Measurements of cell density and Chlorophyll a (Chl-a)

Cell density was determined by measuring the optical density at 680 nm (OD₆₈₀) with a visible spectrophotometer (752N; Shanghai Lengguang Industrial Co., Ltd., Shanghai, China) to represent relative cyanobacterial biomass [14]. Cell numbers were determined by a microscope using the hemocytometer counting method. The regression equation between OD₆₈₀ (Y) and the cell density (X, ×10⁶ cell/mL) was established as Y = 0.056X−0.005 (R² = 0.99). The inhibition ratio (IR) was used for expressing the effects of algaecide, which was calculated as follows:

T and C: cell density of treatment and control

The measurement of Chl-a followed the method described by Wang et al. [6]. Briefly, 3-mL samples were filtered through a 0.45-μm Millipore filter and then extracted with 3 mL of 90% acetone at 4°C for 24 h. The extract was then centrifuged and the Chl-a content was measured with a spectrophotometer at wavelengths of 750, 663, 645, and 630nm. Chl-a content was calculated as follows:

where A₇₅₀, A₆₆₃, A₆₄₅, and A₆₃₀ are the absorbance of the mixtures at 750, 663, 645, and 630nm, respectively; V1: volume of 90% acetone (mL), V: volume of water sample (L).