Eutrophic Lake Taihu Research Articles

Biotic factors shape the structure and dynamics of denitrifying communities within cyanobacterial aggregates.

Eutrophication caused by human activities has severely impacted freshwater ecosystems, leading to harmful cyanobacterial blooms that threaten water quality and ecosystem stability. During blooms, denitrification is a key process for nitrogen removal, which can occur both in the sediment and in the waterbody mediated by cyanobacterial aggregate (CA)-associated microorganisms. In this study, the structure, dynamics and assembly mechanisms of CA-associated nirK-, nirS-, and nosZ-encoding denitrifying communities were investigated in the eutrophic Lake Taihu across the bloom season. External environmental factors showed limited influence on denitrifying community structure, which were more strongly shaped by biotical factors. Network analysis revealed both monofunctional and multifunctional modules, with a small subset of linker OTUs playing a critical role in maintaining these multifunctional modules by mediating interactions among different functional communities. Biotic regulation was further demonstrated by coupling patterns among denitrifiers associated with specific microbial lineages, as well as niche partitioning driven by cyanobacterial composition. While the assembly of the total phycospheric communities was governed by stochastic processes, the denitrifying communities were predominantly shaped by homogeneous selection. These findings indicate biological processes, particularly synergistic relationships and autotroph-heterotroph interactions, were key drivers of the structure and dynamics of denitrifying communities within CAs. Moreover, the key taxa identified in this study may provide potential targets for regulating nitrogen cycling in the management of cyanobacterial blooms.

Intra-colony light regulates morphology diversity of colonial cyanobacteria

Morphological diversity widely exists in colonial cyanobacteria, which poses a challenge to monitor and predict population dynamics. Light-induced adaptation plays a key role in the underlying mechanisms. Here, to demonstrate how morphological parameters adapt to physiological features (such as pigments and gas vesicles) under light limitation, a model of intra-colony light regimes (ICLR) was developed based on the colonies of Microcystis Kützing. that caused cyanobacterial harmful blooms worldwide. Cell pigments and gas vesicles positively regulated cellular light attenuation coefficient (D(λ)cell), and both accounted for the D(λ)cell variation of 68.2 % and 30.1 %, respectively. According to the ICLR model, D(λ)cell negatively correlated to colony thickness, and a positive relationship existed between colony thickness and cell-to-cell distance (an indicator of colony compactness). Therefore, increasing cell-to-cell distance is an effective strategy when the D(λ)cell is high or colonies are light-limiting, facilitating the formation of loose, large colonies. This pattern was also observed in Microcystis populations in eutrophic Lake Taihu (China) and cyanobacterial Nostoc sphaeroides Kützing, showing cell-to-cell distance positively related to colony thickness or colony depth. This suggests that the predicted morphological adaptation patterns based on the ICLR model are general in colony-forming cyanobacteria. These findings provide new insights into the morphology diversity of colonial cyanobacteria toward changing environments.

Community Assembly Mechanisms of nirK- and nirS-type Denitrifying Bacteria in Sediments of Eutrophic Lake Taihu, China.

Denitrifying bacteria, particularly nirK- and nirS-type, are functionally equivalent and could occupy different niches, but their community assembly mechanisms and responses to environmental heterogeneity are poorly understood in eutrophic lakes. In this study, we investigated the community assembly mechanisms of nirK- and nirS-type denitrifying bacteria and clarified their responses to sediments environmental factors in Lake Taihu, China. The quantitative real-time PCR and Illumina HiSeq-based sequencing revealed that the abundance and composition of two types of denitrifyingbacterial communities varied among different sites in the sediments of Lake Taihu. The functions of these two types of denitrifying bacteria were assigned to mainly nitrogen cycling along with carbon, oxygen, and sulfur cycling, indicating their diverse ecosystems functions. Neutral community model showed that majority of nirK- and nirS-type denitrifying bacteria were neutrally distributed, while dispersal and selection were the dominant drivers in shaping community assembly of nirK-type bacteria. The community assembly of nirS-type was mainly driven by homogeneous selection. We found complex network interactions between nirK- and nirS-type denitrifying bacteria with other bacterial communities, indicating the importance of otherbacterial coexistence for ecosystem functions by denitrifying bacteria in lake sediments. Keystone taxa of other bacteria showed the highest interactions with denitrifying bacteria; further, a strong significant correlation between keystone taxa with environmental factors and sediment enzyme content revealed by Mantel tests. Specially, total phosphorous was the key environmental factor determining the composition and diversity of nirK and nirS-type denitrifying bacteria in lake sediments, whereas NAR, AmoA, and NIR were the key reductase enzymes directly or indirectly affected to them. Our results provide significant insights into understanding the effects of changing nirK- and nirS-type denitrifying bacterial diversities and underlying community assembly mechanisms under changing environmental conditions in eutrophic lake ecosystems.

