Toxic Blooms Research Articles

Modelling Pseudo-nitzschia events off southwest Ireland

Toxic and non-toxic Pseudo-nitzschia blooms are common in coastal waters worldwide including Ireland. Off southwest Ireland, the timing of blooms on a weekly scale is highly variable, while the seasonal pattern is more regular with a bimodal distribution. Upwelling conditions are closely linked to Pseudo-nitzschia blooms. The work presented here describes a mathematical model, a Zero-Inflated Negative Binomial Model, employed to forecast the onset, abundance and duration of Pseudo-nitzschia blooms in the bays of southwest Ireland. Variables used in the model included field observations of Pseudo-nitzschia, sea surface temperature and wind. The estimated model reveals that, on average, cell levels on a given day depend on sea surface temperature, the value of a wind index on the previous day and the number of Pseudo-nitzschia in the water the previous week. The model forecast performed well for the onset and duration of blooms. However, the magnitude of blooms was sometimes underestimated by the model.

Effects of increasing nitrogen and phosphorus concentrations on phytoplankton community growth and toxicity during Planktothrix blooms in Sandusky Bay, Lake Erie.

Sandusky Bay experiences annual toxic cyanobacterial blooms dominated by Planktothrix agardhii/suspensa. To further understand the environmental drivers of these events, we evaluated changes in the growth response and toxicity of the Planktothrix-dominated blooms to nutrient amendments with orthophosphate (PO4) and inorganic and organic forms of dissolved nitrogen (N; ammonium (NH4), nitrate (NO3) and urea) over the bloom season (June - October). We complemented these with a metagenomic analysis of the planktonic microbial community. Our results showed that bloom growth and microcystin (MC) concentrations responded more frequently to additions of dissolved N than PO4, and that the dual addition of NH4 + PO4 and Urea + PO4 yielded the highest MC concentrations in 54% of experiments. Metagenomic analysis confirmed that P. agardhii/suspensa was the primary MC producer. The phylogenetic distribution of nifH revealed that both heterocystous cyanobacteria and heterotrophic proteobacteria had the genetic potential for N2 fixation in Sandusky Bay. These results suggest that as best management practices are developed for P reductions in Sandusky Bay, managers must be aware of the negative implications of not managing N loading into this system as N may significantly impact cyanobacterial bloom size and toxicity.

Toxic cyanobacterial bloom triggers in missisquoi bay, lake champlain, as determined by next-generation sequencing and quantitative PCR.

Missisquoi Bay (MB) is a temperate eutrophic freshwater lake that frequently experiences toxic Microcystis-dominated cyanobacterial blooms. Non-point sources are responsible for the high concentrations of phosphorus and nitrogen in the bay. This study combined data from environmental parameters, E. coli counts, high-throughput sequencing of 16S rRNA gene amplicons, quantitative PCR (16S rRNA and mcyD genes) and toxin analyses to identify the main bloom-promoting factors. In 2009, nutrient concentrations correlated with E. coli counts, abundance of total cyanobacterial cells, Microcystis 16S rRNA and mcyD genes and intracellular microcystin. Total and dissolved phosphorus also correlated significantly with rainfall. The major cyanobacterial taxa were members of the orders Chroococcales and Nostocales. The genus Microcystis was the main mcyD-carrier and main microcystin producer. Our results suggested that increasing nutrient concentrations and total nitrogen:total phosphorus (TN:TP) ratios approaching 11:1, coupled with an increase in temperature, promoted Microcystis-dominated toxic blooms. Although the importance of nutrient ratios and absolute concentrations on cyanobacterial and Microcystis dynamics have been documented in other laboratories, an optimum TN:TP ratio for Microcystis dominance has not been previously observed in situ. This observation provides further support that nutrient ratios are an important determinant of species composition in natural phytoplankton assemblages.

