Nitrogen Removal,Fixation And Eutrophication Management Of Shallow Lakes

Eutrophication and harmful cyanobacterial blooms threaten the water resources all of the world.In the lake eutrophication management,there is a great controversy on whether the nitrogen should be controlled.The main argument against the dual nutrient control of eutrophication is that nitrogen fixation can compensate for the loss of nitrogen.Therefore,it is necessary to study the main factors affecting the nitrogen fixation and removal capacities in lakes.Due to the difference of climate and human influence,there are also great differences in water quality（such as water temperature（WT）and ratio of nitrogen to phosphorus（N:P））and hydrological conditions（such as lake water residence time（LWRT））.Numerous studies show that low N:P can easily induce the growth of cyanobacteria and enhance the nitrogen fixation capacity.However,the effect of WT and LWRT on nitrogen fixation and the balance between nitrogen fixation and removal remain unclear.In this study,30 shallow lakes in eastern plain of China were selected as our study area.The concentrations of dissolved nitrogen gas（N₂）and Argon（Ar）in water samples were measured by membrane inlet mass spectrometer based on N₂:Ar method.The nitrogen fixation and removal capacities were quantitatively analyzed,and the effects of WT and LWRT on the nitrogen fixation capacity were discussed.Meanwhile,taking Lake Taihu as an example,undisturbed sediment cores were collected in different lake areas in spring and summer.The purpose is to study the regional differences of nitrogen removal capacity and its influencing factors,and provide suggestions for lake eutrophication management.In addition,a large number of studies show that the nitrogen input in the upstream rivers of lakes is an important source of nitrogen pollution.Lots of studies focused on the rivers with many muds,but for rivers with no mud,the internal nitrogen cycle,exogenous nitrogen pollution and its potential impact on downstream lakes are not clear.Therefore,an aerobic river（River Xin’an）,which has no mud,was selected as the study area.The water samples were collected and then taken back to the laboratory to analyze the physical and chemical parameters and the concentrations of dissolved N₂and Ar.The effects of exogenous nitrogen pollution and the internal nitrogen budget（nitrogen fixation and removal）on nitrogen loading were discussed.The main results are as follows:（1）WT has a great influence on the capacities of nitrogen fixation and removal.During low temperature period,the nitrogen removal capacity of the lake was far higher than that of the nitrogen fixation capacity,resulting in the supersaturation of dissolved N₂in the lake water;During high temperature period,the growth of nitrogen-fixing cyanobacteria and the decrease of dissolved inorganic nitrogen concentration shift the lakes to be dominated by nitrogen fixation,and the dissolved N₂in the water tends to be unsaturated.This may be one of the important reasons for the increase of TN concentration with the increase of WT in many lakes of the eastern plain of China.For lakes with high temperature,controlling nitrogen is useless.（2）LWRT significantly affected the nitrogen fixation and removal capacities.Short LWRT can significantly inhibit the growth of cyanobacteria.In addition,for lakes with short LWRT（such as Lake Dongting and Poyang）,the N:P,dissolved inorganic nitrogen and suspended solids are generally high.These factors can significantly inhibit the growth of nitrogen fixing cyanobacteria,and shift the lakes to be dominated by nitrogen removal.For such lakes,if the lake is limited by nitrogen,nitrogen control is feasible.（3）The results showed that the spatial distribution of denitrification and anammox rates in different areas of Lake Taihu in spring was quite different,the denitrification rate was（27.74±8.45）～（142.43±35.54）μmol·（m²·h）^-1,the anammox rate was（2.35±1.06）～（17.95±8.66）μmol·（m²·h）^-1,the contribution of anammox to the total nitrogen removal was relatively low and was（7.82±1.71）%～（11.20±1.53）%.During summer,the nitrogen removal rate of Zhushan Bay was the highest,and the denitrification and anammox rates were（165.68±62.14）and（33.56±10.66）μmol·（m²·h）^-1,respectively.The contribution of anammox was（16.85±1.78）%.The nitrogen removal rates of other lakes were relatively low,and there was no significant spatial difference.Generally,the denitrification rate in summer was generally lower than that in spring,while the anammox rate did not decrease significantly compared with that in spring.Statistical analysis showed that denitrification and anammox rates were significantly correlated with nitrogen concentration（P<0.01）,indicating that nitrogen concentration was the main controlling factor that caused the different nitrogen removal rates among different lake areas.These results indicate that the increase of temperature may not be the main factor that affect the nitrogen removal capacity,the nitrogen removal rate may also be limited by the concentration of substrate nitrogen.Therefore,controlling nitrogen in lakes with low nitrogen concentration should be cautious,because it may induce the growth of nitrogen-fixing cyanobacteria.（4）For River Xin’an,the nitrogen fixation capacities of all sampling sites were much higher than the nitrogen removal capacities,indicating that nitrogen fixation has a potential impact on nitrogen loading.The TN concentration increased significantly after flowing through the city,indicating that nitrogen pollution was mainly caused by human activities rather than nitrogen fixation.The reason of low nitrogen removal capacity is that the river is in aerobic state,and there is no mud to provide suitable anoxic environment for denitrifying bacteria.Furthermore,the low concentration of organic carbon in the water and the lack of mud indicate that the sediment could not provide sufficient carbon source for denitrification process.In addition,the TN concentration at the downstream of river was much higher than that in the lake,which indicates that the river could have a significant impact on the nitrogen pollution of the downstream lake.This study revealed the effects of WT and LWRT on nitrogen fixation and removal,and found that there were great differences in removal capacities among different areas even in a single lake due to different water qualities.Through the balance between nitrogen fixation and nitrogen removal,we discussed whether it is necessary to control nitrogen in lake eutrophication management,and if not,what types of lakes do not need to control nitrogen.In addition,by studying the process of nitrogen budget of an upstream river,it is found that exogenous pollution has a great impact on the nitrogen pollution,which can significantly affect the water qualities of the downstream lakes.