Environmental variables influencing phytoplankton communities in hydrologically connected aquatic habitats in the Lake Xingkai basin

Eukaryotic plankton community and assembly processes in a large-scale water diversion project in China

The Middle Route of the South to North Water Diversion Project (MRP) and its water source, the Danjiangkou Reservoir (DJK), play a pivotal role in mitigating the chronic water scarcity challenges faced by northern China. Eukaryotic plankton are widespread in aquatic ecosystems, which are crucial for the water quality stability of DJK and MRP, yet comparative studies on their contemporaneous dynamics and assembly processes are scarce. In this study, amplicon sequencing was used to investigate the eukaryotic plankton communities. The results revealed that the similarity in community composition of DJK is significantly higher than that of MRP, exhibiting distance-decay patterns. Environmental heterogeneity exhibits significant differences between DJK and MRP, and it significantly influences community composition and alpha diversity. Additionally, the assembly processes of eukaryotic plankton in both DJK and MRP are predominantly influenced by stochastic processes. However, in comparison to DJK, deterministic processes have a more pronounced impact on MRP, accounting for 39.29% and 1.82%, respectively. The variations in total nitrogen (TN), chlorophy IIa (Chl.a), and conductivity (Spc) have led to a transition in the assembly of eukaryotic phytoplankton communities in MRP from a stochastic process to a deterministic process. This study extends insights into the dynamics and assembly processes of eukaryotic plankton communities in the large, engineered drinking water diversion project and its water source, which is also useful for the management and regulation of the DJK and MRP.

Environmental and hydrological synergies shaping phytoplankton diversity in the Hetao irrigation district

Phytoplankton are crucial primary producers in freshwater ecosystems, driving matter and energy flow across trophic levels, essential for biodiversity and ecological balance. Most research emphasizes environmental factors shaping their diversity, while the role of hydrological connectivity remains poorly understood. This study collected 81 phytoplankton samples from the Hetao Irrigation District along a gradient from upstream to downstream and utilized high-throughput sequencing to evaluate the spatial distribution patterns of phytoplankton diversity. The study analyzed the impacts of environmental factors, hydrological connectivity (water surface ratio, Wp), and human activities (land-use intensity, LUI) on phytoplankton diversity. The results revealed that the phytoplankton community comprised 9 phyla, 158 families, 378 genera, and 1189 species. There were significant differences in phytoplankton diversity among different water bodies, with a gradual increase in phytoplankton diversity from west to east across the five major irrigation areas. Lake Ulansuhai had relatively low phytoplankton diversity. The ASV number, Chao1 index, and ACE index showed significant positive correlations with dissolved oxygen (DO), pH, and water temperature (WT). The Shannon index and Pielou'e evenness (Pielou_e) index showed significant positive correlations with the water surface ratio (Wp). The partial least squares model indicated that environmental factors directly influenced phytoplankton diversity. Hydrological connectivity indirectly affected phytoplankton diversity by altering environmental factors. We emphasize that hydrological connectivity is as important as environmental factors in driving phytoplankton diversity in the Hetao Irrigation District. This study provides key insights for water quality assessment and biodiversity conservation in the region.

Precipitation input increases biodiversity of planktonic communities in the Qinghai-Tibet Plateau

Planktonic communities in aquatic ecosystems are crucial water quality indicators, with their growth dependent on runoff chemical and hydraulic characteristics (e.g., nutrient availability and turbidity). Previous studies have indicated that runoff components (i.e., proportions of precipitation, groundwater, snowmelt, etc.) play a vital role in regulating runoff characteristics, potentially affecting planktonic communities. However, the response of these communities to runoff components, particularly in mountainous regions, remains underexplored. In this study, we conducted four sampling campaigns from 2017 to 2020 in a watershed on the Qinghai-Tibet Plateau. Combined with laboratory incubation experiments, we examined the impact of various runoff components on the diversity and abundance of phytoplankton and zooplankton. We found that a higher proportion of precipitation in runoff contributed to an increase in the diversity of plankton communities. Laboratory experiments with unified water samples incubated with different runoff components demonstrated that the significant influence of precipitation on planktonic diversity primarily stems from the influx of abundant exogenous particulate material into rivers. Using a path analysis, we further confirmed that the impact of precipitation on diversity is primarily through chemical pathways, notably by increasing nutrient concentrations. Our study enhances our understanding of the interactions between the hydrological cycle and aquatic ecosystems, offering valuable insights for effectively maintaining and managing these natural environments.

