Seasonal Nutrient Limitation And Short-term Early Warning Decision Support System Of Cyanobacterial Blooms In Lake Taihu

Regulation and control of cyanobacteria blooms based on the study of nutrient kinetics was always the hot topic in the lake management field.Controlling nitrogen(N),phosphorus(P)or N&P,which was always a bone of contention in limnology research.The "phosphorus reduction paradigm" was practiced in past many years,which was not control cyanobacteria bloom in some large shallow lakes in the worldwide.For the implementation of N/P regulation in large shallow lakes,the in-depth study of seasonal nutrient limiting process was always ignored,especially the drive and regeneration mechanism of nutrient in cyanobacteria bloom period.Since the middle of 1980s,the large shallow lake-Taihu,has been undergoing heavy eutrophication,especially the harmful cyanobacteria blooms occuring nearly every year in Meiliang Bay at the north end of the lake,which always troubled the decision-makers of Lake Taihu.Recently,the finding of seasonal nutrient limitation transforming in the Meiliang Bay provided the new perspective for the study of cyanobacteria bloom and nutrient kenetics.Therefore,based on the continuous limnology data sets in Lake Taihu,the present study showed the spatial-temporal distribution of algal communities and nutrient(nitrogen and phosphorus),constructed the responsive model related to cyanobacteria bloom and seasonal nutrient limitation,demonstrated the long-term nutrient limitation pattern in the Meiliang Bay,and explored the seasonal response relationship between the nutrient limitation and algal biomass.On the basis,to further reveal the formation process and key mechanism of nutrient limitation in the more concerned cyanobacteria bloom period,the study constructed hydrodynamic-water quality-algal growth kinetics integrated model,and put forward N/P reduction strategy to control cyanobacteria bloom by setting different scenarios.In addition,along with the long-term nutrient regulation,this study conducted the short-term early warning for the heavy cyanobacteria bloom areas and water source field based on the integrated model of Lake Taihu.The above studies expected to provide scientific references and decision supports for the nutrient regulation and short-term forecast of cyanobacteria bloom in Lake Taihu,which can help the lake management decision-makers to put forward new thinking on the more targeted regulation solutions for the cyanobacteria blooms.The main contents and results are as follows,The spatio-temporal distribution of environmental factors,chlorophyll-a(Chla),microcystins(MCs)and algal community abundance in a shallow lake,Lake Taihu(China),were investigated from 2009 to 2011 on a monthly basis at nine sampling stations.TN and NO3-were present at the highest levels in late spring and early summer and the lowest levels appeared in autumn.However,the highest levels of Chl-a,total phosphorus(TP)and soluble reactive phosphorus(SRP),were showed an opposite seasonal changes.The phytoplankton communities consisted of Cyanbacteria,Chlorophyta,Bacillariophyta,Euglenophyta and Cryptophyta in Lake Taihu.The algal communities shifted from the dominance of Cyanbacteria(Microcystis),Chlorophyta and Bacillariophyta to dominance of Microcystis from winter-spring to summer-autumn.Additionally,the distribution of MCs was northern lake areas>western lake areas>central lake areas>macrophyte-dominated areas.During the study period,the maximum MCs concentration reached 2.75 ±?.27 ?g/L in the bloom period in the northern lake areas,which is more than two times the safety limit of 11.g/L MCs required for drinking water and threaten the water source of Gonghu Bay.Therefore,the part of study provided detailed data materials and scientific basis for exploring the nutrient limitation and regulation of cyanobacterial bloom in Meiliang Bay.To elucidate the potential relationship between seasonal nutrient limitation and phytoplankton proliferation,a 20-year(1991-2012)time series of nutrient limitation in Meiliang Bay was analyzed for deviations between trophic state index(TSI)parameters.Results showed that patterns of nutrient limitation in Meiliang Bay were distinctly seasonal,where phytoplankton growth was generally phosphorus(P)-limited in winter and spring,but nitrogen(N)-limited mainly occurred in summer and fall.The resulting variation in external N and P inputs from the tributaries of Meiliang Bay as well as internal P release from the sediment caused winter and springtime P limitation and summer and autumnal N limitation in Meiliang Bay.Using routine monitoring data,a generalised additive model(GAM)with time and deviations between trophic state indexes for nitrogen and phosphorus(TSIN-TSIP)as explanatory variables was used to explore which nutrient was responsible for limitation of phytoplankton chlorophyll-a(Chl-a)in different seasons.Surprisingly,the model revealed a weak N limitation(TSIN-TSIP =-10)corresponded to peak values of Chl-a in summer-autumn season.The shift of nutrition limitation during winter-spring would partially explain high values of Chl-a throughout 1996.This study suggests that seasonal patterns of nutrient limitation must be considered to develop effective management measures to control cyanobacterial blooms.From the perspective of ecological system in Lake Taihu,the present study constructed hydrodynamic-water quality-algal growth kinetics integrated model,calibrated and verified the dynamic process of cyanobacteria bloom and nutrient kinetics in summer-autumn season,and demonstrated distinctly nitrogen-limitation fluctuation in cyanobacteria bloom period.For the further study,this study set four different scenarios according to the external inflow transferring of nutrient and internal nutrient release from the sediment,and revealed the key mechanism concentrated on the shortage of nitrogen source and complementary phosphorus source in the nutrient limiting process.The forming of nitrogen-limitation main result from nitrogen and phosphorus consumed largely due to the cyanobacteria bloom,denitrification in the water column,and phosphorus released from sediment.On the bisis,aim to the recovery of the water quality in Lake Taihu,we put forwad more targeted nutrient reduction strategy from the seasonal perspective.In the winter-spring season,strengthen to external inflowing phosphorus reduction can inhibit the scale of cyanobacterial recovery.Meanwhile,the reduction of nitrogen source in the watershed can cut down the nitrogen load inflowing into the lake,which can inhibit the degree and frequency of cyanobacteria bloom;In the summer-autumn season,strengthen to the recution and dilution of nitrogen may be the more effective and economic approch to control the cyanobacteria bloom.Modelling analysis and exploring approach to these process mechanisms provided a new perspective for the study of the response relationship between cyanobacteria bloom and nutrient kinetics.In addition,the proposed seasonal nutrient reduction strategy will be a detailed theoretical complement for the nutrient regulation and control of cyanobacteria bloom in the large shallow lake-Taihu.Based on forecast principle and procedure of cyanobacteria bloom,the present study presented prediction system integrated into decision support system platform.We constructed the hydrodynamic-water quality-algal growth kinetics integrated model and developed visual decision support system platform of cyanobacteria bloom forecasting.In order to forecast the short-term(3 days)cyanobacteria bloom in Lake Taihu,we collected data sets,digged the data informatics,configured the integrated model and estimated the predicted results.Meanwhile,we established the more visual indicators of peak value and areas,and conducted the study of cyanobacteria bloom accumulated areas in different wind direction.In addition,we integrated these modules into the decision support system platform and achieved the data sharing and uniqueness,rapid data accessing and query,and implemented forecasting of cyanobacteria bloom in Lake Taihu.In the present study,we analyzed the advantage and shortage in the current forecasting procedure of cyanobacteria bloom,and further discussed technology of forecasting model,data monitoring and digging,calibration and verification of model,forecasting estimation and information issues,which heralded more accurate and effective early-warning of cyanobacteria bloom in the future.Therefore,the present study may provide ideological and methodological references for the long-term nutrient regulation and short-term early-warning of cyanobacteria bloom in Lake Taihu.