Local nutrient regimes determine site-specific environmental triggers of cyanobacterial and microcystin variability in urban lakes

Abstract. Toxic cyanobacterial blooms in urban lakes present serious health hazards to humans and animals and require effective management strategies. Managing such blooms requires a sufficient understanding of the controlling environmental factors. A range of them has been proposed in the literature as potential triggers for cyanobacterial biomass development and cyanotoxin (e.g. microcystin) production in freshwater systems. However, the environmental triggers of cyanobacteria and microcystin variability remain a subject of debate due to contrasting findings. This issue has raised the question of whether the relevance of environmental triggers may depend on site-specific combinations of environmental factors. In this study, we investigated the site-specificity of environmental triggers for cyanobacterial bloom and microcystin dynamics in three urban lakes in Western Australia. Our study suggests that cyanobacterial biomass, cyanobacterial dominance and cyanobacterial microcystin content variability were significantly correlated to phosphorus and iron concentrations. However, the correlations were different between lakes, thus suggesting a site-specific effect of these environmental factors. The discrepancies in the correlations could be explained by differences in local nutrient concentration. For instance, we found no correlation between cyanobacterial fraction and total phosphorous (TP) in the lake with the highest TP concentration, while correlations were significant and negative in the other two lakes. In addition, our study indicates that the difference of the correlation between total iron (TFe) and the cyanobacterial fraction between lakes might have been a consequence of differences in the cyanobacterial community structure, specifically the presence or absence of nitrogen-fixing species. In conclusion, our study suggests that identification of significant environmental factors under site-specific conditions is an important strategy to enhance successful outcomes in cyanobacterial bloom control measures.

“MT-PCR – A rapid, reliable and effective tool for assessing toxic ‘algal’ blooms in Victorian water supplies: aiding protection and preservation”

This project addressed a need for a sensitive, accurate and reliable testing method to aid assessment of the toxicity of algal blooms and assist water management. Increasingly, diagnostic dilemmas are resolved through the use of DNA-based technologies which often provide high sensitivity and specificity and are efficient both in terms of costs and time. However to date, no such test was available to the Victorian water industries. This project sought to bridge this gap by developing an automated DNA-based diagnostic assay for cyanobacterial bloom assessment blooms in Victorian waters. The assay exceeds expectation in its ability to accurately quantify levels of toxigenic cyanobacteria in bloom samples, retains exceptionally high specificity and sensitivity and each assay out-performs common conventional PCR approaches established in the literature. Four toxigen assays (microcystin, nodularin, cylindrospermopsin and saxitoxin) were designed, tested and optimised. This book is co-published with Water Research Australia This title belongs to WaterRA ISBN: 9781780407562 (eBook) ISBN: 9781780407555 (Print)

ELECTRODIALYTIC PROCESS OF NANOFILTRATION CONCENTRATES – PHOSPHORUS RECOVERY AND MICROCYSTINS REMOVAL

Toxic cyanobacteria blooms are associated with nutrient enrichment of surface waters. Nanofiltration (NF) is a pressure-driven process that can be used in water treatment plants, to effectively remove particulate matter and organic contaminants, including cyanobacteria toxins, and nutrients. NF produces a nutrient and toxin-enriched stream, e.g. with microcystin-LR (MC-LR), that has to be safely disposed of.The suitability of the electrodialytic (ED) process for phosphorus recovery and decrease of MC-LR concentrations from NF waste stream was assessed. In ED experiments running between 5 and 12h, phosphorus electromigrated towards the anode compartment promoting an isolated clean phosphorus product. The maximum obtained phosphorus recovery was 84%, varying with the NF waste stream (membrane concentrate) characteristics, cell design and treatment time. After the application of the ED process the concentration of MC-LR was also reduced. These results encourage further tests, aiming the inclusion of this hybrid technology (NF followed by ED) in water treatment plants, contributing not only for the phosphorus recovery, but also for the decrease in the toxicity of the NF concentrate, lowering the costs and promoting safe disposal.

