Study On The Geochemical Behavior Of Phosphorus,Iron And Sulfur At The Sediment-water Interface And The Exchange Flux Of Phosphorus In Silty Tidal Flat Of Estuary

Phosphorus is an important nutrient element,which is an important factor in the primary productivity and food chain.Iron is a redox sensitive element,The reduction of iron in sediments controls the release of phosphorus,Sulfur is a nonmetallic element which is widely distributed in the crust and is closely related with the geochemical behavior of phosphorus and iron.The estuary is a transition zone between rivers and oceans,wave effects,tidal effects,and biological activities make the redox environment of the surface sedimnets changes.Circulating seawater is an important part of SGD,and it also plays an important role in the exchange of material between sea and land,Therefore,it is important to study the biogeochemical behavior of phosphorus,iron and sulfur in estuarine region and estimate the exchange flux of phosphorus by circulating seawater to study the geochemical cycle of phosphorus,iron and sulfur and the biogeochemistry of estuaries.This paper relying on the National Natural Science Foundation "Geochemical processes,material transport and its contribution to the oceans of submarine groundwater discharge(SGD)".The study aera of this paper is in the mudflat of Julong River and the mangrove tital flat of Tonagan Bay.The concentration of the dissolved phosphorus,iron and sulfur in porewater was obtained by using the diffusive gradients in thin-films technique(DGT)in situ and high-definitionly.Combined with the physical and chemical properties of sediments,the distribution,variation and coupling relationship of dissolved phosphorus,iron and sulfur in porewater of mudflat were studied,and the flux of circulating seawater and the exchange flux of phosphorus of the Jiulong River estuary were also calculated.Get the following points:(1)The DRP and phosphorus speciation in the sediments of the Jiulong River estuary were analyzed,The content of inorganic phosphorus in sediments was higher than that in organic phosphorus,there was a significant positive correlation between total phosphorus,inorganic phosphorus and Fe/Al-bound phosphorus,because phosphorus has a higher mineralization rate in summer.In Summer,the accumulation of organic phosphorus in the sediments is greater than the consumption,probably because of the strong biological activity,so that the inorganic phosphorus in the sediments is converted into organic phosphorus.Based on the comparison of DRP in the sediments of the mudflat of Julong River and the mangrove tital flat of Tonagan Bay,the concentration of DRP in the sediments is related to the phosphorus speciation in the sediments,The concentration of DRP in the porewater of the Jiulong River estuary is higher than that in the Tongan Bay mangrove tidal flat due to the higher release of the available phosphorus source in the sediments.(2)The concentration of dissolved Fe2+ in pore water of the Jiulong River estuary is positively correlated with the concentration of DRP,Fe(Ⅲ)controls the release of phosphorus,the concentration of Fe2+ has a significant negative correlation with the concentration of S2-,and there is a competitive relationship between Fe2+ and S2-.The chemical iron reduction and dissimilatory iron reduction exsit at the same time,in the surface,the reduction rate of sulfate is higher,the chemical reduction reaction of Fe(Ⅲ)occurs mainly.In deep sediments,the reduction rate of sulfate is reduced and Fe(Ⅲ)mainly occurs dissimilatory reduction.(3)The activity of sulphate-reducing bacteria in summer is higher than that in winter,and the sediments contain more organic matter,which makes the sulfate reduction in sediments stronger,and the concentration of S2-increases.At the same time,the dissimilatory iron reduction is more intense,leading to the addition of Fe2+and DRP in the sediments in summer than in winter.(4)Circulating seawater is the main way of the submarin grongdwater discharge(SGD),the flux of circulating seawater caused by the tide is much higher than that caused by the wave because the tidal range is larger and the permeability coefficient of the sediments is low.The flux of circulating seawater caused by the tide is 28.09m3/(m·d).In summer,the flux of phosphorus in high tide position and medium tide position are 0.64 mmol/(d·m2)and 0.36 mmol/(m2·d),and which in winter are-0.79 mmol/(d·m2)and-0.78mmol/(m2·d),that means high concentrations of dissolved phosphorus in the sea enter the sediments.