Effects of ice cover on diversity and community assembly of benthic algae in a stream system of northern China

doi:10.21203/rs.3.rs-8292142/v1

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Effects of ice cover on diversity and community assembly of benthic algae in a stream system of northern China

https://doi.org/10.21203/rs.3.rs-8292142/v1

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Ice cover is common phenomena in aquatic systems of cold regions; however community succession and community assembly of aquatic organism in relation to ice cover have received less attention. Since ice cover usually dominant a majority period of freshwater ecosystem, especially in high latitude or high altitude regions, it is crucial to evaluate the impacts of ice cover on aquatic systems. In this study, effects of ice cover on environments and benthic algal communities were studied with a six-month field investigation in a stream system of northern China to verify ecological succession between the ice-free period and the ice-covered period. Results showed that ice cover significantly altered environmental conditions, manifest as decreased dissolved oxygen and water temperature, as well as increased total phosphorous (TP). We also observed obvious changes in benthic algal community structure, characterized by a rise in Chlorophyta and Euglenophyta, alongside declines in species richness and multiple facets of functional diversity. In addition, functional homogenization in algal community was found because ice cover reduced β-functional diversity. At last, we observed that community assembly of benthic algae was governed by deterministic processes both before and after ice cover, although strength of these processes was significantly weaker during ice cover. This work demonstrates that ice cover can act as a strong environmental filter, reducing biodiversity and altering community assembly, which may provide crucial insights for forecasting ecological responses in seasonally frozen streams.

Benthic algae

ice cover

α-diversity

β-diversity

community assembly

In recent years, environmental changes driven by the intensifying human activities have disrupted the biodiversity of aquatic ecosystems and impaired the homogenization of biological community structures. Besides, the emergence of seasonal variations have had adverse effects on the structure of biological communities and biodiversity patterns(Virta et al., 2020). It has been reported that the community structure of benthic algae is significantly influenced by seasonal physical and chemical factors, such as water temperature, dissolved oxygen, nitrogen, phosphorus, and changes in hydrological conditions (Palmer and Ruhi, 2019; Zhao et al., 2020; Zhao et al., 2022). Of note, under the climatic conditions in northern regions, where climatic conditions are unique and seasonal variations are distinct, the ecosystems are sustained by external energy inputs, such as runoff and organic matter from seasonal leaf litter(Baker et al., 2023). Although researchers have demonstrated that winter is a "dormant season" for freshwater ecosystems, there is evidence that this period is characterized by significant biological activities. In the low temperature condition of the ice cover, the composition of biological communities significantly changes due to reduced nutrient concentration(Cruaud et al., 2020). Furthermore, studies have demonstrated that the growth and succession patterns of benthic algae is influenced by various factors under different seasons. For example, the abundance and chlorophyll-a levels of benthic algae is higher in winter than in summer, characterized by reduced Cyanophyta abundance and increased abundance of filamentous Chlorophyta in later stages(Davie et al., 2012).

Streams are important freshwater systems accounting for ~ 77% of global river networks(Golden et al., 2025). The physical, chemical, and biological processes occurring in streams exhibit marked changes from headwaters to trunk stream before and after ice cover. Ice cover environment is characterized by reduced water temperature and dissolved oxygen levels, as well as reduced external inputs of dissolved organic matter and nutrients. Research has demonstrated that the effects of ice cover on streams are influenced by stream size, geomorphology, and temperature variations(Thellman et al., 2021). Under ice cover conditions, the dissolved oxygen content in lake bottom sediments or silt decreases, with anaerobic decomposition of organic matter being the most prevalent activity (Romankevich et al., 2023). Moreover, the dissolved oxygen triggers the conversion of organic phosphorus to inorganic phosphorus, which increases the concentration of available phosphorus in the water(Van Dael et al., 2021). These changes promote the proliferation of phosphorus-favoring algal groups, such as Cyanophyta, Chlorophyta, and Euglenophyta. However, excessive levels of phosphorus can decrease the diversity of algal community structures and enhance functional homogenization(Dunck et al., 2019). Studies show that the β-diversity may be influenced by nutrient levels, with evidence showing that β-diversity is significantly reduced at high nutrient concentrations. Indeed, even minor changes may strongly affect the community structures(Leboucher et al., 2019). Therefore, we speculate that the prolonged ice cover-induced increase in total phosphorus (TP) content can have profound effects on the structure of benthic algal communities, creating a more homogenized community structure.

