Increasing drying changes the relationship of biodiversity and community assembly with ecosystem multifunctionality in temporary streams

doi:10.21203/rs.3.rs-5221322/v1

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Increasing drying changes the relationship of biodiversity and community assembly with ecosystem multifunctionality in temporary streams

https://doi.org/10.21203/rs.3.rs-5221322/v1

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Increased drought in rivers under the impact of global climate change is leading to biodiversity loss. However, it is not clear whether biodiversity loss affects river multifunctionality. In this study, we investigated the changes in community structure and ecological functions of biofilm communities in an artificially simulated stream after different drought durations.A drought period of about 60 days is a critical time point for changes in the structure and functions of river ecosystems under drought stress. Therefore, different drought durations were divided into short-term drought (0 ~ 20 d) and long-term drought (60 ~ 130 d) to analyse the maintenance mechanism of benthic community structure in terms of multifunctionality. In summary, biodiversity showed a significant dominant relationship in maintaining community stability after short-term drought, while the dominant relationship got uncoupled after long-term drought. For the maintenance of multifunctionality in benthic ecosystems, community assembly has been dominant with drought intensification, rather than biodiversity as traditionally perceived. This study reveals the importance of community assembly in maintaining multifunctionality in intermittent river and ephemeral stream ecosystems, extending theoretical knowledge of B-EMf relationships in extreme environments.

Biological sciences/Microbiology/Biofilms

Biological sciences/Microbiology/Communities

Biological sciences/Microbiology/Environmental microbiology

Global Climate Change

Intermittent Rivers and Ephemeral Streams

Benthic Biofilm

Community Assembly

Biodiversity and Ecosystem Multifunctionality Relationship

Extreme events (e.g. drought) induced by climate warming and human activities significantly affected river ecosystems and caused degradation of ecosystem functions ^1,2. River disconnection and drying are becoming more frequent and severe under the influence of extreme droughts, such as intermittent rivers and ephemeral streams (IRES) which experience intermittent dryings, and this alteration in river hydrology and resource availability significantly affects the entire benthic community ^3,4. As a crucial ecological indicator in river ecosystems, benthic biofilm is paramount in unraveling the intricate interplay between biodiversity and ecosystem functions ⁵. In response to various environmental pressures, exploring the relationship between biodiversity and ecosystem function (BEF) has become a priority for ecologists ^6–8. For example, Maestre, et al. ⁹ found that plant multifunctionality was positively and significantly related to species richness in drylands, and this relationship became progressively stronger and more positive as environmental stress increased ¹⁰. However, Alvarez, et al. ¹¹ observed a decoupling of diversity from ecosystem function during environmental disturbances recovery in a river ecosystem, possibly due to the community instability caused by rapid biotic turnover during the recovery period.

It was determined that multiple dimensions of biodiversity (alpha diversity, microbial beta and network diversity) are significantly correlated with ecosystem processes ¹². So, systematically exploring the relationship between multiple biodiversity dimensions and ecosystem functions may provide deeper insights than focusing on a single biodiversity dimension in isolation ¹³. In addition, actual ecological communities are undoubtedly governed by rules such as niche assembly and ecological drift, and different community assembly mechanisms were demonstrated to also significantly affect ecosystem functions ^14,15. Based on the niche-based theory and the neutral theory ^16,17, ecologists broadly accepted that both stochastic (‘neutral’) and deterministic processes influence community assembly simultaneously ¹⁸, which includes five driving mechanisms: homogeneous selection, homogenising dispersal, undominated, dispersal limitation and heterogeneous selection ^19,20. However, the relationship between community assembly processes and ecosystem multifunctionality (CA-Mf) are unexpectedly neglected. Consequently, exploring the determinism and stochasticity in the community assembly process is critical to comprehending the dynamic change of biodiversity and its contribution to ecosystem functions ^7,21.

