Study On Migration And Transformation Mechanisms Of Internal Phosphorus Via Resuspension Of Estuarine Sediment

Estuaries are transition zones that link up terrestrial rivers and marineecosystems. The biogeochemical behavior of the nutrient elements in estuary zoneshas significant impact on their output to the ocean. Phosphorus is one of the nutrientsthat limiting the growth of phytoplankton in the water, and the release of internalphosphorus is an important factor leading to eutrophication. Due to the low depth ofwater, the sediments in estuary zones periodically undergo resuspension under theaction of bottom shear stress caused by waves and tides. The resuspension of estuarysediments is usually accompanied by the release of internal phosphorus to theoverlying water, which is an important way of migration and transformation ofinternal phosphorus. The sediments in the estuary of Licun River, a typicalpollutant-receiving municipal river in Qingdao, are heavily polluted. The migrationand transportation of the internal phosphorus during sediment resuspension events hassignificant effect on the biological production process in the estuary zone and evenJiaozhou Bay. Based on field monitoring, sampling and analysis, the temporal andspatial distribution patterns of the phosphorus, as well as other main biogenicelements, in sediment and its overlying water are obtained. By simulating theresuspension process of the sediments, the respond characteristics of the sediments toerosion shear stress are studied and the influencing mechanism of resuspension on thephosphorus load of the overlying water and sediment P fractions is discussed. Themineralization mechanism of sediment organic phosphorus is investigated bylaboratory culture experiment and the subsequent study on the impact of sedimentproperties and environmental factors on mineralization rate is conducted. Thephotolysis characteristics of the sadiment organic phosphorus and the influence ofdifferent factors on the degradation process are explored by studying the photolysis process of suspended sediments. The main conclusions of the study are as follows:1. The physico-chemical properties of sediment and over overlying water in thestudy area are obtained through field observation and laboratory measurement. Duringall seasons, the concentration of dissolved total phosphorus (DTP), dissolvedinorganic phosphorus (DIP), NH+4-N and NO-2-N in the over overlying waterdecreases as the tide rises, and increases as the tide ebbs. While salinity, pH，dissolvedoxygen and redox potential of the over overlying water change in the opposite trend.The quality of the over overlying water is significantly affected by tide. The watercontent, loss on ignition and nutrient content of estuarine sediment do not exhibitsignificant seasonal variation. Sediment water content and inorganic phosphoruscontent decrease with depth，while the content of NH+4-N and NO-3-N in sedimentfluctuates strongly with depth. The concentration of DTP and DIP in porewaterincreases firstly and then decreases with depth. In summer, the concentrations of totaldissolved nitrogen and NO-x-N in porewater decrease with depth, while they fluctuatestrongly with depth in spring.2. The resuspension process of intertidal sediments is simulated and the shearstress of tide scouring on the sediment surface is calculated based on field monitoringdata. The critical shear stress for the resuspension of fine sediments ranges between0.117N/m2and0.156N/m2in different seasons. Yet the shear stress induce by tidalcurrent could reach0.4N/m2, indicating that fine sediments could be eroded under thedisturbance of tidal current. The thickness of the sediments with enough resuspensionpotential in the study area is less than1mm and fluctuated wildly with the seasons. Inaddition, the intensity of the hydrodynamic disturbance in the estuary area wasinsufficient for the resuspension of coarse sediments.3. The influencing mechanism of resuspension on phosphorus load of water bodyand sediment phosphorus speciation is analyzed. In winter, sediment resuspensionfacilitates scavenging of DIP from overlying water, while it benefits the release ofinternal DIP to overlying water in other seasons. During all seasons, resuspension ofsediment leads to release of internal dissolved organic phosphorus (DOP). Theamounts of DIP and DOP released via resuspension are comparable. However, they are all an order of magnitude lower than the release of particulate phosphorus. Beforeand after resuspension, no significant differences in the content of Fe-bound P,authigenic P, detrital P and organic P are observed, indicating resuspension do notpromote the transformation of sediment phosphorus speciations.4. The effects of environmental conditions on the mineralization process ofsediment organic phosphorus are investigated. Hydrodynamic disturbances canpromote the mineralization of intrenal phosphorus. Relative to intermittentdisturbance, continuous disturbance is more conducive to phosphorusmineralization.The release of phosphorus from sediment increases with the increaseof sediment organic matter content during the mineralization process.Bothmicrobialactivities and external carbon source can significantly promote the mineralization ofsediment organic phosphorus. Reduced salinity inhibited the mineralization process.Under hydrodynamic disturbances, the mineralization rate of sediment organicphosphorus decreases with time.5. The photolysis characteristics of the sediment organic phosphorus areinvestigated along with its influencing factors. The results show that the photolysisprocess of the organic matter in the sediments can increase the contents of DIP andDOP in the water. The increment of DIP might derive directly from the degradation ofthe particulate organic phosphorus in the sediment, or originate from the furtherdegradation of DOP, which is one of the photolysis products. The release amount ofDIP and DOP from sediment increases with the increase of sediment organicphosphorus content and the decrease of the sediment grain size. The photolysisrelease amount of phosphorus increases with the increase of suspended sedimentconcentration when its concentration was relatively low. However, the promotioneffect of increasing suspended sediment concentration on the photolysis release ofphosphorus is no more when the suspended sediments come to a certain concentration.The photolysis of the sediment organic phosphorus is highly spectrum-sensitive.During the process of photolysis, the generation of DIP is mainly due to theirradiation of ultraviolet. Ultraviolet and visible light both account for about50%ofthe total generation amount of DOC and DOP.