Environmental factors controlling colony formation in blooms of the cyanobacteria Microcystis spp. in Lake Taihu,China

Nitrogen (N) and phosphorus (P) over-enrichment has accelerated eutrophication and promoted cyanobacterial blooms worldwide. The colonial bloom-forming cyanobacterial genus Microcystis is covered by sheaths which can protect cells from zooplankton grazing, viral or bacterial attack and other potential negative environmental factors. This provides a competitive advantage over other phytoplankton species. However, the mechanism of Microcystis colony formation is not clear. Here we report the influence of N, P and pH on Microcystis growth and colony formation in field simulation experiments in Lake Taihu (China). N addition to lake water maintained Microcystis colony size, promoted growth of total phytoplankton, and increased Microcystis proportion as part of total phytoplankton biomass. Increases in P did not promote growth but led to smaller colonies, and had no significant impact on the proportion of Microcystis in the community. N and P addition together promoted phytoplankton growth much more than only adding N. TN and TP concentrations lower than about TN 7.75–13.95 mg L⁻¹ and TP 0.41–0.74 mg L⁻¹ mainly promoted the growth of large Microcystis colonies, but higher concentrations than this promoted the formation of single cells. There was a strong inverse relationship between pH and colony size in the N&P treatments suggesting CO₂ limitation may have induced colonies to become smaller. It appears that Microcystis colony formation is an adaptation to provide the organisms adverse conditions such as nutrient deficiencies or CO₂ limitation induced by increased pH level associated with rapidly proliferating blooms.     

Controlling Cyanobacterial Blooms in Hypertrophic Lake Taihu,China: Will Nitrogen Reductions Cause Replacement of Non-N2 Fixing by N2 Fixing Taxa?

Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China’s third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N₂ fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N₂ fixing with N₂ fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N₂ fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N₂ fixing with N₂ fixing cyanobacteria, and N₂ fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N₂ fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N₂ fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.     

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.     

Dominant genera of cyanobacteria in Lake Taihu and their relationships with environmental factors

Cyanobacterial blooms in freshwaters have become one of the most widespread of environmental problems and threaten water resources worldwide. Previous studies on cyanobacteria in Lake Taihu often collected samples from one site (like Meiliang Bay or Zhushan Bay) and focused on the variation in patterns or abundance of Microcystis during the blooming season. However, the distribution of cyanobacteria in Lake Taihu shows differing pattern in various seasons. In this study, water samples were collected monthly for one year at five sites in Lake Taihu with different trophic status and a physicochemical analysis and denaturing gradient gel electrophoresis (DGGE) were conducted. DGGE fingerprint analysis showed that Microcystis (7/35 bands) and Synechococcus (12/35 bands) were the two most dominant genera present during the study period at all five sites. Cyanobium (3/35 bands) was the third most common genus which has seldom been previously reported in Lake Taihu. Redundancy analysis (RDA) indicated that the cyanobacterial community structure was significantly correlated with NO₃^--N, COD_Mn, and NH₄⁺-N in the winter and spring, whereas it was correlated with water temperature in the summer and autumn. Limiting the nutrient input (especially of N and C loading) in Lake Taihu would be a key factor in controlling the growth of different genera of cyanobacteria.     

Urea dynamics during Lake Taihu cyanobacterial blooms in China

Lake Taihu, the third largest freshwater lake in China, suffers from harmful cyanobacteria blooms caused by Microcystis spp., which do not fix nitrogen (N). Reduced N (i.e., NH₄⁺, urea and other labile organic N compounds) is an important factor affecting the growth of Microcystis. As the world use of urea as fertilizer has escalated during the past decades, an understanding of how urea cycling relates to blooms of Microcystis is critical to predicting, controlling and alleviating the problem. In this study, the cycling rates of urea-N in Lake Taihu ranged from non-detectable to 1.37 μmol N L⁻¹ h⁻¹ for regeneration, and from 0.042 μmol N L⁻¹ h⁻¹ to 2.27 μmol N L⁻¹ h⁻¹ for potential urea-N removal. The fate of urea-N differed between light and dark incubations. Increased ¹⁵NH₄⁺ accumulated and higher quantities of the removed urea-¹⁵N remained in the ¹⁵NH₄⁺ form were detected in the dark than in the light. A follow-up incubation experiment with ¹⁵N-urea confirmed that Microcystis can grow on urea but its effects on urea dynamics were minor, indicating that Microcystis was not the major factor causing the observed fates of urea under different light conditions in Lake Taihu. Bacterial community composition and predicted functional gene data suggested that heterotrophic bacteria metabolized urea, even though Microcystis spp. was the dominant bloom organism.     

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.     

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.     