During winters, the phenomenon of ice cover significantly influences the structure and diversity of biological communities in aquatic ecosystems. Notably, the formation, duration, and degree of stream ice cover strongly alter the physical and chemical characteristics of water bodies, such as water temperature, light conditions, dissolved organic matter, and nutrient levels(Fell et al., 2021; Fell et al., 2018; Prowse, 2001). This challenges the traditional view that algal activity is low in temperate lakes during winter, confirming the presence of active and diverse algal communities beneath the ice(Phillips and Fawley, 2002). The ice cover not only alters the community structure of organisms in streams but also modifies its biodiversity. Other scholars have investigated the adaptability of algae under ice cover conditions, observing that winter Chl-a concentrations are influenced by the TN concentration in sub-ice water and the TP concentration difference between water and ice, which provide evidence that changes in nutrient levels under the ice may affect algal biomass(Wen et al., 2020). The effects of ice cover on aquatic ecosystems are complex, involving changes to the structure, diversity, and assembly mechanisms of benthic algal communities.

Benthic algae, as the key primary producers in streams and rivers, play crucial roles in material and energy cycling(Wetzel, 1964). They not only serve as a primary food source for aquatic organisms but in some cases, it exceeds phytoplankton as the major source of food(PEI, 2006). Benthic algae may serve as an indicator of the health status of ecosystems and water quality. Research indicates that with their productivity is affected by environmental factors such as water temperature, nutrient concentrations, water turbidity, and wind(Deng et al., 2017). Moreover, the composition of benthic algae displays variations in spatial distribution under the influence of environmental factors such as TN and TP(Hu et al., 2022). Previously, several studies have investigated stream ecology, however, they focused on responses of benthic algal communities to water quality and natural habitat changes(ZHANG et al., 2012; ZHUANG et al., 2003). Therefore, the impact of winter ice cover on benthic algal communities has not been clarified. Considering that stream ice cover is prevalent in the Northern Hemisphere, the changes in hydrology, habitat, and physio and chemical factors induced by this phenomenon may have profound changes to their biodiversity. In this study, we investigated the impact of ice cover on environmental factors, the diversity and assembly of benthic algal communities in streams. To address these questions, we selected the Mijiang River, a natural stream in northeastern China, as our study area and chose the benthic algae as the research subject. The main objectives of this study are: (1) to profile the effects of ice cover on stream environmental factors, (2) to dissect the impact of ice cover on the structure, α- and β-diversity of benthic algal communities, and (3) to elucidate the impact of ice cover on benthic algal community assembly mechanisms.

Study area

Mijiang stream is a typical stream system located in Jilin Province, China(ZHANG, 2023). The stream stretches for about 56 kilometers, the basin area is approximately 771 square kilometers, and the average annual flow is 218 million cubic meters(wang, 2024). In this work, 10 sampling sites were set up along the Mijiang stream (Fig. 1), and the community structure and biodiversity of the stream were systematically studied for six months from September 2021 to February 2022. This region is influenced by the northern temperate continental monsoon climate, with the non-ice cover period ranging from September to November, and the typical ice cover period ranges from December to February of the subsequent year.

Sampling of benthic algae

A circular hole with a diameter of 30-50cm was cut using an ice axe into the ice at each sampling site upstream, midstream, and downstream of the stream. Three stones with similar volume and smooth surface were selected, and the algae on the sunny side of the stone were gently brushed using a soft-brush. During this process, the brush and the stone were repeatedly rinsed with a small amount of pure water to ensure all the algae were collected. Three groups of parallel samples were taken by repeating the steps, named Quantitative 1, Quantitative 2 and quantitative 3. The collected algae samples were transferred to a 100ml fine-mouth sampling bottle, into which 1.5ml of Lugo's solution and a 2%-4% formaldehyde mixture were added, and the process was repeated to collect three sets of pure samples. Subsequently, the samples were transported to the laboratory, allowed to stand for 48 hours, and concentrated to volume of 100ml. Next, 0.1ml samples were placed in a quantitative plankton counting box and the species were confirmed using a 400× microscope(HU and WEI, 2006; HU et al., 1980; ZHAO, 2016). Each sample was counted in parallel 2–3 times and averaged to convert the number of cells per liter of water into cell density per square centimeter(ZHOU et al., 2017).

