Large Shallow Eutrophic Lake Research Articles

Teleconnection Between Early Winter Monsoon System and Harmful Algal Blooms in Shallow Lake Taihu

AbstractThere is a long history of observation and monitoring at local scales with a focus on meteorological modulation of lakes, however, the features at the local scale are heavily influenced by large‐scale atmospheric circulation. Harmful algal blooms (HABs) in large shallow eutrophic lakes are susceptible to local meteorological conditions, but their response to climate system variability is rarely considered. This study reveals that HABs in Lake Taihu, the third largest freshwater lake in subtropical China, are promoted by the weakened East Asian monsoon system in precursory early winter, but show no response in late winter with the low temperature unfavorable for algal metabolism. The weak early winter monsoon system is characterized as the northward‐shifted polar front jet and eastward‐tilted East Asian Trough. The accompanied anomalous descending motion decelerates surface winds over the middle to lower reaches of the Yangtze River including Taihu basin by strengthening atmospheric stratification and decreasing turbulent activity in the lower troposphere. The weakened surface winds reduce sediment resuspension and improve underwater light availability in Lake Taihu, providing favorable light condition for phytoplankton growth. It is indicated that the early‐winter surface winds rather than simultaneous nutrient supply, play a dominant role on the interannual variability of algal biomass in Lake Taihu. The positive algal biomass anomaly in early winter tends to persist and promote HABs in the following warmer seasons. Our finding validates the interannual teleconnection between climate system and HAB variation in Lake Taihu, and provides insights on predicting HABs in subtropical East Asian shallow lakes.

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.

Lacustrine groundwater discharge-derived carbon and nitrogen to regulate biogeochemical processes as revealed by stable isotope signals in a large shallow eutrophic lake

Eutrophic shallow lakes are hotspots of carbon (C) and nitrogen (N) accumulation and transformation, and are increasingly recognized as important sources of greenhouse gases (GHGs: CO2, CH4 and N2O). Lacustrine groundwater discharge (LGD) is a crucial component of the water budget and terrestrial material delivery for lakes, but its interplays with intrinsic CN biogeochemical processes remain less tackled. In this study, C and N ingredients and multiple stable isotopes (δ2H, δ18O, δ13C, and δ15N) were measured seasonally in groundwater, river water and lake water of a large eutrophic shallow lake in eastern China. The results revealed that groundwater is enriched with various forms of C and N that have similar sources and pathways as surface water in the lake and rivers. The isotope balance model also indicated that LGD derived C and N contribute significantly to lake inventories in addition to river runoff. These allochthonous C and N provide extra substrates for related biogeochemical processes, such as algae proliferation, organic matter degradation, methanogenesis and denitrification. Simultaneously, the excess oxygen consumption leads to depletion and hypoxia in the lake, further facilitating the processes of methanogenesis and denitrification. LGD functions not only as an external source of C and N that directly increases GHG saturations, but also as a mediator of internal CN pathways, which significantly affect hypoxia formation, GHG productions and emissions in the eutrophic lake. This study highlights the unrevealed potential regulation of LGD on biogeochemical processes in the eutrophic lake, and underscores the need for its consideration in environmental and ecological studies of lakes both regionally and globally.

Is the impact of groundwater on lake greenhouse gas dynamics underestimated? A comparative analysis of subsurface and ecological factors