Microcystis Genotype Succession and Related Environmental Factors in Lake Taihu during Cyanobacterial Blooms

From spring to autumn, heavy Microcystis blooms always occur in Lake Taihu, although environmental conditions vary markedly. We speculated that Microcystis genotype succession could play an important role in adaptation to environmental changes and long-term maintenance of the high Microcystis biomass. In this study, we investigated Microcystis genotype succession pattern and the related environmental variables in Lake Taihu during cyanobacterial blooms. Denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction -amplified the genus-specific cpcBA and mcyJ gene fragments was used to monitor the variations of Microcystis genotype and potential microcystin (MC)-producing Microcystis genotype compositions during blooms biweekly in three sites (Meiliang Bay, lake center, and Gonghu Bay) and CANOCO 4.5 for Windows were used for the multivariate statistical analysis of their relationships to environmental variables. DGGE patterns indicated that the number of dominant cpcBA genotype per sample increased from spring to autumn. Principal component analysis ordination plots of DGGE profiles showed clear temporal distribution pattern, but not spatial distribution pattern based on both cpcBA and mcyJ genotype compositions. These results indicated there were relatively gradual successions of Microcystis cpcBA and mcyJ genotype compositions in each site, and no distinct spatial difference among the three sites. Redundancy analyses of the gel patterns showed that, in all the three sites, three environmental factors (nitrate, pH, and chemical oxygen demand) were correlated significantly to successions of both cpcBA and mcyJ genotypes except for mcyJ genotype in the lake center. Spearman’s correlations indicated that the three environmental variables were also strongly correlated with chl a and MC concentrations. These results suggested that the environmental factors affecting succession of Microcystis community composition might also influence the growth of Microcystis and MC production.     

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.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

太湖水华期间有毒和无毒微囊藻种群丰度的动态变化

采用荧光定量PCR技术分析太湖3个湖区(梅梁湾、贡湖湾和湖心)水体中有毒和无毒微囊藻基因型丰度及有毒微囊藻比例的季节变化(2010年4-9月),并与环境因子进行统计分析。结果表明,有毒微囊藻基因型丰度及所占比例存在季节和空间差异:从4-8月,有毒微囊藻基因型丰度及其比例呈逐渐增加趋势,到9月开始下降;梅梁湾水体中有毒微囊藻基因型丰度及其比例高于贡湖湾和湖心。梅梁湾、贡湖湾和湖心有毒微囊藻在微囊藻种群中的比例变化范围分别为(26.2±0.8)%-(64.3±2.2)%、(4.4±0.2)%-(22.1±1.8)%和(10.4±0.4)%-(20.6±1.5)%。相关分析结果表明,有毒微囊藻丰度、总微囊藻丰度和叶绿素a浓度呈极显著正相关(P<0.01),均与温度呈显著正相关(P<0.05);有毒微囊藻比例与磷浓度呈显著正相关(P<0.05),与温度呈极显著正相关(P<0.01)。研究结果表明,温度和磷浓度是决定太湖有毒微囊藻种群丰度及其比例的关键因子。     

Genetic analysis on Dolichospermum (Cyanobacteria; sensu Anabaena) populations based on the culture-independent clone libraries revealed the dominant genotypes existing in Lake Taihu,China

Lake Taihu has been severely eutrophied during the last few decades and dense cyanobacterial blooms have led to a decrease in phytoplankton diversity. The cyanobacterial blooms in Lake Taihu were mainly composed of unicellular colony-forming Microcystis and filamentous heterocystous Dolichospermum (formerly known as planktonic species of Anabaena). In contrast to that of Microcystis spp., the fundamental knowledge about diversity, abundance and dynamics of Dolichospermum populations in Lake Taihu is lacking. The present study was conducted to understand genotypic distribution, dynamics and succession of Dolichospermum populations in Lake Taihu. By sequencing 688 internal transcribed spacer (ITS) regions between the 16S and 23S rRNA genes of Dolichospermum, we were able to confirm that all the sequences were Dolichospermum rather than Aphanizomenon. 118 different genotypes were identified from the obtained sequences, and two genotypes (W-type and L-type) were found to dominate in the lake, representing 36.6% and 26.2% of the total sequences, respectively. These two dominant genotypes of Dolichospermum displayed the significant seasonal pattern. Stepwise regressions analysis revealed that water temperature was associated with the two dominant genotypes. The combined results implied the possible existence of ecotypes in bloom-forming cyanobacteria, probably triggered by water temperature in the lake.     

