourse\nregarding the potential impact of summer sea ice disappearance on spectral niches and microalgae\nselection in polar oceans, without sufficiently grounding its arguments in existing literature.\nResponse: We thank the reviewer for the thoughtful comments. We have now written more explicitly\nin the Abstract that light attenuation in sea ice is dominated by scattering (line 19) and we have\nincreased the contribution of scattering in the model by adding a surface scattering layer.\nFurthermore, we have lowered the maximum CDOM concentration by a factor four to values\nrepresentative of the river plumes of Arctic rivers, the sea ice thicknesses simulated by the model are\nnow in the realistic range (i.e., a few decimeters to a few meters), and we have rewritten large parts\nof the Discussion to better connect our results to changes in microalgal composition that are\ncurrently being observed in Arctic and Antarctic waters. All these changes are firmly based upon the\nexisting literature, which has increased the number of references (from 55 references in our original\nsubmission to 73 references in the revised manuscript).\nOur own changes:\n1. We shortened the Abstract to 150 words, in line with the formatting instructions of Nature\nCommunications.\n2. In our original submission, we argued in the Discussion that the results did not show a spectral\nniche in the violet region (< 450 nm) in Figure 4d, because the low solar elevation in polar\nregions largely removed the shortest wavelengths by Rayleigh scattering in the atmosphere.\nHowever, this explanation was not correct. Of course, the sun does have a low elevation in polar\nregions, but this was not the explanation for the lack of the violet wavelengths. Instead, we had\ninadvertently applied a relatively high absorption coefficient for non-algal particles (aNAP(440) =\n0.01 m-1), which effectively removed the violet wavelengths even at low CDOM concentrations.\nWe have now used a lower absorption coefficient for non-algal particles of aNAP(440) = 0.001 m-\n1, which is representative for clear ocean waters in the Arctic (Matsuoka et al. 2011).\nAs a consequence, the results now also show a spectral niche in the violet region of the\nspectrum (see the new Figure 4d). The reference to Matsuoka et al. (2011) has been added in the\nMethods section (Ref 66 on line 413) and the new value of aNAP(440) = 0.001 m-1 is\nimplemented in Extended Data Table 2.\nMatsuoka, A., Hill, V., Huot, Y., Babin, M. & Bricaud, A. (2011). Seasonal variability in the light\nabsorption properties of western Arctic waters: parameterization of the individual components of\nabsorption for ocean color applications. Journal of Geophysical Research: Oceans 116: C02007.\n3. We stored all data of our figures in figshare and added a Data Availability statement.\n4. As explained in the Methods, all our model calculations were done with the commercial\nsoftware package Hydrolight-Ecolight version HE6.0 (lines 332-334), which is a state-of-the-\nart radiative transfer model that solves the radiative transfer equation numerically. The\nparameter settings for these calculations are summarized in Extended Data Tables 1 and 2.\nHence, we did not write our own code, and therefore did not add a Code Availability\nstatement.\n9"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately responded to my concerns and have made appropriate technical corrections. I believe this manuscript is now ready to be published as revised.", "reviewer_4": "This is the first time I am reviewing the paper that has already gone through a previous round of reviews. While I find the idea that there can be a shift in spectra between ice-covered and ice-free waters that would in turn impact primary producers, I do not feel this manuscript is noteworthy enough for Nature Communications. My reasons are outlined below.\nThis is a modeling study that simulates light under sea ice and in the open ocean as a function of CDOM levels. I have several issues with the approach and its relevance:\n1. Realistic sea ice is not used in this simulation. Sea ice is covered by snow until summer melt and plays the most important role in light entering the upper ocean under the sea ice. However, under low light levels, which can happen for thin snow, there is enough light already for ice algae to grow (see recent paper by Clara Hoppe in Nature using the MOSAiC data). How concentrations of ice algae influence then the light regime entering the in water column is important in the narrowing of the spectra yet is not discussed.\n2. When looking at Figure 2 there is no discussion on what is a good fit of the model to observations. When there is a broader spectrum say for the Baltic Sea case, there is a pronounced shift between the modeled and observed spectra towards shorter wavelengths. How does this influence your results and interpretation? A shift is also seen in the Antarctic case.\n3. Regarding Figure 3, while it is clear that at depth in the ocean, the model simulates a narrower distribution of light there does not appear to be a clear shift in the peak wavelength between the light under the ice and at depth in the water column. But besides this, I don't understand why this comparison is made. Instead the figure should show the same depths in the water column with and without sea ice, rather than showing the light under the ice and then state this doesn't compare to the light at depth in the water column. You are not comparing the same things. If you want to talk about sea ice vs. open water then the same comparison should be made, i.e. show the irradiance at depth in the water column with and without sea ice.