Lakes are recognized as important sources of greenhouse gases (GHGs) emissions to the atmosphere, influenced by various ecological processes. Groundwater discharge into lakes, despite its small volume, has high concentrations of dissolved carbon and nitrogen that significantly affect the production and emission of GHGs from lakes. Therefore, a comprehensive investigation of the mechanisms behind lake GHGs emissions under the influence of groundwater discharge and ecological processes, holds substantial scientific importance. However, due to uncertainties and quantification challenges associated with groundwater discharge, related research is currently limited. Here, we conducted year-round field observations on Honghu Lake, a large shallow eutrophic lake in Hubei Province, China. We analyzed the seasonal variation of GHGs emissions and quantified the groundwater discharge flux using radon (222Rn). The fluxes of methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) at the water–air interface were estimated to be 31.1 ± 4.88 mg m−2 d−1, 386 ± 90.7 mg m−2 d−1, and 0.327 ± 0.072 mg m−2 d−1, respectively. CH4 is the primary greenhouse gas in Honghu Lake, contributing to 82.2 % and 62.2 % of the lake’s total emissions over 20-year and 100-year frames. The average rate of groundwater discharge was 8.19 ± 0.471 mm d−1, with the highest discharge rate in winter and the lowest in spring. The daily groundwater discharge volume accounts for 0.649 % of the lake’s total water volume. The daily contributions of groundwater discharge to the lake’s total emissions of CH4, CO2, and N2O were 0.318 %, 12.1 %, and 2.59 %, respectively. The facilitative role of groundwater discharge in CO2 emissions primarily manifests through the transport of dissolved organic carbon (DOC) and high concentrations of CO2 into the lake. Meanwhile, CH4 emissions depend on the activity of methanogenic bacteria, substrate availability, and anaerobic conditions, whereas N2O emissions are influenced by temperature and nutrient levels. Our study reveals that in the short term, the effect of groundwater on lakes is relatively minimal. Yet, its long-term role as a steady supplier of carbon and nutrients to lakes should not be overlooked. This study reveals the ways in which groundwater discharge and internal lake dynamics collectively fuel GHGs emissions. Our findings offer a new perspective and critical theoretical support for the assessment of lake greenhouse gas emissions.

Synergy of lacustrine groundwater discharge and algal biomass on CH4 and CO2 pathways and emissions in a large shallow eutrophic lake

Inland lakes are known to be a significant component of the global carbon (C) cycle, with C pathways in these lakes potentially changing alongside their eutrophic states. One factor contributing to eutrophication is lacustrine groundwater discharge (LGD), which is also a key conduit for CH4 and CO2 to enter lakes. However, the regulation of LGD on CH4 and CO2 behavior in these lakes remains unclear. To address this gap, we conducted continuous monitoring of 222Rn, CH4 and CO2, and δ13C at two sites with distinct algal densities in Lake Taihu, a large shallow eutrophic lake in China, and aimed to investigate the synergy of LGD and algal biomass on CH4 and CO2 pathways and emissions. We found that organic matter (OM) degradation is the dominant factor controlling CH4 and CO2 production in the lake, and this process appears to be facilitated by the presence of ample OM and reduced oxygen level at the site with high algal density. In addition, LGD is found to be a significant contributor of CH4 and CO2 to the lake, and the nutrients derived from LGD play a crucial role in regulating the proliferation of algae in the lake. As algal density increases, gas emissions such as CH4 ebullition and CO2 diffusion are amplified, while processes such as CH4 oxidation and atmospheric CO2 exchange within lakes are attenuated. The contribution of LGD to CH4 and CO2 inventories in the lake also decreases as OM degradation becomes a more significant contributor. This study highlights LGD regulation on the C flux and underscores the importance of synergistical studies from both ecological and hydrogeological perspectives in comprehending lacustrine C flux dynamics.

Wind-driven post-bloom dispersion of Microcystis in a large shallow eutrophic lake: A case study in Lake Taihu

To clarify the wind-driven post-bloom dispersion range of Microcystis, which originally clustered on the water surface, an Individual-Based Model (IBM) of Microcystis movement considering the combined effects of wind and light was developed based on actual hydrodynamic data and Microcystis biomass. After calibrating the effects of hydrodynamics and light, 66 cases of short-term (within a week) post-bloom with satellite images from 2011 to 2017 were simulated. The results showed that there were three short-term post-bloom types: vertical reduction (VR), horizontal reduction (HR) and mixed reduction (MR). For VR type, the cyanobacterial bloom reduction rate was rapid (>160 km2/day), but the dispersion range of Microcystis was limited (<2 km/day), and a larger bloom area was likely to form in the original location when wind speed decreased. For HR type, the cyanobacterial bloom reduction rate was slow (<10 km2/day), but Microcystis exhibited a broad dispersion range (>4 km/day), often leading to smaller, thicker, and longer-lasting cyanobacterial blooms downwind, albeit with a lower probability of occurrence. The characteristics of MR lay between the two aforementioned types.