Ecological assessment of water quality in freshwater wetlands based on the effect of environmental heterogeneity on phytoplankton communities in Northeast China

Phytoplankton community characterized by strong vitality response to environmental change in freshwater ecosystems. This study aims to evaluate the effectiveness of using phytoplankton diversity as a water quality indicator in wetlands, and find out the main environmental variables affecting the distribution of phytoplankton. From 2020 to 2021, we examined phytoplankton assemblages and water environmental variables in spring, summer, and autumn at eight sampling sites from Hulanhe Wetland, Northeast (NE) China. The results showed that Bacillariophyta was the dominant species. Phytoplankton composition and abundance differed among sampling sites in each season; the abundance in summer (613.71 × 104 ind. L-1) was higher than that in autumn and spring. The water quality assessment of the trophic state index (TSI) based on the four physicochemical indicators was compared with phytoplankton diversity indices, which indicated that the phytoplankton community was stable, and these two indices were significantly lower in summer than in spring and autumn. According to redundancy analysis (RDA), total phosphorus (TP) and nitrogen (TN) were the main environmental variables affecting the distribution of phytoplankton. Temperature and dissolved oxygen (DO) changes also played a role, and their impact on the community was discussed. This work can provide relevant scientific references on the usefulness of phytoplankton diversity structure in assessing water quality in cold regions, in which the succession can be significantly affected by nutrients and temperatures.

Linking seasonal plankton succession and cellular trace metal dynamics in marine assemblages

Factors affecting trace metal dynamics in marine plankton still need to be fully understood. Underlying mechanisms affecting cellular metal distribution, seasonal changes, and the influence of plankton community structure are poorly explored. This study comprehensively analyzed the seasonal changes in environmental factors, plankton community structure, and their impact on plankton cellular metal dynamics. Plankton samples were isolated, and trace metals (Cr, Mn, Fe, Co, Ni, Cu, As, Cd, Hg, and Pb) were analyzed with an inductively coupled plasma mass spectrometer (ICP-MS). Plankton community structure significantly changed with seasons (p < 0.05), which were mainly driven by temperature (seasonal change) and nutrients (eutrophication). Mean plankton cellular trace metals did not significantly change (p > 0.05) in the study area but were higher along estuaries likely due to differences in metal influx from rivers. However, their distribution patterns significantly differ between the wet and dry seasons, likely influenced by the changes in community structure and anthropogenic influx. Cellular trace metals, particularly in phytoplankton, strongly correlated with selected species suggesting the impacts of community structure in trace metal distribution. Hence, the influence of environmental factors in driving plankton succession may have caused a ripple effect on cellular trace metal distribution, especially in phytoplankton. However, both blooming species Skeletonema and Chaetoceros (diatoms) showed a contrasting relationship with cellular metals, suggesting the cooccurrence of bioaccumulation or biodilution mechanisms. This study shows the potential influence of community structure in cellular trace metal dynamics for marine plankton assemblages. However, more than plankton abundance and functional diversity, i.e., species diversity, might be needed to assess the community-level impacts on cellular metals.

Spatial and Temporal Characteristics of Phytoplankton Communities in Drinking Water Source Reservoirs in Shenzhen, China

Phytoplankton diversity and community characteristics are closely associated with aquatic environmental factors. Understanding these dynamics can provide insights into the ecological health of water bodies. We investigate the spatial and temporal characteristics of phytoplankton communities in 27 drinking water source reservoirs in Shenzhen, China. As a method, we collected samples during the dry season in 2021 and the wet season in 2022, analyzed the alpha and beta diversities of phytoplankton communities, and correlated these with the environmental factors. The results reveal that Cyanobacteria dominate the phytoplankton communities in the Shenzhen reservoirs. Phytoplankton diversity is greater during the dry season. The algal composition varies spatially, and the phytoplankton diversity tends to decrease with increasing eutrophication. A co-occurrence network analysis indicates denser and stronger correlations among phytoplankton nodes during the wet season than dry season. Reservoirs with moderate eutrophication levels exhibit denser nodes and stronger correlations compared to those with low or high eutrophication levels. The chemical oxygen demand, water temperature, pH, and total nitrogen are identified as key influencers of the phytoplankton community structure. Our results contribute to the enhanced understanding of the spatial and temporal dynamics of phytoplankton communities in reservoirs in South China and provides insights into the management and conservation of these drinking water reservoirs.

Dissimilatory iron reduction contributes to anaerobic mineralization of sediment in a shallow transboundary lake

Dissimilatory iron reduction (DIR) coupled with carbon cycling is increasingly being recognized as an influential process in freshwater wetland soils and sediments. The role of DIR in organic matter (OM) mineralization, however, is still largely unknown in lake sediment environments. In this study, we clarified rates and pathways of OM mineralization in two shallow lakes with seasonal hydrological connectivity and different eutrophic situations. We found that in comparison with the domination of DIR (55%) for OM mineralization in Lake Xiaoxingkai, the contribution of methanogenesis was much higher (68%) in its connected lake (Lake Xingkai). The differences in rates and pathways of sediment OM mineralization between the two lakes were attributed to higher concentrations of carbonate associated iron oxides (Fecarb) in Lake Xiaoxingkai compared to Lake Xingkai (P = 0.002), due to better deposition mixing, more contributions of terrigenous detrital materials, and higher OM content in Lake Xiaoxingkai. Results of structural equation modeling showed that Fecarb and total iron content (TFe) regulated 25% of DIR in Lake Xiaoxingkai and 76% in Lake Xingkai, accompanied by a negative effect of TFe on methanogenesis in Lake Xingkai. The relative abundance and diversity of Fe-reducing bacteria were significantly different between the two lakes, and showed a weak effect on sediment OM mineralization. Our findings emphasize the role of iron minerals and geochemical characterizations in regulating rates and pathways of OM mineralization, and deepen the understanding of carbon cycling in lake sediments.

