Temporal variation in zooplankton and phytoplankton community species composition and the affecting factors in Lake Taihu-a large freshwater lake in China

From disruption to adaptation: Response of phytoplankton communities in representative impounded lakes to China's South-to-North Water Diversion Project

Impounded lakes are often interconnected in large-scale water diversion projects to form a coordinated system for water allocation and regulation. The alternating runoff and transferred water can significantly impact local ecosystems, which are initially reflected in the sensitive phytoplankton. Nonetheless, limited information is available on the temporal dynamics and assembly patterns of phytoplankton community in impounded lakes responding to continuous and periodic water diversion. Herein, a long-term monitoring from 2013 to 2020 were conducted to systematically investigate the response of phytoplankton community, including its characteristics, stability, and the ecological processes governing community assembly, in representative impounded lakes to the South-to-North Water Diversion Project (SNWDP) in China. In the initial stage of the SNWDP, the phytoplankton diversity indices experienced a decrease during both non-water diversion periods (8.5 %∼21.2 %) and water diversion periods (5.6 %∼12.2 %), implying a disruption in the aquatic ecosystem. But the regular delivery of high-quality water from the Yangtze River gradually increased phytoplankton diversity and mediated ecological assembly processes shifting from stochastic to deterministic. Meanwhile, reduced nutrients restricted the growth of phytoplankton, pushing species to interact more closely to maintain the functionality and stability of the co-occurrence network. The partial least squares path model revealed that ecological process (path coefficient = 0.525, p < 0.01) and interspecies interactions in networks (path coefficient = -0.806, p < 0.01) jointly influenced the keystone and dominant species, ultimately resulting in an improvement in stability (path coefficient = 0.878, p < 0.01). Overall, the phytoplankton communities experienced an evolutionary process from short-term disruption to long-term adaptation, demonstrating resilience and adaptability in response to the challenges posed by the SNWDP. This study revealed the response and adaptation mechanism of phytoplankton communities in impounded lakes to water diversion projects, which is helpful for maintaining the lake ecological health and formulating rational water management strategies.

Recognizing the variation of DNA-P during and after the algal bloom in lake Hulun

Eutrophication has spread from shallow lakes in temperature zones to lakes in cold regions as a result of a continuous warm climate and human activities. Little proof for the importance of dissolved organic phosphorus (DOP) in contributing to phosphorus cycling and algae growth has been generated for aquatic ecosystems, particularly in cold eutrophic lakes. In this study, a comprehensive in situ study was conducted in overlying water, suspended particulate matter, and sediment during and after algal bloom (in July and September, respectively) in Lake Hulun. Multiple methods of 31P NMR, enzymatic hydrolysis, and UV-visible technologies were combined to detect phosphorus occurrence, bioavailability, and molecular structure from a novel angle. The 31P NMR analysis results showed that DNA-P is mainly stored in the dissolved phase and has not been detected in suspended particulate matter or sediment. Enzymatic hydrolysis was used to determine the bioavailability of DOP, which revealed that in July and September, respectively, 85% and 79% of DOP were hydrolyzable. UV-visible analysis represented that the degree of humification and molecular weight of DOP were high during the algal bloom, but these values considerably dropped following the algal bloom. The large amount of DNA-P present in the overlying water is the main reason for the high degree of humification and high molecular weight of the water body. Besides, Lake Hulun's DNA-P remains highly bioavailable during algal blooms, despite its high degree of humification and molecular weight. These findings can serve as a theoretical basis for understanding the migration and transformation of DOP, as well as the persistence of algal blooms in eutrophic lakes located in cold regions.

Development and testing of a planktonic index of biotic integrity (P-IBI) for Lake Fuxian, China

Lake Fuxian has the largest reserves of high-quality water resources in China, and understanding its ecological health status is the basis of its environmental protection. Based on a seasonal field investigation of the plankton community, we established a planktonic index of biotic integrity (P-IBI) evaluation system to assess the lake's ecosystem health. The biological integrity of Lake Fuxian was relatively good during winter and spring, but gradually deteriorated from summer to autumn. Areas with poor biological integrity were mainly distributed near tourist attractions along the lake's west coast. Redundancy analysis (RDA) was performed to explore the relationships between the P-IBI, its selected indicators, and the environmental variables. Water temperature (WT), pH, ammonia nitrogen (NH3-N), and dissolved oxygen (DO) significantly influenced the P-IBI and its selected indicators. NH3-N and DO were significantly positively correlated with the P-IBI, indicating that it could be used as a water quality indicator to indirectly reflect lake biological integrity. We demonstrated that the P-IBI can effectively reflect temporal and spatial variations of biological integrity and could be used as a potential tool to evaluate Lake Fuxian ecosystem health.

Responses of Zooplankton Community Pattern to Environmental Factors along the Salinity Gradient in a Seagoing River in Tianjin, China

As the primary consumers in aquatic organisms, zooplankton play an important role in aquatic ecosystems. It is valuable for management and researchers to have an insight into the responses of zooplankton community patterns to environmental factors. In this study, RDA and variation partitioning analysis were adopted to determine the important environmental factors affecting zooplankton abundance and biomass, as well as the relative importance of different environmental factors. The findings reveal that TN (total nitrogen), WD (water depth), pH, and SAL (salinity) were all important abiotic factors shaping the zooplankton community pattern in the study area. TN affected protozoa by influencing Stentor amethystinus, while the effects of WD on copepods may have been mainly induced by the responses of Calanus sinicus and Paracyclopina nana. By inhibiting Stentor amethystinus and Vorticella lutea, pH significantly affected protozoa. In addition, Rotifera and copepods were affected by SAL mainly through the responses of Brachionus calyciflorus, Calanus sinicus, and Ectocyclops phaleratus. Importantly, fundamental alternations in the variation trends of zooplankton abundance and biomass along the salinity gradient were found when the salinity was approximately 4-5. By combining these results with the findings on phytoplankton responses to salinity in previous studies, it can be concluded that salinity may influence the river ecosystem by influencing zooplankton abundance and biomass rather than phytoplankton.

Pulses outweigh cumulative effects of water diversion from river to lake on lacustrine phytoplankton communities

Due to the allochthonous input of nutrients and species, the cumulative effects of water diversion on water-receiving lakes deserve attention. Taking the water diversion project from the Yangtze River to Lake Taihu (WDYT) as an example, we explored the temporal effects of WDYT on the phytoplankton community and physicochemical habitat of Lake Taihu in autumn and winter from 2013 to 2018. Although the short-term diversion significantly increased the risk of importing nutrients, the relatively high quality of the diversion water compared with other inflow rivers had improved the water quality of the water-receiving lake region. The seasonal water diversion significantly increased phytoplankton diversity and community network complexity and reshaped the lacustrine community to be diatom-dominated with their relative proportions of 24.1-64.9% during water diversion periods. The contributions of physicochemical habitat changes induced by water diversion to variations in phytoplankton communities were 24.0-28.0%. The differences in phytoplankton diversity, community composition and physicochemical habitat in the water-receiving lake region between the diversion and non-diversion years were more evident than those between the non-diversion years in the same season, when comparing the multivariate dispersion indices among them. However, the lacustrine phytoplankton community during non-diversion periods still has not been essentially altered after several years of diversion, so the pulse effects of short-term water diversion were more obvious than the long-term cumulative impacts. Better control of allochthonous nutrients, appropriate increase in inflow water, adhering to the long-term operation, should be effective to enhance ecological benefits of such water diversion projects.

