Changes in phytoplankton species composition
The abundance and biomass of the phytoplankton in Lake Datong have undergone relatively considerable changes but have, nevertheless, been at low levels (Table 2). Conversely, the resident phytoplankton species have changed markedly over the past 60 years (Figure 2). From the 1860s, dinoflagellates have been the dominant species, accounting for 69.7% of all phytoplankton species; however, these have decreased to less than 5% in recent years. In 2019, Chlorophyta formed the dominant species, accounting for 40–50%; Euglenophyta increased from 0% (prior to 2005) to 34.69% and then decreased rapidly to less than 10%. Cyanophyta increased from 0% in the 1960s to 19.08% in the 2000s, slowly increased to 23.75% in 2013–2014, and then dropped to around 20% in 2019. In addition, Bacillariophyta changed minimally from the 1960s to 2008–2009, increased to 27.59% by 2010, and then decreased gradually to 16.24% by 2019. The Shannon diversity index (H-value) decreased from 2.045 in 2008-2009 to 1.24 in 2013, and then increased to 3.549 by 2019. The Margalef richness index (D-value) decreased from 3.11 in 2008-2009 to 0.75 in 2013, and then increased to 6.773 in 2019. The Pielou evenness index (J-value) decreased from 1.037 in 2008-2009 to 0.68 in 2012-2013 and further decreased to 0.555 by 2019 (Table 2). In terms of spatial distribution, the phytoplankton abundance gradually decreased from south to north. In contrast, mollusc biomass decreased from northeast to southwest  (Figure 3).
Succession of diatoms in sediments
A total of 43 species from 21 genera were identified in the sedimentary column of Lake Datong. Among them, planktonic and benthic genera were dominant, and epiphytic genera were also common (Figure 4). It can be seen from Figure 4 that Aulacoseira granulata is one of the dominant species. At approximately 10 cm depth (approximately corresponding to the year 1980), the content of Aulacoseira granulata and the diatom assemblage in the sedimentary column changed significantly. Below 10 cm depth, Aulacoseira granulata is a dominant species, accounting for 60-80% of the total content, and it is found at the bottom of the borehole, where there are few diatoms. The populations of planktonic species with low nutrition, including Aulacoseira ambigua and Cyclotella bodanica, and epiphytic species, including Fragilaria Capucina, Cocconeis placentula, Gyrosigma acuminatum, and Eunitia accounted for less than 15% of the total population. At a depth of 10 cm, the proportion of Aulacoseira granulata decreased rapidly. From 1 to 9 cm in the sediment column of Lake Datong, the content of diatoms began to increase. Simultaneously, the numbers of eutrophic species such as Cyclotelaphanos thaliformis, Cyclotella meneghiniana, Stephanodiscus hantzschii, and Stephanodiscus minutulus began to increase.
The zooplankton in Lake Datong have been dominated by copepods since the 1960s. Among them, Calanoids along with a few rotifers and protozoa accounted for 84% of the total biomass. In Lake Datong, during the aquaculture period, the structure of its zooplankton community was characterized by small zooplankton protozoa, and the number of species, abundance, and biomass of rotifers accounted for a higher proportion. The large and medium-sized zooplankton Cladoceras and Copepods account for a lower proportion. Especially after 2011, a trend of miniaturization is increasingly evident. However, the number of species, biomass, and abundance of zooplankton surveyed recently have shown an increase (Table 3). The average abundance of zooplankton was 19,904 ind/L and the average biomass was 966.38 mg/L. Of these, the average abundance and biomass values of small zooplankton protozoa and rotifers were 19870 ind/L and 5.471 mg/L, respectively. In case of large and medium-sized zooplankton, the average abundance and biomass of copepods were 31 ind/L and 912.91 mg/L, respectively. The highest values of abundance and biomass were 41100 ind/L and 6587.713 mg/L, respectively at the sampling point D19. The minimum value was at D5, where the abundance was 9000 ind/L, and the biomass was 5.961 mg/L. The abundance and biomass of zooplankton in Lake Datong demonstrated a significant variation in spatial distribution, with relatively high abundance in the east and relatively low abundance in the south (Figure 5). Simultaneously, the zooplankton H-index in Lake Datong was 1.06, with a range of 0.31-1.88. The average value of the D-index was 0.67, ranging from 0.11 to 1.39. The mean value of the J-index was 0.65, with a range of 0.45-0.90.
