2.1 Study area
The
Wannan Mountains, locating at the south of Anhui Province, China, are
composed mainly of the Jiuhua Mountain, Huangshan Mountain, and Tianmu
Mountain, with the highest elevations of
1,342 m; 1,841 m; and 1,787 m,
respectively. Due to the influence of the subtropical monsoon climate,
this area is characterized by asymmetric seasonal temperature and
precipitation distributions. Annual air temperature ranges from –2.1 °C
(January) to 27.5 °C (July) with the mean of approximately 17.8 °C.
Annual rainfall is 1,100–2,500 mm/year, with a mean of approximately
1,900 mm/year, of which more than 60% of the rainfall occurs from May
to August. The upland headwater streams in this area are highly complex
and abundant, these streams are confluent to the Xin’an, Chang, Qiupu,
and Qingyi rivers, among which the Xin’an River belongs to the Qiantang
watershed, and the other three rivers belong to the Yangtze River.
Fifty-three low-head dams in the
first-order headwater streams of
the Wannan Mountains were surveyed, of which 21, 9, 4, and 19 low-head
dams were located in the Xin’an,
Chang, Qiupu, and Qingyi rivers, respectively (Figure 1). Overall,
low-head dams (< 5 m in dam height) were built in the
tributaries for agricultural irrigation and water supply in this region
(Yan et al., 2013; Li et al., 2021). For more details about the dam
size, see the results of Li et al., 2021.
Given the constraints of time and
lack of historical data, biotic homogenization can be studied
effectively as a spatial process (reviewed in Dar & Reshi, 2014). The
spatial patterns may also provide tentative evidence for changes in
current fish assemblages (Liu et al., 2019). As fish dispersal is
restricted to via-watercourse routes in the river ecosystems, low-head
dams can affect stream fish assemblages by changing habitat conditions
and by blocking upstream movement (Porto et al., 1999; Yan et al.,
2013). Compared with the free-flowing segments below dams, choosing the
references above dams usually ignore the dams blocking fish movement,
which may underestimate the ecological effects of low-head dams on
current fish assemblages (Li et al., 2021). Therefore, two types of
sampling sites were set for each low-head dam: the impoundments created
by low-head dams (i.e., the treatment sites) and the free-flowing
segments below low-head dams (i.e., the reference sites). In addition,
the effects of low-head dams on geomorphological footprints are often
less than 2 km (Fencl et al., 2015), and the free-flowing segments were
ensured at least 2 km from the dams.