Field and experimental studies on the combined impacts of cyanobacterial blooms and small algae on crustacean zooplankton in a large,eutrophic, subtropical,Chinese lake

Field and experimental studies were conducted to evaluate the combined impacts of cyanobacterial blooms and small algae on seasonal and long-term changes in the abundance and community structure of crustacean zooplankton in a large, eutrophic, Chinese lake, Lake Chaohu. Seasonal changes of the crustacean zooplankton from 22 sampling stations were investigated during September 2002 and August 2003, and 23 species belonging to 20 genera were recorded. Daphnia spp. dominated in spring but disappeared in mid-summer, while Bosmina coregoni and Ceriodaphnia cornuta dominated in summer and autumn. Both maximum cladoceran density (310 ind. l⁻¹) and biomass (5.2 mg l⁻¹) appeared in autumn. Limnoithona sinensis, Sinocalanus dorrii and Schmackeria inopinus were the main species of copepods. Microcystis spp. were the dominant phytoplankton species and formed dense blooms in the warm seasons. In the laboratory, inhibitory effects of small colonial Microcystis on growth and reproduction of Daphnia carinata were more remarkable than those of large ones, and population size of D. carinata was negatively correlated with density of fresh large colonial Microcystis within a density range of 0–100 mg l⁻¹ (r = −0.82, P< 0.05). Both field and experimental results suggested that seasonal and long-term changes in the community structure of crustacean zooplankton in the lake were shaped by cyanobacterial blooms and biomass of the small algae, respectively, i.e., colonial and filamentous cyanobacteria contributed to the summer replacement of dominant crustacean zooplankton from large Daphnia spp. to small B. coregoni and C. cornuta, while increased small algae might be responsible for the increased abundance of crustacean zooplankton during the past decades.     

Spatial and temporal diversity of microcystins and microcystin-producing genotypes in Oneida Lake, NY

Oneida Lake is a shallow, eutrophic lake with a well-established cyanobacterial population with reported toxic blooms containing hepatotoxic microcystins (MC). Peak bloom events from the summers of 2002 and 2003 were analyzed to determine the principal cyanobacterial genera containing microcystin synthetase (mcy) genes. Sequence analysis of a partial mcyA amplicon targeting Microcystis, Anabaena and Planktothrix sp. indicated that Microcystis sp. was the dominant mcy genotype. This Microcystis clade was split into two distinct sub-clades. Bloom events contained members of both sub-clades with the higher MC concentrations found when both sub-clades were present in near equal proportions. The proportion of Microcystis containing the mcyD gene ranged from 0 to 37% of the total Microcystis population as determined by quantitative PCR (qPCR). The total concentration of Microcystis containing mcyD genes was linearly related to the concentration of MCs (r² = 0.63). The relationship between mcy genotype and physiochemical variables was examined to determine the factor(s) controlling the periodicity in MC production in Oneida Lake. Multivariate statistical analyses, used to correlate the continuous-response variables, revealed a strong relationship between chlorophyll a, MCs and total Microcystis.     

Effects of Microcystis blooms on the crustacean plankton community: enclosure experiments in a subtropical lake

An enclosure experiment was conducted in July–September 2001 in subtropical eutrophic Lake Donghu (China) to test a hypothesis that a moderate cyanobacterial biomass would have a positive effect on small-sized cladocerans. Eight enclosures (12.5 m³) were arranged with different nutrient concentrations using the lake water, tap water, and sediment from Lake Donghu. Microcystis blooms appeared in enclosures with higher nutrient concentrations and the average fresh weight biomass of Microcystis spp. ranged from 4.6 to 30.4 mg l^?1 during the bloom period. Three cladocerans (Moina micrura, Diaphanosoma brachyurum, and Ceriodaphnia cornuta) and two cyclopoids (Mesocyclops dissimilis and Thermocyclops taihokuensis) dominated the crustacean plankton community during the experimental period. The C. cornuta biomass constituted the greatest percentage (55.9–90.0%) of cladoceran biomass in the Microcystis bloom treatments. When the Microcystis biomass increased, the average biomass of C. cornuta increased and the biomass of M. micrura and D. brachyurum decreased, whereas the cyclopoid biomass did not change significantly. The total biomass of cladoceran and crustacean plankton were significantly positively correlated with the Microcystis biomass. Our results indicate that a moderate biomass of Microcystis spp. can favor crustacean plankton to some extent and, furthermore, may impact food web structures in a eutrophic lake.     

