Abstract: To examine the relationships among ammonium nitrogen (NH₄⁺-N), phytoplankton and submerged macrophytes, an outdoor simulation experiment was conducted on the shores of Dianchi Lake. Theeffects of NH₄⁺-N on the growth and physiological characteristics of Myriophyllum spicatumwere investigated at different NH₄⁺-N concentrations (0, 0.05, 0.15, 0.50, 1.50, 5.00 mg·L^-1) and 100 μg·L^-1 of the initial chlorophylla. The growth and physiological indices of M. spicatum were measured. At the NH₄⁺-N concentration of 1.50 mg·L^-1, the height and biomass of M. spicatum were significantly higher than those under other treatments, while the total chlorophyll concentration, photosynthetic activity, and soluble protein concentration of M. spicatum could maintain the initial level. In contrast, these indices decreased in other treatments. Under combined conditions of NH₄⁺-N and phytoplankton, 1.50 mg·L^-1 of NH₄⁺-N was beneficial to the growth of M. spicatum whereas high concentration of NH₄⁺-N (5.00 mg·L^-1) and N deficiency (NH₄⁺-N≤0.50 mg·L^-1) were not beneficial to the growth of M. spicatum. When the NH₄⁺-N concentration reached 5.00 mg·L^-1, the massive phytoplankton blooms could bring some correlative effects, including the increases of water turbidity and the decreases of underwater light intensity, which resulted in the lower tolerance threshold of M. spicatumcompared with single pollution of NH₄⁺-N. Meanwhile, the inhibitory effects of submerged plants on phytoplankton would be relieved, leading to algae blooms, in which the chlorophyll-a concentration reached up to (948.1±313.0) μg·L^-1.

Key words: biocrust, sand burial, greenhouse gases, desert.