In the winter of 1960, there were more than 21 species of Mollusca in Lake Datong, including 11 species of Gastropoda and 10 species of bivalves. In terms of both abundance and biomass, bivalves are dominant. The composition of molluscs in Lake Datong decreased to 15 species belonging to 5 families by 2008–2009 (Table 4). Bellamya purificata, Corbicula fluminea, and Uniodoug lasiae are the dominant species. During the aquaculture period, Lake Datong had high abundance and high biomass of molluscs (Figure 6). However, their average abundance and biomass had evident temporal and spatial differences, showing a gradual increase over time from spring and summer to autumn and winter and a gradual decrease from northeast to southwest. During the period, the molluscs Gyraulus albus, Segmentina nitida,and Radix, which prefer habitats with aquatic macrophytes, and Stenothyra divalis, Limnoperna lacustris, and Solenaia oleivara, which prefer the lotic habitat, disappeared. In addition, the dominant mollusc type changed from Lamellibranchia to Gastropoda. A recent survey in 2019 demonstrated that Lake Datong had 7 species of molluscs from 2 orders, 3 families, and 7 genera. It had one gastropod species and 6 bivalve species from 1 order, 2 families, and 6 genera. A total of 163 molluscs were collected during the investigation, and the total quantity of fish caught was 2824 g. The number of ring snails of species Bellamya purificata was 89, and the catch was 188 g, accounting for 6.66% of the total catch. Among bivalves, the number of the Lamellibranch species Unio douglasiae was 50. The catch of Cristaria plicata was the largest, weighing 1945 g, accounting for 68.87% of the total catch; Corbicula fluminea was the least, weighing only 20 g, accounting for a mere 0.71% of the total catch (Table 5). Comparing the distributions of molluscs in Lake Datong during aquaculture can shed light on their distinct temporal and spatial heterogeneity. Mollusca are mostly distributed in the northern and eastern parts of the lake, with a gradual decrease from northeast to southwest. There have been significant differences in their abundance and biomass in Lake Datong over different years. In 2019, our survey revealed that the biomass of large molluscs had dropped sharply (Figure 6) to less than 90% of the biomass prevalent during aquaculture, as compared to previous years. Additionally, we investigated many sampling points that did not collect mollusca (only 55.56% of the sampling points collected molluscs), especially in the western waters where aquatic hydrophytes are abundant.
3.4 Aquatic hydrophytes
In the early 1960s, Lake Datong maintained its natural lake form, with a 30% cover of aquatic hydrophytes on the lake surface. The dominant species were Potamogeton wrightii Morong, Myriophyllum spicatum Linn., Vallisneria natans, and Hydrilla verticillata. Additionally, Ceratophyllum demersum, Najas minor, Potamogeton maackianus, Trapa bispinosa, and Nymphoides peltatum were associated species. Nelumbo nucifera, Azolla imbricata, Salvinia natans, Phragmites communis, and Leersia hexandra Swartz grew in the coastal shallow water marsh. In the 1980s, the main species of aquatic hydrophytes were Vallisneria natans, Trapa bispinosa, Nelumbo nucifera, Potamogeton wrightii Morong, Ceratophyllum demersum, Hydrilla verticillata, and Zizania latifolia. In 2000, the coverage was only 10.60%. During 2011-2012, the coverage of aquatic hydrophytes was approximately 5% (420 hm2), and they were mainly distributed in the littoral zone of the lake and in the river–lake confluence areas. Among them, submerged macrophytes accounted for 44%, emerged macrophytes accounted for 30%, floating-leaved macrophytes accounted for 14%, and floating macrophytes accounted for 12% of the coverage. In 2017, there were no aquatic macrophytes in the entire lake. Subsequently, by the end of 2017, comprehensive management of Lake Datong was undertaken and aquatic hydrophytes were planted. By 2019, the coverage of aquatic macrophytes had reached 48% (Table 6).