\n4. I do not understand the choice of ice albedos used, and why the Antarctic ice albedo is higher than that for the Arctic. It all depends on the thickness of the ice in the absence of snow but for 1.6 and 2m of ice, the bare ice albedo will not be different. 20% for 20cm ice may be realistic though it has to be backed by observations or a reference. The thickness of the SSL should also be specified.\n5. I do not find that a sufficient sensitivity study has been performed as a function of ice characteristics to make this study significant. How sensitive are the results to the scattering coefficients used for example? There are many observations made by different studies that have different coefficients than those of Biegrib and Light. I realize extended Figure 3 tries to do a sensitivity study by assuming different thicknesses of the SSL, DL and ICE but that is not the same as considering the impact of the actual scattering coefficients used. These will be different for the different ice types.\n6. It is difficult to make real implication statements as there are so many factors that influence primary productivity, including the initial blooms that happen under very low light levels, nutrient availability and timing of predatation. Without real observations to support these statements it does feel a bit underwhelming.\nOverall, I feel such a modeling study as this manuscript presents should be published somewhere, but to make it a Nature paper, I would have expected to see actual results on the taxonomy changes observed between a sea ice-covered regime and open water."}, "author_response": "Response to the reviewer’s comments\nResponse to Reviewer 1:\nThe authors have adequately responded to my concerns and have made appropriate technical\ncorrections. I believe this manuscript is now ready to be published as revised.\nResponse: We thank Reviewer #1 for the positive feedback and kind appreciation of our manuscript.\nResponse to Reviewer 4:\nThis is the first time I am reviewing the paper that has already gone through a previous round of\nreviews. While I find the idea that there can be a shift in spectra between ice-covered and ice-free\nwaters that would in turn impact primary producers, I do not feel this manuscript is noteworthy\nenough for Nature Communications. My reasons are outlined below.\nResponse: We thank the reviewer for the time invested in our manuscript. We are pleased to read\nthat the reviewer appreciates the idea that there can be a shift in spectra between ice-covered and\nice-free waters that would in turn impact primary producers. Obviously, we do feel that our novel\nfindings are noteworthy for Nature Communications and we also think the reviewer misinterpreted\nsome of our results, as we will outline in our responses below. We hope our explanations below as\nwell as the changes and new analyses in the manuscript will clarify most of the issues.\nThis is a modeling study that simulates light under sea ice and in the open ocean as a function of\nCDOM levels. I have several issues with the approach and its relevance:\n1. Realistic sea ice is not used in this simulation. Sea ice is covered by snow until summer melt and\nplays the most important role in light entering the upper ocean under the sea ice. However, under\nlow light levels, which can happen for thin snow, there is enough light already for ice algae to grow\n(see recent paper by Clara Hoppe in Nature using the MOSAiC data). How concentrations of ice algae\ninfluence then the light regime entering the in water column is important in the narrowing of the\nspectra yet is not discussed.\nResponse: Indeed, our model considers sea ice in late spring and summer, when the snow cover has\nmelted and a surface scattering layer develops due to ablation of the top layer of sea ice. The\nseasonal decrease in snow cover and subsequent development of the surface scattering layer are\nnicely illustrated in Figure 1 of Macfarlane et al. (Elementa Science of the Anthropocene 2023), which\nis also based on the MOSAiC expedition. In our view, investigating how the spectra are affected by\nthe loss of sea ice in the summer season is a perfectly valid scenario, especially because the loss of\nsea ice is most dramatic towards the end of the summer period. We now explain the seasonal setting\nof our model at the start of the Results section (lines 87-89): “We consider bare sea ice,\nrepresentative of late spring and summer when the snow has melted. After snow melt, often a\nporous granular structure develops in the top layer of melting sea ice known as the surface scattering\nlayer (Macfarlane et al. 2023).”\ndiminish light levels below sea ice. It was interesting to read the recent findings of Hoppe et al. (2024)\nthat the very low light levels below snow-covered ice can already provide sufficient irradiance for\nalgae to grow. Incidentally, we note that the paper of Hoppe et al. (2024) was published on 4\nSeptember 2024, i.e., after we had submitted our revised manuscript to the journal. Hence, their\nfindings were not available to us earlier.