Controlling Processes Of Dissolved Oxygen,organic Carbon,nitrogen And Phosphorus Cycles And Budgets In The Coastal Area Of Rushan Bay And Bohai And Yellow Seas

Essential biogenic elements such as oxygen(O),carbon(C),nitrogen(N)and phosphorus(P)play key roles in marine ecosystems.The intensity and frequency of coastal eutrophication and hypoxia have been accelerating by enhanced anthropogenic activities,which would result in long-term ecosystem effects and environmental problems.It is thus important to explore the biogeochemical cycle of biogenic elements,which has a great significance to the regional material cycling and environmental evolution and protection.This study focused on the coastal area of Rushan Bay(mariculture area)and the Bohai and Yellow Seas(BSYS).The concentration and distribution of dissolved oxygen(DO),organic carbon(OC),N and P in the adjacent area of Rushan Bay were determined.Combined with the hydrological and biological data as well as incubation experiments,budgets of DO,organic carbon,N and P were established as well.Aslo,budgets of N and P were established based on the measurements in the Changjiang and Yellow rivers,in the BSYS,and additional data from literature.And the critical role of riverine N and P supply in biogeochemical cycling in coastal marine ecosystems were evaluated.Our results would provide the basis on the study of regional biogeochemical processes and land-sea interaction under human disturbance.Major results in the thesis are summarized as follows:(1)The concentration of DO in summer 2009 ranged from 3.21 to 11.8 mg/L,with an average value of 6.99 mg/L.The DO concentration was higher in July,and lower in August on the basis of their contents.DO concentration in the surface water showed no significant difference,while bottom water DO concentration varied greatly.The low oxygen phenomenon was found with the minimum value of 3.24 mg/L in the bottom water.The DO budget shows that the major oxygen depletion is oxygen consumption processes in water column,accounting for 79% of the total oxygen consumption.And the sediment oxygen demand contributes to 21% of total oxygen consumption.The amount of oxygen input from the air-sea interface exchange accounts for 74% of the total oxygen input;the contribution of primary productivity to DO pool cannot be ignored,accounting for about 22% of the total amount of oxygen supply.The water mass transport driving oxygen exchange are relatively a low contribution for the whole survey area,only accounting for about 4% of the total oxygen input,respectively.The low DO phenomenon in the adjacent area of Rushan Bay is mainly controlled by the physical process..(2)The concentration of dissolved organic carbon(DOC)in summer 2009 ranges from 0.70 to 3.19 mg/L,with an average concentration of 1.80 mg/L.Concentration of DOC was high in August,and low in June.Time-dependent variation of DOC and particulate organic carbon(POC)were ranged in 1.79~15.2 mg/L and 0.04~1.33 mg/L,respectively.DOC patterns were mainly affected by human activities,primary production and tide.Estimated POC sedimentation rate was(25±0.8)g/m2 in summer,accounting for 66% of the carbon fixation by phytoplankton related to primary production.DOC concentrations in the pore water of the upper sediment(0~4cm)were 8~9 times larger than those in the overlying water,and benthic DOC flux at the sediment-water interface ranged from 14.4 to 97 g/(m2 a),accounting for 1.1%~13.4% of DOC in the water DOC pool.Anthropogenic activities were the key factors that influence the OC distribution and cycling in the coastal areas of Rushan Bay.The OC budget shows that regeneration of sedimentary TOC contributes to a large share of diffusive DOC flux,indicating that the sediment is a significant source of DOC to water column in the study area.More than 50% of OC can be transported by tides and currents.The net burial of OC in sediments represents 13% of the carbon pool that yield by the primary production in summer.Low-oxygen level may enhance benthic diffusive of DOC and other diagenetic processes,which would play a key role in controlling OC conservation in the sediment.(3)Distributions of DIN and DIP in summer showed significant variability in the coastal area of Rushan Bay,with fairly high concentrations of DIN and DIP in the mouth of Rushan Bay,and the nutrient distribution was also influenced by tides and currents.The N form of DIN in the study area was dominated by NO3-.The gradient of DIN and DIP concentrations at the sediment-water interface showed efflux of nutrient from the sediment to the overlying water,resulting in higher concentrations of DIN and DIP than those in the surface water.The budget of nitrogen showed that internal cycling was the dominant source of DIN for the primary production,accounting for 86% of uptake by primary production,followed by water exchange(11%),and the benthic efflux contributed to <3%.The removal of DIN in the water column was dominant by sedimentation(80%),export to offshore(16%),and denitrification(4%).The regeneration of ON in the sediment after sedimentation only accounted for 6%,while 70% of ON were burial permanently,and 24% of ON were mineralized.The budget of phosphorus showed that internal cycling in the water column was the dominant source of DIP for the primary production,accounting for 91% of uptake by primary production,followed by water exchange(9%),and the benthic efflux contributed to <1%.The removal of DIP in the water column was dominant by sedimentation(67%)and export to offshore by water exchange(33%).The burial and mineralization of OP in the sediment after sedimentation were the main processes for preservation,accounted for 90% and 10%,respectively.Based on the budgets of DIN and DIP,the internal recycling was the dominant source for DIN and DIP supplies,however,the different nutrient structure of external and internal source may result in long-term effects for nutrient balance.(4)Concentrations of DIN and DIP in the study area are affected by riverine input and benthic flux.The major sources of the DIN in the water column of the BSYS are benthic diffusion,followed by the atmospheric deposition and inputs from adjacent rivers the groundwater and the East China Sea;while the dominant removals of DIN from water column in the BSYS are phytoplankton utilization and sedimentation.The key sources of the DIP are phosphorus minerals desorption and weathering,accounting for 91% of external input,followed by benthic flux and atmospheric deposition,then riverine flux and groundwater input;while the dominant outputs of DIP are sedimentation and the outflow to the East China Sea.There was a 11Gmol/a of nitrogen accumulated in the BSYS,which would result in an increasing of DIN concentration by 0.6 ?mol/(L·a).DIN concentrations and N/P ratio in the Bohai Sea and the Yellow Sea kept increasing in recent decades due to the increasing of nitrogen accumulation and phosphorus sedimentation.Terrestrial nitrogen loadings might enhance the primary production and alter the structure and function of ecosystem,which had significant effects on the ecosystem of BSYS in the long-time scale.We,therefore,should reduce the riverine nitrogen loads to maintain the ecosystem stability in the BSYS.Therefore,the impact of significant changing terrestrial input to the coastal marine ecosystems need to be further evaluated.