Studies on the Effects of Silver Carp (Hypophthalmichthys molitrix) on the Phytoplankton in a Shallow Hypereutrophic Lake through an Enclosure Experiment

The responses of nutrients, water transparency, zooplankton and phytoplankton to a gradient of silver carp biomass were assessed using enclosure methods. The gradient of four silver carp biomass levels was set as follows: 0, 116, 176 and 316 g m^—2. Nutrients did not show any statistically significant differences among the treatments. An outburst of Daphnia only occurred in fishless enclosures where phytoplankton biomass was the lowest and water clarity significantly increased. While among fish enclosures, the small‐sized Moina micrura dominated throughout the experiment and both zooplankton and phytoplankton biomasses decreased with increased fish biomass. No large colonial cyanobacterial blooms occurred in the fishless enclosures as predicted. This might be due to low water temperature, short experiment time and the occurrence of large bodied Daphnia in our experiment. Cryptophyta was the most dominant group in most of the enclosures and the lake water throughout the experiment. The fishless enclosure had much lower proportion of Cyanophyta but higher proportion of Trachelomonas sp.     

Assessment of Environmental Conditions That Favor Hepatotoxic and Neurotoxic Anabaena spp. Strains Cultured under Light Limitation at Different Temperatures

Abstract Toxic cyanobacterial mass occurrences have caused animal poisonings worldwide and may pose a health hazard for humans. Strains of the genus Anabaena are either non-toxic or produce hepatotoxins, microcystins (MCYST), or neurotoxins (such as anatoxin-a). In order to study which growth conditions favor hepatotoxic vs neurotoxic strains and how production of toxins varies, we compared the responses of two microcystin- and two anatoxin-a-producing Anabaena strains in continuous turbidostat cultures, at different temperatures, under growth-limiting light levels. Growth rates consistently remained <0.8 divisions per 24 h. Differences were strain-specific and not associated with hepatotoxicity or neurotoxicity. Thus, differential adaptation of strains to temperature and to growth-limiting light levels cannot explain why, in some cyanobacterial water blooms, hepatotoxic strains, and in others, neurotoxic ones become dominant. A statistical analysis of field data showed that the most significant discriminating factors between different types of blooms were the concentrations of dissolved PO₄-phosphorus and NO₃-nitrogen. Anabaena blooms with unknown neurotoxicity associated with low PO₄-phosphorus and high NO₃-nitrogen concentrations. Among other Anabaena blooms, the hepatotoxic ones associated with the lowest, and most of the non-toxic ones with higher concentrations of PO₄-phosphorus. Anabaena blooms that contained anatoxin-a and hepatotoxic Microcystis blooms showed tendencies towards the highest concentrations of PO₄-phosphorus. Non-toxic blooms dominated by genera other than Anabaena occurred over a wide range of growth conditions. In turbidostat cultures, maximal production of microcystins correlated with maximal growth rates. Light regulated the production of MCYST-LR variants, and temperature affected the production of MCYST-RR variants. Anatoxin-a seemed to be produced most under temperatures and light levels slightly suboptimal for growth. Under low light, considerable amounts of extracellular anatoxin-a were detected while microcystins consistently remained intracellular. Received: 25 August 1997; Accepted: 2 December 1997     

