Study On The Exchange Flux Of Nutrients At The Sediment-Water Interface In Estuary Area

As an interface of land-ocean interaction, estuaries are characterized by strongwater dynamic action, frequent sediment transportation, constant variations indeposition and erosion, and abundant biodiversity, et al., At the same time, estuaries,which widely affected and regulated by the increasing anthropogenic activities, areconsidered as the most fragile ecological environments. As a sort of environmentboundary between water and sediment, the sediment-water interface is a veryimportant region where physical, chemical and biological parameters of nature watervary sharply. Simultaneously, numerous substance transportation and exchange takeplace in this interface. Nowadays, researches on the tidal flat sediment-water interfacebecomes a hotspot in environment science.Licun Estuary is located in the east of Jiaozhou Bay, it belongs to extratropicalmonsoon clima-tic region. As a typical urban pollutant carrying rivers, Licun Riverestuary polluted seriously. The migration and transformation of nutrient elements intidal flat sediments have important implications on biological production process inthe estuary and Jiaozhou bay. As a result, it is necessary to study the distribution ofnutrients of sediments in Licun Estuary and their release inhibition technology toprovide evidence for the treatment of Licun Estuary.Based on the natural environment in the region, the nutrients exchange fluxbetween bottom sediment and water layer at3stations in Licun Estuary is studied inthis paper form May to Dec in2010, and the lab incubation method is adopted tosimulate and measure the nutrients exchange flux under different conditions in orderto study the effect of related factors. To cope with the treatment of Licun River, theimpacts of different sludge dredging degree on the nutrients exchange flux areanalyzed. Based on these, the effect of the in situ capping technology is also studied. Based on the field observation and the laboratory simulations, we get followingmain results:(1) As seasonal alternate, the chemical and physical parameters of overlyingwater are also present significant seasonal variation characteristics. The temperature ishighest but salinity、Eh and dissolved oxygen are generally lowest in summer.Because of the upstream water, the contents of nitrate nitrogen and ammoniumnitrogen are high in summer. But the influence of the season on nitrite nitrogen andphosphate are small. The contents of nutrients of overlying water in C sampling siteare higher than those in Licun Estuary. The sediments are mainly for fine particles.There is obviously negative correlation between moisture content and particle size.(2) According to the consecutive monitor within a tide cycle, we find that thechemical and physical properties of overlying water are significantly affected by theoceanic tide. The change of depth, salinity and dissolved oxygen content and pH issimilar with The pattern of tide, presenting the trend of first decreases then increases.Whereas the change law of water temperature is negative related with tide height.(3) The results form laboratory incubation revealed that the exchange of nutrientshave obvious seasonal variety and regional difference. The sediment is a source ofnitrite nitrogen, ammonium nitrogen and phosphate in the three seasons of a year. Insampling site A and B the flux of nitrate nitrogen is removed form overlying water tosediment. But in sampling site C, nitrate nitrogen is removed form sediment to waterin spring and summer. Generally speaking, the average exchange flux of nutrients ishigher in summer.(4) Study on the exchange flux of nutrients at the sediment-water interface underdifferent environment factors showed that the exchange flux of nitrate nitrogen,ammonium nitrogen and phosphate is higher in anoxic situation than in aerobicsituation. However, the effects of DO levels on nitrite nitrogen were unclear. Asincrease of temperature, the exchange flux of nitrate nitrogen, ammonium nitrogenand phosphate were higher, but nitrite nitrogen was just showed opposite trend. Theexchange flux of nitrite nitrogen and ammonium nitrogen were high when salinitywas increased. The exchange flux of phosphate were influenced few with salinity. The biological activity has an important effect on the exchange flux of dissolved nutrients.(5) Experimental results from the study of sectional property of sedimentrevealed that sediment in weak reducing environment and reducing environment. Thevertical distributions of nitrate nitrogen, ammonium nitrogen and TP decrease with thedepth. In terms of the vertical distribution in the interstitial water, ammonium nitrogenis still the chief constituent of nitrogen. The vertical distribution of nitrate nitrogenand nitrite nitrogen content are decreased with depth increased. The concentration ofammonium nitrogen in interstitial water is increased with depth increased in samplingsite A and B. But in sampling site C, the concentration of ammonium nitrogen washigher in surface. phosphate content is increased with depth increased in site C.(6) After dredging, the exchange fluxes of nutrients between the new layer ofsediment and water are related on the concentration of nutrients in interstitial water.The final concentration of nitrate nitrogen, nitrite nitrogen and phosphate in theoverlying water decreased with the15~30cm depth of sludge dredging, but the releaserate of ammonium nitrogen are not in direct ratio with the dredging degree in study.The effects of the in situ capping technology are different for different nutrient. Theregulation effects of ammonium nitrogen and phosphate are best.