| With being an important part of coastal wetlands,lagoons have an extremely important existence value.As a special landform between the sea-land interaction zone,the brackish water exchange environment makes lagoons have a unique biome,which have unique ecological significance.At the same time,lagoons also provide fishing,sightseeing and leisure places for human in coastal areas,which have great social and economic value.However,since lakes are often an important sink of nutrients contained in rivers and human activities,so lagoons are often strongly influenced by human activities in the drainage and along the lakes.Similar to other major river deltas in the world,the Nile Delta has a series of lagoons on its northernmost Mediterranean coast,with four lagoons such as Manzala,Burullus,Edku and Mariut being the most prominent.For a long time,because the water quality and ecological environment of the lagoons have been affected by factors such as land reclamation around the lagoons and in the lagoons,unrestricted aquaculture,large amounts of agricultural wastewater,and untreated industrial wastewater and domestic sewage and so on,they have had a great change.Especially after the completion of the Aswan Dam,the inflow of water and sediment from the drainage decreased dramatically,and the human activities intensified greatly.Now the ecological environment of the lagoons has become more fragile.Among these lagoons,the Burullus Lagoon in the center of the estuary delta absorbs most of the agricultural drainage and domestic sewage in the central part of the Nile Delta.Its water quality deteriorates significantly and the ecological environment tends to deteriorate.The study of the ecological environment changes and influencing factors in the Burullus Lagoon over the past 100 years,especially after the Aswan Dam built,can not only have a clear understanding of the lake’s past environmental change processes and mechanisms,but also provide some scientific advice for the future protection and management of the lake,which has certain practical significance.Through the 210Pb dating of two cores’samples collected from west of the Burullus Lagoon,a chronological framework is constructed,and combines with various environmental proxy indicators such as particle size,organic matter（TOC（total organic carbon）,TN（total nitrogen）,TP（total phosphorus））,BSi（biosilicon）and heavy metals,mainly discussing the following issues:（1）Changes and effects of human activity as reflected by changes in deposition rates;（2）The characteristics of nutrient salts over time and the impact of human wastewater discharge activities reflected by them;（3）The characteristics of heavy metals over time and their potential ecological hazard;（4）Analysis of impact factors of ecological environment changes in the Burullus Lagoon over the past 100 years.The following conclusions were obtained:（1）The 210Pbex of B-3 core in the Burullus Lagoon tended to decrease exponentially with depth.The CRS model-calculated sedimentation rate showed that the construction of the Aswan Dam had not significantly affected the sedimentation rate of Burulus Lagoon.The sedimentation rate increased rapidly in the mid-1980s,reaching a maximum of more than 1.5 cm/yr,after which it decreased and stabilized at about 1cm/yr.It was related to enhanced human activities such as agricultural irrigation networks construction,agricultural reclamation,fish pond culture,and wastewater discharge.（2）The contents of TOC,TN,and TP in the sediments of the central lagoon increased by 41%,38%,and 41%,respectively,and BSi decreased by 12%after the Aswan Dam built.The C/N values indicated that the organic matters in the lagoon were mainly from the terrestrial input.The TN/TP values indicated that the Burullus lagoon exhibited significant phosphorus limitation.The increased N and P fluxes from the emissions of downstream human activities had greatly exceeded that intercepted by the water and sediment retention effect of the Aswan Dam,which made the TOC,TN and TP in the Burullus Lagoon increase after the Aswan Dam built.But the contents of BSi in the lagoon sediments were significantly reduced after dam built because of the obvious retention effect of dam on DSi（dissolved silicon）.（3）The results of heavy metals in sediments showed that,before the normalization,except for Mn and Pb in the west and Fe,Pb and Mn in the middle of the Burullus Lagoon increasing,other contents of heavy metals were reduced.After normalization,nearly all heavy metals below the mixed layer remained the same trend as before normalization,but in the mixed layer,almost all heavy metals showed an increasing trend.The EF values showed that Mn in the Burullus Lagoon presented serious pollution.Pb in the central and western lagoon was moderately polluted,but the eastern region did not reach the risk of pollution.Fe and Ni was slightly or moderately polluted,and Cr and Zn were below the pollution risk.A single potential ecological risk index values indicated that all elements were below a slight risk,and the total potential ecological hazard values also indicated that the ecology of the Burullus Lagoon was slightly threatened.（4）The cluster analysis showed that,Zn、Al、Fe、Cr、Ni、BSi were natural sources and Mn、Pb、TN、TOC、TP were anthropogenic sources.The results of factor analysis showed that BSi,Fe,Zn in the western part of the lagoon and BSi,Fe and Cr in the central part were mainly derived from natural input.TOC,TN in the western part of the lagoon and TOC,TN and TP in the central part of the lagoon were mainly affected by agricultural wastewater and domestic sewage.Ni and Zn were likely to be imported from the same mineral source.Pb and Mn were mainly affected by human activities,but the human activities source of Pb was more complicated. |
