Discussion
The understanding on ecological status of lakes helps determine the
ecosystem services provided by them (Grizzetti, Lanzanova, Liquete,
Reynaud, & Cardoso, 2016). Therefore, it is important to assess them.
Ecological status and water quality is affected by a complex interaction
of environmental variables. Understanding the relative effects of these
environmental variables is a necessary step in determining the
activities required for lake management. The macroinvertebrates
community is an ideal indicator to assess impacts as they respond to a
wide variety of physical, chemical and biological factors (Rai, Shah,
Shah, & Milner, 2019). This study aimed to investigate the response of
macroinvertebrate community structure to environmental variables of
Lianhuan Lake.
The community of macroinvertebrate were used to classify the spatial
patterns of the lake for the purpose of biogeographical division. Based
on the species composition, the SOM grouped the sampling sites into four
groups. This classification could imply that spatial variation
correlated with the macroinvertebrate communities on a small-scale.
Kruskal-Wallis test indicated that the environmental variables differed
significantly among the 5 groups. Groups with similar macroinvertebrates
communities were placed close to each other in the SOM. Groups I and V
were placed furthest in the SOM, and this was further depicted in the
final result of principal component analyses (PCA).
Group I which principally
encompasses samples that were taken in autumn in the top-left area of
the Lianhuan Lake, was characterized by lower WT and high values of DO,
COND, TP, and NO3-N. The high DO, COND, TP, and
NO3-N, could be attributed to the fact these sampling
sites were located near to the river mouth which discharges water into
the lakes from the surrounding agricultural farms (Fu-hua & Shu-ying,
2010; Xiao et al., 2014). The indicator species for Group I were
Chironomidae and C. fluminea which are more tolerant to
eutrophication conditions (Zhijun, Ping, Huijuan, & Shida, 2001).C. fluminea has been documented to tolerate low temperature
(Gerard et al., 2009; Min, 2008). Group II included sampling point in
was Habuta lake was characterized by high CODMn, TP,
NH4-N and NO3-N. Interestingly,
annelids
such as B. sowerkyi and Herpobdella sp related to lake
excess nutrients (Cai et al., 2017; Du et al., 2021) were remarkable
high in Group II. This implies that even though Habuta Lake is located
far away from the other lakes, their source of the water is the same as
the other lakes. The status of the Habuta Lake could be at the stage of
degradation due to eutrophication.
A significant variability of environmental variables was recorded in
Group IV which encompasses sampling points from the eastern part of
Lianhua Lake. CODMn, TP, and NH4-N were
markedly high, while DO was notably low in Group IV likely because of
the dominant human activities such as crop farming on the eastern part
of Lake (Xiao et al., 2014). Increased agriculture and urban land uses
can significantly change lake physicochemical characteristics (Huang et
al., 2014; Johnson, Furse, Hering, & Sandin, 2007; Mathur, Agarwal, &
Nag, 2007). Increases in urban and agricultural land uses have been
proven to increase nutrients and change in macroinvertebrate indicator
species (Johnson, Wiederholm, & Rosenberg, 1993; Kubosova, Brabec,
Jarkovsky, & Syrovatka, 2010; S. Yu, Xu, Wu, & Zuo, 2016). G.
pervia , Clinotarypus sp and Tanypus sp indicators species
were grouped in Group IV possible due to the fact that they are tolerant
to high levels of pollution (J. Wang, 2003). These species respond to
organic pollution by increase in abundance. Being grouped in Group IV is
an attestation to this fact. They can live in extremely polluted waters
with very low oxygen levels (Uwadiae, 2016).
This study also revealed that Delong Lake, Yangcaohao Lake and Beiqin
Lake grouped in Group V were characterized by good water quality. This
is despite the fact that these three lakes were located in the upper
reaches of Lianhuan lake and sampled during summer
when large amount of water are discharged by through surface runoff.
This clearly shows that there is spatial variation in terms of water
quality. The fact that many indicator species were grouped in Group V
also implies that the conditions in Group V allowed many organisms to
thrive.
