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).