Water environment quality of Dongyu River and estuary area of Nansi lake (Shandong Province, China) based on the characteristics of diatom community

Abstract The Dongyu River is a national site for monitoring the inflow of Nansi Lake, located in the southwest of Shandong Province, China. Water quality has a significant impact on the safety of the South-to-North Water Diversion Project. To investigate the water quality of Dongyu River and the estuary area of Nansi Lake, we collected water samples and surface sediments from Dongyu River and the estuary area in May 2019 and analysed the characteristics of the diatom communities and water quality. The results showed that 23026 diatom shells were identified at 15 sampling points in the study area, belonging to 161 species of 23 genera. The dominant diatom taxa were Cyclotella meneghiniana, Fragilaria fasciculata, Fragilaria ulna, Amphora libyca, and Cocconeis placentula. Redundancy analysis explained 43.2% of diatom change information. The Monte Carlo permutation test showed that NH4+-N and DO were significant environmental factors that affected diatom assembly, explaining 34.78% and 8.38% of the diatom variability, respectively. The results of the biological evaluation showed that the water quality of Dongyu River and the estuary area of Nansi Lake were in a light-medium pollution state. The RDA analysis of the diatom community and principal component analysis showed that the water quality of the middle and lower reaches of Dongyu River was poor. In general, the middle and lower reaches of Dongyu River are severely polluted by agricultural non-point sources and controlling the use of nitrogen and phosphorus fertilisers and the emission of nitrogen-containing organic matter can preserve the aquatic water quality of Dongyu River and the water safety of the East Route of the South-to-North Water Transfer Project.


Introduction
As an important impounded lake on the East Route of the South-to-North Water Diversion Project and one of six freshwater lakes in China, Nansi Lake involves 8 districts, 4 cities, and 16 counties.Water quality is important for the safety of the eastern line and for the normal production and life of coastal residents.As the largest inlet river in Nansi Lake, the water quality of Dongyu River has received widespread attention since Nansi Lake became an important reservoir for the transfer of water from South to North (Li et al. 2022).
In recent years, with the development of local economies and population explosion, water demand has grown.Paradoxically, humans unscrupulously discharge wastewater into rivers and abuse fertilisers, which worsen the fragile ecosystem of the Dongyu River.Therefore, there is an urgent need to accelerate the water environment management of the Dongyu River.
Diatoms belong to the bacillariophyta and approximately 16,000 types of diatoms are distributed worldwide.As a primary producer in the food chain, it provides bait for primary consumers through photosynthesis.Diatoms are characterised by rapid reproduction (mainly three breeding modes of nutrition, asexuality, and sexual reproduction), small individuals, and a short life cycle (1-7 weeks) (Bradshaw et al. 2002).They are important unicellular plants in the ocean with silica-rich cell walls and are sensitive to environmental changes.Fossil diatoms in sediments record interdecadal paleoenvironmental information and are important tools for studying paleoenvironmental changes (Li et al. 2015).Diatoms are extremely sensitive and affected by various environmental factors.Changes in temperature, light, nutrients, and other factors may alter the composition of diatom communities.The geographical location and water environment of the study area are complex.The temperature and nutrient composition of different watersheds vary, and the water environment is unstable.The estuary area of the lake is affected by various environmental factors.Diatoms of different genera show different tolerance values for different water quality indicators.From a spatial perspective, China's research on diatoms has mainly focused on the Qinghai-Tibet Plateau, the middle and lower reaches of the Yangtze River, and the large and medium-sized lakes in Yunnan (Tao et al. 2015).Chen et al. (2019) systematically analysed the relationship between diatom community characteristics and the water environment factors of 23 river bottoms in Shanghai (Chen et al. 2019).Based on the relationship between diatoms and water environments, Kock et al. (2019) successfully analysed the water quality of Sibaya Lake in South Africa (Kock et al. 2019).Based on the results of modern lake diatoms and total phosphorus transfer function of the Yangtze River, Dong et al. (2006) quantitatively reconstructed the change in the total phosphorus concentration of Taibai Lake over the past 400 years (Dong et al. 2006).In China, my research on diatoms mainly focuses on the middle and lower reaches of the Yangtze River, most lakes in Yunnan, and the Qinghai-Tibet Plateau, etc., while the lower reaches of the Yellow River, which is the mother river of human beings, are rarely studied.
In May 2019, upper 2 cm thick sediment samples and surface water samples were collected from the Dongyu River and estuary area in the lake.We used samples to discriminate diatom species and analysed the community features of diatoms and the water environment quality of Dongyu River and estuary area of Nansi lake.The main water pollution factors of the Dongyu River and the estuary area in the lake are discussed and are expected to provide an important water quality assessment method for ensuring the safety of the East Route of the South-to-North Water Diversion Project and the production and life of residents along the Dongyu River (Fan et al. 2012).

