Spatiotemporal Dynamics Of Cyanobacterial Bloom In Erhai Lake And Its Response To The Adjustment Of Planting Structure

Under the global trends of lake eutrophication caused by human activities,the over growth of cyanobacteria as dominant species in a wide range of inland waters leads to aquatic ecosystem imbalance and frequently cyanobacterial blooms.Especially in the watershed where the agricultural production dominates,the eutrophication caused by agricultural non-point pollution for drinking water sources in lakes and the occurance of cyanobacterial bloom are particularly prominent,which has threatened the lake environment security,and the treatment efforts needs to be promoted.Therefore,how to coordinate agricultural production and water environment protection is an important problem to realize the sustainable development of global watershed.In the early stage of eutrophication,it is urgent to control the non-point source pollution of Erhai Lake in Yunnan Guizhou Plateau.Local government has implemented a series of rescue protection policies including adjustment of planting structure in the basin,especially the compulsory prohibition of planting crops with large amount of water and fertilizer represented by garlic in the whole basin which has attracted widespread public attention.However,it is rare to systematically analyze the spatiotemporal dynamics of cyanobacterial bloom in Erhai Lake and its influencing factors,and to explore the relationship between cyanobacterial bloom and the adjustment of planting structure in the basin.These three key issues are important for the water environment protection policy being implemented in the local area.This study systematically obtains long-term dataset indicating cyanobacteria by satellite remote sensing,explores key environmental factors inducing phenological changes of cyanobacteria bloom,analyzes the spatiotemporal distribution of crop types in the basin and its influence on cyanobacterial bloom during the adjustment of planting structure action,so as to explore the coordinated management of "Basin-Lake" for water environment protection,agricultural planting structure transformation and provides scientific basis and information support for "Basin-Lake" collaborative management and the ecological and efficient industrial goal.The main research results are as follows:The long-term dynamic characteristics of cyanobacteria were indicated by the ratio of cyanobacterial biomass to total phytoplankton biomass and phycocyanin(PC)retrieved by MERIS and OLCI.The spatiotemporal variability of cyanobacteria was analyzed with an empirical orthogonal function over the period 2003-2011 and 2016-2019,and the potential factors fueling cyanobacterial bloom were investigated.The results indicate that there is a shift of cyanobacterial aggregation from northern and central regions to the northern bay regions,and alleviated PC level in summer and elevated PC level in winter were observed during 2016-2019.The nutrient limitation condition in summer for cyanobacteria proliferation characterized by total nitrogen(TN)and total phosphorus(TP)colimitation during 2003-2011 has transited to TP limitation condition during 2016-2019,while enhanced TP limitation condition in winter was observed in the latter period 2016-2019.Additionally,lake water level,TN/TP ratio,and air temperature were identified as potentially significant modulating factors.The combined modulating factors involving water level and TN/TP ratio has shifted to single factor TN/TP ratio in summer,while air temperature turns out to be a significant modulating factor in winter instead of water level.Our results reveal that given the effective control of nitrogen pollution,the implementation of phosphorus limitation strategy and suitable water level manipulation is needed to confirm the sustainability of Lake Erhai,particularly under a consistent global warming scenario.This study explored the spatio-temporal characteristics of phenological metrics and coverage extent for surface cyanobacterial bloom,based on phycocyanin pigment series retrieved from satellite and investigated the bloom responses to environmental forces.The results showed that higher intensity and earlier onset bloom with a larger coverage extent mainly occurred in the northern region.We identified three distinct cyanobacterial bloom growth patterns with large inter-annual variability in their bloom initiation timing(BIT).The earlier BIT is highly linked to elevated total nitrogen level and decreased wind speed.The bloom amplitude is mainly related to summer TN in2003-2011,while associated with winter TN/TP ratio during 2016-2019.Additionally,we found direct evidence linking agricultural topdressing and the BIT of a typical autumn bloom.After one week for the topdressing practice,the BITs lagged by approximately average 23 days,coinciding with high rainfall intensity.Our research demonstrates the topdressing activity should be reduced through agricultural planting adjustments.High TN and TP loss crops should be prohibited;planting adjustments should decrease high TP loss crop plantings in response to regional environmental forces.Landsat7/8 and sentinel1/2 images were used to monitor the agricultural planting structure(planting intensity and crop types)in Erhai Lake Basin(ELB),and then the export coefficient model was used to calculate the nitrogen and phosphorus load for summer and winter crops.According to the phenological characteristics of crops,a random forest classification model was developed to classify the crop types in the basin.The results showed that the main rice(summer crop)planting in the northern ELB did not change significantly from 2017 to 2020,but changed greatly in the northwest ELB,from maize planting to other crops planting.The distribution of winter crops in ELB has changed significantly.After strict adjustment of planting structure,since 2018,there has been no garlic planting represented by using large amount of water and fertilizer in ELB.Instead,it has been transferred to planting crops such as ivy mosses and flowers which need low amount of water and fertilizer.However,from the perspective of the whole basin,the cultivation of winter crops is mainly broad bean.The spatiotemporal distribution of nitrogen and phosphorus load showed that the total nitrogen load of winter crops in the watershed decreased significantly,but the total phosphorus load did not change significantly;the nitrogen and phosphorus load of summer crops,especially in the northern ELB,decreased significantly in 2019 and 2020.The average discharge coefficient of total phosphorus of winter crops is twice as high as that of summer crops.Therefore,the total nitrogen emission of winter crops decreased significantly,while the total phosphorus emission was still an important problem.In this study,the spatial effects of the adjustment of planting structure in the watershed on the spatial distribution of cyanobacterial blooms were studied by using eigenvector spatial filtering model and geographic detector.The results showed that the effect of the spatial distribution of nitrogen and phosphorus load for summer crops on autumn PC distribution was lagging behind,and there was a significant spatial relationship between autumn PC distribution and the spatial distribution of nitrogen and phosphorus load for winter crops.The effect of nitrogen and phosphorus load distribution for planting structure on autumn PC distribution is statistically obvious,which leads to the occurrence of typical autumn bloom.Especially for cyanobacterial blooms in wintertime 2017,the nitrogen and phosphorus load for winter crops in the the watershed had a great contribution to the increasement of winter PC concentration.In addition,based on the our research,the collaborative management scheme of "BasinLake" is proposed,combined with site observation and remote sensing integrated monitoring covering "Basin-Lake",so as to realize the comprehensive evaluation of the spatial pattern of nitrogen and phosphorus load from cropland through surface runoff and cyanobacterial bloom,and provide information support for the sustainable development of ELB.Finally,the feasibility of introducing nitrogen and phosphorus load for crops in the basin and using naive bayesian model to predict the risk probability of cyanobacterial bloom in Erhai Lake was explored based on the long-term phycocyanin data estimated from satellite images.