Effects of Summer and Autumn Drought on Eutrophication and the Phytoplankton Community in Dongting Lake in 2022

Since July 2022, the Yangtze River basin has experienced the most severe hydro-meteorological drought since record collection started in 1961, which has greatly affected the ecological environment of the Dongting Lake (DTL) basin. To investigate the effects of drought events on the eutrophication and phytoplankton community structure of DTL, the lake was sampled twice in August and September 2022 based on the water level fluctuations resulting in 47 samples. Furthermore, we combined the comprehensive trophic level index (TLI) and phytoplankton Shannon-Wiener diversity index (H) to characterize and evaluate the eutrophication status. The key influencing factors of the phytoplankton community were identified using redundancy analysis (RDA), hierarchical partitioning, and the Jaccard similarity index (J). Our results showed that the TLI of DTL changed from light-moderate eutrophication status (August) to mesotrophic status (September), whereas the H changed from light or no pollution to medium pollution. The phytoplankton abundance in August (122.06 × 104 cells/L) was less than that in September (351.18 × 104 cells/L) in DTL. A trend in phytoplankton community succession from Bacillariophyta to Chlorophyta and Cyanophyta was shown. The combination of physiochemical and ecological assessment more accurately characterized the true eutrophic status of the aquatic ecosystem. The RDA showed that the key influencing factors in the phytoplankton community were water temperature (WT), pH, nitrogen and phosphorus nutrients, and the permanganate index (CODMn) in August, while dissolved oxygen (DO) and redox potential (ORP) were the key factors in September. Hierarchical partitioning further indicated that temporal and spatial variations had a greater impact on the phytoplankton community. And the J of each region was slightly similar and very dissimilar, from August to September, which indicated a decreased hydrological connectivity of DTL during drought. These analyses indicated that the risk to the water ecology of DTL intensified during the summer-autumn drought in 2022. Safeguarding hydrological connectivity in the DTL region is a prerequisite for promoting energy flow, material cycle, and water ecosystem health.

Human impact on current environmental state in Chinese lakes

Anthropogenic and natural disturbance to inland aquatic ecosystems displays a notable spatial difference, yet data to measure these differences are scarce. This study encompasses 217 lakes distributed over five lake regions of China and elucidates the environmental factors determining the spatial variability of the water quality and trophic status. A significant correlation between human modification index in surrounding terrestrial systems (HMT) and trophic status of lake ecosystems (TSI) was found, and the regression slope in each region was similar except in the Qinghai-Tibet Plateau region. It was further noted that the pattern of environmental factor network (EF network) differed among freshwater and saline lakes. The EF network was complex for freshwater lakes in less human-influenced areas, but intensive man-made influence disrupted most relationships except for those between total nitrogen, total phosphorus, chlorophyll-a, and water turbidity. As for regions including saline lakes, correlations among water salinity and organic forms of carbon and nitrogen were apparent. Our results suggest that HMT and EF network can be useful indicators of the ecological integrity of local lake ecosystems, and integrating spatial information on a large scale provides conservation planners the option for evaluating the potential risk on inland aquatic systems.

pH Drives Differences in Bacterial Community β-Diversity in Hydrologically Connected Lake Sediments

As microorganisms are very sensitive to changes in the lake environment, a comprehensive and systematic understanding of the structure and diversity of lake sediment microbial communities can provide feedback on sediment status and lake ecosystem protection. Xiao Xingkai Lake (XXL) and Xingkai Lake (XL) are two neighboring lakes hydrologically connected by a gate and dam, with extensive agricultural practices and other human activities existing in the surrounding area. In view of this, we selected XXL and XL as the study area and divided the area into three regions (XXLR, XXLD, and XLD) according to different hydrological conditions. We investigated the physicochemical properties of surface sediments in different regions and the structure and diversity of bacterial communities using high-throughput sequencing. The results showed that various nutrients (nitrogen, phosphorus) and carbon (DOC, LOC, TC) were significantly enriched in the XXLD region. Proteobacteria, Firmicutes, and Bacteroidetes were the dominant bacterial phyla in the sediments, accounting for more than 60% of the entire community in all regions. Non-metric multidimensional scaling analysis and analysis of similarities confirmed that β-diversity varied among different regions. In addition, the assembly of bacterial communities was dominated by a heterogeneous selection in different regions, indicating the important influence of sediment environmental factors on the community. Among these sediment properties, the partial least squares path analysis revealed that pH was the best predictor variable driving differences in bacterial communities in different regions, with higher pH reducing beta diversity among communities. Overall, our study focused on the structure and diversity of bacterial communities in lake sediments of the Xingkai Lake basin and revealed that high pH causes the β-diversity of bacterial communities in the sediment to decrease. This provides a reference for further studies on sediment microorganisms in the Xingkai Lake basin in the future.

