Responses of macroinvertebrate functional trait structure to river damming: From within-river to basin-scale patterns
Introduction
Freshwater ecosystems have been increasingly exposed to multiple anthropogenic disturbances recent decades (Grill et al., 2019; Heino et al., 2020). Dam constructions, for example, result in negative ecological and environmental consequences to river ecosystems and biodiversity worldwide (Zhang et al., 2018; Arantes et al., 2019; Maavara et al., 2020; Ahmed et al., 2022). The dam-associated modifications have changed geomorphology, hydrology, and water quality (Calapez et al., 2021), resulting in the loss of river connectivity, altered sediment transportation and changed habitat heterogeneity of rivers, and subsequently affecting species composition and their functional responses, and ultimately the biodiversity in river systems (Petrin et al., 2013; Zhang et al., 2019; Wang et al., 2020, 2022).
Incorporating functional trait information into biomonitoring has improved our understanding of the effects of anthropogenic disturbance (e.g., dams) on biodiversity (McGill et al., 2006; Mouillot et al., 2013; Garcia-Giron et al., 2021). Traits indicate the morphological, physiological and behavioral attributes of organisms, and the combinations of multiple traits that allow organisms to match corresponding habitats to overcome disturbance and coexist in local communities (Belmar et al., 2019, Liu et al., 2021, Southwood, 1977). Environmental changes induced by disturbance can act as a filter, removing species with traits that are poorly adapted to novel environmental conditions and allowing colonization by better-adapted species (Mouillot et al., 2013; Kuzmanovic et al., 2017; Jiang et al., 2019). These ‘signature’ traits can make species resilient or vulnerable to disturbances (Edegbene et al., 2021a). Identifying such functional traits and associated environmental conditions via revealing trait-environment relationships would be useful in river biomonitoring and conservation programs, especially for dammed rivers under global dam-building blooms (Zarfl et al., 2015). For example, Arantes et al. (2019) reported that large fishes with a periodic life-history and long-distance migratory behavior were more seriously affected by dam construction and suggested that conservation decisions should involve protecting related environmental conditions (such as suitable flow conditions and large tributaries) to ensure the completion of fish life cycles.
The immediate changes in temperature regimes directly below dams, or localized changes in benthic substrate size, may have a larger influence in functional traits in tailwaters, i.e., some traits are lost and others increase in prevalence (Martínez et al., 2013; Bruno et al., 2019). Such within-river changes have often been examined in previous relevant studies (e.g., Martínez et al., 2013; Belmar et al., 2019; Arantes et al., 2022). However, Ruhi et al. (2018) suggested that the impact of dams on functional structure can extend longitudinally beyond the immediate tailwater, which can be examined from the metacommunity and meta-ecosystem perspectives (Cid et al., 2022). On the one hand, this may be because the effect of dams on sediment transportation and habitat conditions much further downstream may influence species functional traits at broader scales. Such a trait distribution scenario can be formed by a deterministic processes, filtering taxa with certain trait, or via a stochastic pathway, such as through passively drifting organisms (Ruhi et al., 2018). On the other hand, some species with certain traits can actively search for suitable habitats suitable for survival beyond the prevailing environmental conditions (Heino et al., 2015; Tonkin et al., 2018; Dominguez Almela et al., 2022), thereby affecting the trait structure at little-impacted locations far from dams. Therefore, one can deduce that the changes in the trait structure at certain locations within rivers can resemble the whole-river metacommunity dynamics under the impact of dams (Ziv et al., 2012). This idea suggests that the trait structure may be different when they were examined at different scales (Mokany and Roxburgh, 2010; Da et al., 2022). Given the importance of understanding how trait resembled when they affected by dams at different scales (i.e., within river and across basin level), the mapping of the biota along a river continuum would provide foundations for visualizing rivers driving the distribution of taxa (Torgersen et al., 2022). Such comparisons of aquatic biota in locations upstream and downstream of dams vs between entire rivers may provide new insights into variation of functional trait structure and increase efficacy of biomonitoring of dam impacts in river systems (Biswas et al., 2016). However, only rarely studies have such examinations considered entire drainage basins (Van Looy et al., 2014).