Significant methane ebullition from large shallow eutrophic lakes of the semi-arid region of northern China

Eutrophic lakes are a major source of the atmospheric greenhouse gas methane (CH4), and CH4 ebullition emissions from inland lakes have important implications for the carbon cycle. However, the spatio-temporal heterogeneity of CH4 ebullition emission and its influencing factors in shallow eutrophic lakes of arid and semi-arid regions remain unclear. This study aimed to determine the mechanism of CH4 emission via eutrophication in Lake Ulansuhai, a large shallow eutrophic lake in a semi-arid region of China.To this end, monthly field surveys were conducted from May to October 2021, and gas chromatography was applied using the headspace equilibrium technique with an inverted funnel arrangement. The total CH4 fluxes ranged from 0.102 mmol m−2 d−1 to 59.296 mmol m−2 d−1 with an average value of 4.984 ± 1.82 mmol m−2 d−1. CH4 ebullition emissions showed significant temporal and spatial variations. The highest CH4 ebullition emission was observed in July with a grand mean of 9.299 mmol m−2 d−1, and the lowest CH4 ebullition emissions occurred in October with an average of 0.235 mmol m−2 d−1. Among seven sites (S1–S7), the maximum (3.657 mmol m−2 d−1) and minimum (1.297 mmol m−2 d−1). CH4 ebullition emissions were observed at S2 and S7, respectively. As the main route of CH4 emission to the atmosphere in Lake Ulansuhai, the CH4 ebullition flux during May to October accounted for 69% of the total CH4 flux. Statistical analysis showed that CH4 ebullition was positively correlated with temperature (R = 0.391, P < 0.01) and negatively correlated with air pressure (R = 0.286, P < 0.00). Temperature and air pressure were found to strongly regulate the production and oxidation of CH4. Moreover, nutritional status indicators such as TP and NH4+-N significantly affect CH4 ebullition emissions (R = 0.232, P < 0.01; R = −0.241, P < 0.01). This study reveals the influencing factors of CH4 ebullition emission in Lake Ulansuhai, and provides theoretical reference and data support for carbon emission from eutrophic lakes. Nevertheless, research on eutrophic shallow lakes needs to be further strengthened. Future research should incorporate improved flux measurement techniques with process-based models to improve the accuracy from regional to large-scale estimation of CH4 emissions and clarify the carbon budget of aquatic ecosystems. In this manner, the understanding and predictability of CH4 ebullition emission from shallow lakes can be improved.

Seasonal sediment phosphorus release across sediment-water interface and its potential role in supporting algal blooms in a large shallow eutrophic Lake (Lake Taihu, China)

Seasonal sediment internal phosphorus (P) release is known to affect annual algal blooms in eutrophic lakes. In this study, a year-long field investigation and laboratory sediment core incubation were conducted to study the relationship between sediment internal P cycling and algal growth in Lake Taihu. The results indicated that the concentrations of water total phosphorus (TP) and chlorophyll-a (Chla) correlated with seasonal temperature and were assumed to be caused by internal P release. From cold winter to warm seasons, sediment internal P (porewater P concentration and P flux) exhibits dynamic changes. Sediment porewater soluble reactive phosphorus (SRP) and its flux in the summer were approximately five times and eight times those during winter, respectively. The release of sediment mobile P in the summer decreases its concentration and can supply SRP for algal blooms. Laboratory core incubation indicated that Chla and phycocyanin concentrations in the overlying water showed similar changes to sediment porewater P and P flux when cores were incubated from low to high temperature. The results of this study indicated that warmer conditions could increase the sediment porewater P concentration and sediment P flux into the bottom waters and consequently enhance sediment P availability to algae. This study provides new insights into the relationship between internal sediment P cycling and algal blooms in Lake Taihu.

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.

Interactions between Phosphorus Enrichment and Nitrification Accelerate Relative Nitrogen Deficiency during Cyanobacterial Blooms in a Large Shallow Eutrophic Lake.

