Shallow Eutrophic Lake In China Research Articles

Bloom-induced internal release controlling phosphorus dynamics in large shallow eutrophic Lake Taihu, China

High phosphorus (P) concentrations are commonly observed in lakes during algal blooms despite massive efforts on external nutrient reduction. However, the knowledge about the relative contribution of internal P loading linked with algal blooms on lake phosphorus (P) dynamics remains limited. To quantify the effect of internal loading on P dynamics, we conducted extensive spatial and multi-frequency nutrient monitoring from 2016 to 2021 in Lake Taihu, a large shallow eutrophic lake in China, and its tributaries (2017–2021). The in-lake P stores (ILSP) and external loading were estimated and then internal P loading was quantified from the mass balance equation. The results showed that the in-lake total P stores (ILSTP) ranged from 398.5 to 1530.2 tons (t), and exhibited a dramatic intra- and inter-annual variability. The annual internal TP loading released from sediment ranged from 1054.3 to 1508.4 t, which was equivalent to 115.6% (TP loading) of the external inputs on average, and responsible for the fluctuations of ILSTP on a weekly scale. High-frequency observations exemplified that ILSTP increased by 136.4% during algal blooms in 2017, while by only 47.2% as a result of external loading after heavy precipitation in 2020. Our study demonstrated that both bloom-induced internal loading and storm-induced external loading are likely to run counter significantly to watershed nutrient reduction efforts in large shallow lakes. More importantly, bloom-induced internal loading is higher than storm-induced external loading over the short term. Given the positive feedback loop between internal P loadings and algal bloom in eutrophic lakes, which explains the significant fluctuation of P concentration while nitrogen concentration decreased. It is emphasized that internal loading and ecosystem restoration are unignorable in shallow lakes, particularly in the algal-dominated region.

Arrhenius equation construction and nitrate source identification of denitrification at the Lake Taihu sediment − water interface with 15 N isotope

Total nitrogen in Taihu Lake, China has gradually decreased since 2015 while the total phosphorus concentration has exhibited an increasing trend, indicating an asynchronous change. The dominant nitrogen removal process in freshwater ecosystems is denitrification which primarily occurs at the sediment-water interface. In this study, 15N isotope incubation experiments were attempted to analyze the effect of water temperature on denitrification, to construct the regional denitrification Arrhenius equations considering water temperature, and to identify the nitrate source of denitrification in Lake Taihu sediments. The results indicated that the potential N2 production rates and denitrification rates generally decreased in the west to east direction, which was significantly positively correlated with the nitrate concentration of overlying water by Pearson correlation coefficient analysis (P < 0.05). In addition, when the water temperature was lower than 30°C, the rates of the potential N2 production and denitrification were higher with an increase in water temperature, but when the water temperature was overhigh, denitrification was inhibited. The ratio of the total denitrification rate of nitrate from the water column in the sediment to the total denitrification rate during the incubation experiment was above 0.5 at each sampling site. This indicated that the denitrification in the Lake Taihu sediment primarily occurred at the expense of nitrate from the water column. The research results of Arrhenius equation construction and nitrate source identification of denitization can be applied to improve the accuracy of water quality model of Taihu Lake, which is of great significance to improve Taihu Lake water quality, and can act as a reference for the water environment treatment of other shallow eutrophic lakes in China and abroad.

Three-dimensional observations of particulate organic carbon in shallow eutrophic lakes from space

Particulate organic carbon (POC) storage play an essential role in determining carbon sinks and water quality in shallow eutrophic lakes. Although satellite data have been adopted to observe surface POC content, the previous remote sensing results cannot indicate water column-integrated POC storage in shallow eutrophic lakes with vertically nonuniform POC profiles. Based on 272 in-situ POC profiles collected from 19 shallow eutrophic lakes in China's Yangtze-Huai Plain (YHP), this study developed a novel process-oriented method to derive water column-integrated POC storage from OLCI/Sentinel-3A satellite data. The method included three critical steps: remotely estimating surface POC contents, identifying POC profile types (uniform, exponential decay, or power decay) through a binary decision tree, and parameterizing POC profiles using surface POC concentrations. The developed algorithms showed a bias of -25.79% for in-situ match-ups from 15 lakes (N = 88). The POC in the eutrophic lakes was mainly produced by phytoplankton (R2 = 0.87). As a result, the surface POC content presented a unimodal peak in summer. However, in addition to air temperature regulations (0.52 – 68.53%), the water level also showed apparent effects on increasing POC storage, with relative contributions of 29.29 – 95.51% for the six largest lakes. Therefore, multisource satellite data are needed to derive POC storage in global eutrophic lakes remotely. This study is the first attempt to three-dimensionally observe lake POC contents from space and is important for understanding the carbon cycle in shallow eutrophic lakes.

