3.5 Relationships between environmental factors and fish
community composition
The correlation analysis between
environmental factors (see Table S1 in the Supplementary Material) and
the composition of fish communities showed that 44 (Fig. 6A), 41 (Fig.
6B), and 40 (Fig. 6C) species had significant relationships with
environmental factors according to the eDNA, individual number, and
biomass data types, respectively.
In addition, for eDNA-based fish community composition, at the order,
family, genus, and species levels, there were 5, 18, 41, and 44 units,
respectively, that significantly (P < 0.05) correlated
with the environmental factors (see
Fig. S1 in the Supplementary
Material). This result indicated that the species-level identification
of the fish community compositions had the strongest response to the
environmental changes, followed by genus-level identification.
Interestingly, regardless of input data types,
there were two environmental
gradients that significantly influenced the longitudinal distribution of
fish communities. The first environmental gradient was mostly composed
of physical water parameters and habitat factors, including elevation
(m), dissolved oxygen (mg/L), flow velocity (m/s), riffle area (%), and
vegetation cover (%). The high values of these factors indicated
pristine habitats in the headwaters and upper reaches, where there was
less human disturbance. The second environmental gradient was mostly
composed of the chemical water parameters (e.g., high
CODMn [mg/L] and BOD5 [mg/L]),
conductivity (μs/cm), bacteria levels (total number of bacteria and
coliform), and heavy metals (e.g., Hg, As, and Cu [mg/L]). The high
values of these factors indicated water pollution and extraneous
interference that commonly occurred in urban and industrial areas of the
lower reaches. These results indicated that eDNA-based analysis of fish
community composition could reflect the longitudinal changes in
environmental factors along the river, which was similar to the pattern
reflected by individual number and biomass.