The Effects Of Decomposition Of Pistia Stratiotes On Water Quality Under Different Dissolved Oxygen Conditions

The decomposition of aquatic plant litter is the key of nutrient cycling and energy flow in wetland,and is one of the main processes to maintain wetland ecosystem function.Decomposition of plant litter is a very complicated physical,chemical and biological process,which is affected by the surrounding environment and the composition and structure of the litter.Among them,DO is an important water quality index,affecting the microbial communities in water and mud,as well as the number and species of macro invertebrates.It affects the carbon cycle and nitrogen cycle of the entire ecosystem by affecting the activity of microorganisms,enzymes,and macro invertebrates.Another important factor is plant coverage,which directly determines the mass of plant residues in water.It is the nutrient source of microorganisms and macro invertebrates,and also an important carbon source of nitrogen cycle.When the water has the appropriate plant residue,will promote the nitrogen removal of water;but when there are a lot of plant residues,decomposition process will consume a large amount of dissolved oxygen,destroy the carbon cycle and nitrogen cycle in the ecosystem of normal water,weaken the entire ecosystem function,reduces the stability of the ecosystem,and increases the difficulty of water ecological management and restoration.It can be said that the decomposition of plant residues and the change of decomposition rate will directly or indirectly influence the process of global change.Therefore,it has important ecological significance to study the decomposition process of plant residues under the condition of different dissolved oxygen and plant residues and their effects on watr quality.Understanding the differences in decomposition process of aquatic plants and its water quality under different dissolved oxygen level had an important role in management of plant residues and water quality control.In this paper,we studied the changes of DO,pH,COD,TSS,NH₄⁺-N,NO₃^--N,NO₂^--N,PO₄^3--P,and other water quality indices under different levels of Pistia stratiotes coverage under non-infllated aeration（NA）and inflated aeration（IA）conditions.The experiment lasted 95 days.Results showed that:（1）The longer residence time of Pistia stratiotes residues in low dissolved oxygen groups（2-4 mg·L1-）,high dissolved oxygen groups（8-9 mg-L-1）of plant decomposition rate significantly faster than low dissolved oxygen groups,but the water quality indices changed synchronously.In the initial stage（1-25d）,the concentration of NO₃^--N and NO₂^--N in the IA groups significantly remained higher than in the NA groups,the concentration of PO₄^3--P in the IA groups remained lower than the NA groups（P<0.05）;In the subsequent stage（26-95d）,in addition to NH₄⁺-N,the main indices of IA groups were lower than NA groups.（2）High dissolved oxygen conditions can lead to NO₃^--N and NO₂^--N concentration increased rapidly at the initial stage,but PO₄^3--P remained relatively low concentration;low oxygen conditions can maintain lower NO₃^--N,NO₂^--N concentration,and PO43-P concentration was higler and lasted for a long time.Different dissolved oxygen conditions had significant influence on the main water quality indices at the initial stage,but had little influence on the water quality in the subsequent stage.（3）Under high dissolved oxygen condition,except for TSS and PO₄^3--P concentration in the initial stage,there were significant differences in different plant coverage,while other water quality indices were less affected by plant coverage.Under low dissolved oxygen condition,the plant coverage had a certain influence on the maximum concentration of NO₃^--N,NO₂^--N and PO₄^3--P,and there was a delay of maximum in NO₃^--N,NO₂^--N,PO₄^3--P concentration and the delay time increased as initial plant coverage increased,especially seen in the variation of PO₄^3--P concentration.（4）Compared with the plant coverage,DO had more significant effects on plant decomposition process and water quality.NO₃^--N,NO₂^--N,NH₄⁺-N,PO₄^3--P and TSS had higher sensitivity to DO,and were easily changed by DO;while most of the water quality indices were less sensitive to plant cover.（5）Most of the water quality indices tended to be relatively stable at the end of the experiment,but the system stability of the decomposition process was low,especially in the initial stage of the rapid decomposition rate.On the whole,compared with the NA groups,the C V value of the water quality indices in IA groups was lower,indicating that the system stability was relatively high under the high dissolved oxygen condition.As a whole,the concentration of main water quality indices was relatively low and tended to be stable early in the high dissolved oxygen condition,while the difference of water quality indices between different plant coverage was small.Compared with dissolved oxygen,plant coverage had little influence on water quality indices during plant decomposition process.