Urumqi Glacier No. 1, located at 43°06′ N and 86°49′ E, lies within the Tien Shan Mountains of the Xinjiang Uyghur Autonomous Region in western China (Fig. 1a). This northeast-facing glacier spans elevations from 3752 to 4445 m above sea level (m a.s.l.). This glacier lost approximately 20 % of its volume between 1962 and 2003, and it split into eastern and western branches in 1993 as a result of its continued retreat (Ye et al., 2005). The total catchment area is approximately 1.618 km2 (Yue et al., 2022). Mass accumulation primarily occurs during the summer months, corresponding with peak precipitation rates. Hourly meteorological data, including air temperature, downwards solar radiation, precipitation, and snowmelt (in water equivalent), were obtained from ERA5-Land (0.1°×0.1° resolution, 2 m above surface, https://cds.climate.copernicus.eu/datasets/reanalysis-era5-land?tab=download, last access: 16 July 2025) for 1 July–31 August 2013, and converted to daily means by averaging hourly values (Fig. 2). The mean annual equilibrium line altitude (ELA) in 2013 was 4240 m a.s.l. (https://wgms.ch/products_ref_glaciers/urumqi/, last access: 7 March 2026). This region is frequently influenced by dust storm activity due to the surrounding major deserts, with dust primarily originating from the Taklimakan Desert (Takeuchi et al., 2011; Li et al., 2010; Nagatsuka et al., 2014). The glacier’s ablation zone is notably characterized by the widespread presence of cryoconite granules, with a mean dry weight of 335 gm-2 and an organic matter content of 9.4 % ± 1.6 % (Takeuchi and Li, 2008). Cryoconite holes are generally absent from the glacier surface (Takeuchi and Li, 2008).

Two taxa of algae and six taxa of cyanobacteria were identified on the glacier surface through microscopic observation (Table 1, Fig. 3). The morphological characteristic of each taxon was described as follows, in conjunction with findings from previous DNA-based studies (Segawa et al., 2017, 2023). Molecular analyses previously detected a total of 20 cyanobacterial operational taxonomic units (OTUs) on the glacier surface. These previous molecular frameworks were utilized as a reference to guide our morphological classification of the dominant active taxa.

The total biovolume-based biomass of algae and cyanobacteria on the glacier surface remained relatively stable through the melt season (two-way ANOVA, p=0.62>0.05, Fig. 4). However, a marked increase in total biomass was observed when the snow surface transitioned to bare ice at the midstream sites (S4 and S5). At S4, biomass increased markedly from P1 to P2 (from 4.7 to 419.1 µLm-2). At S5, although the biomass in P1 (3.7 µLm-2) was not significantly different from that in P2 (1.2 µm-2), a significant increase was observed in P3, with biomass reaching 278.3 µLm-2. During P5, biomass on ice surface (S1–S5) remained at comparable levels. Overall, the total biovolume-based biomass on the ice surface was significantly higher than that on the snow surface (two-way ANOVA, p=0.03<0.05).

The relative dominance of algae and cyanobacteria varied primarily with surface status rather than sampling period (Fig. 5). PERMANOVA analysis detected no significant differences in community composition among sampling periods (p>0.05), whereas significant differences were observed between snow-covered and ice surfaces (p<0.01). Across all sampling periods, snow surfaces were consistently dominated by algal taxa, primarily Chloromonadinia species, whereas cyanobacteria predominated on bare ice surfaces.

A clear upstream shift in cyanobacterial dominance accompanied the seasonal expansion of bare ice. During P1, filamentous cyanobacteria were dominant on downstream ice-covered sites (S1–S3), accounting for 61 %–75 % of the total biovolume-based biomass, whereas the snow-covered upstream sites (S4–S6) were prevailed by Chloromonadinia species, contributing 65 %–92 % in biovolume-based community composition. By P2, cyanobacterial dominance had expanded upstream and Chloromonadinia species remained dominant only at S5 and S6. From P3 onward, filamentous cyanobacteria consistently dominated sites S1–S5, while Chloromonadinia species remained dominant solely at the uppermost site (S6). The only exception occurred during P4, when filamentous cyanobacteria dominated all sites, accounting for 61 %–84 % of the total biomass.

Among the algal taxa, Chloromonadinia species were the principal constituents of snow communities throughout the study, accounting for 16 %–92 % of the total biomass. In contrast, Cylindrocystis brébissonii was detected exclusively on ice surfaces and occurred sporadically at one or two sites during each sampling period, representing less than 1.5 % of the total biomass.