The involvement of α-proteobacteria Phenylobacterium in maintaining the dominance of toxic Microcystis blooms in Lake Taihu,China

Lake Taihu in China has suffered serious harmful cyanobacterial blooms for decades. The algal blooms threaten the ecological sustainability, drinking water safety, and human health. Although the roles of abiotic factors (such as water temperature and nutrient loading) in promoting Microcystis blooms have been well studied, the importance of biotic factors (e.g. bacterial community) in promoting and meditating Microcystis blooms remains unclear. In this study, we investigated the ecological dynamics of bacterial community, the ratio of toxic Microcystis, as well as microcystin in Lake Taihu. High-throughput 16S rRNA sequencing and principal component analysis (PCA) revealed that the bacteria community compositions (BCCs) clustered into three groups, the partitioning of which corresponded to that of groups according to the toxic profiles (the ratio of toxic Microcystis to total Microcystis, and the microcystin concentrations) of the samples. Further Spearman's correlation network showed that the α-proteobacteria Phenylobacterium strongly positively correlated with the toxic profiles. Subsequent laboratory chemostats experiments demonstrated that three Phenylobacterium strains promoted the dominance of the toxic Microcystis aeruginosa PCC7806 when co-culturing with the non-toxic PCC7806 mcyB⁻ mutant. Taken together, our data suggested that the α-proteobacteria Phenylobacterium may play a vital role in the maintenance of toxic Microcystis dominance in Lake Taihu.     

Estimating spatial variation in the abundance of potential microcystin-producing Microcystis spp. using real-time PCR during summer bloom in Lake Taihu

Lake Taihu, which is the third largest freshwater lakes 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, quantitative real-time PCR assays targeting on phycocyanin intergenic spacer (PC-IGS) and a microcystin synthetase gene mcyD were established, respectively. Water samples collected from eight sampling sites (including Zhushan Bay (N5), Meiliang Bay (N2), Gonghu Bay (N4), West lake areas (W2 and W4), south-middle lake areas (S2, S4 and S5)) in August of 2009 and 2010 were analyzed using real time PCR for the distribution and abundance of toxic and total Microcystis populations. The results showed that Microcystis exists as a mixed population of potential toxic and non-toxic genotypes, and there was significant spatial changes in the abundance of potential toxic Microcystis on the basis of quantification by quantitative real-time PCR analysis: the abundance of toxic Microcystis population in 2009 and 2010 varied from 4.08 × 10⁴ to 8.28 × 10⁶ copies mL^?1, from 4.45 × 10⁵ to 5.22 × 10⁷ copies mL^?1, respectively. Meanwhile the ratio of the mcyD subpopulation to the total Microcystis varied considerably, from 5.7% to 41.1% in 2009 and from 10.3% to 65.8% in 2010 in all sampling sites, and the value is high in Zhushan Bay and Meiliang Bay with the high level of eutrophication. Correlation analysis showed the abundance of toxic and total Microcystis being strongly related (P < 0.01). However, there is different effects of environmental factors on the abundance of toxic and non-toxic Microcystis populations. The abundance of toxic and total Microcystis populations were positively correlated with chlorophyll-a (Chl-a) concentration (P < 0.01) suggesting that Microcystis is dominated genera of cyanobacterial bloom in Lake Taihu. It was also found that the abundance of toxic Microcystis and the proportion of toxic subpopulation to the total Microcystis were positively correlated with total phosphorus and orthophosphate concentrations (P < 0.01), whereas there was no significant correlation with total nitrogen and nitrate concentration (P > 0.05). All data suggest that phosphorus concentration is a critical factor for determining the abundance of toxic Microcystis population.     

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.     

Nutrient addition effects on chlorophyll a,phytoplankton biomass,and heterocyte formation in Lake Erie’s central basin during 2014–2017: Insights into diazotrophic blooms in high nitrogen water

1. Phosphorus (P) usually is the primary limiting nutrient of phytoplankton biomass, but attention towards nitrogen (N) and trace nutrients, such as iron (Fe), has surfaced. Additionally, N-fixing cyanobacterial blooms have been documented to occur in N-rich, P-poor waters, which is counterintuitive from the paradigm that low N and high P promotes blooms. For example, Lake Erie's central basin has Dolichospermum blooms when nitrate concentrations are high, which raises questions about which nutrient(s) are selecting for Dolichospermum over other phytoplankton and why an N-fixer is present in high N waters?
2. We conducted a 4-year (2014–2017) study in Lake Erie's central basin to determine which nutrient (P, N, or trace nutrients such as Fe, molybdenum [Mo], and boron [B]) constrained chlorophyll concentration, phytoplankton biovolume, and nitrate assimilation using nutrient enrichment bioassays. The enriched lake water was incubated in 1-L bottles in a growth chamber programmed at light and temperatures of in situ conditions for 4–7 days. We also quantified heterocytes when N-fixing cyanobacteria were present.
3. Compared to the non-enriched control, the P-enriched (+P) treatment had significantly higher chlorophyll and phytoplankton biovolume in c. 75% of experiments. Combination enrichments of P with ammonium-N, nitrate-N, Fe, Mo, and B were compared to the +P treatment to determine secondary limitations. +P and ammonium-N and +P nitrate-N resulted in higher chlorophyll in 50% of experiments but higher phytoplankton biovolume in only 25% of experiments. These results show that P was the primary limiting nutrient, but there were times when N was secondarily limiting.
4. Chlorophyll concentration indicated N secondary limitation in half of the experiments, but biovolume indicated only N secondary limitation in 25% of the experiments. To make robust conclusions from nutrient enrichment bioassays, both chlorophyll and phytoplankton biovolume should be measured.
5. The secondary effects of Fe, Mo, and B on chlorophyll were low (<26% of experiments), and no secondary effects were observed on phytoplankton biovolume and nitrate assimilation. However, +P and Fe resulted in more chlorophyll than +P in experiments conducted during Dolichospermum blooms, and +P and B significantly increased the number of heterocytes in Dolichospermum. These results indicate that low Fe availability might select for Dolichospermum, and low B constrains heterocyte formation in the central basin of Lake Erie. Furthermore, these results could apply to other lakes with high N and low P where diazotrophic cyanobacterial blooms occur.

