Do the regular annual extreme water level changes affect the seasonal appearance of Anabaena in Poyang Lake?

Background: Poyang Lake is an exceptional system exhibiting a water level change of up to 14 m every year. In the years 2013 and 2014 water level changes were 8.03 m and 11.22m respectively. The biomass of Anabaena and heterocyst frequency increased in the summers of recent years. Methods: . A weekly or bi-weekly monitoring period from June to November 2013 and 2014 was set up to explain the reason for this phenomenon. Results: . Anabaena was observed throughout the year. The average relative biomass of Anabaena in the study period was over 40%, being most abundant in summer. The average heterocyst frequency was 0.226% in 2013 and 0.760% in 2014. The SPSS analysis indicated a positive correlation of Anabaena biomass with water temperature and water level and negative one with TN, which is the reason for the increase of heterocyst frequency from 2013 to 2014. It also indicated that heterocyst frequency of Anabaena was positively correlated with water temperature, water level and PO 4 -P, and negatively with DIN/DIP, NO 3 -N and TN. At the same time, water temperature and DIN/DIP were significantly correlated with water level, which means that water level changes have a direct effect on Anabaena and its heterocyst formation in Poyang Lake. Conclusions: . The results of the study support the hypothesis that increasing biomass and heterocyst formation of Anabaena were primarily caused by changes in the environmental parameters, such as high water temperature and suitable nutrients in summer and autumn, driven by the regular annual extreme water level change of Poyang Lake. The results can provide a theoretical basis for the management and utilization of Lake Poyang.