Effects Of Salinity On The Determination Of Nitrogen And Phosphorus In Coastal Waters And The Effects Mechanism Of Nitrogen And Phosphorus Release In Sediments

Nitrogen and phosphorus are essential nutrients for the growth of aquatic organisms,and they are also important indicators to evaluate the quality of water quality.However,the excess of nitrogen and phosphorus will lead to water eutrophication,resulting in rapid reproduction of plankton,reduction of dissolved oxygen and increase of turbidity in the water.It has caused great damage to the aquatic ecosystem.With the increasingly strict municipal sewage discharge standards,the exogenous input of nitrogen and phosphorus in coastal waters has been controlled effectively.However,the release of nitrogen and phosphorus from endogenous sediments has become the main cause of water eutrophication in coastal waters.As a result,ecological disasters such as red tide and green tide occur frequently in coastal zones.The task of ecological protection in the coastal zone is complex and arduous.Therefore,the accurate determination of nitrogen and phosphorus concentrations in coastal waters is of great significance to the sustainable development of coastal zones.According to different water bodies,China has set the national standard methods for nitrogen and phosphorus determination as“freshwater method”（applicable to surface water）and“seawater method”（applicable to seawater）.It should be noted that the coastal water and sediment are in the alternating zone of land water and sea water,and their salinity（SAL）changes obviously.The determination method for the nitrogen and phosphorus in the water of salinity transition area is not clear.Moreover,the research on the influence of salinity on the measurement results of nitrogen and phosphorus is not discussed.Therefore,in this paper,the effect of salinity on the accurate determination of nitrogen and phosphorus in coastal waters was explored,and the basis for the selection of accurate nitrogen and phosphorus determination methods was provided.Furthermore,on the basis of the above,the release mechanism and influencing factors of nitrogen and phosphorus in the sediments of heavily polluted rivers in the coastal zone were studied.It can be expected that this study will provide a theoretical basis for the prevention and control of eutrophication in coastal waters.The contents of this thesis are as follows:1.Determination method and accurate determination of nitrogen and phosphorus in coastal waters.Accurate determination of nitrogen and phosphorus in coastal zone plays a vital role in land-sea coordination and coastal zone management.The purpose of this study is to explore the influence of salinity on determination accuracy for different forms of nitrogen and phosphorus and to clarify the basis for the selection of nitrogen and phosphorus determination methods in salinity transition zone of estuarine and coastal waters.The concentration of different forms of nitrogen and phosphorus in both simulated seawater samples and natural seawater samples were analyzed by the national standard fresh water and seawater methods.The results indicated that the salinity had a certain influence on the determination results using fresh water and seawater methods.To determine the TN,a SAL of 5.0 was the threshold for selecting either fresh water method or seawater method.And for determining the NO₂^-,there was no obvious difference between using fresh water method or seawater method.To determine NH₄⁺,when the sample SAL was less than 4.0,using both fresh water and seawater methods can accurately determine the concentration of NH₄⁺in the coastal water.However,when the SAL was more than 4.0,to adjust water samples pH was an indispensable step for using the seawater method.To determine the PO₄^3-in coastal waters,the influence of salinity was negligible under the low salinity and low phosphorus concentration(SAL≤35.0 and C_PO43-≤0.2 mg·L^-1)conditions,but for water samples with high salinity and high phosphorus concentration(SAL>100.0and C_PO4^3->0.5 mg·L^-1),it was more accurate to choose more sensitive seawater method.To determine the TP,the seawater method was of higher accuracy.2.Effect of salinity on the determination of non-reactive phosphorus and total phosphorus in coastal waters.The second chapter explores the influence of salinity on TP,focusing on the influence of salinity on reactive phosphate（RPs）in TP.However,the salinity may affect the phosphorus determination accuracy in coastal waters,especially for the non-reactive phosphorus（NRP）and total phosphorus（TP）.In this work,the competition mechanism between NaCl and NRP for oxidants（K₂S₂O₈,the most commonly used and recognized oxidant）was identified in different NRP determinations.Furthermore,salinity influences on determinations of tetrasodium pyrophosphate decahydrate（PP）,glyphosate（Gly）,phytic acid sodium salt hydrate（IHP6）,adenosine-5’-nomophosphate disodium（AMP）and TP were investigated.The results indicated that approximately 10%IHP6 and AMP would be transferred to reactive phosphorus（RPs）during digestion without K₂S₂O₈.When NaCl increased from 0%to 3.5%with fresh water method,the determination of Gly+K₂HPO₄ and IHP6+K₂HPO₄ decreased by 8.0%±0.00%and 24±0.01%,respectively.In addition,the determinations of NRPs and TP with different salinities in natural coastal waters by fresh water method and seawater method were performed.It showed that when the salinity>5.0,the NRPs and TP determination results presented deviations.At a salinity of 35.0,the TP（KH₂PO₄+Gly+IHP6+AMP）reduction measured by two methods was more than 12.3±0.46%.Furthermore,oxidants with higher digestion efficiency than K₂S₂O₈ should be developed.3.Release of nitrogen and phosphorus from heavily polluted river sediments in coastal zone and its influencing factors.Salinity not only affects the accurate determination of nitrogen and phosphorus in coastal water,but also may affect the release of nitrogen and phosphorus in coastal sediments.Based on the accurate determination of various forms of nitrogen and phosphorus in coastal water,the release mechanism and influencing factors of heavily polluted river sediments in coastal zone have been studied.It was found that TN,NO₃^-,NO₂^-and NH₄⁺in the overlying water decreased to the same concentration as the initial value after reaching the maximum concentration on the 8th,19th,14th and 6th days,respectively,and basically stopped releasing after the 42nd day.The concentrations of TP and PO₄^3-released from heavily polluted river sediments in the coastal zone could reach the release equilibrium after the 14nd days.The release of nitrogen and phosphorus from heavily polluted river sediments in coastal zone would be affected by environmental factors such as resuspension,SAL,pH,dissolved oxygen（DO）and temperature（T）.The effects of various environmental factors on sediment nitrogen of heavily polluted rivers in the coastal zone were listed as follows:resuspension(160 r·min^-1)（34.5%）>SAL（35.0）（21.0%）>pH（7）（10.5%）>DO(8mg·L^-1)（10.1%）>T（30℃）（9.4%）.The effects of various environmental factors on phosphorus in heavily polluted river sediments in the coastal zone were as follows:pH（7）（13.5%）>resuspension(160 r·min^-1)（9.9%）>DO(1 mg·L^-1)（6.7%）>SAL（35.0）（6.0%）>T（30℃）（3.6%）.