\nIn response to these comments, we added a new sensitivity analysis in which we compared\nirradiance spectra for different degrees of snow cover and extended our analysis to lower light levels\n(see the new Extended Data Figure 5). The results are described on lines 180-187:\n“Furthermore, in early spring, sea ice is often covered by a layer of snow (Ardyna & Arrigo 2020,\nMacfarlane et al. 2023), which has a very high albedo and can strongly diminish light levels (Perovich\n2007, Lund-Hansen et al. 2014). Our calculations revealed that even a thin layer of only 3 cm of snow\ncan remove almost 90% of the incident irradiance, in line with field observations (Hamre 2004; Lund-\nHansen et al. 2014). Yet, the low light levels below snow-covered ice can already provide sufficient\nirradiance for ice algae to grow (Hancke et al. 2018; Hoppe et al. 2024). We therefore also compared\nirradiance spectra below ice for different degrees of snow cover, and extended the comparison to\nlower light levels. The results show that sea ice covered by snow will generate even broader\nirradiance spectra than bare sea ice without snow cover (Extended Data Figure 5).”\nOur model does include light absorption and scattering by ice algae and phytoplankton. See\nequations (3) and (4) in the Methods section. Their effect on the light regime depends on their\nconcentration. For example, in our case study of the Central Arctic Ocean, the chlorophyll a\nconcentration in sea ice is very low and has hardly any effect on the calculated spectrum (Figure 2a).\nHowever, the chlorophyll a concentrations of ice algae were higher at the Baltic Sea (Figure 2b) and\nvery high at McMurdo Sound in Antarctica (Figure 2c), where it had an effect on the calculated\nspectra. How these ice algae are important in narrowing the spectra entering the water column is\nexplicitly discussed in the Results section. On lines 115-119, we wrote:\n“At McMurdo Sound, in Antarctica …. … … the chlorophyll a concentration of ice algae was\nvery high (15.2 mg m-2). Hence, both violet-blue and red wavelengths were largely absorbed by\nchlorophyll, and the light spectrum underneath the ice cover was dominated by green and yellow\nwavelengths.”\nIn the next section of the Results, entitled ‘Comparison of light spectra below sea ice and in\nopen water’, we model light regimes without ice algae or phytoplankton. We do this on purpose, to\npredict irradiance spectra generated by the abiotic environmental conditions, because these spectra\nprovide the template for new algal photosynthesis and growth. We now explain this at the beginning\nof this section: “The concentration of ice algae and phytoplankton is set to zero to predict the light\nspectra generated by the abiotic environment (i.e., by sea ice, seawater, CDOM and NAP). In other\nwords, we calculate the spectra that will be available for aquatic photosynthesis when new\npopulations of ice algae or phytoplankton start to grow.” (lines 130-134).\n2. When looking at Figure 2 there is no discussion on what is a good fit of the model to observations.\nWhen there is a broader spectrum say for the Baltic Sea case, there is a pronounced shift between\nthe modeled and observed spectra towards shorter wavelengths. How does this influence your\nresults and interpretation? A shift is also seen in the Antarctic case.\nIn response to this comment, we made several changes in our model simulations for Figure 2:\n In our original submission we used a 4-layered system of ice to model the irradiance spectra\nin Figure 2. We now changed it to a 3-layered system (SSL, DL, INT) in order to make the\n2\napproach fully consistent throughout the manuscript, since we used a 3-layered system for\nthe model calculations in all other figures. The thicknesses of the different layers (SSL, DL,\nINT) are specified in Extended Data Table 1.\n We now adopted the empirical relation of Babin et al. (2003) to convert the absorption\ncoefficient of NAP (a (440)) to the NAP concentration (see the new sentence added on line\nNAP\n446-447); this affected the NAP concentration used to calculate the spectral scattering by\nNAP (see Extended Data Table 1).\n The Baltic Sea study of Kari et al. (2020) measured SPM concentrations for both sea ice and\nseawater, and we now use these data to distinguish between a (440) for sea ice versus\nNAP\nseawater in the Baltic Sea (see Extended Data Table 1).\n Our co-author Dr. Lars C. Lund-Hansen provided unpublished data of the albedo (65%), ice\nthickness (198 cm) and chl-a concentration in water (1.1 mg/m3) at McMurdo Sound,\nAntarctica, which he had measured on location simultaneously with the irradiance spectrum\nin Figure 2c. We now incorporated these data in the model (see Extended Data Table 1).\n After this new parameterization, we re-estimated the value of aCDOM(440) at McMurdo\nSound and the values of the scattering coefficients at all three locations by a least-squares fit\nof the model to the measured spectra (see Extended Data Table 1).\nIndividually, the effects of each of these changes were small, but together they helped to further\nimprove the model fits (see new Figure 2). There are still slight differences between the modeled and\nobserved spectra, but they are not systematic and they are less pronounced.