4.1 Analysis of drivers of water quality parameters over time
Consistent with previous studies, the influence of natural and
anthropogenic factors plays an irreplaceable role in the process of
changing water quality parameters in reservoirs (Dodds and Cole, 2007;
Wang et al., 2020).
Because of differences in natural geographic location, there is some
variability in WT changes among the three reservoirs. Huanglishu
Reservoir is located in Chuzhou on the border with Hefei City and
Maanshan City, as well as geographically situated to the south, with
higher temperatures and greater exchange of heat in the atmospheric
cycle. As a result, Huanglishu Reservoir has the highest water
temperatures. Shahe Reservoir is in a populated area, where air
circulation is difficult, heat dissipation is weak, for relatively high
water temperatures. Chengxi Reservoir borders Mt. Langya and is
surrounded by large vegetation cover, with less solar radiation absorbed
by the ground, leading to the lowest water temperatures. As shown by
Pearson Correlation Analysis, WT showed significant correlations
(P < 0.01 or P < 0.001) with pH, DO,
CODMn, TP, and Chl-a (Fig. 2). Lower WT reduces the rate
of planktonic algal blooms, thereby slowing their ability to consume DO
(Varol et al., 2012; Naveedullah et al., 2016). This is the primary
explanation for the higher DO concentrations in the Chengxi Reservoir
than in the other two reservoirs. Appropriate WT increases phytoplankton
abundance in the reservoir, enhances photosynthesis, and reduces
CO2, ultimately resulting in higher water pH (Wu et al.,
2014). Moreover, the Chengxi Reservoir has the highest phytoplankton
population in its own right and hence with a higher pH than the other
two reservoirs. It is well known that the biological oxidative
decomposition of CODMn needs to consume a large amount
of DO, however, Excessive WT causes planktonic algae to preemptively
deplete DO, which in turn alters the CODMn concentration
(Yin et al., 2011). Furthermore, ANOVA showed that CODMnwas remarkably different (P < 0.05) between clusters
(Tables 3 to 5), which may be related to the occurrence of water bloom
in the reservoirs. During the outbreak of blooms, phytoplankton will
rapidly multiply and die, generating large amounts of organic matter and
causing elevated CODMn concentrations in reservoirs
(Klemas, 2012; Smayda, 2008).
Under anthropogenic interference, the elevated discharge of industrial
wastewater and the rise in agricultural fertilizers and livestock manure
have to a large extent contributed to the enrichment of reservoir waters
with large quantities of organic matter and inorganic salts, which has
led to an increase in the concentration of BOD (Liu et al., 2022). Of
these, the area surrounded by Huanglishu Reservoir is mostly villages
and towns, with relatively backward industrial and agricultural
technology and slow development. Therefore, the reservoir does not
present the characteristic. As everyone knows, the removal of nitrogen
sources mainly relies on nitrification under aerobic conditions and
denitrification under anaerobic conditions (Ma et al., 2016; Zhou and
Hosomi, 2008). However, when the water environment is polluted by
humans, the water DO is no longer stabilized and both effects are
repulsive, resulting in the accumulation of NH3-N and TN
from factory effluents and municipal wastewater discharges (Yao et al.,
2011). Owing to the higher DO concentrations in Chengxi Reservoir, the
nitrogen source concentrations (TN and NH3-N) for water
quality are lower than the other two reservoirs. As for the variation of
TP concentrations, all three reservoirs characterized by higher
concentrations in spring and summer (March to August) than in fall and
winter (September to December). That is primarily attributed to the fact
that the former period is a busy farming season, agricultural
cultivation requires the use of large quantities of phosphorus
fertilizers, which, after being dissolved by rainfall, sink into the
reservoirs along with pollutants enriched with highly concentrated
sources of phosphorus, ultimately resulting in higher TP concentrations
than in the latter period (Kim et al., 2001). Chl-a can reflect the
growth of phytoplankton in water (Wurtsbaugh et al., 2019). As revealed
by the TLI method, the eutrophication level of the Huanglishu Reservoir
and the Shahe Reservoir was higher than Chengxi Reservoir (Fig. 6 and
Table 2). The former two reservoirs would receive more inputs of
exogenous nutrients, phytoplankton biomass can be dramatically boosted.
Hence, it was relatively easy to increase the Chl-a concentration.
(Saluja and Garg, 2017).
Certainly, natural and anthropogenic factors can simultaneously affect
water quality parameters. The irregularity exhibited by the parameter SD
over time is mainly related to the erratic rainfall in the area and the
irregular water demand of the population. There is less small-scale
vegetation along the reservoir, large amounts of sediment are easily
washed into the reservoir by rainwater, impeding light penetration and
causing changes in SD. The water demand of the local population can lead
to irregular opening and releasing of the reservoir, which greatly
affects the stabilization of the phytoplankton population in the
reservoir. Excess phytoplankton can green the water and reduce the SD of
the water body (Sommaruga and Augustin, 2006). Moreover, the different
light intensities under sunny and cloudy days are another important
factor contributing to the erratic SD variation. Again, because of the
difference in reservoir eutrophication, clean water quality is less
likely to experience algal overgrowth and does not easily prevent light
from penetrating the water layer (Guo et al., 2022), thus SD changes are
more stable in Chengxi Reservoir.