Contrasting diversity patterns and community assembly mechanisms of bacterioplankton among different aquatic habitats in Lake Taihu, a large eutrophic shallow lake in China

Eutrophication leads to the degradation of lake habitat types from macrophyte-dominated habitats (MDH) to algae-dominated habitats (ADH), which is a common environmental problem faced by many lakes. However, the variations in diversities and community assembly processes of bacterioplankton in the process of lake eutrophication have not been thoroughly elucidated. Here, we contrasted bacterial diversity patterns and processes of community assembly among ADH, MDH, and other habitats (OH) of Lake Taihu, a large shallow eutrophic lake in China with strong wind-induced disturbances. We found that the bacterial diversity patterns and potential functions between ADH and MDH were significantly different. Moreover, the contributions of purely environmental variables to the bacterial diversity patterns of all habitat types were much higher than those of spatial variables. However, the relative importance of stochasticity in the bacterial community assembly of each habitat type was much higher than that of determinism. Intriguingly, ‘undominated’ stochastic processes shape the diversity patterns of bacterioplankton in ADH, MDH, and OH of Lake Taihu. These findings demonstrate that the degradation of lake habitats caused by eutrophication can profoundly change the diversity and potential function patterns of the bacterioplankton community in lake ecosystems. Although the distinct diversity patterns of the bacterioplankton among the different aquatic habitats in Lake Taihu can be affected by deterministic processes (local environmental variables), they were dominated by stochastic processes (drift). Our study confirms that strong, disordered, wind-induced disturbances in shallow lakes could lead to strong hydrologic mixing, thus increasing the randomness of bacterial community assembly in each habitat.

Response of planktonic diversity and stability to environmental drivers in a shallow eutrophic lake

Plankton is an important component of a healthy shallow lake aquatic system, and the diversity and stability of its community structure are impacted by numerous environmental factors. However, knowledge about the relative importance of these factors is limited. This study presents a case study conducted in semi-arid, shallow eutrophic lakes in China to evaluate the relative importance of physical, chemical, and nutritional characteristics of water quality and light effects on the spatiotemporal distributions of diversity and stability of plankton communities. The findings showed that the plankton community was the least diverse in winter and the most stable during the spring-to-summer transition. The results of partition variation showed that physical and chemical indicators, nutritional parameters, and light effects explained >70 % of the zooplankton community dynamics, compared with 37 % for phytoplankton. For phytoplankton community composition, physical and chemical parameters, followed by light, had the greatest influence. Further analysis showed that phytoplankton were influenced primarily by nutrients, water depth, and pH. Similarly, the diversity and stability of the zooplankton exhibited physical and chemical parameter responses, and the influence of nutrients also played a significant role. Structural equation modelling showed that light had minimal direct effect on the plankton and affected the plankton indirectly through changes in total nitrogen and NO3-N. This study estimates the distribution of plankton community in a shallow eutrophic lake, quantifying the relative effects of different environmental drivers on plankton diversity and stability, and provides an in-depth understanding for improving the lake water quality and controlling nutrients pollution.

Sensitivity of phytoplankton to climatic factors in a large shallow lake revealed by column-integrated algal biomass from long-term satellite observations