Mounting evidence suggests that biodiversity could enhance multiple ecosystem functions in terrestrial and freshwater ecosystems ^13,21,22, such as carbon storage, productivity, and other metabolic functions of biogeochemical element cycling ²³. Ecosystems are highly valued for supporting various ecological functions, yet most research focuses on single processes in isolation ²⁴. As a result, there is an inaccurate estimate of the level of biodiversity required to maintain ecosystem multifunctionality and a lack of knowledge about the relationship between biodiversity and ecosystem multifunctionality (B-EMf). Due to the key role of stream biofilm in fluvial ecosystems, the research should focus on exploring the change in biofilm B-EMf relationship response to environmental changes in artificially simulated river channels. This controlled environment enables the examination of complex interactions within biofilm communities and their multifunctionality, which are vital for understanding the dynamics of ecosystem processes and the impacts of environmental changes.

Ultimately, the sustainability of both the multifunctionality and biodiversity rendered by the benthic ecosystem are dependent on a relatively stable microbiome (Griffiths and Philippot, 2013), defined as the degree of variation of the microbial community ^25,26. Therefore, it is important to investigate the stability of the microbial community and the possible functional consequences, which can provide mechanistic insights into the effects of climate change on the biogeochemical processes governing benthic ecosystems ^27,28. In order to analyse the dominance in driving biofilm multifunctionality during flow phase after different drought duration. We simultaneously conducted an assessment of 19 distinct ecosystem functions related to the vital cycling and storage of carbon, nitrogen, and phosphorus, considering their pivotal role in regulating ecosystem function and the necessity for biofilm energy cycling ^6,13. Furthermore, we employed partial least squares path models (PLS-PMs) to investigate the relationship of B-EMf and CA-Mf, along with the potential link of biodiversity and community assembly with community stability.

2.1 Biofilm colonisation and laboratory experiment

Cobbles (2–3 cm in diameter) were colonized in Qin Huai River (32°03 36.2 N; 118°44 38.1 E), and the colonisation and water quality parameters are detailed in the supporting information (Text S1). After 44 days, these cobbles with biofilm were translocated to acclimatized to the artificial streams(160 cm long, 20 cm wide, and 30 cm high) for four additional days under constant water quality through the addition of a nutrient solution per week^29,30. The channels were equipped with a pump (BT100-1L, Longer Precision Pump Co., Ltd., Baoding, China) to ensure water recycling and that the velocity of 0.14 m/s was maintained (consistent with the velocity of the Qin Huai River) ³¹. Biofilm resilience was studied using artificial channels that simulate key environmental factors of riverine ecosystems, enabling controlled experimentation on the effects of non-flow periods.

In order to study the effect of drought duration on the recovery of river ecosystems, we set up four experimental groups with four replicate channels of each experimental group respectively (after 20, 60, 100, and 130 days of flow disconnection duration, with measurements of biofilm community structure and ecological functioning during the 21 days of flow restoration), and a control group with the constant flow for analyzing the effect of intermittent river flow on the ecosystem^32,33. Samples were collected from experimental groups at five different times (four replicates per time) by randomly selecting specific amounts of cobbles from the channels and sampled from the control group at four different flow recovery periods (Fig. 1). More details are provided in the supporting information (Text S1).

2. 2 DNA sequencing and analyses
Four biofilm replicates were collected at five different times from four experimental groups during the rewetting and sampled from the control group at four different rewetting periods (Fig. 1). Detailed descriptions of the DNA extraction, eukaryotic (algal and fungal) 18S and bacterial 16S amplifications, and MiSeq sequencing are available in the supporting information Text S2³⁴. The MagaBio Soil/Fecal Genomic DNA Purification kit (Bioer, Hangzhou, China) was used to extract the total DNA. The primer pair 515F (5`-GTGCCAGCMGCCGCGGTAA-3`) and 806R (5`-GGACTACHVGGGTWTCTAAT-3`)^35,36 was chosen to amplify the V4 hypervariable regions of the 16S rRNA gene for bacteria. For eukaryotes, we used 528F (5’ GCGGTAATTCCAGCTCCAA) and 706R (5’ AATCCRAGAATTTCACCTCT) primer sequences to amplify the V4 region of the 18S rRNA gene³⁴. Meanwhile, the fungus and algae were distinguished from each other at the kingdom level for analyze; then the number of effective tags (no. of seqs) and the operational taxonomic units (OTUs) synthesis information table (OTU_table) were obtained for the subsequent analysis.

