Feeding dynamics of the copepod Diacyclops thomasi before, during and following filamentous cyanobacteria blooms in a large, shallow temperate lake

Cyanobacteria blooms are an increasing problem in temperate freshwater lakes, leading to reduced water quality and in some cases harmful effects from toxic cyanobacteria species. To better understand the role of zooplankton in modulating cyanobacteria blooms, from 2008 to 2010 we measured water quality and plankton abundance, and measured feeding rates and prey selectivity of the copepod Diacyclops thomasi before, during and following summertime cyanobacteria blooms in a shallow, eutrophic lake (Vancouver Lake, Washington, USA). We used a combined field and experimental approach to specifically test the hypothesis that copepod grazing was a significant factor in establishing the timing of cyanobacteria bloom initiation and eventual decline in Vancouver Lake. There was a consistent annual succession of zooplankton taxa, with cyclopoid copepods (D. thomasi) dominant in spring, followed by small cladocerans (Eubosmina sp.). Before each cyanobacteria bloom, large cladocerans (Daphnia retrocurva, Daphnia laevis) peaked in abundance but quickly disappeared, followed by brief increases in rotifers. During the cyanobacteria blooms, D. thomasi was again dominant, with small cladocerans abundant in autumn. Before the cyanobacteria blooms, D. thomasi substantially consumed ciliates and dinoflagellates (up to 100% of prey biomass per day), which likely allowed diatoms to flourish. A shift in copepod grazing toward diatoms before the blooms may have then helped to facilitate the rapid increase in cyanobacteria. Copepod grazing impact was the highest during the cyanobacteria blooms both years, but focused on non-cyanobacteria prey; copepod grazing was minimal as the cyanobacteria blooms waned. We conclude that cyclopoid copepods may have an indirect role (via trophic cascades) in modulating cyanobacteria bloom initiation, but do not directly contribute to cyanobacteria bloom decline.     

Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.     

Predicting cyanobacterial dynamics in the face of global change: the importance of scale and environmental context

There is growing concern that harmful cyanobacterial blooms are increasing in frequency and occurrence around the world. Although nutrient enrichment is commonly identified as a key predictor of cyanobacterial abundance and dominance in freshwaters, several studies have shown that variables related to climate change can also play an important role. Based on our analysis of the literature, we hypothesized that temperature or water‐column stability will be the primary drivers of cyanobacterial abundance in stratified lakes whereas nutrients will be the stronger predictors in frequently mixing water bodies. To test this hypothesis, as well as quantify the drivers of cyanobacteria over different scales and identify interactions between nutrients and climate‐related variables, we applied linear and nonlinear mixed‐effect modeling techniques to seasonal time‐series data from multiple lakes. We first compared time series of cyanobacterial dominance to a published lake survey and found that the models were similar. Using time‐series data of cyanobacterial biomass, we identified important interactions among nutrients and climate‐related variables; dimictic basin experienced a heightened susceptibility to cyanobacterial blooms under stratified eutrophic conditions, whereas polymictic basins were less sensitive to changes in temperature or stratification. Overall, our results show that due to predictable interactions among nutrients and temperature, polymictic and dimictic lakes are expected to respond differently to future climate warming and eutrophication.     

Developing a two-step method for retrieving cyanobacteria abundance from inland eutrophic lakes using MERIS data

Cyanobacteria are the main dominant species in inland eutrophic lakes during algae blooms, and measures of cyanobacteria abundance can be used for monitoring and early detection of algal blooms by remote sensing. During May 2013 and August 2016, a total 137 water samples were collected from Lake Taihu and Lake Chaohu. Remote-sensing reflectance was measured, surface water was collected in the field, and chlorophyll-a concentration, phycocyanin concentration, suspended-matter concentration and phytoplankton pigment absorption parameters were measured in the laboratory. The composition and density of planktonic algae were also detected by microscope examination. The remote-sensing reflectance at 15 MERIS bands was simulated based on our measured spectral data, and a two-step method for detecting cyanobacteria abundance using the partial least squares model based on 5 MERIS bands was developed. The results showed that the estimation algorithm can predict cyanobacteria abundance in inland eutrophic lakes with satisfactory accuracy, with RMSE of 7.56 and MAPE of 13.44 %. This algorithm was successfully applied to the MERIS image acquired on August 12, 2010, and showed a reasonable spatial distribution of cyanobacteria abundance in Lake Taihu. It demonstrated that the developed estimation method was an effective way to monitor cyanobacteria abundance in water with a potential to be successfully applied to Sentinel-3 images.     

