3.1. Community structure of macrobenthos in East Taihu Lake
During the four seasons between January 2021 and November 2021, a total of 266 macrobenthic organisms was collected, consisting of three major groups: Mollusca, Annelida and arthropods, were collected from five sampling areas in the main functional area of East Taihu Lake. In terms of the number of organisms, there were 58 Mollusca, accounting for 21.80%; 164 Annelida, accounting for 61.65%; and 44 Arthropoda, accounting for 16.54%. After further observation and identification, there were 28 species of macrobenthos, belonging to 3 phyla, 6 classes, 13 orders, 18 families and 25 genera. There were 10 species of Mollusca in the three phyla, accounting for 35.71%, including 7 species of Gastropoda and 3 species of Lamellibranchia; 7 species of Annelida, accounting for 25.00%, including 6 species of Oligochaeta and 1 species of Polychaeta; 11 species of Arthropoda, accounting for 39.27%, including 1 species of Chironomus plumosus, 3 species of Crustacea, and 7 species of other small number of aquatic insects (Fig. 2.).
The dominant macrobenthic species in East Taihu Lake also varied between seasons and across functional areas (Table 3 and Table 4). Seasonally, Bellamya purificata and Limnodrilus were dominant species throughout the year. Among them, Limnodrilus had the highest IRI index values in spring, summer and winter, and Bellamya purificata had the highest IRI index values in autumn. In addition to this, Bellamya aeruginosa was also more dominant in the spring; Tubifex was more dominant in the summer and autumn; and Chironomus plumosus was more dominant in the autumn, and thus autumn had the highest number of dominant species.
Table 3
Seasonal changes of dominant macrobenthic species in East Taihu Lake.
Species | IRI |
Spring | Summer | Autumn | Winter |
Bellamya purificata | 2830.05 | 2949.69 | 2381.91 | 2732.04 |
Bellamya aeruginosa | 1157.55 | — | — | — |
Limnodrilus | 4155.21 | 2967.02 | 1265.87 | 3065.19 |
Tubifex | — | 2194.08 | 1985.47 | — |
Chironomus plumosus | — | — | 1103.22 | — |
Bold font indicates the maximum IRI value for the season.
Spatially, Limnodrilus was the only dominant species in all functional areas of East Taihu Lake and had the highest IRI indexes in three areas: the conservation area, the pelagic area and the original enclosure aquaculture area. Bellamya purificata was the most dominant species in the wetland area, and was also a relatively important species in the original enclosure aquaculture area. Each functional area had 2 ~ 3 kinds of macrobenthic species with IRI index values of 1000 or more, and only the wetland area was dominated by Mollusca, while the entrance area, the conservation area and original enclosure aquaculture area were dominated by Annelida (Oligochaeta, Polychaeta) and Chironomus plumosus.
Table 4
Spatial variation of dominant macrobenthic species in East Taihu Lake.
Species | IRI |
S1 | S2 | S3 | S4 | S5 |
Bellamya purificata | 2071.38 | 2262.94 | — | — | 9709.04 |
Bellamya aeruginosa | — | — | — | — | 2314.05 |
Limnodrilus | 1217.68 | 4762.96 | 3008.53 | 4969.43 | 1349.56 |
Tubifex | — | 2134.77 | — | 3642.47 | — |
Chironomus plumosus | 1628.25 | — | — | 1224.06 | — |
Polymoe | 3467.21 | — | — | — | — |
Corbicula fluminea | — | — | 2602.66 | — | — |
Bold font indicates the maximum IRI value for the area.
3.2. Relationship between macrobenthic community structure and environmental factors
3.2.1. Impact of water environmental factors
Changes in macrobenthic community structure are affected by various environmental factors in the habitat and human activities, and the influencing factors and mechanisms are very complex. The heterogeneity of habitats in different habitats and seasons leads to different environmental influencing factors and feedback mechanisms for macrobenthos.
