Composition And Transport Of Carbon And Silicon In Rivers And Wetlands Around The Bohai Sea And The Budget Of Silicon And Carbon In The Bohai

The carbon（C）cycle is closely linked to the silicon（Si）cycle and plays an important role in controlling global biogeochemical cycles and climate change.Si plays an essential role in the healthy growth of terrestrial plants,as well as the primary production and the population structure of phytoplankton in the ocean.Si can control the transport and cycling of surface multi-layered materials through coupling with C.And Si‘pumps’,combined with C‘pumps’plays a central role in the carbon sink process,which is a hot topic in environmental science and marine science research.In terrestrial ecosystems,the formation of phytolith can compound organic carbon（OC）and preserve it for a long time;in the ocean,the growth and deposition of diatoms can transport particulate matter（including C）from the surface to the deep sea,thereby contributing to carbon sequestration.Both of the above processes are thus important for mitigating global warming.The increasing human activities in recent years however,such as dam building,excessive use of nitrogen and phosphorus fertilizers,and urbanization,have profoundly changed the concentration and composition of C and Si flowing into the sea,which could affect the biogeochemical processes of silicon and carbon in rivers,estuaries and offshore waters,and change the structure and function of ecosystems.Coastal wetlands play an important role in the global carbon cycle and climate change because of their high productivity and potential carbon sequestration.Besides,the carbon decomposition rate is slow in wetlands owing to the anoxic and humid environments.Therefore,the analysis of the key processes and interactions between Si and C cycles in rivers,coastal wetlands and offshore ecosystems can help to further understand the mechanisms of carbon and silicon coupling and the main processes of sea-land interaction,as well as providing scientific reference for the management of marine environment under the influence of human activities.This study concentrated on 24 rivers along the Bohai Sea（BS）,2 typical coastal wetlands,and the Bohai Sea（BS）.Combined with the historical data,based on the comprehensive survey of several voyages from 2017 to 2021,the spatial and temporal distribution,composition and preservation of carbon and silicon in water column and sediments were analyzed to discuss the composition and export of C and Si from rivers and their environmental effect on the BS,the preservation and coupling of C and Si in coastal wetlands,and the distribution,budget and response to land changes of C and Si in the BS.The main conclusions are as follows:（1）The DOC concentration ranged from 4.88 to 30.6 mg/L and the high values were concentrated in the Hai River cluster.The mean concentration of DOC in the dry season was lower than that in the flood season.The variation in carbon stable isotope(δ¹³C_DOC)in the flood season was consistent with that in the dry season.Theδ¹³C_DOCvalues indicated the DOC sources were mainly associated with river plankton and aquatic plants,followed by terrestrial plants and sewage.Compared with rivers in Eastern China and others worldwide,the DOC concentrations and specific DOC flux of in rivers around the BS were at high levels.Protein-like,humic-like and unknown components were identified by the Parallel Factor Analysis（PARAFAC approach）.Most of the Chromophoric dissolved organic matter（CDOM）in the rivers around the BS was dominated by autochthonous sources.Approximately 40%of CDOM was from a soluble,microbial byproduct-like material and the contribution of terrestrial plants was minor.The DOC flux transported by rivers to the BS was approximately 0.55 Tg/a（equivalent to 55×10~4t/a）,mainly controlled by the runoff.Approximately 0.15 Tg/a（equivalent to 15×10~4t/a）of refractive C was discharged to the BS.Rivers around the BS had fairly high DOC yields in terms of unit drainage area,which could have a large influence on the C cycle in the BS under increasing human activities.（2）The dissolved silicon（DSi）concentration in rivers around the BS varied in different seasons with the range from 0.86 to 148μmol/L during the dry season in2017,the flood season in 2019 and the flood season in 2020.The DSi concentration in flood season was higher than that in the dry season.The ratio of dissolved silicon to dissolved inorganic nitrogen（DSi/DIN）and dissolved silicon to dissolved inorganic phosphorus（DSi/DIP）deviated from the Redfield ratio（16/1）.The variation of biogenic silica（BSi）content in the water was consistent with the suspended particulate matter（SPM）content,indicating SPM was the main control factor of BSi content.The OC content of surface sediment in rivers around BS ranged from 0.001 to 17.7mg/g during the dry season in 2017,the flood season in 2019 and the flood season in2020.The variation of OC concentration in these three periods was relatively consistent,which reflected rivers with high OC concentration in the flood season also had higher OC contents in the dry season.The BSi content ranged from 0.39 to 11.5mg/g.There was a good correlation between OC and BSi contents,indicating that they had a similar source.The high OC and BSi contents were occurred in Hai River cluster and Luan River cluster（except the Luan River）which mainly received urban sewage and domestic wastewater.The low OC and BSi contents were occurred in rivers with serious soil erosion such as the Yellow River and the Luan River.The reactive silicon（RSi=BSi+DSi）flux of rivers around the BS was 30.5×10~4t/a,of which the BSi flux was 20.0×10~4t/a and DSi flux was 10.5×10~4t/a.It is observed that BSi was an important component of RSi.The DSi flux of the Yellow River,which is the main contributor of RSi flux,had greatly reduced from a long time series.Besides,the lower Si/N ratio and higher Si/P ratio in the Yellow River may cause potential effects on the primary production in the BS.