Integrative Monitoring of Marine and Freshwater Harmful Algae in Washington State for Public Health Protection

The more frequent occurrence of both marine and freshwater toxic algal blooms and recent problems with new toxic events have increased the risk for illness and negatively impacted sustainable public access to safe shellfish and recreational waters in Washington State. Marine toxins that affect safe shellfish harvest in the state are the saxitoxins that cause paralytic shellfish poisoning (PSP), domoic acid that causes amnesic shellfish poisoning (ASP) and the first ever US closure in 2011 due to diarrhetic shellfish toxins that cause diarrhetic shellfish poisoning (DSP). Likewise, the freshwater toxins microcystins, anatoxin-a, cylindrospermopsins, and saxitoxins have been measured in state lakes, although cylindrospermopsins have not yet been measured above state regulatory guidance levels. This increased incidence of harmful algal blooms (HABs) has necessitated the partnering of state regulatory programs with citizen and user-fee sponsored monitoring efforts such as SoundToxins, the Olympic Region Harmful Algal Bloom (ORHAB) partnership and the state’s freshwater harmful algal bloom passive (opportunistic) surveillance program that allow citizens to share their observations with scientists. Through such integrated programs that provide an effective interface between formalized state and federal programs and observations by the general public, county staff and trained citizen volunteers, the best possible early warning systems can be instituted for surveillance of known HABs, as well as for the reporting and diagnosis of unusual events that may impact the future health of oceans, lakes, wildlife, and humans.

Competition between toxic and non-toxic Microcystis aeruginosa and its ecological implication.

The frequency of toxic cyanobacterial blooms has increased in recent decades, but the factors that regulate the dominance of toxin-producing cyanobacteria over non-toxin-producing strains of one species are still obscure. This study examined the effects of temperature, light intensity, nitrate and phosphate on the dominance of MC-producing and non-MC-producing strains of Microcystis aeruginosa in monoculture and co-culture experiments. In the monoculture experiments, growth rates of the non-MC-producing strain were higher than those of the MC-producing strain under the same growth conditions. However, at the end of the co-culture experiments, the MC-producing strain became surprisingly dominant in all treatments except when treated with extreme low phosphate concentrations. Higher temperatures and nutrient levels can shift the dominance more quickly towards the toxic strain. The dominance may be explained by allelopathic interactions through allelochemicals and other secondary metabolites, but not MC. Environmental factors such as extremely low phosphate content may exert an indirect effect on strain dominance by changing the production of allelochemicals. Our findings highlight the complications in predicting competitive outcome for cyanobacterial strains in natural environments.

Identification of a new microcystin-degrading bacterium isolated from Lake Chaohu, China.

A microcystin-LR (MC-LR)-degrading bacterium was isolated from Lake Chaohu, a eutrophic freshwater lake containing toxic cyanobacterial blooms. Based on the analysis of 16S rDNA gene sequence and physiobiochemical characteristics, the isolated strain, most likely belongs to the genus Bacillus with the highest sequence similarity value with Bacillus nanhaiencis strain K-W39 (JQ799091.1), was named B. nanhaiencis strain JZ-2013. The strain JZ-2013 could grow on mineral salt medium supplied with MC-LR as sole carbon and nitrogen sources. The optimal temperature and pH for strain JZ-2013 growth and MC-LR biodegradation were 30°C and 8.0, respectively. The MC-LR with the initial concentration of 15 mg/L could be consumed 80 % by strain JZ-2013 within 9 days. The existence of exogenous carbon and nitrogen sources could significantly increase the removal efficiency of MC-LR. The strain JZ-2013 can efficiently removed MC-LR of low concentration in real water sample.

Effects of microcystin-LR, cylindrospermopsin and a microcystin-LR/cylindrospermopsin mixture on growth, oxidative stress and mineral content in lettuce plants (Lactuca sativa L.)