Filamentous Oscillatoriaceae cyanobacteria constituted the dominant component of ice-surface communities during sampling period. Osc. cyanobacterium 1 showed a clear preference for ice surfaces, with biomass generally rising from P1 to P4, particularly at mid- to up-glacier ice sites. In contrast, its biomass on snow surfaces remained negligible throughout the sampling period, rarely exceeding 0.1 µLm-2 (Fig. S2).

Osc. cyanobacteria 2 and 3 were the most abundant cyanobacterial taxa, constituting 65 %–95 % of the total cyanobacterial biovolume-based biomass across all ice sites and sampling periods. Their biomass increased progressively from P1 to P4, with the highest values recorded at sites S3 and S4 during P4, where Osc. cyanobacterium 2 exceeded 300 µLm-2 and Osc. cyanobacterium 3 surpassed 470 µLm-2. From P4 onward, a distinct spatial pattern emerged: Osc. cyanobacterium 2 dominated the downstream sites (S1–S2), whereas Osc. cyanobacterium 3 became dominant at the upstream sites (S3–S5). In contrast, both taxa showed very low biovolume-based biomass on snow-covered surfaces, with biomass consistently below 1 µLm-2.

Among the unicellular cyanobacteria, Chr. cyanobacterium 1 showed a noticeable biomass increase by P4 on ice, reaching up to ∼30 µLm-2 at S3 and S4. Chr. cyanobacteria 2 and 3 maintained relatively low abundance levels throughout all sampling sites and periods, with slight elevations observed on ice during P4. All three Chroococcaceae taxa were almost absent on snow surfaces (biomass<0.13 µLm-2).

The concentrations of major ions showed notable temporal variation throughout the five sampling periods (Fig. 6). SO42-, NO3-, K+ and Mg2+ concentrations temporally varied significantly among sampling periods (one-way ANOVA, p<0.01). SO42- and NO3- both peaked in P1 (=12.9 and =6.8 µEqL-1, respectively), declined sharply in P2, and showed secondary increases in P5. In contrast, K+ remained stable in early periods (∼5 µEqL-1) before declining to its lowest level in P5 (=1.7 µEqL-1). Mg2+ exhibited elevated concentrations in P1 and P3 (=20.3 and =25.9 µEqL-1), dropped markedly to 8.6 µEqL-1 in P4, and partially rebounded to 11.3 µEqL-1 in P5.

Significant differences in ion concentrations were also observed between surface status (ice vs. snow) (one-way ANOVA, p<0.01, Fig. 6b). Ice surfaces consistently hold higher mean concentrations of crustally derived ions, including Ca2+ (125.5 vs. 22.6 µEqL-1), Mg2+ (20.6 vs. 5.9 µEqL-1), Na+ (12.5 vs. 5.3 µEqL-1) and K+ (4.9 vs. 1.3 µEqL-1). In contrast, snow surfaces showed higher concentrations of NO3- (2.3 vs. 6.0 µEqL-1), SO42- (6.3 vs. 10.1 µEqL-1), and NH4+ (2.6 vs. 8.0 µEqL-1).

Mineral matter constituted the dominant fraction (>88.2 %) in samples collected on ice surface (S1–S5) during P5, accounting for 185.7–314.6 gm-2, whereas organic matter represented a much smaller proportion, ranging from 15.5 to 42.0 gm-2 (Table S4). No significant differences in mineral fractions were observed among sampling sites (one-way ANOVA, p>0.05), whereas organic fractions varied significantly among sites (one-way ANOVA, p<0.05).

The biomass of snow/glacier algae on Urumqi Glacier No. 1 remained consistently low with minimal temporal variation, suggesting that algal proliferation is highly constrained in this environment. Previous studies have shown that glacier algal blooms on the Greenland Ice Sheet are strongly promoted by the availability of mineral phosphorus released from aeolian dust (McCutcheon et al., 2021). Despite the abundance of mineral particles on the Urumqi Glacier No. 1, glacier algae kept a low biomass on the surface, suggesting that factors other than dust abundance, such as phosphorus bioavailability or local environmental conditions, may limit glacier algal establishment in this region. Chloromonadinia species was found on both snow and ice but showed a clear preference for snow, aligning with its classification as a snow-environment specialist (Takeuchi, 2001), similar to observations in the Himalayas (Yoshimura et al., 1997) and Altai Mountains (Takeuchi et al., 2006). All observed Chloromonadinia cells were in the zygote stage, characterized by orange-red pigmentation. All observed Chloromonadinia cells were in the zygote stage, characterized by orange-red pigmentation. The exclusive occurrence of zygotes indicates that this taxon was present predominantly in a dormant or resistant life stage rather than as an actively growing population.