Bottom-up versus top-down effects on ciliate community composition in four eutrophic lakes (China)

Previous studies have shown that ciliate plankton is generally controlled by food resources (e.g., algae) and predators (e.g., metazooplankton). Among lakes with similar trophic levels but different distributions of phyto- and metazooplankton, the main forces acting on ciliate assemblages may be different. We investigated the relationship between ciliate communities and bottom-up versus top-down variables based on a survey of four subtropical eutrophic lakes (China). Two of the lakes (Chaohu, Taihu) are located on the Mid-lower Yangtze Plain near sea level, and the other two (Dianchi, Xingyunhu) on the Yunnan-Kweichow Plateau at 1700 m above sea level. Blooms of cyanobacteria developed during summer in Lakes Chaohu and Taihu and throughout the year in Lakes Dianchi and Xingyunhu. Ciliate functional feeding groups differed significantly between lakes. The results of canonical correspondence analysis (CCA) and variation partitioning showed that cyanobacteria significantly influence ciliate species, whereas ‘edible’ algae (cryptophytes, diatoms) and cladocerans were the important variables in explaining the ciliate community structure of Lakes Dianchi and Xingyunhu compared with Lakes Taihu and Chaohu. Our results highlight the importance of consistent cyanobacterial blooms in shaping the ciliate community in subtropical eutrophic shallow lakes by interacting with top-down and bottom-up factors.     

Targeted deep sequencing reveals high diversity and variable dominance of bloom-forming cyanobacteria in eutrophic lakes

Cyanobacterial blooms in eutrophic lakes are severe environmental problems worldwide. To characterize the spatiotemporal heterogeneity of cyanobacterial blooms, a high-throughput method is necessary for the specific detection of cyanobacteria. In this study, the cyanobacterial composition of three eutrophic waters in China (Taihu Lake, Dongqian Lake, and Dongzhen Reservoir) was determined by pyrosequencing the cpcBA intergenic spacer (cpcBA-IGS) of cyanobacteria. A total of 2585 OTUs were obtained from the normalized cpcBA-IGS sequence dataset at a distance of 0.05. The 238 most abundant OTUs contained 92% of the total sequences and were classified into six cyanobacterial groups. The water samples of Taihu Lake were dominated by Microcystis, mixed Nostocales species, Synechococcus, and unclassified cyanobacteria. Besides, all the samples from Taihu Lake were clustered together in the dendrogram based on shared abundant OTUs. The cyanobacterial diversity in Dongqian Lake was dramatically decreased after sediment dredging and Synechococcus became exclusively dominant in this lake. The genus Synechococcus was also dominant in the surface water of Dongzhen Reservoir, while phylogenetically diverse cyanobacteria coexisted at a depth of 10 m in this reservoir. In summary, targeted deep sequencing based on cpcBA-IGS revealed a large diversity of bloom-forming cyanobacteria in eutrophic lakes and spatiotemporal changes in the composition of cyanobacterial communities. The genus Microcystis was the most abundant bloom-forming cyanobacteria in eutrophic lakes, while Synechococcus could be exclusively dominant under appropriate environmental conditions.     

To bloom or not to bloom: contrasting responses of cyanobacteria to recent heat waves explained by critical thresholds of abiotic drivers

Past heat waves are considered harbingers of future climate change. In this study, we have evaluated the effects of two recent Central European summer heat waves (2003 and 2006) on cyanobacterial blooms in a eutrophic, shallow lake. While a bloom of cyanobacteria developed in 2006, consistent with our expectations, cyanobacterial biomass surprisingly remained at a record-low during the entire summer of 2003. Critical thresholds of abiotic drivers extracted from the long-term (1993–2007) data set of the studied lake using classification tree analysis (CTA) proved suitable to explain these observations. We found that cyanobacterial blooms were especially favoured in 2006 because thermal stratification was critically intense (Schmidt stability >44 g cm cm⁻²) and long-lasting (>3 weeks). Our results also suggest that some cyanobacterial species (Anabaena sp.) benefitted directly from the stable water column, whereas other species (Planktothrix sp.) took advantage of stratification-induced internal nutrient loading. In 2003, conditions were less favourable for cyanobacteria due to a spell of lower temperatures and stronger winds in mid-summer; as a result, the identified thresholds of thermal stratification were hardly ever reached. Overall, our study shows that extracting critical thresholds of environmental drivers from long-term records is a promising avenue for predicting ecosystem responses to future climate warming. Specifically, our results emphasize that not average temperature increase but changes in short-term meteorological variability will determine whether cyanobacteria will bloom more often in a warmer world.     