Are you filtering enough? Unveiling the impact of water sample volumes on eukaryotic microbial community in freshwater lakes

The appropriate volume of filtered water samples is crucial for accurately reflecting microbial community characteristics. Previous research has shown that changes in water sample volumes significantly affect bacterial diversity and the relative abundance of dominant taxa in marine environments. However, the specific impact on freshwater microbial diversity and community structure, particularly for eukaryotic microorganisms, remains unclear. To address this gap, we collected water samples from eutrophic Lake Taihu and mesotrophic Lake Bosten. We filtered the samples through 0.2 μm filters using volumes ranging from 0.1 to 2 L for Lake Taihu and 0.1 to 3.2 L for Lake Bosten. Analysis of 18S rRNA gene amplicons revealed that variations in filtered water sample volume significantly affected the diversity, community structure, and composition of eukaryotic microorganisms. In light of these findings, we recommend using a filtered water sample volume of approximately 0.2 L for eutrophic aquatic ecosystems and 0.8 L for mesotrophic aquatic ecosystems in microbial analyses. These recommendations underscore the importance of selecting the appropriate filtered water sample volume to ensure representative and reliable results in the study of eukaryotic microorganisms in freshwater ecosystems.

Contrasting but interconnecting metatranscriptome between large buoyant and small suspended particles during cyanobacterial blooming in the large shallow eutrophic Taihu Lake

Large cyanobacterial colonies as visible particles floating on the water surface provide different microbial niches from small particles suspended in the water column in eutrophic freshwaters. However, functional potential differences among microbes colonizing on these contrasting particles are not well understood. Here, the metatranscriptome of microbes inhabiting these two kinds of particles during cyanobacterial bloom (dominated by Microcystis spp.) was analyzed and compared. Community compositions of active bacteria associated with small suspended particles (SA, aggregates dominated by small cyanobacteria colonies, other algae and detritus, etc.) were much more diverse than those associated with large buoyant cyanobacterial colonies (LA), but functional diversity was not significantly different between them. Transcripts related to phosphorus and nitrogen metabolism from Proteobacteria, and respiration from Bacteroidetes were enriched in LA, whereas many more pathways such as photosynthesis from Cyanobacteria, cofactors, and protein metabolism from all dominant phyla were enriched in SA. Nevertheless, many transcripts were significantly correlated within and between LA and SA. These results indicated interconnection of bacteria between LA and SA. Moreover, many transcripts in SA were significantly correlated with transcripts from cyanobacterial phycobilisome in LA, indicating that bacterial metabolism in SA may influence cyanobacterial biomass in LA. Thus, the prediction of cyanobacterial blooms by bacterial activity in SA may be possible when there is no visible bloom on the water surface.

Microbial nitrogen cycling in Microcystis colonies and its contribution to nitrogen removal in eutrophic Lake Taihu, China