Phytoplankton and Microzooplankton Community Structure and Assembly Mechanisms in Northwestern Pacific Ocean Estuaries with Environmental Heterogeneity and Geographic Segregation

Phytoplankton and microzooplankton are crucial players in marine ecosystems and first responders to environmental changes, but their community structures and how they are shaped by environmental conditions have rarely been studied simultaneously. In this study, we conducted an eDNA metabarcoding sequencing combined with multiple statistical methods to simultaneously analyze the phytoplankton and microzooplankton in Liaohe (LH) and Yalujiang (YLJ) estuaries. The major objective was to examine how plankton community structure and assembly mechanism may differ between two estuaries with similar latitudinal position and climate but geographical segregation and differential level of urbanization (more in LH). Clear differences in diversity and composition of phytoplankton and microzooplankton communities between LH and YLJ estuaries were observed. Richness of phytoplankton was significantly higher in LH than YLJ, while richness of microzooplankton was higher in YLJ. The magnitude of intrahabitat variations in phytoplankton communities was significantly stronger than that of microzooplankton. Some phytoplankton and microzooplankton taxa also showed interhabitat differences in their relative abundances. Phytoplankton showed a stronger geographic distance-decay of similarity than microzooplankton, while significant environmental distance-decay of similarity in microzooplankton was found in the less urbanized YLJ estuary. Community assembly of phytoplankton was, based on the neutral community models, driven primarily by stochastic processes, while deterministic processes contributed more for microzooplankton. Furthermore, we detected wider habitat niche breadths and stronger dispersal abilities in phytoplankton than in microzooplankton. These results suggest that passive dispersal shapes the phytoplankton community whereas environmental selection shapes the microzooplankton community. IMPORTANCE Understanding the underlying mechanisms shaping a metacommunity is useful to management for improving the ecosystem function. The research presented in the manuscript mainly tried to address the effects of habitat geography and environmental conditions on the phytoplankton and microzooplankton communities, and the underlying mechanisms of community assembly in temperate estuaries. In order to achieve this purpose, we developed a metabarcoding sequencing method based on 18S rRNA gene. The phytoplankton and microzooplankton communities from two estuaries with similar latitude and climatic conditions but obvious geographical segregation and significant environmental heterogeneity were investigated. The results of our study could lay a solid foundation for ascertaining phytoplankton and microzooplankton communities in estuaries with obvious environmental heterogeneity and geographic segregation and mechanisms underlying community assembly.

Temporal continuous monitoring of cyanobacterial blooms in Lake Taihu at an hourly scale using machine learning

Cyanobacterial blooms in most lakes exhibit extraordinary changes in time and space. Herein, a cyanobacterial prediction model was designed for Lake Taihu based on a machine learning method. This method can generate temporally continuous (24 moments throughout the day) cyanobacterial data at a fine spatial scale of 9 km. The hourly meteorological data for 24 moments of the day were obtained from ERA5-Land data. Areal coverage of cyanobacterial blooms was derived from the hourly Geostationary Ocean Color Imager reflectance data observed only eight times a day (from ~8:00 to ~15:00, UTC+8). The cyanobacterial and meteorological data of eight moments in Lake Taihu from 2011 to 2020 were used to design the prediction model. The results were compared and validated employing nine training strategies to determine the best cyanobacterial prediction model for Lake Taihu (R = 0.42; root mean square error = 0.10). With the best-fitted model utilizing meteorological data (2011-2020), the area coverage of cyanobacterial blooms at the other 16 moments during a day were estimated. Based on this, the regional and temporal characteristics of diurnal bloom variation were evaluated at an hourly scale. The results indicated that the hourly variations in the areal coverage of cyanobacterial blooms at 24 moments of the day had similar patterns in each subregion of Lake Taihu with minor seasonal variations. The six meteorological variables adopted to construct the model had similar diurnal changes but with diverse value ranges among the seasons. Further analysis revealed that three meteorological variables (temperature, surface pressure, and evaporation) were positively related to diurnal bloom variations at an hourly scale. Overall, these results illustrate that meteorological conditions can affect the occurrence of cyanobacterial blooms at multiple time scales (e.g., hourly, daily, or monthly). The developed cyanobacterial prediction model can provide cyanobacterial data when cyanobacterial data is unavailable for the target waterbody.

Novel insights in seasonal dynamics and co-existence patterns of phytoplankton and micro-eukaryotes in drinking water reservoir, Northwest China: DNA data and ecological model

Although associations between phytoplankton and micro-eukaryotes have been studied in aquatic ecosystems, there are still knowledge gaps in comprehending their dynamics and interactions in drinking water reservoirs. Here, the seasonal dynamics of phytoplankton and micro-eukaryotic diversities and their co-existence patterns were studied in a drinking water reservoir, Northwest China. The highest phytoplankton diversity was observed in summer, and Chlorella sp. that belongs to Chlorophyta was the most abundant genus. The highest eukaryotic diversity was also detected in summer, and Rimostrombidium sp. that belongs to Ciliophora was the most dominant genus. Mantel test showed that the phytoplankton diversity was significantly correlated with ammonia nitrogen (r = 0.561, p = 0.001) and dissolved organic carbon (r = 0.267, p = 0.017), while the eukaryotic diversity was significantly associated with ammonia nitrogen (r = 0.265, p = 0.034) and temperature (r = 0.208, p = 0.046). PLS-PM (Partial Least Squares Path Modeling) further revealed that nutrients (P < 0.01) significantly affected the phytoplankton diversity, while nutrients (P < 0.01) and temperature (P < 0.01) significantly influenced the eukaryotic diversity. Co-occurrence network displayed the primarily positive interactions (77.66% positive and 22.34% negative) between phytoplankton and micro-eukaryotes. These findings could deepen our understanding of interactions between phytoplankton and micro-eukaryotes and their driving factors under changing aquatic environments of drinking water reservoirs.

Response of zooplankton to nutrient reduction and enhanced fish predation in a shallow eutrophic lake

As a key link between top-down regulators and bottom-up factors, zooplankton responds sensitively to environmental variations and provides information on the ecological state of freshwater systems. Although the response of zooplankton to anthropogenic pressures and fluctuating natural conditions, such as nutrient loading and climate change, has been extensively examined, findings have varied markedly. The mechanistic basis for the correlation between environmental variability and the zooplankton community is still debated, particularly for subtropical eutrophic lakes. We used two methods to analyze physicochemical and selected biological variables derived from long-term monitoring of Lake Taihu, a subtropical shallow lake in China. We first applied random forest regression to examine how changes in zooplankton were related to a set of environmental variables on interannual time scales. Then we used the results to guide the construction of a conceptual model for piecewise structural equation modeling (pSEM) to quantify more precisely the zooplankton-environment relationship. Zooplanktivorous fish and nutrient concentrations were the most important predictors of long-term trends in zooplankton in RF regression. Intensification of planktivorous fish predation led to a lower zooplankton biomass and smaller individuals through the removal of larger crustaceans. Moreover, suppression of zooplankton can in part be explained by increases in inedible algae, triggered by a combination of reduced nutrient concentrations and weakened grazer control. These results were also confirmed in the pSEM, which further indicated that top-down regulators might be more important than bottom-up factors for the zooplankton community in Lake Taihu. Our results suggest that stocking of filter-feeding fish in the lake did not meet the expectation that they would control algae, but that the use of biomanipulation measures considering both water quality and fishery management seems promising. This study offers insights into how indicator metrics of zooplankton can improve our understanding of the associations between plankton communities and ecosystem alterations.

