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Alpha and beta diversity of macroinvertebrate community in relation to river freezing: a case study in the mountainous stream system of north China

This study demonstrates that river freezing in a North Chinese mountain stream significantly alters environmental conditions and macroinvertebrate community structure by shifting diversity patterns from being jointly shaped by water chemistry and spatial distance to being dominated by specific abiotic filters and dispersal limitation, respectively.

Original authors: Tongxiao Pu, Yuanyuan Song, Yiming Liu, Xuwang Yin

Published 2026-06-28

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CC BY 4.0

Original authors: Tongxiao Pu, Yuanyuan Song, Yiming Liu, Xuwang Yin

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine a mountain river in northern China as a bustling city. In the autumn (before winter), the city is full of life: thousands of different types of "citizens" (aquatic insects and worms) are going about their day, interacting, and building complex neighborhoods.

Then, winter hits. The river freezes over, turning the water into a solid block of ice. This paper is like a detective story investigating how this "city" changes when the ice locks the doors.

Here is the breakdown of what the researchers found, using simple analogies:

1. The Great Filter: Who Survives the Freeze?

Think of the ice cover as a strict bouncer at a club.

Before the freeze: The club is open to everyone. You see a huge variety of species (high "species richness").
During the freeze: The bouncer (the ice and cold water) kicks out anyone who can't handle the cold or the lack of oxygen. The total number of different species drops significantly.
The Survivors: The ones who stay are the "tough guys." They are the insects that love the cold, can survive with less oxygen, or have special tricks like laying dormant eggs. The paper found that these tough, cold-loving groups actually became more common relative to the others.

2. The Three Ways to Measure Diversity

The researchers didn't just count heads; they looked at the community in three different ways, like looking at a crowd through three different pairs of glasses:

Taxonomic Diversity (The "Name" Glasses): This just counts how many different names of species are there.
- Result: The number of names dropped. The "city" had fewer distinct species.
Functional Diversity (The "Job" Glasses): This looks at what the creatures do (e.g., do they crawl? do they swim? do they eat plants?).
- Result: Surprisingly, this went up. Even though there were fewer species, the ones that survived had very different "jobs" and skills. It's like a small team where everyone has a unique, specialized superpower to survive the winter.
Phylogenetic Diversity (The "Family Tree" Glasses): This looks at how closely related the creatures are to each other on the evolutionary family tree.
- Result: This also went up. Before winter, the survivors were often close cousins (phylogenetically clustered). During winter, the survivors were distant relatives from different branches of the tree. It seems the ice forced the community to keep only the most distantly related, hardiest survivors.

3. The "City" vs. The "Neighborhoods" (Alpha vs. Beta Diversity)

Alpha Diversity (Inside one spot): As mentioned, inside any single spot in the river, the variety of species dropped, but the variety of skills and family trees increased.
Beta Diversity (Comparing different spots): This asks: "How different is Spot A from Spot B?"
- Before Winter: The differences between spots were driven by both the water chemistry (local conditions) and how far apart the spots were (dispersal).
- During Winter: The differences between spots became mostly about distance. The ice acted like a wall, making it hard for creatures to move between spots. So, Spot A and Spot B became very different from each other simply because they couldn't swap citizens. However, because the "bouncer" (ice) was so strict everywhere, the types of jobs and family trees became more similar across the whole river (functional and phylogenetic beta diversity went down).

4. The Two Forces at Play

The paper explains that two main forces shape these communities:

The Filter (Environment): The ice and cold water pick and choose who survives based on their traits.
The Wall (Dispersal Limitation): The ice stops creatures from moving around.

Before Winter: Both forces were working together.
During Winter: The forces split up. The "Filter" became the boss of what lived inside a single spot (Alpha diversity), while the "Wall" became the boss of how different the spots were from each other (Beta diversity).

The Bottom Line

The paper argues that winter is not just a time when nature "sleeps." It is an active, intense season that reshuffles the deck. The ice acts as a powerful filter that strips away the weak, leaving behind a smaller, tougher, but highly specialized group of survivors.

Key Takeaway: If you only count the number of species (Taxonomic diversity), you might think the river is "empty" in winter. But if you look at the skills and family history of the survivors, you see a community that has reorganized itself to be incredibly resilient and diverse in its own unique way.

Technical Summary: Alpha and Beta Diversity of Macroinvertebrate Community in Relation to River Freezing

Problem Statement
River freezing is a dominant seasonal disturbance in high-latitude mountainous streams, yet its impact on aquatic community diversity remains understudied. While mountainous streams constitute approximately 77% of the global river network, winter is often viewed as a dormant season, and the spatial-temporal dynamics of ice cover are frequently overlooked in ecological assessments due to logistical sampling challenges. Existing literature has primarily focused on freeze-thaw dynamics affecting taxonomic structure and alpha ( $\alpha$ ) diversity, with limited understanding of how river freezing influences functional and phylogenetic diversity dimensions, as well as beta ( $\beta$ ) diversity patterns. Furthermore, the relative roles of deterministic processes (environmental filtering) and stochastic processes (dispersal limitation) in structuring these communities during freezing versus pre-freezing periods require clarification.

Methodology
The study was conducted in the Mijiang River, a mountainous tributary of the Tumen River in northeastern China, characterized by minimal anthropogenic disturbance. Thirteen sampling sites were established across the basin.