Improving the qualities of the trophic magnification factors (TMFs): A case study based on scaled Δ15N trophic position framework and separate baseline species

Scientific understanding of trophic magnification factors (TMFs) is conducive to formulating environmental management measures. Trophic position (TP) of species is the key parameter in TMFs assessment. Nitrogen stable isotopes (δ¹⁵N) provide a powerful tool to estimate TP. However, some limitations could introduce considerable uncertainty into TP and TMFs assessment which mainly includes: 1) determination of Δ¹⁵N between two adjacent trophic positions; 2) determination of baseline species. Different from the widely used constant Δ¹⁵N (3.4 ‰) between two adjacent trophic positions, which is called additive Δ¹⁵N framework, Δ¹⁵N gradually decreases as trophic position increases under scaled Δ¹⁵N framework, which has been confirmed by more and more laboratory studies and meta-analyses. In this study, we sampled in two similar littoral ecosystems separated by one natural dam, which is called Small Xingkai Lake and Xingkai Lake, analyzed the δ¹⁵N and total mercury (THg) of each species. On the one hand, we compared the TP of species under the additive Δ¹⁵N framework and scaled Δ¹⁵N framework with the White shrimp (Exopalaemon modestus) as baseline species in two lakes respectively. On the other hand, we explored the possible changes in TMFs based on TP. Our results show, under the scaled Δ¹⁵N framework, the trophic position of the same species is higher, while TMFs is lower compared with the additive Δ¹⁵N framework; even if in the two interconnected lakes, distributed the same baseline species, in the similar ecosystem, separate baselines should also be used. In this study, two frameworks of the food chain were compared in two interconnected freshwater ecosystems for the first time. The difference between TMFs of two lakes was obvious under scaled framework but not under additive framework. We also recommend that future TMFs assessments should be based on the scaled Δ¹⁵N framework because it has improved the accuracy of trophic position assessment.

Elemental composition and microbial community differences between wastewater treatment plant effluent and local natural surface water: A Zhengzhou city study

Wastewater treatment plant (WWTP) effluent is discharged into rivers as supplemental water, which may result in ecological risk. This study compares the element composition and microbial community of WWTP effluent and natural surface water (NSW) and reveals the potential ecological risk of WWTP effluent discharge. Twenty recently upgraded WWTPs and three relatively large reservoirs in Zhengzhou city, China, were selected. The contents of N, P, S, K, Ca, Mg, B, Si, Na, Fe, Mn, Co, Ni and Sn were significantly higher in WWTP effluent than in NSW, while those of Mo, V, Pb and Cd were significantly lower. There was no significant difference between WWTP effluent and NSW in terms of the element imbalance index (IMI) (representing the extent of imbalance of element proportions) relative to the class IV surface water quality standard (the control standard for most Chinese rivers). The macronutrient IMI relative to the Hoagland formula was significantly lower in WWTP effluent than in NSW, and WWTP effluent discharge could significantly lower this index in NSW; this may be an important cause of primary productivity explosion. The microbial diversity was significantly higher in WWTP effluent than in NSW. The predicted relative abundances of mobile genetic elements and oxidative-stress-tolerant phenotypes were significantly higher in WWTP effluent than in NSW, whereas the abundance of gram-negative phenotypes was significantly lower, and that of potential pathogenic phenotypes was slightly lower. The effluent from upgraded WWTPs exhibited a low risk of pathogen diffusion but a high risk of antibiotic resistance gene diffusion. The element composition and microbial community should be considered when evaluating the ecological risk of WWTP effluent discharge.

Historical reconstruction of aquatic vegetation of typical lakes in Northeast China based on an improved CA-Markov model

Aquatic vegetation is an important marker of the change in the lake ecosystem. It plays an important supporting role in the lake ecosystem, and its abundance and cover changes affect the ecosystem balance. Collecting accurate long-term distribution data on aquatic vegetation can help monitor the change in the lake ecosystem, thereby providing scientific support for efforts to maintain the balance of the ecosystem. This work aimed to establish an improved CA-Markov model to reconstruct historical potential distribution of aquatic vegetation in the two typical lakes (Xingkai Lake and Hulun Lake) in Northeast China during 1950s to 1960s. We firstly analyzed remote sensing data on the spatial distribution of aquatic vegetation data in two lakes in six periods from the 1970 to 2015. Then, we built a transfer probability matrix for changes in hydrothermal conditions (temperature and precipitation) based on similar periods, and we designed suitability images using the spatial frequency and temporal continuity of the constraints. Finally, we established an improved CA-Markov model based on the transfer probability matrix and suitability images to reconstruct the potential distributions of aquatic vegetation in the two northeastern lakes during the 1950s and 1960s. The results showed the areas of aquatic vegetation in the 1950s and 1960s were 102.37 km2 and 100.7 km2 for Xingkai Lake and 90.81 km2 and 88.15 km2 for Hulun Lake, respectively. Compared with the traditional CA-Markov model, the overall accuracy of the improved model increased by more than 50%, which proved the improved CA-Markov model can be used to effectively reconstruct the historical potential distribution of aquatic vegetation. This study provides an accurate methodology for simulating the potential historical distributions of aquatic vegetation to enrich the study of the historical evolution of lake ecosystem.