Benthic macroinvertebrates are widely used as indicators of environmental conditions (Rosenberg and Resh, 1993; Hu et al., 2022). In recent years, researchers have gradually transitioned from taxonomical to functional trait approaches to reveal the responses of macroinvertebrate assemblages to dams (Martínez et al., 2013; Bruno et al., 2019; Ruhi et al., 2018; Belmar et al., 2019; Wang et al., 2020). These studies demonstrated that macroinvertebrate assemblages at sites downstream of dams were colonized by taxa with novel sets of functional traits, such as multivoltine taxa with small body size, resistance forms and aerial dispersion, and loss of certain traits, such as scraping and semivoltine or univoltine cycles. The changes in these traits thus extend to studies of differences between sites upstream and downstream of dams. However, these studies are typically concerned with dam impacts on macroinvertebrate assemblages by using longitudinal comparisons of upstream and downstream sites, and a focus on trait response at the basin scale under dam construction is uncommon (Van Looy et al., 2014). Meanwhile, macroinvertebrates play many important roles in ecosystem functioning, such as linking energy and materials from basal resources to higher consumers, decomposition of organic matter, filtering of nutrients, exporting energy and materials to terrestrial ecosystem, and more (Allan and Castillo, 2007). The impact of dams on macroinvertebrate assemblages may further affect the food web dynamics and ecosystem processes across multiple scales in river systems (Mor et al., 2018). Therefore, there is an urgent need to improve the acquisition and accessibility of biomonitoring data from different spatial scales as well as to demonstrate the potential variation in macroinvertebrate functional traits that could affect the whole river functional structure, and to determine which traits confer sensitivity of species to dam disturbance (Arantes et al., 2019).
Tropical rivers support high levels of aquatic biodiversity (Dudgeon, 1999) but suffer from dramatic biodiversity decline due to rapidly increasing dam construction (Arantes et al., 2019; Encalada et al., 2019). Studies on the impacts of dams on tropical river biota have been largely conducted on fish assemblages (e.g., Carvalho and Araújo, 2020; Arantes et al., 2022), while few studies have been conducted on macroinvertebrate assemblages (Tupinambás et al., 2016). In this study, we investigated dam impacts at both the within-river (downstream vs. upstream sites in a dammed river) scale and the basin scale (dammed vs. undammed rivers) to reveal the responses of macroinvertebrate functional trait structure in two tributaries of the Lancang-Mekong River. We aimed to detect (1) how the functional trait structure changed in dam-impaired sites compared with non-impaired sites, and between dammed and undammed rivers, (2) which traits are sensitive to dam influence, and (3) how within-river changes in traits and the basin-scale trait structure are spatially organized. The results of our study would enhance our understanding of how macroinvertebrates respond to damming in tropical dammed river systems.
Section snippets
Study area
This study was conducted by comparing two adjacent tributaries of the middle Lancang-Mekong River (Fig. 1), the dammed Nanla River (NLR) and the undammed Buyuan River (BYR), due to the lack of pre-damming data. Both rivers experience similar climatic conditions and land use intensities at the basin scale (Wang et al., 2020, Zhang and Cao, 1995). In addition, relatively similar species richness in the undisturbed upper reaches of the two rivers showed that they share the same species pool and
Environmental conditions within the NLR and between the NLR and the BYR
The PCA plots showed a clear tendency of grouping dam-affected sites and each river along environmental gradients. The first and second PCA axes accounted for 77.9%, 59.9%, 57.5%, 46.4% and 16.3%, 19.4%, 24.4%, 32.2% of the total variance, respectively (Fig. 2). Within the dammed NLR increasing river width was more associated with downstream sites in both dry and wet seasons, whereas the upstream sites were more strongly related to higher pH in the dry season (Fig. 2a) and to higher TN in the
Discussion
We observed that the response of macroinvertebrate functional trait structure to damming differed between the two scales of analysis, supporting the idea that trait-based filtering processes operating at large spatial scales can be quite distinct from those operating at smaller scales (Mokany and Roxburgh, 2010). The fluctuation of the environmental conditions at sites downstream of dams is usually severe (Ahearn et al., 2005), caused either by pronounced hydropeaking or exceptional drought due
Conclusions
This study extends the examinations of dam impacts on macroinvertebrate assemblages from within-river to basin scales based on analyzing functional trait structure. Dam-induced environmental change leading to loss of specific traits and increase others within a dammed river may influence the whole-basin trait structure. Life history, mobility, food availability and armoring-related traits were more sensitive to water nutrients and habitat quality in the dammed river, which is consistent with
Credit author statement
J. W. and J. T. conceived and designed the research; J. W. SM. B. and K. Z. analyzed the data and wrote the manuscript; J. H., XM. J., ZY, L. and J. T., contributed to the discussion and, subsequently, various versions of the manuscript; all authors reviewed the manuscript before submission.
Data accessibility statement
The datasets analyzed during the current study are available from the corresponding author upon reasonable request.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank Liuyong Ding, Wan Su, Minrui Huang, Meiling Chen, and Xinyu Cheng for their help in field sampling, and Prof. Chengzhi Ding and the three anonymous reviewers for their suggestions in the improvement of the manuscript. This study was financially supported by the National Natural Science Foundation of China (No. 41907400), China Postdoctoral Science Foundation (No. 2019M663583), and the Yunnan Applied Basic Research Projects (No. 2019FB131).
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