There are some uncertainties of using chlorophyll a (Chla) concentrations in water surface to address phytoplankton dynamics, especially in large shallow lakes, because of the dramatic vertical migration of phytoplankton. The column-integrated algal biomass (CAB) can reflect the whole water column information, so it is considered as a better indicator for phytoplankton total biomass. An algal biomass index (ABI) and an empirical algorithm were proposed previously to measure algal biomass inside and outside euphotic zone from the Moderate Resolution Imaging Spectrometer (MODIS) data. A long-term CAB time series was generated in this study to clarify the temporal and spatial changes in phytoplankton and address its sensitivity to climatic factors in Lake Chaohu, a shallow eutrophic lake in China, from 2000 to 2018. Overall, the CAB for Lake Chaohu showed significant temporal and spatial dynamics. Temporally, the annual average CAB (total CBA within the whole lake) was increased at rate of 0.569 t Chla/y, ranging from 62.06±8.89 t Chla to 76.03±10.01 t Chla during the 19-year period. Seasonal and periodic variations in total CAB presented a bimodal annual cycle every year, the total CAB was highest in summer, followed by that in autumn, and it was the lowest in winter. The pixel-based CAB (total CAB of a unit water column), ranging from 112.42 to 166.85 mg Chla, was the highest in the western segment, especially its northern part, and was the lowest in the central parts of eastern and central segments. The sensitivity of CAB dynamics to climatic conditions was found to vary by region and time scale. Specifically, the change of pixel-based algal biomass was more sensitive to the temperature change on the monthly and annual scales, while wind speed impacted directly on the short-term spatial-temporal redistribution of algal biomass. High temperature and low wind speed could prompt the growth of total CAB for the whole lake, and the hydrodynamic situations affected by wind and so on determined the spatial details. It also indicated that Lake Chaohu may face more severe challenges with the future climatic warming. This study may serve as a reference to support algal bloom forecasting and early warning management for other large eutrophic lakes with similar problems.

Suspended particles phoD alkaline phosphatase gene diversity in large shallow eutrophic Lake Taihu

The bacterial phoD gene encodes alkaline phosphatase plays an important role in the release of bioavailable inorganic phosphorus (P) from organic P in environmental systems. However, phoD gene diversity in suspended particles in shallow freshwater lakes is poorly understood. In this study, we explored the potential relationship between environmental factors and phoD phosphatase gene in suspended particles in different ecosystem types (lake zones) in Lake Taihu, a large shallow eutrophic lake in China. Quantitative PCR and high-throughput sequencing were used to analyze phoD gene abundance and the phoD-harboring bacterial community composition. Our results indicate that the distribution of phoD gene abundance in suspended particles had a high spatiotemporal heterogeneity. The phoD gene abundance in each lake zone decreased significantly from June to September. The dominant phoD-harboring phylum in all samples was Actinobacteria, followed by Proteobacteria, Cyanobacteria and Gemmatimonadetes. The first predominant phoD-harboring genera varied among samples, but most of them belonged to phylum Actinobacteria. Driven by different environmental factors, the phoD-harboring bacterial community structure varied with sampling month and ecosystem type. Nitrate and ammonia nitrogen were the main environmental drivers of phoD-harboring bacterial community in suspended particles in the river mouth zone, while water pH and dissolved oxygen were important factors for the algae-dominated, macrophyte-dominated and central lake zones.

Effects of sediment oxidation on phosphorus transformation in three large shallow eutrophic lakes in China.

Oxidation of surface sediments is an important means for altering phosphorus (P) exchanges across sediment-water interface (SWI) in shallow lakes. In this study, the potential and composition of regenerated oxidation capacity (OC) of surface sediments were evaluated in three large shallow lakes (Tai Lake, Chao Lake, and Dianchi Lake) in China; the transformation of sedimentary P was quantified through P fractionation scheme. The composition of the regenerated OC differed among these three lakes, with Fe(III) and SO42- dominant in Dianchi Lake, Mn(IV) and Fe(III) in Chao Lake and Tai Lake. Oxidation of sediments enhanced the transformation of sedimentary P and altered P exchanges across the SWI. In Chao Lake, the HCl-P was transformed to BD-P; in Tai Lake, the NaOH-P was involved too, and transformed to BD-P; whereas in Dianchi Lake, an increase in NH4Cl-P was also observed except for the transformation from HCl-P to BD-P. The sediment-to-water flux of P was enhanced with 0.17mg/g DW in Dianchi Lake and 0.08mg/g DW in Chao Lake, while a contrary water-to-sediment flux of P was observed in Tai Lake, reaching 0.01mg/g DW. This study advances our knowledge on the impacts of sediment oxidation on P cycles in lakes, which will be beneficial to eutrophication control.

Spatial-Temporal Variation of Bacterial Communities in Sediments in Lake Chaohu, a Large, Shallow Eutrophic Lake in China.

