The Seasonal And Spatial Variability Of Ammonium Uptake And Its Influential Factors In A Mountainous Catchment

With the rapid development of economy,human activities have emitted a large number of nutrients such as nitrogen and phosphorus into rivers and lakes,leading to frequent pollution incidents such as eutrophication of water bodies.Natural rivers,especially headwater streams can uptake nutrients with the aquatic ecosystem.However,this process could be affected by the spatial heterogeneity of catchment and the seasonal variations of runoff and aquatic ecosystem activity.The impacts of spatial and seasonal variation on nutrient uptake processes haven’t been fully understood.This research took the Xiaogang Catchment,a headwater stream in the southwestern mountainous area of Zhejiang Province,as the study area.At the spatial scale from upstream to downstream and at seasonal scales within a year,thirteen nutrient addition experiments were conducted to quantitatively calculate the ammonia nitrogen retention rate of rivers at different locations in the catchment during different seasons.Catchment characteristic data,including land use,hydrology,water quality,and aquatic ecology were also collected.The distribution characteristics and influential factors of spatial and temporal ammonia nitrogen uptake capacity in the catchment were investigated.The main results are as follows:(1)The study captured significant seasonal variations in the capability of ammonia uptake:the uptake capacity increased from early spring,gradually weakened after reaching its peak in September at the end of summer,and reached a bottom in January and winter.Subsequently,the uptake metrics began to rise with warming temperature,forming a periodic cycle throughout the year.At the seasonal scale,the uptake capacity was significantly affected by the ambient ammonia concentration:the ambient ammonia uptake rate and maximum ammonia uptake rate are positively correlated with the ambient ammonia concentration.(2)The uptake capacity of ammonia in the catchment also changed with spatial scale,and the uptake capacity of different experiment sites was more affected by discharge:the ambient uptake rate,ambient uptake velocity and maximum uptake rate of ammonia were positively correlated with flow discharge significantly.In the same stream,the ambient uptake length increased with the discharge,which was consistent with the trend predicted by regression models based on literature data.However,in different streams within the catchment,the measured ambient uptake length increased and then decreased with the discharge,indicating that the traditional regression model will underestimate the uptake capacity under larger discharge conditions downstream.(3)The impacts of the discharge and ambient ammonia concentration on uptake capacity were different at seasonal and spatial scales.Based on the characteristics of water ecological environment,land use,and other data,it was found that the uptake rate of rivers under the ambient concentration of ammonia increased with discharge(especially at spatial scale),because discharge was positively correlated with dissolved organic carbon(DOC)and stream chlorophyll a concentration.The impact of discharge on uptake capacity at seasonal scale was less influential than the ambient ammonia concentration.Because there’s a marginal effects in the wet perimeter increase caused by the increase of flow,resulting in the limited growth area of benthos.Therefore,the impact of discharge on uptake capacity in the same river channel is limited.Additionally,the uptake temperature had an important impact on the growth conditions of algae(using chlorophyll a concentration as an indicator),and the optimal uptake temperature in this study area was about 17℃.Moreover,land use had a dominant impact on stream ion concentrations.