Conversely, the green alga C. brébissonii was restricted to ice surfaces. Despite its presence, its sporadic and low abundance suggest it functions as an ice-specialist that fails to establish stable, large-scale populations on this glacier, contrasting with earlier classifications that often regard certain Cylindrocystis strains as opportunists with broader habitat ranges (Kol, 1942; Remias et al., 2012).

On this glacier, the role of algae in surface darkening is habitat-dependent. On snow, despite low biomass, Chloromonadinia species is likely the primary biological agent for albedo reduction due to the absence of cryoconite. However, on bare ice, the direct contribution of eukaryotic algae to surface darkening may be secondary to that of cyanobacteria and mineral particles, primarily due to the overwhelming biomass dominance of filamentous cyanobacteria and the heavy loading of lithogenic dust observed throughout the melt season. This is consistent with studies suggesting that while snow algae can significantly lower albedo in the early melt season, their impact is often overshadowed by other light-absorbing particles (e.g., cryoconite) once the ice is exposed (Lutz et al., 2016; Di Mauro et al., 2020).

Previous DNA metabarcoding studies on Urumqi Glacier No. 1 detected approximately 8 cyanobacterial operational taxonomic units (OTUs) (Segawa et al., 2017), whereas microscopic observation in this study identified six cyanobacterial morphotypes and two algal taxa. This numerical difference reflects the complementary nature of the two approaches rather than an inconsistency between the datasets. DNA metabarcoding is highly sensitive and captures the full taxonomic inventory of glacier microorganisms, including rare, dormant, or transient taxa introduced by atmospheric deposition. In contrast, microscopic observation coupled with chlorophyll autofluorescence selectively captures structurally intact, pigment-bearing phototrophs, enabling the direct quantification of cell abundance and biomass.

Consistent with previous molecular data, our dominant filamentous morphotypes corresponded with major filamentous cyanobacterial OTUs previously reported from these sites, such as Microcoleus, Pseudanabaena, and Geitlerinema (Table 1; Segawa et al., 2017). Despite the greater OTU richness revealed by high-throughput sequencing, the bulk of the phototrophic biomass during the melt season was contributed by only two dominant Oscillatoriaceae morphotypes, which drove the clear spatial and seasonal succession observed across the glacier. These findings demonstrate that while DNA metabarcoding provides a comprehensive taxonomic inventory, microscopic quantification is indispensable for identifying the specific dominant taxa responsible for actual biomass accumulation and community dynamics. Taken together, integrating molecular checklists with morphological biomass estimation yields a more holistic understanding of supraglacial phototrophic communities.

In contrast to the ephemeral nature of snow algae, the cyanobacterial community on Urumqi Glacier No. 1 exhibited remarkable temporal stability in total biovolume-based biomass, albeit with significant spatial and taxonomic shifts. While unicellular Chroococcaceae maintained low biomass, filamentous cyanobacteria (Oscillatoriaceae) sustained dominance on ice surfaces throughout the melt season. This persistent presence suggests their potential role in surface darkening through two primary pathways: direct organic pigmentation and the structural aggregation of mineral particles into stable, dark cryoconite granules (Takeuchi, 2009, 2013).

The spatial distribution of these dominant filamentous cyanobacteria revealed a distinct altitudinal zonation from P4 onwards, despite uniform geochemical characteristics across the glacier. Osc. cyanobacterium 2 dominated the downstream sites (S1–S2), whereas Osc. cyanobacterium 3 prevailed at the mid-to-up-glacier sites (S3–S5). This niche partitioning is likely driven by physical factors associated with altitude, specifically the duration of bare-ice exposure. The downstream sites (S1–S2) experience the earliest snow disappearance and the longest continuous melt season, allowing for the prolonged accumulation and structural stabilization of ice-surface microbial communities (Hodson et al., 2010; Uetake et al., 2010). The dominance of Osc. cyanobacterium 2 at these lower-elevation sites suggests an association with more developed, stable cryoconite habitats characterized by cumulative dust aggregation. In contrast, the mid-to-up glacier sites (S3–S5) encounter a much shorter exposure window. The dominance of Osc. cyanobacterium 3 in these upper areas, including a sharp increase in detectable biomass (exceeding 470 µLm-2) during P4, indicates that this taxon is a potential pioneer microorganism well-adapted to recently exposed ice surfaces (Ortiz-Álvarez et al., 2018; Bradley et al., 2023). This apparent biomass spike was likely driven by the removal of the seasonal snow cover, which newly uncovered the underlying resident cyanobacterial cells on the ice surface. Alternatively, its prevalence at higher elevations may reflect a greater physiological tolerance to recurrent freeze–thaw cycles.