（3）The carbon,phosphorus（P）and silicon contents of plants in the Yellow River estuary wetland and Liao River estuary wetland increased in the order of roots,stems and leaves.The mean contents of OC,total silicon（TSi）and BSi-occluded OC(OC_BSi)were 369.2 mg/g,8.57 mg/g and 0.10 mg/g for plants in the Yellow River estuary wetland and 332.1 mg/g,9.51 mg/g and 0.25 mg/g for plants in the Liao River estuary wetland,respectively,where the OC and Si contents of reeds were higher than those of other plants.The average OC content of surface sediments in the Yellow River estuary wetland was 2.58 mg/g,and the average BSi content was1.67 mg/g.The average OC content of surface sediments in the Liao River estuary wetland was 3.72 mg/g,slightly higher than that in the Yellow River estuary wetland,and the average BSi content was 3.54mg/g,which was about twice as high as that of the Yellow River estuary wetland.The contents of BSi and OC in sediments from vegetated areas were higher than that in bare beach areas,indicating vegetation may be important for the preservation of OC and BSi.There was a good correlation between OC,BSi and organic phosphorus（Org-P）in wetlands,indicating the sources of carbon,phosphorus and silicon in wetland sediments may be related to the primary production of higher plants.A large amount of OC and BSi were stored in the vegetation and sediment of the Yellow River estuary and Liao River estuary wetlands.The accumulations of OC,OC_BSiand BSi in the aboveground vegetation of the Yellow River estuary wetland were 23.6×10~4t,80.0 t and 0.10×10~4t,respectively;while in the Liao River estuary wetland were 71.0×10~4t,360 t and 0.62×10~4t.The accumulation of OC,OC_BSiand BSi in the stable layer（per 10 cm）of Yellow River estuary wetland were 27.5×10~4t,0.99×10~4t and 29.9×10~4t,respectively;while in the Liao River estuary wetland were18.3×10 ~4t,940 t and 24.5×10~4t,respectively,which suggested wetlands were of great significance for the storage of OC and BSi.（4）The DSi concentration in the BS was higher in eastern China seas.The DSi concentrations in the surface and bottom waters were 11.8μmol/L and 11.4μmol/L in spring（April 2018 and March 2020）,21.8μmol/L and 25.88μmol/L in summer（July2018）,and 4.37μmol/L and 5.30μmol/L in autumn（November 2018 and November2020）,respectively.The DSi concentration in the bottom water column was slightly higher than that in the surface,and the seasonal variation was in the order of summer>spring>autumn.The OC content of surface sediments ranged from 0.33 to 11.6 mg/g with a mean value of 3.15 mg/g in spring,summer and autumn of 2018 and spring and autumn of2020.The BSi content ranged from 1.04 to 7.64 mg/g,with a mean value of 3.46mg/g,and the seasonal differences of BSi content were also quite small.The spatial distribution of OC and BSi was similar.As for the vertical distribution of core sediment,the sample from the Yellow River estuary was taken as an example.The BSi content of BH-0 core ranged from 0.14%to0.66%,with a gradual decrease from the surface to the bottom.The OC content ranged from 0.16%to 0.72%,with an average content of 0.44%.There was a significant correlation among the BSi,OC,organic nitrogen（ON）and Org-P contents,indicating a consistent source.Combined with the C/N ratio,the OC in the upper layer was mainly derived from marine organisms and the OC in deep layer was from terrestrial higher plants.This difference was mainly due to the sedimentation change caused by the shift of Yellow River’s route.（4）The silicon and carbon budget in the BS showed that the riverine input and benthic flux at the sediment-water interface were the main sources of RSi in the BS,which accounted for about 80%of the total input.The input from the North Yellow Sea and riverine input were the main sources of OC,which accounted for more than90%of the total input.While the sedimentation and burial was the main way to remove OC and RSi.For a long time,the DSi concentration in the BS waters had decreased by nearly 2/3and the Si/N ratio also was decreasing.The imbalance of nutrient structure might affect the primary production of diatoms.The carbon and silica transport in rivers around the BS are greatly influenced by human activities,resulting the flux and composition of carbon and silica have changed significantly,which may affect the phytoplankton population and structure in the estuary and offshore,and then affect the environmental evolution of the BS ecosystem.