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.