Effects of experimentally induced cyanobacterial blooms on crustacean zooplankton communities

SUMMARY 1. Large in situ enclosures were used to study the effects of experimentally induced cyanobacterial blooms on zooplankton communities. A combination of N and P was added to shallow (2 m) and deep enclosures (5 m) with the goal of reducing the TN : TP ratio to a low level (∼5 : 1) to promote cyanobacterial growth. After nutrient additions, high biomass of cyanobacteria developed rapidly in shallow enclosures reaching levels only observed during bloom events in eutrophic lakes.
2. In the shallow enclosures, particulate phosphorus (PP) was on average 35% higher in comparison with deep enclosures, suggesting that depth plays a key role in P uptake by algae. Phytoplankton communities in both deep and shallow enclosures were dominated by three cyanobacteria species – Aphanizomenon flos-aquae , Anabaena flos-aquae and Microcystis aeruginosa – which accounted for up to 70% of total phytoplankton biomass. However, the absolute biomass of the three species was much higher in shallow enclosures, especially Aphanizomenon flos-aquae . The three cyanobacteria species responded in contrasting ways to nutrient manipulation because of their different physiology.
3. Standardised concentrations of the hepatotoxic microcystin-LR increased as a result of nutrient manipulations by a factor of four in the treated enclosures. Increased biomass of inedible and toxin producing cyanobacteria was associated with a decline in Daphnia pulicaria biomass caused by a reduction in the number of individuals with a body length of >1 mm. Zooplankton biomass did not decline at moderate cyanobacteria biomass, but when cyanobacteria reached high biomass large cladocerans were reduced.
4. Our results demonstrate that zooplankton communities can be negatively affected by cyanobacterial blooms and therefore the potential to use herbivory to reduce algal blooms in such eutrophic lakes appears limited.     

Cyanobacterial chemical warfare affects zooplankton community composition

1. Toxic algal blooms widely affect our use of water resources both with respect to drinking water and recreation. However, it is not only humans, but also organisms living in freshwater and marine ecosystems that may be affected by algal toxins. 2. In order to assess if cyanobacterial toxins affect the composition of natural zooplankton communities, we quantified the temporal fluctuations in microcystin concentration and zooplankton community composition in six lakes. 3. Microcystin concentrations generally showed a bimodal pattern with peaks in early summer and in autumn, and total zooplankton biomass was negatively correlated with microcystin concentrations. Separating the zooplankton assemblages into finer taxonomic groups revealed that high microcystin concentrations were negatively correlated with Daphnia and calanoid copepods, but positively correlated with small, relatively inefficient phytoplankton feeders, such as cyclopoid copepods, Bosmina and rotifers. 4. In a complementary, mechanistic laboratory experiment using the natural phytoplankton communities from the six lakes, we showed that changes in in situ levels of microcystin were coupled with reduced adult size and diminished juvenile biomass in Daphnia. 5. We argue that in eutrophic lakes, large unselective herbivores, such as Daphnia, are ‘sandwiched’ between high fish predation and toxic food (cyanobacteria). In combination, these two mechanisms may explain why the zooplankton community in eutrophic lakes generally comprise small forms (e.g. rotifers and Bosmina) and selective raptorial feeders, such as cyclopoid copepods, whereas large, unselective herbivores, such as Daphnia, are rare. Hence, this cyanobacterial chemical warfare against herbivores may add to our knowledge on population and community dynamics among zooplankton in eutrophic systems.     

