4.3 The reasons for the differences in the eutrophication level of
the three reservoirs in Chuzhou City
According to the CA method, the sampling time groups of all three
reservoirs were divided into three statistically significant clusters,
primarily because the three reservoirs belong to the same city of
Chuzhou, have the same geographic characteristics, and natural
background, and are influenced by similar natural environments and human
activities. However, in this case, three reservoirs did not show a
consistent degree of eutrophication. The eutrophication degree of Shahe
Reservoir and Huanglishu Reservoir was significantly higher than that of
Chengxi Reservoir, both in the course of the annual change (Fig. S4) and
the monthly change (Fig. S5). The reason for this is mostly due to the
biased utilization of reservoir functions by the region. Shahe Reservoir
is a large reservoir with comprehensive benefits in Chuzhou City, which
not only serves as an important local aquaculture site but also
undertakes irrigation and water supply for neighboring agriculture.
Among that, aquaculture often requires abundant imported food and feed,
which is highly susceptible to imbalances in N concentrations in the
water body (Zhou et al., 2011). The average TN concentration in the
reservoir reached 0.98 mg/L in November, which already belongs to Class
III water (≤1.00 mg/L) in China’s environmental quality standard for
surface water (GB3838-2002). Not only that, the secretions and
excretions produced by fish and shrimp contain a large number of
nutrients, which are dispersed from the nets or suspended or deposited
at the bottom of the reservoir, greatly increasing the P concentration
in the water body. TP concentrations in Shahe Reservoir reached 0.05
mg/L in multiple months and were notably higher than those in Huanglishu
Reservoir and Chengxi Reservoir. It was pointed out that the discharge
from farmed fish accounted for 10 − 20% of the total phosphorus load in
some reservoirs and had a much larger actual impact on eutrophication
than other sources of phosphorus (Zhou et al., 2011). Besides, the
agricultural development around the Shahe Reservoir has been using
chemical fertilizers for a long time. After being irrigated by the
Reservoir, the chemical fertilizers will be completely dissolved, and
the nutrients in its composition will slowly flow into the Reservoir,
which will ultimately cause the N and P content of the water body to
exceed the standard, and aggravate the eutrophication of the Reservoir.
It is worrying that the chemical fertilizer application rate has been
increasing to date (Kim et al., 2001). Huanglishu Reservoir also serves
as a comprehensive reservoir that not only has the two functions of the
Shahe Reservoir mentioned above, but also has the title of a 4A-level
scenic area. After being renamed Biyun Lake, it became a famous tourist
resort in Quanjiao County, Anhui Province, attracting countless
tourists, which has become an important cause of the serious
eutrophication of the water body. It is well known that human activities
contribute significantly to the occurrence of eutrophication in
reservoir waters (Huang et al., 2014). With the rise in the number of
tourists, the amount of domestic sewage discharged from scenic tourist
areas has increased dramatically, and the water bodies are prone to
enrich a large number of highly concentrated nutrients, which in turn
causes eutrophication in the reservoirs (Le et al., 2010).
CODMn can indicate the extent of organic pollution in
water bodies (Li et al., 2018). As expected, Huanglishu Reservoir
exceeded the remaining two reservoirs in average CODMnconcentrations each year. Although Chengxi Reservoir has multiple
functions, its main function is to undertake the water supply needs of
the residents of Chuzhou City, and other functions only play a role in a
small number of areas. This is the important reason why the
eutrophication level of Chengxi Reservoir is the lowest. According to
the TLI method, the concentrations of all five indicators in Chengxi
Reservoir are more reasonable and stable than the other two reservoirs.
The degree of eutrophication in Huanglishu Reservoir is slightly higher
than that in Shahe Reservoir (Fig. 3), due in large part to the fact
that the Huanglishu Reservoir is much older and more severely aged,
resulting in slower water movement and decreased self − purification
capacity (Holz et al., 1997), which affects the degree of eutrophication
in the reservoir.