Atmospheric Wet Deposition Characteristics Of Nitrogen And Phosphorus Nutrients In Taihu Lake And Its Influence On Phytoplankton

Atmospheric wet deposition is one of the most important ways which transport nitrogen and phosphorus into water system(like lakes).These may change the lakes nutrition structure,the composition of the phytoplankton community and bring important influence to the change of lake ecosystem.This article focused on the influence which atmospheric wet deposition bring to the Taihu water ecological system.First of all,we examined the wet deposition characteristics of nitrogen and phosphorus nutrients of Taihu Lake watershed.We conducted a one-year wet deposition monitoring survey from January,2017 to December,2017.We collected precipitation samples from eight sampling site,and measured the physical and chemical indicators of the wet precipitation samples in laboratory.We discussed the composition of the nitrogen and phosphorus wet deposition around Taihu Lake,their temporal and spatial distribution characteristics,deposition rates and their contributions to Taihu Lake.Secondly,to explore the impact of short-term wet deposition on taihu phytoplankton and the relationship between phytoplankton and environmental factors,we conducted a intensive experiment in west littoral zone of Taihu Lake.Then we analyzed the influence of short-term wet deposition on phytoplankton of Taihu Lake.Finally,we designed the experimental enclosure in west littoral zone of Taihu Lake and conduct some experiments in them.During the time that algal blooms happened,we captured typical rainfall and analyzed the changes of Taihu water temperature,nitrogen and phosphorus concentration,variations of algae species,discussed the impact of the wet deposition on water quality and algae community.The main conclusions are as follows:(1)Nitrogen nutrient was a main component of atmospheric wet deposition around Taihu,phosphorus nutrient was relatively small.Total nitrogen concentration in wet deposition was about several dozen times than that of the total phosphorus,which could be speculated that a massive amount of rain may change nutrient composition of the lake,and have a certain influence on phytoplankton.In wet deposition,different forms of nitrogen and phosphorus nutrient concentration changed with the seasons change.Nitrogen nutrient concentration in spring and summer were low,and relatively high in other season.Different forms of nitrogen and phosphorus nutrient deposition rate were high in spring and summer,and relatively low in other season,which in contrast to their concentration change and consistent with the seasonal change of the rainfall.Which indicated that nitrogen and phosphorus nutrient wet deposition rate were mainly influenced by the rainfall.Nitrogen nutrient concentration in the northwest of Taihu is higher than that of the southeast.Phosphorus nutrient concentration in the south of Taihu is higher than that of the north.The spatial distribution of wet deposition rate of nitrogen and phosphorus nutrient were consistent with spatial distribution of their concentration,these might be related to the local climate,economic development and industrial distribution.The annual settlements of total nitrogen and total phosphorus in the precipitation were 7693 t and 200 t,accounted for 33%and 16.7%of the load of rivers into the lake at the same period.The settlement decreased significantly than ever,but the ratio load of rivers into the lake rose.(2)Wet deposition had significant impacts on the water of Taihu Lake,it could change the water temperature in a short time,and added a certain amount of nutrients to Taihu at the same time,which promoted the growth of the chlorophyll a content.According to the influence of environmental factors on the chlorophyll a content,The orders of their contribution to chlorophyll-a content variation were in sequence of water temperature,wind speed and nitrate.Chlorophyll-a content increased with water temperature(>20~oC).Wind speed was one of the key factors which influenced chlorophyll-a content,low wind speed(<3 m/s)could promote algae breeding and make the algae gather,these led to the algae blooms and high content of chlorophyll-a.Nitrate concentration decreased significantly during the period of algae bloom.Temperature and wind speed were key environmental factors influencing chlorophyll-a content.To some degree,the research can be used for prediction and forewarning of algal bloom in west littoral zone of Taihu Lake.(3)Wet deposition had significant effects on water quality of Taihu Lake.Ammonia nitrogen and nitrate content in enclosure which accepted wet deposition rose obviously,because ammonia nitrogen and nitrate content in wet deposition is higher than that of Taihu Lake.When the rain entered Taihu Lake,ammonia nitrogen and nitrate content of Taihu Lake were enhanced by the rain.Wet deposition had significant effects on algae in Taihu Lake.On the one hand,the precipitation process had a certain impact to the algae,influence the distribution of algae in vertical direction,on the other hand,wet deposition reduced the temperature of the water,which had a significant effect on algae growth.That might lead to the change of the dominant algae species,even the change of the phytoplankton community structure.In general,wet deposition has the most significant impact on cyanophyta.After the rain,cyanophyta biomass recovered quickly with the increase of temperature.At the same time,the rain brought some nitrogen and phosphorus to Taihu Lake,which promoted the growth of cyanobacteria.