In late May, 2007, a drinking water crisis took place in Wuxi, Jiangsu Province, China, following a massive bloom of the toxin producing cyanobacteria Microcystis spp. in Lake Taihu, China’s third largest freshwater lake. Taihu was the city’s sole water supply, leaving approximately two million people without drinking water for at least a week. This cyanobacterial bloom event began two months earlier than previously documented for Microcystis blooms in Taihu. This was attributed to an unusually warm spring. The prevailing wind direction during this period caused the bloom to accumulate at the shoreline near the intake of the water plant. Water was diverted from the nearby Yangtze River in an effort to flush the lake of the bloom. However, this management action was counterproductive, because it produced a current which transported the bloom into the intake, exacerbating the drinking water contamination problem. The severity of this microcystin toxin containing bloom and the ensuing drinking water crisis were attributable to excessive nutrient enrichment; however, a multi-annual warming trend extended the bloom period and amplified its severity, and this was made worse by unanticipated negative impacts of water management. Long-term management must therefore consider both the human and climatic factors controlling these blooms and their impacts on water supply in this and other large lakes threatened by accelerating eutrophication.

A Drinking Water Crisis in Lake Taihu, China: Linkage to Climatic Variability and Lake Management

https://www.waterra.com.au/publications/document-search/?download=718

Effects of rainfall patterns on toxic cyanobacterial blooms in a changing climate: Between simplistic scenarios and complex dynamics

Cyanobacterial toxins: Modes of actions, fate in aquatic and soil ecosystems, phytotoxicity and bioaccumulation in agricultural crops

The persistence of cyanobacterial hepatotoxin, microcystin-LR, was investigated in Lake Tuusulanjarvi in southern Finland from August to October, 1993 and 1994. The amount of toxin in particulate material and dissolved in water were determined by HPLC from samples collected from mesocosm enclosures and from the surrounding lake water. In the beginning of the experiments over 80% of the phytoplankton biomass consisted of cyanobacteria. The main species were Microcystis wesenbergii (Chroococcales, Cyanobacteria), M. viridis and M. aeruginosa. The microcystin-LR concentration in particulate material varied from 2.7 to 3.2 μg l−1 and the corresponding concentration of microcystin LR dissolved in water from 0.06 to 0.21 μg l−1. The cyanobacterial biomass decreased towards the middle of September and simultaneously the microcystin concentration in freeze dried particulate material decreased below the detection limit of 10 μg g−1, corresponding 0.01 μg l−1. Dissolved microcystin-LR was detected in a concentration range of 1 to 5 ng l−1 even at the end of the experiments in October, when the cyanobacterial biomass was less than 1 mg l−1. Thus, dissolved microcystin was more persistent compared to microcystin in particulate material: the decimal reduction time for dissolved toxin was 30 d and for toxin in particulate material about 15 d.

Persistence of cyanobacterial hepatotoxin, microcystin-LR in particulate material and dissolved in lake water

Microcystis is a cosmopolitan genus of cyanobacteria and occurs in many different forms. Large surface blooms of the cyanobacterium are well known in eutrophic lakes throughout the globe. We evaluated the role of microcystins (MCs) in promoting and maintaining bloom-forming cell aggregates at environmentally relevant MC concentrations (0.25-10 mu g l(-1)). MCs significantly enhanced Microcystis colony sizes. Colonial diameters in microcystin-RR (MC-RR)-treated cultures (at 1 mu g l(-1)) were significantly larger than control colonies, by factors of 1.5, 2.6 and 2.7 in Microcystis wesenbergii DC-M1, M. ichthyoblabe TH-M1 and Microcystis sp. FACHB1027 respectively. Depletion of extracellular MC concentrations caused Microcystis colony size to decrease, suggesting that released MCs are intimately involved in the maintenance of Microcystis colonial size. MC-RR exposure did not influence Microcystis growth rate, but did significantly increase the production of extracellular polysaccharides (EPS). In addition, MC-RR exposure appeared to trigger upregulation of certain parts of four polysaccharide biosynthesis-related genes: capD, csaB, tagH and epsL. These results strongly indicate that induction of polysaccharides by MC-RR was the major mechanism through which MCs enhanced colony formation in Microcystis spp. Cellular release of MCs, therefore, may play a key role in the persistence of algal colonies and the dominance of Microcystis.

The role of microcystins in maintaining colonies of bloom-forming Microcystis spp.

Distribution and bioaccumulation of microcystins in water columns: a systematic investigation into the environmental fate and the risks associated with microcystins in Meiliang Bay, Lake Taihu

Reduction in microcystin concentrations in large and shallow lakes: water and sediment-interface contributions.

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 x 107 cells mL-1 to 3.23 x 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 mu g microcystins mg1 dw1) 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.

Xiaoming Zhang

Papers

Distribution and bioaccumulation of microcystins in water columns: a systematic investigation into the environmental fate and the risks associated with microcystins in Meiliang Bay, Lake Taihu

Reduction in microcystin concentrations in large and shallow lakes: water and sediment-interface contributions.

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

The freshwater algae culture collection at the Institute of Hydrobiology (FACHB): algal resources for fundamental and applied research