Insighting Into The Bohai Sea From The Phosphorus Dimension: A Study Of Fluxes And Budget Of Phosphorus From Land To Sea With Implication For The Environment Change

Phosphorus(P)is one of the most important nutrient element,which plays a key role in controlling eutrophication.The composition and abundance of primary productivity and its associated phytoplankton population are also directly controlled by the change in P forms and contents in seawater with a rapid response to the variation of P.In comparison with other biogenic elements,P has a physical-chemical interaction with particles.Thus,most riverine P pool associated with particles would be deposited and buried on the offshore continental shelf.Therefore,the changes in forms and contents of P in offshore sediments and the processes of deposition and burial could reflect the water environment and regional climate change.However,the enhanced human activities has led to changes in P cycling in marine systems,resulting in changes in the structure and function of offshore ecosystems,and thus restricting the evolution of the marine environment.Based on the scientific concepts of land-sea interaction,this study concentrated on the rivers around Bohai Sea(BS),the Yellow River Estuary(including the wetland)and the Bohai Sea.In this study,the sedimentary records,numerical models and other technical means are employed to discuss the key processes of P in river-estuary-ocean with an aim to explore the marine environment evolution of BS.The key geochemical processes of P in sediments were analyzed,and the corresponding relationship between river transportation and ocean P burial was established.The transportation process of P under the influence of human activities involves the quantifying P fluxes from the rivers around the BS and their contributions to the P budget in the water column of the BS.The main conclusions are as follows:(1)In the Bohai Rim River-Yellow River Estuary-Bohai Sea system,the total P content in the water body gradually decreased from land to sea.Total P(TP)content in surface sediments showed no significant difference from land to sea,and the main form of P in surface sediment was detrital P(Detr-P).The TP content in the surface sediments of the BS was higher in the nearshore waters than in the central BS and the Bohai Strait;Due to great influence of terrigenous matter carried into the BS by the Yellow River,the sedimentation rate,TP deposition flux and the Detr-P content in sediments were significantly higher in the Yellow River estuary and Laizhou Bay than in other regions of the BS,which shows that nearshore areas such as estuaries were the sea areas where sedimentation processes are affected more intensively by rivers around the BS.(2)The total P fluxes transported into the BS from rivers in 2017,2019 and 2020were 26.4×10~8 mol/a,45.0×10~8 mol/a and 37.5×10~8 mol/a,respectively,mainly dominated by the transport of particulate P,accounting for(88.6±8.11)%.Among all of the rivers around the BS,the Yellow River had the highest P flux to the sea,with an average percentage amounted to(48.2±11.5)%for the three hydrological periods.(3)The P budget in the water column of the BS shows that the supplies amounts of terrestrial sources of P for BS were less than the removal of P through sedimentation and burial from water column of the BS.The P transportation by rivers to the BS was the main source of P(89.4%)which was discharged into the BS.The major P sinks in water column were the sedimentation and burial process,accounting for about 95.9%of the total expenditure.External P inflow was not sufficient to support P consumption for primary production in the BS,and the efficient turnover of P within the BS water column was an important source of nutrients to sustain primary production in the upper water column.The supplementation of exogenous P into the BS water body was smaller than the output of P from the water of the BS such as deposition and burial,showing the P depletion process in the BS water body.(4)The exchange fluxes of P in the sediment-water interface had significant regional differences.The deposition and burial processes of P in the nearshore areas of the BS were significantly influenced by the input of terrigenous matters.The cycling of P in the sediment of the BS was likely to be the most critical process for controlling P content in the water column.(5)The main forms of P in the sediments of the Yellow River estuary are detrital P and authigenic P.The Yellow River estuary exhibits different burial characteristics/rates in different periods and corresponds significantly to the position change of the Yellow River estuary and the intensity of human activities.There could be obvious stoichiometric imbalance in rates of nutrient in the BS,especially the high atomic ratios of nitrogen and P in Laizhou Bay and Bohai Bay.The trend of potential P limitation in some regions of the BS is becoming more and more severe,and the driving mechanism of nutrient imbalance in the BS needs to pay more attention in the future.