|Other Abstract||Toxic cyanobacterial bloom in freshwater lakes and ponds has been reported frequently due to the increasing eutrophication, and it became a worldwide serious environmental problem. Cyanotoxins, one of the disasters originated from bloom-forming cyanobacteria may cause the major health concerns to humans, as well as to the wildlife. Aphanizomenon flos-aquae, one of the dominant bloom-forming species of cyanobacteria in freshwaters, can synthesize toxins belonging to alkaloid called the paralytic shellfish poisons (PSPs), of them the LD50 of STX is only 10μg/kg to mouse. In recent years, Aphanizomenon blooms were frequently observed from freshwater bodies in China. However, there was little information on the distribution of PSPs toxins in freshwater lakes. In this paper, the spatio-temporal distribution of PSPs in Lake Dianchi and other lakes such as Aha, Hongfeng and Erhai, as well as the bloom-forming cyanobacteia community succession were studied. The main results are as follows:
1. The spatio-temporal distribution of PSPs from Aphanizomenon in Lake Dianchi was investigated for one year. The PSPs toxin in Dianchi began to appear in March, and then increased gradually to the highest level (20.65 ng/mgDW) in mid April. After a short term of decreasing, the content of PSP toxins keeping a steady level (15.29 ng/mg.DW) till May. And then PSPs content in cyanobacterial biomass drop quickly to the detecting limit in July. Moreover, toxin analogues varied greatly during the water bloom occurring process. In the early stage of Aphanizomenon bloom, Gonyautoxin-4 (GTX4) was detected first, and in the fast growing stage of Aphanizomenon bloom the two main analogues were saxitoxins (STX) and Gonyautoxin-4 (GTX4), then GTX4 was replaced by GTX1. GTX5 also appeared early, but the content was very low. And a few dcGTX2,3 was also detected during the blooming process. In general, the total content of STXs was much higher than GTXs in Lake Dianchi.
2. The bloom-froming cyanobacteria community succession and the physical and chemical parameters of water in Lake Dianchi were investigated from Dec.2006 to Nov.2007. Results showed that the dominant genera of cyanobacterial bloom were Microcystis and Aphanizomenon. The temperatures (15-19℃) for Aphanizomenon bloom occurring were lower than those (20-23℃) for Microcystis bloom. N/P ratios in Lake Diachi were not very high, with an average value of 15.2 for one year. For most months it was lower than 29, only raised in May and June and generally higher than 40. The ratio of N/P was low when Aphanizomenon bloom occurred, it domonstrated that Aphanizomenon was more competitive than Microcystis when TN was the limiting factor.
3. The toxicity of Aphanizomenon blooms from lakes Aha, Hongfeng and Erhai was also investigated. Standard mouse bioassay showed the Aphanizomenon blooms from lakes Aha and Hongfeng were toxic, while the toxicity of the Aphanizomenon bloom from Erhai is very low. The Oshima’ method for PSPs analysis by HPLC revealed Aphanizomenon blooms from these lakes produce PSPs toxins. The contents of PSPs in lakes Aha, Hongfeng and Erhai were 23.62 ng/mgDW, 4.10 ng/mgDW，2.63 ng/mg?DW respectively, with respective main toxins were STX, neoSTX and dcGTX3.
For reducing contamination and assessing the health risk caused by PSPs, the current study is of crucial importance, and further researches concerning the ecotoxicology of PSPs and control methods for Aphanizomenon bloom are necessary.|