Microbial community beta diversity (community heterogeneity) was calculated through the dissimilarity between experimental communities based on the Bray–Curtis index ³⁷. Network analyses of different biofilm components (bacteria, fungi, and algae) were conducted on species abundance data at the OTU level ³⁸, with Pearson’s correlation p-value ≤ 0.05 and |cor|>0.5. Additionally, biofilm microbial community stability was evaluated in terms of average variation degree, which was calculated using the deviation degree from the mean of the normally distributed OTU relative abundance ²⁵. We analysed the biofilm community assembly based on the null model using the iCAMP v1.5.12 package ³⁹ and the neutral community model ¹⁸. The R² value indicates the goodness of fit to the neutral community model, which means that the community assembly is fully consistent with stochastic processes when R² is close to 1 ⁴⁰. The supporting information provides the detailed experimental determination, including statistical analyses of sequencing data (Text S2).

2.3 Biofilm multifunctionality

To assess functional changes in river ecosystems, we calculated z-scores for 19 ecosystem functions related to the cycling of carbon, nitrogen, and phosphorus. These functions include the ecosystem metabolism (the ratio of gross primary production (GPP) and community respiration (CR)), which governs the fixation and mineralisation of organic carbon ⁴¹. Other functions include CO₂-fluxes ²⁹, biomass ⁴², and enzyme activities such as α-glucosidase (AGLU), β-glucosidase (BGLU), cellobiose hydrolase (CBH), glucokinase pentoses (GK), as well as N₂O-fluxes, leucine aminopeptidase (LAP), and various nitrogen-related enzymes ^7,43,44. CO₂-fluxes and N₂O-fluxes were measured using closed chamber methods ⁴⁵. Ecosystem metabolism was estimated from the drop and rise in dissolved oxygen concentration ⁴⁶. A fixed area of biofilm (30 ± 2 cm²) was dried (105°C, 24h) and calcined (450°C, 5h) to obtain biofilm biomass ⁴⁷, reflecting the accumulation of organic matter. And other enzyme functions were assessed based on the corresponding ELISA kits. More detailed experimental determinations are provided in the supporting information (Text S3).

To obtain an averaging ecosystem multifunctionality index ⁸, we averaged the standardised scores of all individual ecosystem functions and calculated Z-scores of the 19 functions evaluated ⁴⁸ as in Eq. (1–2). Based on related functions, carbon metabolic, nitrogen metabolic and phosphorus metabolic also be calculated in the same way. The multifunctionality index was the average Z-score for all functions measured.
$$\:{x}_{z-score}=\frac{{x}_{i}-\mu\:}{\sigma\:}$$
1
$$\:MF=\sum\:_{1}^{n}{X}_{z-score\:i}$$
2

Where $\:{x}_{i}$ is each biofilm function, $\:\mu\:$ refers to the mean of the $\:{x}_{i}$ and σ refers to the standard deviation of the $\:{x}_{i}$. $\:{X}_{z-score\:i}$ is the z-score of each biofilm function, and n is n is the number of all ecological functions monitored.

2.4 Partial Least Squares Path Models (PLS-PM)

The PLS-PM was constructed to calculate the effect of biofilm biodiversity on ecosystem functions, as well as the impact of community assembly, for analysing the relationship between B-EMf and CA-Mf. We analysed the biodiversity of different biofilm components (algae, bacteria, and fungi) by computing Z-scores of their alpha (richness), beta (community heterogeneity), and network diversity (AvgK, AvgCC, GD, and modularity) as in Eq. (1–2) ⁴⁸. We took the biofilm biodiversity, carbon cycle index, nitrogen cycle index, phosphorus cycle index, community stability and community assembly of different microbial components (fungi, algae, and bacteria) as different potential variables in PLS-PM ^49,50. During the model fitting process, insignificant biofilm functional and structural parameters were excluded based on their outer loadings by employing SmartPLS software (Version 3, Boenningstedt, Germany), with the validity of the fitting results (Model_Fit, Path Coefficients, p-values for both direct and indirect effects, and other indicators ³⁴).