This altitudinal segregation implies that the cyanobacterial community is a strategically partitioned system rather than a monolithic assemblage. For instance, different cyanobacterial taxa may contribute uniquely to cryoconite granule formation (Chen et al., 2025) and vary in their capacities for light utilization and nutrient cycling (Murakami et al., 2022). The presence of multiple dominant taxa with distinct spatio-temporal peaks ensures that the glacier surface maintains high biological biomass across a wide range of environmental conditions – from stable lower reaches to dynamic, highly disturbed upper reaches. Consequently, the functional diversity within the Oscillatoriaceae contributes to the persistence and expansive spatial extent of biological darkening across the entire ablation zone.

Unlike the ephemeral algal blooms on Arctic glaciers (Takeuchi, 2013), the biological impact on Urumqi Glacier No. 1 is stabilized by the formation and multi-year persistence of cryoconite granules driven by filamentous cyanobacteria. While polar glacier algae, such as Anancylonema nordenskiöldii, exhibit explosive seasonal growth dynamics (Stibal et al., 2017; Onuma et al., 2023), the cyanobacterial biomass on Urumqi Glacier No. 1 remained relatively constant throughout the melt season. This lack of a seasonal spike suggests a fundamentally distinct ecological strategy: while polar ice algae thrive through opportunistic, rapid seasonal blooms, the cyanobacteria here rely on the long-term retention of stable structural aggregates. These filamentous taxa produce extracellular polymeric substances (EPS) that trap mineral dust, forming dark, spherical granules (Musilova et al., 2016; Segawa et al., 2017) that can persist and grow over multiple melt seasons (Takeuchi et al., 2010). This prolonged development facilitates the accumulation of refractory humic substances, significantly enhancing the cumulative light-absorbing capacity of the aggregates (Takeuchi, 2002).

Crucially, the spatial manifestation of these aggregates differs fundamentally from polar systems. On polar ice sheets, cryoconite is often concentrated within localized cryoconite holes, rendering its sheet-wide albedo impact secondary to vast, distributed ice algal blooms (Cook et al., 2020). Conversely, the bare-ice surface of Urumqi Glacier No. 1 is extensively covered by dispersed cryoconite rather than discrete holes (Takeuchi and Li, 2008). The high spatial occupancy of this dispersed material, combined with its structural stability provided by the cyanobacterial matrix, helps retain these dark, humic-rich aggregates on the ice surface even during minor scouring events. While literature suggests that consolidated organic-inorganic aggregates exert localized radiative impacts (Cook et al., 2016; Hotaling et al., 2021), the proliferation and retention of these filamentous taxa represents a key biological factor, interacting alongside heavy mineral dust loading, that drives the persistent and cumulative surface darkening observed on this Central Asian glacier.

The distinct community compositions on snow and ice surfaces suggest different environmental controls. We found significant negative correlations between inorganic nitrogen concentrations (NO3- and NH4+) and the relative abundance of dominant filamentous cyanobacteria (Oscillatoriaceae) (p<0.05; Fig. 7). This statistical trend indicates a close coupling between inorganic nitrogen availability and cyanobacterial proliferation on the ice surface. Rather than implying a one-way causal advantage, this negative correlation may reflect the progressive biological drawdown of available nutrients by the high-biomass cyanobacterial mats during their active growth phase, a process supported by their metabolic pathways for efficient nutrient utilization and recycling within the cryoconite micro-habitats (Segawa et al., 2014, 2020; Murakami et al., 2022).