Arsenic and Mercury Distribution in an Aquatic Food Chain: Importance of Femtoplankton and Picoplankton Filtration Fractions

Arsenic (As) and mercury (Hg) were examined in the Yellowstone Lake food chain, focusing on two lake locations separated by approximately 20 km and differing in lake floor hydrothermal vent activity. Sampling spanned from femtoplankton to the main fish species, Yellowstone cutthroat trout and the apex predator lake trout. Mercury bioaccumulated in muscle and liver of both trout species, biomagnifying with age, whereas As decreased in older fish, which indicates differential exposure routes for these metal(loid)s. Mercury and As concentrations were higher in all food chain filter fractions (0.1-, 0.8-, and 3.0-μm filters) at the vent-associated Inflated Plain site, illustrating the impact of localized hydrothermal inputs. Femtoplankton and picoplankton size biomass (0.1- and 0.8-μm filters) accounted for 30%-70% of total Hg or As at both locations. By contrast, only approximately 4% of As and <1% of Hg were found in the 0.1-μm filtrate, indicating that comparatively little As or Hg actually exists as an ionic form or intercalated with humic compounds, a frequent assumption in freshwaters and marine waters. Ribosomal RNA (18S) gene sequencing of DNA derived from the 0.1-, 0.8-, and 3.0-μm filters showed significant eukaryote biomass in these fractions, providing a novel view of the femtoplankton and picoplankton size biomass, which assists in explaining why these fractions may contain such significant Hg and As. These results infer that femtoplankton and picoplankton metal(loid) loads represent aquatic food chain entry points that need to be accounted for and that are important for better understanding Hg and As biochemistry in aquatic systems. Environ Toxicol Chem 2023;42:225-241. © 2022 SETAC.

Phytoplankton Community Response to Environmental Factors along a Salinity Gradient in a Seagoing River, Tianjin, China

A river-estuary ecosystem usually features a distinct salinity gradient and a complex water environment, so it is enormously valuable to study the response mechanism of living organisms to multiple abiotic factors under salinity stress. Phytoplankton, as an important part of aquatic microorganisms, has always been of concern for its crucial place in the aquatic ecosystem. In this study, phytoplankton data and 18 abiotic factors collected from 15 stations in Duliujian River, a seagoing river, were investigated in different seasons. The results showed that the river studied was of a Cyanophyta-dominant type. Salinity (SAL) was the key control factor for phytoplankton species richness, while water temperature (WT) was critical not only for species richness, but also community diversity, and the abundance and biomass of dominant species. Apart from WT, the abundance and biomass of dominant species were also driven by total nitrogen (TN), nitrate (NO₃^-), pH, and water transparency (SD). Moreover, total dissolved phosphorus (TDP), pH, and chemical oxygen demand (COD) were crucial for community diversity and evenness. The bloom of dominant species positively associated with TDP led to lower diversity and evenness in autumn. In addition, when available nitrogen was limited, Pseudoanabaena sp. could obtain a competitive advantage through the N₂ fixation function. Increased available nitrogen concentration could favor the abundance of Chlorella vulgaris to resist the negative effect of WT. The results show that Oscillatoria limosa could serve as an indicator of organic contamination, and nutrient-concentration control must be effective to inhibit Microcystis bloom. This could help managers to formulate conservation measures.

Seasonal Variation Characteristics and the Factors Affecting Plankton Community Structure in the Yitong River, China

To explore how environmental factors affected the plankton structure in the Yitong River, we surveyed the water environmental factors and plankton population in different seasons. The results showed high total nitrogen concentrations in Yitong River throughout the year, while the total phosphorus, water temperature (WT), and chemical oxygen demand in summer were significantly higher than those in other seasons (p < 0.05), and the dissolved oxygen (DO) concentrations and TN/TP ratio were significantly lower (p < 0.01) than those in other seasons. There was no significant seasonal change in other environmental factors. Cyanophyta, Chlorophyta, and Bacillariophyta were the main phytoplankton phylum, while Protozoa and Rotifera were the main zooplankton phylum. The abundance and biomass of zooplankton and phytoplankton in the summer were higher than those in other seasons. Non-Metric Multidimensional scaling methods demonstrated obvious seasonal variation of phytoplankton in summer compared to spring and winter, while the seasonal variation of the zooplankton community was not obvious. The results of the redundancy analysis showed that WT, DO and nitrate nitrogen were the main environmental factors affecting phytoplankton abundance. In contrast to environmental factors, phytoplankton was the main factor driving the seasonal variation of the zooplankton community structure. Cyanophyta were positively correlated with the changes in the plankton community.

Application of agricultural pesticides in a peak period induces an abundance decline of metazoan zooplankton in a lake ecosystem

The contamination of pesticides has been recognized as a major stressor in fresh water ecosystems in terms of the losses of services and population declines and extinctions. However, information on the adverse effects of pesticides on zooplankton communities under natural field conditions are still lacking, although zooplankton is quite sensitive to most of pesticides in laboratory studies. In this study, a natural lake ecosystem (Liangzi Lake) was used to determine the relationship between pesticide contamination and abundance decline of metazoan zooplankton. In August 2020, the comprehensive trophic level indexes and the abundance of phytoplankton in the 14 sampling sites of Liangzi Lake were comparable, but the abundance of metazoan zooplankton showed significant variations across two orders of magnitude. These results suggested that other factors, such as pesticide contamination, might be responsible for the variations of metazoan zooplankton community. Furthermore, the responsible pesticides were screened, and totally 29 pesticides were obtained. Finally, five pesticides were identified to provide more than 99.4% toxic contributions and chlorpyrifos and cypermethrin were two main causal agents. These results were further supported by laboratory exposure experiments using D. magna and field study in November 2020, where the concentrations of the 29 pesticides were strongly decreased and the abundance of metazoan zooplankton was comparable across the 14 sites of Liangzi Lake. Taken together, this work provided an evidence that the contamination of pesticides might be responsible for the abundance decline of metazoan zooplankton in a natural freshwater ecosystem.

The effect of graphene photocatalysis on microbial communities in Lake Xingyun, southwestern China

Graphene photocatalysis is a new method for harmful algae and water pollution control. However, microbial communities undergoing graphene photocatalysis treatment in freshwater lakes have been poorly studied. Here, using 16S rRNA and 18S rRNA gene high-throughput sequencing, the responses of microbial communities to graphene photocatalysis were analyzed in the eutrophic lake, Lake Xinyun, southwestern China. For microeukaryotes, we found that Arthropoda was dominant in summer, while its abundant level declined in spring under natural conditions. The evident reduction of Arthropods was observed after graphene photocatalysis treatment in summer and then reached a relatively stable level. For bacteria, Cyanobacteria decreased in summer due to the graphene photocatalysis-mediated inactivation. However, Cyanobacteria was higher in the treated group in spring with a genera group-shift. Functional analysis revealed that microeukaryotes showed higher potential for fatty acid oxidation and TCA cycle in the treated group in summer, but they were more abundant in control in spring. Pathways of starch and sucrose metabolism and galactose metabolism were more abundant in control in summer, while they were enriched in the treated group in spring for bacteria. This study offers insights into the effects of graphene photocatalysis on microbial communities and their functional potential in eutrophic lake.

Dominance of evaporation on lacustrine groundwater discharge to regulate lake nutrient state and algal blooms

As global threats to freshwater lakes, eutrophication and harmful algal blooms (HABs) are governed by various biogeochemical, climatological and anthropogenic processes. Groundwater is key to join these processes in regulating HABs, but the underlying mechanisms remain unclear. Here, we leveraged basin-wide field data of Lake Taihu (China's largest eutrophic lake) and global archives, and demonstrate the dominance of evaporation on lacustrine groundwater discharge (LGD) in shallow lakes. We extrapolated decadal LGD and the derived nutrient loadings and found that HABs promptly consume ubiquitous groundwater borne nutrients, leading lake water N: P ratios 2-3 months time lagged behind LGD N: P ratios. We conclude that evaporation dominated LGD is an unraveled but crucial regulator of nutrient states and HABs in shallow lakes, which advocates synergistical studies from both climatological and hydrogeological perspective when restoring lake ecosystems.