Sediment bacterial communities are critical for the circulation of nutrients in lake ecosystems. However, the bacterial community function and co-occurrence models of lakes have not been studied in depth. In this study, we observed significant seasonal changes and non-significant spatial changes in the beta diversity and community structure of sediment bacteria in Lake Chaohu. Through linear discriminant analysis effect size (LEfSe), we observed that certain taxa (from phylum to genus) have consistent enrichment between seasons. The sudden appearance of a Firmicutes population in spring samples from the Zhaohe River, an estuary of Lake Chaohu, and the dominance of Firmicutes populations in other regions suggested that exogenous pollution and environmental induction strongly impacted the assembly of bacterial communities in the sediments. Several taxa that serve as intermediate centers in Co-occurrence network analysis (i.e., Pedosphaeraceae, Phycisphaeraceae, Anaerolineaceae, and Geobacteraceae) may play an important role in sediments. Furthermore, compared with previous studies of plants and animals, the results of our study suggest that various organisms, including microorganisms, are resistant to environmental changes and/or exogenous invasions, allowing them to maintain their community structure.

Analysis of environmental drivers influencing interspecific variations and associations among bloom-forming cyanobacteria in large, shallow eutrophic lakes

Non-diazotrophic Microcystis and filamentous N2-fixing Aphanizomenon and Dolichospermum (formerly Anabaena) co-occur or successively dominate freshwaters globally. Previous studies indicate that dual nitrogen (N) and phosphorus (P) reduction is needed to control cyanobacterial blooms; however, N limitation may cause replacement of non-N2-fixing by N2-fixing taxa. To evaluate potentially counterproductive scenarios, the effects of temperature, nutrients, and zooplankton on the spatio-temporal variations of cyanobacteria were investigated in three large, shallow eutrophic lakes in China. The results illustrate that the community composition of cyanobacteria is primarily driven by physical factors and the zooplankton community, and their interactions. Niche differentiation between Microcystis and two N2-fixing taxa in Lake Taihu and Lake Chaohu was observed, whereas small temperature fluctuations in Lake Dianchi supported co-dominance. Through structural equation modelling, predictor variables were aggregated into ‘composites’ representing their combined effects on species-specific biomass. The model results showed that Microcystis biomass was affected by water temperature and P concentrations across the studied lakes. The biomass of two filamentous taxa, by contrast, exhibited lake-specific responses. Understanding of driving forces of the succession and competition among bloom-forming cyanobacteria will help to guide lake restoration in the context of climate warming and N:P stoichiometry imbalances.

A Historical Sedimentary Record of Mercury in a Shallow Eutrophic Lake: Impacts of Human Activities and Climate Change

Abstract Mercury and its derivatives are hazardous environmental pollutants and could affect the aquatic ecosystems and human health by biomagnification. Lake sediments can provide important historical information regarding changes in pollution levels and thus trace anthropogenic or natural influences. This research investigates the 100-year history of mercury (Hg) deposition in sediments from Chao Lake, a shallow eutrophic lake in China. The results indicate that the Hg deposition history can be separated into three stages (pre-1960s, 1960s–1980s, and post-1980s) over the last 100 years. Before the 1960s, Hg concentrations in the sediment cores varied little and had no spatial difference. Since the 1960s, the concentration of Hg began to increase gradually, and showed a higher concentration of contamination in the western half of the lake region than in the eastern half of the lake region due to all kinds of centralized human-input sources. The influences of anthropogenic factors and hydrological change are revealed by analyzing correlations between Hg and heavy metals (Fe, Co, Cr, Cu, Mn, Pb, and Zn), stable carbon and nitrogen isotopes (δ13C and δ15N), nutrients, particle sizes, and meteorological factors. The results show that Hg pollution intensified after the 1960s, mainly due to hydrological change, rapid regional development and urbanization, and the proliferation of anthropogenic Hg sources. Furthermore, the temperature, wind speed, and evaporation are found to interactively influence the environmental behaviors and environmental fate of Hg.

Application of Bayesian network including Microcystis morphospecies for microcystin risk assessment in three cyanobacterial bloom-plagued lakes, China