The distribution of the correlation between the physical and chemical factors of the water environment and the density and biomass of macrobenthic species on the annual scale of East Taihu Lake during the study period is shown in Fig. 3. The correlations between macrobenthos density, biomass and environmental factors were basically consistent, but there were some differences in the strength of significance. The correlations between species of similar taxonomic units (e.g., Bellamya purificata and Bellamya aeruginosa, Limnodrilus and Tubifex, etc.) and the various water environmental factors were somewhat similar, showing similar environmental impact effects.
The aquatic environmental factor that reached the most significant correlation with inter-species correlations at the year-round time scale was water depth, indicating that water depth is one of the most important environmental influences on macrobenthic communities in East Taihu Lake. This result is consistent with previous studies (Chatzinikolaou et al., 2018; Li et al., 2017; Zhang et al., 2023). For shallow lakes with large perennial water level fluctuations, water depth will mostly be the primary environmental influence factor for differences in the distribution of macrobenthic communities, and underwater transmitted light decreases with increasing depth of the water column, with a consequent decrease in macrobenthic densities and biomass, which is especially more pronounced for the Mollusca(e.g., Gastropoda and Lamellibranchia) (Cai et al., 2017). Deep water is more likely to be occupied by phytoplankton, which may sometimes even form blooms due to the input of nutrients and other pollutants, and the low-oxygen environment is more favorable for the survival of some macroinvertebrates (e.g., Oligochaetes, Chlamydia, etc.) with high stress tolerance (Li et al., 2017).
A similar pattern was presented in this study, in which the density and biomass of Bellamya aeruginosa and Radix Auricularia were significantly negatively correlated with water depth (p < 0.05), and other Mollusca species also had a negative correlation with water depth; whereas the density and biomass of the Oligochaeta, as well as that of Chironomus plumosus, showed a significant or highly significant positive correlation with water depth.
The relationship between water depth and macrobenthic species is significant, not only caused by water depth alone, but also due to how water depth varies between different seasons and functional areas, which leads to the regular changes of water transparency, aquatic plant growth and coverage, hydrodynamic conditions and dissolved oxygen. The distribution of macrobenthic animals is affected by various factors at the same time, mostly associated with water depth, and macrobenthos is a thermotropic animal preferring different optimal growth temperatures, and its growth, reproduction and metabolism are largely affected by water temperature, which is closely related to water depth (Arscott et al., 2003; Dudgeon, 1985; Mantelatto et al., 2022). In addition, the concentration of Chl-a and nitrogen and phosphorus of water are also important environmental factors affecting macrobenthos in East Taihu Lake. More information needs to be analyzed in relation to the correlation between macrobenthos and water environmental factors in different seasons.
3.2.2. RDA analysis of macrobenthos and environmental factors
Using the descending trend correspondence analysis (DCA) between the main environmental factors that have a greater impact on macrobenthos in the correlation analysis and the macrobenthic species ranked in the top 20 of IRI values, the gradient lengths of the four ranking axes were analyzed to be 3.02, 2.90, 1.88, and 1.55, which were all less than 4, and the maximum gradient length was close to 3. Therefore, the choice of redundancy analysis (RDA) to further reveal the effect of environment on macrobenthic communities.
The ordination plot between water environmental factors and each species and sample site is shown in Fig. 4., the length of the arrows represents the magnitude of the influence of environmental and species factors on the ordination, thus species located at the skewed edges of the ordination plot will respond to specific environmental factors (Shen et al., 2024). Species with large positive values in the first axis include mollusc species such as Bellamya purificata, Bellamya aeruginosa, Cipangopaludina ussuriensis, Bellamya quadrata, and Unio douglasiae, which are distributed in the functional areas with higher water transparency, shallower water depths, lower concentrations of TP, and lower levels of TSS, and the angles between the arrows of the above species are small, which indicates that the relationships between them and the water environmental factors have high similarity, which is in line with the results of the correlation heatmap analysis. In fact, all these species belong to the low and medium pollution-tolerant taxa, which indicates a better water quality environment, and the wetland sampling sites in each season are located in the positive direction of the first axis in the sample point ranking map, which indicates that Mollusca occur most frequently in this functional area and have the best water quality.