Grazing on toxic cyanobacterial blooms by tadpoles of edible frog Rana grylio

Tadpoles of Rana grylio were raised as edible frogs in fishponds of Guanqiao in Wuhan City, Hubei, China, during cyanobacterial blooms from June to October. The dominant cyanobacterial species was Microcystis, which was found to be lethally toxic by intraperitoneal (i.p.) mouse bioassay. Little is known about the effect of tadpoles on toxic cyanobacterial blooms. To evaluate the potential of the tadpoles to graze on cyanobacterial blooms, the tadpoles were fed on Microcystis collected from the field in the laboratory. The Microcystis cells decreased from 1.19 × 10⁷ cells mL^?1 to 3.23 × 10⁶ cells mL^?1, with a sharp reduction of 73% of the initial Microcystis population observed in the first 24 h after introduction of the tadpoles. The ponds containing tadpoles had a markedly lower density of Microcystis than those lacking tadpoles. Tadpoles exposed to either cultured Microcystis aeruginosa (NIES–90, 2.768 µg microcystins mg^–1 dw^–1) cells or lysed M. aeruginosa cells grew well, however, indicating that they were unaffected by Microcystis toxins. We found a significant increase in tadpole body weight after feeding on either field Microcystis or cultured M. aeruginosa. The mean increase in individual body weight was 20 mg day^?1 when fed on Microcystis from the pond, and 7 mg day^?1 when fed on M. aeruginosa from culture. Our study strongly suggested that there is a direct trophic relationship between R. grylio tadpoles and toxic Microcystis blooms and they possess the potential to graze on toxic Microcystis. The results imply that R. grylio tadpoles may play an important ecological role in reducing toxic cyanobacterial blooms caused by Microcystis.     

Incorporation of Different N Sources and Light Response Curves of Nitrogenase and Photosynthesis by Cyanobacterial Blooms from Rice Fields

In this work, we estimate the contributions of the different sources of N incorporated by two N₂-fixing cyanobacterial blooms (Anabaena sp. and Microchaete sp.) in the rice fields of Valencia (Spain) during the crop cycles of 1999 and 2000, and evaluate the response of nitrogenase and C assimilation activities to changing irradiances. Our results show that, far from the generally assumed idea that the largest part of the N incorporated by N₂-fixing cyanobacterial blooms in rice fields comes from N₂ fixation, both cyanobacterial blooms incorporated about three times more N from dissolved combined compounds than from N₂ fixation (only about 33–41% of the N incorporated came from N₂ fixation). Our results on the photodependence of C and N₂ fixation indicate that in both cyanobacterial blooms, N₂ fixation showed a steeper initial slope (α) and was saturated with less irradiance than C fixation, suggesting that N₂ fixation was more efficient than photosynthesis under conditions of light limitation. At saturating light, N₂ fixation and C fixation differed depending on the bloom and on the environmental conditions created by rice plant growth. Carbon assimilation but not nitrogenase activity appeared photoinhibited in the Anabaena but not in the Microchaete bloom in August 1999, when the plants were tall and the canopy was important, and there was no limitation of dissolved inorganic carbon. The opposite was found in the Microchaete bloom of June 2000, when plants were small and produced little shade, and dissolved inorganic carbon was very low.     

Quantitative assessment of toxic and nontoxic <Emphasis Type="Italic">Microcystis</Emphasis> colonies in natural environments using fluorescence <Emphasis Type="Italic">in situ</Emphasis> hybridization and flow cytometry

Toxic cyanobacterial blooms constitute a threat to human safety because Microcystis sp. releases microcystins during growth, and particularly during cell death. Therefore, analysis of toxic and nontoxic Microcystis in natural communities is required in order to assess and predict bloom dynamics and toxin production by these organisms. In this study, an analysis combining fluorescence in situ hybridization (FISH) with flow cytometry (FCM) was used to discriminate between toxic and nontoxic Microcystis and also to quantify the percentage of toxic Microcystis present in blooms. The results demonstrate that the combination of FISH and flow cytometry is a useful approach for studying the ecology of Microcystis toxin production and for providing an early warning for toxic Microcystis blooms.     

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.     

Diversity and dynamics of microcystin—Producing cyanobacteria in China's third largest lake, Lake Taihu

Microcystins (MC), the most prevalent group of harmful cyanobacterial hepatotoxins, are primarily produced by strains of cyanobacteria in Microcystis, Anabaena and Planktothrix. Lake Taihu, which is the third largest freshwater lake in China, is a hypertrophic shallow lake in eastern China that has experienced lake-wide cyanobacterial blooms annually during the last few decades. In this study, PCR-DGGE was used to evaluate the diversity of potential MC-producing cyanobacteria and real-time PCR was used to analyze the dynamics of this population based on the presence of the mcy gene in samples collected during a year long study. The results revealed that all MC-producing genotypes detected belonged to the genus Microcystis. In addition, the MC-producing genotype communities were more diverse during the bloom season than the non-bloom season, and the diversity in the late bloom period was lower than the diversity in the early bloom period. Furthermore, the abundance of MC-producing genotypes increased dramatically during the bloom development period, reaching its peak in late summer (September). The results also suggested that the highest mcy gene concentration lagged behind the highest MC concentration, and the potential MC-producing cyanobacterial community shift lagged behind the development of blooms.     