The efficiency of combined coagulant and ballast to remove harmful cyanobacterial blooms in a tropical shallow system

We tested the hypothesis that a combination of coagulant and ballast could be efficient for removal of positively buoyant harmful cyanobacteria in shallow tropical waterbodies, and will not promote the release of cyanotoxins. This laboratory study examined the efficacy of coagulants [polyaluminium chloride (PAC) and chitosan (made of shrimp shells)] alone, and combined with ballast (lanthanum modified bentonite, red soil or gravel) to remove the natural populations of cyanobacteria collected from a shallow eutrophic urban reservoir with alternating blooms of Cylindrospermopsis and Microcystis. PAC combined with ballast was effective in settling blooms dominated by Microcystis or Cylindrospermopsis. Contrary to our expectation, chitosan combined with ballast was only effective in settling Cylindrospermopsis-dominated blooms at low pH, whereas at pH ≥ 8 no effective flocculation and settling could be evoked. Chitosan also had a detrimental effect on Cylindrospermopsis causing the release of saxitoxins. In contrast, no detrimental effect on Microcystis was observed and all coagulant-ballast treatments were effective in not only settling the Microcystis dominated bloom, but also lowering dissolved microcystin concentrations. Our data show that the best procedure for biomass reduction also depends on the dominant species.     

Physical and chemical aspects of Chew Valley and Blagdon Lakes, two eutrophic reservoirs in North Somerset, England

Chew Valley and Blagdon Lakes are two shallow, hard-water, lowland eutrophic reservoirs, situated near Bristol in the south-west of England. Chew Lake is more turbid and richer in plankton but poorer in macrophytes than Blagdon, and is more exposed to wind action. Both lakes show evidence of transient stratification in the summer coupled to deoxygenation of the bottom water, which has led to release of nutrients from the sediments. Severe algal blooms in Chew Lake in 1968 and the consequent filtration difficulties led to the establishment of a collaborative research programme between the University of Bristol and the Bristol Waterworks Company, to define the present biological status of the lakes, and to predict their possible future development. The work covers a wide range of measurements and observations on the limnological conditions of the lakes, which form an elegantly contrasted pair, and this paper describes their general physical and chemical features.     

Modelling lake cyanobacterial blooms: Disentangling the climate‐driven impacts of changing mixed depth and water temperature

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Increased risk of cyanobacterial blooms in northern high‐latitude lakes through climate warming and phosphorus enrichment

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EVALUATION OF ENVIRONMENTAL FACTORS ON CYANOBACTERIAL BLOOM IN EUTROPHIC RESERVOIR USING ARTIFICIAL NEURAL NETWORKS1

Cyanobacterial blooms are a common issue in eutrophic freshwaters, and some cyanobacteria produce toxins, threatening the health of humans and livestock. Microcystin, a representative cyanobacterial hepatotoxin, is frequently detected in most Korean lakes and reservoirs. This study developed predictive models for cyanobacterial bloom using artificial neural networks (ANNs; self‐organizing map [SOM] and multilayer perceptron [MLP]), including an evaluation of related environmental factors. Fourteen environmental factors, as independent variables for predicting the cyanobacteria density, were measured weekly in the Daechung Reservoir from spring to autumn over 5 years (2001, 2003–2006). Cyanobacterial density was highly associated with environmental factors measured 3 weeks earlier. The SOM model was efficient in visualizing the relationships between cyanobacteria and environmental factors, and also for tracing temporal change patterns in the environmental condition of the reservoir. And the MLP model exhibited a good predictive power for the cyanobacterial density, based on the environmental factors of 3 weeks earlier. The water temperature and total dissolved nitrogen were the major determinants for cyanobacteria. The water temperature had a stronger influence on cyanobacterial growth than the nutrient concentrations in eutrophic waters. Contrary to general expectations, the nitrogen compounds played a more important role in bloom formation than the phosphorus compounds.     

