Keywords

1 Introduction

Eutrophication of water bodies is one of the major water environment problems facing the world today. The eutrophication of rivers and lakes will become more and more severe due to factors such as changes in precipitation frequency and rapid human urbanization in this century (Sinha et al. 2017), which will further aggravate the deterioration of water quality in rivers and lakes, the outbreak of water blooms and black odor of lake floods, and other disasters, and seriously threaten the regional economic development and ecological health of the basin (Qin et al. 2013; Cooke et al. 2016; Paerl et al. 2016; Monchamp et al. 2018). Therefore, how to quickly and effectively improve the water quality of eutrophic water bodies and mitigate cyanobacterial bloom disasters has become an extremely important research topic in the field of ecological restoration of eutrophic water bodies at home and abroad (Søndergaard et al. 2007; Vander Zanden et al. 2016; Zhang et al. 2016). The transfer and diversion of transit guest water to promote the flow of river and lake water bodies and strengthen the connectivity of river, river and lake systems within the watershed provides new ideas to mitigate cyanobacterial bloom disasters in eutrophic water bodies from an ecohydrological perspective.

While water diversion projects are effective in mitigating lake water hazard, the impact of exogenous phosphorus nutrient input from the diverted guest water on the water ecology of the receiving lake is also controversial (Wu et al. 2018; Yao et al. 2018; Qin et al. 2019; Dai et al. 2016; Yang et al. 2018). For example, the total phosphorus content of the Water Diversion from Yangtze River to Lake Taihu (WDYT) is higher than that of the Lake Taihu water body (Pan et al. 2015a), and the influx of phosphate and other pollutants along the diversion channel also makes the nutrient content under the Wangting Hdro-junction higher than that of the lake body (Dai et al. 2018; Dai et al. 2020). The migration and transformation of exogenous phosphorus nutrients from the passenger water in the receiving lake may have an impact on the phosphorus material cycling mechanism of the lake (Yao et al. 2018) as well as the growth and community structure of the primary producer planktonic algae (Amano et al. 2010; Huang et al. 2015), which in turn increases the aquatic ecological risk of the diversion (Zhai et al. 2010; Zhang et al. 2018; Pan et al. 2015b).

In fact, not all forms of phosphorus in nature contribute directly to lake primary productivity, depending on the biological effectiveness of phosphorus. Biologically active phosphorus (BAP) is the active phosphorus that can be used directly or indirectly by aquatic organisms (Wang et al. 2010). In general, BAP in lake waters is dominated by soluble reactive phosphorus (SRP) and enzymatically soluble phosphorus (EHP) (Gao et al. 2006; Chen et al. 2019), while BAP in sediments mainly includes DGT-P, water-soluble phosphorus (WSP), algal-available phosphorus (AAP), and NaHCO3-extractable phosphorus (Olsen-P) types (Wang et al. 2016). Usually, phosphorus in river waters is predominantly in the particulate form due to the relatively high sediment content, and the potential biological effectiveness of particulate phosphorus is relatively high (Zhou et al. 2018). However, the impact of water diversion on the biological effectiveness of phosphorus in receiving rivers and lakes and its contribution to the production and elimination of planktonic algae in lakes are still not clarified, which is the primary issue limiting the scientific evaluation of the ecological and environmental impacts of rivers and lakes in water diversion projects such as WDYT.

Lake Taihu is one of the largest freshwater lakes in the middle and lower reaches of the Yangtze River, and the rapid economic development of the basin has led to serious pollution of the lake water bodies, with frequent occurrence of cyanobacterial blooms and “lake flooding” in the local lake area (Qin 2020). As an important emergency measure to alleviate the water bloom disaster in Lake Taihu and ensure water security in the basin, the river diversion project has been operating on a regular basis in recent years. The Taihu Basin is located in the subtropical monsoon region, and the ecological environment of rivers and lakes in the basin usually shows significant spatial and temporal variation (Wu et al. 2019). In this study, the spatial distribution and seasonal variation of phosphorus in the receiving areas of rivers and lakes during autumn and winter diversions were analyzed by means of field surveys, and the effects of autumn and winter diversions on the biological effectiveness of phosphorus and planktonic algae in the receiving areas of rivers and lakes were compared and studied to provide a basis for the prevention and control of exogenous phosphorus pollution in lakes by diversions of rivers and lakes. This study provides a basis for the diversion of water from the river to the lake.

