Seasonal Variability Of Air-sea CO₂ Fluxes And Its Influential Factors In Adjacent Seas Of Changjiang Estuary

Since the industrial revolution,by the influence of the human activity,the concentration of atmospheric Carbon Dioxide?CO₂?continued to increase,which will cause a series of climatic,environmental,ecological problems,and the research of the carbon cycle were in full swing.As the planet's largest carbon storehouse,the oceans performed as an atmospheric CO₂ sink on the whole,to some extent alleviated the increase of the anthropogenic atmospheric CO₂,which plays an important role in the process of the global carbon balancing.Especially,the primary productivity in the continental shelf seas was significantly higher than that in the oceans,which was more important to the atmospheric CO₂ source/sink acting.The East China Sea was a typical river-dominated continental shelf sea,and the Changjiang River carried large amounts of fresh water,nutrients into the sea,by adjusting the primary production of phytoplankton to affect the operation of marine biological pump to control the distribution of the air-sea CO₂ flux in the Changjiang estuary and adjacent seas.In the recent 50 years,the nutrient concentrations of the dissolved inorganic nitrogen and dissolved inorganic phosphorus in the Changjiang River continued to increase,while the concentration of the silicate continued to decrease,which had impact on the ecosystem of the Changjiang estuary and adjacent seas and also caused disturbances of the air-sea CO₂ flux.The climatological modeled results in adjacent seas of the Changjiang estuary showed that,dissolved inorganic nitrogen?NIT?,dissolved phosphate?PHO?,silicate?SIL?were mainly distributed in the area less than 30 m,especially near the Changjiang estuary and Subei Shoal.Air-sea CO₂ flux distribution indicated that in winter the sea acted as a strong sink in adjacent seas of the Changjiang estuary;in spring the sea was a weak sink;in summer the Changjiang Bank acted as a weak sink while others were sources,and the whole sea were a weak sink;in fall the sea acted as a moderate sink,other seas were sinks besides the Changjiang Bank and the Subei Shoal areas acted as a source;in annual,the sea acted as a net sink of the atmospheric CO₂ with the average of-2.8 ± 1.5 mmol m-2 d^-1.Simulated the scenario of the nutrient of the Changjiang River increased,assuming that each nutrient concentration of the Changjiang River increased by 50%without considering structural changes in nutrient.The results showed that:compared to the reference case,increased nutrient concentrations of the Changjiang River made atmospheric CO₂ seaward flux a positive change in the Changjiang Bank in summer,with the carbon sink strengthened,and the sea surface absorbed more atmospheric CO₂ of about 68%,while in winter the seaward flux of the atmospheric CO₂ decreased in the Changjiang Bank and Subei Shoal,that is to say,carbon sink had weakened in winter.The CO₂ absorption capacity had an enhancement of about 6%in annual in the Changjiang Bank.Due to the increased nutrient concentrations of the Changjiang River,the biological effects mainly controlled the changes of dissolved inorganic carbon in adjacent seas,which further influenced the air-sea CO₂ flux.Compared the experiment with the Changjiang River nutrient concentration in 2000s to the one with the Changjiang River nutrient concentration in 1990s,increased NIT,PHO mainly concentrated in the seas less than 30 m,especially in the Changjiang estuary and Subei Shoal.However,in the shallow seas less than 30 m SIL showed a decreasing trend,due to the establishment of the Three Gorges Dam which made the sediment flux decrease.Due to the changed nutrient concentrations of the Changjiang River,the biological effects mainly controlled the changes of dissolved inorganic carbon in adjacent seas,which further influenced the air-sea CO₂ flux.In summer the carbon sink enhanced about 185%in the Changjiang Bank;in fall,the carbon source weakened 104%,which indicated the source became into a weak sink;in winter and spring the carbon sink also weakened;in the full year,the carbon sink enhanced about 11%in the Changjiang Bank,which had positive effect to the absorption of the atmospheric CO₂.