Cyanobacterial blooms induced by excessive loadings of nitrogen (N) and other nutrients are a severe ecological problem in aquatic ecosystems. Previous studies of N removal have primarily focused on sediment-water interface, yet the role of cyanobacterial colonies has recently been attracting more research attention. In this study, N cycling processes were quantified for cyanobacterial colonies (primarily Microcystis colonies) and their contribution to N removal was estimated for a large, shallow eutrophic lake in China, Lake Taihu. Various N cycling processes were determined via stable 15N isotope, together with 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) chip. Denitrification was found to be the most prominent process, estimated to be 36.63, 9.85, 3.35, and 3.15 times higher than dissimilatory nitrate reduction to ammonium (DNRA), nitrification, ammonium (NH4+) uptake and nitrate (NO3−) uptake rates, respectively. Denitrifiers accounted for a large part of the bacterial taxa (35.50 ± 24.65%), and the nirS gene was the most abundant among N cycling-related genes, with (2.54 ± 0.51) × 109 copies g−1Microcystis colonies. A field investigation revealed a positive correlation between the potential denitrification rate and the Chl-a concentration (mostly derived from Microcystis colonies). Based on a multiple stepwise regression model and historical data from 2007 to 2015 for Lake Taihu, the total amount of N removed via denitrification by Microcystis colonies was estimated at 171.72 ± 49.74 t yr−1; this suggests that Microcystis colonies have played an important role in N removal in Lake Taihu since the drinking water crisis in 2007. Overall, this study revealed the importance of denitrification within Microcystis colonies for N removal in eutrophic lakes, like Lake Taihu.

Co-accumulation characteristics and interaction mechanism of microplastics and PFASs in a large shallow lake

Microplastics (MPs) and per- and polyfluoroalkyl substances (PFASs) coexist widely in lakes and affect ecological security. The coexistence characteristics and adsorption-desorption mechanisms between MPs and typical PFASs were explored in a typical eutrophic shallow lake (Taihu Lake). Polyvinyl chloride (PVC) and polyethylene (PE) are the primary types of MPs in Taihu Lake, with average abundances in water and sediment of 18630 n/m3 and 584 n/kg, respectively. The average concentrations of PFASs in water and sediment are 288.93 ng/L and 4.33 ng/g, with short-chain PFASs (C4-C7) being the main pollutants. Perfluorobutanoic acid (PFBA) in both water and sediment contributed 38.48 % and 44.53 %, respectively, followed by hexafluoropropylene oxide dimer acid (HFPO-DA). The morphological characteristics of MPs influence the distribution of long-chain PFAS in lake water, while the presence of HFPO-DA and perfluorohexanoic acid (PFHxA) in sediment is directly linked to the concentration and size of MPs. A combination of field investigations and indoor experiments revealed that the irreversible adsorption characteristics between MPs and HFPO-DA may promote the high cumulative flux of HFPO-DA in sediment, and the biofilm on the surface of MPs significantly accelerates this accumulation process. The results provide a new perspective on the co-transport behavior of emerging pollutants in aquatic environments.

Response of sulfate concentration to eutrophication on spatio-temporal scale in freshwater lakes

The dramatical increase of sulfur concentration in eutrophic lakes, especially sulfate (SO42−), has brought attention to the impact on the lake ecosystem; however, the mechanisms driving the intensification of eutrophication and the role of SO₄2− concentrations remain poorly understood. To assess the impact of eutrophication on SO42− dynamics in lakes, this study monitored SO42− concentrations in water and sediments across seven lakes with varying trophic statuses on a spatial scale, and in the eutrophic Lake Taihu over one year on a temporal scale, as well as a series of microcosms with different initial SO42− concentrations. Exogenous sulfur input is the primary driver of increased SO42− concentrations in lakes, the highest SO42− concentration in overlying water was 100 mg/L, as well as which reached 310.9 mg/L in sediment. The concurrent input of nutrients such as nitrogen and phosphorus exacerbated eutrophication, resulting in the destabilization of the sulfur cycle. Eutrophication promoted the SO42− concentration on the spatio-temporal scale, especially in sediment, and trophic lake index (TLI) showed a positive correlation with the SO42− in sediments (R2 = 0.99; 0.88). The SO42− concentration in water and TLI showed a nonlinear correlation on the temporal scale (R2 = 0.44), and showed a positive correlation on the spatial scale (R2 = 0.49). Microscopic experiments demonstrate that the anaerobic environment created by cyanobacteria decomposition induced sulfate reduction and significantly reduces SO42− concentrations. Concurrently, the anaerobic environment facilitates the coupling of iron reduction with sulfate reduction, leading to a substantial increase in Acid Volatile Sulfides (AVS) in the sediment. These findings reveal that eutrophication has a dual effect on the dynamic change of SO42− concentrations in overlying water, which is helpful to accurately evaluate and predict the change of SO42− concentrations in lakes.