Environmental factor

The wet width of the stream was measured at 3 sections at each sampling site, and the average value of all the data was obtained as the corresponding value of each environmental factor in the section. At all sampling sites, the water depth was measured with an ultrasonic sounder (ZMSS-100), the stream width was measured using a rangefinder (SW-600A), the flow velocity was recorded with a flowmeter (LS300-B), and the water temperature, pH, dissolved oxygen and conductivity were determined with a water quality analyzer (YSL Pro2000). Two 1L parallel water samples were collected from three sections at each sampling site and placed in a low temperature incubator. Total suspended solids (TSS) were calculated using a suspended solids tester (SS-500B). Other water environment indicators were measured in the laboratory, and the concentrations of total nitrogen, total phosphorus, ammonia nitrogen, nitrate nitrogen, phosphate and permanganate were quantified using parallel samples according to the standards of "Monitoring and Analysis Method of Water and Wastewater (4th Edition)" and "Surface Water Environmental Quality Standards of the People's Republic of China"(China; State, 2002).

A total of 15 physical and chemical indexes of water environment were subjected to correlation analysis, including six chemical factors: total phosphorus, total nitrogen, ammonia nitrogen, nitrate nitrogen, phosphorus and permanganate index, three indicators of the water environment: water temperature, dissolved oxygen, and pH, three indicators of habitat: water depth, stream width, and flow velocity, and three physical indicators: conductivity, total suspended solids, and chlorophyll-a.

Data analysis

Several parameters were used to characterize the composition of benthic algal species communities before and after ice cover. They included species richness, density, biomass, and relative abundance percentage. Functional richness (FRic), functional dispersion (FDis), functional evenness (FEve), functional divergence (FDiv), and Rao's quadratic entropy (RaoQ) were used to assess the α-functional diversity of benthic algal community structures(SONG et al., 2011; Villéger et al., 2008).

The β-diversity was separated into species turnover and nestedness to assess changes in benthic algal communities. Using a functional group (FG) classification, we categorized their traits into " cell size, life form, attachment type, growth form, motility and mode of reproduction," identifying 23 functional groups and calculating their relative abundances(Piggott et al., 2015; Schneck et al., 2011; Wang et al., 2022)(Table S1). Taxonomic diversity indices (Δ), taxonomic distinctness (Δ*), average taxonomic distinctness (Δ+), and variation in taxonomic distinctness (Λ+) were selected as indicators of α-phylogenetic diversity(Liu et al., 2021).

The α-taxonomic and functional diversity were calculated using the "vegan" package in the R 4.3.3 software environment, while the "Cluster" and "BAT" packages were employed to compute β-taxonomic and functional diversity. The "taxa2dist" and "taxon-dive" functions in the vegan package were used to calculate α-phylogenetic diversity. Subsequently, we integrated the vegan, picante, and BAT packages to design a detailed distance matrix for assessing the β-phylogenetic diversity.

The Dispersal-Niche Continuum Index (DNCI) was computed using the "DNCImper" package to quantitatively assess the differences in community assembly mechanisms of benthic algae between the ice-covered and ice-free periods (Vilmi et al., 2021). As a quantitative metric derived from the PER-SIMPER framework, the DNCI evaluates the relative importance of niche-based deterministic versus dispersal-based stochastic processes in structuring communities between pre-defined groups, based on species presence-absence data. The index is calculated as the difference between the standard effect sizes of E-metrics from null models simulating dispersal alone and niche-based processes alone. A positive DNCI value indicates the dominance of deterministic processes, whereas a negative value signifies the dominance of stochastic processes in governing community differences between groups. In our study, the DNCI was specifically applied to unravel the shifts in assembly processes of benthic algal communities associated with ice cover.

Moreover, variations in the environmental factors and biodiversity indices were analyzed using the IBM SPSS Statistics 23. Specific, normality testing was conducted, in which ice cover states before and after were used as fixed factors. The changes before and after ice cover were compared and analyzed using the independent sample t-tests or non-parametric tests, with statistical significance set at P < 0.05. Boxplots were generated using Origin 21 whereas the RDA plots were prepared using Canoco5.

