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SUMMARY In eutrophicated water, cyanobacteria massively grow and release an abundance of volatile organic compounds (VOCs) that contribute to the water odor. To uncover the effects of different phosphorus (P) nutrients on the formation of cyanobacteria VOCs and water odor, the cell growth and VOC emissions of Microcystis aeruginosa were investigated under different P nutrient conditions. Among K2HPO4, Na4P2O7, and (NaPO3)6, K2HPO4 showed the largest increase in cell density, while a reduction in P concentration decreased the cell density. There were 26, 23 and 22 compounds in M. aeruginosa VOCs with K2HPO4, Na4P2O7 and (NaPO3)6 as the sole P source, respectively, including sulfocompounds, terpenoids, benzenes, hydrocarbons, alcohols, aldehydes, and esters. Non‐P markedly promoted the VOC emission, and six additional compounds were observed: α‐pinene, 1‐phenyl‐1‐butanone, 1H‐1‐ethylidene‐indene, 2,6,10‐trimethyl‐tetradecane, 2‐ethyl‐hexanal, and acetic acid 2‐ethylhexyl ester. It can be deduced that cyanobacteria release different VOC blends using various P forms in eutrophicated waters, and the reduction of P amount promotes VOC emission and increases the water odor.

Natural allelochemicals are considered as a source of algaecides. To uncover the anti-algal activity of Cinnamomum camphora fallen leaves and promote their usage as algaecides, the composition of their water and methanol extracts was analyzed, and the inhibitory effects of extracts on the growth of Microcystis aeruginosa and Chlamydomonas reinhardtii, and chlorophyll (Chl) content and photosynthetic abilities in C. reinhardtii were investigated. Twenty-five compounds were detected in the water extracts, mainly including terpenoids, esters, alcohols, and ketones. Compared to water extracts, there were more compounds and higher concentration in methanol extracts. Both water and methanol extracts inhibited the growth of the two algae, and 15 mg·ml methanol extracts killed the algal cells after 48 h. The levels of Chl a and Chl b, as well as maximum quantum yield of photosystem II photochemistry (Fv/Fm) in C. reinhardtii cells reduced gradually with increasing the concentration of extracts, while the maximum quantum yield of non-photochemical de-excitation (φD) increased gradually. At the same concentration, methanol extracts showed stronger inhibitory effects than water extracts, due to their higher number of compounds and higher concentration. Therefore, C. camphora fallen leaves have a potential value as an algaecide.