Application of Phytoplankton Taxonomic α-Diversity Indices to Assess Trophic States in Barrier Lake: A Case of Jingpo Lake

Phytoplankton taxonomic α-diversity indices are useful tools to characterize the trophic states in freshwater ecosystems. However, the application of these indices to assess trophic states in large barrier lakes is rare, especially in China. To test the usefulness of phytoplankton taxonomic α-diversity indices in trophic state assessments, we investigated the taxonomic α-diversity-Comprehensive Trophic Level Index (TLI) relationships in the second largest alpine lava barrier lake (Jingpo Lake, China) in the rainy and dry season from 2017 to 2018. Based on a two-year dataset, we found that there was a significant difference in the phytoplankton community, α-diversity indices, and TLI dynamic between the rainy season and the dry season. First, there was significant variation in phytoplankton abundance, the Margalef index, and the Shannon-Wiener index in different hydrological periods (p < 0.05). Second, the mean TLI in the rainy season (44 ± 5) was higher than in the dry season (41 ± 5) (p < 0.05). Lastly, the response characteristics of the Margalef and Shannon-Wiener index with TLI were different in different hydrological periods, and the relationship between the Pielou evenness index and TLI was weak. This study highlights that phytoplankton taxonomic α-diversity indices are relevant tools in water quality assessments but selecting the fit index is necessary. The current study provides key information about phytoplankton community, α-diversity, and trophic states in the largest alpine lava barrier lake, and the results of the study will benefit water quality management and biodiversity conservation in barrier lakes.

Phytoplankton Community in Relation to Environmental Variables in the Tidal Mangrove Creeks of the Pasur River Estuary, Bangladesh

The Pasur River estuary (PRE) provides vital fishery resources and supports millions of livelihoods in the southwestern coastal region of Bangladesh. Our research focused on phytoplankton community assemblages, alpha diversity indices, and the seasonal succession of major phytoplankton species in relation to physicochemical parameters in the tidal mangrove creeks of the Pasur River estuary. Spatial and temporal variations were assessed by water sampling at 17 stations in the study area from January to December 2019. The mean salinity level in the tidal mangrove creeks of the PRE was significantly (p < 0.05) higher during the dry season than during the wet season. Spatially, no significant variation (p > 0.05) was observed in the dissolved inorganic nitrogen and dissolved inorganic phosphorus between PRE and mangrove creeks, but temporally, the variables varied significantly (p < 0.05). Spatially, no significant variation (p > 0.05) was observed in the alpha diversity of the phytoplankton community but significantly (p < 0.05) varied temporally. Blue-green algae became dominant in the oligohaline conditions during the wet season, while diatoms were dominant during the dry season which severely depleted dissolved silica. In terms of phytoplankton species diversity, our study classifies the study areas as highly diversified zones. Phytoplankton succession from diatoms (dry season) to blue-green algae (wet season) is attributed to the changes in the physicochemical and nutrient parameters depending on seasonal environmental parameter fluctuations. This study illustrated that phytoplankton diversity and density varied with the degrees of habitat and seasonal changes, implying the potential impacts of anthropogenic activities and natural causes on their community structure in tropical estuaries and mangrove creeks.

Phytoplankton community variation and ecological health assessment for impounded lakes along the eastern route of China's South-to-North Water Diversion Project

Interbasin water diversion projects have been proven to effectively alleviate water resource shortages in areas along water diversion lines, but few studies have focused on ecological health in impounded lakes compared with research on water quality and pollutants. Herein, monitoring data were collected during the nonwater diversion period (NWDP) and the water diversion period (WDP) from 2018 to 2019, and the index of biological integrity (IBI) method based on phytoplankton communities was used to evaluate the ecological health of the impounded lakes (Nansi Lake and Dongping Lake) along the eastern route of the South-to-North Water Diversion Project. The results demonstrated that water diversion improved the water quality of the impounded lakes during the WDP, especially total nitrogen and ammonia nitrogen. Meanwhile, the water diversion affected the phytoplankton community structure and diversity, and network analysis further revealed water diversion could be beneficial to the ecological health of impounded lakes. Furthermore, the P-IBI showed that the overall ecological health assessment was "good" during the WDP. Water diversion substantially improved the ecological health status and stability of the impounded lakes during the dry season. Finally, the direct correlations between the water quality parameters and the P-IBI were weak, and water quality parameters could indirectly affect the P-IBI by changing the phytoplankton community structure. These findings will enhance our understanding of the ecological health of the impounded lakes of the South-to-North Water Diversion Project. Furthermore, this study will provide a reference to support the ecosystem security of impounded lakes in other large water diversion projects.