2 Materials and Methods

2.1 Study Area

Since 2002, Taihu Basin has been implementing the “River Diversion Project” to transfer water from the Yangtze River, which has relatively good water quality, to Lake Taihu by using the Wangyu River, the backbone of the basin’s water conservancy project, and to supply water from Lake Taihu to Shanghai and other downstream areas through the Taipu River Project. The water supply from Lake Taihu to Shanghai and other downstream areas through the Taipu River project, thus driving the optimal scheduling of many water conservancy projects in the basin and promoting the flow of water in Lake Taihu and the river network. The Wangyu River, which starts from the Yangtze River in the north and ends at Gonghu Bay in the south, has a total length of 60.8 km and a dense river network on both sides, and is one of the main diversion and drainage channels of the River Diversion and Tai project (Chu et al. 2014). Changshu water conservancy hub and Wangting water conservancy hub are the two control sections of the main stem section of the Wangyu River, of which Wangting water conservancy hub is the direct control gate for the diversion of water from the Wangyu River into the lake. The tributary gates on the east bank of the Wangyu River are basically controlled, while the tributary gates on the west bank are open for the drainage of flood water in the area, except for the section north of Fushan Pond near the Yangtze River in the north and some tributaries south of Jialing Dang in the south; in the process of water transfer, the diversion flow of the gates on the east bank of the Wangyu River is controlled by not exceeding 30% of the amount of river water transferred or the maximum diversion flow not exceeding 40 m3/s. The vast majority of the Yangtze River water diverted by the Changshu hub of the Wangyu River flows into Lake Taihu and the tributaries on the west bank (Ma et al. 2014).

Gonghu Bay is a large lake and bay type water in the northeast of Lake Taihu (30°55′40′′–31°32′58′′ N, 119°52′32′′–120°36′10′′ E), with an area of about 150 km2 and a year-round average water depth of 1.8 m in the bay (Zhong et al. 2012). The southwestern part of the bay is connected to Meiliang Bay and the center of Lake Taihu, and the northeastern corner is connected to the Wangyu River, which is the primary receiving lake for the diversion project of the Wangyu River. In recent years, with the continuous deterioration of the water environment in Meiliang Bay, Gonghu Bay has become the main water source of Wuxi City and one of the important water sources of Suzhou City. Gonghu Bay is a typical mixed grass-algae lake ecosystem, with both cyanobacterial bloom accumulation and a large amount of aquatic vegetation cover in the bay, but the aquatic vegetation is mostly concentrated in the east shore zone of Gonghu Bay (Zhao et al. 2015). The complexity of the ecological structure within Gonghu Bay makes its water environment and aquatic ecological elements show obvious spatial heterogeneity.

In this study, the river channel of Wangyu River into the lake is selected to study the characteristics of the water source of the diversion project into the lake, and the waters of Gonghu Bay are used as a sensitive receiving lake. The distance between the lake center of Lake Taihu and Gonghu Bay is far, and the open water is used as the control lake area of Gonghu Bay affected by the diversion. Nine monitoring points (Y, W-1–W-8, Fig. 1a) were placed along the entire route from the mouth of the Yangtze River to the Wangting Water Conservancy Hub in Lake Taihu to monitor the physicochemical parameters of the water bodies in the Wangyu River. The sampling sites in the lake area are as shown in Fig. 1b. Seven sampling sites are placed at equal distances along the axis of the bay of Gonghu Bay (G1–G7), and 3 monitoring points (C1–C3) are placed in the lake center area as reference points for the influence of water diversion in Gonghu Bay.

Fig. 1.
figure 1

Location of sampling sites in the Yangtze River, Wangyu River (a) and Lake Taihu (b).

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2.2 Sampling and Physicochemical Parameters Measurement

According to the operation of the Wangyu River diversion project from 2014 to 2016, from November 2014 to January 2016, we selected the autumn and winter seasons, i.e. November and mid-January each year for field observation and sample collection, in which there were water diversions from the Wangyu River to the lake in November 2014 and mid-January 2015, and no water diversions in the rest of the monitoring time. The spatial and temporal distribution patterns of phosphorus and phytoplankton community elements from the Wangyu River to the center of Lake Taihu were analyzed in different seasons, and the monitoring time started from 9:00 am and ended before 12:00 noon.