2.5 Statistical analyses

To evaluate significant differences in biofilm functions and structures among groups, a one-way ANOVA analysis was performed (n = 4), considering a p-value of < 0.05 as indicative of statistical significance. PLS discriminant analysis (PLS-DA) was used to distinguish the biofilm multifunctionalities of different groups based on PERMANOVA (Adonis) analyses (Anderson, 2001), as well as analysed the differences in elemental metabolic (carbon metabolic, nitrogen metabolic and phosphorus metabolic). To study the influence of the biodiversity of different biofilm components (algae, bacteria, and fungi) on biofilm functions, the mantel test was conducted based on Pearson`s correlation using the ggcor package in R ⁵¹.

3.1 The responses of biofilm functions during rewetting after different drought duration

The biofilm multifunctionality index increased with increasing drought duration up to 60 days (Figure S1 a). After the 60-day mark, however, the index began to level off and remained relatively stable despite the continued increase in drought duration (Figure S1 a). Similar responses were also observed in the nitrogen metabolic index (Figure S1 b), carbon metabolic index (Figure S1 c), and phosphorus metabolic index (Figure S1 d). The cluster analysis between groups after z-score standardisation with different biofilm functions showed that the longer the duration of drought, the stronger the functional difference with control, with a critical period of 60 days for rivers to dry up (Fig. 2a). Specifically, the biofilm metabolic activities of BGLU, NADH, NOS, AMO, LAP, NAR and ACP peaked after 60 days of drought and then decreased following prolonged drought (Fig. 2a), while the responses of other biofilm functions (ecosystem metabolism, biofilm biomass, N₂O fluxes and CO₂ fluxes) were exactly the opposite, with decreases after 60 days of drought followed by increases (Fig. 2a). Additionally, there was a significant difference between the biofilm multifunctionality after moderate drought disturbances (control, 20 days, and 60 days) and after prolonged drought disturbances (100 days and 130 days) (Fig. 2b, Table S1, PERMANOVA, p < 0.05). Meanwhile, the same difference was also found in the biofilm carbon cycle and the nitrogen cycle after different drought times (Fig. 2b, Table S1, PERMANOVA, p < 0.05); and the gradually increased difference with increasing drought between the control with each experimental group was found in the phosphorus cycle (Fig. 2b, Table S1, PERMANOVA, p < 0.05).

3.2 The changes of microbial network and community assembly mechanisms

All samples approached saturation in rarefaction curves analysis, indicating that the sequencing effort was sufficient to estimate the responses of these biofilm microbial communities to drying and rewetting alternation (Figure S2-S3). Neutral community model fits (R²) showed that the effects of stochasticity gradually dominated in the biofilm community assembly, especially after 60 days of drought (Fig. 3a and Figures S7-9). Meanwhile, the findings of the null model showed that undominated processes and dispersal limitation were the significant drivers of algal and bacterial community assembly, and the undominated processes always dominated the fungal community assembly (Fig. 3b-d). The network modularity of the different biofilm fractions responded consistently to the different drought times, which decreased and then increased with drought duration, with a significant inflexion point at around 60 days of drought. At the same time, the fungal network was more sensitive with a considerable inflexion point occurring after 20 days of drought (Fig. 3e, f). However, the networks, Avg K and GD of the bacteria and algae were precisely the opposite of the fungal network, as the average K of the fungal network gradually decreased, and the GD gradually increased with increasing drought (Fig. 3f). Nevertheless, they both showed a significant inflexion point at around 20 days of drought (Fig. 3f). In addition, the eukaryotic community stability decreased and then increased significantly with increasing drought, while the stability of bacteria showed no significant change, indicating that eukaryotic microorganisms were significantly affected by drought and unable to recover rapidly during the rewetting period (Fig. 3g).

