Author(s): Takashi Asaeda; Harun Rashid; Lekkala Vamsi-Krishna

Linked Author(s): Takashi Asaeda

Keywords: Submerged macrophytes; Environmental stress; Habitat preferences; Ecophysiology

Abstract: In recent years, invasive submerged macrophytes have overwhelmingly colonized some midstream reaches of Japanese rivers. This study was designed to determine how submerged macrophytes have been able to colonize these areas, to assess the environmental conditions that limit or even prevent colonization, and to develop a prediction model. Under experimental conditions, invasive species (Egeria densa and Elodea nuttallii), and Japanese native species (Myriophyllum spicatum, Ceratophyllum demersum, and Potamogeton crispus) were kept in experimental tanks and a flume with different environmental conditions. In addition, plants were sampled in rivers across Japan, and environmental conditions were measured. Tissue hydrogen peroxide (H₂O₂) concentrations of sampled plant samples were measured responding to either individual or multiple environmental factors of light intensity, water temperature, and water flow velocity/turbulence. Then, the followings were resulted: 1) The H₂O₂ concentration increased in parallel to the increment of unpreferable levels of each abiotic factor, and the trend was independent of other factors. 2) The total H₂O₂ concentration is provided by the sum of contribution of each stress. 3) Under increased total H₂O₂ concentration, plants first started to decrease in chlorophyll concentration, then reduce their growth rate, and subsequently reduce their biomass. 4) The H₂O₂ concentration threshold, beyond which degradation is initiated, was between 15 and 20 μmol/gFW regardless of the environmental factors. These results highlight the potential efficacy of total H₂O₂ concentration as a proxy for the overall environmental condition. In Japanese rivers, major environmental factors limiting macrophyte colonization were identified as water temperature, high solar radiation, and flow velocity/turbulence. The relationship between the unpreferable levels of these factors and H₂O₂ concentration was empirically obtained for these species. Then a mathematical model was developed to predict the colonization area of these species as a function of environmental conditions. The tissue H₂O₂ concentration decreases with increasing temperature for E. densa and increases for other species, including native species. Therefore, native species grow intensively in spring; however, they often deteriorate in summer. For E. densa, on the other hand, H₂O₂ concentration decreases with high water temperature in summer, allowing intensive growth. High solar radiation increases the H₂O₂ concentration, deteriorating the plant in surface. Although the H₂O₂ concentration of E. densa increases with low water temperature in winter, it can survive in deep water with low H₂O₂ concentration due to low solar radiation. Currently, river rehabilitation has created a deep zone in the channel, which supports the growth and spreading of invasive E. densa.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022719

Year: 2022