Characteristics of Phytoplankton Productivity in Three Typical Lake Zones of Taihu, China

In lake aquatic ecosystems, which form the material and energy base of lakes, primary production is critical. The present study addresses the characteristics of primary phytoplankton productivity and its relationship with environmental physicochemical factors in three typical zones (algae-type, transition, and grass-type) of the eutrophic Lake Taihu. Seasonal sampling was conducted, and black–white bottle oxygen measurement was used to determine the primary productivity in different water layers in the lake. The results show obvious temporal and spatial differences in the physicochemical factors and phytoplankton productivity in Lake Taihu. The water column productivity and respiration conformed to the following seasonal descending order, summer > fall > spring > winter, and the following regional descending order, algae-type zone > transition zone > grass-type zone. The seasonal proportions of primary productivity were approximately 40% in the summer, 25% in the fall, 20% in the spring, and less than 15% in the winter. The highest values of water layer productivity and respiration were mainly at a depth of 0.2 m and decreased with the water depth. The percentage of productivity at different water layers was 23% (0 m), 31% (0.2 m), 23% (0.4 m), 11% (0.6 m), 7% (0.8 m), and 5% (1 m). The optical compensation depth for Lake Taihu was about 0.8 times the transparency (SD). Spearman correlation indicated that the temperature (T) and water depth (D) had an obvious impact on productivity in all three lake zones. Multiple stepwise regression suggested that T, D, SD, and chlorophyll-a (Chl-a) can be used to estimate productivity in different seasons/regions. The main influencing factors on phytoplankton productivity are T, D, Chl-a, and SD in the algae-type and transition zones and T, D, and total suspended solids (TSSs) in the grass-type zone.

A new model to estimate shallow lake nitrogen removal rate based on satellite derived variables

Lake nitrogen (N) removal, mainly resulting from bacterial denitrification that converts nitrate (NO3 −) to gaseous N (N2), is important for lake water quality and eutrophication control. However, quantifying lake N removal is challenging due to the high background atmosphere N2 concentration and the heavy burden of field surveys, leading to a decoupling of watershed N management and water quality improvement. Here, we developed and validated an innovative nonlinear model for lake N removal rate estimation by linking the N removal rates with remote sensing-derived variables (chlorophyll-a, chromophoric dissolved organic matter, and lake surface water temperature). The model was validated in shallow eutrophic Lake Taihu in the Yangtze River basin and at the global scale. Based on the new N removal model, we estimated that an annual average of 3.21 × 104 t N yr‒1 was removed in Lake Taihu from 2011 to 2020, accounting for 53%–66% of the total lake N loading. The remaining N loading after denitrification removal in Lake Taihu would be approximately 2.37 mg N l‒1, and 0.79 × 104 t N y‒1 of lake N loading still needs to be removed to meet the target of class IV water quality (1.5 mg N l‒1). This is the first study linking lake N removal in sediment microcosm incubations with reach-scale remote sensing derived variables, providing timely-much insights into lake N removal. This approach can be easily applied in other lakes with satellite derived data, to better understand lake N budget, drivers of eutrophication control, and watershed N management.

Ecological water diversion activity changes the fate of carbon in a eutrophic lake

Lake eutrophication mitigation measures have been implemented by ecological water diversion, however, the responses of carbon cycle to the human-derived hydrologic process still remains unclear. With a famous river-to-lake water diversion activity at eutrophic Lake Taihu, we attempted to fill the knowledge gap with integrative field measurements (2011–2017) of gas carbon (CO2 and CH4) flux, including CO2-equivalent, and dissolved carbon (DOC and DIC) at water-receiving zone and reference zone. Overall, results showed the artificial water diversion activity increased gas carbon emissions. At water-receiving zone, total gas carbon (expressed as CO2-equivalent) emissions increased significantly due to the occurring of water diversion, with CO2 flux increasing from 9.31 ± 16.28 to 18.16 ± 12.96 mmol C m−2 d−1. Meanwhile, CH4 emissions at water-receiving zone (0.06 ± 0.05 mmol C m−2 d−1) was double of that at reference zone. Water diversion decreased DOC but increased DIC especially at inflowing river mouth. Temporal variability of carbon emissions and dissolved carbon were linked to water temperature, chlorophyll a, and nutrient, but less or negligible dependency on these environment variables were found with diversion occurring. Water diversion may increase gas carbon production via stimulating DOC mineralization with nutrient enrichment, which potentially contribute to increasing carbon emissions and decreasing DOC at the same time and the significant correlation between CO2 flux and CH4 flux. Our study provided new insights into carbon biogeochemical processes, which may help to predict carbon fate under hydrologic changes of lakes.

