Characteristics And Mechanisms Of The Decomposition Of Organic Matter In The Bay-shelf Triggered By Typhoons

Typhoons are powerful extreme weather event that can alter not only ocean dynamics but also biogeochemical processes.Under global warming,the increase in ocean surface and subsurface temperatures and the enhancement of salinity stratification caused by precipitation result in a substantial increase in typhoon intensity,which will inevitably change the carbon and nitrogen cycle in the oceans.Therefore,it is very necessary to study the response of marine biogeochemical processes to typhoon processes,which has important scientific significance not only for comprehensively understanding the coupling mechanism between biogeochemical and physical processes in the upper ocean and lower atmosphere,but also for improving the theory of marine carbon and nitrogen cycle.Previous studies based on satellite remote sensing data have found that typhoon-induced vertical mixing,entrainment,and upwelling processes lead to the transport of nutrient-rich deep water to the nutrient-depleted upper ocean,thereby stimulating phytoplankton growth and increasing productivity.Thus,the typhoon is considered as an important role in increasing marine carbon sinks.In contrast,the field observations from our previous studies found that regardless of whether phytoplankton blooms or not after a typhoon,the enhancement of water mixing caused by the typhoon will lead to strong decomposition of organic matter,leading to a strong oxygen consumption in the water column.Strong decomposition can release large amounts of carbon and nitrogen that contribute to warming,contradicting previous conclusions about carbon sequestration.At present,the characteristics and mechanisms of decomposition in the bay-shelf caused by typhoons need to be further studied,that is,whether the decomposition process is dominated by dissolved organic matter（DOM）or particulate organic matter（POM）,and whether the decomposition in the bay-shelf is the same and what its mechanism is.To address these scientific issues,this study scarried out two typhoons,"Barijat"and"Wipha",to conduct four voyages of sample collection in Zhanjiang Bay and western Guangdong before and two weeks after the typhoon.Various stable isotopes（hydrogen and oxygen isotopes of water,carbon and nitrogen stable isotopes of POM）and spectral characterization（colored dissolved organic matter CDOM and fluorescent dissolved organic matter FDOM）,were investigated to discuss the marine ecological environmental effects caused by typhoons,and to reveal the characteristics and mechanisms of decomposition of marine organic matter caused by typhoon processes.The results are as follows:（1）Two weeks after the typhoon,there were no blooms of phytoplankton in Zhanjiang Bay,which was attributed to the strong stirring in Zhanjiang Bay caused by the typhoon.However,in the nearshore area of western Guangdong,terrigenous inputs and Ekman pumping caused by typhoon provide abundant nutrients to coastal waters,resulting in an increased amount of Chl a.By contrast,the level of Chl a slightly decreased in the offshore area,as the weak typhoon was unable to upwell the bottom eutrophic waters to the upper water,and the restriction of inorganic nitrogen nutrients that hinder the growth of phytoplankton.Two weeks after the typhoon,whether or not phytoplankton blooms in the Zhanjiang Bay and the coast of western Guangdong,severe oxygen consumption and significant nutrient addition behavior were observed in the water column in these two regions,which is mainly related to the decomposition of organic matter.（2）In Zhanjiang Bay,the significant correlation between the POM and Chl a suggested that the POM mainly originated from the production of marine phytoplankton before and after the typhoon.Higher Chl a level was responsible for the higherδ¹³C andδ¹⁵N values in the upper bay and lower bay during pre-typhoon.However,the decrease ofδ¹³C andδ¹⁵N suggested that decomposition occurred during post-typhoon.In the nearshore area of western Guangdong,the significant correlation between Chl a and POM indicated that POM mainly originated from the production of marine phytoplankton during pre-typhoon and post-typhoon.However,phytoplankton blooms were responsible for higherδ¹³C and lower C/N of POM during the post-typhoon.In the offshore area,the lower Chl a level was responsible for the lowerδ¹³C andδ¹⁵N values.Less chance of nutrient,PN,and POC levels in the offshore area is mainly related to the internal circulation of nutrients and biological respiration.However,strong stirring during typhoons promoted the decomposition of POM in Zhanjiang Bay and the coast of western Guangdong.In particular,the decomposition of POM in the upper bay of Zhanjiang Bay（72%）and the nearshore area of western Guangdong（23%）is relatively strong,which is related to the more fresh POM produced by phytoplankton.（3）In the upper bay of Zhanjiang Bay,the level of DOM in the post-typhoon period was lower than in the pre-typhoon period,which is associated with the decease of terrestrial input and less impact of biological activity.The results of the two end-member models suggested that the stirring after a typhoon promoted the decomposition of the DOM,resulting in a lossy behavior of some DOM.On the contrary,there was a significant addition of DOM in the lower bay during post-typhoon.The increase of intrusion of high-salinity water into Zhanjiang Bay formed a strong marine front and hindered the transport of material from the upper bay to the lower bay,providing favorable conditions for phytoplankton growth in the lower bay.A significant positive correlation between Chl a andΔC3,indicating that biological release was the main reason for DOM addition in the lower bay during post-typhoon.In addition,significant negative correlations between POC andΔC1,PN andΔC1 were observed in the lower bay during post-typhoo,indicating that POM decomposition was one of the reasons for the addition of DOM.In the coastal area of western Guangdong,the heavy rainfall after the typhoon promoted more terrestrial input and phytoplankton blooms,making the DOM levels in the coastal area much higher than during pre-typhoon period.The results of three end-member models indicated that the DOM exhibited a positive addition in the nearshore during post-typhoon period,with sediment release and biological production being responsible for the positive addition of DOM.In the offshore waters during post-typhoon,DOM showed a loss（7-18%removal）due to microbial decomposition caused by typhoon stirring.（4）Before the typhoons,the distribution of temperature,salinity and water isotopic values indicated weak mixing and significant stratification in the water column of both Zhanjiang Bays and the coast of western Guangdong Province.However,strong stirring breaks the stable stratification of the water column after the typhoon,and thus enhancing vertical water mixing.The calculation of air and sea fluxes show that the vertical mixing of water induces a large amount of oxygen to entering into the water,maintaining the decomposition of organic matter in Zhanjiang Bay and the coast of western Guangdong.In addition,in Zhanjiang Bay,the increased intrusion of high salinity water after the typhoon and the formation of a strong ocean front in the bay also provide favorable conditions for the decomposition of organic matter in the upper bay.In summary,typhoon induced decomposition of organic matter was the dominant process regardless of whether phytoplankton bloom occurred in the bay-shelf area.There is strong decomposition in the upper bay of Zhanjiang Bay（72%）and the nearshore area of western Guangdong（23%）after the typhoon;in the offshore area,the decomposition mainly degraded DOM（7-18%removal）due to less fresh POM.The decomposition of organic matter in the bay-shelf preferentially degraded POM,and the mechanism is mainly related to the strong mixing effect caused by the typhoons,which induce a large amount of oxygen dissolving into the seawater to sustain the decomposition.