Physical and chemical characteristics of streams

The results shown in Fig. 2 indicate that there were significant changes in water chemistry parameters between the two periods: before ice cover (BIC) and after ice cover (AIC). The chlorophyll-a(Chl-a) content increased after ice cover (Z=-2.234, P = 0.025; Fig. S1); whereas the level of COD_Mn was significantly decreased after ice cover (t = 3.967, P < 0.001; Fig. 2). A similar change occurred in dissolved oxygen (DO) levels (t = 3.937, P < 0.001; Fig. 2) and water temperature (Temp) (Z=-3.781, P < 0.001; Fig. 2). After ice cover, pH levels increased significantly (Z = -2.949, P = 0.003; Fig. 2). Conversely, nutrient dynamics showed a decrease in TN (t = 3.028, P = 0.007; Fig. 2), while TP levels rose substantially (Z = -3.781, P < 0.001; Fig. 2). In contrast, habitat parameters, including water depth, stream width, and flow velocity, remained stable without significant variation (Fig. S1).

Composition of benthic algae community and functional group before and after ice cover

In this study, 275 species of benthic algae belonging to 6 phyla and 65 genera were identified. Subsequent analyses at the specific study periods (Fig. 3) showed that the relative abundance of the Cyanophyta varied as follows between September and February: 35.46%, 41.63%, 18.44%, 15.48%, 26.42%, 24.03%. Moreover, the proportion of Bacillariophyta was relatively stable, estimated at 41.64% in September and increased to 65% in November and December, after which it demonstrated a downward trend to 48.21% in February of the following year. The percentage of relative abundance of Chlorophyta increased monthly from 3.36% in September to 23.26% in February of the following year. Notably, the change in Euglenophyta was highly pronounced, with the proportion before ice cover being 0.02%, which increased to 1.23% after ice cover.

Analysis of functional traits revealed distinct shifts in the algal assemblages following ice formation (Fig. S2). The proportion of filamentous colonies decreased markedly. In contrast, the relative abundance of taxa characterized by an erect growth form or those possessing a mucilaginous stalk showed a clear increasing trend. A shift in reproductive strategy was also evident, with the community exhibiting a pronounced rise in the proportion of asexual reproduction. Furthermore, the representation of algae with intermediate attachment strength declined during the ice-covered period.

Variation in α-taxonomic diversity, α-functional diversity and α-phylogenetic diversity before and after ice cover

Results indicated that the species richness was significantly decreased after ice cover compared with levels before ice cover (t = 2.397, P < 0.05; Fig. 4a), but the density and biomass were not significantly changed (P > 0.05; Figs. 4j and k). Furthermore, the α-functional diversity, functional richness (FRic), functional dispersion (FDis), and RaoQ indices of benthic algae communities decreased significantly (t = 3.725, P < 0.01; t = 10.377, P < 0.001; t = 8.741, P < 0.001; Figs. 4b ,e and l), and the functional evenness (FEve) index was significantly improved (t=-3.538, P < 0.01; Fig. 4c), while the functional divergence (FDiv) index did not change significantly after ice cover (P > 0.05; Figs. 4d). It was also observed that the ice cover significantly decrease the average taxonomic distinctness index (Δ+) in the α phylogenetic diversity index (t = 2.227, P < 0.05; Fig. 4g), but it did not alter the taxonomic diversity index (Δ), taxonomic distinctness index (Δ*), and the variation in taxonomic distinctness index (Λ+) (Fig. 4f, h, i).

Changes of β-taxonomic diversity, β-functional diversity and β-phylogenetic diversity before and after ice cover

Our analysis of β-taxonomic diversity in benthic algal communities revealed notable changes. Specifically, the total β-taxonomic diversity was significantly increased (t = -4.931, P < 0.001; Fig. 5c), accompanied by a marked rise in the taxonomic nestedness index (t = -3.381, P < 0.01; Fig. 5b). However, the taxonomic turnover index remained stable, showing no significant variation. In addition, the functional turnover index decreased significantly after ice cover (t = 2.945, P < 0.01; Fig. 5d), as did the total β-functional diversity (t = 5.451, P < 0.001; Fig. 5f). However, but the functional nestedness was not markedly changed.

