Decomposition Of Algae Residue And Its Effect On Water Quality

Aquatic plants in eutrophic water body included higher aquatic plants and algae and so on. Decomposition of aquatic plants was the important process of aquatic ecosystems material cycle, it not only affected lake water environment quality, and at the same time also could affect the stability of the lake ecosystem. Therefore, the study of the decomposition process of higher aquatic plants and algae as well as its impact on water quality had important theoretical and practical significance.Algal bloom from Lake Taihu and such macrophytes as Phragmites australis, Potamogeton malaianus and Limnanthemun nymphoides were selected to do simulative lab experiment. The decomposition process of aquatic plants and its subsequent influence on water quality were discussed and the results were as follows:(1) There existed significant discrepancy between the residues decomposition of Phragmites australis, Potamogeton malaianus and Limnanthemun nymphoides (P<0.0i).The decomposition of Limnanthemun nymphoides was the fasted, with Potamogeton malainusas the second, and Phragmites australis the slowest. The significant positive correlation (P<0.05) was found between the decomposition rate and initial concentrations of phosphorus. The author also found a significant negetive correlation (P<0.05) between the decomposition rate and carbon content, C/N and C/P.(2) The decomposition of higher aquatic plants had two obvious stages:fast decomposition and slow decomposition, the decomposition rate of Phragmites australis, Potamogeton malaianus and Limnanthemun nymphoides reached 0.051, 0.046 and 0.123 d-1 during the fast period, while the average decomposition rate was only 0.001,0.009 and 0.016 d-1 in the slow decomposition period. During the decomposition of plant residues, the accumulation index (NAI) of C, N and P were significantly less than 100%, indicating a net release, whereby, the releaserate of P was higher than that of C and N.(3) The influence of higher aquatic plants decomposition on water quality at low temperature were with obivious stages. In the residues of Phragmites australis, Potamogeton malaianus and Limnanthemun nymphoides, the concentration of total N had increased by 216.0%,270.8% and 270.8%, while total P reached 0.14,0.31 and 0.30 mg-L’1 during the fast decomposition stage. In the slow decomposition stage, nutrient concentrations decreased obviously,was just a little higher than the control grop in the end of the experiment..(4) The responses of water quality to residues decomposition were varied by plantspecies with the same biomass. For the water covered with Limnanthemun nymphoides, the value of pH and dissolved oxygen were the lowes. The concentration of total notrogen was the lowest in the first 8 days, while the highest at last. For the water covered with Potamogeton malaianus, the concentrations of NH4+-N, NO3--N, total notrogen and total phosphorus were higher than other groups. For the water covered with Phragmites australis, total phosphorus was the lowest, and the concentrantions of NH4+-N, NO3--N became the highest at the end. Moderate intervention of plant residues could enhance water denitrification, which was helpful for nitrogen removal.(5) The joint decomposition of Potamogeton malaianus and algal bloom could result in the phenomenon of malodorous black water. On this condition, pH and DO displayed a quick reduction, creating an anaerobic environment for the decompostion. The influence of the joint decomposition of Potamogeton malaianus and algal bloom on water quality were with obivious stages. C, N, P were quickly released into the water in the first 8 days, with TDN, TDP and TOC reached the maximum, specificly 41.88,0.63 and 294.82 mg·L-1 on the 8th day, and then presented a downward trend from day 8 to day 64.(6) NH4+-N kept on a high level during the joint decomposition of Potamogeton malaianus and algal bloom in the first 32 days, and then presented a downward trend from day 32 to day 64. For experiment groups, the highest concentration of NH4+-N reached 15.6,36.72 and 41.30 mg·L-1, which indicated that NH4+-N was the advantageous nitrogen fraction of inorganic nitroge, accounting for more than 90%. In addition, low dissolved oxygen accelerated the denitrification, with concentrations of NO3--N and NO2--N keeping on a low level throughout the whole process.