Quantification of Diffusive Methane Emissions from a Large Eutrophic Lake with Satellite Imagery.

Lakes are major emitters of methane (CH4); however, a longstanding challenge with quantifying the magnitude of emissions remains as a result of large spatial and temporal variability. This study was designed to address the issue using satellite remote sensing with the advantages of spatial coverage and temporal resolution. Using Aqua/MODIS imagery (2003-2020) and in situ measured data (2011-2017) in eutrophic Lake Taihu, we compared the performance of eight machine learning models to predict diffusive CH4 emissions and found that the random forest (RF) model achieved the best fitting accuracy (R2 = 0.65 and mean relative error = 21%). On the basis of input satellite variables (chlorophyll a, water surface temperature, diffuse attenuation coefficient, and photosynthetically active radiation), we assessed how and why they help predict the CH4 emissions with the RF model. Overall, these variables mechanistically controlled the emissions, leading to the model capturing well the variability of diffusive CH4 emissions from the lake. Additionally, we found climate warming and associated algal blooms boosted the long-term increase in the emissions via reconstructing historical (2003-2020) daily time series of CH4 emissions. This study demonstrates the great potential of satellites to map lake CH4 emissions by providing spatiotemporal continuous data, with new and timely insights into accurately understanding the magnitude of aquatic greenhouse gas emissions.

Why do algal blooms intensify under reduced nitrogen and fluctuating phosphorus conditions: The underappreciated role of non‐algal light attenuation

AbstractLight availability for phytoplankton in shallow lakes is closely related to non‐algal light attenuation (KdNALA, the fractional light absorption by non‐algal substances, such as suspended sediments); thus, significant changes in global wind speed in recent decades may have a profound effect on light availability and algal blooms in shallow lakes. Herein, the eutrophic shallow Lake Taihu was selected to investigate the long‐term dynamics of light availability and its effect on algal blooms. The results showed that KdNALA decreased from approximately 3.5 to 2.5 m−1 with decreasing wind speed from 2005 to 2021, indicating the significantly increased light availability for phytoplankton despite surface photosynthetically active radiation exhibiting limited variability during the study period. In addition, both experimental (i.e., in situ nutrient enrichment experiments) and statistical approaches (i.e., deviations of Trophic State Index subindices) indicated that phytoplankton growth was primarily light‐limited during the study period in Lake Taihu. Consequently, considerable increases in fraction of observed and maximal chlorophyll a yield at given nitrogen or phosphorus concentration were observed and were mostly related to decreasing KdNALA, which implied that decreasing KdNALA allowed phytoplankton utilize “unused nutrient‐capacity” until the additional algal‐turbidity induce further light limitation or nutrient limitation. As the effect of changes in global wind speed on KdNALA and algal growth received limited attention in the existing research, we revealed an underappreciated mechanism by which global changes in wind speed significantly affects algal biomass by influencing light availability, which may have profound effects on future algal bloom mitigation efforts in shallow lakes.

Variable promotion of algae and macrophyte organic matter on methanogenesis in anaerobic lake sediment

Shallow lakes are an important natural source of atmospheric methane (CH4), and the input of autochthonous organic matter (OM) into their sediments encourages methanogenesis. Although algal- and macrophytic-originated OM in these lakes are expected to have different impacts on methanogenesis and methanogenic archaeal communities in lake sediments owing to their various properties, their specific influence and role in sediment remain unclear. In this study, a 148-day incubation was carried out by adding algal- and macrophytic-OM to the sediments of shallow eutrophic Lake Chaohu and Lake Taihu in China. CH4 was periodically monitored, while the methanogens were examined via qPCR and high-throughput sequencing at the end of incubation. Algal-OM stimulated CH4 production more than macrophytic-OM in both sediments, with the rates initially increasing and then decreasing before reaching a relative constant. Macrophytic-OM promoted CH4 production to a comparable extent in both lakes, while algal-OM promoted greater CH4 in Lake Chaohu than in Lake Taihu. However, algal-OM did not significantly increase mcrA gene copies, while macrophytic-OM did by 17.0–20.1-fold. Algal-OM potentially promoted the methylotrophic pathway in Lake Taihu but did not change the methanogenic structure in Lake Chaohu. Comparatively, macrophytic-OM promoted CH4 production mainly by acetoclastic methanogen proliferation in both lakes. More CH4 release with algal-OM compared to macrophytic-OM deserves further attention owing to the prevailing increasing algal blooms and the declining macrophyte population in lakes.