Survey of the research area
Dongyu River (N34°54′59″~35°08′12″, E115°43′47″~116°09′50″) is located in Shandong Province and is the largest artificial river in the Lunan area.Excavation from 1967 to 1969, formerly known as Red Guard River.The Dongyu River starts from Liulou, Dongming County, and Heze City, and is injected into Zhaoyang Lake east of Yutai County.The total length is 172.1 km and the drainage area is 5923 km 2 .The ecological environment of the Dongyu River drainage is fragile and human activities are frequent.The river relies primarily on rainfall recharge (Dou et al. 2018).

Data acquisition and processing
In May 2019, the experiment set seven and eight Sampling points in the Dongyu River and lake estuary, respectively, numbered D-1, D-2, D-3…D-7, N-1, N-2, N-3…N-8.Sampling points D-1, D-2, D-3, and D-7 are scattered in the Dongyu River drainage, and Sampling points N-1, N-2, N-3…N-8 are scattered in the estuary area of the lake (Figure 1).The Kajka gravity mud collector was used to collect the upper 2-cm-thick loose bottom sediments of the Dongyu River while collecting 1 L water samples with a cover pressure-type water collector.At the same time, the water surface width, water temperature, dissolved oxygen, vegetation type, shore width, slope, and water depth were recorded.The sampling point information is shown in Table 1.A spoon was used to extract 1 g of Figure 1.river sampling sites and geographical region.
the sediment sample into a small beaker, and distilled water was added and stirred with a glass rod.The sample glasses were placed on the hot plate at 100 °C for 4 h, and CaCO 3 was removed from the diatom cell wall by adding a small amount of hydrochloric acid.Then continuous small-scale participation of H 2 O 2 is allowed into the beaker until no more bubbles are generated in the reaction.After the reaction is complete, the sample is centrifuged for 10 min at 2500 r•min -1 speed.The supernatant after centrifugation is poured out and then added to distilled water; and this is repeated twice.After the end of the centrifugation, the sample is diluted by adding distilled water and is dropped by a straw to a cover glass placed at 170 °C (to the maximum surface tension of the cover glass).Subsequently, filming is performed using naphrax gum after steaming.Diatom species were identified with reference to the European Diatom Classification System under a 1,000-fold Leica biomicroscope.One-and-a-half of the shells were recorded as one, and the other cases were not counted.

Principal component analysis
The PCA method is an effective way to analyse multi-attribute decision-making problems.It can be used to calculate several comprehensive indicators, usually called principal components, by studying the internal structural relationships of the original variable correlation matrix.The principal components not only retain the main information of the original variables, but also have superior properties to the original variables, which makes it easy to solve the original problem (Fang 2015;Cao et al. 2020).In this study, SPSS22 software was used to conduct principal component analysis of the water quality data of Dongyu River and the estuary area into the lake.Many complex water quality data were decomposed into a few comprehensive indicators to reveal the main environmental gradient in the water quality data and improve the accuracy of the environmental quality evaluation of Dongyu River.

Biological evaluation
In this study, diversity analysis was performed using Primer 5.0 software to calculate the diatom diversity index.The Margalef index (dm) (Death 2008), Shannon-Wiener index (H′) (Wang et al. 2015), and Pielou index (Je) (Pielou 1995)were used to calculate richness, diversity, and evenness.The calculation formulas and water quality evaluation standards are listed in Table 2.