Spatial and temporal variations of phytoplankton composition and biomass in Lake Bunyonyi, South-Western Uganda

The purpose of this study was to examine the spatial and temporal variations of phytoplankton species composition and biomass in Lake Bunyonyi, South-Western Uganda. Samples were collected monthly from nine fixed stations in the lake from October 2019 to September 2020. Based on the morphological characterization, 52 different species of phytoplankton were recorded. These were dominated by cyanobacteria (21 species) and chlorophytes (15 species) followed by diatoms (10 species), euglenophytes (4 species), dinoflagellates (2 species), and cryptophytes (1 species). The biomass (Chl-a concentration) ranged from 0.019 ± 0.009 mg/L at Heissesero station to 0.045 ± 0.013 mg/L obtained at Nyombe station. On a temporal basis, the highest mean Chl-a concentration of 0.044 ± 0.03 mg/L was recorded in March 2020 while the least concentration of 0.015 ± 0.011 mg/L was obtained in September 2020. Significant differences existed in the Chl-a concentration values between stations and across sampling months. Chl-a concentration was significantly positively correlated with dissolved oxygen (DO), turbidity but negatively correlated with temperature. The Shannon-Wiener index and evenness put it clear that the distribution of phytoplankton species in the lake is inequitable. Besides, 94.2% of the phytoplankton species revealed had never been reported by the previous studies in the study area. The dominance of species cyanobacteria (such as Microcystis spp., Cylindrospermopsis raciborskii, Anabaenopsis sp., and Anabaena sp.) presents potential future challenges to water quality management. Therefore, the establishment of a strong and committed committee dubbed "Lake Bunyonyi Water Management Committee" to oversee the activities and avert potential water quality challenges is strongly recommended. The existence of some toxic phytoplankton species calls for regular monitoring and careful use of the lake and its food products.

Phycocyanin-rich Synechococcus dominates the blooms in a tropical estuary lake

Cyanobacterial blooms challenge the safe water supply in estuary reservoirs. Yet, data are limited for the variation of phytoplankton dynamics during an algal bloom event at refined scales, which is essential for interpreting the formation and cessation of blooms. The present study investigated the biweekly abundances and dynamics of pico- and nano-phytoplankton in a tropical estuary lake following a prolonged bloom event. Flow cytometry analysis resolved eight phenotypically distinct groups of phytoplankton assigned to nano-eukaryotes (nano-EU), pico/nano-eukaryotes (PicoNano-EU), cryptophyte-like cells (CRPTO), Microcystis-like cells (MIC), pico-eukaryotes (Pico-EU) and three groups of Synechococcus-like cells. Total phytoplankton abundance ranged widely from 2.4 × 104 to 2.8 × 106 cells cm-3. The phytoplankton community was dominated by Synechococcus-like cells with high phycocyanin content (SYN-PC). Temporal dynamics of the phytoplankton community was phytoplankton- and site-specific. Peak values were observed for SYN-PC, SYN-PE2 (Synechococcus-like cells with low levels of phycoerythrin) and Pico-EU, while the temporal dynamics of other groups were less pronounced. Redundancy analysis (RDA) showed the importance of turbidity as an abiotic factor in the formation of the current SYN-PC induced blooms, and Spearman correlation analysis suggested a competitive relationship between SYN-PC and Pico-EU.

Toxic metal pollution and ecological risk assessment in water and sediment at ship breaking sites in the Bay of Bengal Coast, Bangladesh

Quantification of four toxic metals (As, Cr, Cd, and Pb) in water and sediments at the Sitakunda ship breaking area in Bangladesh was studied. Along with this, sediment quality and ecological risk were evaluated for the metal intrusion to the study area. A total sample number of 120 (water; n = 60 and sediment; n = 60) were analyzed for both winter and summer seasons using atomic absorption spectrophotometer (AAS). The trace metal concentration in both water and sediment showed decreasing trend as follows; Cr (mean-W: 0.118 mg/L; mean-S:121.87 mg/kg) > Pb (mean-W: 0.064 mg/L; mean-S: 65.31 mg/kg) > As (mean-W: 0.03 mg/L; mean-S: 32.53 mg/kg) > Cd (mean-W: 0.004 mg/L; mean-S: 4.81 mg/kg). However, in both segments, the concentrations of the toxic metals exceeded the recommended acceptable limits. As and Cd showed significant variation (water and sediment) between the seasons, while Pb and Cr had no seasonal impact. Metal pollution index (MPI) and contamination factor (CF) was evaluated and revealed that the study area exhibited the critical score of water quality (MPI > 100). The cumulative effect of the metal concentrations was high (CI > 3). The assessed mean geoaccumulaiton index (I_geo) revealed that the study area was moderate to strongly polluted except for Cr. According to the contamination factor (CF), the sediment samples were moderate to highly contaminated by Cd, Pb, and As. Moreover, the explored range of pollution load index (PLI) in all sampling sites in the ship breaking region was from 1.75 to 3.10, suggesting that the sediment in the study area was highly polluted by heavy metals (PLI > 1). The risk index and the potential ecological risk index (PERI) suggested that the study area was at high risk due to metals pollution. Therefore, it is obligatory to maintain some crucial efforts for the betterment of the surrounding environment near the investigated sites.

How habitat heterogeneity shapes bacterial and protistan communities in temperate coastal areas near estuaries

In this study, we investigated microbial communities (bacteria and protist) in two coastal areas near the estuaries of the Liaohe (LH) River and Yalujiang (YLJ) River in the Northwestern Pacific Ocean. Due to the existence of Liaodong Peninsula and different levels of urbanization, geographical segregation and significant environmental heterogeneity were observed between these two areas. There were significantly different regional species pools and biogeographic patterns for both bacterial and protistan communities between LH and YLJ coastal areas. Species turnover was the main mechanism driving β-diversity patterns of both bacterial and protistan communities in each area. In addition, the contributed ratio of nestedness to the β-diversity patterns was significantly higher for protists compared to bacteria. Variation in regional species pools was found to be the dominant driver of differences of bacterial and protistan communities between the LH and YLJ coastal areas. For a single-studied area, local community assembly mechanisms, including heterogeneous selection and dispersal limitation, were found to shape the bacterial and protistan communities through calculation of the β-deviation index. Among them, the relative importance of heterogeneous selection and dispersal limitation on the community assembly varied according to microorganism type and habitat.

