Seasonal dynamics of microcystins with associated biotic and abiotic parameters in two bays of lake Taihu, the third largest freshwater lake in China.

The occurrence of heavy cyanobacterial blooms in eutrophic freshwater has been a worldwide problem (Carmichael 1992). Cyanobacteria are a nuisance in inland water bodies mainly because of bloom formation and toxin production (Chorus and Bartram 1999). Microcystins (MCs), which are secondary metabolites and are produced by some cyanobacteria (Carmichael 1992), are health hazard to livestock, wildlife and even humans (Carmichael 2001), because these cyclic heptapeptide are hepatotoxic by virtue of their accumulation in the liver via multispecific bile acid transporters (Eriksson et al. 1990) and inhibition of serine/threonine protein phosphatases 1 and 2A (Mackintosh et al. 1990). Because of public health concerns, many scientists have tried to elucidate the factors controlling the toxicity of cyanobacteria. External nutrient concentrations are suggested as an important factor for different toxicity of water blooms (Rapala and Sivonen 1998). There have been extensive laboratory studies to evaluate effects of environmental factors on MC production, such as the incidence of light on MC biosynthesis (Hesse and Kohl 2001), the response of MC-producer strains at different temperatures (Sivonen and Jones 1999; Marı́a et al. 2003), and the production of MC under different nutrient limitation, mainly P and N restriction (Oh et al. 2000; Hesse and Kohl 2001; Long et al. 2001; Vézie et al. 2002; Marı́a et al. 2003). However, some of these results are contradictory. It is likely that concentrations of phosphorus and/or nitrogen act indirectly via an influence on growth rates. It is reported that MC concentrations are correlated with total and dissolved phosphorus (Kotak et al. 1995; Lahti et al. 1997), total nitrogen (Lahti et al. 1997), and NO3-nitrogen (Vézie et al. 1997). On the other hand, negative correlations between MC concentration and NO3-nitrogen (Kotak et al. 1995; Lahti et al. 1997) have been also reported. The main purposes of this study were to describe seasonal dynamics of M. aeruginosa and to analyze the relationship between MC production and major biological and physical–chemical parameter in Lake Taihu. This study was conducted in two major bays with different nutrient levels. Such information is essential for us to develop a monitoring program of water quality in Lake Taihu.