     

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.     

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.     

The Role of Nitrogen Fixation in Cyanobacterial Bloom Toxicity in a Temperate,Eutrophic Lake

Toxic cyanobacterial blooms threaten freshwaters worldwide but have proven difficult to predict because the mechanisms of bloom formation and toxin production are unknown, especially on weekly time scales. Water quality management continues to focus on aggregated metrics, such as chlorophyll and total nutrients, which may not be sufficient to explain complex community changes and functions such as toxin production. For example, nitrogen (N) speciation and cycling play an important role, on daily time scales, in shaping cyanobacterial communities because declining N has been shown to select for N fixers. In addition, subsequent N pulses from N₂ fixation may stimulate and sustain toxic cyanobacterial growth. Herein, we describe how rapid early summer declines in N followed by bursts of N fixation have shaped cyanobacterial communities in a eutrophic lake (Lake Mendota, Wisconsin, USA), possibly driving toxic Microcystis blooms throughout the growing season. On weekly time scales in 2010 and 2011, we monitored the cyanobacterial community in a eutrophic lake using the phycocyanin intergenic spacer (PC-IGS) region to determine population dynamics. In parallel, we measured microcystin concentrations, N₂ fixation rates, and potential environmental drivers that contribute to structuring the community. In both years, cyanobacterial community change was strongly correlated with dissolved inorganic nitrogen (DIN) concentrations, and Aphanizomenon and Microcystis alternated dominance throughout the pre-toxic, toxic, and post-toxic phases of the lake. Microcystin concentrations increased a few days after the first significant N₂ fixation rates were observed. Then, following large early summer N₂ fixation events, Microcystis increased and became most abundant. Maximum microcystin concentrations coincided with Microcystis dominance. In both years, DIN concentrations dropped again in late summer, and N₂ fixation rates and Aphanizomenon abundance increased before the lake mixed in the fall. Estimated N inputs from N₂ fixation were large enough to supplement, or even support, the toxic Microcystis blooms.     

越冬和复苏时期太湖水体蓝藻群落结构的时空变化

为研究太湖蓝藻在越冬与复苏时期群落结构的时空变化规律,于2008年11月,2009年2月,2009年4月,在太湖富营养化较严重的湖区选取8个采样点 (梅梁湾、竺山湾、贡湖湾、大浦、西太湖、南太湖、湖心和湖湾交汇处),分3层采水样,过滤并提取样品DNA经PCR扩增蓝藻16S rDNA序列,采用T-RFLP(末端标记的限制性酶切片段长度多样性)技术分析蓝藻群落结构和多样性变化。共得到87个不同的T-RFs(末端限制性酶切片段),表明太湖蓝藻具有丰富的基因多样性。T-RF相对丰度和聚类分析结果表明,太湖蓝藻群落结构在垂直空间上相似性较高,相似度 > 50%;在水平空间,与Microcystis spp.对应的信号峰在8个采样点均为最强峰(相对丰度为17.7% 47.5%)。竺山湾蓝藻多样性最低,西太湖最高,但其余采样点间蓝藻群落和Shannon多样性指数没有显著差异(P>0.05)。Microcystis相对丰度与Shannon多样性指数呈显著负相关(皮尔逊相关系数为-0.958)。在时间尺度上,相似性分析(Analysis of similarity,ANOSIM)结果显示太湖蓝藻群落结构存在极显著差异( P<0.01) 。春季复苏时蓝藻多样性最高,秋季衰亡时最低。聚类分析表明样品聚成两大特征类群,秋季衰亡时样品独自聚为1支,而春季复苏期和冬季越冬期样品彼此混杂。