Controlling phytoplankton blooms in a canyon-shaped drinking water reservoir via artificial and induced natural mixing: Taxonomic versus functional groups

Water-lifting aerators (WLAs) were often deployed in reservoirs to achieve artificial mixing (WLA activation) and induced (natural) mixing (early occurrence of complete natural mixing after WLA deactivation) for water quality improvement. Here, the mechanisms controlling phytoplankton growth via artificial and induced mixing were explored using a combination of taxonomic and functional classifications based on two-year monitoring data (i.e., non-operational and operational years of the WLAs). Artificial mixing resulted in a decrease of 99.2 % in phytoplankton cell density compared to that of the non-operational year, which continuously diminished to (3.06 ± 0.59) × 10⁶ cells/L during induced mixing. The succession of phytoplankton structure in taxonomic and functional classification levels changed from Cyanobacteria to Chlorophyta and Bacillariophyta, from groups F, J, H1, and L_M to A and X1, respectively, by comparison of the non-operational and operational years. Decreases in surface water temperature, total phosphorus concentration, and light availability, and increases in mixing depth via artificial and induced mixing were responsible for phytoplankton control, especially for cyanobacterial blooms, depending on a shift in phytoplankton composition from large or colonial, low surface to volume (S/V) to small, high S/V genera. Artificial and induced mixing also improved the trophic/ecological status of the reservoir, from "hyper-eutrophic and bad level" to "light-eutrophic and excellent level", based on an assessment of the trophic level index (TLI) and phytoplankton assemblage (Q) index. This study demonstrates that the suitable combination of artificial and induced mixing plays a crucial role in the maintenance and extension of healthy ecosystems in reservoirs.

Surface oil is the primary driver of macroinvertebrate impacts following spills of diluted bitumen in freshwater

The response of freshwater invertebrates following accidental releases of oil is not well understood. This knowledge gap is more substantial for unconventional oils such as diluted bitumen (dilbit). We evaluated the effects of dilbit on insect emergence and benthic invertebrates by conducting experimental spills in limnocorrals (10-m diameter; ~100-m³) deployed in a boreal lake at the IISD-Experimental Lakes Area, Canada. The study included seven dilbit treatments (spill volumes ranged from 1.5 L [1:66,000, oil:water, v/v] to 180 L [1:590, oil:water, v/v]), two controls, and additional lake reference sites, monitored for 11 weeks. Invertebrate emergence declined at the community level following oil addition in a significantly volume-dependent manner, and by 93-100 % over the 11 weeks following the spill in the highest treatment. Dilbit altered community structure of benthic invertebrates, but not abundance. One-year post-spill and following oil removal using traditional skimming and absorption techniques, benthic richness and abundance were greater among all treatments than the previous year. These results indicate that recovery in community composition is possible following oil removal from a lake ecosystem. Research is needed concerning the mechanisms by which surface oil directly affect adult invertebrates, whether through limiting oviposition, limiting emergence, or both. The response of benthic communities to sediment tar mats is also warranted.

Contrasting the assembly of phytoplankton and zooplankton communities in a polluted semi-closed sea: Effects of marine compartments and environmental selection

Understanding the underlying mechanisms of community assembly is a major challenge in microbial ecology, particularly in communities composed of diverse organisms with different ecological characteristics. However, very little is known about the effects of marine compartments in shaping marine planktonic communities; primarily, how they are related to organism types and environmental variables. In this study, we used multiple statistical methods to explore the mechanisms driving phytoplankton and zooplankton metacommunity dynamics at the regional scale in the Bohai Sea, China. Clear geographic patterns were observed in both phytoplankton and zooplankton communities. Zooplankton showed a stronger distance-decay of similarity than phytoplankton, which had greater community differences between locations with further distances. Our analyses indicated that the zooplankton communities were primarily governed by species sorting versus dispersal limitation than the phytoplankton communities. Furthermore, we detected that zooplankton exhibited wider habitat niche breadths and dispersal abilities than phytoplankton. Our findings also showed that environmental pollution affected high trophic organisms via food webs; the presence of heavy metals in the Bohai Sea altered the abundance of some phytoplankton, and thus modified the zooplankton that feed on them.

Tempo-spatial variations of zooplankton communities in relation to environmental factors and the ecological implications: A case study in the hinterland of the Three Gorges Reservoir area, China

To expand the knowledge on the tempo-spatial patterns of zooplankton and the key modulated factors in urban aquatic ecosystem, we investigated zooplankton and water quality from April 2018 to January 2019 in the hinterland of the Three Gorges Reservoir area, Wanzhou City of China. The results indicated that water quality indicated by the trophic state index (TSI) reached a state of mesotrophication to light eutrophication in the Yangtze River, and a state of moderate- to hyper- eutrophication in its tributaries. Based on the biomass of zooplanktons, Asplanchna priodonta was the most common specie in April; Encentrum sp., Filinia cornuta and Epiphanes senta were the most noticeable species in summer; Cyclopoida Copepodid, Sinocalanus dorrii and Philodina erythrophthalma became the dominant species in winter. Generally, rotifers prevailed in April and August, and copepods became the most popular in January. According to canonical correspondence analysis, nitrate, temperature (T), ammonia, water level and permanganate index (CODMn) significantly influenced the community structure of zooplankton (p < 0.05). The dominant species shifts of zooplankton were partly associated with nutrient level (nitrate and ammonia) under periodic water level fluctuations. Rotifers and protozoans were characterized as high T adapted and CODMn-tolerant species comparing with cladocerans and copepods. The ratio of microzooplankton to mesozooplankton (Pmicro/meso) has presented a strongly positive relationship with T (p < 0.001), as well as Pmicro/meso and CODMn (p < 0.001). It implied that zooplankton tended to miniaturize individual size via species shift under high T and/or CODMn conditions induced by global warming and human activities. The information hints us that climate change and human activities are likely to produce fundamental changes in urban aquatic ecosystem by reorganizing biomass structure of the food web in future.

Uncertainty and Sensitivity Analysis of Input Conditions in a Large Shallow Lake Based on the Latin Hypercube Sampling and Morris Methods

We selected Tai Lake in China as the research area, and based on the Eco-lab model, we parameterized seven main external input conditions: discharge, carbon, nitrogen, phosphorus, wind speed, elevation, and temperature. We combined the LHS uncertainty analysis method and the Morris sensitivity analysis method to study the relationship between water quality and input conditions. The results showed that (1) the external input conditions had an uncertain impact on water quality. Among them, the uncertainties in total nitrogen concentration (TN) and total phosphorus concentration (TP) were mainly reflected in the lake entrance area, and the uncertainties of chlorophyll-a (Chl-a) and dissolved oxygen (DO) were mainly reflected in the lake center area. (2) The external input conditions had different sensitivities to different water layers. The bottom layer was most clearly and stably affected by input conditions. The TN and TP of the three different water layers were closely related to the flux into the lake, with average sensitivities of 83% and 78%, respectively. DO was mainly related to temperature and water elevation, with the bottom layer affected by temperatures as high as 98%. Chl-a was affected by all input factors except nitrogen and was most affected by wind speed, with an average of about 34%. Therefore, the accuracy of external input conditions can be effectively improved according to specific goals, reducing the uncertainty impact of the external input conditions of the model, and the model can provide a scientific reference for the determination of the mid- to long-term governance plan for Tai Lake in the future.

Response of planktonic communities to environmental stress in the eutrophic waters of Xiaoping Island in China

Phytoplankton blooms were affected by external environmental nutrient input, while the interspecific interactions in plankton (phytoplankton and mesozooplankton) under the nutrient pollution gradient remain largely unknown. Here, we systematically collected samples for 9 months (from April to December 2018) in the coastal waters of Xiaoping Island in China to analyze the planktonic community structure and identify the main driving environmental factors along the nutrient gradient from the sewage outlet to the open sea. The results indicated that there existed obvious seasonal and spatial variations in the planktonic community. Procrustes test analysis showed that temperature, transparency, dissolved oxygen, nitrate (NO₃-N), phosphate (PO₄-P), and silicate (SiO₃-Si) significantly affected the community compositions and diversity of plankton (p < 0.05). Co-occurrence network showed that seasons and nutrients pollution had an important influence on the inter-specific interactions between phytoplankton and mesozooplankton. In different nutrient pollution gradients, diatom was the most associated with Copepods in Section 1 (9.38%), Section 2 (9.84%), and Section 3 (5.38%), respectively, and it was also associated with Planktonic larva in Section 1 (7.81%), followed by in Section 3 (4.30%) and 2 (1.64%). Dinoflagellates were associated with Chaetognatha only in Section 1 (4.69%). This study may provide new insights into the plankton dynamics and facilitate nearshore environmental management.