\nWe now added the coefficient of determination (R2 value) in the new Figure 2 to quantify the\nfit of the model to the observed spectrum. The R2 values of the Arctic, Baltic and Antarctic ranged\nfrom 0.93 to 0.99 (which is pretty impressive in our view!). Furthermore, we now added in the main\ntext: “Probably, the model predictions could be even further improved by fine-tuning the parameter\nvalues (e.g., by small changes in CDOM or chlorophyll concentration), but we preferred to base our\nmodel predictions as much as possible on measured parameter values (Extended Data Table 1).”\n(lines 121-124)\nHence, the relatively small differences between modeled and observed spectra in Figure 2 are\nessentially a parameterization issue; they are very different from the systematic differences between\nthe spectra generated by sea ice versus open ocean water reported in Figures 3 and 4.\n3. Regarding Figure 3, while it is clear that at depth in the ocean, the model simulates a narrower\ndistribution of light there does not appear to be a clear shift in the peak wavelength between the\nlight under the ice and at depth in the water column. But besides this, I don't understand why this\ncomparison is made. Instead the figure should show the same depths in the water column with and\nwithout sea ice, rather than showing the light under the ice and then state this doesn't compare to\nthe light at depth in the water column. You are not comparing the same things. If you want to talk\nabout sea ice vs. open water then the same comparison should be made, i.e. show the irradiance at\ndepth in the water column with and without sea ice.\nResponse: The full width at half maximum (FWHM), mean wavelength and peak wavelength of the\nirradiance spectra in Figure 3 are reported in Table 1. We agree that the shifts in mean wavelength of\nthe spectra below sea ice versus open water are more pronounced than the shifts in peak\nwavelength. Still, there are clear shifts in peak wavelength, although they are better visible at the\n25% optical depth than in the tiny spectra at the 1% optical depth. For instance, at the 25% optical\ndepth, the peak wavelength of the oligotrophic ocean shifts from 533 nm below ice to 478 nm in\nopen water. Likewise, the peak wavelength shifts from 548 nm to 498 nm for the mesotrophic ocean,\nand from 608 nm to 583 nm for coastal waters (see Table 1). These are substantial changes!\nwater in the columns of the table and the different ocean ecosystems in the rows of the table. This\nmakes it much easier to read the table. Furthermore, in the text, we now describe the shift in the\nmean wavelength of the spectra instead of the peak wavelength, because the spectral shifts are\nindeed more pronounced for the mean wavelength. That is, on lines 157-159, we now write: “For\nexample, at an optical depth of 25%, the mean wavelength of the irradiance spectrum in the\noligotrophic ocean is shifted from the green part of the spectrum (586 nm) below ice to the blue part\nof the spectrum (486 nm) in open ocean water (Figure 3a,d; Table 1).” This is a shift of 100 nm!\nWe compare spectra at the same optical depths, not at the same physical depths. This is a standard\napproach commonly used for the comparison of different spectra. See e.g., the Wikipedia pages on\noptical depth or the textbooks on marine and atmospheric optics. The advantage of comparing\nspectra at the same optical depth is that the shape of the spectra can be compared while the total\nirradiance integrated over the full spectra (their ‘total light intensity’) is the same. This was also\nemphasized by Reviewer #2 in the previous round of review, who wrote: “The comparison of change\nbetween under an ice cover to light available in the open ocean is really only comparable at the same\nintensity levels”. Hence, we are “comparing the same things”.\nWe now added a sentence in the Results section to explain this more clearly (lines 127-130):\n“The comparisons are made at the same optical depth, i.e., at the same total irradiance integrated\nover the spectrum. In this way, the shape of the spectra can be compared without potentially\nconfounding effects of differences in total irradiance.” Furthermore, we changed the term “photic\ndepth” to “optical depth” throughout the manuscript, because the latter term is more widely used.\n4. I do not understand the choice of ice albedos used, and why the Antarctic ice albedo is higher\nthan that for the Arctic. It all depends on the thickness of the ice in the absence of snow but for 1.6\nand 2m of ice, the bare ice albedo will not be different. 20% for 20cm ice may be realistic though it\nhas to be backed by observations or a reference. The thickness of the SSL should also be specified.\nResponse: Good point. The value that we used for the albedo of the Central Arctic Ocean is based on\nmeasurements of Lund-Hansen et al. (2015). Unfortunately, we made an error in Extended Data\nTable 1, for which we apologize. The albedo for the Central Arctic Ocean should indeed not be 40%.\nLund-Hansen et al. (2015) measured an albedo value of 55% and that is the value that we have used\nin our model simulations. Hence, we have now corrected the albedo for the Central Arctic Ocean to\n55% in Extended Data Table 1.