The 2014–2016 Wangyu River water inflow and Lake Taihu water level data were quoted from the Taihu Basin Authority Annual Report on River Diversion to Taihu (TBA 2014, 2015 & 2016). The wind direction at the sampling points was determined by a boat-mounted wind speed and direction meter, and weather and rain conditions were determined by the monitors on site. The water temperature and pH values at each sampling point were determined by HACH portable multi-parameter water quality meter HQ30d on site. Plexiglass column sampler was used to collect water samples from rivers and lakes 50 cm below the water surface, and one mixed sample was collected from each sampling point, totaling 1 L of water samples, collected in pre-washed transparent plastic bottles, placed in a holding tank with ice, transported to the laboratory, and total phosphorus (TP), total dissolved phosphorus (DTP) and soluble reactive phosphorus (SRP) and chlorophyll a (Chl-a) were measured within 24 h.

The planktonic algae samples were collected directly in the field by first collecting 1 L of mixed water samples (50 cm below the lake surface) from Lake Taihu, preserving them in washed plastic bottles, adding 15 ml of Lugol’s reagent at 1% mass concentration to fix the algal cells, and transporting them to the laboratory for storage at room temperature and protected from light (Jin and Tu 1990).

TP and DTP were determined by referring to the relevant methods in the Specification for Lake Eutrophication Investigation (2nd edition), and DTP and SRP were determined by filtering the original water samples through 0.70 μm pore size GF/F glass fiber membrane beforehand and using the filtered water. The Chl-a content was determined by the hot ethanol method (Chen et al. 2003).

2.3 Identification of Phytoplankton Community

The 1 L of fixed sample was shaken and transferred to a partition funnel, and after 24 h of resting, 50 ml of the concentrated sample was separated from the bottom of the funnel and stored in an 80 ml glass vial at room temperature and protected from light for microscopic imaging and counting of planktonic algae (Jin and Tu 1990). The identification and enumeration of planktonic algal populations in water samples were carried out using automatic algal identification and enumeration software. 0.1 ml of the above static concentrated sample was taken in a 0.1 ml algal enumeration frame, imaged and enumerated under a 16 × 40 × biological microscope, and 100 available fields of view were read randomly and uniformly in the enumeration frame for each sample. The obtained field of view images were processed by the automatic algal identification and counting software for identification of planktonic algal populations and calculation of species numbers. The identification of planktonic algal species was carried out with reference to the literature (Hu and Wei 2006) method.

2.4 Statistical Analyses

Spatial differences in the monitoring indicators of the three waters of the Wangyu River, Gonghu Bay and the lake core area, as well as differences in the water environment parameters of Gonghu Bay during the diversion and non-diversion periods were compared using two-tailed t-tests in the numerical statistical software SPSS (IBM, Armonk, USA) 16.0. Data were mapped using Sigmaplot v14.0 scientific data mapping software (Systat Software Inc., London, UK).

The community composition of planktonic algae was based on the cell density of algae, and the percentage of the total cell density of each phylum or species of algae in the total cell density of a sample site was calculated. A ranking method was used to analyze the correlation between physicochemical parameters, phosphorus and planktonic algal community structure in the water column of Gonghu Bay during the diversion and non-diversion periods. The calculation procedure was performed by CANOCO 4.53 for Windows (Microcomputer Power) software (Ter Braak and Šmilauer 2002). Before analysis, the data of the planktonic algal community structure matrix based on algal density and the physicochemical parameters were transformed with open square roots to meet the requirements of statistical analysis. The data were selected by forward selection in the CANOCO software for those parameters that had a significant effect on the structure of the planktonic algal community. The significance of the calculated results (p < 0.05) was verified using 499 unrestricted Monte Carlo permutations.

3 Results and Discussion

3.1 Monitoring the Meteorological and Water Level Profile of the Lake

The wind direction, wind level, direction of lake flow and duration of water diversion in the waters of Gonghu Bay during the field survey are shown in Table 1. The wind direction in Gonghu Bay during the monitoring period in autumn and winter was mostly from the north, and the wind level was mainly 3, which had less influence on the lake flow. The duration of water diversions in both diversion periods up to the day of monitoring did not exceed one month.