As per the survey of aquatic hydrophytes from July to November in 2019, 7 species, 6 genera, and 6 families of aquatic hydrophytes were recorded. There were 5 families, 5 genera, and 5 species of dicots, namely Myriophyllum verticillatum, Ceratophyllum demersum, Trapa bispinosa, Nelumbo nucifera, and Halerpestes cymbalaria. The monocots had a representation of 1 family, 1 genus, and 1 species, namely, Hydrilla verticillata. Myriophyllum verticillatum grows faster in the summer and slower in winter, making it tolerant of low temperatures. The tolerance of water temperature for Ceratophyllum demersumis wider. Trapa bispinosa requires a warm, humid, and sunny environment and is not resistant to frost. The temperature required for its flowering and fruiting is 20-30 ℃ during daytime and 15 ℃ at night. Nelumbo nucifera is a hydrophyte that requires long hours of sunshine, a light and warm environment, and is intolerant to shade. Growing very slowly below 16 ℃, Hydrilla verticillata likes an environment with sufficient light and warmth, and being cold resistant, it grows well in the temperature range of 15-30 ℃. Vallisneria natans is a heliad and grows well at 18-22 ℃. Therefore, from July to November, with the weakening of light intensity, decrease in light duration, and gradual decrease in temperature, the biomass of this aquatic hydrophyte also decreases (Table 7).
The survey results in 2019 (Figure 7) demonstrated that aquatic hydrophytes are mainly distributed in the western part of Lake Datong. In July, the aquatic hydrophytes were found to be distributed over an area of 39.67 km2, which amounts to 48% of the total water surface area of Lake Datong. In September, they were distributed over an area of 27.2 km2, and in November over 23.99 km2,which amount to 33% and 29% of the total water surface area of Lake Datong, respectively. It can be observed that the range of distribution of aquatic hydrophytes gradually reduced from July to November. The total biomass of aquatic hydrophytes in Lake Datong in July, September, and November 2019 was 362000 t, 173000 t, and 70500 t, respectively, also displaying a gradual decrease.
Before 1980, Lake Datong was utilized only for natural fishing, by feeding seedlings into the river, with the development of aquaculture being established in the early 1980s. From 1981 to 1989, the average annual fish production was 585 t. At that point, there were 109 species of fish belonging to 23 families and 8 orders in Lake Datong. In 1989, fertilizers were beginning to be used in aquaculture. From 1990 to 1999, the average annual output of fish products from Lake Datong was approximately 1000 t, and the highest annual output was above 1600 t. In 1997, there were 8 orders, 23 families, and 109 species representation of fish. Since 2000, approximately 4000 ~ 5000 t of various inorganic fertilizers have been utilized in Lake Datong annually. From 2000 to 2004, the average annual output of fish products from Lake Datong was approximately 2000 t, with the highest annual output being 3000 t. After 2004, the annual input of various inorganic and compound fertilizers was approximately 12000 ~ 15000 t, and the aquatic output of Lake Datong increased rapidly. The average annual output was approximately 5000 t, and the highest annual yield exceeded 8000 t. In 2011, Lake Datong began to fence grass carp culture, adopt fertilization programs, and intensify cultivation. The fish yield that year exceeded 10000 t, with the yield maintained at approximately 12000 t. By 2012, the number of fish species declined to 45 species of 39 genera, 13 families, and 7 orders. From 2011 to 2015, 4500 ~ 5500 t of fish were placed into Lake Datong every year, of which Hypophthalmichthys molitrix and Hypophthalmichthys nobilis accounted for 60% of the species, Ctenopharyngodon idella accounted for 30% of the species, and other fish species accounted for approximately 10%.