Microcystis spp., which occur as colonies of different sizes under natural conditions, have expanded in temperate and tropical freshwater ecosystems and caused seriously environmental and ecological problems. In the current study, a Bayesian network (BN) framework was developed to access the probability of microcystins (MCs) risk in large shallow eutrophic lakes in China, namely, Taihu Lake, Chaohu Lake, and Dianchi Lake. By means of a knowledge-supported way, physicochemical factors, Microcystis morphospecies, and MCs were integrated into different network structures. The sensitive analysis illustrated that Microcystis aeruginosa biomass was overall the best predictor of MCs risk, and its high biomass relied on the combined condition that water temperature exceeded 24 °C and total phosphorus was above 0.2 mg/L. Simulated scenarios suggested that the probability of hazardous MCs (≥1.0 μg/L) was higher under interactive effect of temperature increase and nutrients (nitrogen and phosphorus) imbalance than that of warming alone. Likewise, data-driven model development using a naïve Bayes classifier and equal frequency discretization resulted in a substantial technical performance (CCI = 0.83, K = 0.60), but the performance significantly decreased when model excluded species-specific biomasses from input variables (CCI = 0.76, K = 0.40). The BN framework provided a useful screening tool to evaluate cyanotoxin in three studied lakes in China, and it can also be used in other lakes suffering from cyanobacterial blooms dominated by Microcystis.

Spatial heterogeneity of food web structure in a large shallow eutrophic lake (Lake Taihu, China): implications for eutrophication process and management

The knowledge of food web spatial heterogeneity is important for ecologists and lake managers to understand ecosystem complexity and lake management. Lake Taihu, a large shallow eutrophic lake in China, has two distinct zones: algae- and macrophyte-dominated. In this study, we assessed the spatial heterogeneity of food webs in the two lake zones using stable isotope analysis and isotope mixing model. The basal sources and consumers showed significant differences in δ13C and δ15N ratios between the two lake zones, except for the filter-feeding fishes. Overall, more δ13C-depleted and δ15N-enriched ratios were found in the algae- than the macrophyte-dominated zones for basal sources and consumers. Although the consumers in the algae-dominant zone had higher average δ15N values, the food web of the macrophyte-dominated zone had longer food chain length and more diverse trophic linkages. These spatial differences may have resulted from resource availability and environmental stress of Lake Taihu ecosystem. Factors associated with spatial trophic heterogeneity should be considered for the management and restoration of this shallow eutrophic lake.

High-resolution characterization of arsenic mobility and its correlation to labile iron and manganese in sediments of a shallow eutrophic lake in China

Understanding the status and dynamics of labile arsenic (As), iron (Fe), and manganese (Mn) across the sediment/water interface (SWI) is essential for As cycling in eutrophic lakes. This study conducted systematic monitoring of labile As, Fe, and Mn in water and sediments in regions of varying eutrophic levels in Lake Taihu to investigate Fe- or Mn-coupled cycling of As in sediments. We arranged high-resolution combined ZrO-Chelex diffusive gradients in thin films (DGT) probes in intact sediment cores to measure labile As, Fe, and Mn (i.e., DGT-labile As, Fe, and Mn) under static condition for 24 h. After retrieval, we rinsed each probe first with lake water and then with deionized water to remove any visible sediment particles for analysis. Results showed that DGT-labile As in overlying water and sediments was relatively higher in the middle or hyper-eutrophic regions compared to the mesotrophic region. Diffusive flux across the SWI further demonstrated that the internal dissolution of As in the middle or hyper-eutrophic region was stronger than the mesotrophic region, indicating that eutrophic environments may facilitate As release in sediments. We observed a positive correlation between DGT-labile As and Fe or Mn, which indicated that the reductive dissolution of As binding Fe/Mn oxyhydroxides in sediments was responsible for the internal release of As, which strengthens under eutrophic environments. Furthermore, the DGT-labile molar ratio of Fe/As or Mn/As had a significant positive relation with Fd, further confirming that labile Fe or Mn is much more relevant for sediment As cycling. Eutrophic environments facilitated reductive dissolution of As binding Fe/Mn oxyhydroxides in sediments, resulting in the release of more labile As in the middle or hyper-eutrophic region.

Unionid shells (Hyriopsis cumingii) record manganese cycling at the sediment-water interface in a shallow eutrophic lake in China (Lake Taihu)