The unprecedented 2022 extreme summer heatwaves increased harmful cyanobacteria blooms

Heatwaves are increasing and expected to intensify in coming decades with global warming. However, direct evidence and knowledge of the mechanisms of the effects of heatwaves on harmful cyanobacteria blooms are limited and unclear. In 2022, we measured chlorophyll-a (Chla) at 20-s intervals based on a novel ground-based proximal sensing system (GBPSs) in the shallow eutrophic Lake Taihu and combined in situ Chla measurements with meteorological data to explore the impacts of heatwaves on cyanobacterial blooms and the potential relevant mechanisms. We found that three unprecedented summer heatwaves (July 4–15, July 22–August 16, and August 18–23) lasting a total of 44 days were observed with average maximum air temperatures (MATs) of 38.1 ± 1.9 °C, 38.7 ± 1.9 °C, and 40.2 ± 2.1 °C, respectively, and that these heatwaves were characterized by high air temperature, strong PAR, low wind speed and rainfall. The daily Chla significantly increased with increasing MAT and photosynthetically active radiation (PAR) and decreasing wind speed, revealing a clear promotion effect on harmful cyanobacteria blooms from the heatwaves. Moreover, the combined effects of high temperature, strong PAR and low wind, enhanced the stability of the water column, the light availability and the phosphorus release from the sediment which ultimately boosted cyanobacteria blooms. The projected increase in heatwave occurrence under future climate change underscores the urgency of reducing nutrient input to eutrophic lakes to combat cyanobacteria growth and of improving early warning systems to ensure secure water management.

Contributions of meteorology and nutrient to the surface cyanobacterial blooms at different timescales in the shallow eutrophic Lake Taihu

Quantitative assessments of the contributions of various environmental factors to cyanobacterial blooms at different timescales are lacking. Here, the hourly cyanobacterial bloom intensity (CBI) index, a proxy for the intensity of surface cyanobacterial biomass, was obtained from the geostationary satellite sensor Geostationary Ocean Color Imager (GOCI) over the years 2011–2018. Generalized additive model was applied to determine the responses of monthly and hourly CBI to the perturbations of meteorological factors, water stability and nutrients, with variation partitioning analysis used to analyze the relative importance of the three groups of variables to the inter-monthly variation of diurnal CBI in each season. The effects of environmental factors on surface cyanobacterial blooms varied at different timescales. Hourly CBI increased with increasing air temperature up to 18 °C but decreased sharply above 18 °C, whereas monthly CBI increased with increasing air temperature up to 30 °C and stabilized thereafter. Among all the environmental factors, air temperature had the largest contribution to the intra-daily variation in CBI; water stability had the highest explanation rate for the inter-monthly variation of diurnal CBI during summer (42.3 %) and autumn (56.9 %); total phosphorus explained the most variation in monthly CBI (18.5 %). Compared with cyanobacterial biomass (CB) in the water column, high light and low wind speed caused significantly lower CBI in July and higher CBI in November respectively. Interestingly, cyanobacterial blooms at the hourly scale were aggravated by climate warming during winter and spring but inhibited during summer and autumn. Collectively, this study reveals the effects of environmental factors on surface cyanobacterial blooms at different timescales and suggests the consideration of the hourly effect of air temperature in short-term predictions of cyanobacterial blooms.

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.

