Decadal biodiversity trends in rivers reveal recent community rearrangements

Stream macroinvertebrate communities in restored and impacted catchments respond differently to climate, land-use, and runoff over a decade

Identifying which environmental drivers underlie degradation and improvements of ecological communities is a fundamental goal of ecology. Achieving this goal is a challenge due to diverse trends in both environmental conditions and ecological communities across regions, and it is constrained by the lack of long-term parallel monitoring of environmental and community data needed to study causal relationships. Here, we identify key environmental drivers using a high-resolution environmental - ecological dataset, an ensemble of the Soil and Water Assessment Tool (SWAT+) model, and ecological models to investigate effects of climate, land-use, and runoff on the decadal trend (2012-2021) of stream macroinvertebrate communities in a restored urban catchment and an impacted catchment with mixed land-uses in Germany. The decadal trends showed decreased precipitation, increased temperature, and reduced anthropogenic land-uses, which led to opposing runoff trends - with decreased runoff in the restored catchment and increased runoff in the impacted catchment. The two catchments also varied in decadal trends of taxonomic and trait composition and metrics. The most significant improvements over time were recorded in communities of the restored catchment sites, which have become wastewater free since 2007 to 2009. Within the restored catchment sites, community metric trends were primarily explained by land-use and evaporation trends, while community composition trends were mostly associated with precipitation and runoff trends. Meanwhile, the communities in the impacted catchment did not undergo significant changes between 2012 and 2021, likely influenced by the effects of prolonged droughts following floods after 2018. The results of our study confirm the significance of restoration and land-use management in fostering long-term improvements in stream communities, while climate change remains a prodigious threat. The coupling of long-term biodiversity monitoring with concurrent sampling of relevant environmental drivers is critical for preventative and restorative management in ecology.

When Do Traits Tell More Than Species about a Metacommunity? A Synthesis across Ecosystems and Scales

AbstractLinking species traits with the variation in species assemblages across habitats has often proved useful for developing a more mechanistic understanding of species distributions in metacommunities. However, summarizing the rich tapestry of a species in all of its nuance with a few key ecological traits can also lead to an abstraction that provides less predictability than when using taxonomy alone. As a further complication, taxonomic and functional diversities can be inequitably compared, either by integrating taxonomic-level information into the calculation of how functional aspects of communities vary or by detecting spurious trait-environment relationships. To remedy this, we here synthesize analyses of 80 datasets on different taxa, ecosystems, and spatial scales that include information on abundance or presence/absence of species across sites with variable environmental conditions and the species' traits. By developing analyses that treat functional and taxonomic diversity equitably, we ask when functional diversity helps to explain metacommunity structure. We found that patterns of functional diversity explained metacommunity structure and response to environmental variation in only 25% of the datasets using a multitrait approach but up to 59% using a single-trait approach. Nevertheless, an average of only 19% (interquartile range = 0%-29%) of the traits showed a significant signal across environmental gradients. Species-level traits, as typically collected and analyzed through functional diversity patterns, often do not bring predictive advantages over what the taxonomic information already holds. While our assessment of a limited advantage of using traits to explain variation in species assemblages was largely true across ecosystems, traits played a more useful role in explaining variation when many traits were used and when trait constructs were more related to species' status, life history, and mobility. We propose future research directions to make trait-based approaches and data more helpful for inference in metacommunity ecology.

Algal or bacterial community: Who can be an effective indicator of the impact of reclaimed water recharge in an urban river

Reclaimed water has been widely utilized for water resource replenishment, yet little is known regarding its impacts on various microorganisms in the receiving water. To address this knowledge gap, we systematically investigated the responses of bacteria and algae to the recharge of reclaimed water by using the high-throughput sequencing technology in the urban Chaobai River. After the inputs of reclaimed water, lower contents of NO2--N, NH4+-N, and TP were observed in the downstream section compared to that of upstream without reclaimed water, indicating that reclaimed water could improve the water quality of the receiving water. Correspondingly, both bacterial and algal communities showed the decreased network complexity in the downstream section, but many common freshwater bacteria and typical bloom-forming algae were dominant in the downstream, potentially suggesting that algae were more sensitive to the local environmental conditions. More importantly, although nitrogen and phosphorus served as the paramount factors in shaping both bacterial and algal communities, environmental selection contributed more to algal rather than bacterial community, and simultaneously algal variations could further affect bacterial dynamics in the urban river. Overall, these findings revealed distinct characteristics of bacteria and algae in responding to the reclaimed water recharge, highlighting the superiority of algae in indicating environmental changes, especially in monitoring and regulating the replenishment of reclaimed water in urban rivers.