3.3 The effects of different drought times on the relationship between biodiversity and multifunctionality

Based on the critical period of 60 days for rivers to dry up, we divided the experimental group with different drought durations into short-term (control and D20) and long-term drought groups (D60, D100 and D130) (Fig. 4). Increasing drought had a significant effect on the biodiversity-ecosystem functions (BEF) relationship and the relationship of the community assembly mechanism with ecosystem functions (CA-Mf) (Fig. 4). More importantly, biodiversity is decoupled with ecosystem functions after long-term droughts, while community assembly progressively contributes to explaining the variations in biofilm ecosystem functions with increasing drought (Fig. 4). With the increasing drought, the most significant effect of community assembly on biofilm ecosystem functions shifted to the phosphorus cycle from the nitrogen cycle (Fig. 4). In addition, the explanatory power of biodiversity was always vital for community assembly.

The PLS-PM results showed that the increasing drought duration had a significant effect on the relationship of biofilm multifunctionality with community assembly (from − 0.289 to 0.446) and biodiversity (from − 0.28 to 0.191) during rewetting (Fig. 5). The most important predictor for biofilm multifunctionality was community assembly rather than biodiversity (Fig. 5). In addition, the relationship of multifunctionality with community stability similarly changed from a negative to a positive effect (Fig. 5). With drought increasing, the significant impact of biofilm biodiversity on community stability during rewetting was weakened (Fig. 5).

In response to the escalating challenges presented by climate change, specifically the increased intermittency of river flows, this study suggests the impact on benthic communities. This investigation commenced with a multidimensional biodiversity analysis of biofilms and assessments of ecosystem multifunctionality. Employing partial least squares path modelling, the research explored the intrinsic relationships between biodiversity, ecosystem stability, community assembly, and ecosystem multifunctionality (BE-Mf and CA-Mf relationship). The results provide evidence regarding the relationship between biodiversity and multifunctionality in intermittent streams, considering both the non-flow period and the recovery period.

In conclusion, the observed shifts in microbial communities could precipitate substantial changes in ecosystem processes and services, accentuating the need to preserve microbial diversity and functionality to ensure ecosystem robustness amid climate variability. These findings illuminate the complex interplay between biodiversity and ecosystem functionality in intermittent streams across both drying and recovery phases.

4.1 Thecritical period for rivers to dry up of biofilm biodiversity and multifunctionality

Consistent with the cluster analysis of biofilm functions and the result of the PLS-DA from multifunctionality, there was also a critical period for rivers to dry up at around 60 days for biofilm community stability and community assembly (Figs. 1–3), which may have been induced by the temporary transformation of aquatic ecosystems into terrestrial systems after 3 months of drying². The phosphorus cycle of biofilm showed higher drought resistance than the carbon and nitrogen cycles (Fig. 2b), likely because biofilm always uses phosphorus preferentially to generate energy during the recovery phase after drying⁵², reflecting the importance of phosphorus as a key element of microbial metabolism during rewetting⁵³. In contrast, the carbon and nitrogen cycles of the biofilm could not maintain the same activity with control during the rewetting period after a prolonged drought, likely because the prolonged drought led to the change of redox conditions in the biofilm habitat⁵. Based on the correlative nature of the individual functions⁵⁴, biofilm multifunctionality was more sensitive to increasing drought than single function (Fig. 2), reflecting the similar underlying processes to increasing drought⁴³. The significant difference in multifunctionality before and after 60 days of drought may have been caused by the temporary transformation of aquatic ecosystems into terrestrial systems, which leads to a significant change in resource availability and turnover efficiency of biofilm².