Toxic cyanobacterial blooms are documented worldwide as an emerging environmental concern. Recent studies support the hypothesis that microcystin-LR (MC-LR) and cylindrospermopsin (CYN) produce toxic effects in crop plants. Lettuce (Lactuca sativa L.) is an important commercial leafy vegetable that supplies essential elements for human nutrition; thus, the study of its sensitivity to MC-LR, CYN and a MC-LR/CYN mixture is of major relevance. This study aimed to assess the effects of environmentally relevant concentrations (1, 10 and 100µg/L) of MC-LR, CYN and a MC-LR/CYN mixture on growth, antioxidant defense system and mineral content in lettuce plants.In almost all treatments, an increase in root fresh weight was obtained; however, the fresh weight of leaves was significantly decreased in plants exposed to 100µg/L concentrations of each toxin and the toxin mixture. Overall, GST activity was significantly increased in roots, contrary to GPx activity, which decreased in roots and leaves. The mineral content in lettuce leaves changed due to its exposure to cyanotoxins; in general, the mineral content decreased with MC-LR and increased with CYN, and apparently these effects are time and concentration-dependent. The effects of the MC-LR/CYN mixture were almost always similar to the single cyanotoxins, although MC-LR seems to be more toxic than CYN. Our results suggest that lettuce plants in non-early stages of development are able to cope with lower concentrations of MC-LR, CYN and the MC-LR/CYN mixture; however, higher concentrations (100µg/L) can affect both lettuce yield and nutritional quality.

Biodiversity of cyanobacteria in Tunisian freshwater reservoirs: occurrence and potent toxicity – a review

In spite of the great environmental and sanitary importance of cyanobacteria, their biodiversity is little known in Tunisia. In this work, a review was carried out, based on literature data, of potentially toxic cyanobacteria occurrence in Tunisia. Microcystis, one of the most widely distributed toxic bloom-forming cyanobacteria genera, was represented by Microcytis wesenbergii, found only in Lebna water reservoir, and Microcytis aeruginosa recorded in different water bodies. The invasive potentially toxic cyanobacterium Cylindrospermopsis raciborskii , reported for the first time in Tunisia in October 2004, was observed in the semi-arid to arid areas. The harmful genus Planktothrix was represented in Tunisian freshwater by the green-pigmented species P. agardhii . The filamentous cyanobacteria dominance is increasingly reported in Tunisia in eutrophic water bodies. This dominance increases especially during the summer–autumn period. Recently, potentially toxic cyanobacteria blooms have been reported in some reservoirs in the north east of the country. These blooms were generated by the potentially toxic Chroococcale Microcystis aeruginosa. Harmful cyanobacteria tend to spatio-temporal expansion in the Tunisian inland waters. The toxicological potential evaluated by several methods showed that none of the Tunisian strains were proved to be cylindrospermopsin nor saxitoxin producers. However, the majority of Microcystis were able to synthesize microcystin.

Constitutive cylindrospermopsin pool size in Cylindrospermopsis raciborskii under different light and CO2 partial pressure conditions.

Cylindrospermopsin (CYN) and 7-deoxy-cylindrospermopsin (dCYN) are potent hepatotoxic alkaloids produced by numerous species of cyanobacteria, including the freshwater Cylindrospermopsis raciborskii. C. raciborskii is an invasive cyanobacterium, and the study of how environmental parameters drive CYN production has received significant interest from water managers and health authorities. Light and CO2 affect cell growth and physiology in photoautotrophs, and these are potential regulators of cyanotoxin biosynthesis. In this study, we investigated how light and CO2 affect CYN and dCYN pool size as well as the expression of the key genes, cyrA and cyrK, involved in CYN biosynthesis in a toxic C. raciborskii strain. For cells growing at different light intensities (10 and 100 μmol photons m(-2) s(-1)), we observed that the rate of CYN pool size production (μCYN) was coupled to the cell division rate (μc) during batch culture. This indicated that CYN pool size under our experimental conditions is constant and cell quotas of CYN (QCYN) and dCYN (QdCYN) are fixed. Moreover, a lack of correlation between expression of cyrA and total CYN cell quotas (QCYNs) suggests that the CYN biosynthesis is regulated posttranscriptionally. Under elevated CO2 (1,300 ppm), we observed minor effects on QCYN and no effects on expression of cyrA and cyrK. We conclude that the CYN pool size is constitutive and not affected by light and CO2 conditions. Thus, C. raciborskii bloom toxicity is determined by the absolute abundance of C. raciborskii cells within the water column and the relative abundance of toxic and nontoxic strains.