There were 28 species from 24 genera, 8 families, and 5 orders represented in 2019 (Table 8). Among them, Cypriniformes have the most species of Cyprinidae, accounting for 89.29% of the total population, with 24 species belonging to 21 genera, 5 families, and 4 orders, found in the eastern part of China. The total number and weight of the catch were 1486 fish and 43684.6 g, respectively. Among them, the most abundant species were goblins and minnows, the total number and weight of the catch being 1059 tails and 21486.7 g, respectively. There were 22 species belonging to 19 genera, 5 orders, 5 families, found in the northern part of the lake. The total number and weight of the catch were 935 and 53860 g, respectively. Among them, the largest number and weight of the carnivorous fish Coilia brevicaudus were 248 and 5768 g respectively, followed by Carassius auratus, which produced sticky eggs and omnivorous hydrophyte food, with 193 fish weighing 16022 g. There were 21 species belonging to 18 genera, 3 orders, and 4 families found in the western parts of lake. The number and total weight of the catch were 1180 and 77780 g respectively. Among them, the most omnivorous hydrophyte-feeding crucian carp was 598 and 47983 g in number and total weight of the catch, respectively, followed by the omnivorous hydrophyte-eating gobiocyphus and minnows at 204 and 2552 g, in number and weight respectively (Table 9). Simultaneously, there are no aquatic plants in the eastern lake area; however, there are more carnivorous fishes. The number of the Culter Basilewsky and Erythroculter were higher in the eastern Lake area, while the number of Coilia brachygnathus was the largest in the Northern Lake area, followed by Culter Basilewsky and Erythroculter. This is because of the few aquatic hydrophytes in the eastern Lake area, which is not conducive for the hiding of small fish, but conducive to the predation and survival of carnivorous fish. Conversely, there are abundant aquatic hydrophytes in the Western Lake area. Parabramis, Megalobrama, and Carassius, which produce sticky eggs and omnivorous hydrophyte food are the most abundant, while the number of carnivorous fish is lower. A large number of aquatic hydrophytes provide abundant food for the omnivorous and vegetarian fishes and a suitable habitat for their reproduction, and also provide shelter for small fish, ensuring their abundance. However, compared with other lakes of the same type, the number of predatory fish in the lake is lower.
3.6 Succession in aquatic environment
The sediment records of Lake Datong reveal the succession of its water environment, and the phosphorus trends in the sediment reflect the eutrophication trends (Figure 4). According to the criteria of trophic status, Lake Datong has experienced four different stages of nutrition. The first stage: from 1847 to 1930, Lake Datong was expansive, extending in all directions and connected with the Yangtze River. Aulacoseira granulata was the dominant species of diatoms, and it usually inhabited moderately eutrophicated aquatic environments. Its high silicification degree, high sedimentation rate, and a preference for strong disturbance in the water, indicated strong hydrodynamic conditions. The sediment suspension caused by hydrodynamic effects also affected the underwater light and primary productivity. At the time, the biomass of epiphytic diatoms and sample LOI were low, and the development of aquatic hydrophytes was not high. It is speculated that the water environment of the lake was good and the ecosystem was stable at this stage. The second stage: from 1930 to 1980, the TP began to increase slowly. Especially after 1949, Lake Datong began large-scale reclamations around the lake, and the deposition rate fluctuated greatly at this time and peaked; the numbers of planktonic species Stephanodiscus hantzschii and Stephanodiscus minutulus increased slightly. They are good indicators of eutrophication in the middle and lower reaches of the Yangtze River, and the epiphytic diatoms Eunotia and Fragilaria increased. This demonstrates that aquatic vegetation increased at this stage, and the LOI reached a peak. The development of aquatic vegetation can control