Index analysis
Dimensionality Correspondence Analysis (DCA) is a numerical analysis method for selecting linear or single-peak models based on the gradient length shown and is used to analyse the variation in diatom community composition, revealing the difference in diatom composition data at different sample points.The diatom data were first analysed using DCA to determine the response model of the diatoms to environmental factors, and the maximum lengths of the gradient of species change (SD) of each sorting axis in the results were analysed using DCA.The experimental analysis showed that SD (=2.17) was less than 3, and the response of diatom to environmental factors is a linear model.Therefore, redundancy analysis is more suitable for analysing the relationship between the community structure of diatoms and environmental elements.To avoid occasional errors, we retained the diatom species with abundances greater than 0.2%.Environmental factors were used as explanatory variables, and diatom community composition was used as the response variable.Monte Carlo permutation (p < 0.05, n = 499) was used to test and analyse the significance of the environmental indicators.The explanatory and response variables were subjected to log (x + 1) conversion before performing RDA.The DCA and RDA were performed using CANOCO 5 (Zuur et al. 2007;Lai 2013;Liu et al. 2022, Table 3).Non-binding ordination analysis (DCA) was used to analyse the data of diatoms and extract the maximum variability of the data, which was used to identify the response patterns of diatoms to various physical and chemical factors, and the main environmental factors affecting the composition of genera and species were obtained.First, the diatom data were analysed by DCA, and the response model of the diatoms to environmental factors was judged by the maximum gradient (SD) of the order axis (Chen et al. 2015;Zhang et al. 2015;Min et al. 2020).

Analysis of spatial variation of water quality
As shown in Table 4, in the study area, the maximum value of SAL in the sampling sites of N-5 is 0.3 mg•L −1 , the maximum value of DO in the sampling sites of D-3 is 16.08 mg•L −1 , the minimum value in D-6 is 6.53 mg•L −1 , the maximum value of NH 4+ -N in the sampling sites of D-4 is 1.14 mg•L −1 , the minimum value in the sampling sites of D-2 is 0.22 mg•L −1 , the minimum value of NO 3− appears in the sampling sites of D-2 is 2.61 mg•L −1 , the minimum value in the sampling sites of N-2 is 0.06 mg•L −1 , the maximum value of NO 2− in the sampling sites of D-2 is 0.06 mg•L −1 .The minimum value in the sampling sites of N-7 is 0.000039 mg•L −1 .The extreme values of the mass fractions of SOP, STP, TP, TOC, and As appeared at sampling sites D-6.The maximum values are 0.91 mg•L −1 , 1.01 mg•L −1 , 1.04 mg•L −1 , 15.3 mg•L −1 , 0.03192 mg•L −1 respectively; the minimum values are all in the upper reaches of the Dongyu River, and the variation range is 0.01-0.91mg•L −1 , 0.02-1.01mg•L −1 , 0.11-1.04mg•L −1 , 0.87-15.3mg•L −1 , 0.0004-0.03192mg•L −1 ; Hg is in The maximum value in the sampling sites of N-1 is 0.00046 mg•L −1 , and the minimum value in the sampling sites of N-4 is 0.00009 mg•L −1 , the variation range is between 0.00009 and 0.00046 mg•L −1 .
Table 5 lists the results of water quality evaluation using the Margalef index (dm), the Shannon-Wiener index (H′), and the Pielou index (Je) on the Dongyu River and the estuary of the lake.The variation range of the Margalef index is between 1.62 and 3.30, the average value is 2.47, the maximum value is 3.30 at the sampling sites of D-6, and the minimum value is 1.62 at the sampling sites of N-4, indicating that the diatoms richness at sampling sites of D-6 was higher and the diatom richness of N-4 is lower; the variation range of Shannon-Wiener index is between 1.76 and 2.34, the average value is 2.02, and the maximum value is 2.34 at the sampling sites of D-4 (Table 5).The minimum value appears at the sampling sites of N-4 is 1.76; the variation range of the Pielou index is between 0.60 and 0.87, the average value is 0.73, the maximum value is 0.87 at the sampling sites of N-1, and the minimum value is 0.60 at the sampling sites of N-8 (Nong et al. 2020).