Horizontal and vertical migration of cyanobacterial blooms in two eutrophic lakes observed from the GOCI satellite

Under the variations of natural conditions (temperature, wind speed, light, et al.) and self-regulation of buoyancy, cyanobacterial blooms can change rapidly in a short time. The Geostationary Ocean Color Imager (GOCI) can provide hourly monitoring of the dynamics of algal blooms (eight times per day), and has potential in observing the horizontal and vertical movement of cyanobacterial blooms. Based on the fractional floating algae cover (FAC), the diurnal dynamics and migration of floating algal blooms were evaluated, and the horizontal and vertical migration speed of phytoplankton was estimated from the proposed algorithm in two eutrophic lakes, Lake Taihu and Lake Chaohu in China. The locations, number, and area of algal bloom patches showed the hotspots and horizontal movement of bloom patches. The spatial and seasonal variations of the vertical velocities indicated that both the rising and sinking speed were higher in summer and autumn than those in spring and winter. The factors affecting diurnal horizontal and vertical migrations of phytoplankton were analyzed. Diffuse horizontal irradiance (DHI), direct normal irradiance (DNI), and temperature had significant positive relationships with FAC in the morning. Wind speed contributed 18.3 and 15.1% to the horizontal movement speed in Lake Taihu and Lake Chaohu, respectively. The rising speed was more related to DNI and DHI in Lake Taihu and Lake Chaohu with contribution of 18.1 and 16.6%. The horizontal and vertical movement of algae provide important information for understanding phytoplankton dynamics and the prediction and warning of algal blooms in lake management.

Long-term remote observations of particulate organic phosphorus concentration in eutrophic Lake Taihu based on a novel algorithm

Monitoring the long-term spatiotemporal variations in particulate organic phosphorus concentration (CPOP) is imperative for clarifying the phosphorus cycle and its biogeochemical behavior in waters. However, little attention has been devoted to this owing to a lack of suitable bio-optical algorithms that allow the application of remote sensing data. In this study, based on Moderate Resolution Imaging Spectroradiometer (MODIS) data, a novel absorption-based algorithm of CPOP was developed for eutrophic Lake Taihu, China. The algorithm yielded a promising performance with a mean absolute percentage error of 27.75% and root mean square error of 21.09 μg/L. The long-term MODIS-derived CPOP demonstrated an overall increasing pattern over the past 19 years (2003–2021) and a significant temporal heterogeneity in Lake Taihu, with higher value in summer (81.97 ± 3.81 μg/L) and autumn (82.07 ± 3.8 μg/L), and lower CPOP in spring (79.52 ± 3.81 μg/L) and winter (78.74 ± 3.8 μg/L). Spatially, relatively higher CPOP was observed in the Zhushan Bay (85.87 ± 7.5 μg/L), whereas the lower value was observed in the Xukou Bay (78.95 ± 3.48 μg/L). In addition, significant correlations (r > 0.6, P < 0.05) were observed between CPOP and air temperature, chlorophyll-a concentration and cyanobacterial blooms areas, demonstrating that CPOP was greatly influenced by air temperature and algal metabolism. This study provides the first record of the spatial-temporal characteristics of CPOP in Lake Taihu over the past 19 years, and the CPOP results and regulatory factors analyses could provide valuable insights for aquatic ecosystem conservation.

Patterns of free-living and particle-attached bacteria along environmental gradients in Lake Taihu.

To elucidate the effects of environmental heterogeneity on diversity, composition, and degree of overlap between free-living (FL) and particle-attached (PA) bacteria, we sampled large, shallow, eutrophic Lake Taihu, China across gradients spanning riverine inflow, cyanobacterial blooms, and the open limnetic area. Using high-throughput sequencing of the 16S rRNA gene, we show that (i) bacterial communities near riverine inflow had high α-diversity and a high degree of overlap between FL and PA lifestyles, (ii) communities in cyanobacterial blooms have reduced α-diversity within the PA lifestyle, and (iii) communities from the limnetic area had the lowest bacterial α-diversity within the FL lifestyle and a medium degree of overlap between the FL and PA lifestyles. Redundancy analysis showed that the variation of the FL bacterial community was shaped by suspended solids and total phosphorous, while the variation of the PA bacterial community was shaped by suspended solids, dissolved oxygen, and the percentage of organic matter in suspended solids. This study highlights the importance of environmental heterogeneity, riverine input, cyanobacterial blooms, and nutrient status on the spatial distribution patterns of FL and PA bacterial communities in freshwater lakes.