Quantitative study on the survivability of Microcystis colonies in lake sediments

Microcystis colonies can overwinter in sediment and may inoculate the water column through recruitment, promoting the annual recurrence of Microcystis blooms. Through microcosm experiments, the survivability of benthic Microcystis was quantitatively investigated by elucidating the dynamics of its cell numbers, photosynthetic activity, and toxin content under various sediment conditions. The abundance of Microcystis colonies in the sediment samples did not decrease significantly over the 22-week experiment at 5 A degrees C, whereas more than 90% of the benthic population died within 3 months at 25 A degrees C. The survivability of the Microcystis colonies under the aerobic conditions of surface sediments was significantly lower than under the anaerobic conditions of deep sediments. Toxic M. aeruginosa and non-toxic M. wesenbergii exhibited similar survivability, although the final survival efficiency of M. aeruginosa was higher than that of M. wesenbergii at 15 and 25 A degrees C. Microcystin quotas remained stable at 5 and 15 A degrees C but decreased significantly at 25 A degrees C. The photosynthetic activity of the benthic Microcystis colonies decreased gradually, with a larger decline rate at higher temperatures. Our study suggests that most benthic Microcystis colonies can overwinter successfully, with microcystins preserved and photosynthetic activity maintained, significantly increasing the risk of bloom formation. However, they were unable to accumulate in the sediments of subtropical shallow lakes over a period of years.