The Ecological Effects Of The Sediment Dredging And Dilution On Shallow Urban Eutrophic Lake

Eutrophication has become a widespread environmental problem. Consequences of eutrophication include excessive plant production, blooms of harmful algae, increased frequency of anoxic events, and fish kills. Economic losses attributed to eutrophication include costs of water purification for human use, losses of fish and wildlife production, and losses of recreational amenities. Over the past 30–40 years much research on lake has focused on the effects of various controlling measures.Sediments serve both as reservoirs and potential sources of contaminants to the water column. Contaminated sediments now represent the main hazard for the environment. The sediment contaminants include a wide variety of organic compounds and heavy metals. In addition, nutrients in the sediment are of great concern because they can be a major internal source of eutrophication and influence water quality. Environmental dredging is a primary remedial option for removal of the contaminated material from aquatic environment. Of primary concern in environmental dredging is the effectiveness of the intended sediment removal. Dilution and flushing technique can achieve improved in eutrophic lakes by reducing the concentration o limiting nutrient and by increasing the water exchange rate. The effects of sediment dredging on lake ecological systems have been uncommonly reported for shallow lake in China.Located in Changchun, Jilin Province, P.R.China, Nanhu Lake is a shallow urban eutrophic lake. A project of environmental sediment dredging had been carried at the Lake Nanhu during 2000 to 2004 in order to control its eutrophication. Following completion the sediment dredging, dilution and flushing was employed in 2005. A 5-year field monitoring study was conducted to assess the effectiveness of the environmental dredging and dilution project in Nanhu Lake.The dissertation focused on the effects of the sediment dredging and dilution project on shallow urban eutrophication lake. There are five sections in this dissertation. Chapter one reviewed the progress of lake eutrophication and ecological effects of sediment dredging and dilution on lake system. Chapter two studied the effects of dredging and dilution on water quality. Chapter three focused on the effects of dredging on the nutrient and heavy metals. The chapter four reported the effects of dredging and dilution on the plankton in Nanhu Lake. The chapter five is the conclusion.The water quality, nutrients and heavy metals of sediment and plankton of Nanhu Lake were analyzed. Based on the monitoring data and collected of pre-dredging data (1998-1999), physical-chemical factors, nutrients and heavy contents, concentration of chl-a and the number, density, biomass of plankton were compared. The changes of physical-chemical characteristics of water, contents of nutrients and heavy metals and succession of plankton were observed. The causes of nutrients increased during and after dredging were discussed. The findings from the research can provided field evidences for controlling lake eutrophication.At the research of water quality section, nine physi-chemical characteristics including pH value, dissolved oxygen(DO), suspended solid(SS), Secchi Depth(SD), biochemical oxygen demanded for five days (BOD5), chemical oxygen demanded (COD_cr), ammonium-N (NH₄⁺-N), total nitrogen (TN)and total phosphorus (TP) were analyzed. The main conclusions were as follows:1.Some physi-chemical characteristics greatly improved after sediment dredging project. SD increased 36% compared the pre-dredging. pH value was also increased after dredging. While SS contents did not increased as pre-anticipated. Dilution project increased the SD of water. However, the changes of pH value DO and SS were not significant.2.The effects of sediment dredging on BOD5 and COD_cr were not significant. However,the dilution project effectively reduced the contents of COD_cr.3.Dredging activities could result in the concentration increase of NH4+-N, TN and TP over short-term period after dredging. TN and TP in the surface water of Nanhu Lake after dredging was reduced approximately 74%,44% and 47%. From the long-term monitoring, the role of the sediment in recycling nutrients was reduced substantially following dredging.At the research of sediment section, the concentrations of total nitrogen, total phosphors and heavy metals (Zn, Pb, Cd, Cu, Cr, Ni,Hg and As) before and after dredging in sediment were determined and compared. Multiple ecological risk indices were employed to assess the contamination of heavy metals before and after dredging. Our results showed that:1.The total phosphorus levels reduced 42% following dredging. Similar changes for Hg, Zn, As Pb, Cd, Cu, Cr and Ni were observed, with reduction percentages of 97.0, 93.1, 82.6, 63.9, 52.7, 50.1, 32.0 and 23.6, respectively,2.The quality of sediment improved based on the criterion of Sediment Quality Guidelines by USEPA and contamination degree values (Cd) decreased significantly (paired-t-test, p < 0.05).3.The total nitrogen increased 49% after dredging compared to before dredging. Findings from the study demonstrated that the environmental dredging was an effective mechanism for removal of total phosphorus and heavy metals from South Lake. Nevertheless, the dredging was ineffective to remove total nitrogen from sediment.At the research of plankton section, the characteristic of the water the plankton had been investigated 4 sampling sites in Nanhu lake. The main conclusions were as follows:1.Compared the species number of phytoplankton pre-dredging; the species number of phytoplankton increased two fold. Dredging caused the succession for composition of phytoplankton community in Nanhu Lake. The phytoplankton community took succession from chlorophyta-cyanophyta-diatom of prior to dredging to chlorophyta-diatom-cyanophyta of post dredging.2.The diversity of phytoplankton community increased after dilution. The number of phytoplankton community increased 1.5 fold compared the completion of dredging. The species of Dinobryon, Mallomonas, Synura were investigated after dilution provided an evidence that the water quality had been improved.3.The concentration of chl-a decreased significantly(p<0.05), which decreased up to 36% compared the pre-dredging. The concentration of chl-a (44.86ug/L) was the lowest in 2006. The responses of chl-a to dredging and dilution were consistent with the variation of nutrients.4.The zooplankton community was mainly consisted of protozoa and rotifer, and the number of cladocera and copepoda was little. After dilution, the he number of cladocera and copepoda increased. TheTrichocerca cylindrica became the dominant species of zooplankton in Nanhu Lake. The phenomena demonstrated that the water quality had been improved.5.The number of zooplankton decreased 16% and 45% compared the pre-dredging and pre-dilution. Margalef diversity increase from 1.96 to 5.99.In sum, the 5-year monitoring results indicated that the environmental dredging in South Lake was an effective mechanism for removal of nutrients and heavy metals, though the nutrients might increase during shortly dredging. The most likely reason was ammonia release from the sediment during dredging impairing the effectiveness. We therefore suggest that, prior to consideration of dredging as remedial alternative for rich-sediment lake, the potential ammonia release from the sediment caused by dredging operation must be carefully considered. Additionally, long-term monitoring effects of the project carried out in lake is needed.