Analysis of intracellular and extracellular microcystin variants in sediments and pore waters by accelerated solvent extraction and high performance liquid chromatography-tandem mass spectrometry

The fate and persistence of microcystin cyanotoxins in aquatic ecosystems remains poorly understood in part due to the lack of analytical methods for microcystins in sediments. Existing methods have been limited to the extraction of a few extracellular microcystins of similar chemistry. We developed a single analytical method, consisting of accelerated solvent extraction, hydrophilic–lipophilic balance solid phase extraction, and reversed phase high performance liquid chromatography-tandem mass spectrometry, suitable for the extraction and quantitation of both intracellular and extracellular cyanotoxins in sediments as well as pore waters. Recoveries of nine microcystins, representing the chemical diversity of microcystins, and nodularin (a marine analogue) ranged between 75 and 98% with one, microcystin-RR (MC-RR), at 50%. Chromatographic separation of these analytes was achieved within 7.5min and the method detection limits were between 1.1 and 2.5ngg−1 dry weight (dw). The robustness of the method was demonstrated on sediment cores collected from seven Canadian lakes of diverse geography and trophic states. Individual microcystin variants reached a maximum concentration of 829ngg−1 dw on sediment particles and 132ngmL−1 in pore waters and could be detected in sediments as deep as 41cm (>100 years in age). MC-LR, -RR, and -LA were more often detected while MC-YR, -LY, -LF, and -LW were less common. The analytical method enabled us to estimate sediment-pore water distribution coefficients (Kd), MC-RR had the highest affinity for sediment particles (log Kd=1.3) while MC-LA had the lowest affinity (log Kd=−0.4), partitioning mainly into pore waters. Our findings confirm that sediments serve as a reservoir for microcystins but suggest that some variants may diffuse into overlying water thereby constituting a new route of exposure following the dissipation of toxic blooms. The method is well suited to determine the fate and persistence of different microcystins in aquatic systems.

Environmental effects of modified clay flocculation on Alexandrium tamarense and paralytic shellfish poisoning toxins (PSTs)

Among various mitigation strategies for harmful algal blooms (HABs), the flocculation of algal cells by using modified clay (MC) has been widely applied in the field, particularly in Japan, Korea and China. However, to examine the long-term effects and the environmental safety of this method, we investigated alterations in macronutrients and paralytic shellfish poisoning toxins (PSTs) induced by the application of MC treatment to a toxic bloom, Alexandrium tamarense. The control, algal cells grew in nature condition (A1), was compared to the only MC flocculation (A2) and the MC-sediment co-matrix systems of A. tamarense (A3). The low-dosage of 0.25gL−1 MC could efficiently remove >90% of the A. tamarense cells within 3.5h. The mechanisms underlying the effects elicited by MC flocculation on nutrient cycling, PSTs and Chl-a degradation were also discussed. This study demonstrated that MC treatment was able to significantly remove the macronutrients (43–60% TP removal and 17–30% TN removal) and scavenge most of the PSTs from seawater, thereby speeding up the nutrient settling and the transformation and degradation of PSTs (83% decreasing in A2). Simultaneously, the study firstly demonstrated the potential detoxification of PSTs by using MC treatment, from the high toxicity of gonyautoxin 1 and 4 (GTX1 and GTX4) to the lower toxicity decarbamoyl gonyautoxins (dcGTX3) and gonyautoxin 2 (GTX2), particularly within the water-sediment environment during the two month incubation.

Development of a real-time PCR assay for the quantification of Ma-LMM01-type Microcystis cyanophages in a natural pond.

