Water quality and transparency characteristics during the Planktothrix abundance period in shallow Lake Kasumigaura, Japan

摘要

Cyanobacterial blooms in lakes cause serious environmental problems on a globalrnscale. Planktothrix (a filamentous cyanobacterium) blooms occurred during winterrnand spring from 2007 to 2011 in Lake Kasumigaura, a eutrophic lake located in Japan.rnThis study analysed water quality during the period of Planktothrix abundance (2007–rn2010) at the centre of Lake Kasumigaura and compared it to that in the succeedingrnperiod, which did not have Planktothrix blooms (2012–2015). The average phytoplanktonrnbiovolume during the abundance period was larger than that in the succeedingrnperiod, which would contribute to the high chemical oxygen demandrn(CODMn; annual average of 9.7 mg/L) in the abundance period, compared to the succeedingrnperiod (7.6 mg/L). The total phosphorus concentration peaked in spring andrnsummer in the abundance period, but only in summer in the succeeding period,rnwhereas the seasonal variation in total nitrogen concentration between the two periodsrnwas relatively small. The annual average transparency (Secchi disc depth) increasedrnbefore the abundance period due to a decrease in fixed suspended solid,rnwhich is comprised mainly of inorganic matter. It is likely that the change in irradiancernconditions affected the occurrence of Planktothrix blooms.