\nFurthermore, the albedo of 60% for McMurdo Sound, Antarctica, indicated in our previous\nmanuscript version was merely an estimate. We have now updated this with actual measurements of\nthe albedo at McMurdo Sound, Antarctica by our co-author Lars Lund-Hansen (unpublished data),\nmeasured on location simultaneously with the irradiance spectrum of Figure 2c. These measurements\ngave an Antarctic ice albedo of 65%, which we have now incorporated in our model simulations and\nExtended Data Table 1.\nThe albedo of 21% for the Baltic Sea was based on reflectance spectra of bare ice in Figure 6b\nof Kari et al. (2020), where we used the lowest value of the range. We now added a reference to Kari\net al. (2020) for the albedo of the Baltic Sea in Extended Data Table 1.\nThe thicknesses of the different layers (SSL, DL, INT) are specified in Extended Data Table 1.\n5. I do not find that a sufficient sensitivity study has been performed as a function of ice\ncharacteristics to make this study significant. How sensitive are the results to the scattering coefficients used for example?\nThere are many observations made by different studies that have different coefficients than those of Biegrib and Light.\nI realize extended Figure 3 tries to do a sensitivity study by assuming different thicknesses of the SSL, DL and ICE but that is not the same as\nconsidering the impact of the actual scattering coefficients used. These will be different for the different ice types.\nResponse: This comment is not correct. For some reason, the reviewer believes that the sensitivity\nanalysis in our Extended Data Figure 4 (which was Extended Data Figure 3 in the previous manuscript\nversion) assumed different thicknesses of the SSL, DL and ICE. However, this was not the case. Our\nExtended Data Figure 4 investigated the sensitivity of the model predictions to different values of the\nscattering coefficients of ice (b ). Therefore, we already performed the sensitivity analysis requested\nice\nby the reviewer!\nWe have ensured that Extended Data Figure 4 clearly states that it investigates the scattering\ncoefficients. In the panels of Extended Data Figure 4 (our former Ext Data Figure 3), we specify the\ndifferent values of b (which has units of m-1). Furthermore, the caption of Extended Data Figure 4\nice\nexplicitly states “Sensitivity of the model predictions to the scattering coefficient of ice (b ).”\nice\nFurthermore, we have now conducted additional sensitivity analysis. We added a sensitivity analysis\nof the albedo of ice in the new Extended Data Figure 3. And in response to comment #1 above, we\nadded a sensitivity analysis of the effect of snow cover in the new Extended Data Figure 5.\nWe moved the sensitivity analysis from the Discussion to a new section in the Results called\n‘Variation in optical properties of sea ice’, where we present and discuss the results of these new\nsensitivity analyses (lines 171-190).\n6. It is difficult to make real implication statements as there are so many factors that influence\nprimary productivity, including the initial blooms that happen under very low light levels, nutrient\navailability and timing of predatation. Without real observations to support these statements it does\nfeel a bit underwhelming.\nOverall, I feel such a modeling study as this manuscript presents should be published somewhere,\nbut to make it a Nature paper, I would have expected to see actual results on the taxonomy changes\nobserved between a sea ice-covered regime and open water.\nResponse: We fully agree that primary production is affected by many factors, including light levels,\nnutrient availability, predation, and so on. Hence, the reviewer is preaching to the choir😉. We have\nalso extensively published about effects of light, nutrients, grazing, viruses, etc, on primary\nproduction ourselves (see previous publications by several of our team members).\nOur present manuscript is indeed a model study. It presents new insights on how the loss of\nice cover will affect irradiance spectra for aquatic photosynthesis according to current state-of-the-\nart models in marine optics. However, we do compare our model predictions with real observations.\nSpecifically, the light spectra predicted by our model are compared with light spectra measured in\nthe Arctic Ocean, Baltic Sea, and at Antarctica (Figure 2). Furthermore, we extensively compare our\nmodel findings with major taxonomic changes observed in polar waters by other studies, such as the\nshift from diatoms to Micromonas spp in the Canada Basin of the Arctic Ocean (lines 283-293) and\nthe increase of cryptophytes in coastal waters of the West Antarctic Peninsula (lines 294-307).\nIt is clearly beyond the scope of our current manuscript to organize a series of polar\nexpeditions to collect new data on taxonomic changes of ice algae and phytoplankton. However, we\ndo hope our novel findings will inspire new polar expeditions, to investigate how the loss of sea ice\nwill affect marine primary producers and to collect new data linking irradiance spectra with changes\nin algal pigmentation and taxonomy in Arctic and Antarctic waters.\n5"}, {"round": 3, "reviewer_comments": {"reviewer_5": "Review of manuscript ‘Loss of sea ice alters light spectra for aquatic photosynthesis’ by Woźniak et al. submitted to Nature Communications.