Effect of Indian Ocean Dipole signal on freshwater cyanobacterial dynamics

The proliferation of freshwater cyanobacteria is a serious environmental problem that often reduces water quality. In this study, we observed the possible influence of the Indian Ocean Dipole (IOD), represented by the Dipole Mode Index (DMI), on the dynamics of cyanobacterial blooms in a river system (the lower Nakdong River). Korean monsoon rainfall (KMR) was preceded by 5-month moving average DMI of ~1.5 years, and the KMR and DMI were negatively associated ( r 2 = 0.53, n = 15, p < 0.01). In turn, a decrease in KMR was responsible for a decrease in total dam discharge ( r 2 = 0.64, n = 15, p < 0.01), resulting in an abrupt increase in cyanobacterial cell density at the study site ( r 2 = n = 15, p < 0.01). We found a strong and significant positive correlation between cyanobacterial density of the Nakdong River and a 5-month moving average DMI ( r 2 = 0.95, n = 15, p < 0.01). These correlations suggest that a positive DMI decreased KMR and therefore water discharge, resulting in an excessive proliferation of the cyanobacterial density during the monsoon period. Despite a variety of uncertainties, the presence of the IOD is believed to play a role in triggering freshwater cyanobacterial proliferation through a signal propagation pathway during summer drought occurrence in the monsoon period.