Influence of cyanobacterial blooms and environmental variation on zooplankton and eukaryotic phytoplankton in a large, shallow, eutrophic lake in China

Harmful cyanobacterial blooms are a widespread destruction to the processes and function of aquatic ecosystems. To study effects of cyanobacterial blooms on plankton diversity and composition, we analyzed data of cyanobacterial, eukaryotic phytoplankton, metazoan zooplankton, and physicochemical samples collected from 24 sites for four seasons in 2017 and 2018 from the large, shallow Lake Taihu. We found that cyanobacterial abundance significantly correlated with phytoplankton biomass, species richness, functional richness and evenness, and zooplankton biomass, Shannon's diversity, Simpson's evenness, and functional evenness and richness. High cyanobacterial abundance during summer did not result in low species and functional diversities for both phytoplankton and zooplankton compared with other seasons. Species and functional diversities of sites with high cyanobacteria abundance were not significantly lower than other sites with relatively low cyanobacteria abundance. Structure equation modeling indicated that cyanobacteria had direct influence on phytoplankton and zooplankton compositions. Physicochemical and temporal-spatial factors had direct influence on phytoplankton and zooplankton, and had indirect influence on phytoplankton and zooplankton through direct influence on cyanobacteria. Variance partitioning analysis quantified that cyanobacteria alone and interactions with physicochemical and spatial-temporal factors explained about 10% of phytoplankton variation and 26% of zooplankton variation. Our results indicate that cyanobacteria have substantial effects on phytoplankton and zooplankton biodiversity and community composition. Physicochemical and spatial-temporal factors could potentially obscure the detection of cyanobacterial effects on plankton in Lake Taihu that has cyanobacterial blooms in all seasons. Our findings may improve the understanding of dynamics and responses of plankton communities to environmental changes and cyanobacterial bloom disturbance and enhance the capability of assessing the effectiveness of eutrophication management and restoration of aquatic ecosystems.

Spatial and temporal variations of faecal indicator bacteria in Lake Bunyonyi, South-Western Uganda

Background Microbial water quality serves to indicate health risks associated with the consumption of contaminated water. Nevertheless, little is known about the microbiological characteristics of water in Lake Bunyonyi. This study was therefore undertaken to examine the spatial and temporal variations of faecal indicator bacteria (FIB) in relation to physicochemical parameters in Lake Bunyonyi. Result The FIB concentration was consistently measured during sampling months and correlated with each other showing the presumed human faecal pollution in the lake. The highest concentration values for E. coli (64.7 ± 47.3 CFU/100 mL) and enterococci (24.6 ± 32.4 CFU/100 mL were obtained in the station close to the Mugyera trading centre. On a temporal basis, the maximum values were recorded during the rainy season in October 2019 (70.7 ± 56.5 CFU/100 mL for E. coli and 38.44 ± 31.8 CFU/100 mL for enterococci. FIB did not differ significantly among the study stations (p > 0.05) but showed significant temporal variations among the months (p < 0.05) with concentrations being significantly high in wet season than dry season (U = 794, p < 0.0001 for E. coli; U = 993.5, p = 0.008 for enterococci). Spearman’s rank correlation revealed that FIB concentrations were significantly positively correlated with turbidity and DO concentration levels (p < 0.05). Approximately 97.2% of the water samples had E. coli and enterococci concentrations levels below USEPA threshold for recreational waters. Likewise, 98.1 and 90.7% of samples recorded E. coli and enterococci counts exceeding the UNBS, APHA, WHO and EU threshold values for drinking water. Conclusion The FIB counts show that the Lake Bunyonyi water is bacteriologically unsuitable for drinking unless it is treated since the FIB pose health risks to consumers. Besides, the water can be used for recreational purposes.

Spatio-temporal variations in physicochemical water quality parameters of Lake Bunyonyi, Southwestern Uganda

The current study was carried out to examine the spatial and temporal variations of physicochemical water quality parameters of Lake Bunyonyi. The observations were made on the surface water of Lake Bunyonyi for 1 year to determine the water quality. The basic 12 variables used to determine the quality of water were measured monthly at nine stations. Water temperature, dissolved oxygen (DO), turbidity, electric conductivity (EC), pH and Secchi depth (SD) were measured in the field, while parameters like total nitrogen (TN), total phosphorus (TP), nitrite-nitrogen (NO2-N), nitrate-nitrogen (NO3-N), soluble reactive phosphorus (SRP) were determined following APHA 2017 standard guidelines for physicochemical analysis. Taking into account standard guidelines for drinking water by the Uganda National Bureau of Standards (UNBS) and the World Health Organization (WHO), the water quality index (WQI) was used to determine the water quality. Temperature, DO, pH, turbidity and EC did not differ significantly among the study stations (p > 0.05) but showed significant temporal variations among the study months (p < 0.05). Likewise, TN, TP, NO2-N, NO3-N and SRP did not differ significantly among the study stations (p > 0.05) but showed significant temporal variations among the study months (p < 0.05). The WQI values ranged from 28.36 to 49 across and from 28.2 to 56.2 between study months with an overall mean value of 36.9. The measured water quality variables did not exceed the UNBS and WHO standards for drinking water in all months and at all stations. According to these values, the water quality of Lake Bunyonyi generally belongs to the ‘good’ class in terms of drinking water quality based on the WQI classification. The study findings are fundamentally important for policy makers in setting guidelines for effective lake management.

Rapid Changes in the Phytoplankton Community of a Subtropical, Shallow, Hypereutrophic Lake During the Rainy Season

Lake Cajititlán is a small, shallow, subtropical lake located in an endorheic basin in western Mexico. It is characterized by a strong seasonality of climate with pronounced wet and dry seasons and has been classified as a hypereutrophic lake. This eutrophication was driven by improperly treated sewage discharges from four municipal wastewater treatment plants (WWTPs) and by excessive agricultural activities, including the overuse of fertilizers that reach the lake through surface runoff during the rainy season. This nutrient rich runoff has caused algal blooms, which have led to anoxic or hypoxic conditions, resulting in large-scale fish deaths that have occurred during or immediately after the rainy season. This study investigated the changes in the phytoplankton community in Lake Cajititlán during the rainy season and the association between these changes and the physicochemical water quality and environmental parameters measured in the lake’s basin. Planktothrix and Cylindrospermopsis were the dominant genera of the cyanobacterial community, while the Chlorophyceae, Chrysophyceae, and Trebouxiophyceae classes dominated the microalgae community. However, the results showed a significant temporal shift in the phytoplankton communities in Lake Cajititlán induced by the rainy season. The findings of this study suggest that significant climatic variations cause high seasonal surface runoff and rapid changes in the water quality (Chlorophyll-a, DO, NH₄⁺, and NO₃^–) and in variations in the composition of the phytoplankton community. Finally, an alternation between phosphorus and nitrogen limitation was observed in Lake Cajititlán during the rainy season, clearly correlating to the presence of Planktothrix when the lake was limited by phosphorus and to the presence of Cylindrospermopsis when the lake was limited by nitrogen. The evidence presented in this study supports the idea that the death of fish in Lake Cajititlán could be mainly caused by anoxia, caused by rapid changes in water quality during the rainy season. Based on our review of the literature, this is the first study on the phytoplankton community in a subtropical lake during the rainy season using high throughput 16S rRNA and 18S rRNA amplicon sequencing.