the release of nutrients. At the time, the nutrient level had increased (DI-TP ranged from 50 to 60 μg/L). The dominant diatom was still Aulacoseira granulata, with the maximum abundance observed in 1930–1940. In the winter of 1949, a dike was built on Lake Datong, encircling the embankment to become an inner lake and cutting off the water exchange with Yangtze River. The hydrodynamic conditions consequently declined, with a decrease in the abundance of this species (DI-TP ranged from 66 to 83 μg/L). During the third stage, i.e., after 1980, aquaculture and fertilizer farming began in the lake in 1989. During this period, the sediment TP increased rapidly, and it rose to its highest point around 2001. Simultaneously, compared with 1960, the alkalinity of the water in Lake Datong significantly increased (from 7.0–7.5 to 8.4–8.8), and the redox potential of the water changed from + 592 ~ 859 to - 67.88 ~ -110.80 mv. Electrical conductivity increased 17-fold, from 0.138 to 2.363 mS/cm. The lake environment shifted from oxidized to reduced [44, 46] The TP content of the sediment also peaked around 2006, reaching 202 μg/L. At that time, Aulacoseira granulata biomass dropped sharply, and the biomass of epiphytic species, such as Eunotia and Fragilaria, decreased gradually, whereas planktonic species, such as Stephanodiscus hantzschii and Stephanodiscus minutulus, were replaced by the dominant species. It was accompanied by the emergence of typical eutrophic species such as Cyclostephanos tholiformis and Cyclotella meneghiniana. This indicates that Lake Datong was in a state of eutrophication (the annual average DI-TP concentration was 202 μg/L). The fourth stage: after 2010, the eutrophication level of the water declined (DI-TP ranged from 127 to 152 μg/L).
The latest survey in 2019 revealed that TP was higher between July and August, and lower between September and November. Conversely, July and August are wet months, with the farmland receding, causing a large quantity of TP to accumulate in the ditches and eventually enter Lake Datong. However, in July and August, the growth of aquatic hydrophytes in several areas of Lake Datong was too dense, resulting in an ecological imbalance of its water. Several aquatic hydrophytes began to decompose. Additionally, the water temperature of the lake was high in July and August, which accelerated the decomposition rate. The nutrients absorbed by aquatic hydrophytes during the growth period were released back to the water, causing secondary pollution.
3.7 Food web structure and trophic levels
The trophic level of each functional group in the aquatic food web of Lake Datong varied between 1 (primary producer) and 3.685 (mandarin fish) (Table 10). Primary producers included submerged hydrophytes and phytoplankton. Carnivorous fish, such as mandarin fish and calamari, occupied higher trophic positions of the Lake Datong ecosystem.
The Ecopath model analysis demonstrated that the theoretical trophic level of Lake Datong in 2019 was Level IV (Figure 8). The primary producers and detritus of the food web were defined as Level Ⅰ, and the nutritional level of consumers increased sequentially [34, 36]. The trophic levels of the three primary producer functional groups in Lake Datong were all Level I, and the trophic levels of molluscs and zooplankton were all Level II. The energy flow of the Lake Datong food web had three main paths, including two pastoral food chains and one detrital food chain. Debris was the main energy source of the Lake Datong aquatic food web (Figure 9).
Lake Datong trophic Level Ⅱ had 11 functional groups. Among them, molluscs and small zooplankton were at Level Ⅱ, while the proportions of crucian, bream, cladocerans, and copepods at the second integrated trophic level were more than 50%. Twelve functional groups were occupying nutritional Level Ⅲ, and seven functional groups integrated at nutritional Level III, at more than 50%. Eight functional groups were occupying nutritional Level Ⅳ, and only three functional groups were in nutritional Level Ⅴ. The proportion of mandarin fish was 3.09%, and all other proportions were less than 1%, indicating that the energy flow of trophic Level Ⅴ and above can be ignored.