Cyanobacterial blooms and water quality in Greek waterbodies

The cyanobacterial species composition of nine Greek waterbodies of different type and trophic status was examined during the warm period of the year (May–October). Cyanobacterial water blooms were observed in all waterbodies. Forty-six cyanobacterial taxa were identified, 11 of which are known to be toxic. Eighteen species are reported for the first time in these waterbodies, 8 of which are known to produce toxins. Toxin producing species were found in all of the waterbodies and were primarily dominant in bloom formations (e.g., Microcystis aeruginosa, Anabaena flos-aquae, Aphanizomenon flos-aquae and Cylindrospermopsis raciborskii). Cosmopolitan species (e.g., M. aeruginosa), pantropic (e.g., Anabaenopsis tanganyikae) and holarctic species (e.g., Anabaena flos-aquae) were encountered. Shallow, eutrophic waterbodies had blooms dominated by Microcystis species and were characterized by phytoplankton association M. Anabaena and Aphanizomenon species of association H were dominant in waterbodies with low dissolved inorganic nitrogen and thermal stratification in the summer. Total cyanobacterial biovolumes (CBV) ranged from 7 to 9,507 cm³ m⁻³ and were higher than Alert Level 2 and Guidance Level 2 (10 cm³ m⁻³; World Health Organization; WHO) in seven of the waterbodies. Chlorophyll a concentrations ranged from 6 to 90,000 mg m⁻³ and were higher than Alert Level 2 and Guidance Level 2 (50 mg m⁻³; WHO) in eight of the waterbodies. There is also an elevated risk of acute toxicosis (Guidance Level 3; WHO) in five waterbodies. Water of an undesirable quality, hazardous to humans and animals occurs in several Greek waterbodies.     

Impacts of two biomanipulation fishes stocked in a large pen on the plankton abundance and water quality during a period of phytoplankton seasonal succession

Silver and bighead carp were stocked in a large pen to control the nuisance cyanobacterial blooms in Meiliang Bay of Lake Taihu. Plankton abundance and water quality were investigated about once a week from 9 May to 7 July in 2005. Biomass of both total crustacean zooplankton and cladocerans was significantly suppressed by the predation of pen-cultured fishes. There was a significant negative correlation between the N:P weight ratio and phytoplankton biomass. The size-selective predation by the two carps had no effect on the biomass of green alga Ulothrix sp. It may be attributed to the low fish stocking density (less than 40 g m⁻³) before June. When Microcystis dominated in the water of fish pen, the pen-cultured carps effectively suppressed the biomass of Microcystis, as indicated by the significant decline of chlorophyll a in the >38 μm fractions of the fish pen. Based on the results of our experiment and previous other studies, we conclude that silver and bighead carp are two efficient biomanipulation tools to control cyanobacterial (Microcystis) blooms in the tropical/subtropical eutrophic waters. Moreover, we should maintain an enough stocking density for an effective control of phytoplankton biomass.     

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day⁻¹, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K _s (0.12 μg P l⁻¹ 10⁻⁶ cells) and lower V _max (9.63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), than Anabaena (K _s 0.02 μg P l⁻¹ h⁻¹ 10⁻⁶ cells; V _max 46.25 63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin. Received: July 31, 2000 / Accepted: October 18, 2000     

Response of bacterial communities to cyanobacterial harmful algal blooms in Lake Taihu,China

Cyanobacterial harmful algal blooms are prevalent around the world, influencing aquatic organisms and altering the physico-chemical properties in freshwater systems. However, the response of bacterial communities to toxic cyanobacterial blooms and associated microcystins (MC) remain poorly understood even though global concentrations of MC have increased dramatically in the past few decades. To address this issue, the dynamics of bacterial community composition (BCC) in the water column and how BCC is influenced by both harmful cyanobacterial blooms and environmental factors were investigated on a monthly basis from August 2013 to July 2014 in Lake Taihu, China. Non-metric multidimensional scaling (NMDS) revealed that seasonal variation in BCC was significant, and that the succession of BCC greatly depends on changes in environmental conditions. Redundancy analysis (RDA) results showed that the overall variation of BCC was explained mainly by dissolved oxygen (DO), nitrate nitrogen (NO₃⁻-N), and Microcystis. The alpha biodiversity of the bacterial community was different among months with the highest diversity in February and the lowest diversity in October. Furthermore, significant negative relationships were found between alpha biodiversity indices and Microcystis abundance as well as with intracellular MC concentrations, indicating that Microcystis and associated MC may influence the bacterial community structure by reducing its biodiversity. This study shows that potential associations exist between toxic cyanobacterial blooms and bacterial communities but more investigations are needed to obtain a mechanistic understanding of their complex relationships.     