Analysis of the β -phylogenetic diversity index showed that the phylogenetic nesting index was significantly enhanced (t=-3.381, P < 0.01; Fig. 5h), indicating common phylogenetic paths among different species. The total β-phylogenetic diversity was also significantly enhanced (t=-4.867, P < 0.001; Fig. 5i), suggesting that the difference in phylogeny between communities has enhanced. However, the phylogenetic turnover index was not altered (Fig. 5g).

Analysis of the association of environmental factors with benthic algae community

The environmental factors and benthic algae species that exhibited significant changes during the entire study period were subjected to redundancy analysis (RDA) analysis. This analysis uncovered a unique pattern of multiple species (denoted by corresponding numbers as SP) associated with various environmental gradients described by vectors. The RDA ordination explained 67.6% of the variation in the structural composition of the benthic algae community (Table 1). The structural composition of the benthic algae community was strongly influenced by environmental factors throughout the ice cover. Most species were primarily distributed along the positive side of the first axis, DO had a strong positive correlation with species, while TP was negatively correlated with species, exhibiting significant habitat characteristics (Fig. S3).

Table 1

Summarizes the statistical characteristics of the four-axis RDA analysis
Statistic	Axis 1	Axis 2	Axis 3	Axis4
Eigenvalues Explained variation (cumulative) Pseudo-canonical correlation Explained fitted variation (cumulative) Total variation	0.2214 22.14 0.9435 32.73 49.27	0.1551 37.65 0.9868 55.66	0.128 50.45 0.9597 74.58	0.0717 57.61 0.9648 85.18
explanatory variables	67.6%

Divergence in benthic algae assembly mechanisms before and after ice cover

The analysis of the Dispersal-Niche Continuum Index (DNCI) revealed a divergence in assembly mechanisms between the two periods (Fig. 6). The DNCI values were positive both before and after ice cover, indicating that deterministic processes predominantly governed the assembly of benthic algal communities during both periods. However, the magnitude of the DNCI was significantly higher prior to ice formation, indicating a marked reduction in the relative contribution of deterministic processes under ice-covered conditions. In contrast to the total community, the assembly mechanisms for Chrysophyta and Xanthophyta exhibited stability, with their respective DNCI values showing minimal change between the two periods.

In this, we found that ice cover had a significant effect on certain environmental parameters in water bodies. For instance, it increased TP, although slightly, leading to alterations in the community structure of benthic algae. These results demonstrate that TP influences changes in the composition of benthic algal communities. Although the biomass and density of benthic algal communities were not significant differences before and after ice cover, the relative abundance of Chlorophyta and Euglenophyta was markedly increased, with the Chl-a content in the water showing a similar trend. A stable and significant correlation was observed between TP and Chl-a concentration, particularly when the Chl-a concentration was below 0.051 mg/L, where this relationship was even more pronounced(Liu, 2017).

Research has demonstrated that the species composition of benthic algal communities is influenced by the concentrations of nutrients such as phosphorus and nitrogen(Dalkıran and Zünbülgil-Ünsal, 2023). In this stud, we observed a significant shift in algal community composition following ice cover compared to the composition before ice cover, which was accompanied by a significant decline in species richness, demonstrating the impact of extreme weather on the biodiversity of stream ecosystems(Julian et al., 2025). The analysis showed that benthic algal species richness, functional richness (FRic), and functional dispersion (FDis) were decreased after ice cover relative to before, indicating a reduction in both the number of species and functional traits, with species functional traits becoming more concentrated. The increase in functional evenness (FEve) indicates a more uniform distribution of species in terms of functionality, with each functional trait being more evenly represented within the community. Notably, the growth and structural composition of spring algal communities is influenced by the Winter climate condition, with studies showing that winter nutrient levels can affect the composition of spring algal communities(Deng et al., 2020). As nutrient levels increase, the diversity of biological communities tends to decrease, and excessive nutrient levels may promote the functional homogenization of communities(Wang et al., 2021).

