Patterns Of Phytoplankton Phenology And Its Response To Lake Surface Water Temperature In Lake Taihu Based On MODIS Data

Phytoplankton phenology is able to reflect the growth and changes of phytoplankton and the changes of lake ecosystems.Water temperature,nutrient concentration and other factors have important effects on phenology.For Lake Taihu with high nutrition level,the effect of nutrients on phytoplankton tends to be saturated,and the effect of water temperature is more significant.Therefore,the monitoring of phenology and water temperature is of great significance for understanding,controlling and improving the Taihu ecosystem.Based on MODIS remote sensing data from 2003 to 2018,this study extracted the annual phytoplankton phenology indicators of Lake Taihu and calculated the long-term serial of lake surface water temperature(LSWT).The time trends and spatial distribution characteristics of each phenology indicator and water surface temperature were analyzed.From the perspectives of field,areas,and pixels,and integrating information from both time and space,the response characteristics of phytoplankton phenology to changes in water surface temperature were analyzed.The response of phytoplankton growth in different Lake Taihu areas to changes in water surface temperature was revealed.Results try to provide technical support for prediction of phytoplankton growth trends and bloom occurrence.The paper mainly draws the following conclusions:(1)Temporal and spatial changes of phytoplankton in Lake TaihuFrom 2003 to 2007,the number of blooms and peak Chlorophyll a(Chla)concentrations continued to increase,and the start time of blooms generally advanced;from 2008 to 2018,the number of blooms and peak Chla concentrations first decreased and then increased,and the bloom time,the peak time and peak Chla was delayed and then advanced,reaching the extreme value in 2017;the average duration and the total duration of the blooms fluctuated and did not show a significant trend.The high values of the number of blooms,the peak Chla concentration,and the total duration of blooms are generally distributed in the western coastal areas,followed by Meiliang Bay and Zhushan Bay,and the values are lower toward the center of the lake;phytoplankton growth start time and peak Chla appears more complicated in time and space,and the coastal area is relatively early.(2)Characteristics of interannual variation of phytoplankton phenology in different lake areasBased on the phenological change characteristics,Lake Taihu can be divided into four types: ?,?,? and ?.Type I area is mainly located in Gonghu Bay,the eastern coast and the central part of Lake Taihu.The annual Chla concentration in this area is low and fluctuating gently.The number of blooms is the lowest,the time is the latest,and the duration is the shortest.Type II area is mainly distributed on the western coast of Lake Taihu,the annual Chla concentration in this area changes drastically,with the most frequent blooms,the earliest onset,and the longest duration.Type III areas are mainly distributed in Meiliang Bay,Zhushan Bay,and their exits.Type IV areas are mainly located on the southern coast and central Taihu.The number of blooms,average duration of blooms and total blooms remained basically stable in area ?,and increased in some waters in areas ?,?,and ?,and the area ratio showed ?> ?> ?.The phytoplankton growth initiation time and peak Chla appeared in 50% of the waters in each area in advance;the peak Chla concentration increased in most waters in the four types of areas.(3)Spatiotemporal evolution of lake surface temperatureFrom the perspective of the inter-annual change trend,from 2003 to 2008,Taihu LSWT first fell and then rose,reaching a local peak in 2007,and an overall upward trend in 2009-2018,reaching a peak in 2017.From the perspective of time series composition,the relative contributions of seasonal components,irregular components and interannual variation components of Taihu LSWT time series account for 95-97%,2.57-5.10%,and 0.20-1.01%,respectively.Regular periodic changes dominate.The relative contribution rate of irregular components in Meiliang Bay and Zhushan Bay and the corresponding bay exits is relatively high;the composition of interannual variation presents the characteristics of high open areas and low coastal areas.In terms of spatial structure,the LSWT spatial structure of Lake Taihu is simple in winter and the most complicated in summer.The interannual change trend of LSWT in different regions of Lake Taihu was basically the same in each month,with a significant upward trend in summer,and the interannual change in the remaining months fluctuated and trend was weak.At the same time,there were differences in the inter-annual change sensitive areas of LSWT in each month,and the distribution of sensitive areas in winter was scattered;in spring,it was mainly concentrated in the western coast of Lake Taihu,Meiliang Bay,and Zhushan Bay;the overall change of Lake Taihu in summer and autumn was relatively consistent,and there were no significant sensitive areas.(4)Phytoplankton phenology in response to lake surface temperature changesAt the field scale,the response of Chla concentration to LSWT is relatively consistent in space,mainly showing a positive correlation.The rise of LSWT in Lake Taihu will promote the growth of phytoplankton.The Meiliang Bay and Zhushan Bay exits and western area are sensitive area.At the lake level,the growth of phytoplankton is significantly promoted by LSWT in type II lakes with high nutrient levels.And the increasing trend of inter-annual changes in LSWT has a more significant impact on the increase of phytoplankton growth and biomass in this area.However,in Type I lakes with low nutrient level,the impact is weakened.At the pixel scale,there is a dynamic seasonal change in the relationship between Chla concentration and LSWT,and the negative correlation is generally lower within the year,and only a high negative correlation occurs during spring and summer.The positive correlation between the two is the highest in winter,and the positive correlation is absolutely dominant,and the dominant position is less affected by the movement of nutrients in the water body.But the positive correlation in summer is the lowest,the dominant position of it is the weakest,and it is vulnerable to the movement of nutrients in water.This indicates that the effect of LSWT on phytoplankton growth in winter is stronger than that of nutrient,and that the effect of nutrient on phytoplankton growth in summer is more significant than that of LSWT.