DNA metabarcoding of zooplankton communities: species diversity and seasonal variation revealed by 18S rRNA and COI

Background

Zooplankton is an important component of aquatic organisms and has important biological and economical significance in freshwater ecosystems. However, traditional methods that rely on morphology to classify zooplankton require expert taxonomic skills. Moreover, traditional classification methods are time-consuming and labor-intensive, which is not practical for the design of conservation measures and ecological management tools based on zooplankton diversity assessment.

Methods

We used DNA metabarcoding technology with two different markers: the nuclear small subunit ribosomal RNA (18S rRNA) and mitochondrial cytochrome c oxidase (COI), to analyze 72 zooplankton samples collected in 4 seasons and 9 locations from the Sanmenxia Reservoir. We investigated seasonal changes in the zooplankton community and their relationship with water environmental factors.

Results

A total of 190 species of zooplankton were found, belonging to 12 phyla, 24 classes, 61 orders, 111 families, and 174 genera. Protozoa, especially ciliates, were the most diverse taxa. Richness and relative abundance of zooplankton showed significant seasonal changes. Both alpha and beta diversity showed seasonal trends: the diversity in summer and autumn was higher than that in winter and spring. The zooplankton diversity was most similar in winter and spring. By correlating metabarcoding data and water environmental factors, we proved that water temperature, chemical oxygen demand, total nitrogen and ammoniacal nitrogen were the main environmental factors driving the seasonal changes in zooplankton in the Sanmenxia Reservoir. Water temperature, followed by total nitrogen, were the most influential factors. This study highlights the advantages and some limitations of zooplankton molecular biodiversity assessment using two molecular markers.

A burning issue: The effect of organic ultraviolet filter exposure on the behaviour and physiology of Daphnia magna

Ultraviolet (UV) filters are compounds utilized in many manufacturing processes and personal care products such as sunscreen to protect against UV-radiation. These highly lipophilic compounds are emerging contaminants of concern in aquatic environments due to their previously observed potential to bioaccumulate and exert toxic effects in marine ecosystems. Currently, research into the toxic effects of UV filter contamination of freshwater ecosystems is lacking, thus the present study sought to model the effects of acute and chronic developmental exposures to UV filters avobenzone, oxybenzone and octocrylene as well as a mixture of these substances in the freshwater invertebrate, Daphnia magna, at environmentally realistic concentrations. Median 48-hour effect and lethal concentrations were determined to be in the low mg/L range, with the exception of octocrylene causing 50% immobilization near environmental concentrations. 48-hour acute developmental exposures proved to behaviourally impair daphnid phototactic response; however, recovery was observed following a 19-day post-exposure period. Although no physiological disruptions were detected in acutely exposed daphnids, delayed mortality was observed up to seven days post-exposure at 200 μg/L of avobenzone and octocrylene. 21-day chronic exposure to 7.5 μg/L octocrylene yielded complete mortality within 7 days, while sublethal chronic exposure to avobenzone increased Daphnia reproductive output and decreased metabolic rate. 2 μg/L oxybenzone induced a 25% increase in metabolic rate of adult daphnids, and otherwise caused no toxic effects at this dose. These data indicate that UV filters can exert toxic effects in freshwater invertebrates, therefore further study is required. It is clear that the most well-studied UV filter, oxybenzone, may not be the most toxic to Daphnia, as both avobenzone and octocrylene induced behavioural and physiological disruption at environmentally realistic concentrations.

Disentangling effects of river inflow and marine diffusion in shaping the planktonic communities in a heavily polluted estuary

Estuarine ecosystems are important in terms of biodiversity processes because there are intense interactions between the river and sea environments. Phytoplankton and zooplankton have been shown to be ecological indicators of the water quality status in estuary ecosystems. Therefore, a comprehensive evaluation of the effects that multiple pressures have on the phytoplankton and zooplankton communities in estuarine ecosystems is essential. In this study, water samples from 29 stations were collected from the Liaohe Estuary over three different seasons, and biotic factors (i.e., phytoplankton and zooplankton) were obtained and compared. The results showed that there were significant temporal and spatial variations in the phytoplankton and zooplankton communities from the Liaohe Estuary. The correlation analyses showed that water temperature was the most important factor regulating the variation in phytoplankton communities, whereas the main driving force for the zooplankton was nutrient concentrations. Large amounts of nutrients entered the estuary in spring and summer due to intensive human activities in the Liaohe River basin. The inflows by the Liaohe River introduced some phytoplankton and zooplankton into the estuary, such as Coscinodicus asteromphalus, Chaetoceros decipiens, and Schmacheria poplesia. The impacts of Liaohe inflows on the estuary region gradually decreased as the distance from the inlet increased and this change was mediated by marine diffusion. The results from this study will improve knowledge about planktonic communities in estuarine ecosystems and provide a theoretical foundation for estuary environmental management.

Temporal variations and pond age effect on plankton communities in semi-intensive freshwater marron (Cherax cainii, Austin and Ryan, 2002) earthen aquaculture ponds in Western Australia

The abundance and diversity of the plankton community represents the health of the aquatic ecosystem, and plays an important role in the growth of cultured animals under aquaculture conditions. The temporal variations of plankton abundance, taxonomic composition, diversity, evenness and species richness were studied in three old and three new semi-intensive marron (Cherax cainii, Austin and Ryan, 2002) ponds. Water parameters such as temperature, dissolved oxygen, pH, turbidity, TAN, nitrite, nitrate and reactive phosphate were recorded, and plankton samples were collected every two months, for one year of juvenile production cycle. A total of twenty-six phytoplankton and seven zooplankton genera were recorded. Chlorophyceae was the dominant class of phytoplankton throughout the year, followed by Trebouxiophyceae. Rotifera comprised 49.8% of the total zooplankton community (individuals L⁻¹), the largest proportion of any group. Temporal variations impacted the plankton abundance and community structure, and plankton abundance were more abundant during summer. The pond age did not influence the phytoplankton abundance, whereas zooplankton abundance was higher in older ponds.

Composition characterization and biotransformation of dissolved, particulate and algae organic phosphorus in eutrophic lakes

Characteristics and transformation of organic phosphorus in water are vital to biogeochemical cycling of phosphorus and support of blooms of phytoplankton and cyanobacteria. Using solution ³¹P nuclear magnetic resonance (NMR), combined with field surveys and lab analyses, composition and structural characteristics of dissolved phosphorus (DP), particulate phosphorus (PP) and organic P in algae were studied in two eutrophic lakes in China, Tai Lake and Chao Lake. Factors influencing migration and transformation of these constituents in lake ecosystems were also investigated. A method was developed to extract, flocculate and concentrate DP and PP from lake water samples. Results showed that orthophosphate (Ortho-P) constituted 32.4%-81.3% of DP and 43.7%-54.9% of PP, respectively; while monoester phosphorus (Mono-P) was 13.2%-54.0% of DP and 32.9%-43.7% of PP, respectively. Phosphorus in algae was mostly organic P, especially Mono-P, which was ≥50% of TP. Environmental factors and water quality parameters such as temperature (T), electrical conductivity (EC), pH, secchi depth (SD), dissolved oxygen (DO), chemical oxygen demand (COD_cr), chlorophyll-a (Chl-a), affected the absolute and relative concentrations of various P components in the two lakes. Increased temperature promoted bioavailable P (Ortho-P and Mono-P) release to the lake waters. The results can provide an important theoretical basis for the mutual conversion process of organic P components between various media in the lake water environment.

