Study On Nitrogen And Carbon Cycle Of Lakes In The Middle And Late Stages Of Algal Blooms

Cyanobacterialblooms have afflicted lakes all over the world over the past few decades.While tremendous efforts have been made to understand the effects ofnutrients on the abundance and composition of phytoplankton in lakesand the potentially harmful effects of cyanobacterial blooms,it is less well known whetherphytoplankton can change the nutrient dynamics in lake ecosystems.Therefore,the mesocosmsexperimentswerecarried out on the north shore of Lake Taihu,in combination with molecular biology methods,to investigate l)theresponse of algal bloom to exogenous inputs of nitrateor ammonium,and 2)the ultimate fates of those assimilatednitrogen by algae,the specific work is as follows:1.In the systemwithexogenous nitrate added,treatment with cyanobacteria led toremoval of about 3.62 mg N L-1 total nitrogen,40%ofwhich was organic nitrogen,indicating a nitrogentransformation and removal mechanism was present inthe system.Variations in the biogeochemical propertiessuggested that remineralization and coupling nitrificationand denitrification by epiphytic and pelagicmicroorganisms was the primary pathway throughwhich organic nitrogen was removed.The algal-bacterial symbiotic system reduced the cycling pathway of nitrogen in lake water.The resultsrevealed that algal blooms can acceleratenitrogen removal efficiency,which may be the primaryreason thatnitrogen is limited in summer in Lake Taihu.2.In the addition of exogenous ammonia nitrogen system,algae were dominant in competition with epiphytic bacteria to ammoniumin the early stage of algae growth.However,nitrifiers gained a competitive advantage for ammonium over cyanobacteria with time.Nitrate concentrations in the samples collected at dawn were always higher than those collected later in the morning of the same day or the next day,indicating that ammonium was oxidized to nitrate during the day and subsequently reduced at night.When additional ammonium was depleted,internal remineralization became the primary nitrogen source for coupled nitrification and denitrification,leading to efficient nitrogen removal by the elimination of previously assimilated nitrogen.There were no significant differences in the nitrate concentrations between the control and treatments with added cyanobacteria(P<0.01),suggesting that epiphytic coupled nitrification and denitrification would occur in many regions of Lake Taihu,where relatively low cyanobacterial biomass levels occur in summer.Thus,the remineralization-nitrification-denitrification process that occurs in cyanobacterial aggregates may be the cause behind high denitrification efficiency under low nitrate concentrations during the summer in Lake Taihu.3.In the system withoutexogenous nitrogen added,the concentration of DOC increased continuously and reached a peak value of 72 mgL-1 at day 15,which was 12.5 timeshigher than that in LakeTaihu.There were significant differencesbetween the intensity of 3 fluorescent peaks and Chla(P<0.01),which indicated that the growth and degradation of cyanobacteria are important sources of the dissolved organic matter in Lake Taihu.The UV spectra of E3/E4 were greater than 3.5 at the middle and early stage of the experiment,and decreased at the end of the experiment,indicating that the humification degree is low in the early and middle period of cyanobacterial bloom.