Decomposition of Potamogeton crispus and its effect on the aquatic environment of Dongping Lake

The mass loss of Potamogeton crispus during 40 days of its decomposition and the dynamics of nutrient elements (C, N, S and P) in the litter were studied from June to July in 2011 by litterbag method. The results showed that Potamogeton crispus decomposed rapidly in first 14 days. About 80% of the biomass (dry weight) was lost after 40 days and 95% after 365 days. Compared with the single exponential model W_t/W₀=e^-0.157t (R²=0.402), the double exponential model W_t/W₀ = 0.785e^-0.386t+0.215e^-0.004t (R²=0.876) could describe the mass loss more accurately. The loss of the nutrient elements in litter was not synchronized with dry mass losing. The content of C, N and S in litter generally increased during the first 20 days, then decreased obviously from the 20th day to 30th day and then increased slowly from the 30th day to 40th day. The content of P in litter reduced by 73.3% in the first 6 days and then increased very slowly. The accumulative release rates (ARR) of C, N, S and P had a similar change, and had two peaks in the 14th day and 25th day, respectively. The values of ARR were in the order of P>S>C>N in the initial stage of decomposition (0~8 days), and P>C>S>N on 10~40 days. According to the change of ARR, it could be inferred that the release of nutrient elements was mainly occurred in the early period (0~14 days) of the decomposition. The total release amount of C, N, S and P in this stage (0~14 days) was about 12510 t, 1187 t, 147 t and 235 t, respectively, which was equivalent to the increase of 29.9 mg·L^-1 (C), 2.84 mg·L^-1 (N), 0.35 mg·L^-1 (S) and 0.56 mg·L^-1 (P) in the overlying water. This could cause the secondary pollution of water body. The annual residual amount of Potamogeton crispus was about 2090t (dry weight), and it only accounted for 0.6% of annual depositive flux of Dongping Lake, indicating less impact on sedimentary of Dongping Lake.