Regime shifts between nitrogen (N) and phosphorus (P) limitation, which trigger cyanobacterial succession, occur in shallow eutrophic lakes seasonally. However, the underlying mechanism is not yet fully illustrated. We provide a novel insight to address this from interactions between sediment P and nitrification through monthly field investigations including 204 samples and microcosm experiments in Lake Chaohu. Total N to P mass ratios (TN/TP) varied significantly across seasons especially during algal bloom in summer, with the average value being 26.1 in June and descending to 7.8 in September gradually, triggering dominant cyanobacterial succession from Microcystis to Dolichospermum. The regulation effect of sediment N/P on water column TN/TP was stronger in summer than in other seasons. Iron-bound P and alkaline phosphatase activity in sediment, rather than ammonium, contributed to the higher part of nitrification. Furthermore, our microcosm experiments confirmed that soluble active P and enzymatic hydrolysis of organic P, accumulating during algal bloom, fueled nitrifiers and nitrification in sediments. These processes promoted lake N removal and led to relative N deficiency in turn. Our results highlight that N and P cycles do not exist independently but rather interact with each other during lake eutrophication, supporting the dual N and P reduction program to mitigate eutrophication in shallow eutrophic lakes.

Effects of nutrient reduction and habitat heterogeneity on benthic macroinvertebrate assemblages in a large shallow eutrophic lake

The Taihu Lake ecosystem has been subjected to numerous anthropogenic stressors during the past decades, leading to substantial changes in nutrient dynamics and habitat quality. For instance, the northwestern lake bays receive large amounts of nutrient-rich wastewater and have frequently experienced algal blooms, while the eastern lake region is still dominated by submersed macrophytes. Such changes in environmental characteristics can greatly impact benthic macroinvertebrate communities. We used a 15-year monitoring data series collected by the Taihu Laboratory for Lake Ecosystem Research to examine the spatial and temporal variations of the benthic invertebrate fauna and evaluate its status and trends. We found that three major communities could be distinguished based on taxonomic group composition and abundance, and these corresponded well with three lake habitat types: algal-dominated, macrophyte-dominated, and open-lake zone. An analysis of temporal trends showed major changes in the macroinvertebrates during the study period, largely driven by a lake-wide and significant decline in the abundance of pollution-tolerant taxa. The spatial and temporal variations of macroinvertebrate communities were mainly explained by nutrients (e.g., total nitrogen and ammonium concentrations) and habitat factors (e.g., sediment substrates and macrophyte biomass) as indicated by Random Forests regression, but the major drivers of macroinvertebrate density differed among the three lake zones at the temporal scale. Moreover, our findings suggest that benthic invertebrates were more sensitive to the improvement of the lake's environmental conditions than the pelagic community was. This study provides insights into the responses of macroinvertebrates to ecological dynamics in lakes and highlights the importance of continued monitoring for tracking long-term changes.

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.

Spatiotemporal pattern and biodegradation process of amino acids in the large shallow eutrophic lake Taihu, China.