β-diversity, an important indicator of variations in species composition (Whittaker, 1960), may reflect two phenomena: spatial species turnover and community nestedness. Therefore, decomposing β-diversity into its ecologically additive components may allow accurate distinction between the contributions of spatial turnover and nestedness to β-diversity(Baselga, 2010). In this study, the proportion of benthic algal functional groups adapted to "mesotrophic, still water " and "eutrophic" conditions significantly increased following ice cover, which may be survival strategies adopted by benthic algal communities to thrive under such hostile environmental conditions. Under low nutrient conditions, β-diversity is influenced by environmental distance, whereas under high nutrient conditions, it is primarily influenced by spatial distance(Leboucher et al., 2019). Our findings revealed that rising TP levels following ice cover were associated with a notable decline in the β-functional diversity of benthic algae. This shift appears to be driven by the loss of specialized species and their subsequent replacement by those better suited to extreme environmental conditions. As a result, the benthic algal communities underwent a process of homogenization, reducing their structural diversity. Eutrophication can cause homogenization of diatom community biodiversity, mainly because the biological communities in eutrophic water bodies become subsets of those in oligotrophic and mesotrophic water bodies(Zorzal-Almeida et al., 2021). In our study, the nestedness index of β-taxonomic diversity gradually increased, suggesting that community compositions at different sites became more similar. The decrease in the turnover index of β-functional diversity may be an indicator of decreased replacement rate of functional traits among different sites or communities. In contrast, increased total β-taxonomic diversity showed enhanced species differentiation among communities, leading to the emergence of new unique species at each site and promoting the overall species diversity and distribution breadth of the ecosystem. The decrease in total β-functional diversity is also an indication of reduced overall differentiation of functional traits among benthic algal communities, with different communities becoming functionally homogenized and less heterogeneous. Based on these observations, we infer that even a slight increase in TP content may decrease benthic algal community diversity and functional homogenization. This conclusion is supported by the decline in β-functional diversity of benthic algal communities.

To overcome the shortcomings of using only species richness, which ignores evolutionary and functional differences, researchers are increasingly turning to phylogenetic diversity as a more comprehensive measure in biodiversity studies(Wong et al., 2018). To quantify phylogenetic α-diversity (α-PD), we employed four key indices: taxonomic diversity index (Δ), taxonomic distinctness index (Δ*), average taxonomic distinctness index (Δ+), and variation in taxonomic distinctness index (Λ+)(Liu et al., 2021). These indices collectively form a framework for comprehensively assessing the phylogenetic dimension of biodiversity. Climatic instability may cause a decrease in the turnover indices and increase in nestedness indices of phylogenetic and functional diversity, probably by inducing species extinctions during glacial periods and the selective processes of phylogenetic and functional diversity during post-glacial diversity explosions(Xu et al., 2023). In our study, ice cover induced significant environmental changes decreasing the average taxonomic distinctness (Δ+) index of α-phylogenetic diversity in benthic algal communities, while the taxonomic diversity (Δ), taxonomic distinctness (Δ*), and variation in taxonomic distinctness (Λ+) indices were not significantly changed. This may be attributed to the increase in TP, which reduced α-taxonomic and functional diversity, decreased species numbers, and eliminated phylogenetically similar species, thereby reducing the average phylogenetic distance among species. The results revealed a significant increase in β-phylogenetic diversity after the ice cover, indicating that although different communities contain distinct different species, these species often originate from the same higher taxonomic levels (e.g., genus or family), which improves the phylogenetic homogenization among communities(Dehling and Dehling, 2023). Specifically, the increase in the nestedness index of β-phylogenetic diversity suggests that species across different communities tended to be phylogenetically similar. Although species at different sites may be different, they share more common ancestors phylogenetically.