Macroinvertebrates are important part of the lake ecosystem and the
characteristics of community structure were related to lake
environmental variables. This study revealed that that pH, TP,
NO3-N, WT, DO, COND, CODMn, and
NH4-N had a significant effect on the classification of
macroinvertebrate community (Fig. 4). pH has been shown to play a role
in influencing the composition and abundance of macroinvertebrate
communities. Study by Feldman and Connor (1992) indicate that acid water
has reduced macroinvertebrate abundance, biomass, and diversity.
Moreover, the tolerance studies have revealed that the tolerance to pH
varies between macroinvertebrates species (Ormerod et al., 1987). Based
on the RDA and spearman correlation analysis, the biomass of
macroinvertebrates mainly mollusks was significantly negatively
correlated with pH in this lake. This could be attributed by the high pH
values in the range of 8 to 10 which is experienced in the lake all year
round (Jing et al., 2009). It has been documented that an extreme pH
environment can directly produce toxic effects on Mollusca, and under
certain conditions, it can endanger the normal survival of organisms (Wu
et al., 2018). Peiffer, Beierkuhnlein, Sandhage-Hofmann, Kaupenjohann,
and Bär (1997) also noted that decrease of pH will not only directly
affect the birth rate of benthic invertebrates, reduce their
biodiversity, but also cause benthic invertebrate poisoning by
triggering the release of heavy metals. This could be another possible
explanation for the negative influence of pH to macroinvertebrates
because study by Jing et al. (2009) in the Lianhuan Lake revealed that
the acidity and alkalinity of the lake has changed with the differential
enrichment of heavy metals caused by the discharge of industrial sewage.
Water temperature (WT) affects the physiological processes of organisms,
so temperature dynamics may change life cycle patterns and trophic
interactions (F. Li, Cai, Jiang, & Qu, 2012). This may alter the
community composition and biodiversity. According to the RDA results,
most of the macroinvertebrates were significantly positively correlated
with WT which is in agreement with other studies (Buss, Baptista,
Nessimian, & Egler, 2004). Water temperature (WT) is an important
factor for embryonic development, larval growth, emergence, metabolism
and survivorship of macroinvertebrate (Haidekker & Hering, 2008). The
fact that many indicator species were recorded in Group V further shows
that WT is an important factor because these sites were sampled during
summer and spring.
Nutrients are essential for maintaining an ecosystem’s structure and
function. However, excessive nutrients can also reduce water quality
causing problems and can deplete dissolved oxygen, leading to death of
aquatic organisms (Ouyang, Qian, Becker, & Chen, 2018). On average,
macroinvertebrates species richness and abundance in this study exhibits
a subsidy-stress relationship with nutrients mainly TP and
NO2-N. This result
is consistent with the conclusion that a high level of nutrient
concentrations negatively affects benthic invertebrate species richness
and abundance, a conclusion also found in manipulative experiments and
observational studies (Dodson, Arnott, & Cottingham, 2000; L. Wang,
Robertson, & Garrison, 2007). When nutrients are excessive in a lakes
as observed in Group IV, the diminished water quality and the depleted
oxygen caused by decomposition of algal bloom biomass will likely reduce
species richness (L. Wang et al., 2007), leading to a negative
association between nutrients and species richness. The high nutrients
concentration recorded in this lake could be attributed to the use of
phosphatic fertilizers to increase agricultural
production.
Conclusion
In this study, the analysis of macroinvertebrate assemblage identifies
gradient of macroinvertebrate diversity in Lianhuan Lake. It also
captured the spatiotemporal variation in macroinvertebrate community
structure and indicator species in the Lake. The SOM analysis of the
macroinvertebrate communities revealed that eutrophication causes
serious impacts to the macroinvertebrate communities. The differences in
the community structure and environmental variables between Groups I and
V is remarkable and the indicator species are reflecting the
environmental characteristics of each group of communities. The high
alkalinity characteristics and eutrophication of the lake may have a
serious impact on the macroinvertebrate community. This is clearly shown
by the significant negatively correlation between the biomass of
macroinvertebrates and pH, as well as the negative correlation between
species richness and Shannon’s diversity with TP. High-intensity human
interference and unreasonable industrial and surface runoff from
agricultural farms destroy the ecological environment and affect the
community structure of macroinvertebrate. Thus, the improvement of the
macroinvertebrate’s community structure should be carried out by
improving the Lianhuan Lake watershed ecological environment and
controlling watershed environmental pollution (Non-point source
pollution).