The genera of Limnodrilus and Tubifex were located in the second axis with the largest negative value, and the genera of Aulophorus tonkinensis belonging to the same Oligochaeta were also located in the same position, and they were all distributed in the functional areas with high Chl-a concentration, and at the same time, in the same area in the sample sorting map were the conservation area and the original enclosure aquaculture area, which was in line with the high Chl-a concentration in the water bodies of these two functional areas. Chironomus plumosus was located in similar positions and distributed in waters with deeper water depths and high concentrations of TP and TN, same as Pristina longiseta and Aeolosoma hemprichii, which belong to the Oligochaeta. These species with high fouling tolerance values prefer to live in low oxygen environments of deeper water depth, which can be indicative of a more severe eutrophication status of the water body (Gao et al., 2023). Organic matter provides an important food source and shelter for macrobenthos, so the organic matter content of habitats is crucial for macrobenthic community structure (Shen et al., 2024). It has been shown that the removal of organic matter will lead to a significant decrease in the abundance of Chironomidae Diptera in freshwater ecosystems, such as rivers, and a significant change in the community structure (Entrekin et al., 2007). However, higher organic matter content will be accompanied by a decrease in habitat pH, water transparency and dissolved oxygen, and an inhibitory effect on the growth of macrobenthos (Luoto et al., 2016). Cai also found that in Zhushan Bay, Meiliang Bay and estuarine areas of Taihu Lake, which are heavily polluted and highly eutrophic, the substrate is silt-clay and high in organic matter, and Limnodrilus hoffmeisteri is the main dominant macrobenthic fauna (Cai et al., 2010).
Alphabetical labels: Bellamya purificata Be.p, Limnodrilus Li, Tubifex Tu, Chironomus plumosus Ch, Bellamya aeruginosa Be.a, Polymoe Po, Unio douglasiae Un, Pristina longiseta Pr, E.modestus E.mo, Cipangopaludina ussuriensis Ci.u, Cipangopaludina chinensis Ci.c, Corbicula fluminea Co, Bellamya quadrata Be.q, Aulophorus tonkinensis Au, Radix auricularia Ra, Cipangopaludina ampulliformis Ci.a, N.denticulata N.de, Aeolosoma hemprichii Ae, Macrobrachium Mac, Cuneopsis heudei Cu.
3.3. Response of macrobenthos in East Taihu Lake to changes in different habitats
In order to further analyze and verify the effects of various environmental factors on macrobenthos in East Taihu Lake, the sediments of the three most representative functional areas of the entrance area, the original enclosure aquaculture area and the wetland area were selected as the substrate environment, and the raw water of East Taihu Lake as the aqueous environment. Three groups of parallel experiments were set up in each functional area to form a micro-ecosystem with no aquatic plants, submerged plants, submerged plants and floating leaf plants coexisting, to investigate the response of macrobenthos to the environmental changes in different habitats in each experimental group.
The macrobenthos included Bellamya (Bellamya purificata, Bellamya aeruginosa) and Radix auricularia in Mollusca, Limnodrilus, Tubifex, and other detected annelid species in Annelida, as well as Chironomus plumosus in Arthropoda. These macrobenthic species in the three phyla were selected as the relatively dominant and representative species found in the field sampling of East Taihu Lake, with Bellamya purificata being the first species in the relative importance index. The responses of these macrobenthos to environmental changes in different functional areas can effectively verify the correlation between macrobenthos and environmental factors in East Taihu Lake.