Consequences of changes in thermal regime for plankton diversity and trait composition in a polymictic lake: a matter of temporal scale

1. Changes in plankton species diversity and community structure as a result of global warming are of growing concern in ecological studies, as these properties contribute substantially to key ecosystem processes. 2. We analysed the effect of short‐term temperature rise and changes in the thermal regime during summer on plankton diversity of the eutrophic and polymictic Müggelsee in Germany, from 26 years of summer records (1982–2007). We tested for changes in community properties, such as species richness, evenness and population size of phyto‐ and zooplankton, during alternating periods of thermal stratification and mixing, which were between 2 and 8 weeks long. Moreover, we tested for overall long‐term temporal trends in annual averages of the community properties during stratified and mixed events. 3. We found that the overall number of stratification events increased significantly across the study period. When the lake was stratified, consistently higher surface water temperatures and lower epilimnetic nutrient concentrations were found. As the length of thermal stratification increased, the phytoplankton shifted towards a higher proportion of buoyant cyanobacteria capable of N‐fixation (Aphanizomenon, Anabaena). Diatoms were at a disadvantage because of high temperature, exceeding their upper lethal limit and sedimentation losses. Zooplankton species with high thermal tolerances (i.e. Thermocyclops oithonoides, Thermocyclops crassus) and/or those that grow quickly at high temperatures (i.e. rotifers) became more common. 4. During periods of continuous mixing, the community remained largely unchanged, except for some minor increase in the biomass of diatoms. 5. While a noticeable shift towards N‐fixing cyanobacteria was observed with increasing length of stratified events, and rotifers and copepods became the main predators, there were minimal changes in diversity, except for an increase in cyclopoid copepods and a decrease in diatom diversity. As for cyanobacteria, the net short‐term effect on their diversity was neutral as a result of species replacements. In the long term, however, the diversity of cyanobacteria and cladocerans declined while that of rotifers increased. 6. Overall, our study presents a cautionary example of how we might fail to foresee the impact of climate‐induced changes on ecosystem processes if we restrict our studies to seasonal or yearly temporal scales, thus neglecting the impact of substantial changes operating at smaller temporal scales.     

Lake Level Fluctuations Boost Toxic Cyanobacterial “Oligotrophic Blooms”

Global warming has been shown to strongly influence inland water systems, producing noticeable increases in water temperatures. Rising temperatures, especially when combined with widespread nutrient pollution, directly favour the growth of toxic cyanobacteria. Climate changes have also altered natural water level fluctuations increasing the probability of extreme events as dry periods followed by heavy rains. The massive appearance of Dolichospermum lemmermannii ( = planktonic Anabaena), a toxic species absent from the pelagic zone of the subalpine oligotrophic Lake Maggiore before 2005, could be a consequence of the unusual fluctuations of lake level in recent years. We hypothesized that these fluctuations may favour the cyanobacterium as result of nutrient pulses from the biofilms formed in the littoral zone when the lake level is high. To help verify this, we exposed artificial substrates in the lake, and evaluated their nutrient enrichment and release after desiccation, together with measurements of fluctuations in lake level, precipitation and D.lemmermannii population. The highest percentage of P release and the lowest C∶P molar ratio of released nutrients coincided with the summer appearance of the D.lemmermannii bloom. The P pulse indicates that fluctuations in level counteract nutrient limitation in this lake and it is suggested that this may apply more widely to other oligotrophic lakes. In view of the predicted increase in water level fluctuations due to climate change, it is important to try to minimize such fluctuations in order to mitigate the occurrence of cyanobacterial blooms.     

Ten-year survey of cyanobacterial blooms in Ohio’s waterbodies using satellite remote sensing

Cyanobacterial blooms are on the rise globally and are capable of adversely impacting human, animal, and ecosystem health. Blooms dominated by cyanobacteria species capable of toxin-production are commonly observed in eutrophic freshwater. The presence of cyanobacterial blooms in selected Ohio lakes, such as Lake Erie and Grand Lake St. Marys, has been well studied, but much less is known about the geographic distribution of these blooms across all of Ohio’s waterbodies. We examined the geographic distribution of cyanobacterial blooms in Ohio’s waterbodies from 2002 to 2011, using a nested semi-empirical algorithm and remotely sensed data from the Medium Resolution Imaging Spectrometer (MERIS) onboard the European Space Agency’s Envisat. We identified: 62 lakes, reservoirs, and ponds; 7 rivers; 6 marshes and wetlands; and 3 quarries with detectable cyanobacteria pigment (phycocyanin) concentrations. Of the 78 waterbodies identified in our study, roughly half (54%; n = 42) have any reported in situ microcystins monitoring results from state monitoring programs. Further, 90% of the waterbodies identified reached phycocyanin pigment concentrations representative of levels potentially hazardous to public health. This gap in lakes potentially impacted by cyanobacterial blooms and those that are currently monitored presents an important area of concern for public health, as well as ecosystem health, where unknown human and animal exposures to cyanotoxins may occur in many of Ohio’s waterbodies. Our approach may be replicated in other regions around the globe with potential cyanobacterial bloom presence, in order to assess the intensity, geographic distribution, and temporal pattern of blooms in lakes not currently monitored for the presence of cyanobacterial blooms.     