Sampling Design: Macroinvertebrates and environmental variables were collected during two distinct periods: the pre-freezing period (September–November 2021) and the freezing period (December 2021–February 2022).
Data Collection: Benthic macroinvertebrates were sampled using a D-frame Surber sampler (0.3 × 0.3 m, 500 µm mesh) in three river sections per site. During the freezing period, access holes were cut through the ice to maintain methodological consistency. Samples were identified to the lowest practical taxonomic level (mostly genus or species).
Environmental Variables: Eighteen variables were measured, including water chemistry (pH, temperature, conductivity, dissolved oxygen, TN, TP, CODMn, NH4-N, NO3–-N), hydrology (depth, velocity, width), and substrate composition (boulders, cobbles, gravel, etc.).
Diversity Metrics:
- Taxonomic Diversity: Species richness (SR), Shannon–Wiener index (H), and Pielou's evenness.
- Functional Diversity (FD): Nine functional traits were analyzed across life history, ecology, resistance/resilience, and morphology. Indices included functional richness (FRic), evenness (Feve), divergence (Fdiv), dispersion (FDis), and Rao's quadratic entropy.
- Phylogenetic Diversity (PD): Calculated using a Linnaean taxonomy tree as a surrogate for phylogeny. Metrics included Mean Nearest Taxon Distance (MNTD) and the Nearest Taxon Index (NTI) to assess clustering vs. overdispersion.
Statistical Analysis: $\alpha$ -diversity was analyzed using variance tests and correlations. $\beta$ -diversity (taxonomic, functional, phylogenetic) was decomposed into turnover and nestedness components. Multiple Regression on Distance Matrices (MRM) was employed to quantify the relative contributions of environmental variables and spatial distance (dispersal limitation) to $\beta$ -diversity.

Key Results

Environmental Shifts: Significant alterations occurred between periods. Conductivity, flow velocity, temperature, dissolved oxygen, TN, and coarse substrate proportions were higher pre-freezing. Conversely, TP, nitrate nitrogen, and pH were significantly higher during the freezing period.
Community Structure: Species richness and density decreased significantly during the freezing period, while biomass remained unchanged. The community composition shifted distinctly, with a higher abundance of stress-tolerant functional trait groups (e.g., cold-preferring, herbivorous, crawlers, and passive dispersers) during freezing.
Phylogenetic Assembly: Pre-freezing communities exhibited phylogenetic clustering (species more closely related than expected by chance). During freezing, assembly patterns became heterogeneous: some sites showed random distribution, others overdispersion (species more distantly related), and some maintained clustering.
$\alpha$ -Diversity Patterns:
- Taxonomic: Species richness and Shannon diversity were significantly lower during freezing.
- Functional & Phylogenetic: Functional richness and mean nearest taxon distance (MNTD) were significantly higher during freezing.
- Correlations: Pre-freezing, taxonomic and functional $\alpha$ -diversity were strongly correlated. During freezing, correlations shifted, with taxonomic diversity showing significant links to functional evenness and phylogenetic distance.
$\beta$ -Diversity Patterns:
- Taxonomic total $\beta$ -diversity increased during freezing, driven primarily by species turnover.
- Functional and phylogenetic $\beta$ -diversity decreased during freezing, with nestedness contributions declining and turnover contributions increasing.
Drivers of Diversity:
- Pre-freezing: Both $\alpha$ - and $\beta$ -diversity were shaped by a combination of water chemistry variables and spatial distances.
- Freezing: $\alpha$ -diversity was primarily influenced by dissolved oxygen, total nitrogen, nitrate, and coarse substrate proportion. $\beta$ -diversity was predominantly governed by spatial distance (dispersal limitation), though environmental filtering (e.g., dissolved oxygen) still influenced functional $\beta$ -diversity.

Key Contributions
This study provides a multidimensional assessment (taxonomic, functional, and phylogenetic) of macroinvertebrate responses to river freezing. It demonstrates that while taxonomic diversity declines under ice cover, functional and phylogenetic diversity can increase, suggesting that surviving species possess unique adaptive traits and evolutionary histories. The research highlights a shift in community assembly mechanisms: pre-freezing communities are structured by both environmental filtering and dispersal limitation, whereas freezing conditions intensify environmental filtering for $\alpha$ -diversity while spatial constraints (dispersal limitation) become the dominant driver of $\beta$ -diversity.

Significance and Claims
The authors claim that these findings offer new insights into the mechanisms underlying biodiversity patterns in frozen streams, challenging the traditional view of winter as a period of ecological dormancy. The study emphasizes that:

Winter is an Active Filter: River freezing acts as a strong environmental filter that selectively shapes community assembly, altering the balance between deterministic and stochastic processes.
Multidimensional Necessity: Relying solely on taxonomic metrics is insufficient; functional and phylogenetic dimensions respond differently to freezing, providing a more complete picture of ecosystem resilience.
Conservation Implications: The results suggest that biodiversity monitoring and conservation strategies for cold-region river ecosystems must incorporate seasonal freezing effects. Specifically, maintaining habitat connectivity and water quality (e.g., dissolved oxygen, nutrients) during the freezing period is critical for preserving ecosystem functions, even as taxonomic diversity declines.

The paper concludes that while it is limited to a single river system and two time points, it serves as a meaningful expansion into a domain requiring further exploration, urging the integration of winter dynamics into global biodiversity assessments and climate change impact models.