Submerged macrophytes constitute an important natural component in shallow aquatic ecosystems. The submersed aquatic plants are characterized by leaf morphology with a high area-to-volume ratio and a thin cuticula, which facilitates a rapid exchange of gases between the plants and bulk water. The micro-boundary layer around the submerged macrophyte surface plays a significant ecological role in plant growth and nutrient transformation in the aquatic environment. Potamogeton crispus is one of the dominant species in eutrophic shallow lakes in China, and oxygen (O₂) is the key parameter shaping the oxidation-reduction heterogeneous microenvironment around submerged plants. Thus, the characterization of the O₂ microgradients in the micro-boundary layer around submerged macrophytes is of particular interest. Using micro-optodes, O₂ in the micro-boundary layer around P. crispus leaves and stems was investigated across different growing stages at various leaf positions relating to diurnal variations. The periphyton and rapid light curves were measured by conventional methods and the pulse amplitude modulated fluorometer (Diving-PAM), respectively. Results showed that significant spatio-temporal variations in O₂ concentration gradients existed in the micro-boundary layer around P. crispus leaves. In the vertical direction from the stem/leaf surface, the O₂ concentration in the micro-boundary layer increased markedly with decreasing distance from the surface of leaf/stem and peaked at the leaf/stem surface. At the temporal scale, O₂ concentration in the micro-boundary layer varied significantly among different growing stages within the entire life cycle. The fluctuation in O₂ concentrations in the micro-boundary layer was the lowest during seedling stages (9.08-9.65 mg/L) while its amplitude peaked (9.28-13.16 mg/L) at stable growing stages. However, O₂ gradients markedly differentiated spatially at declining stages, and O₂ concentrations decreased significantly relative to that of stably growing stages. The O₂ concentration at the surface of the leaf displayed diurnal variations with a significant unimodal pattern. The O₂ concentration increased gradually and reached the maximum of 16.68 mg/L at 15:00, and then decreased with the decreasing light intensity. The O₂ concentration dropped rapidly after sunset and reached the minimum of 6.01 mg/L at 05:10. The O₂ concentration was effected mainly by light and water temperature during the diurnal cycle. At the spatial scale, marked differences in the O₂ concentration were observed in the micro-boundary layer of leaves at different parts of individual plants. O₂ concentrations in the micro-boundary layer around young leaves at the shoot apex fluctuated slightly, while those of the mature leaves at the middle shoot were steep, with the greatest amplitude of fluctuation. However, O₂ concentrations in the micro-boundary layer around stems at the middle shoot and around senescent leaves at the basal shoot maximized at the surface of periphyton which possessed higher dense, and declined slightly when entering the periphyton layer.
In conclusion, among different growing stages, O₂ concentration gradients in the micro-boundary layer around P. crispus leaves and stems were mainly affected synergistically by the photosynthetic capability and the attached periphyton. However, among different positions on individual plants, O₂ in the micro-boundary layer was mainly affected synergistically by plant physiological characteristics and the periphyton. The micro-optodes are ideal oxygen microsensors for investigating the micro-boundary layer around submerged macrophytes for fine spatial ( < 50 μm) and temporal (s) resolutions. This study provides methods for better understanding the ecological role of the micro-boundary layer around submersed macrophytes and for verifying the processes within the micro-boundary layer for regulating nutritional cycling in eutrophic waters.