Spatiotemporal variation and ecological risk assessment of sediment heavy metals in two hydrologically connected lakes

Excessive accumulation of heavy metals in global lake sediments poses a serious threat to lake water quality and ecosystem security. However, there is still a knowledge gap in comparison of heavy metal variation and pollution in hydrologically connected lakes. In this study, concentrations of As, Cd, Cr, Cu, Hg, Pb, and Zn in sediments of two hydrologically connected lakes, Xingkai Lake and Xiaoxingkai Lake, were determined during the hydrologically connected periods (May and September) and disconnected period (January and July) in 2021. We found the range of As was 2.58∼14.35 mg/kg, Cd was 0.050∼0.21 mg/kg, Cr was 28.58∼262.3 mg/kg, Cu was 3.12∼28.05 mg/kg, Hg was 0.0030∼0.14 mg/kg, Pb was 10.87∼58.86 mg/kg, and Zn was 18.21∼90.73 mg/kg. Heavy metal concentrations were lower than grade I level in Chinese soil quality standards with significant spatial and temporal differences in the basin. Overall, most of the sampling sites in Xingkai Lake and Xiaoxingkai Lake were at the uncontaminated level and moderate ecological risk during the sampling period. Two lakes showed different heavy metal compositions, accompanied by higher contamination level and higher potential ecological risk in the small lake than those in the large lake based on analysis of the geo-accumulation index and potential ecological risk index. Besides, the contamination level and potential ecological risk in May and September were higher than those in January and July, mainly due to human activities and hydrological connectivity. The ecological risks were moderate for Cd and Hg, and low for As, Cr, Cu, Pb, and Zn. Correlation and PCA analyses showed that Cd mainly originated from anthropogenic sources, while other metals mainly came from natural sources. These findings elucidate the effects of agriculture and hydrological connectivity on heavy metals in sediments, and provide scientific basis for the reasonable management of lake ecosystem.

Ecological health evaluation of rivers based on phytoplankton biological integrity index and water quality index on the impact of anthropogenic pollution: A case of Ashi River Basin

Based on the phytoplankton community matrices in the Ashi River Basin (ASRB), Harbin city, we developed an evaluation method using the phytoplankton index of biotic integrity (P-IBI) to evaluate ecological health while investigating the response of P-IBI to anthropogenic activities. We compared the effectiveness of P-IBI with that of the water quality index (WQI) in assessing ecological health. Between April and October 2019, phytoplankton and water samples were collected at 17 sampling sites in the ASRB on a seasonal basis. Our results showed that seven phyla were identified, comprising 137 phytoplankton species. From a pool of 35 candidate indices, five critical ecological indices (Shannon–Wiener index, total biomass, percentage of motile diatoms, percentage of stipitate diatom, and diatom quotient) were selected to evaluate the biological integrity of phytoplankton in the ASRB. The ecological status of the ASRB as measured by the P-IBI and WQI exhibited a similar spatial pattern. It showed a spatial decline in ecological status in accordance with the flow of the river. These results highlighted that P-IBI was a reliable tool to indicate the interaction between habitat conditions and environmental factors in the ASRB. Our findings contribute to the ecological monitoring and protection of rivers impacted by anthropogenic pollution.

Comparison of the Intestinal Structure and Intestinal Microbiome between Two Geographically Isolated Populations of Culter alburnus

Geographical populations of Culter alburnus inhabiting different regions of China present substantial differences in their reproduction and development characters. This study compared the intestinal structures, digestive enzyme activities, and intestinal microbiomes in the Xingkai (XK) Lake and the Dianshan (DS) Lake populations of C. alburnus collected in two isolated and contrasting river systems. We wanted to discover whether the intestinal structure and functional divergence were formed in the two populations due to adaptive evolution caused by geographical isolation. Our study indicated that higher intestinal villi, thicker intestinal mucosa layer and intestinal muscle layer, and significantly higher activity of α-amylase were identified in the XK population. Moreover, quite different intestinal microbiomes were presented in the two populations, with the higher abundance of Bacteroidetes and Firmicutes in the XK population. The significantly different intestinal microbiome in the XK population was functionally enriched in carbohydrate, lipid, and amino acid metabolism by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Our findings indicated that substantial adaptative divergence in the intestinal structures and intestine microbiomes was formed in the two populations due to long-term geographical isolation, which may have strongly affected the digestion and absorption ability of the XK population compared with the DS population.