Redundancy analysis of diatom plant communities and environmental factors
Redundancy analysis explained 43.2% of diatom change information.As shown in the DCA analysis in Table 3, the eigenvalues of the first (0.226) and second axes (0.092) accounted for 29.63% and 12.09% of the species change in the diatom combination, respectively, and the eigenvalues explained a total of 76.4% of the diatom changes.The maximum gradient length of species change in each sorting axis is 2.17 less than 3; hence, it is appropriate to use RDA to model the data of species and environments (Chen et al. 2014).The Monte Carlo permutation test results showed that only NH 4+ -N and DO could reach significant levels among all the water environmental factors (Figure 2), and diatom variabilities of 34.78% and 8.38% were explained, respectively.Therefore, DO and NH 4+ -N are highly correlated with diatom composition.According to Figure 2(a), NH 4+ -N is closely related to the first axis, and DO is closely related to the second axis and positively correlated.We can eliminate the relationship between diatoms and water environmental elements using the cosine values of NH 4+ -N and DO with the diatom species (Shibabaw et al. 2021).Some species, such as Amphora veneta, Navicula capitata, Fragilaria capucina, Navicula halophila, and Cyclotella meneghiniana have a significant positive correlation with NH 4+ -N, and other species, such as Navicula constans, Cyclotella planctonica, and Achnanthes thermalis with NH 4+ -N showed a significant negative correlation.Some species, such as Amphora coffeaeformis, Surirella patella, Fragilaria fasciculata, and Nitzschia valdecostata have a significant positive correlation with DO.In addition, some species were negatively correlated with the DO (Zelnik and Sušin 2020).For example, the species richness of Nitzschia liebetruthii, Stephanodiscus parvus, Navicula lundii, and Navicula pygmaea decreased with increasing DO content.According to the cosine value of the sample and water environment elements in the sample-environment sorting diagram in Figure 2(b), the Sampling points D-4, D-5, D-6, D-7, and N-3 are scattered in the high concentration of NH 4+ -N, and these regions are more distributed in species Fragilari ulna, Cyclotella planctonica, Cyclotella meneghiniana, and Fragilaria capucina.Sampling points D-1, D-2, N-2, and N-7 were scattered in NH 4+ -N low-concentration areas, and diatom species richness was low in these areas; Sampling points D-2, D-3, N-7, and N-8 were scattered in areas with high DO concentrations; and Sampling points D-5, N-4 N-5, and N-6 were scattered in a region where the DO concentration was low.It can be preliminarily judged that the water quality of Sampling points D-4, D-5, D-6, D-7, and N-3 is poor; that is, the water quality of the middle and lower reaches of the Dongyu River and N-3 is poor (Tables 6 and 7).species abbreviation: nm=Nitzschia modesta; nv=Nitzschia valdecostata; nc=Nitzschia constricta; ni=Nitzschia inconspicua; nl=Nitzschia levidensis; nlie=Nitzschia liebetruthii; go=Gomphonema olivaceum; gp=Gomphonema parvulum; cp=Cyclotella planctonica; cm=Cyclotella meneghiniana; sp=Surirella patella; ff=Fragilaria fasciculata; fc=Fragilaria capucina; fu=Fragilaria ulna; ncap=Navicula capitata; np=Navicula pygmaea; ncry=Navicula cryptofallax; nt=Navicula trivialis; nr=Navicula reichardtiana; nh=Navicula halophila; na=Navicula aquaedurae; ncon=Navicula constans; nlum=Navicula lundii; ac=Amphora coffeaeformis; av=Amphora veneta; al=Amphora libyca; cpl=Cocconeis placentula; spar=Stephanodiscus parvus; at=Achnanthes thermalis; am=Achnanthes minutissima; acoa=Achnanthes coarctata; ah=Achnanthes hungarica; ct=Cymbella tumida; an=Actinocyclus normanii.
To further confirm the accuracy of the diatom community composition in the aquatic environment, we performed a PCA analysis of the aquatic environmental factors.
As shown in Table 8, four principal components were screened according to an accumulated contribution rate of >85%, a total of 4 principal components were screened out.According to the main component comprehensive score ranking, it can be seen that the scores of D-3, D-4, D-5, D-6, D-7, and N-3 are higher in the ranking and the water pollution is more serious.

