Outbreak of 2-methylisoborneol in a drinking water reservoir attributed to the cyanobacterium Pseudanabaena sp.

Taste and odor compounds(T&Os), which is often associated with plankton in water sources,pose significant challenges to water quality in the world. Identifying the potential producers and understanding the driving factors are critical for effective treatment. While most studies focused on outbreak of T&Os linked with summer cyanobacterial blooms, questions remain about the mechanisms and environmental factors driving spring outbreaks when cyanobacteria are typically in low abundance.Therefore, changes in the concentrations of T&Os, i.e., 2-methylisoborneol(2-MIB) and three others, in relation to the phytoplankton community, the physico-chemical and hydrological conditions in a mesotrophic reservoir surrounded by hills in East China were investigated from March to June 2023.Results show that 2-MIB was the main T&O during the spring outbreak of phytoplankton, and peaked at 96.10 ng/L, which is nearly 10 times of its odor threshold concentration(OTC) of 10 ng/L. The filamentous cyanobacterium Pseudanabaena sp. was identified as the primary producer of 2-MIB, and the cell concentration of 4 000 cells/mL of Pseudanabaena sp. could potentially result in 10 ng/L of 2-MIB in this reservoir. The abundance of Pseudanabaena sp. and 2-MIB were correlated significantly with turbidity,suggesting that a lower light environment might be critical to promote the dominance of Pseudanabaena sp.and the subsequent production of 2-MIB. Moreover, the combination of consistently decreased water level,moderate water stratification, optimal water temperature ranging ～14–23 °C, the increase of the chemical oxygen demand using permanganate as an oxidant(CODMn) and total nitrogen(TN) concentration were likely to drive the accumulation and outbreak of 2-MIB. The hydrodynamic disturbance(i.e., a sudden rapid flow of water) triggered the abrupt disappearance of both Pseudanabaena sp. and 2-MIB. This study suggests that proliferation of Pseudanabaena sp. in spring is important in contributing to 2-MIB outbreak,and flushing may be an effective approach to mitigate T&O issues in water sources.