Epipelon, phytoplankton and zooplankton responses to the experimental oligotrophication in a eutrophic shallow reservoir

Epipelon can contribute to the maintenance of shallow lake oligotrophication. Herein, we simulated oligotrophication by diluting eutrophic water and evaluated epipelon biomass and structure and potential relationships with phytoplankton and zooplankton communities. Dilutions of 25-75% negatively impacted phytoplankton biomass and zooplankton diversity and increased Rotifera density. Additionally, the 25% dilution increased Copepoda density, but had no effect on Cladocera. On both experimental days, epipelon chlorophyll-a and algal density responded to oligotrophication, but the algal biomass response was less pronounced after 14 days. Ceratium furcoides was dominant in the phytoplankton, while diatom species were dominant in the epipelon. We observed that experimental oligotrophication can influence both the biomass and taxonomic structure of the algal and zooplankton communities. Overall, we concluded that experimental oligotrophication negatively impacted the phytoplankton biomass and favored the development of the phototrophic epipelon; however, a large reduction in eutrophication (>50%) is required for a significant algal response in the benthic environment of a shallow tropical reservoir.

Nutrient addition bioassay and phytoplankton community structure monitored during autumn in Xiangxi Bay of Three Gorges Reservoir, China

The increasing freshwater ecosystem nutrient budget is a critical anthropogenic factor promoting freshwater eutrophication and episodic bloom of harmful algae which threaten water quality and public health. To understand how the eutrophic freshwater ecosystem responds in term of phytoplankton community structure dynamics to a sudden rise in nutrient concentrations, a microcosm study by nutrient addition bioassay was implemented in Xiangxi Bay (XXB) of Three Gorges Reservoir, China. Our results showed that dissolved trace elements supply adequately altered the phytoplankton community structure creating a regime shift from cyanobacteria-dominated to essentially Chlorophytes-dominated system, relative abundance (>70%). Combined N, P, and Si led to maximum growth stimulation accompanied by the highest chlorophyll yield (82.7 ± 14.01 μgL^-1) and growth rate (1.098 ± 0.12 μgL^-1d^-1). N separate additions resulted in growth responses which did not differ while P -addition differed significantly (p∠0.05) with the control justifying a P limited system. Si enrichment stimulated diatom growth, relative abundance (20.62%) and maximum utility rate (U_Si = 83.37 ± 0.33%). This study also reveals that increasing nutrient loading from anthropogenic sources adequately decrease the ecological diversity (H < 1) and community overlap (CC ≤ 0.5) intensifying competition and succession which then select the fast-growing taxa to dominate and expand. Result points to the need for multiple nutrient control of N, P and Si loading into XXB through a prudent nutrient management protocol for lasting bloom mitigation in the tributary bay.

Towards the comprehensive water quality control in Lake Taihu: Correlating chlorphyll a and water quality parameters with generalized additive model

In this study, the generalized additive model (GAM) was used to analyze seasonal monitoring data from Lake Taihu, collected from 2010 to 2014, with the aim to explore the correlation between chlorophyll a (Chla) and other water quality parameters. The selected optimal multivariable GAM could effectively explain the concentration variation of Chla occurring during each season, and the interpretation degree followed the order: summer > autumn > spring > winter. The fitting results indicated that the concentration variation of Chla could reflect that of biochemical oxygen demand and chemical oxygen demand in all seasons. In addition, the total phosphorus showed strong ability to explain the concentration change of Chla in spring and summer, as the growth of algae would be affected when the concentration of phosphorus shifted high or low. Nitrogen showed strong ability to explain the variations in Chla concentration in autumn. The conclusions of the optimal multivariable GAM could provide decision basis for the eutrophication control. In other words, the prevention of eutrophication outbreaks could be carried out via the targeted control of key water pollutants. According to these results, the concentration of Chla was higher in northern and western lake during summer and autumn, the management should focus on nutrient input of adjacent rivers.

Community Compositions of Phytoplankton and Eukaryotes during the Mixing Periods of a Drinking Water Reservoir: Dynamics and Interactions

In deep drinking water reservoir ecosystems, the dynamics and interactions of community compositions of phytoplankton and eukaryotes during the mixing periods are still unclear. Here, morphological characteristics combined with high-throughput DNA sequencing (HTS) were used to investigate the variations of phytoplankton and the eukaryotic community in a large canyon-shaped, stratified reservoir located at the Heihe River in Shaanxi Province for three months. The results showed that Bacillariophyta and Chlorophyta were the dominant taxa of the phytoplankton community, accounting for more than 97% of total phytoplankton abundance, which mainly consisted of Melosira sp., Cyclotella sp., and Chlorella sp., respectively. Illumina Miseq sequencing suggested that the biodiversity of eukaryotes increased over time and thatspecies distribution was more even. Arthropoda (6.63% to 79.19%), Ochrophyta (5.60% to 35.16%), Ciliophora (1.81% to 10.93%) and Cryptomonadales (0.25% to 11.48%) were the keystone taxa in common, contributing over 50% of the total eukaryotic community. Cryptomycota as a unique fungus was observed to possess significant synchronization with algal density, reaching a maximum of 10.70% in December (when the algal density distinctly decreased) and suggesting that it might affect the growth of algae through parasitism. Co-occurrence network patterns revealed the complicated and diverse interactions between eukaryotes and phytoplankton, suggesting that eukaryotes respond to variations in dynamic structure of the phytoplankton community, although there might be antagonistic or mutualistic interactions between them. Redundancy analysis (RDA) results showed that environmental variables collectively explained a 96.7% variance of phytoplankton and 96.3% variance of eukaryotic microorganisms, indicating that the temporal variations of phytoplankton and eukaryotic microorganisms were significantly affected by environmental conditions. This study shows that potential interactions exist between phytoplankton and eukaryotic microorganism communities, andcould improve our understanding of the ecological roles of phytoplankton and eukaryotic microorganisms in changing aquatic ecosystems. However, long-term investigations are necessary in order to obtain comprehensive understandings of their complicated associations.

Blooming cyanobacteria alter water flea reproduction via exudates of estrogen analogues

Cyanobacteria blooms are increasing globally, with further increases predicted in association with climate change. Recently, some cyanobacteria species have been identified as a source of estrogenic effects in aquatic animals. To explore possible estrogenic effects of Microcystis aeruginosa (an often-dominant cyanobacteria species) on zooplankton, we examined effects of cyanobacteria exudates (MaE, 2 × 10⁴ and 4 × 10⁵ cells/ml) on reproduction in Daphnia magna. We analyzed physiological, biochemical and molecular characteristics of exposed Daphnia via both chronic and acute exposures. MaE at both low and high cell density enhanced egg number (15.4% and 23.3%, respectively) and reproduction (37.7% and 52.4%, respectively) in D. magna similar to 10 μg/L estradiol exposure. In addition, both MaE of low and high cell densities increased population growth rate (15.8% and 19.6%, respectively) and reproductive potential (60% and 83%, respectively) of D. magna. These exudates promoted D. magna reproduction by stimulating 17β-hydroxysteroid-dehydrogenase (17β-HSD) activity and production of ecdysone and juvenile hormone, and by enhancing vitellogenin biosynthesis via up-regulating expression of Vtg1 and Vtg2. However, increased expression (6.6 times higher than controls) of a detoxification gene (CYP360A8) indicated that MaE might also induce toxicity in D. magna. Reproductive interference of zooplankton by blooming cyanobacteria might negatively affect foodwebs because MaE-induced zooplankton population increase would enhance grazing and reduce abundance of edible algae, thereby adding to the list of known disruptive properties of cyanobacterial blooms.