Identification of major odor-causing compounds and algal causes in the Huangpu River

Offensive odor issues caused by harmful algae are a long-term challenge impacting freshwater and water supply systems worldwide, while the key odor-causing compounds and algae responsible for these issues are not fully understood. A multifactorial water quality dataset, including information on odor characteristics, algae species, natural organic matter composition, and basic water quality (∼20000 measurements), was generated in Huangpu River watershed over a year to identify the major odorants and algal sources for odor issues caused by harmful algae. By integrating analytical tools, three predominant odor profiles were systematically characterized: (i) earthy/musty odor associated with 2-methylisoborneol (n.d.-74.1 ng/L); (ii) swampy/septic odor mainly linked to thioethers (n.d.-355 ng/L), and (iii) fishy odor from heptanal and dimethyl sulfide. Correlation network showed Cyanophyta's distinct spatiotemporal distribution compared to Bacillariophyta, Euglenophyta, and Chlorophyta, and they are self-generated sources of newly produced organic matter. Data analysis by Structural Equation Modeling and Principal Component Analysis delineated the odorant-algae associations: (1) Oscillatoria sp. and Pseudanabaena sp. as potential 2-methylisoborneol producers; (2) Cryptomonas sp. positively correlated with dimethyl sulfide concentrations, which might also be contributed by biological methylation of SO₄^2-; (3) Phytoplankton belonging to Chlorophyta, Bacillariophyta, and Cryptophyta associated with heptanal synthesis. These algae were mainly from the input of Taihu Lake or the tributary at sampling point S13. The study first employs multiple data analytical tools to provide an operational paradigm for identifying the collective odor contributors and major odor-producing algae, which will benefit the management of algal-derived odor problems in drinking water.