Organic matter (OM) and nutrient inputs generated by human activities promote the development of eutrophication. Amino acids (AAs) are an integral part of OM, and studying their patterns will provide new insights into organic matter dynamics in lakes. Four seasonal field campaigns in eutrophic Lake Taihu and a 14-day phytoplankton degradation experiment were carried out to determine the variability and bioavailability of amino acids. The quality and quantity of AAs varied among different seasons and lake types. The concentrations of particulate (PAA) and dissolved (DAA) AA were 14.67 ± 13.25μM (carbon- and nitrogen-normalized PAA yields: PAA-C%, 23.8 ± 13.5%; PAA-N%, 22.8 ± 2.1%) and 2.95 ± 1.05μM (carbon- and nitrogen-normalized DAA yields: DAA-C%, 3.7 ± 1.1%; DAA-N%, 12.6 ± 11.2%) in Lake Taihu, respectively. PAA and DAA showed high mean values in the algal-dominated northern area in summer. Glutamic acid (Glu), alanine (Ala), aspartic acid (Asp) and glycine (Gly) contributed to nearly 50% of PAA, while Gly, accounting for 17-24%, was the main component of DAA. During a 14-day phytoplankton degradation period, 98% of Chl a, 63% of POC and 92% of PAA were removed in the dark treatment, and a more obvious downward trend was recorded than in the light treatment. Principal component analysis of the indices of PAA indicated that Glu, arginine (Arg) and histidine (His) were dominant on day 0, while Gly and lysine (Lys) were dominant on day 14. There were notable serine + threonine (Ser + Thr [mol%]) and aspartic acid/glycine ratio (Asp/Gly [mol%]) end-member divisions among different organic matter sources. DAA had higher Ser + Thr [mol%] and lower Asp/Gly [mol%] values than PAA. The amino acid degradation index (DI) of PAA and DAA was 0.97 ± 0.28 and - 1.04 ± 0.43, respectively. The fresh DAA from the algae degradation incubation also had DI values similar to those of field DAA in the northern algae-dominated lake region. Amino acid parameters (AA-C%, DI values, Ser + Thr [mol%] and Asp/Gly [mol%]) were calculated to indicate the source, freshness and bioavailability of organic matter in eutrophic shallow Lake Taihu.

Intrahabitat Differences in Bacterial Communities Associated with Corbicula fluminea in the Large Shallow Eutrophic Lake Taihu.

The Asian clam Corbicula fluminea is a keystone zoobenthos in freshwater ecosystems. However, its associated microbiome is not well understood. We investigated the bacterial communities of this clam and its surrounding environment, including sediment and water simultaneously, in a large lake by means of 16S rRNA gene sequencing. Approximately two-thirds of the bacterial operational taxonomic units (OTUs) associated with clams were observed in the surrounding environment and mostly from particle-associated samples. The associated bacterial communities were site specific and more similar to environmental bacteria from the same site than those at other sites, suggesting a local environmental influence on host bacteria. However, the significant differences in bacterial diversities and compositions between the clam and the environment also indicated strong host selection pressure on bacteria from the surrounding environment. Bacteria affiliated with Firmicutes, Spirochaetes, Tenericutes, Bacteroidetes, Epsilonbacteraeota, Patescibacteria, and Fusobacteria were found to be significantly enriched in the clams in comparison to their local environment. Oligotyping analyses of the core-associated bacterial OTUs also demonstrated that most of the core OTUs had lower relative abundances and occurrence frequencies in environmental samples. The core bacterial OTUs were found to play an important role in maintaining the stability of the bacterial community network. These core bacteria included the two most abundant taxa Romboutsia and Paraclostridium with the potential function of fermenting polysaccharides for assisting host clams in food digestion. Overall, we demonstrate that clam-associated bacteria were spatially dynamic and site specific, which were mainly structured both by local environments and host selection. IMPORTANCE The Asian clam Corbicula fluminea is an important benthic clam in freshwater ecosystems due to its high population densities and high filtering efficiency for particulate organic matter. While the associated microbiota is believed to be vital for host living, our knowledge about the compositions, sources, and potential functions is still lacking. We found that C. fluminea offers a unique ecological niche for specific lake bacteria. We also observed high intrahabitat variation in the associated bacterial communities. Such variations were driven mainly by local environments, followed by host selection pressure. While the local microbes served as a source of the clam-associated bacteria, host selection resulted in enrichments of bacterial taxa with the potential for assisting the host in organic matter digestion. These results significantly advance our current understanding of the origins and ecological roles of the microbiota associated with a keynote clam in freshwater ecosystems.