3.3.1 Physical and chemical indicators of water bodies and sediments
The changes of Chl-a concentration in the system of each experimental group during the 15th, 30th, 45th and 60th time periods of the experimental period are shown in Fig. 5. Since the experimental light intensity was not as sufficient as under natural conditions, the Chl-a concentration in the system of each experimental group was not high, and only diatoms grew, but still showed a certain pattern. Submerged plants can inhibit algae by secreting chemosensory chemicals and also indirectly by reducing sediment resuspension and increasing water transparency (Liu et al., 2024). The overall Chl-a concentration within the three experimental environmental groups in the entrance area, the original enclosure aquaculture area and the wetland area all showed that the no-plant group < submerged plant group < submerged × floating plant group. In the early stage of the experiment (15 d), the Chl-a concentrations in the two treatment groups with aquatic plant growth were similar and significantly higher than those in the no-plant group. In the middle and late stages (30 d ~ 60 d) under constant water replenishment, the Chl-a concentrations of the three functional areas without aquatic plants treatment groups were all in an increasing trend, but the increase was not large, and the concentrations were still lower than 10 ug/L; the Chl-a concentrations of the three submerged plant treatment groups were significantly lower than those in the early stage (p < 0.05), and stabilized in the range of 8.30 ug/L − 9.82 ug/L; the Chl-a concentrations of the three submerged × floating plant treatment groups Chl-a concentrations had a decreasing trend but was not obvious, so the Chl-a concentration of the submerged plant treatment group was more similar to that of the no-plant group in the late stage of the experiment and had a greater difference from that of the submerged × floating plant group.
Chl-a concentration can characterize phytoplankton biomass to a certain extent and is also an important component of organic suspended particulate matter in the water column. Aquatic plants can directly absorb nutrients from the lake, and also regulate the microbial community through exudation from their roots and further influence nitrification (Yin et al., 2020), thus the experimental group with aquatic plant growth had less total suspended particulate matter than the group without plants, and the water body had lower turbidity (Table 5) and higher transparency. Aquatic plants can inhibit the growth of algae to a certain extent, and under natural conditions, when aquatic plants decline, it will lead to an increase in the Chl-a concentration in the water body; but on the other hand, having aquatic plant growth will increase the percentage of organic suspended particulate matter in the water body, which is greater than that of inorganic particulate matter. In this experiment, the second process was more obvious due to the insufficient light conditions, which were not favorable for algal bloom growth. Meanwhile, scrapers have an inhibitory effect on algae, which can effectively remove suspended particles from the water body and improve water transparency (Waajen et al., 2016; Y. Zhang et al., 2023). Overall, the Chl-a concentration was similar between the same aquatic plant treatment groups in the three functional areas in this experiment, while there was a significant difference in Chl-a concentration between the different aquatic plant treatment groups, suggesting that there is a close relationship between Chl-a concentration and aquatic plants.
Table 5
Physicochemical indicators of water and sediment in the final state for each experimental group.
Experimental group | Water index | Sediments |
NTU | TN (mg/L) | NO3−-N (mg/L) | TP (mg/L) | PO43−-P (mg/L) | STN (mg/kg) | STP (mg/kg) |
S1་no-plant | 9.64 | 7.37 | 7.31 | 0.06 | 0.03 | 195.07 | 75.18 |
S1་submerged plant | 1.89 | 6.79 | 5.75 | 0.29 | 0.25 | 351.38 | 69.92 |
S1་submerged×floating plant | 1.51 | 4.26 | 3.71 | 0.34 | 0.28 | 361.63 | 73.73 |
S4་no-plant | 18.49 | 9.99 | 9.90 | 0.04 | 0.03 | 1562.80 | 136.12 |
S4་submerged plant | 5.23 | 9.11 | 7.43 | 0.03 | 0.02 | 1529.64 | 120.74 |
S4་submerged×floating plant | 3.22 | 7.39 | 5.26 | 0.24 | 0.13 | 1341.94 | 98.14 |
S5་no-plant | 8.25 | 9.37 | 8.28 | 0.03 | 0.00 | 249.82 | 49.10 |
S5་submerged plant | 2.70 | 7.65 | 7.40 | 0.51 | 0.14 | 512.47 | 51.49 |
S5་submerged×floating plant | 1.97 | 5.65 | 5.18 | 0.61 | 0.30 | 443.83 | 50.74 |
TN in the water column of the experimental system was mainly in the form of nitrate nitrogen (NO3−-N). Nitrogen concentrations in the water column were similar overall in the original enclosure aquaculture area and the wetland area, while the entrance area was slightly lower overall than the original enclosure aquaculture area and the wetland area. Within the experimental groups of each functional area, the concentrations of TN and nitrate nitrogen in the three treatment groups of no plant, submerged plant, and submerged × floating plant decreased sequentially, indicating that the uptake of nutrients by aquatic plants in the experimental system mainly manifested itself as the uptake of nitrogen in the water column. While TP and PO43−-P concentrations in each functional area showed that the group with plants was significantly higher than the group without plants, which may be the reason that Chl-a concentration was higher in the group with plants, and higher phosphorus concentration was more suitable for the growth of algae.
