Responses of macroinvertebrate assemblages to environmental variations in the river-oxbow lake system of the Zoige wetland (Bai River, Qinghai-Tibet Plateau)
Graphical abstract
Introduction
Understanding the biodiversity value of river-oxbow lake systems in high plateau peatlands is critical for the regional ecological protection and restoration, which, however, has been little yet recognized. As one of the largest high plateau peat swamps around the world, the Zoige wetland has recently gain much attention due to its rapid deterioration (Bai et al., 2008; Xiang et al., 2009; Li et al., 2010; Huo et al., 2013). It is located in the bi-province area of the Sichuan and the Gansu, China, on the northeastern margin of the Qinghai-Tibet Plateau, and constitutes an important part of the Yellow River Source Region. This wetland plays an ecologically important role in balancing regional carbon budget by depositing approximately over 1.9 million Gt of carbon through plant absorption (Cui et al., 2015). Additionally, it provides refugia for many endemic and endangered species, including Grus nigricollis, the only plateau crane (IUCN: VU, CITES Appendix I; Scott, 1993). However, over 80% of the pristine Zoige wetland has degraded since 1970s as a combined result of neotectonics uplifting, global warming, artificial drainage, peat overexploitation, livestock overgrazing, and rodent disaster, which severely threatened the ecological health of the Yellow River Source Region (Jiao et al., 2007; Xiang et al., 2009).
Oxbow lakes play central roles in increasing complexity of floodplain and structuring its successional patterns (Tockner et al., 1999), and are very important with respect to improving basin-scale biodiversity by providing aquatic biota with unique riparian refugia (Bornette et al., 1998). “Flood pulse” process in its adjoining river provides major water supply for the oxbow lake (Obolewski, 2011a), whereas it cannot prevent the shallow lake (aquatic system) from inevitable conversion to swamp, and then to swampy meadow, and finally to meadow (terrestrial system), as a result of sediment accumulation brought by flood events, and intensified evapotranspiration by vegetation (Xiang et al., 2009). This process, on the other hand, is critical for taxonomic turnover from aquatic to terrestrial systems within the floodplain (Nicolet et al., 2004).
Although previous studies have preliminarily described ecological characteristics of oxbow lakes by using macroinvertebrates as indicators (e.g. Brock and Van der Velde, 1996; Merritt et al., 2002; Pan et al., 2008), few have made clear how environmental conditions of river-oxbow lake systems influence macroinvertebrates. Notable exceptions have studied relationships between lateral hydrological connectivity and macroinvertebrate assemblages (Gallardo et al., 2008; Obolewski, 2011b; Pan et al., 2012). Gallardo et al. (2008) pointed out, through investigations in the middle Ebro River floodplain (Spain), that the highest macroinvertebrate diversity were found in semi-open oxbows as a result of their high habitat heterogeneity. Similar observations were reported by Obolewski (2011b) in the middle Słupia River floodplain (Poland), owing to changes in nitrogen and sulfur concentrations. Such unimodal patterns of macroinvertebrate diversity along the gradient of hydrological connectivity, were in consistence with the intermediate disturbance hypothesis (Connell, 1978). However, two important preconditions were not addressed clearly in those studies: (i) their adjoining rivers were at good baseline status in terms of macroinvertebrate diversity, and (ii) their oxbow lakes were generally oligotrophic or at least not densely covered by macrophytes.
In contrast, riverine ecological baseline status in the Zoige wetland was poor with respect to macroinvertebrates in comparison to those in other regions (Zhou et al., 2017; Yang et al., 2018). Besides, many oxbow lakes in the Zoige were comparatively macrophyte-abundant and with peat-substrate (Zhao et al., 2017). Due to these facts, how macroinvertebrate assemblages were influenced by hydrological connectivity and its associated environmental variables in the river-oxbow lake systems in this unique plateau peat wetland might differ from the previous studies, and requires careful investigation for statutory habitat protection.
To this end, we carried out integrated field surveys in the Zoige wetland from 2015 to 2016. Given the poor riverine ecology and dense-macrophyte coverage in oxbow lakes, we hypothesized that the hydrological connectivity would be an essential determinant affecting macroinvertebrate diversity in this region, although the diversity might not peak in oxbows with moderate connectivity to rivers, and that the macrophyte biomass, associated with connectivity, would be a direct determinant affecting macroinvertebrate assemblages. Based on the previous work in oxbow lake (e.g. Tockner et al., 1999), we also assumed that oxbow lakes would have positive ecological effects on the Zoige. Our objectives were: (i) to classify main types of aquatic habitats, and to characterize each type using environmental conditions and macroinvertebrate assemblages; (ii) to analyze macroinvertebrate-environment relationships and to identify the patterns how hydrological connectivity, macrophyte and other environmental variables affecting macroinvertebrate assemblages; (iii) to show ecological significance of oxbow lakes to peat-substrate wetland on the Qinghai-Tibet Plateau.
Section snippets
Study area
The Zoige wetland is located on the northeastern margin of the Qinghai-Tibet Plateau (N32°40′ – 34°10′, E101°45′ – 103°20′), spanning on the Ruoergai, Hongyuan and Aba Counties of Sichuan Province and the Maqu County of Gansu Province. This region, with an average elevation of over 3500 m a.s.l., covers an area of 300,000 km2, in which the peat-substrate swamp covers currently 4200 km2. Two major tributaries of the Yellow River Source Region, the Bai and Hei Rivers, flow through the Zoige
Results
A total of 24,468 macroinvertebrate specimens were collected and identified, belonging to 14 orders, 37 families and 73 genera. A summary of taxonomical composition of macroinvertebrates is available in the supplementary material (Table S2). Large variations were found among the different sampling sites with respects to macroinvertebrate richness, density, biomass, and the improved Shannon-Wiener Index. Taxa richness ranged from 3 (S01, S02, S03) to 22 (S22) with an average of 12 ± 1
Ecological characteristics of oxbow lakes in the Zoige
Results of ANOVA, Kruskal-Wallis, perMANOVA and IndVal all showed a convergence pattern in development of river origins to oxbow lakes. Even though assemblage difference was observed between the sand-bed river habitat and cobble-bed river habitat of the Bai River, indicating a high β diversity, the headwater remnants — the lentic oxbow lake habitats were environmentally and biologically similar regardless of their larger scale landscape differences. Similar findings were reported on lentic
Conclusions
In contrast to the commonly believed unimodal influential pattern reported in previous studies that medium hydrological connectivity supports highest biodiversity in oxbow lakes, reduced hydrological connectivity of the river-oxbow lake system was observed to benefit macroinvertebrate assemblages in the Zoige wetland. This inconsistency mainly resulted from that macrophyte played a direct role in influencing macroinvertebrates in this high plateau oligotrophic wetland, while hydrological
Acknowledgement
The study was financially supported by the National Science Fund (91547204, 51779120, 41790434), the National Key Research and Development Program of China (2016YFC0402407, 2016YFC0402406), Tsinghua University Project (2015THZ02-1), State Key Laboratory of Hydroscience and Engineering Project (2016-KY-04), and the Yellow River Institute of Hydraulic Research (HKY-JBYW-2016-03). We especially thank Lijian Qi, Chendi Zhang, Luo Sun, Xing Liu, and Xiangchao Wang for their assistances in the field
Author contributions
M. Z. Xu and Z. Y. Wang conceived the idea; X. D. Zhou carried out the investigations, and analyzed the data, and interpretated the results; X. D. Zhou and M. Z. Xu led the writing; all authors contributed to writing of the manuscript.
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