Simulation Study Of Tidal Cycle On Nitrogen,Phosphorus And DOM Release From Sediment In Dagu River-Jiaozhou Bay Estuary

Estuarine sediment is a vital part of estuarine ecosystem and an important storage place for carbon,nitrogen and phosphorus in water.Estuarine sediment is strongly affected by tides all year round.Tide has a great influence on the migration,transformation and distribution of estuarine sediment.However,there are few studies on the effect of tidal cycle on nutrient release from estuarine sediment.In this study,the tidal cycle device is used to simulate the tidal disturbance of different intensity.The effects of tide cycle on nitrogen,phosphorus and dissolved organic matter（DOM）release in sediment of Dagu River-Jiaozhou Bay estuary are studied.A model with stepwise regression analysis is established to estimate the short-term release of nitrogen and phosphorus from sediment under tidal cycle.Besides,the variation of microbial abundance and community structure in sediment under tidal cycle is analyzed to further explain the mechanism of nutrient release in sediment under tidal cycle.The main conclusions are as follows:（1）The simulated tidal cycle promotes the release of NH₄⁺-N and NO₃^--N in sediment.Under no tidal cycle disturbance（static release）,the maximum release content of NH₄⁺-N and NO₃-^-N is only 360.15 mg/g and 36.34 mg/g,respectively.While under tidal cycle disturbance,the maximum release of NH₄⁺-N and NO₃^--N reaches 513.45 mg/g and 194.94mg/g,respectively.In addition,the release content of NO₃-^-N decreases with the increase of tidal cycle intensity.This is because low tidal cycle intensity results in higher dissolved oxygen content and stronger aerobic nitrification activity,while high tidal cycle intensity leads to higher abundance of denitrification microorganism and promotes denitrification.The tidal cycle also promotes the release of phosphorus in sediment.The release content of TP fluctuates over time and maintains at a low level,which is attributed to the low phosphorus content in sediment of the Dagu River estuary.The stepwise regression model is used to fit the nitrogen and phosphorus release data.The fitting of NH₄⁺-N,NO₃^--N,NO₂^--N release is accurate,and the correlation coefficient（R²）is 0.8887,0.9051 and 0.7991,respectively,illustrating that the introduced model can predict the nitrogen release content at certain time.While the fitting effect of TP release is poor,which can be attributed to the low release content of TP and the fluctuation of experimental data under tidal cycle.（2）The simulated tidal cycle promotes the release of DOM in sediment.The DOM release content of tidal cycle group is always higher than that of static release group,and the maximum release content of tidal group is 2.14 times that static release group.This is because tidal cycle promotes the mineralization of DOM in sediment and promotes the diffusion of DOM into overlying water,leading to the increase of DOM content in overlying water.When the tidal cycle intensity is the smallest,the DOM release content is the highest,which may be due to the double effect of the overlying water height and tidal intensity.Besides,under simulated tidal cycle,the DOM fluorescence intensity of tidal group is larger than that of control group,indicating that tidal cycle has a great influence on DOM fluorescence intensity,and humic A peak is the strongest in all tidal groups.（3）The simulated tidal cycle has a significant effect on the abundance and community structure of microorganisms in sediment.The abundance of microbial communities in static groups is significantly higher than that in tidal groups,indicating that tidal cycle greatly inhibits the abundance of microbial communities.The decrease of microbial abundance in tidal groups is due to the change of dissolved oxygen during tidal process.Besides,through the analysis of microbial community structure,it is found that the tidal cycle is beneficial to the growth of Gammaproteobacteria.Gammaproteobacteria microorganisms have the ability of denitrification,which can reduce nitrate to nitrite.The denitrification activity is strong in tidal gtoups,which further explains the negative correlation between tidal disturbance intensity and NO₃^--N release content.The tidal cycle also greatly affects the abundance of Marinobacteraceae,thereby enhances the mobility of carbon,nitrogen and phosphorus.The microbial community studies further explain the mechanism of nutrient release during the tidal process.