Aquatic eutrophication is becoming a serious environmental problem throughout the world. The utility of bivalves as bio-filters to improve water quality and reduce algal blooms has been widely acknowledged, but the potential usefulness of bivalve shells as retrospective monitors of eutrophication-induced environmental change has received little attention. Here, we present the first multi-year, high-resolution Mn/Cashell records of the freshwater mussel, Hyriopsis cumingii (Lea, 1852) from a shallow eutrophic lake (Lake Taihu, China). Mn/Cashell time-series of the two studied shells exhibit a high degree of synchrony after being placed in a precise temporal context by means of growth pattern analysis. There is a large inter-annual variability of Mn/Cashell records during 2011–2015, with the highest value occurring in 2013. Mn/Cashell also displays a pronounced intra-annual variability with maxima consistently occurring during late spring/early summer. The high reproducibility of Mn/Cashell time-series among contemporaneous specimens highlights the existence of strong environmental rather than biological control on the incorporation of Mn into the shells. In particular, the striking feature of late spring/early summertime Mn/Cashell maxima is synchronous with the occurrence of reducing conditions beneath the sediment-water interface (SWI). The latter results in substantial increases of biologically available Mn2+ in the sediment pore water and organic particles, the element being rapidly taken up from these sources by the mussels with subsequent incorporation of Mn into their shells. Therefore, Mn/Cashell can potentially serve as a high-resolution proxy of the mobility of Mn at the SWI. As demonstrated by the present study, documenting and understanding geochemical properties of bivalve shells can help to retrospectively monitor eutrophication-induced environmental change in aquatic ecosystems.

Effects of climate change on bioaccumulation and biomagnification of polycyclic aromatic hydrocarbons in the planktonic food web of a subtropical shallow eutrophic lake in China

To date effects of climate change on bioaccumulation and biomagnification of chemical pollutants in planktonic food webs have rarely been studied. Recruitments of plankton have shifted earlier due to global warming. Global warming and precipitation patterns are projected to shift seasonally. Whether and how the shifts in plankton phenology induced by climate change will impact bioaccumulation and biomagnification of chemical pollutants, and how they will respond to climate change are largely unknown. Here, we combine data analysis of the past seven decades, high temporal resolution monitoring and model development to test this hypothesis with nine polycyclic aromatic hydrocarbons (PAHs) in the planktonic food web of a subtropical shallow eutrophic lake in China. We find biphasic correlations between both bioconcentration factors and bioaccumulation factors of the PAHs and the mean temperature, which depend on the recruitment temperatures of cyanobacteria, and copepods and cladocerans. The positive correlations between bioconcentration factors, bioaccumulation factors and the mean temperature will be observed less than approximately 13–18 days by 2050–2060 due to the shifts in plankton phenology. The PAHs and their bioaccumulation and biomagnification will respond seasonally and differently to climate change. Bioaccumulation of most of the PAHs will decrease with global warming, with higher decreasing rates appearing in winter and spring. Biomagnification of most of the PAHs from phytoplankton to zooplankton will increase with global warming, with higher increasing rates appearing in winter and spring. Our study provides novel insights into bioaccumulation and biomagnification of chemical pollutants in eutrophic waters under climate change scenarios.

Precipitation and temperature drive seasonal variation in bioaccumulation of polycyclic aromatic hydrocarbons in the planktonic food webs of a subtropical shallow eutrophic lake in China.

Hydrophobic organic contaminants (HOCs) are toxic and ubiquitous in aquatic environments and pose great risks to aquatic organisms. Bioaccumulation by plankton is the first step for HOCs to enter aquatic food webs. Trophic status is considered to dominate variations in bioaccumulation of HOCs in plankton in temperate and frigid deep oligotrophic waters. However, long-term driving factors for bioaccumulation of HOCs in planktonic food webs of subtropical shallow eutrophic waters have not been well investigated. China has the largest subtropical lake density in the Northern Hemisphere. Due to limited field data, long-term variations in the bioaccumulation of HOCs in these lakes are almost unknown. Here we take Lake Xuanwu as an example to investigate long-term variations in the bioaccumulation, and biomagnification of polycyclic aromatic hydrocarbon (PAHs) in planktonic food webs of subtropical shallow eutrophic lakes in China, and elucidate the driving factors. Our results indicate that temperature rather than nutrients dominates long-term dynamics of planktonic biomass in this lake. Precipitation significantly enhances the concentrations of the PAHs, and total suspended particles, and consequently affects the distribution of the PAHs in the water column. Biomass dilution induced by temperature dominates bioaccumulation of the PAHs by both phytoplankton and zooplankton (copepods and cladocerans). Biomagnification of the PAHs from phytoplankton to zooplankton is positively correlated with temperature. Our study suggests that temperature and precipitation drive long-term variations in the bioaccumulation of the PAHs in the planktonic food webs of this subtropical shallow eutrophic lake. Lake Xuanwu has a similar mean annual temperature, annual precipitation, sunshine duration, and nutrient levels as other subtropical shallow eutrophic lakes in China. This study may also help to understand the bioaccumulation of HOCs in planktonic food webs of other subtropical shallow eutrophic lakes.

