Research On The Response And Indicator Significance Of Typical Aquatic Organisms To Environmental Changes In The Pearl River Estuary

In recent years,with the rapid development of urbanization in coastal areas,estuaries have been subjected to high-intensity human activities,resulting in changes in estuarine environments and severe impacts on the stability of ecosystems.The protection of estuarine aquatic organisms and environments is crucial for maintaining ecosystem stability.Therefore,research on the response of estuarine aquatic organisms to environmental changes and their indicator significance is of great theoretical significance.However,due to the lack of longterm ecological monitoring data,current studies have focused more on the current status of estuarine aquatic organisms and environments,with limited research that comprehensively evaluates the characteristics of estuarine changes in both present and historical periods.Additionally,due to the complexity of estuarine changes and the overlapping effects of multiple environmental factors on aquatic organisms,it is challenging to identify key indicator species and explore their indicative significance regarding environmental changes.Therefore,in this study,the Pearl River Estuary is chosen as the study area to comprehensively evaluate the response of typical aquatic organisms to environmental changes in the Pearl River Estuary in both present and historical periods and to identify indicator species for analyzing the indicator significance of aquatic organisms regarding environmental changes.First,24 sampling sites were uniformly set up along the Pearl River estuary,and the water quality and existing organisms(benthic macroinvertebrates and diatoms)were sampled and analyzed for three periods(January,April and August)to understand the current status of aquatic organisms and environment in the Pearl River Estuary.Classification and regression tree models(CART)were used to identify key water quality factors affecting aquatic organisms,and generalized additive models(GAMs)response curves between key water quality factors and aquatic organisms were fitted to determine the suitable range of key water quality factors.Then,sediment cores were collected from sites with good depositional conditions,and sediment chronology theory was applied to date the sediment cores,and identify sediment physicochemical indicators and biological remains(diatom fossils)to reconstruct the changes of aquatic organisms and environment in the Pearl River Estuary during the historical period.Redundancy analysis(RDA)was used to identify key physicochemical indicators affecting diatom communities,and GAMs were used to fit the response of diatom diversity and abundance to key physicochemical indicators to determine the appropriate range of sediment physicochemical indicators.Finally,to explore the indicator significance of aquatic organisms on estuarine changes,the k-means method was used to spatially divide the Pearl River Estuary into different environmental characteristic types,and the biological indicator value method was used to identify diatom indicator species for each environmental characteristic type.The GAMs model was used to determine the suitable range of key environmental factors for indicator species,and a random forest model was used to construct a conversion function between indicator species and key environmental factors to simulate the range of diatom abundance for different environmental scenarios.This study identified the response and indicator significance of typical aquatic organisms to environmental changes in the Pearl River Estuary,and the results provide important theoretical and practical support for estuarine environmental management.The main results of this study are as follows:(1)The spatial distribution of water quality factors and aquatic community characteristics in the Pearl River Estuary varied significantly,and the spatial differences of water quality factors such as salinity(SAL),total phosphorus(TP),total nitrogen(TN),total organic carbon(TOC),p H,dissolved oxygen(DO),and three types of inorganic nitrogen(NO2-N,NO3-N,NH3-N)were more obvious.In general,the water quality near the outer sea was better than that in the river and estuary.The results of CART analysis showed that the key water quality factors affecting macro-benthic invertebrates were SAL,TP,p H and NH3-N,while those affecting diatoms were SAL,TP and DO.The suitable range for macro-benthic invertebrates was SAL=21.51‰~32.32‰,TP=0.05~0.20 mg/L,p H=7.7~8.2,TN=0.40~1.50 mg/L,and for diatoms was SAL=18.42‰~32.32‰,TP=0.20~0.35 mg/L,DO=5.30 ~11.0 mg/L.(2)Based on the sediment chronology method,the obtained sediment column can identify the change process of historical periods from 1925 to 2020 in the Pearl River Estuary,in which the sediment physicochemical indicators TP,TC,Cr,Zn,Ni,Cu,Pb,Cd and D50 changed significantly in the historical periods,and most of them increased over the years.The results of the sediment column diatom assemblage zone delineation showed that the characteristics of diatom assemblage in Pearl River Estuary changed significantly after 1986.In terms of diatom abundance,it increased substantially after 1986 and was approximately 10 times in2019.The results of RDA analysis indicated that TP,Zn,Cu and Pb were the key physicochemical indicators affecting sediment diatoms.The results of GAMs showed that the suitable ranges of key substrate factors for sediment diatoms were TP=710~770 mg/kg,Pb=50~72 mg/kg.(3)The results showed that there were 15 indicator species sensitive to water quality changes,among which the occurrence frequency was more than 20%,including Coehorn criophilum,Skeletonema costatum,Guinardia striata,Lauderia annulata and Pleurosigma formosum,14 indicator species sensitive to sediment physicochemical indexes,among which the occurrence frequency was more than 20%,including Aulacoseira granulata,Coscinodiscus blandus,Coscinodiscus jonesianus,Coscinodiscus oculatus,Coscinodiscus rothii and Coscinodiscus subtilis.The results of GAMs analysis showed that the responses of different indicator species to environmental changes were different,and the random forest model was used to construct the conversion relationship between indicator species and key environmental factors,and the determination coefficient(R2)showed that the model had good prediction performance.Simulations of different environmental characteristics scenarios showed that TP concentrations should be maintained at lower levels in low salinity areas;when DO concentrations in estuarine waters are high,lower TP concentration levels(i.e.,TP < 0.21mg/L)should also be maintained.The pollution levels of sampling sites HU1,HU2,HU4,JM2 and LD9 were high,while the pollution levels of sampling sites HM2,HQ1,HU3,LD4,LD7,LD8 and LD10 were low.This study identified the response and the indicator significance of the typical aquatic organisms to the environmental variations in the Pearl River estuary,and the results provide theoretical and practical support for the environmental management of the estuary.