Phytoplankton alpha diversity indices response the trophic state variation in hydrologically connected aquatic habitats in the Harbin Section of the Songhua River

The relationship between biodiversity and ecological functioning is a central issue in freshwater ecology, but how this relationship is influenced by hydrological connectivity stress is still unknown. In this study we analyzed the dynamic of the phytoplankton alpha diversity indices and their relationships with trophic state in two hydrologically connected aquatic habitats (Jinhewan Wetland and Harbin Section of the Songhua River) in the Songhua River Basin in northeast China. We hypothesized that the phytoplankton alpha-diversity indices have the potential to provide a signal linking trophic state variation in hydrologically connected aquatic habitats. Our results showed the Cyanophyta and Bacillariophyta were abundant at most stations. T-test showed that phytoplankton alpha diversity indices varied significantly between rainy season and dry season. Trophic State Index recorded that a meso-trophic to eutrophic states of two connected habits during study period. Multivariate statistical analysis revealed that the dynamic of phytoplankton alpha diversity index was closely associated with trophic states change. Our result indicated that hydrological connectivity is a key factor influenced phytoplankton community assembly. In addition, it is beneficial to develop an integrated approach to appropriately describe and measure the trophic state variations of hydrologically connected aquatic habits in freshwater ecosystem.

Shifts in Diatom Dominance Associated with Seasonal Changes in an Estuarine-Mangrove Phytoplankton Community

A study on seasonal phytoplankton abundance and composition in a mangrove estuary, Matang Mangrove Forest Reserve (MMFR), Malaysia, was carried out to determine the phytoplankton structure in this ecosystem, and to identify potential indicators of environmental changes. Phytoplankton samples were collected bimonthly from June 2010 to April 2011, to cover both dry (June to October) and wet (November to April) seasons, at four selected sampling sites along the river. Diatoms showed the highest number of species (50 species) from a total of 85 phytoplankton species from 76 genera. Diatoms contributed more than 90% of the total phytoplankton abundance during the dry season (southwest monsoon) and less than 70% during the wet season (northeast monsoon) as dinoflagellates became more abundant during the rainy season. Two diatoms were recorded as dominant species throughout the sampling period; Cyclotella sp. and Skeletonema costatum. Cyclotella sp. formed the most abundant species (62% of total phytoplankton) during the dry period characterized by low nutrients and relatively low turbidity. Skeletonema costatum contributed 93% of the total phytoplankton in October, which marked the end of the dry season and the beginning of the wet season, characterized by strong winds and high waves leading to the upwelling of the water column. Massive blooms of Skeletonema costatum occurred during the upwelling when total nitrogen (TN) and total phosphorus (TP) concentrations were highest (p < 0.05) throughout the year. The abundance of diatom species during the wet season was more evenly distributed, with most diatom species contributing less than 12% of the total phytoplankton. Autotrophic producers such as diatoms were limited by high turbidity during the northeast monsoon when the rainfall was high. During the wet season, Cyclotella and Skeletonema costatum only contributed 9% and 5% of the total phytoplankton, respectively, as dinoflagellates had more competitive advantage in turbid waters. This study illustrates that some diatom species such as Cyclotella sp. and Skeletonema costatum could be used as indicators of the environmental changes in marine waters.

Geographic Patterns of Bacterioplankton among Lakes of the Middle and Lower Reaches of the Yangtze River Basin, China

The revolution of molecular techniques has revealed that the composition of natural bacterial communities normally includes a few abundant taxa and many rare taxa. Unraveling the mechanisms underlying the spatial assembly process of both abundant and rare bacterial taxa has become a central goal in microbial ecology. Here, we used high-throughput sequencing to explore geographic patterns and the relative importance of ecological processes in the assembly of abundant and rare bacterial subcommunities from 25 lakes across the middle and lower reaches of Yangtze River basin (MLYB), located in Southeast China, where most of the lakes are interconnected by river networks. We found similar biogeographic patterns of abundant and rare subcommunities which could significantly distinguish the community compositions of the two lake groups that were far from each other but which could not distinguish the community compositions of the nearby lakes. Both abundant and rare bacteria followed a strong distance-decay relationship. These findings suggest that the interconnectivity between lakes homogenizes the bacterial communities in local areas, and the abundant and rare taxa therein may be affected by the same ecological process. In addition, based on the measured environmental variables, the deterministic processes explain a small fraction of variation within both abundant and rare subcommunities, while both neutral and null models revealed a high stochasticity ratio for the spatial distribution patterns of both abundant and rare taxa. These findings indicate that the stochastic processes exhibited a greater influence on both abundant and rare bacterial subcommunity assemblies among interconnected lakes.IMPORTANCE The middle and lower Yangtze Plain is a typical floodplain in which many lakes connect with each other, especially in the wet season. More importantly, with the frequent change of regional water level in the wet season, there is a mutual hydrodynamic exchange among these lakes. The microbial biogeography among these interconnected lakes is still poorly understood. This study aims to unravel the mechanisms underlying the assembly process of abundant and rare bacteria among the interconnected lakes in the middle and lower Yangtze Plain. Our findings will provide a deeper understanding of the biogeographic patterns of rare and abundant bacterial taxa and their determined processes among interconnected aquatic habitats.

