Effect of Vallisneria spiralis on Water Quality and Sediment Nitrogen at Different Growth Stages in Eutrophic Shallow Lake Mesocosms

Submerged macrophytes are commonly highlighted as the key step to restore eutrophic shallow lakes. However, how submerged macrophytes affect water quality and sediment nitrogen at different growth stages remains unclear. In this study, the dynamics of water quality, sediment nitrogen, and denitrification during the four growth periods of Vallisneria spiralis (V. spiralis), namely, the rapid growth period (stage I), stable growth period (stage II), withering and death period (stage III), and decay and decomposition period (stage IV), were investigated using mesocosms to simulate eutrophic shallow lakes. The results showed that the purification effects of V. spiralis on water quality parameters in the four periods clearly differed. Compared with the control group (in the absence of V. spiralis), the treatment group presented a lower concentration of Chlorophyll a (Ch1.a) in stage I; total nitrogen (TN), ammonia nitrogen (NH4+-N), and Ch1.a in stage II; and TN, total phosphorus (TP), chemical oxygen demand (COD), and Ch1.a in stage III. However, the water quality in stage IV deteriorated for a short-period in the group with V. spiralis, especially the increases in NH4+-N and Ch1.a. Moreover, the effect of V. spiralis on the forms of nitrogen in sediment primarily occurred from stages II to IV. The content of sediment TN in treatment group was much lower than that in control group, with a final decrease of 40.7%. In addition, the denitrification rate (DNR) differed significantly between the two groups, and the DNR in treatment group increased by 4.8 % compared with that in the control group. The DNR of treatment group decreased in stage I but increased in stages II and IV. Our study provides empirical evidence that re-established submerged macrophytes can contribute differently to eutrophication mitigation in shallow lakes at their various growth stages.