\nGeneral comment\nThis is the first time that I review the paper Loss of sea ice alters light spectra for aquatic photosynthesis by Woźniak et al.\nThe manuscript already went through two rounds of revisions and I was asked to evaluate the manuscript as well as the responses to Rev#4.\nThe paper is well written, the analysis properly conducted and the figures are of good quality. I find the results of the paper interesting and worth publishing, but I agree with Rev#4 that such study is not suited for Nature Communications. I will refer in the following to the general comments of Rev#4:\n1. In response to Rev#4 the authors did include a sensitivity test accounting for the presence of snow, however there is no specification in the text on how the snow was included. As they showed, snow strongly attenuates light, however the effect of snow is wavelength-dependent (Mundy et al., 2007; Lange et al., 2016). I think this latter aspect is even more relevant for the present study rather than attenuation itself.\n2. I find that the authors address this comment thoroughly\n3. The authors compare spectra at the same optical depths in sea ice and open water, arguing on the comment of the reviewer to compare the spectra at the same depth in the water. The authors approach of comparing the spectra at optical depths is reasonable in order to assess the difference that the two environments (sea ice VS water) have on the light spectrum. However, for these findings to be relevant for phytoplankton communities in the water, I agree with Rev#4 that the authors should show the irradiance at the same depth in the water, with and without sea ice. I assume that the difference ice VS no-ice will be visible only at the surface where ice has the strongest effect. At depths larger than a couple of meters under the ice most likely the effect of the above water column will overcome any effect due to the absence of ice. This is an important aspect to then infer to which extent (to which depth) ecosystem may be affected by the disappearance of sea ice.\n4. Addressed\n5. Rebutted\n6. The authors include in the discussion the point that light only, but also other factors (nutrients) do have effect on ocean primary productivity. However, I agree that with lack of data this remains a purely modeling study, interesting and worth publication, but not in nature communications.\nI also add that the authors should address comment 3 of Rev#4 to show the extent at which phytoplankton communities can really be affected by changes in the light spectrum due to sea-ice loss.\nDetailed comments:\n- L 61: it was discovered -> it has been shown\n- L 115: remove ‘sheet’\n- L 126: Change ‘Now that … ’ in ‘Once …’\n- L 202 euphotic depth (zone) is defined here but it is mentioned already above in the manuscript, so this definition should appear already before\n- L 695: bice appears here for the first time, it needs to be defined in the text or at least here in the caption\nReferences:\nMundy, C. J., J. K. Ehn, D. G. Barber, and C. Michel (2007), Influence of snow cover and algae on the spectral dependence of transmitted irradiance through Arctic landfast first-year sea ice, J. Geophys. Res., 112, C03007, doi:10.1029/2006JC003683.\nLange, B. A., C. Katlein, M. Nicolaus, I. Peeken, and H. Flores (2016), Sea ice algae chlorophyll a concentrations derived from under-ice spectral radiation profiling platforms, J. Geophys. Res. Oceans, 121, 8511–8534, doi:10.1002/2016JC011991."}, "author_response": "Response to the reviewer’s comments\nResponse to Reviewer 5:\nGeneral comment\nThis is the first time that I review the paper Loss of sea ice alters light spectra for aquatic\nphotosynthesis by Woźniak et al. The manuscript already went through two rounds of\nrevisions and I was asked to evaluate the manuscript as well as the responses to Rev#4.\nThe paper is well written, the analysis properly conducted and the figures are of good\nquality. I find the results of the paper interesting and worth publishing, but I agree with\nRev#4 that such study is not suited for Nature Communications.\nResponse: We thank Reviewer #5 for the kind words of appreciation for our manuscript, pointing out\nthat the paper is well written, the analysis properly conducted, and the figures are of good quality.\nWe are also most grateful for the reviewer’s comments. Obviously, we disagree on one point, in the\nsense that we do feel that our novel findings are highly suited for Nature Communications (see our\nreply to point 6 below). However, we appreciate the reviewer’s constructive approach that has\nhelped us to further improve (and hopefully finalize) the manuscript.\nI will refer in the following to the general comments of Rev#4:\n1. In response to Rev#4 the authors did include a sensitivity test accounting for the\npresence of snow, however there is no specification in the text on how the snow was\nincluded. As they showed, snow strongly attenuates light, however the effect of snow is\nwavelength-dependent (Mundy et al., 2007; Lange et al., 2016). I think this latter aspect is\neven more relevant for the present study rather than attenuation itself.