Response of benthic macroinvertebrates in highland rivers to the lateral hydrological connectivity: Taking the Quanji River as an example

Lateral hydrological connectivity alters the hydrodynamic and trophic conditions of a river system, which further affects the diversity, structure, and function of biotic assemblages. Previous studies focusing on lateral hydrological connectivity are mostly conducted on lowland river systems, whereas the ecological effects on highland rivers have yet to be explored. The objectives of this study are as follows: (1) to investigate the benthic macroinvertebrate assemblages in a typical highland river to facilitate the identification of different biotopes based on the macroinvertebrate traits; (2) to characterize biotopes according to the environmental conditions (especially in response to lateral hydrological connectivity), macroinvertebrate diversity, and morphological and functional structures; and (3) to analyze the pattern of how the benthic macroinvertebrate assemblages respond to variation in lateral hydrological connectivity.

We adopted a combined approach of field sampling, in situ measurement, and laboratory observation. Using benthic macroinvertebrate assemblages as indicators, we analyzed the ecological differences among biotopes with different lateral hydrological connectivities in the Quanji River, a typical highland river in northeastern Qinghai Lake, Qinghai Province, China. Ecological data were analyzed using the following methods: (1) ordination (canonical correspondence analysis, CCA) to analyze the general distribution patterns of macroinvertebrate taxa along critical environmental variable gradients; (2) hierarchical clustering and nonmetric multidimensional scaling (NMDS) based on the pairwise Bray-Curtis distance of macroinvertebrate assemblages to identify the representative biotopes; and (3) ANOVA and Kruskal-Wallis analysis to detect the significant differences in environmental variables and macroinvertebrate indices and to quantify how ecological characteristics respond to the variations in the lateral hydrological connectivity.

Our results showed the following: (1) a total of 122 195 macroinvertebrate specimens were collected from the Quanji River, representing 33 families and 61 genera; (2) macroinvertebrate taxa exhibited different preferences to the environmental conditions and formed a featured distribution along the environmental gradients; (3) results of hierarchical clustering identified four types of biotopes (e.g., G1—G4) for the Quanji River based on the macroinvertebrate dissimilarity as measured by the Bray-Curtis distance, and samples of the four biotopes were also discriminated by the results of NMDS in the 2-D gradients; (4) significant differences in environmental conditions, including physicochemical and trophic conditions, were detected among biotopes, and in particular, the lateral hydrological connectivity from G1/G2 to G3 to G4 showed a clear decreasing pattern (e.g., "open" to "semi-open" to "closed"); (5) macroinvertebrate assemblage characteristics, including biodiversity and morphological and functional structures, were also found to be significantly different among biotopes, which indicated that the lateral hydrological connectivity was strongly related to diversity, structure, and function of macroinvertebrate assemblages; and (6) in the Quanji River basin, biodiversity responded to variations in lateral hydrological connectivity in a "single-valley" pattern, which was markedly distinguished from the unimodal pattern observed in lowland river systems. We argue that the hypometabolic and oligotrophic conditions in the highland river system lead to the increased sensitivity of macroinvertebrate assemblages to disturbances. Therefore, the intermediate disturbance preferred by macroinvertebrate assemblages shifts from the median connectivity environment to the low connectivity environment in highland rivers, which accounts for the "single-valley" response pattern.

Our study on bethic macroinvertebrate assemblages in different biotopes of the Quanji River has contributed considerably to our understanding of the highland invertebrates' response to variations in lateral hydrological connectivity. We find that the individual biotopes in the Quanji River respond differently to the lateral hydrological connectivity. One of the most intriguing results of our study is that biological indices, in particular the diversity index, demonstrate a single-valley response pattern to the lateral hydrological connectivity, which is in contrast with the unimodal pattern commonly observed in lowland rivers. This study not only reveals the critical roles of lateral hydrological connectivity variation in structuring highland river ecosystems from biological perspectives but also suggests that the management strategies of highland rivers should be different from those of lowland rivers.