The Formation Of Algal Deposits In Eutrophic Shallow Lakes And Its Impact On The Water Environment

In shallow eutrophic lakes,the benthic bioclastic deposit accumulates abundant organic matter derived from algae and macrophytes debris,which have attracted much public attention.Different from the slow natural deposition process,large scale of bloomed algae accumulated every year at specific areas,then decay with emission of nutrient,taste and odor and subsequently form benthic black warp when deposited.The characteristics and composition of benthic bioclastic deposit,including the impact of interfacial physiochemical processes of aggregation layer and water-sediment interfaces on the decay of algae and the transport of odorants,are still poorly understood.Take Taihu as an example,according to the spatiotemporal changes of algal and macrophytes debris.Lakeside trenches formed by water erosion,fenced flora region and lake shore pond as places are choosed for studying deposition.Various investigative and experimental methods are employed,including field investigation,field experiment and laboratory simulation.In addition,innovative techniques such as microelectrode technique,microchemical detection and molecular biology approach are also applied.The goals of this project include to identify the formation and dynamic of lake bioclastic deposit,to determine the coupling process between surficial aggregation layer and benthic deposition layer,and to reveal the relationship between bioclastic deposit geochemical process and lake ecosystem succession.The main results are summarized as follows:（1）The concept,characteristics and composition of benthic bioclastic mat in typical habitats were defined.In shallow eutrophic lakes,large amount of debris from algae and aquatic plants accumulated at specific regions,including fenced flora region and lake-shore pond,formed kind of“benthic bioclastic mat”.By collecting sediment cores from different habitats and combining microelectrode technology,the characteristics and material composition of benthic bioclastic mat were explored.Results showed that the massive phytodetritus trapped by aquatic plants gradually evolved into benthic bioclastic mat which was characterized by anoxic,reductive and low p H.The moisture concentration of the top layer was as high as65.3%from 0 to 10 cm,and the thickness can range from 20 to 30 cm or even 1 m.The C/N ratio ranged from 6.3 to 11.6.The proportion of biodegradable organic matter accounted for46%to 77%of the total organic matter.It was mainly lipids,followed by sugars,with the lowest protein content.In contrast,in areas dominated by debris from aquatic plants,the C/N ratio ranged from 22.4 to 29.3,and biodegradable organic matter accounted for only 20%to47%of total organic matter.（2）The coupling process between sedimentation and resuspension was investigated.A time series sediment trap was applied,according to the complicated hydrodynamic conditions and various interference factors of large shallow lakes.Based on the characteristics of suspended particles and organic matter composition in 4 seasons,combined with high-frequency meteorological data,the sedimentation and resuspension processes of organic debris in typical habitats in the northwestern lake area of Taihu Lake were analyzed.Results showed that wind speed was the main factor that causes the increasing concentration of suspended solids in water body,and the critical wind speed that caused sediment resuspension was about 3.6 m/s.The observed gross resuspension and sedimentation rate exhibited significant spatial and temporal variations depending on the different ecotypes in the littoral zone of Zhushan Bay.The gross sedimentation and resuspension rates across all sites were generally higher during spring and summer than during autumn and winter.And gross sedimentation rate was markedly higher in areas containing no macrophytes than in macrophytes areas.Almost 70%of the resuspension sediment originated from sedimentation.（3）The superposition of temperature and algae accumulation led to the accelerated formation of benthic bioclastic mat was found.Based on the laboratory simulation experiment,the chemical and physical indicators of the water body during the algal sedimentation and decomposition were measured,and the composition of nutrients and organic matter in the overlying water and sediment was measured.The effect of temperature and algae accumulation on the formation and development of benthic bioclastic mat were clarified.Results showed that increasing temperature can accelerate the algae decomposition process and promoted the release process of nitrogen and phosphorus nutrients.The maximum release rates of TN and TP were6.40 and 0.63 mg/（L d）,respectively.The algae decomposition not only released lots of nutrients to the water body,but also supplied a large amount of algae debris which increased the nutrient load of the sediment.The decomposition rate was greater than the sedimentation rate in the early stage,and some highly active organic matter is preferentially degraded,such as pigments,proteins,sugars,etc.,and the nutrient content of the water body was rapidly increased.In the middle of the decomposition,the sedimentation rate was gradually increased as the organic debris gradually increases.With the increase of organic debris and gradually reached the equilibrium stage where the short-term decomposition rate was equal to the sedimentation rate.The later-decomposed organic matter will gradually accumulate,the sedimentation rate was greater than the decomposition rate.Nutrient content in water slowly rised and nutrient load in sediments continued to increase.（4）The secondary effects of benthic bioclastic mat formation and development were assessed.Based on previous investigations and laboratory simulation experiments,the effects of benthic bioclastic mat development on typical habitat water quality conditions,sediment nutrient load,and odorant release were further explored,and ecological health assessments were performed.The results showed that the development of benthic bioclastic mat not only increased the endogenous load,but also provided the main source of odorants in eutrophic shallow lakes.The surface sediment S_TN was in the range of 0.19～2.87,the comprehensive pollution index FF is between 0.36～4.31,and the whole region was between moderate to severe pollution.It was worth noting that the odorant content in the sediment and the odorant content in the overlying water have a symmetrical distribution.All of them showed that the concentration of odorant in the algae-dominated area was higher than that in the open area and the estuary area.In terms of the composition of odorant,β-Cyclocitral andβ-Ionone were the main components,and Geosmin and DMDS were the lowest.The contents ofβ-Cyclocitral andβ-Ionone in the algae accumulation area were 1.24 and 93.79 times the odor threshold,respectively,indicating a risk of diffusion from the sediment to the water column.In summary,algal blooms are occurring globally at an increasing frequency,magnitude and duration under the dual effects of eutrophication and climate change.A close coupling of this phenomenon to rapid sedimentation of algal detritus in the benthic ecosystem were investigated.Year after year,the accumulation of algae debris becomes deeper and deeper.With the continuous rise in temperature and the increasing decomposition rate of loose algae debris on the surface layer,the abundance and composition of algae-derived bioclastic accumulation in lake sediments varies considerably with local conditions,exerting different effects on the biogeochemical processes in lake ecosystems.Firstly,the aggregation and decomposition of algal detritus can potentially change the biological and physical characteristics of the benthic environment,such as the p H,redox state（Eh）and dissolved oxygen（DO）of the water-sediment profiles,which can significantly affect nutrient cycling.Secondly,because algal detritus contains a large amount of nutrients,such as carbon and nitrogen,it serves as a mobile nutrient reservoir.Burial and decomposition of massive algal detritus can fuel carbon and nitrogen sedimentation,which may in turn spur algal blooms that can either be promptly mineralized or assimilated to enter the food web.Thirdly,massive sedimentary algal detritus in static and highly eutrophic water bodies,in combination with excessive nutrient loading,may result in black water blooms that are associated with the release of a large amount of odorants,including dimethyl sulfide（DMS）,dimethyl disulfide（DMDS）,dimethyl trisulfide（DMTS）,2-methylisoborneol（2-MIB）,geosmin,β-ionone andβ-cyclocitral.Therefore,the impact of the formation and development of benthic bioclastic mat should not be ignored.Whether this process will accelerate the succession of lakes needs further study.The achievement of this study can provide supportively theoretical and practical basis for the regulation and improvement of water quality in shallow eutrophic lakes.