The same critical time point of the impact of the duration of river drying on river ecosystems was also observed in the fitting result of the neutral community model (Fig. 3a), and the increasing dominance of stochasticity perhaps resulted from the stimulation from the hydrological change perturbations to stochastic factors ^37,55. Although previous studies have suggested that decreasing community diversity may lead to the dominance of deterministic processes in community assembly ⁵⁶, this is true only at saturated community sizes⁵⁷. However, in small community sizes, such as biofilm, communities are more susceptible to stochasticity with decreasing diversity^58,59, especially under the environmental stimuli of such variable hydrological conditions in IRES⁵⁵ (Figure S8). Of the stochastic influence, “undominated” (meaning that dispersal and drift contribute equally to community composition)⁶⁰ was the major factor for community assembly mechanisms of all components of biofilms (Fig. 3b-d), and it was especially significant for fungi. This might be due to unmeasured ecological processes, such as species competition for similar resources, that may exert a more substantial influence in a low heterogeneous environment and beta diversity (Table S2)^56,61. The varied influence of dispersal limitation on different components of biofilms may be related to their different survival strategies, as the relative abundance of the microbial community could be the major factor influencing dispersal limitation^62,63. The stronger influence of dispersal limitation on bacteria has also been reported by Oono, et al. ⁶⁴. Prolonged droughts reduce microbial heterogeneity, diminishing ecosystem resilience by rendering microbial communities less equipped to face environmental adversities⁶⁵. This homogenization disrupts the efficient cycling of key nutrients such as carbon, nitrogen, and phosphorus—elements critical for ecosystem productivity and health. Such disruptions may impair vital ecosystem services, including soil fertility, water purification, and greenhouse gas regulation^66,67.

Given the important effect of diversity ⁶⁸, such as beta-diversity and ecological networks, the growing theoretical and empirical evidence suggests that the mechanisms driving spatial variation in diversity to the in-depth study of the biodiversity-ecosystem functions (BEF) relationship are of high theoretical importance⁶⁹. Apart from the decreased microbial alpha diversity with increasing drought (Figure S4-S6), the reduction in beta-diversity (Bray–Curtis dissimilarity between groups) with prolonged drought durations indicated that extended drought periods contributed to decreased microbial community heterogeneity (Table S2), which could substantially impact biofilm functions through the “insurance effects” of beta-diversity in varying environments (enabling the dispersal of species with different physiological responses as conditions change)⁷⁰. Specifically, reduced microbial diversity could jeopardize soil carbon sequestration, potentially escalating atmospheric CO₂ concentrations and impacting global climate patterns⁷¹.

Furthermore, the consistent response of the biofilm network modularity of the different components indicated different microbial communities rescaled to form a complex network of interactions following drought disturbance⁷². In this study, the bacterial and algal network’s response mechanism to increasing drought stress reflected a tight and complex network structure³⁴, with a critical period of 60 days for rivers to dry up. The response of the fungal network was the opposite (Fig. 3e and f), which might be due to the higher environmental adaptation for fungi under extreme conditions⁷³, especially under the more impaired conditions of bacterial and algal communities^74,75.

Moreover, microbes constitute essential resources for various organisms, including protozoa, small invertebrates, and filter feeders. Changes in microbial community composition and functionality can alter the availability and nutritional quality of microbial biomass, affecting consumer growth, reproduction, and survival. These alterations may lead to disrupted trophic interactions and reduced biodiversity within food webs^76,77. The stability and resilience of these food webs are intrinsically linked to the diversity and functional capacity of their microbial components⁷⁸. Microbial communities that exhibit limited functional diversity are ill-prepared for environmental changes, which can compromise the resilience of the entire ecosystem⁷⁹. This decreased adaptability may impair the ecosystem's ability to recover from disturbances, resulting in long-term shifts in community structure and function across all trophic levels, ultimately undermining ecosystem health and stability^11,79,80.Considering that biofilm function sustainability depended on a relatively stable microbiome²⁵, community stability was evaluated during rewetting after different drought times. There was also a critical time point of river drying at 60 days for eukaryotic microbial community stability, corresponding to biofilm functions and confirming the community stability's critical effect on biofilm function⁸¹. Meanwhile, bacteria were more resistant to increasing drought, likely due to its most complex network or resistant phenotypes such as spore formation ⁸².

