Abstract:Submerged macrophytes play an important role in nutrient cycling, especially in shallow lakes. Submerged macrophytes can acquire significant amounts of nutrients from the water via shoots and from the sediment via roots. In most natural situations, root uptake is the primary pathway for nutrients, because the absorbable nutrient concentrations are much higher in the sediment than in the water column. However, submerged macrophyte species vary in their root traits. Some submerged macrophytes, such as Vallisneria natans, have large root systems, while other species, such as Hydrilla verticillata, grow only a few roots per plant. Phosphorus (P) is the most critical nutrient limiting lake productivity, and while submerged macrophytes play an important role in P cycling, little is known about the effects of different submerged macrophyte species on the behavior of P. Therefore, studies of the effects of different root characters of submerged macrophytes on P concentrations are important for understanding lake ecosystems. The purpose of this work was to identify how two different typical submerged macrophytes, H. verticillata and V. natans, affect the behavior of sediment P. We examined P concentrations and environmental factors in aquatic systems growing each of these plant species from May to September, 2012. During that time, we collected samples of sediment pore water, sediment, and column water on days 0, 30, 60, 90, and 120 of the experiment to determine P concentrations. The environmental factors of pH and redox potential (Eh) of the sediment were also measured. The results indicated that P concentrations in pore water of the H. verticillata and V. natans treatments were lower than that of the control group. Both H. verticillata and V. natans had obvious effects on water stabilization and reducing P release from sediment. Pore water concentrations of dissolved total P (DTP) and soluble reactive P (SRP) in the H. verticillata and V. natans groups were significantly different, with 0.24 mg/L DTP and 0.22 mg/L SRP in the H. verticillata treatment and 0.01 mg/L DTP and 0.004 mg/L SRP in the V. natans treatment. P concentration increased after the 90th day of the experiment in the H. verticillata group, but it remained at a low level in the V. natans group. The pH was lower in the H. verticillata treatment than in the V. natans treatment, while Eh was higher in the H. verticillata group than in the V. natans group, which might explain why P levels fluctuated differently in the two treatment groups. The main P fractions in pore water were DTP and SRP, while the amount of dissolved organic P (DOP) was relatively low. The submerged macrophytes reduced the P concentration in water, sediment, and pore water during their growth periods. Their absorption of pore water P was one of the main reasons for the decreased P levels. Vallisneria natans could absorb more P than H. verticillata. Both H. verticillata and V. natans could reduce the DOP concentration, but there was no significant difference between the two submerged macrophyte species. There was no significant difference between treatments and control groups in DOP, indicating that H. verticillata and V. natans mainly absorbed DTP and SRP. Submerged macrophyte inhibited the release of P from the sediment into the water column by decreasing the pH and increasing the Eh of the sediment. Overall, the submerged macrophytes H. verticillata and V. natans significantly stabilized water quality and reduced the release of P from the sediment to the water.