Composition and spatial distribution characteristics of diatom community
A total of 23026 diatom shells were identified from 15 sample sites in the Dongyu River and estuary of the lake.They belonged to 161 species from 23 genera, mainly epiphytic benthic diatoms, were abundant in most samples.Species rich in A diatoms included Nitzschia (46), Navicula (37), Amphora (11), Cyclotella (8), Cymbella (9), Achnanthes (8), Pinnularia (6), Gomphonema (5), Surirella (5), and Fragilaria (5).There were 39 diatom species with a species richness greater than 0.2%, and the percentage composition of the major diatom species is shown in Figure 3.The main genera of the diatom community in the Dongyu River and estuary of the lake were Cyclotella meneghiniana, Fragilaria fasciculata, Fragilaria ulna, Amphora libyca, and Cocconeis placentula, accounting for 63.36% of the total counts.Cyclotella meneghiniana had a comparative advantage; the proportions of D-4, D-5, D-6, and N-8 at the Sampling points were relatively high, accounting for 37.53%, 33.97%, 48.18%, and 42.79%, respectively, and were mainly distributed in the middle reaches of the Dongyu River.Cyclotella meneghiniana accounted for 26.38% of all the genera.Most of the diatom species are distributed in the middle and lower reaches of the Dongyu River (Figure 3), and the diatoms biomass of the D-3, D-4, D-5, D-6, and D-7 are amount accounts for 66.50% of the total counts.

Discussion
This study analysed the influence mechanism of water environment conditions on the characteristics of diatom communities in the study area, established the response relationship between species and the environment, and indicated water quality changes by diatom community characteristics, thus judging the water environment quality status of the study area (Eliaszkowalska and Wojtal 2020;Ramstack Hobbs et al. 2022).According to the characteristics of a higher biodiversity index and better water environment, the Margalef and Shannon-Wiener indices were higher in the middle and lower reaches of Dongyu River and lower in the estuary area, which was inconsistent with the results of the water quality assessment.Analysis of the reasons, first, the estuary of Dongyu River is the junction of Dongyu River and Nansi Lake.It is connected to the Beijing-Hangzhou Canal.The river disturbance makes it difficult for diatoms in the sediment to be completely preserved (Yang et al. 2005).Second, the nutrient content in estuaries is low, there are many reeds, and an excellent aquatic environment does not provide a good living environment for diatoms.Simultaneously, the correlation analysis shows that NO 2− is the main environmental factor that promoted the growth of diatoms (Table 6).In contrast, the low concentration of NO 2− , NH 4+ -N in the estuary may inhibit the growth and reproduction of diatoms (Reynolds 1984).Nitrogen and phosphorus are important environmental factors affecting diatom communities and that the diatoms in the sediments of lake estuaries are significantly affected by the sedimentary environment during the preservation process (Yu et al. 2021).A significant positive correlation was observed between the three diatom indices and DO (Table 6).Thus, an increase in the DO content in the water environment index significantly improved the diversity of the diatom community.The dissolved oxygen in the clean water was close to saturation; however, owing to the growth of algae, the dissolved oxygen may have been supersaturated, and the Sampling points D-3, D-4, and N-8 were due to the high DO content in the water due to algal growth.The diatom index was negatively correlated with other environmental factors, which is consistent with the findings of Christie and Smol (1993) that river water pollution has a significant impact on algal structure composition, and the pollution status index is negatively correlated with most indices (Christie and Smol 1993).This study shows that the overall NH 3 -N fluctuation range of Dongyu River and the estuary of the lake is 0.22-1.14mg•L −1 >1.00 mg•L −1 .The range of TP varies from 0.11-1.04mg•L −1 >0.02 mg •L −1 .It can be seen from these two water quality indicators that the concentration of nitrogen and phosphorus in the study area exceeded the standard.The high concentration in water was mainly due to the use of nitrogen and phosphorus fertilisers in coastal agricultural production that enter the river through the infiltration and production of domestic sewage discharge.In addition, the mass concentration fluctuation range of Hg is 0.00009 ~ 0.00046 mg•L −1 >0.0001 mg•L −1 , and the water environmental hazard is not easy to ignore.The high Hg mass concentration was mainly due to the large amount of Hg in the coal.It enters the atmosphere through combustion, enters the soil environment through atmospheric precipitation, and finally enters the water environment through infiltration (Chaudhry and Malik 2017).
The results of species-environment RDA analysis showed that the main water environmental factors affecting the distribution of diatoms in the study area were NH 4+ -N and DO, indicating that the water quality is mainly controlled by nutrients which were NH 3 -N and DO in the study area water significantly affected the combination of benthic diatom communities changes.The dominant species consisted of diatom community in the study area were Cyclotella meneghiniana, Fragilaria fasciculata, Fragilaria ulna, Amphora libyca, and Cocconeis placentula, among which Cyclotella meneghiniana and Cocconeis placentula were ubiquitous in eutrophic water the and eutrophication Indicator Species (Trobajo et al. 2009;Frumin and Gildeeva 2014;Chen et al. 2019).Cocconeis placentula is mainly distributed in the middle reaches and middle and lower reaches of the Dongyu River which is a moderately polluting indicator (Moss 2008).The NH 4+ -N concentration in the water environmental factors of this basin exceeded the standard value for Class III water bodies specified in the National Standard -Surface Water Environmental Quality Standard (2002), reached the Class IV water quality standard, and the water quality was poor.In addition, according to the measured water quality data, the TP and Hg contents in the middle reaches of the Dongyu River exceeded the standard, and the water environment was inferior.This is consistent with the sample-environment RDA ranking analysis, and Cyclotella meneghiniana, Fragilaria fasciculata, Fragilaria ulna, Amphora libyca, and Cocconeis placentula had a significant positive correlation with NH 4+ -N, which supports the above conclusions.High concentrations of ammonia nitrogen are converted to nitrite under certain conditions.Nitrite is a strong carcinogen that poses a significant threat to aquatic organisms and human health.The causes of pollution may be mainly agricultural nonpoint source pollution, such as the use of nitrogenous fertilisers, farmland drainage, and the discharge of domestic wastewater containing nitrogen.The richness of Nitzschia liebetruthii, Stephanodiscus parvus, Navicula lundii, and Navicula pygmaea decreased with increasing DO levels, indicating that these species were anaerobic.In addition, the TP in the study area mainly fluctuated between water quality standards III and V, and the concentration was higher (Leland and Porter 2000).TP is the most important environmental factor affecting the diatom community; however, the results of this study showed that the relationship between TP and diatom community composition was not significant.This indicated that the effect of TP on the diatom community was much smaller than that of NH 4+ -N and DO in the study area.Therefore, the main environmental pollution factor in the study area was the NH 4+ -N excessive concentration.The overall water quality in the estuary area of the lake was better, diatom richness and species richness was lower, and the genus Fragilari fasciculata is more predominant here.Fragilaria prefers to live in aquatic environments with good aquatic vegetation, high transparency, and good water quality (Chen et al. 2014).Reeds growing in lake estuaries are flourishing, which can have a purifying effect on water quality.Human intervention is strictly prohibited in this area for fishing net culture, which improves the aquatic environment.However, the water quality at the sampling sites at point N-3 was poor.This is because observation point N-3 was in the vicinity of the river channel.Ships pass through them year-round, and domestic sewage is often discharged.

