Responses of chlorophyll and MDA of Vallisneria natans to nitrogen and phosphorus availability and epiphytic algae

It is common for submerged plants to become less abundant and disappear in eutrophic lakes in the world. To explore degradation mechanisms of submerged plants during the growing season, we determined chlorophyll and malondialdehyde (MDA) concentrations in Vallisneria natans (Lour.) Hara cultivated under four different nutrient concentrations (N/P, mg/L): T1 = 0.5/0.05, T2 = 2.5/0.25, T3 = 4.5/0.45, and T4 = 12.5/1.25) and under low and high density of epiphytic algae. With the increase in nutrients, the content of chlorophyll in the leaves of V. natans varied in the order T3 > T2 > T1 (T4) at low density (grazed) of epiphytic algae and varied in the order T3 > T2 > T1 > T4 in the high density (ungrazed) of epiphytic algae. With the increase in nutrients, the content of MDA in the leaves of V. natans varied in the order T4 >T1 (T2, T3) at the low density (grazed) of epiphytic algae and varied as T4 > T3 > T2 > T1 at the high density of epiphytic algae. At the same time, chlorophyll content of V. natans was significantly higher (p < 0.01 and the content of MDA was significantly lower (p < 0.01) in the grazed treatment than in the non-grazed treatment under the same nutrient concentrations. During the growing season, total nitrogen and total phosphorus concentrations were low (N < 4.5 mg N/L, P < 0.45 mg P/L) in Lake Taihu, a large shallow eutrophic lake in China; consequently water column nutrient loading benefited the growth of V. natans, but the high biomass of epiphytic algae and planktonic algae caused by the high nutrient loading of water column inhibited the growth of V. natans.