Microcystis aeruginosa forms toxic cyanobacterial blooms throughout the world where its infectious phages are thought to influence host population dynamics. To assess the cyanophage impact on the host dynamics, we previously monitored Ma-LMM01-type phage abundance using a real-time PCR with a primer set designed based on the sequence of Microcystis phage Ma-LMM01; and we estimated the phage-infected host cell abundance. However, a recent study shows the Ma-LMM01 g91 gene sequence belongs to the smallest group, group III, of the three genotype groups, suggesting Ma-LMM01-type phage abundance was underestimated. Therefore, to re-evaluate the effect of Ma-LMM01-type phages on their hosts, we monitored the abundance of Ma-LMM01-type phages using real-time PCR with a new primer set designed based on the sequences of genotype groups I-III. We found phage abundance between 10(3) and 10(4) ml(-1) using the new primer set in samples where previously these phages were not detected using the old primer set. The frequency of Ma-LMM01-type phage-infected cells to Ma-LMM01-type phage-susceptible host cells may be as high as 30%, suggesting the phages may occasionally affect not only shifts in the genetic composition but also the dynamics of Ma-LMM01-type phage-susceptible host populations. Phages are one of the factors that may control the ecology of their host blooms. Therefore, it is essential to estimate phage abundance to understand phage impact on host populations. A real-time PCR assay was improved to detect a larger range of Microcystis cyanophages in natural surroundings where no phages were detected using a previous method by re-designing a new primer set based on sequences from three Ma-LMM01-type phage genetic groups. The new method allows us to determine the distribution, dynamics and infection cycle of the phage to help understand the interaction between the phages and the hosts.

Inhibitory mechanisms of Acacia mearnsii extracts on the growth of Microcystis aeruginosa.

Our previous work revealed that Acacia mearnsii extract can inhibit the growth of Microcystis aeruginosa, the common species forming toxic cyanobacterial blooms in eutrophic freshwater. In the present study, we demonstrated that this plant extract can significantly increase cell membrane permeability and Ca²⁺/Mg²⁺-ATPase activity on the membrane. Long-term exposure to concentrations of 20 ppm A. mearnsii extract led to algal cell membrane leakage or even lysis. Comparison of expression of three photosynthesis-related genes (rbcL, psaB and psbD) in M. aeruginosa with and without plant extract treatment revealed that their expression was remarkably reduced in the presence of the extract. Down-regulation of photosynthesis-related genes could indicate the inhibition of the photosynthetic process. Thus, our results suggested that both photosynthetic systems and membranes of M. aeruginosa are potentially damaged by A. mearnsii extract.

Cyanobacterial bloom management through integrated monitoring and forecasting in large shallow eutrophic Lake Taihu (China)

The large shallow eutrophic Lake Taihu in China has long suffered from eutrophication and toxic cyanobacterial blooms. Despite considerable efforts to divert effluents from the watershed, the cyanobacterial blooms still reoccur and persist throughout summer. To mitigate cyanobacterial bloom pollution risk, a large scale integrated monitoring and forecasting system was developed, and a series of emergency response measures were instigated based on early warning. This system has been in place for 2009–2012. With this integrated monitoring system, it was found that the detectable maximum and average cyanobacterial bloom area were similar to that before drinking water crisis, indicating that poor eutrophic status and cyanobacterial bloom had persisted without significant alleviation. It also revealed that cyanobacterial bloom would occur after the intense storm, which may be associated with the increase in buoyance of cyanobacterial colonies. Although the cyanobacterial blooms had persisted during the monitoring period, there had been a reduction in frequency and intensity of the cyanobacterial bloom induced black water agglomerates (a phenomenon of algal bloom death decay to release a large amount black dissolved organic matter), and there have been no further drinking water crises. This monitoring and response strategy can reduce the cyanobacterial bloom pollution risk, but cannot reduce eutrophication and cyanobacterial blooms, problems which will take decades to resolve.

Arginine kinase in the cladoceran Daphnia magna: cDNA sequencing and expression is associated with resistance to toxic Microcystis

