Characteristics Of Aquatic Ecosystem Degradation And Analysis Of The Key Issue In Longgan Lake Wetland Natural Reserve

Longgan Lake Wetland Natural Reserve is one of the most important freshwater lake wetland in the middle and lower reaches of the Yangtze River,with lots of kinds of animals and birds living and breeding there.Since 2016,because many phenomenon of degradations emerged in Longgan Lake Wetland Natural Reserve,we had conducted an annual characteristic survey of water quality,aquatic vegetation,phytoplankton,zooplankton and fish to understand the current situation.Then we combined these information,analyzed and summarized the key issue of the wetland degradation,and explored its driving foctor.1.The reserve was under moderate eutrophication.The concentration of TN,TP,Chl.a,SS were respectively 1.747mg/L,0.188 mg/L,44.908 mg/m3,37.97 mg/L.The secchi depth was only 30.917 cm,which was 2/3 less than before.In static state,the maximum release of nitrogen and phosphorus from sediment was 1468.072 mg/m2 and 71.591 mg/m2.The excessive concentration of nitrogen and phosphorus in water was caused by endogenous release and exogenous input.There was a significant positive correlation between water transparency and suspended substance,which was mainly composed of inorganic particle matter and easily disturbed by the wind and waves.2.The aquatic vegetation of Longgan Lake Wetland Natural Reserve had seriously degraded,with 18 species in total.The quantity of the submerged plant was almost negligible,of which only four species were observed.The coverage of aquatic vegetation had been severely reduced from 90 percent at the end of the last century to 25 percent.Based on the dominent association species and structures,four association types were categorized:Eichhornia crassipes association,Zizania caduciflora-Polygonum orientale association,Trapa bicornis association,Nelumbo nucifera-Trapa litwinowii+Euryale ferox association.Species of these associations were mainly resistant to pollution.The annual average density of seed bank was 2448 grain per square meter,most of which was Ceratopteris pteridoides(2246 grain per square meter),occupying 91.7 percent.Seventeen species constituted the seed bank,4 of which were submerged plant and 2 were emerged plant.The similarity coefficient between seed bank and aquatic vegetation was 0.499,and the spatial distribution of the two was similar.3.There were 192 phytoplankton species in Longgan Lake Wetland Natural Reserve,of which 49 percent were cyanobacteria.The annual abundance was 32.395 million cells/L,and the dominant species were Merismopedia minima,Cyclotella meneghiniana,Anabaenopsis arnoldii.The dominant species were mainly classified as cyanobacteria.Zooplankton consisted of Rotifera,Cladocera,and Copepoda,which had 85 species in total.The average abundance was respectively 91,21,56 individual per litre,of which the biomass was respectively 0.218,0.376,1.636 milligram per litre.Dominant species of Rotifera were Brachionus angularis orientalis,Brachionus calyciflorus amphiceros,etc,of which many were resistant to pollution.Dominant species of Cladocera were Bosmina coregoni and Moina micrura,of Copepoda was Cyclopoida.4.The species diversity of fish in Longgan Lake Wetland Natural Reserve decreased and the standing crop increased,compared with the end of the last century.Cultrichthys erythropterus,Carassius auratus and Coilia nasus taihuensis were dominant species in Longgan Lake Wetland Natural Reserve.The standing crop was 195.1 kilogram per hectare and the most was Cyprinus carpio.All four fish were omnivorous,and had a certain impact on the restoration of aquatic ecosystem.5.The key issue of Longgan Lake Wetland Natural Reserve was the deterioration of water quality,which showed up in secchi depth,nitrogen and phosphorus concentration.This issue had caught the degradation of aquatic vegetation,and submerged plant could not germinate and grow.