Mesozooplankton and microzooplankton grazing during cyanobacterial blooms in the western basin of Lake Erie

Lake Erie is the most socioeconomically important and productive of the Laurentian (North American) Great Lakes. Since the mid-1990s cyanobacterial blooms dominated primarily by Microcystis have emerged to become annual, late summer events in the western basin of Lake Erie yet the effects of these blooms on food web dynamics and zooplankton grazing are unclear. From 2005 to 2007, grazing rates of cultured (Daphnia pulex) and natural assemblages of mesozooplankton and microzooplankton on five autotrophic populations were quantified during cyanobacterial blooms in western Lake Erie. While all groups of zooplankton grazed on all prey groups investigated, the grazing rates of natural and cultured mesozooplankton were inversely correlated with abundances of potentially toxic cyanobacteria (Microcystis, Anabaena, and Cylindrospermopsis; p < 0.05) while those of the in situ microzooplankton community were not. Microzooplankton grazed more rapidly and consistently on all groups of phytoplankton, including cyanobacteria, compared to both groups of mesozooplankton. Cyanobacteria displayed more rapid intrinsic cellular growth rates than other phytoplankton groups under enhanced nutrient concentrations suggesting that future nutrient loading to Lake Erie could exacerbate cyanobacterial blooms. In sum, while grazing rates of mesozooplankton are slowed by cyanobacterial blooms in the western basin of Lake Erie, microzooplankton are likely to play an important role in the top-down control of these blooms; this control could be weakened by any future increases in nutrient loads to Lake Erie.     

Phosphorus strategy in bloom-forming cyanobacteria (Dolichospermum and Microcystis) and its role in their succession

Dolichospermum (formerly Anabaena) and Microcystis cause harmful cyanobacterial blooms in freshwater ecosystems worldwide. Input reduction of both nitrogen (N) and phosphorus (P) are commonly recognized as basic ways of controlling blooms, but little is known about the roles of nutrients and their using strategy among cyanobacteria in triggering the succession of diazotrophic to non-diazotrophic cyanobacteria. In this study, we investigated in situ responses of cyanobactria to ambient P status during the transition from Dolichospermum flos-aquae to Microcystis spp. in Lake Taihu and Lake Chaohu. While dominant in phytoplankton community, D. flos-aquae experienced P deficiency as evidenced by qualitative detection of extracellular phosphatase via enzyme labeled fluorescence (ELF). The percentage of ELF-labelled D. flos-aquae cells was 33% when it dominated the phytoplankton community, and was 78% when it co-dominated with Microcystis spp., indicating an increase in P deficiency. Meanwhile, no ELF-labelled Microcystis cells were observed while polyphosphate body (PPB) were present, suggesting that Microcystis spp. were not P deficient. Additionally, the percentages of Microcystis cells containing PPB showed an inverted “U-shaped” relationship with concentrations on soluble reactive phosphorus (SRP). To validate the field observation, a laboratory study of the monocultures of the dominant cyanobacteria was conducted. Extracellular alkaline phosphatase activity (APA) and PPB accumulation were regulated by P availability in monocultures of D. flos-aquae. Interestingly, no cell bound extracellular phosphatase was found on Microcystis aeruginasa even in the culture without P supply. Consistently, the expressions of phosphatase encoding gene phoX showed no differences among the treatments. The way in which PPB accumulation occurred in Microcystis spp. in response to P availability in the cultures was similar to that observed in the field, demonstrating a strategy of energy conservation over P accumulation. The competitive advantage of Microcystis spp. was displayed at low P concentrations: where it could rapidly uptake and store inorganic P, which also increased the P deficiency of the coexisting phytoplankton species. Responses of P-transport gene pstS confirmed this hypothesis. The physiological and molecular mechanisms mentioned above enable Microcystis to survive and proliferate in environment with low available P supply more efficiently. In conclusion, different cyanobacterial species have distinct ways of responding to P availability, suggesting that the control of cyanobacterial blooms by targeted nutrient reduction is largely dependent upon the dominant species. P reduction is more effective in controlling diazotrophic cyanobacteria than non-diazotrophic cyanobacteria.     

