Environmental gradient relative to oxbow lake-meandering river connectivity in Zoige Basin of the Tibetan Plateau

Abstract

Oxbow lakes generated from meandering river cutoffs are an indispensable component of riparian ecosystem. This study aimed to examine whether predictable patterns in water properties arise relative to oxbow lake geometry and lake-channel connectivity. We selected 24 oxbow lakes along a meandering river belt in Zoige Basin, the Tibetan Plateau, to conduct field work at the beginning (May) and the end (September) of the flood season, 2019. Six parameters (pH, dissolved oxygen (DO), conductivity, total dissolved organic carbon, nitrogen and phosphorus) were measured to represent water property. Lake surface area, length, width, length/width ratio, distance to active channel, and curvature were measured to represent lake geometry. Shrinkage ratio, defined as current lake length/initial lake length, was measured to represent lake-channel connectivity between oxbow lakes and active channel. We found an environmental gradient relative to connectivity. As oxbow lakes aged and gradually disconnected from the channel, increased DO and nutrients drove the lakes towards eutrophication. This trend however was disturbed by (i) flood pulses that diluted nutrient concentrations in oxbow lakes and narrowed the differences among different types of lakes and the channel through altering lake-channel connectivity, and (ii) lateral meander migration passed through older lakes such that regulated water properties by reconnecting older lakes to newly formed lakes. Maintenance or restoration of connectivity thus should be of great importance to prevent oxbow lakes from eutrophication in meandering river belts in Zoige Basin.

Introduction

Floodplains are an important alluvial complex of a widely inter-connected series of biotopes and ecological gradients that, with their unique landscapes and biotic communities, create riverine wetland ecosystems. Floodplain lakes are a common component of riverscape. Together with terrestrial landscape they generate a mosaic floodplain of specific environmental gradient (Ward, 1998), resulting in highly diverse aquatic habitats, communities, and ecological processes within river corridors (Arscott et al., 2005; Marshall et al., 2006).

Within meander belts of meandering rivers, oxbow lakes are one of the most typical geomorphic units that generated from neck or chute cutoff through lateral bank erosion and overbank flooding (Leopold and Wolman, 1957; Crosato, 2008; Durkin et al., 2015). These lakes are abandoned channels of meander bends and are physically isolated from original channels, usually with U- and Ω-shaped water bodies (Knight et al., 2002; Delhomme et al., 2013). As oxbow lakes age, due to flooding input and sediment deposition, they become shallower, smaller, and gradually disconnected from the active meandering channel (Winemiller et al., 2000; Miranda, 2005). Based on hydrological connectivity of oxbow lakes and meandering rivers, researchers usually divide oxbow lakes into three types: (i) lotic lakes that permanently connect with the active channel at both ends, (ii) semi-lotic lakes that connect at only downstream ends, and (iii) lentic lakes that temporarily connect during high flows (e.g., Obolewski, 2013; Kobus et al., 2016).

Because of this spatial heterogeneity, within a meander belt, existing lakes that at a wide range of aging phases can show large between-lake variation in terms of geometry, trophic state, and aquatic vegetation (Amoros and Bornette, 2002; Miranda, 2005), thereby providing diverse habitats that facilitate biodiversity (Acreman et al., 2007; Obolewski, 2011), as well as offering different socio-economic benefits in terms of fishery, recreation, and flood mitigation. Researchers have done many field surveys concerning oxbow lake ecosystems, and we sum them up in two main aspects.

First, the patterns of association between hydrological connectivity and lake chemical variables, and assemblages of fish, macroinvertebrate, and zooplankton. Previous studies have examined the correlations among aquatic life (e.g., abundance, structure, and diversity), aquatic physical-chemical properties (e.g., pH, dissolved oxygen (DO), turbidity, and dissolved nutrients), and connectivity reflected by lake geometry (e.g., area, depth, and distance from active channel), and have revealed several predictable patterns (e.g., Glinska-Lewczuk, 2005; Gallardo et al., 2008; Kufel and Leśniczuk, 2014; Kobus et al., 2016). For example, Winemiller et al. (2000) found that concentration of dissolved nutrient was the best predictor of fish species diversity, and the negative correlation between dissolved nitrogen and species diversity implied that nutrient-rich oxbow lakes were severe habitats in which many species would not survive.

Second, the evaluation of oxbow lakes restoration projects. The effectiveness of oxbow lakes reconnection has been widely assessed based on the comparison between before and after of these projects, in terms of mainly biodiversity and water quality (e.g., Gallardo et al., 2012; Seidel et al., 2017). Most of the assessments have suggested that the reconnection of oxbow lakes to active meandering channels was of great potential to improve aquatic ecosystem.

Previous studies have certainly extended the knowledge of biogeochemical processes within oxbow lakes, however there are still knowledge gaps. First, the focus mainly has been on organisms. Although nutrients are essential for survival, growth, and recruitment of aquatic species, their associations with lake-channel connectivity and related lake geometry have received relatively less attention. Second, lacking focus of oxbow lakes in highland floodplains (>3000 m), whilst alpine climate may influence the relationship between connectivity and environmental variables. For example, in contrast to the intermediate disturbance hypothesis that suggests the highest biodiversity occurs in lakes with intermediate connectivity, i.e., a unimodal response (Connell, 1978; Ward et al., 1999; Amoros and Bornette, 2001), Zhou et al. (2019) observed that reduced connectivity could benefit macroinvertebrate assemblages in oxbow lakes in the Tibetan Plateau. Therefore, previous lake restoration measures may not be applicable for alpine oxbow lakes. Moreover, within previous studied meander belts, researchers usually chose less than ten oxbow lakes to monitor, thus might not able to represent the broad range of lake-channel connectivity.

To address these questions, we chose an oxbow lake group along a high-sinuosity tributary of White River, a typical meandering river located at Zoige Basin, the northeastern Tibetan Plateau, to conduct field measurement and sampling at the beginning (May) and the end (September) of the flood season in 2019. We selected 24 lakes and recorded their geometry and connectivity, and measured physical-chemical properties of lake water and river water. We aimed to test the hypothesis that predictable patterns in water properties arise relative to lake geometry and connectivity. This work will help to establish principles towards restoring alpine oxbow lakes ecosystem.