Estimation of Kd(PAR) in inland waters across China in relation to the light absorption of optically active components

The comprehensive analysis of the relationships between the attenuation of photosynthetic active radiation (K_d(PAR)) and light absorption is an imperative requirement to retrieve K_d(PAR) from remote sensing data for aquatic environments. The spatial distributions of the K_d(PAR) and light absorption of optically active components (a_OACs) were routinely estimated in China lakes and reservoirs. Spatial K_d(PAR) was relatively dependent on the inorganic particles (average relative contribution of 57.95%). The a_OACs could explain 70-87% of K_d(PAR) variations. A linear model is used to predict K_d(PAR), as a function of light absorption coefficient of phytoplankton (a_phy), colored dissolved organic matter (a_CDOM), and inorganic particles (a_NAP): K_d(PAR) = 0.41 + 0.57 × a_CDOM + 0.96 × a_NAP + 0.57 × a_phy (R² = 0.87, n = 741, p < 0.001). In the lakes with low TSM concentration and non-eutrophic lakes with high TSM, a_CDOM was the most powerful predicting factor on K_d(PAR). In eutrophic lakes with high TSM, a_NAP had the most significant impact on K_d(PAR). This study allowed K_d(PAR) to be predicted from a_OACs values in the inland waters.

Phytoplankton community structure in relation to environmental factors and ecological assessment of water quality in the upper reaches of the Genhe River in the Greater Hinggan Mountains

Phytoplankton assemblages were investigated in 2015 along the seasonal changes of the Genhe River in the Greater Hinggan Mountains. The survey was performed in June (spring), August (summer), and October (autumn) at nine sampling stations to study the community composition, abundance, and biodiversity. The results showed that 61 species belonging to 16 genera were identified, including Bacillariophyta of 31 species, Dinophyta 2 species, Cyanophyta 2 species, Chlorophyta 20 species, Chrysophyta 2 species, and Cryptophyta 1 species; Besides, Bacillariophyta are dominant species. Shannon-Wiener (H') and Pielou (J') indices indicated that phytoplankton community was stable. And these two indices were significantly lower in summer than in spring and autumn. Phytoplankton abundance and biomass show significant differences in each season. The total phytoplankton abundance (1122.3 × 104 ind/L) and biomass (6.5709 mg/L) in summer are much higher than that in spring and autumn. There were few species and low abundance and biomass in the upper reaches of Genhe River; this fact can be explained by the cold climate in the Greater Higgnan Mountains region. Canonical correspondence analysis (CCA) was used to analyze the data. It revealed that Fe³⁺, Cu²⁺, pH, and water temperature (WT) were responsible for most of the variation in space in the phytoplankton community. These environmental parameters play an essential role in the community structure variation of phytoplankton in the upper reaches of Genhe River, the strong association between phytoplankton community structure and ecological factors is varied in each season.

Shifting of phytoplankton assemblages in a regulated Chinese river basin after streamflow and water quality changes

Phytoplankton is critical to river ecosystems. These organisms are sensitive to streamflow and water quality changes and, therefore, used to determine stability of river ecosystems, especially in regulated rivers. However, exactly how such disturbances alter spatial distribution of phytoplankton remains unclear, particularly during different seasons. A thorough understanding of these mechanisms is required to better analyze impact of environmental factors on regulated rivers. Given this, phytoplankton communities, streamflow, and water quality factors were assessed in areas sampled four times from 2015 to 2016 in upper and middle Huai River Basin. Biodiversity indices, as well as cluster and rank analyses, were used to (1) determine phytoplankton composition and distribution and (2) clarify impacts of key streamflow and water quality factors on such communities. It was found phytoplankton composition deteriorated over time, with phyla number decreasing from six to three. Moreover, proportion of Bacillariophyta increased from 51.83% to 68.13%. Phytoplankton in three regions, upstream region (Shannon-Wiener index 1.39-2.95), midstream region (0.70-4.55), and downstream region (0.22 to 2.97), were spatially clustered. The most impact factors impacting variation in composition and distribution were water quality factors and then hydrological factors. Of these, the most important factors in wet seasons were total nitrogen and maximum runoff, while ammonia nitrogen and low flow discharge were the most important factors during dry seasons. Streamflow and water quality contributed the most in midstream region, which was significantly affected by numbers of high and low flow. Contributions of these factors to downstream region were the strongest during dry seasons, which were significantly affected by numbers of low flow. Collectively, these results reveal significant impact of streamflow and water quality factors on phytoplankton deterioration in upper and middle Huai River Basin. Critically, this study provides scientific and technological support for increased biomonitoring and ecohydrological studies in regulated river basins.