Analysis Of Water Quality Variation And Element Release And Regression Of Typical Aquatic Plants In Baiyangdian Lake

Baiyangdian lake is the largest fresh grass lake in the North China Plain,and is known as the"Pearl of North China".Baiyangdian has a wide variety of aquatic plants,such as reeds,potamogeton and other large aquatic plants.Aquatic plants participate in the water nutrient cycle,on the one hand,they play the role of producers in the lake food chain,on the other hand,they are important regulators of lake evolution and ecological balance.Aquatic plants in the growing period have a good maintenance effect on water quality,but aquatic plants tend to affect water quality during decomposition,and the indexes of nutrients and oxygen consumption pollutants tend to increase in a short period of time,and excessive reproduction tends to cause"secondary pollution"of water.Therefore,it is of great significance for lake water protection and ecological restoration to study the water quality effect caused by decomposition of aquatic plants in Baiyangdian lake and explore the migration and transformation of various elements in the decomposition process.Typical submerged plants（Potamogeton crispus）,emergent plants（Phragmites australis）and floating-leaf plants（duckweed）in Baiyangdian Lake were taken as research objects.Based on field measurement data and laboratory simulation research,the effects of natural decomposition of different types of aquatic plants on conventional physical and chemical indexes,oxygen consumption pollution indexes and nutrient indexes of water body were analyzed,which provided data support for salvage and harvesting of aquatic plants,and was of great significance to aquatic plant management and water quality protection in Baiyangdian Lake.The dynamic balance of carbon,nitrogen and phosphorus in different elements such as plant residues,water bodies and sedimentary debris is calculated quantitatively,and the migration and transformation laws of each element in the decomposition process are revealed,which provides a quantitative research basis for determining the dominant water quality process and key parameters in water environment simulation.The main research contents and conclusions are as follows:（1）The release rates of C,N and P of aquatic plants in this experiment were different,and the overall performance was P>C>N.The results of quadratic exponential model fitting showed that the decomposition rate of submerged plants（Potamogeton crispus）was higher than that of emergent plants（Phragmites australis）.In submerged plants（Potamogeton crispus）,the proportion of easily decomposed components was 72.1%,and the corresponding decomposition rate k₁ was 0.082d^-1,while the proportion of refractory components was 27.9%,and the corresponding decomposition rate k₂ was 0.0053d^-1;In emergent plants（Phragmites australis）,the proportion of easily decomposed components was 18.1%,and the corresponding decomposition rate k₁ was 0.047d^-1.The proportion of slowly decomposed components was81.9%,and the corresponding decomposition rate k₂ was 0.0027d^-1.The C content of submerged plants（Potamogeton crispus）and emergent plants（duckweed）increased in a small range at the initial stage of decomposition,and then decreased obviously.The overall change of C content in reed plant residues was stable.The change trend of N element in the three plant residues was different,and the overall performance was that the N element content of Potamogeton crispus and duckweed was lower than that of Phragmites australis in the later stage of decomposition.During the whole decomposition process,the release rate of P was much higher than that of the other two elements,and P in the three plant residues decreased significantly at the initial stage of decomposition.（2）Water indexes of potamogeton crispus,Reed and duckweed all changed during decomposition,and p H,ORP and DO decreased first and then increased.During the decomposition process of Crispus crispus,DOC and COD had similar variation trends and reached the maximum value at the 16th day,39.26mg/L and 52.01mg/L,respectively.N indexes（TN,ammonia nitrogen and nitrate nitrogen）also had the greatest influence on water quality at the 16th day of decomposition.The concentrations of TP and SRP in water reached the maximum on the eighth day of decomposition.At this time,the concentration is 0.51mg/L,0.24mg/L),and then it begins to decrease the proportion of retained water in the water body from 33.7%,32.8%,66.8%to 11.1%,7.2%,8.0%,which has a short duration and a large degree of influence on regional water quality.Reed decay in the whole process,DOC and COD in water has been in a higher status in the decomposition of 20 days after reaching the maximum began to reduce,the overall concentration is higher,the water in the N index（TN）,ammonia nitrogen,nitrate nitrogen,the P index（TP and SRP）concentration change law were reduced after the first rise trend,reed decay slowly,The effect on water quality lasts for a long time and the cumulative effect is large.C indexes（DOC,COD）,N indexes（TN,ammonia nitrogen,nitrate nitrogen）and P indexes（TP,SRP）had the greatest influence on water quality at the 12th,12th and 8th day of duckweed decomposition,respectively.With the end of the decomposition experiment,the effects of duckweed decomposition on water quality were similar to those of submerged plants（Potamogeton crispus）,which lasted for a short time and had a large degree.（3）The C,N and P nutrients of potamogeton crispus,Phragmites australis and duckweed obviously migrate and transform to sediments during the whole decomposition cycle,and some C and N elements will form gas and diffuse to the outside world during the transformation process.At the 16th,16th and 8th day,the decomposition of POTAMOGETON crispus had the greatest effect on C,N and P in water body.40.6%C,31.8%N and 71.5%P in POTAMOGETON crispus were transferred to water body in dissolved and suspended detritus state,which led to the water quality being inferior to Class V water standard.The decomposition amount of POTAMOGETON crispus in 44 days was 72.1%,the proportion of C,N and P in water decreased from 33.7%,32.8%and 66.8%to 11.1%,7.2%and 8.0%,the proportion of C and N leaving water in the form of gas was 29.8%and 16.3%,and the proportion of sedimentary detritus was 38.5%,52.7%and 76.2%,respectively.In the whole process of reed decomposition,the proportion of C in water has been in a low state of 1.9%-4.2%,and the highest proportion of N and P in water is 16.8%and 10.0%.After 90 days of decomposition,the decomposition amount of Phragmites australis was 18.1%.At this time,the proportion of C,N and P in water was less than 3%,and the proportion of plant residues was 62.8%,73.6%and 33.3%,respectively.During the decomposition of duckweed,the indexes of C,N and P in water reached the maximum on the 12th,12th and 8th day,respectively,and the indexes of C,N and P in water reached the highest of 27.9%,23.3%and 29.2%,respectively,and decreased to10.3%,6.1%and 8.2%respectively at the end of the decomposition experiment.The dynamic equilibrium calculation of element destination in the decomposition process shows that the decomposition of submerged plants POTAMOGETON crispus and floating-leaf plants（duckweed）has relatively concentrated influence time on the water quality of Baiyangdian Lake,and the main influence way is that the decomposition of plant residues directly enters the water body in soluble state and suspended debris state;The decomposition rate of emergent plant Phragmites australis is slow and relatively uniform,which has a long influence on the water quality,especially DOC,and has a large cumulative effect.It affects the water quality through two ways:decomposition of plant residues and release of sedimentary debris.