\nResponse: We are well aware that light attenuation by snow is wavelength-dependent, and this was\nincorporated in our model. In the caption of Extended Data Figure 5, in the previous version of our\nmanuscript, we wrote that “The albedo and spectral extinction coefficient of snow are based on\nFigures 3 and 5 of Perovich (2007)”. Figure 5 of Perovich (2007) shows the wavelength-dependence of\nthe extinction coefficient of snow; it is very similar to Figure 4 of Mundy et al. (2007). However, the\ndata of Perovich (2007) spanned the full spectral range from 400 - 850 nm, while Mundy et al. (2007)\ncovered only the range from 400 - 700 nm. Hence, we used the data of Perovich (2007) in our model.\nWe have now added a short paragraph in the Methods section to explain how snow was\nincluded in the model (lines 492-501). Furthermore, in the main text, we now state explicitly that light\nattenuation by snow is wavelength-dependent, with references to both Perovich et al. (2007) and\nMundy et al. (2007) (ref 37 and 38) (lines 204-205).\nPerovich, D. K. Light reflection and transmission by a temperate snow cover. Journal of Glaciology 53,\n201–210 (2007).\nMundy, C. J., Ehn, J. K., Barber, D. G. & Michel, C. Influence of snow cover and algae on the spectral\ndependence of transmitted irradiance through Arctic landfast first-year sea ice. Journal of\nGeophysical Research 112, C03007 (2007).\n1\n2. I find that the authors address this comment thoroughly\nResponse: Thank you!\n3. The authors compare spectra at the same optical depths in sea ice and open water,\narguing on the comment of the reviewer to compare the spectra at the same depth in the\nwater. The authors approach of comparing the spectra at optical depths is reasonable in\norder to assess the difference that the two environments (sea ice VS water) have on the\nlight spectrum. However, for these findings to be relevant for phytoplankton communities\nin the water, I agree with Rev#4 that the authors should show the irradiance at the same\ndepth in the water, with and without sea ice. I assume that the difference ice VS no-ice will\nbe visible only at the surface where ice has the strongest effect. At depths larger than a\ncouple of meters under the ice most likely the effect of the above water column will\novercome any effect due to the absence of ice. This is an important aspect to then infer to\nwhich extent (to which depth) ecosystem may be affected by the disappearance of sea ice.\nResponse: We have added a new Figure to address this comment (Figure 3). The first two panels of\nthis figure show scalar irradiance spectra at the same depths in the water column, with and without\nsea ice (Figure 3a,b). In line with expectation, the results show that irradiance in water below sea ice\nis much lower than irradiance at the same depths in open ocean water. The model predictions also\nsuggest differences in the spectral composition of ice-covered versus open ocean water, although this\nis difficult to assess in view of the concomitant differences in total irradiance. The new Figure 3a,b is\ndescribed on lines 137-146.\nFor comparison, we have also added two other panels in the new Figure 3, which show scalar\nirradiance spectra at the same optical depths (Figure 3c,d). In this way, the shape of the spectra in\nthe euphotic zone can be compared without potentially confounding effects of differences in total\nirradiance. Moreover, this visualization also helps to address the second part of the reviewer’s\ncomment. At the 10% and 5% optical depth (i.e., at 4.6 m and 16.5 m depth), water below sea ice still\nhas a relatively broad irradiance spectrum that covers violet, blue and green wavelengths (400-600\nnm; Figure 3c), whereas open ocean water at the same optical depths is characterized by a more\npronounced peak largely confined to the violet and blue part of the spectrum (400-500 nm; Figure\n3d). However, deeper down in the euphotic zone, at 2% and 1% optical depth, the water column has\nfiltered out the longer wavelengths and the spectra below sea ice and in open ocean water become\nmore similar. Hence, for the clear ocean water used in this example, the effect of sea ice on the\nunderwater spectrum extends to at least 16.5 m depth but gradually vanishes at greater depths. The\nnew Figure 3c,d is described on lines 147-166.\n4. Addressed\nResponse: Thank you!\n5. Rebutted\nResponse: Thank you!\n6. The authors include in the discussion the point that light only, but also other factors\n(nutrients) do have effect on ocean primary productivity. However, I agree that with lack\nof data this remains a purely modeling study, interesting and worth publication, but not in\nnature communications.\nResponse: We agree that our manuscript is largely a model study. To our knowledge, Nature\nCommunications and many other journals in the Nature family regularly publish model studies, and\n2\nthe development of new models has contributed to major new scientific insights and discoveries in,\ne.g., physics, chemistry, astronomy, earth sciences, biology, and many other fields of science.\nWe emphasize that our model study is firmly based on data. The model assumptions and\nparameter estimates are based on real-world observations (see Extended Data Tables 1 and 2). The\npredicted spectra are compared with light spectra measured in the Arctic Ocean, Baltic Sea, and at\nAntarctica (Figure 2; note that two of the spectra were measured by our co-author Dr. Lars Lund-\nHansen). Furthermore, in the Discussion section, we extensively compare our model predictions with\ntaxonomic changes observed in polar waters by other studies, such as the shift from diatoms to\nMicromonas spp in the Canada Basin of the Arctic Ocean (lines 311-321) and the increase of\ncryptophytes in coastal waters of the West Antarctic Peninsula (lines 322-335).