Eutrophication in Lake Dianchi was accelerated since 1980’s. From then on, cyanobacterial blooms occurred more frequently and heavily. Lake Dianchi was thus regarded as one of most polluted large shallow lakes all over the world. Microcystis aeruginosa is the most popular species dominating this lake. In the past years, some investigation showed that there was an obvious succession (Aphanizomenon flos-aquae to M. aeruginosa) in early spring. In present study, a sediment environments simulation device was designed and modified to trace the cell morphology, photosystem, antioxidant enzyme activities and reculture characters and variations of Microcystis aeruginosa in enclosures and Lake Dianchi. We found some differences between the field studies and simulated studies, and the results could be used to assess the advantages in species competition of M. aeruginosa during its overwintering and recruitment. The novel device showed a good maneuverability in simulation study as well as in field study. And we could expect an excellent performance in future studies.
M. aeruginosa showed acute responses when met sediment environments in the study, the antioxidant system and photosystem could be partly repaired. But as stress went on, cell membranes suffered more than beginning, and the antioxidant enzyme activities decreased finally. Fv/Fm remained a relative stable value after the recover of PS II until the end of the study. Cell morphology was greatly changed during the study, the samples were observed as small colonies and single cells in the end, sheath and gas vacuoles were hardly observed. The re-culture data showed the samples could grow well after the stress in sediments except for a pretty longer lag phase (9 days).
In situ research data indicated that the cell morphology had no significant differences among the samples set in different layers of sediments, and there were a great deal of colonies at the end of study. Colony size of Microcystis in fixation sediments was larger than control. We supposed that the lack of benthos and the anaerobic environment could be primary reasons for this. Vertical diversity in antioxidant activities was obviously observed, and which were coincident with microcystins concentration variations.
Simultaneously, we confirmed that the recruitment of Microcystis did have spatio-temporal diversity via experiments on re-culture. The samples collected at the beginning stage of overwintering showed nothing different with the fresh sample except for the 10-day-lag phase, while the samples taken from the late stage of the overwintering showed no log phase. Recruitment rate of M. aeruginosa in different sediments were quite different. Our data indicated that the sediments taken from the estuary of Fubao Bay were organic polluted heavily, and M. aeruginosa recruitment rate in these kind of sediments was quite lower than others. We hypothesized that the horizontal movements of M. aeruginosa populations which induced by wind-driven current contribute much more than original recruitment to the blooms, and the formation of blooms could be some kind of external loading.
Furthermore, we investigated the succession of in early spring, which was considered to be the most crucial period of M. aeruginosa. All the results from the investigation revealed that M. aeruginosa cells in south bay could recruit later than central and north bay. The biomass of M. aeruginosa moved from overlying water to surface water as the investigation time past, and this biomass shift coupled well with the A. flos-aquae and M. aeruginosa succession. The results implied that the succession could be partly induced by the spatio-temporal patterns of recruitment of M. aeruginosa in Lake Dianchi.