Effects of extreme water levels on nutrient dynamics in a large shallow eutrophic lake (Changhu Lake, China)

Changhu Lake, a large, shallow, eutrophic lake in central China, experienced an extremely low water level event from November 2015 to January 2016 followed by an extremely high water level event in July 2016. In this study, we examined the effects of two extreme water levels on the nutrient dynamics of Changhu Lake over five years. The nutrient parameters in Changhu Lake showed significant interannual variations, and the nutrient concentrations at the sites in the western part of Changhu Lake were 2–41% higher than those at the outlet of the lake. In late 2015, the effects of low water levels led to a 17–74% increase in nutrient concentrations. After July 2016, however, a high water level event occurred, leading to a 34–48% decrease in nutrient concentrations. These changes in nutrient parameters were strongly related to water level fluctuations (p < 0.05). As extreme water levels are likely to become more frequent during the twenty-first century, this work may provide some insights into the conservation and management of lake ecosystems in the face of climate change and human activity.

Monitoring Fractional Floating Algae Cover Over Eutrophic Lakes Using Multisensor Satellite Images: MODIS, VIIRS, GOCI, and OLCI

Coarse-resolution sensors have been used operationally to monitor floating algal blooms with near daily revisit in coastal and inland waters. Most of the current methods in estimating fractional floating algae cover (FAC) were based on the linear pixel un-mixing assumption. In this study, a new FAC model following logistic curve was developed and applied to multisensor satellite data in two large shallow eutrophic lakes, Lake Taihu and Lake Chaohu, in China. The results indicated that after resampling to 250 m, match-up pairs of MODIS (Moderate Resolution Imaging Spectroradiometer), VIIRS (The Visible and Infrared Imager/Radiometer Suite), GOCI (Geostationary Ocean Color Imager) and OLCI (Ocean and Land Color Instrument) possessed consistent Rayleigh corrected reflectance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_rc</i> ) and FAI (floating algae index) or AFAI (alternative floating algae index). The FAC model was developed based on the simulated AFAI data of GOCI using point spread function (PSF) and bloom percent derived from OLI (Operational Land Imager), and then was applied to MODIS, VIIRS, and OLCI. Compared with the linear pixel un-mixing method, the FAC model reflects the asymptotic reflectance saturation in the NIR (near infrared) band with the accumulation of blooms. Besides, the equivalent bloom area (EBA) of GOCI was validated using OLI matched pairs with UPD 37.6% (N=39, R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> =0.96). The spatial-temporal dataset of FAC (2002-2020) shows that Lake Taihu and Lake Chaohu experienced severe algal blooms after 2010, partly resulting from the higher frequency of multisensor data. This study provides a method in building a lasting and comparable FAC dataset using multisensors.

Assessing the risks of harmful algal bloom accumulation atlittoral zone of large lakes and reservoirs: An example from Lake Chaohu

淡水湖库富营养化与蓝藻水华是全球性的突出水环境问题，尤其是滨岸带严重蓝藻水华堆积甚至造成了水体黑臭、威胁饮用水安全等严重危害，科学评估滨岸带蓝藻水华堆积风险、精准识别蓝藻水华易堆积区域是水环境管理与研究中亟待解决的关键科学问题.本研究以我国长江中下游的大型浅水富营养化湖泊巢湖为研究对象，依托流域水文与湖泊水动力模拟、遥感反演、GIS空间分析等技术，综合考虑藻类生物量、岸线形态、湖泊水动力、风速和风向等要素，创新构建了蓝藻水华堆积风险评估指标体系，量化评估了2018-2019年的巢湖滨岸带的蓝藻水华堆积风险，并将滨岸带蓝藻水华堆积风险等级划分为5级（Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ），绘制了蓝藻水华滨岸带堆积风险的空间分布，识别了蓝藻水华的易堆积区域.评估结果表明：巢湖滨岸带蓝藻水华堆积的高风险区域呈连续片状分布于西巢湖西岸与西北岸，占巢湖沿岸区域的12.1%，是巢湖蓝藻水华应急处置管理的关键区域，评估结果与调研结论总体一致.研发的蓝藻水华堆积风险评估方法可应用于其他大型富营养化湖库，为蓝藻水华应急处置管理提供关键技术支撑.;Eutrophication and harmful algal blooms (HABs) in freshwater lakes and reservoirs are severe global problems for aquatic ecosystems. In particular, severe HABs accumulation in the littoral zone is likely to cause hypoxia (black and odorous water), and thus threaten drinking water safety. Therefore, assessing the risks of HABs accumulation and accurately identifying the littoral zones that is prone to HABs accumulation are of particular importance for water management practice. This study took Lake Chaohu, a large shallow eutrophic lake in the middle and lower reaches of the Yangtze River in China as the study area. Based on the technological advances in hydrological and hydrodynamic modelling in the lake-watershed system, remote sensing monitoring, spatial analysis in GIS software, etc., we developed a novel risk assessment index for HABs accumulation by considering the factors of phytoplankton biomass, shoreline shape, hydrodynamic and wind conditions. The index was applied in Lake Chaohu to assess its risk of HABs accumulation during 2018-2019. The assessment results were divided into five levels from I to V, and were mapped to identify the area with a high risk of HABs accumulation. Our investigation results showed that the high-risk areas of HABs accumulation are mainly located in the western and northwest areas of Western Lake Chaohu, accounting for 12.1% of the littoral zone in Lake Chaohu. These areas should be highly concerned in the emergency management of HABs in Lake Chaohu. The assessment results were generally consistent with our survey findings. The developed risk index for HABs accumulation can be easily transferred to other large eutrophic lakes and reservoirs, and can well support the emergency management practice of HABs.