Nutrient loading derived from anthropogenic activities into lakes have increased the frequency, severity and duration of toxic cyanobacterial blooms around the world. Although herbivorous zooplankton are generally considered to be unable to control toxic cyanobacteria, populations of some zooplankton, including Daphnia, have been shown to locally adapt to toxic cyanobacteria and suppress cyanobacterial bloom formation. However, little is known about the physiology of zooplankton behind this phenomenon. One possible explanation is that some zooplankton may induce more tolerance by elevating energy production, thereby adding more energy allocation to detoxification expenditure. It is assumed that arginine kinase (AK) serves as a core in temporal and spatial adenosine triphosphate (ATP) buffering in cells with high fluctuating energy requirements. To test this hypothesis, we studied the energetic response of a single Daphnia magna clone exposed to a toxic strain of Microcystis aeruginosa, PCC7806. Arginine kinase of D. magna (Dm-AK) was successfully cloned. An ATP-gua PtransN domain which was described as a guanidine substrate specificity domain and an ATP-gua Ptrans domain which was responsible for binding ATP were both identified in the Dm-AK. Phylogenetic analysis of AKs in a range of arthropod taxa suggested that Dm-AK was as dissimilar to other crustaceans as it was to insects. Dm-AK transcript level and ATP content in the presence of M. aeruginosa were significantly lower than those in the control diet containing only the nutritious chlorophyte, Scenedesmus obliquus, whereas the two parameters in the neonates whose mothers had been previously exposed to M. aeruginosa were significantly higher than those of mothers fed with pure S. obliquus. These findings suggest that Dm-AK might play an essential role in the coupling of energy production and utilization and the tolerance of D. magna to toxic cyanobacteria.

Nutrient limitation dynamics examined on a multi-annual scale in Lake Taihu, China: implications for controlling eutrophication and harmful algal blooms

Rapidly increasing urban, agricultural, and industrial growth in the Taihu basin during the past four decades has led to accelerated nitrogen (N) and phosphorus (P) loading to the lake. This has caused the lake to shift from oligo-mesotrophic to hypertrophic conditions, symptomized by toxic cyanobacterial blooms, dominated by the non-N2 fixing genus Microcystis. From 2008 to 2013, a series of in situ microcosm and mesocosm nutrient addition bioassays were conducted that were focused on the heavily polluted northern region (i.e., Meiliang Bay) and other lake locations. Bioassays showed that phytoplankton production, as chlorophyll a and photopigments diagnostic of major phytoplankton groups, was controlled by P inputs from spring to early summer, while N played a more dominant controlling role in summer–fall. In most cases, combined N and P additions promoted maximum growth. This pattern proved true for both the highly eutrophic northern region and the less-eutrophic central and southern regions. Cyanobacteria, chlorophytes, and cryptophytes all showed the strongest positive responses to N and N+P enrichment during the summer bloom period, while diatoms were the least abundant then and just moderately stimulated by nutrient additions. Cyanobacteria failed to selectively respond to P inputs during the summer bloom period, contradicting the paradigm that selective P enrichment will favor them, especially the N2-fixing genera. Rather, Microcystis-dominated blooms remained N-limited during summer months and were not replaced by N2-fixing genera, indicating that internal N and P regeneration of previously loaded nutrients must be sustaining blooms. Successful ‘de-eutrophication’ of Taihu will require reductions of both N and P inputs in all lake regions in order to control blooms and counter the legacy of several decades of nutrient over-enrichment.

Composition and Anti-Cyanobacterial Activity of Essential Oils from Six Different Submerged Macrophytes

Eutrophication of water due to pollution is one of the most serious environmental problems. The occurrence of toxic cyanobacterial blooms in eutrophic lakes, reservoirs, and recreational waters has become a worldwide problem. The inhibitory allelopathy of plants on harmful algae, which has drawn extensive attention recently, is low-cost and ecologically safe, and some good results have been reported. In this study, volatile organic compounds as major allelochemicals have been isolated from some aquatic plants and identified. The chemical compositions of essential oil isolated from Potamogeton cristatus, Potamogeton maackianus, Potamogeton lucens, Vallisneria spinulosa, Ceratophyllum demersum, and Hydrilla verticillata were analyzed by GC/MS. Thirty components were identified in the oils, mainly including fatty acids, ester, sterol, and ketone, etc. Inhibitory effects of essential oils on Microcystis aeruginosa were also investigated. The inhibition ratio of essential oils on M aeruginosa was 30.2-41.7% when the treatment concentration of extracts was at a level of 50.0 mg/L. Hence, the essential oils isolated exhibited a significant anti-cyanobacterial activity.