Furthermore, although no significant relationship was observed between Oscillatoriaceae biomass and the mineral fractions during P5, potentially due to the limited sample size, Oscillatoriaceae biomass was positively correlated with mineral-derived ions (Ca2+, Mg2+, and K+, p<0.05, Fig. 7). In Central Asian glacier systems, intense atmospheric dust deposition and subsequent lithogenic mineral weathering strongly govern the meltwater chemistry, making these dissolved cations reliable indicators of mineral influence (Li et al., 2006; Wu et al., 2012). The concentrations of these ions on Urumqi Glacier No. 1 are substantially higher than those reported for Arctic glaciers, such as those in Svalbard, where mineral-derived ion concentrations are significantly lower due to limited dust input (Takeuchi et al., 2019). While glacier algae (such as Ancylonema spp., formerly Ancylonema) are commonly reported to dominate the low-mineral environments of Svalbard glaciers (Stibal et al., 2017), the high mineral availability on Urumqi Glacier No. 1 provides a more suitable substrate and nutrient base for the proliferation of filamentous cyanobacteria and the subsequent development of cryoconite granules. This stark contrast in geochemical conditions explains why the primary biological drivers of albedo reduction differ so fundamentally between Central Asian and Arctic glacial systems – where the former relies on dust-mediated cyanobacterial aggregation (Segawa and Takeuchi, 2010; Takeuchi and Li, 2008) and the latter is driven by widespread blooms of specialized eukaryotic ice algae (Williamson et al., 2018; Cook et al., 2020).

This relationship highlights the critical importance of mineral availability for cyanobacterial colonization and granule formation. Our results mirror observations from the Greenland Ice Sheet, where phosphorus and mineral availability are key drivers of cyanobacterial growth (Uetake et al., 2019), but this effect is likely amplified on Urumqi Glacier No. 1 due to the extreme dust deposition characteristic of Central Asian mountains. In contrast, algal taxa showed no such correlations, suggesting their distribution may be driven by stochastic colonization or micro-scale physical factors like meltwater flow rather than macro-scale chemical parameters (Stibal et al., 2012a).

One of the most significant ecological findings of this study is the complete absence of glacier algae, such as Ancylonema spp., which are the primary drivers of ice darkening on Arctic glaciers (e.g., Alaska, Svalbard) (Takeuchi, 2013; Takeuchi et al., 2019). In polar regions, these Zygnematalean algae bloom on the ice surface, producing dark purple pigments that significantly reduce albedo (Williamson et al., 2018; Remias et al., 2012). However, on Urumqi Glacier No. 1, this niche is predominantly occupied by a stable, cyanobacteria-dominated community.

The absence of Ancylonema and the corresponding dominance of filamentous cyanobacteria (Oscillatoriaceae) are likely attributable to the unique geochemical and physical environment of Central Asia. Takeuchi et al. (2019) demonstrated that Arctic glaciers in Svalbard are characterized by extremely low mineral ion concentrations, a condition that appears to favor the proliferation of free-living glacier algae. In contrast, the high dust flux on Urumqi Glacier No. 1 provides an abundance of mineral cations, resulting in substantially elevated ionic concentrations. For example, mean Mg2+ and Ca2+ concentrations on Urumqi Glacier No. 1 are approximately 13 and 59 times higher, respectively, than those reported for Svalbard (Mg2+: 20.6 vs. 1.6 µEqL-1; Ca2+: 125.5 vs. 2.1 µEqL-1, Takeuchi et al., 2019). These elevated mineral inputs maintain a higher pH on the ice surface. The high influx of carbonate-rich mineral dust from surrounding arid regions acts as a powerful geochemical buffer, maintaining the meltwater pH in a neutral-to-alkaline range (typically 7.5–8.5; Li et al., 2007; Wu et al., 2012). Such alkaline, mineral-rich conditions may be physiologically inhibitory to Anancylonema (formerly Ancylonema), which typically thrives in the more acidic, ultra-oligotrophic environments characteristic of polar ice (Remias et al., 2012; McCutcheon et al., 2021).

Furthermore, the physical presence of massive mineral dust promotes the formation of cryoconite granules. As revealed by Segawa and Takeuchi (2010) on Qiyi Glacier and further elucidated by Chen et al. (2025) on Urumqi Glacier No. 1, filamentous cyanobacteria act as “ecosystem engineers,” trapping mineral particles to form stable, spherical aggregates. This process may physically exclude glacier algae by reducing the availability of “clean” ice surfaces required for their colonization.

These results suggest that biological darkening on Central Asian glaciers may differ from the algal bloom, dominated patterns commonly reported for Arctic glaciers. While Arctic darkening is often ephemeral and sensitive to seasonal snowmelt timing, the darkening on Urumqi Glacier No. 1 is more persistent due to the structural stability of cyanobacterial cryoconite. Therefore, applying Arctic-based bio-albedo models to Central Asian glaciers may lead to significant errors, highlighting the need for region-specific models that account for high mineral loading and its role in shaping microbial community structures.