Functionally similar species of ciliates have similar dynamics: A biennial survey study in a large eutrophic lake

The spatial and temporal planktonic ciliate community structure of a large shallow eutrophic lake was described in detail based on a monthly monitoring campaign over 2 years using the quantitative protargol stain approach. We found that there was a large variety and low constancy of ciliate plankton and a clear advantage of several small species. Balanion planctonicum, Rimostrombidium brachykinetum, and Urotricha farcta contributed 19.7%, 13.4%, and 10.7% to the total abundance, respectively. The ciliate community was significantly varied in different lake regions of different eutrophication levels, and the distribution of some of the main species, especially prostomatids and scuticociliatids, might be closely related to the nutrient level of the lake regions. The seasonal dynamics of ciliate taxa with similar functional feeding habits across regions at different trophic levels are more convergent than those of species with very different functions, and the top-down effect (cladocerans, rotifers), nutrition, and water temperature were the key factors shaping ciliate community structure. The results of this study accentuate the important effects of species functional diversity on community differences and may improve our knowledge of ciliate diversity and functional ecology in shallow eutrophic lakes.

Combined Generalized Additive model and Random Forest to evaluate the influence of environmental factors on phytoplankton biomass in a large eutrophic lake

The over-proliferation of phytoplankton has been a public concern for the last several decades. To evaluate the importance of different environmental factors on phytoplankton biomass variations with their complex response relationships in the large eutrophic Lake Okeechobee, the nonlinear methods including Generalized Additive Model (GAM) and Random Forest (RF) algorithm were employed. A long-term monitoring dataset of 8 sampling sites ranging from January 1996 to December 2010 was applied to explore the driven factors and response relationships of phytoplankton biomass from the different scales in Lake Okeechobee. Results showed a spatially heterogeneous distribution of phytoplankton biomass there, and the western and southern littoral zones occurred heavier algal blooms than the central pelagic zones. Spearman’s correlation results denoted Chlorophyll a (Chla) was negatively correlated with phosphorus in different scales. In the aspect of temporal variations, phytoplankton biomass appeared a three-peak-two-valley variation trend there, which peaked in 1997, 2003, and 2010, respectively. On the lake-wide scale, the RF model indicated that the inorganic nutrients were the primary predictors of phytoplankton biomass, while underwater light availability factors were followed to play essential roles in the prediction. On the local scale, total nitrogen (TN) was the top predictor in the western near-shore zone, while other zones were nitrate and nitrite (NO23) or phosphate (PO4). GAM results suggested that phytoplankton biomass had positive response to TN increase in both lake-wide and local scales. However, its response to water temperature (WT) appeared spatial heterogeneity. This study provided a new perspective to evaluate the primary predictors of phytoplankton biomass with their response relationships from lake-wide scale and local scale in the large shallow eutrophic lake, and the importance of taking spatial heterogeneity into account for lake water quality management was stressed.

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.