Unlike the large differences in water body nitrogen and phosphorus indicators in different aquatic plant treatment groups, sediment TN and TP contents were significantly different among the three functional zones, with sediment nitrogen and phosphorus contents in the original enclosure aquaculture area significantly higher than those in the entrance area and the wetland area (p < 0.05), and higher in the wetland area than those in the entrance area, and the sediment nitrogen and phosphorus contents were basically similar between the treatment groups with and without aquatic plants in all the functional areas with no significant differences.
The nitrogen and phosphorus indices of the water body in the final state of the experiment were higher than that of the raw water of East Taihu Lake used in the experiment at the initial time, while the sediment TN and TP were on the contrary, with the concentration of the sediment nitrogen and phosphorus in the final state lower than that measured at the initial time. The reason for this may be due to the fact that the water movement caused by the oxygenation device promoted the release of nitrogen and phosphorus from the sediments in the relatively small-scale closed experimental environment. While in natural lakes, the input of organic matter occurs continuously due to phytoplankton growth (Zhang et al., 2024).
3.3.2. Density change of macrobenthos and correlation analysis of environmental factors
(1) Changes in the density of various macrobenthic fauna
The densities of Bellamya, Radix auricularia, Annelida (mainly composed of Limnodrilus and Tubifex) and Chironomus plumosus observed and counted in the final state of the experiment are shown in Fig. 6. The densities of Bellamya, except for the submerged × floating plant group in the entrance area where the densities were significantly lower due to higher mortality rates, varied between 188.64 ind/m2 and 264.10 ind/m2 in each group. The biomass of Bellamya was absolutely dominant among the benthos of all experimental groups, while the density was only maximum in the submerged × floating plant group in the entrance and wetland areas, and there was no absolute dominance. The density of Radix auricularia was higher in the group with aquatic plants than in the group without, and its density peaked (877.7 ind/m2) in the group of the submerged × floating plant coexisting in the original enclosure aquaculture area. Its density peaked at 877.19 ind/m2 in the submerged × floating plant group in the wetland area and was the second highest (377.29 ind/m2). The density distribution of Chironomus plumosus and Radix auricularia showed some similarity, and Chironomus plumosus were observed only in the submerged × floating plant group in the entrance area and the original enclosure aquaculture area, which also showed some dependence on aquatic plants.
The density of the Annelida in each experimental group was original enclosure aquaculture area>entrance area>wetland area, and the density of the original enclosure aquaculture area without plant treatment group was the highest, which was 1490.28 ind/m2. The density of the Annelida in each experimental group in each functional area was significantly higher than that of the treatment group with plant growth, and the root growth of aquatic plants damaged the anaerobic environment at the sediment-water interface and within the sediments, increasing the dissolved oxygen concentration in the water and improving the water transparency. The root growth of aquatic plants destroyed the anaerobic environment at the sediment-water interface and inside the sediments, increased the concentration of dissolved oxygen in the water, and at the same time improved the transparency of the water body and the water quality (Li et al., 2024; Yan et al., 2022; Zhang et al., 2014). The higher density of Annelida, which are fouling-tolerant species adapted to polluted water and anaerobic environments, was consistent with the results of the benthic field collection and analysis, and further verified their environmental adaptation characteristics.