Table 1. Wind speed, wind direction, water level and duration of water diversion in Gonghu Bay during the monitoring period.
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3.2 Spatial Distribution of Phosphorus During Diversion and Non-diversion Periods

The spatial distribution of the values of the main physicochemical parameters of the water bodies in the Wangyu River, the bay center axis of Gonghu Bay and the lake center area in autumn and winter are shown in Fig. 2. The difference in water temperature between the diverted and non-diverted Wangyu River in autumn was significant (t-test, p < 0.05) and was significantly higher in the diverted period (November 2014) than in the non-diverted period (November 2015), and there was no significant difference in water temperature between the diverted and non-diverted Wangyu River in winter, but the difference was significant in Lake Taihu waters (t-test, p < 0.05). The pH of Lake Taihu was significantly lower during the diversion period than during the non-diversion period, influenced by the lower pH of Wangyu River waters, and the pH showed a significant increasing trend from Wangyu River to Lake Taihu. Similarly, Chl-a showed a significant increasing trend from the Wangyu River to Lake Taihu waters, but the values in the monitored waters during the diversion period were significantly lower than those in the non-diversion period (t-test, p < 0.05). The spatial variation patterns of the three phosphorus parameters were consistent, all showing a decreasing spatial trend from Wangyu River to Lake Taihu. The phosphorus concentrations in Gonghu Bay were significantly higher than those in the non-diversion period in winter (Fig. 2b, t-test, p > 0.05), while there was no significant difference between the DTP and SRP contents in the monitored waters in the diversion and non-diversion periods in autumn, except for TP (t-test, p > 0.05). The TP in Lake Taihu during the non-diversion period in autumn was significantly higher than that in the diversion period, which was related to the presence of cyanobacterial blooms in Lake Taihu during the non-diversion period, and this study also confirmed that the Chl-a content in Lake Taihu during the non-diversion period was significantly higher than that in the diversion period (Fig. 2a).

Fig. 2.
figure 2

Spatial distribution of physicochemical parameters of water bodies in the monitoring area in November (a) and January (b).

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The proportion of particulate phosphorus in the water bodies of the Yangtze River and the Wangyu River was higher in the fall and winter diversion periods than in the non-diversion periods, but not higher than in the lake core area of Lake Taihu. Water diversions in autumn and winter still increase the biologically active phosphorus content in Gonghu Bay, but the impact area is limited to the inlet waters of the Wangyu River by the diversion flow and duration (Fig. 3). Studies have reported pollutants along the Wangyu River from tributaries into the Wangyu River, especially during non-diversion periods. In recent years, although some control measures have been implemented in the Wangyu River, further control of domestic and agricultural nonpoint pollution along the Wangyu River should be carried out in autumn and winter (Zhang et al. 2010). In autumn and winter, the water level of the Wangyu River during the diversion period is always higher than that of the non-diversion period due to low rainfall and the input of Yangtze River water. During the non-diversion period in winter, nutrients and organic pollutants from the tributaries can flow into the Wangyu River and increase the pollutant concentration in the main channel.

Fig. 3.
figure 3

Spatial distribution of phosphorus composition in the monitoring area during the diversion and non-diversion periods in autumn and winter.

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3.3 Seasonal Variation of Phosphorus in Lake Taihu

The mean values of the three phosphorus concentrations in the monitored waters in different seasons of the diversion and non-diversion periods are shown in Fig. 4. The values of the three phosphorus concentrations in the winter diversion period were significantly higher than those in the non-diversion period in the same season (t-test, p < 0.05). On the contrary, the phosphorus concentration values in the autumn diversion period were significantly lower than those in the non-diversion period (t-test, p > 0.05), which was related to the appearance of cyanobacterial blooms in Lake Taihu in the autumn non-diversion period, which led to a significant increase in phosphorus content in the regional water column due to the accumulation of blooms. The values of all three phosphorus concentrations in the water coming from the Wangyu River in the fall and winter diversion periods were higher than those in the lake center area, and the values of each phosphorus in Gonghu Bay were between the Wangyu River and the lake center area, indicating that the diversion of water from the Wangyu River had the possibility of increasing the risk of phosphorus loading in Gonghu Bay.

Fig. 4.
figure 4

Comparison of concentrations of total phosphorus (a), total dissolved phosphorus (b) and dissolved reactive phosphorus (c) in monitoring areas in autumn and winter.