\nIn our view, the key contribution of our manuscript is that it derives from first principles (from\nthe inherent optical properties) how ice and water shape light spectra in fundamentally different\nways. To our knowledge, these insights are new, they align with available data and observations, and\nthey contribute to a better understanding of how the loss of sea ice will change the photosynthetic\npigments and species composition of primary producers in polar ecosystems. For these reasons, we\nbelieve our manuscript is highly suited for Nature Communications.\nI also add that the authors should address comment 3 of Rev#4 to show the extent at which\nphytoplankton communities can really be affected by changes in the light spectrum due to\nsea-ice loss.\nResponse: Fair point. Our analysis was thus far mostly concerned with spectra directly below the ice\n(Figures 2, 4 and 5), which is the prime habitat for ice algae attached to the bottom of the ice layer.\nHowever, the new Figure 3 now extends this analysis and shows how the spectra change with depth\nin the water column below the ice, which is relevant for phytoplankton communities in this water.\nThe results show that, for clear ocean waters, the effect of sea ice on the spectrum in water below\nthe ice extends to at least the 5% optical depth (16.5 m depth) (compare the spectra in Figure 3c and\n3d). Hence, this new Figure shows that the loss of sea ice will alter light spectra for aquatic\nphotosynthesis, not only for algae attached to the ice but also for phytoplankton communities in the\nwater underneath the ice. See our above response to comment 3 for further details.\nDetailed comments:\n- L 61: it was discovered -> it has been shown\nResponse: Done (lines 60-61).\n- L 115: remove ‘sheet’\nResponse: Done (line 116).\n- L 126: Change ‘Now that … ’ in ‘Once …’\nResponse: The model was validated in the preceding section. Hence, we feel that our wording (‘Now\nthat the model has been validated …’) is more accurate. We consulted a native speaker from the UK\n(a professor in paleo-ecology), who read this part of the text and agreed with us. Therefore, in this\ncase, we have not adopted the reviewer’s suggested wording (line 127).\n- L 202 euphotic depth (zone) is defined here but it is mentioned already above in the\nmanuscript, so this definition should appear already before\nterms “euphotic depth” and “euphotic zone” are related but not exactly identical, we have also\nmaintained the definition of “euphotic depth” at first mention (on lines 228-229).\n- L 695: bice appears here for the first time, it needs to be defined in the text or at least\nhere in the caption\nResponse: bice is mathematically defined as the scattering coefficient of sea ice in Eqn (4); see lines\n404-414 in the Methods section. We have now also defined bice in the figure caption (line 739).\nOur own changes:\n1. Some sentences that were previously in the paragraphs describing the former Figure 3 (current\nFigure 4) have now been incorporated in the paragraphs of the new Figure 3. Therefore, the\nparagraphs describing the current Figure 4 have been shortened and restructured accordingly (lines\n167-193).\n2. We have defined optical depth based on scalar irradiance (see lines 152-153), because scalar\nirradiance is a more relevant measure of light availability for phytoplankton cells than planar\nirradiance (lines 421-423). However, although all graphs in former Figures 3 and 4 (now Figures 4\nand 5) and Extended Data Figures 3-5 correctly showed scalar irradiance spectra, we had erroneously\nused planar instead of scalar irradiance to calculate the optical depths in these graphs. We have now\ncorrected this error, and recalculated the optical depths based on scalar irradiance. This correction\ncaused minor changes in several spectra and for some values in Table 1, but does not have any\nimplications for the main conclusions of the manuscript.\n3. Very recently (on 18 March 2025), a new paper by Ahonen et al. came out in Limnology &\nOceanography that measured irradiance spectra in 127 boreal lakes. They used the 10% optical\ndepth (i.e., the mid-point of the euphotic zone), and confirmed the presence of three distinct spectral\nniches (in the green, orange and red part of the spectrum) in lakes, exactly as predicted by Stomp et\nal. (2007), Holtrop et al. (2021), and our current manuscript. Please note that their study only\nconsiders open water without ice cover; it does not investigate how the loss of ice will alter the\nspectra, so it does not interfere with our present work. However, it is a very neat study confirming the\ntheoretically predicted existence of distinct spectral niches with actual data! We now cite the new\nstudy of Ahonen et al. (2025) as ref 28 on lines 63 and 275.\nAhonen, S. A., Jones, R. I., Seppälä, J., Vuorio, K. M., Tiirola, M., Vähätalo, A. V. Phytoplankton absorb\nmainly red light in lakes with high chromophoric dissolved organic matter. Limnology and\nOceanography, article 70034 (2025). https://doi.org/10.1002/lno.70034\n4"}]

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