4.1 Spatial and temporal patterns of fish assemblage
Several studies have observed significant spatial and temporal variations in fish assemblages in natural streams (Silvano et al. 2000; Fernandes et al. 2013; He et al. 2017). Specially, spatial variations in fish assemblage were usually caused by habitat features changes along the longitudinal gradient, while seasonal variability in fish assemblages were often attributed to flooding-related changes in habitat characteristics and induced seasonal migrations of fish species (Silvano et al. 2000; Fernandes et al. 2013; He et al. 2017).
The present study revealed that fish assemblages in streams of Ren River basin varied significantly with stream order. However, no significant changes in fish assemblages were observed between seasons. These results were consistence with other studies (Habit et al. 2007; Yan et al. 2010; Li et al. 2012; Zhu et al. 2017; Fernandes et al. 2013). For example, Ostrand and Wilde (2002) found spatial components of variation in fish assemblages in the upper Brazos River were greater than seasonal components. Fernandes et al (2013) revealed fish assemblages in Meridional Amazonian streams changed among watersheds (spatial variation), but not seasonally (temporal variation). Fish assemblages in Northwestern Great Plains streams varied more spatially than temporally (Mullen et al. 2011). Similar findings were also observed in the lower Yangtze River, China (Yan et al. 2010; Li et al. 2012; Zhu et al. 2017). Yan et al (2010) observed that fish assemblages in Puxi Stream were significantly different in spatial variation but not in temporal variation. Li et al (2012) found that fish assemblages in a mountain stream of the north Tiaoxi River differed along the stream continuum, but there was little apparent change associated with the seasons. These studies together suggested that seasonal variations in habitat features may not always lead to temporal changes in fish assemblage structure. One of the possible reasons is the structure of fish assemblages in these stream systems were determined more by average or persistent spatial heterogeneity in environmental conditions and environmental variability than by seasonal variation in environmental conditions (Ostrand and Wilde 2002; Mullen et al. 2011; Li et al. 2012).
In this study, some habitat variables (e.g., water temperature, pH, dissolved oxygen, conductivity, velocity and substrate composition) changed markedly from wet season to dry season. However, temporal changes in these habitat variables were probably exceeded by deterministic landscape variations along the longitudinal gradient. As a result, fish assemblages were determined mainly by large-scale spatial variables (e.g., altitude and C-link), while local habitat variables played less important roles or just acted in single season. In addition, fish assemblages in the Ren River basin were dominated mainly by resident fish species, such as R .oxycephalus , G . herzensteini , M. omeiensis ,S. macrolepis and Z. platypus . These species were well-adapted to the instream running water and did not depend on long-distance migration to complete their life cycles. Therefore, the lack of seasonal change in fish assemblages in streams of the Ren River was probably resulted from the natural variability of the river system and the sedentary life habit of fish species.