[Effect of Nutrient Loadings on the Regulation of Water Nitrogen and Phosphorus by Vallisneria natans and Its Photosynthetic Fluorescence Characteristics].
Submerged macrophytes are an important component of aquatic ecosystems. During the growing period, submerged macrophytes can absorb nitrogen and phosphorus nutrients to reduce pollution loadings. Shoots of submerged macrophytes can also promote the adhesion of suspended substances in water, reducing the turbidity. The release of nutrients in sediments can be suppressed by its root system, and the resuspension of sediments caused by disturbance of winds and waves can also be resisted. The role of submerged macrophytes in ecological restoration of eutrophic lakes has attracted widespread attention. In 1960, the submerged plants Vallisneria natans and Potamogeton malaianus had been the dominant species in East Taihu. However after 2002, Nymphoides peltatum, Elodea nattalii, P. malaianus, etc. have gradually taken over the dominant roles along with significant elevations of nitrogen and phosphorus levels. Nutrients in water are not the only key factors causing eutrophication of water bodies; the nutrient source for submerged plant growth affect both the purification efficiency and the photosynthetic characteristics of submerged macrophytes. Excessive nitrogen and phosphorus concentrations can inhibit the photosynthetic physiological activities of submerged macrophytes, affecting the succession of aquatic vegetation. In addition, under high nutrient conditions, the competition from periphytic algae and planktonic algae may also directly poison submerged macrophytes, leading to its degradation and disappearance. Systematic studies on the regulation and photosynthetic fluorescence response mechanism of submerged macrophytes to varied nutrient loadings are helpful in revealing their relationships. The seedlings of submerged macrophyte V. natans were transplanted in a laboratory mesocosm to study the effect of nutrient loadings on its regulation of water nitrogen and phosphorus. Three nitrogen and phosphorus loadings from low, medium, and high levels derived from nitrate, ammonium, and phosphate were setup as the aquatic medium for the plant growth. Twelve harvests were carried out to determine the evolution of nutrient removal performance of V. natans. Its photosynthetic fluorescence characteristics were measured by a pulse-amplitude modulated fluorometer (Diving-PAM). Results showed that the nitrogen and phosphorus adsorption abilities of V. natans were gradually enhanced with the increase of nutrient concentrations in the range of TN ≤ 12 mg·L-1 and TP ≤ 1.0 mg·L-1. In the treatment of high nutrient concentrations (TN=12 mg·L-1 and TP=1.0 mg·L-1), the removal rates of nitrogen and phosphorus reached more than 95%. V. natans preferentially absorbed ammonium nitrogen when its concentration was high. The medium nutrient concentrations (TN:8-12 mg·L-1 and TP:0.6-1.0 mg·L-1) did not significantly affect the Fv/Fm ratio of leaves. However, the low nutrient concentrations (TN=3 mg·L-1 and TP=0.3 mg·L-1) could improve the Fv/Fm ratio of leaves and were beneficial for the growth of V. natans. The inhibition of photosynthetic activity and light tolerance were enhanced with the increase in nutrient concentrations. The photosynthetic activity of V. natans gradually recovered with no significant changes in the capacity for light harvesting, when the nutrient concentrations gradually decreased in the water. Our results indicate that the high nitrogen and phosphorus loadings indeed hamper the photosynthetic capacity, which may subsequently restrain the maintenance of the dominance of V. natans in the submerged macrophyte communities.