Furthermore, we quantified the assembly processes governing the benthic algal community before and after ice cover. The DNCI values indicated that deterministic processes dominated during both periods; however, their relative contribution was significantly attenuated under the ice. This shift can be attributed to the more stable and homogeneous environmental conditions created by the ice cover, which may reduce niche differentiation and competitive exclusion, thereby diminishing the overall strength of deterministic selection(Denison Elizabeth et al., 2024). In contrast to this overarching pattern, the assembly mechanisms for Chrysophyta and Xanthophyta exhibited remarkable stability, with negligible changes in their DNCI values. The consistency in their assembly mechanisms suggests that these taxa are influenced by a stable set of environmental filters or niche constraints that remain largely unaltered by the formation of ice cover. Chrysophytes, for instance, are often associated with cold, oligotrophic waters and are known for their specific habitat preferences, including low conductivity(Bessudova et al., 2021). Their ecological niche may remain stable and consistently filtered by the prevailing conditions, irrespective of the ice cover's influence on the broader habitat. Similarly, certain xanthophyte lineages may possess conserved traits that render their presence and assembly consistently deterministic across both periods(Maistro et al., 2009). Our data highlight that while a significant hydrological transition, such as ice cover, can alter the assembly rules for the entire community, its effects are not uniform across all taxonomic groups, underscoring the importance of taxon-specific autecology in mediating responses to environmental change.

In this study, we conducted a comprehensive analysis of the effects of environmental changes on the structure and diversity of benthic algal communities in a northern stream, both before and after ice cover. The results demonstrated that ice formation significantly altered aquatic environmental conditions, particularly by increasing total phosphorus levels, which subsequently induced substantial changes in community structure and multiple dimensions of diversity. The rise in phosphorus levels was identified as a major driver of the decline in α-diversity and the convergence of functional traits across communities. Furthermore, analysis of community assembly revealed that deterministic processes dominated both before and during ice cover, although their relative contribution significantly decreased under the ice. However, this pattern of reduced determinism did not apply uniformly to all taxonomic groups, as Chrysophyta and Xanthophyta maintained consistent assembly mechanisms regardless of ice conditions. These findings enhance our understanding of how biological communities respond to extreme environmental conditions and provide valuable insights for predicting ecosystem changes and conserving biodiversity in the context of ongoing global climate change.

Acknowledgements

This work was funded by the National Science and Technology Basic Resources Survey Program of China (2019FY101700), and the National Natural Science Foundation of China (41977193). We thank Y. Zhang, B. Zheng, M. Ma, X. Wang and T. Pu for technical assistance during the work.

Author contributions

X.Y. and F.M. conceived the study, F.M., Y.L., Y.S., and M.J. analyzed the data. Y.L., S.Z., H.W and F.M. designed the statistical framework and extended the analysis. F.M., Y.L. and X.Y. wrote the original draft, all authors contributed to manuscript editing.

Competing interests

The authors declare no conflicts of interest.

Data availability

The datasets are available in the following repository

[10.5061/dryad.mpg4f4rd6]

Link:http://datadryad.org/share/LINK_NOT_FOR_PUBLICATION/FFlBOy1BR03yzaf_aM5m8S8x6C99H9lqrC3LizGQv6I

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No competing interests reported.

TableS1.docx
FigS1.pdf
Fig. S1 Physical and chemical characteristics of streams in BIC (Before ice cover) and AIC (After ice cover) with non-significant differences. The middle line of each box plot represents the median, and the boxes represent the range between the first and third quartiles. A P-value indicates significant difference, with P<0.05 indicates statistically significant and P<0.01 indicates highly significant. The statistical parameters t and Z are statistical test were used to compare the distribution of data before and after ice cover.
Fig.S2.pdf
Fig. S2 This chart displays the functional trait composition of benthic algae before and after ice cover. BIC: Before ice cover, AIC: After ice cover. The relative abundance is shown on the left Y-axis as the percentage of different functional groups represented by different colors. The height of each histogram block represents the percentage relative to the total quantity in the period, while the sites on the line chart represent the total biomass of the month.
FigS3.pdf
Fig. S3 The Redundancy analysis (RDA) of environmental factors and benthic algae species that changed significantly during the study period.

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Effects of ice cover on diversity and community assembly of benthic algae in a stream system of northern China

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Abstract

Figures

Introduction

Materials and Methods

Study area

Sampling of benthic algae

Environmental factor

Data analysis

Results

Physical and chemical characteristics of streams

Composition of benthic algae community and functional group before and after ice cover

Variation in α-taxonomic diversity, α-functional diversity and α-phylogenetic diversity before and after ice cover

Changes of β-taxonomic diversity, β-functional diversity and β-phylogenetic diversity before and after ice cover

Analysis of the association of environmental factors with benthic algae community

Divergence in benthic algae assembly mechanisms before and after ice cover

Discussion

Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1