Enhancement of water quality and odor substance (Azulene) release risks in ecological restoration using Vallisneria natans

Vallisneria natans (V. natans), a widely utilized submerged plant for lake ecological restoration, improves water quality but can release odorants. However, research on the characteristics of odorant release from V. natans and its mechanisms of impact on aquatic environments remains insufficient. This study investigated an ecological pond planted with V. natans in a lakeside wetland restoration project in Wuhan. Changes in physical and chemical indicators of total nitrogen (TN), total phosphorus (TP), and chlorophyll a (Chla) in the ecological pond water during the growth process of V. natans were monitored monthly (from April to December, 2023) to analyze the characteristics of the primary odorant, Azulene, released by V. natans. The influence of environmental factors on the release of odor substances was explored by using the structural equation model (SEM). The results showed that after the cultivation of V. natans, the water quality of the ecological pond was significantly improved. The concentrations of TP, TN and Chla decreased by 84.5%, 31.4% and 55.6%, respectively; the water transparency (SD) increased by 315%, and the dissolved oxygen (DO) concentration increased by 21.1%. Concurrently, V. natans released a range of volatile odorants during its growth and metabolism, with Azulene being the main component. The peak concentration of Azulene in the water column reached (293.65±22.51) ng/L in September. With the decrease of water temperature, the concentration of Azulene decreased significantly. Compared with September, it dropped by 78.14% in December, reaching a low of (64.19±15.23) ng/L. Correlation analysis and SEM revealed that water temperature exerted a highly significant positive effect on V. natans coverage (path coefficient=0.97, P<0.001), and increased coverage directly elevated the Azulene concentrations in both the leaf and water (path coefficients=0.95 and 0.96, respectively; P<0.001). Azulene concentrations in the water were significantly negatively correlated with TN, TP, and Chla (P<0.05), and positively correlated with SD, pH, and V. natans coverage (P<0.05). This study indicates that while the cultivation of V. natans significantly improves water quality, it also leads to sensory effects arising from the release of odorant compounds, especially the release of Azulene, which is closely related to the growth of V. natans and water temperature. In the practice of water ecological restoration projects, it is important to balance the dual effects of water quality improvement and the release of odorous compounds, giving consideration to their sensory impacts while also conducting in-depth research into their mechanisms of influence and ecological functions within water bodies and ecosystems.