4.2 The relationship between biodiversity and ecosystem functions decoupled after long-term droughts

Two possible explanations exist for the decoupling between biodiversity and ecosystem functions after prolonged drought (Fig. 4). The first facet to consider is functional plasticity, defined as the ability of microbial communities to adapt to environmental fluctuations by fine-tuning their performance⁷⁵. For example, BGLU, NOS, LAP, AMO and NAR tended to adapt to environmental changes (increase followed by decrease or a decrease followed by the rise with increasing drought duration) (Fig. 2a). While analysed from the perspective of elemental metabolic cycling, the elemental metabolic cycling functions of the biofilm differed significantly before and after the critical period for rivers to dry up (60 days of drought) (Fig. 2b). This was especially true for the carbon and nitrogen element metabolic functions (Fig. 2b), indicating that functional plasticity is not the only explanation for this decoupling. So, functional redundancy might also contribute to the results that some metabolic functions of biofilms could be maintained at a certain level when the biofilm diversity had been irreversibly affected by a prolonged drought, even though they may have behaved differently for a single function⁸³. Moreover, microbial species adapt to environmental change by generating a trade-off between environmental filtering and disposal limitation⁸⁴, which could explain the strong links between biodiversity and community assembly (Fig. 4).

4.3 Community assembly is more important for ecosystem multifunctionality than biodiversity in IRES

The environment selects microbial functional traits rather than species to maintain essential ecosystem functions⁸⁵, which is why community assembly contributed more to biofilm ecosystem functions than biodiversity after long-term droughts (Fig. 4 and Fig. 5). The significant effect of community assembly on biofilm ecosystem functions shifted to the phosphorus cycle from the nitrogen cycle, which could be because the phosphorus cycle was more susceptible to being filtered by environmental conditions⁸⁶ (Fig. 4). Notably, the relationship between biofilm multifunctionality and different factors changed from a negative to a positive correlation following prolonged drought (Fig. 5), indicating that coexisting species, following niche partitioning based on various resources, could positively interact and further improve community functional performance¹⁴. Based on the high relativity between more minor environmental variations and higher microbial phylogenetic diversity²⁵, there was a relatively significant positive effect between biofilm biodiversity and community stability (Fig. 5). After long-term drought, the narrow distribution of community functional groups and the depleting redundancy of biofilm functions may have weakened the relationship between biofilm biodiversity and community stability⁸⁷.

4.4 Limitations and environmental implications

The results highlight that community assembly has a more significant influence on biofilm multifunctionality than biodiversity and that their relative influences vary with increasing drought. Understanding the relationship between community assembly and microbial functions is at the forefront of current ecological research^88,89. Yet, this paradigm has not been broadly studied in river ecosystems. In this study, we examined the impact of different drought times on the B-EMf and CA-Mf relationship through indoor simulation experiments. Nevertheless, it is essential to acknowledge that the controlled trials conducted might potentially restrict the extensive extrapolation of our findings, considering the heterogeneous environmental composition in natural IRES. For example, the dynamics of critical variables, including light, temperature, and organic matter within natural river ecosystems, might coincide with dry-to-wet transitions and affect the extent and nature of biofilm responses⁴⁷. More rigorous tests and further field experiments on the generality of this result in different ecosystems are needed for consideration as a fundamental principle of microbial ecology.

Based on this study, increasingly refined insights have been gained into the critical period for rivers to dry up of biofilm multifunctionality and the vital contribution of community assembly in driving it. Initially, the study observed the critical period for rivers to dry up in biofilm functions and community structure (network, community stability, and community assembly), while the applicability of the critical period for rivers to dry up studies for global nature conservation and policy remains somewhat limited⁹⁰. Addressing the identified research gaps will progressively enhance the future policy relevance of critical periods for rivers to dry up. Implementing timely flow replenishment before reaching the critical period for rivers to dry up is likely the most essential and effective strategy. Additionally, biodiversity decoupled from ecosystem functions after long-term drought disturbances, meaning further work to identify the main drivers of ecosystem functions will be critical for ultimately predicting the response of community functions to environmental changes. More importantly, the increasing prominence of stochastic processes becomes more pronounced with longer drought durations, and community assembly is more important for ecosystem multifunctionality than biodiversity.