European river typologies fail to capture diatom, fish, and macrophyte community composition

Typology systems are frequently used in applied and fundamental ecology and are relevant for environmental monitoring and conservation. They aggregate ecosystems into discrete types based on biotic and abiotic variables, assuming that ecosystems of the same type are more alike than ecosystems of different types with regard to a specific property of interest. We evaluated whether this assumption is met by the Broad River Types (BRT), a recently proposed European river typology system, that classifies river segments based on abiotic variables, when it is used to group biological communities. We compiled data on the community composition of diatoms, fishes, and aquatic macrophytes throughout Europe and evaluated whether the composition is more similar in site groups with the same river type than in site groups of different river types using analysis of similarities, classification strength, typical species analysis, and the area under zeta diversity decline curves. We compared the performance of the BRT with those of four region-based typology systems, namely, Illies Freshwater Ecoregions, the Biogeographic Regions, the Freshwater Ecoregions of the World, and the Environmental Zones, as well as spatial autocorrelation (SA) classifications. All typology systems received low scores from most evaluation methods, relative to predefined thresholds and the SA classifications. The BRT often scored lowest of all typology systems. Within each typology system, community composition overlapped considerably between site groups defined by the types of the systems. The overlap tended to be the lowest for fishes and between Illies Freshwater Ecoregions. In conclusion, we found that existing broad-scale river typology systems fail to delineate site groups with distinct and compositionally homogeneous communities of diatoms, fishes, and macrophytes. A way to improve the fit between typology systems and biological communities might be to combine segment-based and region-based typology systems to simultaneously account for local environmental variation and historical distribution patterns, thus potentially improving the utility of broad-scale typology systems for freshwater biota.

Environmental DNA metabarcoding reveals the impacts of anthropogenic pollution on multitrophic aquatic communities across an urban river of western China

Anthropogenic activities are intensively affecting the structure and function of biological communities in river ecosystems. The effects of anthropogenic pollution on single-trophic community have been widely explored, but their effects on the structures and co-occurrence patterns of multitrophic communities remain largely unknown. In this study, we collected 13 water samples from the Neijiang River in Chengdu City of China, and identified totally 2352 bacterial, 207 algal, 204 macroinvertebrate, and 33 fish species based on the eDNA metabarcoding to systematically investigate the responses of multitrophic communities to environmental stressors. We observed significant variations in bacterial, algal, and macroinvertebrate community structures (except fish) with the pollution levels in the river. Network analyses indicated a more intensive interspecific co-occurrence pattern at high pollution level. Although taxonomic diversity of the multitrophic communities varied insignificantly, phylogenetic diversities of fish and algae showed significantly positive and negative associations with the pollution levels, respectively. We demonstrated the primary role of environmental filtering in driving the structures of bacteria, algae, and macroinvertebrates, while the fish was more controlled by dispersal limitation. Nitrogen was identified as the most important factor impacting the multitrophic community, where bacterial composition was mostly associated with NO₃^--N, algal spatial differentiation with TN, and macroinvertebrate and fish with NH₄⁺-N. Further partial least-squares path model confirmed more important effect of environmental variables on the relative abundance of bacteria and algae, while macroinvertebrate and fish communities were directly driven by the algae-mediated pathway in the food web. Our study highlighted the necessity of integrated consideration of multitrophic biodiversity for riverine pollution management, and emphasized the importance of controlling nitrogen inputs targeting a healthy ecosystem.

Linking Micropollutants to Trait Syndromes across Freshwater Diatom, Macroinvertebrate, and Fish Assemblages

The ecological quality of freshwater ecosystems is endangered by various micropollutants released into the environment by human activities. The cumulative effects of these micropollutants can affect the fitness of organisms and populations and the functional diversity of stream ecosystems. In this study, we investigated the relationships between the joint toxicity of micropollutants and trait syndromes. A trait syndrome corresponds to a combination of traits that could occur together in communities due to the trait selection driven by exposure to these micropollutants. Our objectives were to (i) identify trait syndromes specific to diatom, macroinvertebrate, and fish assemblages and their responses to exposure, taking into account four micropollutant types (mineral micropollutants, pesticides, PAHs, and other organic micropollutants) and nine modes of action (only for pesticides), (ii) explore how these syndromes vary within and among the three biological compartments, (iii) investigate the trait categories driving the responses of syndromes to micropollutant exposure, and (iv) identify specific taxa, so-called paragons, which are highly representative of these syndromes. To achieve these objectives, we analyzed a dataset including the biological and physico-chemical results of 2007 sampling events from a large-scale monitoring survey routinely performed in French wadeable streams. We have identified five (diatoms), eight (macroinvertebrates), and eight (fishes) trait syndromes, either positively or negatively related to an increasing toxicity gradient of different clusters of micropollutant types or modes of action. Our analyses identified several key trait categories and sets of paragons, exhibiting good potential for highlighting exposure by specific micropollutant types and modes of action. Overall, trait syndromes might represent a novel and integrative bioassessment tool, driven by the diversity of trait-based responses to increasing gradients of micropollutant toxic cocktails.