铜绿微囊藻对紫外辐射的生理代谢响应

采用紫外(UV)滤膜过滤日光UV以及紫外灯添加UV的方法,研究了UV辐射对铜绿微囊藻Microcystis aeruginosa单细胞藻株PCC7806和群体藻XW01生长及生理代谢的影响。结果显示,在室内条件下低剂量UV辐射可促进群体微囊藻XW01生长;室外条件下与滤除了UV的光照相比,含有UV的完全日光更有利于微囊藻生长;而相同的UV辐射强度均导致单细胞株死亡,群体株显示了较强的UV抗性;日光中的UV可促进XW01合成抗氧化相关的超氧化物歧化酶(SOD)和过氧化氢酶(CAT)、促进胞外多糖的产生并形成较大的群体、促进UV屏障物质类菌孢素氨基酸(MAAs)和伪枝藻素(Scy)积累。这些生理代谢的改变,消除了阳光辐射中UV对微囊藻的伤害。研究的结果提示,自然条件下阳光中的UV有助于群体微囊藻生长。     

含微囊藻毒素的蓝藻水华提取物对小鼠抗氧化酶的毒性作用

在世界范围内发生的富含营养的废水中生长蓝藻水华已经造成了严重的水污染。已有报道显示水华中的有毒成分是一类单环七肽微囊藻毒素,它们对人和家畜有严重的毒害作用,本文所使用的蓝藻水华采集于我国的太湖地区。我们采用三个亚致死计量(16、32、64 mg冻干藻细胞/kg体重,相当于4.97、9.94、19.88μg藻毒素/kg体重)的蓝藻水华提取物(CBE),分析它们对小鼠肝脏抗氧化酶的影响。经过连续14d每天腹腔注射CBE,小鼠分别在第3、5、7、9、11、13、15天处死并检测三种主要的抗氧化酶:超氧化物歧化酶(SOD)、过氧化氢酶(CAT)和谷胱甘肽巯基转移酶(GST)的活性。SOD和CAT的活性均呈现了抑制效应,GST的活性也发生了明显的变化。为了进一步确定过氧化的程度,我们又通过检测丙二醛的含量分析了脂质过氧化的程度。经过14d CBE处理后脂质过氧化表现出了明显的浓度依赖性的上升趋势。我们的结果表明,CBE对小鼠的抗氧化酶有严重毒害作用,并且能够引起脂质的过氧化。     

The antagonistic relationship between chlorophyll a concentrations and the growth areas of Trapa during summer in a shallow eutrophic lake

The relationship between concentrations of chlorophyll a in an open water area and growing areas of the macrophytes Trapa spp. and Nelumbo nucifera was investigated to clarify the role of macrophytes in phytoplankton growth in summer in a eutrophic shallow sand lake, Lake Honsagata, Japan. The chlorophyll a concentrations formed peaks in June 2007 and July 2008 in summer with cyanobacterial blooms composed of mainly Microcystis, Anabaena, Phormidium, and Oscillatoria, which decreased toward August and were maintained at different annual levels. The decline of blooms in August was caused by the rapid growth of macrophytes. The composition of phytoplankton and the level of bloom development in summertime differed characteristically from year to year. Also the total vegetation area of N. nucifera and Trapa spp. showed annually marked changes. A significant negative correlation between the concentrations of chlorophyll a and the growth areas of Trapa spp. in August was detected, indicating that the floating-leaved plants, Trapa spp., produced irregular, clear and turbid water states in this shallow eutrophicated lake. These antagonistic relations are explained based on a likely scenario of allelopathic effects on the development of cyanobacteria by Trapa spp. vegetation and the nutrient absorption competition among them. Our study demonstrated the potential of Trapa spp. to control cyanobacterial blooms producing harmful toxins.