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3.4 Spatial and Temporal Distribution of Phytoplankton Community Composition

The composition of planktonic algal community in the monitoring area in autumn and winter is shown in Fig. 5. Most of the waters in the axis of the bay center of Gonghu Bay during the non-diversion period in winter are dominated by cyanobacteria, which is very similar to the composition of algal community in Wangyu River, and both the bay mouth and the lake center area are dominated by cyanobacteria. Diatoms dominated in the bay axis of Gonghu Bay during the winter diversion period, with relative proportions ranging from 70.6% to 94.3%, followed by green algae (relative proportions ranging from 2.2% to 13.2%), which is also very similar to the community composition of the incoming water from the Wangyu River, indicating the important influence of exogenous diatom input on the community composition of the receiving lake area of Lake Taihu. Cyanobacteria were still absolutely dominant in the bayou and core areas of Gonghu Bay.

The diatoms were dominant at the mouth of Wangyu River (G1–G5), while the cyanobacteria were dominant at the mouth of the bay and the center of the lake during the diversion period in autumn 2014, and the diatoms were dominant at the mouth of Wangyu River to G5 of Gonghu Bay during the non-diversion period in autumn 2015, while the cyanobacteria were dominant in all other waters (Fig. 5). The main reason for this is that the sampling date in November 2015 was on the third day after the end of the fall water diversion, and the effect of water diversion is continuous.

Fig. 5.
figure 5

Spatial distribution of phytoplankton community composition in the monitoring area in autumn and winter.

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3.5 Contribution of Phosphorus Changes to Lacustrine Phytoplankton Communities

TP (p = 0.002) and DTP (p = 0.008) in autumn were significantly correlated with the difference in planktonic algal community structure between the diverted and non-diverted periods in Gonghu Bay in autumn, and the three dominant environmental factors explained a total of 20.7% of the spatial and temporal variation in algal community structure, with TP and DTP explaining 10.8% and 7.2% of the variation in algal community, respectively, and the highest values of TP and DTP were in the diverted period (Fig. 6a). The highest TP and DTP values belonged to the water diversion period (Fig. 6a). The results of CCA ranking of planktonic algal community structure and water column phosphorus during winter diversion and non-diversion periods in Gonghu Bay are shown in Fig. 6b, DTP (p = 0.002) was significantly correlated with the succession of algal community structure in Gonghu Bay. 11.8% of the differences in algal community structure.

The SRP content in the waters of Gonghu Bay during the non-diversion period in autumn was relatively higher than that in the diversion period, and the higher concentration of SRP in Gonghu Bay supplemented phosphorus nutrients for the proliferation and growth of planktonic algae in the non-diversion period. At the same time, the structure of algal community in Gonghu Bay during the diversion period was also importantly related to the higher DTP content in the incoming water of Wangyu River, and the relative proportion of diatoms and green algae in the bay during the autumn diversion period was higher than that in the non-diversion period. The study confirms that diatoms appear when the concentration of nitrate, phosphate and silicon oxide in the water column is high, and higher concentrations of phosphorus are instead more favorable for the growth of diatoms and other algae, which in turn replace the dominance of cyanobacteria.

Fig. 6.
figure 6

Two-dimensional analysis of the paradigmatic correspondence between phosphorus and phytoplankton communities in autumn and winter.

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4 Conclusions

The most significant effect of water diversion on phosphorus in Gonghu Bay in autumn and winter is at the mouth of the Wangyu River, and the phosphorus concentration decreases from the mouth of the Wangyu River to the mouth of Gonghu Bay during the water diversion period. In autumn and winter, water diversions still increased the biologically active phosphorus content in Gonghu Bay, but the impact area was limited to the mouth of the Wangyu River due to the flow and duration of diversions. The water diversion of Wangyu River significantly changed the community composition of planktonic algae in Gonghu Bay, and the water diversion in autumn and winter significantly promoted the proliferation of diatoms, making them the dominant species of planktonic algae. The difference in biologically effective phosphorus concentration between autumn and winter diversions and non-diversions in Gonghu Bay is an important environmental factor affecting the change of planktonic algae community, with the contribution of biologically effective phosphorus to the change of algal community ranging from 10.8 to 11.8%, and the contribution of phosphorus to the succession of planktonic algal community in Gonghu Bay in different seasons is also around 20%.