Conclusion
1.The dominant species of diatoms in the Dongyu River and the Nansi Lake estuary included Cyclotella meneghiniana, Fragilaria fasciculata, Fragilaria ulna, Amphora libyca, and Cocconeis placentula, accounting for 63.36% of the total counts; redundancy analysis showed that NH 4+ -N and DO were the main environmental factors affecting the changes in diatom assemblages.This indicates that the Dongyu River is mainly affected by nutrient salts.2. The water environment was in a light-medium pollution state, and the main factor leading to pollution was found to be NH 4+ -N.The NH 4+ -N over standard watershed was mainly concentrated in the middle and lower reaches of the Dongyu River.
Agricultural NPS is the primary source of pollution.Controlling the use of nitrogen and phosphorus fertilisers and the emission of nitrogen-containing organic matter are extremely important for the water environment of the Dongyu River and the water safety of the East Route of the South-to-North Water Transfer Project.

Table 1 .
habitat information of sampling sites.

Table 2 .
Biodiversity assessment index and water quality assessment criteria.

Table 3 .
the analysis results of Dca.

Table 5 .
water quality evaluation results of Dongyu river and the entrance to the lake.

Table 6 .
correlation analysis between environment factor variables and diatom community parameters.

Table 9 .
water quality comprehensive evaluation results.