The difference in density of the four benthic species in the no-plant group in the entrance area was distinct, and the density of Annelida was dominant, only the difference in density of the four benthic species in the submerged plan group was reduced, and the difference in density in the submerged × floating plant group was the smallest, due to the diversity of aquatic plants, which provided a complex habitat for the survival of benthic animals, and the benthic animals detected in a single habitat were more diverse, and the density distribution was more uniform. In the original enclosure aquaculture area, the density of benthos varied greatly among the three treatment groups, with Annelida being the dominant species in the no-plant and submerged plant groups, and Radix auricularia dominating in the submerged × floating plant group. On the contrary, there was little difference in the density of each benthic fauna in all three treatment groups in the wetland area, and none of them had a clear density-dominant species.
(2) Correlation analysis of environmental factors
The correlation heatmaps of the pre- and post-experimental rates of change in biomass, final survival and final density of Bellamya and Radix auricularia, and the final density of Annelida with the measured environmental factors are shown in Fig. 7. The three indices, rate of change in biomass, survival and final density of Radix auricularia, were all significantly and positively correlated (p < 0.05) with the Chl-a concentration, while they were negatively or significantly negatively correlated with the TN and NO3−-N concentrations which relate to pollution(Xie et al., 2016). The Chl-a concentration in the water column was closely related to the presence or absence of aquatic plants, and there was no significant difference in the Chl-a concentration among the same aquatic plant treatment groups in the three functional areas in this experiment, while there was a significant difference in the Chl-a concentration among the different aquatic plant treatment groups, which were as follows: no-plant group<submerged plant group<submerged × floating plant group. The growth condition of Radix auricularia was better in the treatment group with aquatic plant growth than in the no-plant group, especially in the original enclosure aquaculture and wetland areas where the biomass and density of the submerged × floating plant treatment group increased dramatically, and the survival rate was also the highest, thus showing a significant positive correlation with the Chl-a concentration in the water column. The relationship between aqutic plant and Radix auricularia with herbivory also play an important role on this phenomenon(Lv et al., 2019). The water column TN and NO3−-N concentrations were also closely related to the presence or absence of aquatic plant distribution, and the TN and NO3−-N concentrations in the treatment group with aquatic plants were reduced due to plant uptake, and thus negatively correlated with the growth status of Radix auricularia.
Bellamya grew better in most of the experimental groups, and densities also did not change significantly in any of the experimental groups, except for the submerged × floating plant group in the entrance area, so the correlations between the three indicators and the environmental factors were not significant, in line with its high environmental adaptability. The density of Annelida was significantly correlated with both the water and sediment environments, and was significantly higher in the no-plant group than in the planted group. The Chl-a concentration in the water body was low in the no-plant group, and the turbidity was significantly higher than that in the planted group, so the density of Annelida was significantly positively correlated with turbidity (p < 0.05) and negatively correlated with Chl-a. The nitrogen concentration in the water body was significantly lower in the planted group than that in the no-plant group, while the concentration of TP and PO43−-P were higher in the planted group, thus, the density of Annelida was positively correlated with the concentration of nitrogen, and negatively correlated with the concentration of TP and PO43−-P. PO43−-P concentration was significantly negatively correlated. Sediment nitrogen and phosphorus content also affected the density of Annelida significantly. The sediment nitrogen and phosphorus content in the original enclosure aquaculture area was significantly higher than that in the entrance and wetland areas, and the density of Annelida was also the highest in the original enclosure aquaculture area, and at the same time, the substrate particle size in the original enclosure aquaculture area was relatively small, so the density of Annelida in the original enclosure aquaculture area was the highest in the group of the treatment without plants.