Nitrification and denitrification by algae-attached and free-living microorganisms during a cyanobacterial bloom in Lake Taihu, a shallow Eutrophic Lake in China

Cyanobacterial blooms may stimulate epiphytic nitrification and denitrification in the water column. To validate this hypothesis, a 4-week floating mesocosms experiment that involved a cyanobacterial decay–growth–decay period was conducted at Lake Taihu. In addition to conventional methods for detecting the physical and chemical properties, quantitative real-time PCR was used to identify the nitrification and denitrification genes (archaeal and bacterial amoA, nirS and nirK). Treatment with cyanobacteria led to removal of about 3.62 mg N L−1 total nitrogen, 40% of which was organic nitrogen, indicating a nitrogen transformation and removal mechanism was present in the system. Variations in the biogeochemical properties suggested that remineralization and coupling nitrification and denitrification by epiphytic and pelagic microorganisms was the primary pathway through which organic nitrogen was removed. The results of this study revealed that algal blooms can accelerate nitrogen removal efficiency, which may be the primary reason that nitrogen is limited in summer in Lake Taihu.

Fluorescent components and spatial patterns of chromophoric dissolved organic matters in Lake Taihu, a large shallow eutrophic lake in China.

Water samples at both surface and bottom layers were taken from 102 sites in Lake Taihu to study the fluorescent components and spatial patterns of chromophoric dissolved organic matters (CDOM). Three-dimensional excitation-emission matrix data obtained from the samples were analyzed by parallel factor approach in which four humic-like and two protein-like fluorescent components (named C1-C6) were identified. The results showed that fluorescence intensities were higher in the northern and western lake regions, and notable declines of fluorescence maxima (F max) were observed from the northwest to the center and then to the southeast of the lake. Calculated biological index (BIX) values ranged from 0.88 to 1.44 and humification index (HIX) values from 0.64 to 3.37 for all the samples. The spatial variations of BIX and HIX values suggested stronger allochthonous CDOM characteristics in Zhushan Bay and the western area and autochthonous characteristics in the southern and eastern areas. Vertically, the average F max value of the surface samples was about 6% less than that of the bottom samples, but noticeable variations existed among different sampling sites and components. These distribution characteristics of CDOM were mainly attributed to the spatial heterogeneity of sources and wind-induced transportation process. Interestingly, the C6 component (Exmax/Emmax=250/455nm) seemed to be unique in samples from Zhushan Bay and probably resulted from the discharge of the Taige River. Therefore, it could be used as an indicator of point-source discharge and a tracer to study the fate of CDOM in the lake.

Photosynthetic characteristics and inferred changes in thylakoid membrane fluidity determine bloom succession between Anabaena and Microcystis in eutrophic lakes

Anabaena and Microcystis are common dominant cyanobacterial genera and frequently form blooms in large shallow eutrophic lakes in China. There is usually a quick succession between these two cyanobacterial blooms in early summer; however, there is currently no solid evidence based on photosynthesis and population competition to explain this phenomenon. This study compared the photosynthetic parameters and the indirectly measured thylakoid membrane fluidity characteristics between Anabaena and Microcystis. The results showed that the growth rate, the maximum photochemical efficiency of PSII (F v/F m), and the maximum relative electron transport rate (r ETR max) of Microcystis cultured at 25 °C were significantly lower than those from cells grown at 15 °C, indirectly suggesting a marked limitation of thylakoid membrane fluidity at low temperatures. Chlorophyll fluorescence dynamics also indicated inhibition of NPQ formation and membrane fluidity when Microcystis was cultured at low temperatures; however, this inhibition was not significant for Anabaena. Fourier transition infrared (FT-IR) spectrometry indicated that the thylakoid membrane of Anabaena can maintain homeostasis fluidity through the upregulation of unsaturated fatty acids when exposed to chilling stress. Increasing the temperature significantly promoted the growth rate of Microcystis, while it had little effect on Anabaena. Moreover, photoinhibition of Anabaena was more easily induced by high light, thus accelerating the bloom succession from Anabaena to Microcystis during warm seasons. An activation-energy model was used to deduce that the threshold temperature of bloom succession was 15.4 °C, which would increase to 16.2 °C and decrease to 14.9 °C under conditions of N-limitation and P-limitation.