The fate of cyanobacterial detritus in the food web of Lake Taihu: a mesocosm study using 13C and 15N labeling

The ecosystem of the highly eutrophic Lake Taihu (China) is seriously affected by recurrent cyanobacterial blooms, but little is known about the contribution made by cyanobacteria to the food web. In this study, we investigated the fate of detritus of the cyanobacterium Microcystis in the food web of Lake Taihu through a 19-day mesocosm experiment using stable-isotopic tracers of carbon (¹³C) and nitrogen (¹⁵N). ¹³C- and ¹⁵N-labeled Microcystis detritus was added to the mesocosm tanks and tracked through different elements of the food web. We found clear enrichment with both ¹³C and ¹⁵N in some zooplankton species, including Daphnia, Diaphanosoma, and Sinocalanus, which suggests that these zooplankters can utilize cyanobacterial detritus as a food source. Benthic animals, chironomid larvae and Limnodrilus, also showed pronounced increases in ¹³C and ¹⁵N, but the isotope increase was relatively smaller in the gastropods, Radix sp. and Bellamya sp., implying that they either exploited this food source differently or responded slower than the zooplankton, which apparently grew faster than the snails. Our study suggests that cyanobacterial detritus, originating almost wholly from the bloom-forming Microcystis, is an important food source for both planktonic and benthic food webs in eutrophic lakes such as Lake Taihu.     

Seasonal succession of phytoplankton in Lake Yogo over 2 years: effect of artificial manipulation

We studied the seasonal variation in concentrations of nutrients and phytoplankton in Lake Yogo for 2 years, from May 2000 to May 2002, in order to clarify the seasonal succession of phytoplankton and the effect of various manipulations on it. It was revealed that in spite of the installation of aeration systems and the pumping of mesotrophic water from Lake Biwa during the summer season, the trophic state of Lake Yogo overall has not improved during the past few decades. However, the pumping of water from Lake Biwa did affect the concentrations of nutrients and the periods of cyanobacterial bloom during the summer. The pumping period was different in each year, and the cyanobacterial bloom occurred during the period without pumping in both years. The aeration destratification was not strong enough to prevent cyanobacterial blooms. Cyanobacteria and Bacillariophyceae contributed most to the phytoplankton biomass in both years. Aphanizomenon, Anabaena, and Microcystis were the main genera among cyanobacteria. The bloom of Aphanizomenon or Anabaena occurred early in the summer, and was then replaced by Microcystis. Aphanizomenon was almost always present, and often formed bloom even in winter. The seasonal succession of Bacillariophyceae was almost the same in both years and was well categorized: winter-growing species such as Aulacoseira pusilla (F. Meister) Tuji et Houki and species of Thalassiosiraceae, spring-growing species such as Asterionella formosa Hassall, Fragilaria crotonensis Kitton, and Synedra cf. acus, and fall-growing species such as Aulacoseira ambigua (Grunow) Simonsen, and Aulacoseira granulata (Ehrenb.) Simonsen.     

Distribution of Hepatotoxic Cyanobacterial Blooms in Belgium and Luxembourg

A survey of the distribution of cyanobacterial blooms in the southern part of Belgium, in Luxembourg as well as in bordering northeastern France was carried out for 4 years (1997, 1999–2001). In the 64 cyanobacterial bloom samples collected, Microcystis as well as Planktothrix were the most frequently encountered dominant bloom formers, followed by Anabaena, Woronichinia, and Aphanizomenon. The relative frequency of (co-)dominant genera was highly correlated to the geology of the catchments. Microcystins were found in 53% of the analysed blooms and their presence was mainly assigned to Microcystis dominance. The highest microcystin concentration of 2231 μg g⁻¹ seston DW was recorded in a sample dominated by Woronichinia naegeliana. Among the 6 investigated microcystin variants, MC-LR was the most frequently detected whereas MC-LY was never revealed.