This study provides an inaugural demonstration of the dominant role of stochastic assembly in shaping community structure and ecosystem multifunctionality. Elucidating the intricate connections among community assembly, biodiversity, and ecosystem functioning is critical for preserving biodiversity and effectively managing ecosystems.

This study elucidates the multifaceted interactions between different drought times and benthic biofilm communities in artificial stream channels, highlighting the intricate dynamics of biodiversity and ecosystem multifunctionality in the face of increasing drying severity. Our results suggest a clear shift in the dominance of biodiversity over ecosystem functioning with increasing drought duration. Notably, a critical period for rivers to dry up of around 60 days significantly altered the biodiversity and multifunctionality of biofilm communities. This shift emphasises the resilience of microbial communities to short-term drying conditions, where biodiversity significantly influences multiple ecosystem processes. However, the dependence of ecosystem multifunctionality on community assembly became more pronounced under long-term drying conditions, suggesting that the structural dynamics of microbial communities play a key role in maintaining ecosystem multifunctionality under environmental stress. Furthermore, this study highlights the importance of stochastic processes in community assembly, especially under prolonged drying conditions, which may lead to reconfiguration of interactions between community assembly and ecosystem multifunctionality. Our study contributes to a deeper understanding of how drying affects freshwater ecosystems, providing crucial insights for predicting and managing the impacts of climate change on biofilm biodiversity and ecosystem services. These insights also provide a comprehensive view of the ecological mechanisms that sustain ecosystem functioning in a changing environment. This study not only deepens our theoretical understanding of the relationships between B-EMf and CA-MF but also provides a valuable framework for future studies of the impacts of other global change factors on freshwater ecosystems.

Acknowledgments, Samples, and Data

Supplemental information includes three texts, nine figures and six tables.
Authors declare that they have no competing interests.
The 16S and 18S sequence data that support the findings of this study are available in the NCBI repository (accession code: PRJNA1014953). The remaining data supporting the findings of this study are available within the article and source data file.
We are grateful for the grants for the project supported by the National Natural Science Foundation of China (No. 52379063), the Fundamental Research Funds for the Central Universities (B240205016), Graduate Research and Innovation Projects of Jiangsu Province (KYCX24_0902), the Excellent Young Team Program for Central Universities “Environmental Risks and Decontamination Technology of Emerging contaminants in Rivers and Lakes”, the Key Research and Development Project of Tibet Autonomous Region Science and Technology Department (XZ202101ZY0016G) and the Open Fund of the State Key Laboratory of Hydraulics and Mountain River Engineering of Sichuan University (SKHL2109).

Author Contribution

Conceptualization: JunHou, MingKongMethodology: YuYao, JunWu, GuoxiangYouInvestigation: SongqiLiu, ZijunYang, TanveerM.AdyelVisualization: YueYu, ChaoranLiSupervision: JunHou,LingzhanMiaoWriting—original draft: ChaoranLiWriting—review & editing: ChaoranLi, JunHou,LingzhanMiao

Data Availability

• The 16S and 18S sequence data that support the findings of this study are available in the NCBI repository (accession code: PRJNA1014953). The remaining data supporting the findings of this study are available within the article and source data file.

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Increasing drying changes the relationship of biodiversity and community assembly with ecosystem multifunctionality in temporary streams

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Abstract

Figures

1 Introduction

2 Materials and Methods

2.1 Biofilm colonisation and laboratory experiment

2.3 Biofilm multifunctionality

2.4 Partial Least Squares Path Models (PLS-PM)

2.5 Statistical analyses

3 Results

3.1 The responses of biofilm functions during rewetting after different drought duration

3.2 The changes of microbial network and community assembly mechanisms

3.3 The effects of different drought times on the relationship between biodiversity and multifunctionality

4 Discussion

4.1 Thecritical period for rivers to dry up of biofilm biodiversity and multifunctionality

4.2 The relationship between biodiversity and ecosystem functions decoupled after long-term droughts

4.3 Community assembly is more important for ecosystem multifunctionality than biodiversity in IRES

4.4 Limitations and environmental implications

5 Conclusion

Declarations

Author Contribution

Data Availability

References

Additional Declarations

Supplementary Files

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