Concentration,Composition And Influencing Factors Of Dissolved Organic Matter In Antarctic Ocean

Marine dissolved organic matter（DOM）is the most active organic carbon reservoir in the world’s oceans,with total pool of about 662–700 Pg C,which is comparable to carbon reservoir in the atmosphere（～750 Pg C）,so the minor change of marine DOM would have a significant impact on global carbon cycle.Intensive study of DOM carbon dynamics and its composition is of great significance for understanding marine biogeochemical processes,such as fueling bacteria respiration,protecting planktons from ultraviolet light damage,inhibiting photosynthesis rate by reducing light penetration,as well as mobilizing and transporting pollutants.Thus,marine DOM has received considerable attentions from tropical to polar oceans.The Southern Ocean（or Antarctic Ocean）is the strongest carbon sink that accounts for about 40%of the global oceanic uptake of anthropogenic CO₂.Ocean waters surrounding the Antarctic continent are dynamic and heterogenous environments,characterized by spatiotemporal variability in ice cover,light,water masses and nutrient.Over the past decades,the Western Antarctic Peninsula（WAP）has undergone significant warming,which have powerful impacts on the regional ecosystem,biogeochemical and hydrological properties,and would have a significant impact on global carbon cycle by regulating the change of marine DOM.A few studies suggest that in polar seas with seasonal ice cover,the melting of sea ice represents an important source for nutrient and DOM in surface water.The annual flux of organic carbon from the melting of sea ice into the Southern Ocean is estimated to be ca.8 Tg,representing an important source of fresh and labile organic matter to surface ocean.In response to significant warming,marine species in the Southern Ocean are extremely sensitive to ambient environment,e.g.,Krill,Salpa thompsoni（salps）,phytoplankton,and microorganism etc.Phytoplankton production contributes to significant accumulation of newly produced dissolved（DOC）and particulate organic carbon（POC）in the surface ocean,and also affects Antarctic macroorganisms（such as salps）,all of which are closely related to the concentration,composition,and bioavailability of DOM in surface water.However,so far,most of the reports on seawater carbon dynamics in the Antarctic marginal waters are based on single-cruise studies with low spatio-temporal resolution,and very limited studies have explored the spatio-temporal differences（East and West Antarctica response differences,summer,and autumn/winter differences）and their regulatory mechanisms in response to rapid climate change.This study has investigated the seawaters of various regions of the Southern Ocean(Prydz Bay located in the East Antarctica sea（2017–2018 cruise and 2018 cruise）,the South Shetland Islands located in the West Antarctica sea（2018 cruise,2019 cruise and2020 cruise）and the Antarctic Peninsula-South Orkney Island-South Georgia Island was carried out during the austral autumn and winter（2018 cruise and 2020 cruises）.We examined the differences in optical proties and the molecular level response of DOM in Southern Ocean to climate change and its influencing factors,using several analytical techniques such as total organic carbon analyzer,UV-vis and fluorescence spectroscopy and fourier transform ion cyclotron resonance mass spectrometry.The results of this paper are lists as follows:（1）East Antarctica-Prydz Bay:The surface waters off Prydz Bay are greatly affected by the seasonal retreat of sea ice.This section collected surface water samples off Prydz Bay,East Antarctic during two cruises in 2017–2018（post-sea ice melting season;post-SIMS）and 2018（sea ice melting season;SIMS）.The mean concentration of DOM during SIMS is 1.3-fold higher than that during post-SIMS,whereas a₃₅₀,an indicator for chromophoric DOM,shows an opposite trend by a factor of 2.8.Five components of DOM including two protein-like and three humic-like components were identified in fluorescence excitation emission matrices-parallel factor analysis,with the enrichment of humic-like refractory components during post-SIMS and protein-like biolabile components during SIMS.Sea ice dynamics driving variability of dissolved organic matter in surface water off Prydz Bay,East Antarctic.（2）West Antarctica-South Shetland Islands:The Western Antarctic Peninsula（WAP）was one of the most rapidly warming regions worldwide,especially,the water around the South Shetland Islands was greatly affected by climate warming.Surface water samples were collected in South Shetland Islands among three cruises（2018,2019 and 2020cruises）.This study found that the mean DOC concentrations of surface water continuously augmented among three cruises.For 2018 cruise,DOM in surface water is characterized by the lowest DOC concentration,the lowest BIX andβ/αvalues and fewer biolabile compounds,but the highest a₃₅₀,SUVA₂₅₄,HIX and AI_mod values,suggesting extensive microbial transformation from protein-rich biolabile DOM to refractory humic-like DOM.The spatial distribution trend of DOC concentrations is opposite to the a₃₅₀value since the microbial conversion of biolabile DOM to CDOM.Besides,there is no coupling between DOC concentration and a₃₅₀ value during the 2019 cruise,likely attributed to that marine primary productivity contributed significantly to seawater DOM in surface of the WAP Ocean.Finally,surface water in 2020 cruise is characterized by higher DOC concentration,higher BIX andβ/αvalues and more biolabile compounds,but lower a₃₅₀,SUVA₂₅₄,HIX and AI_mod values,likely suggesting that the salps leaching microbial DOM is an important contributor to fresh and biolabile DOM in surface WAP Ocean,and the spatial distribution trend of DOC concentration in agreement with the a₃₅₀value trend during the 2020 cruise due to the salps leaching microbial DOM into waters.These data,combined with correlations between environmental factors and DOM,suggest that salps,chlorophyll and microbial activity are responsible for substantial changes of DOM concentrations and compositions in surface water in South Shetland Islands.Our study suggests a rapid and great change of DOM biogeochemistry in Antarctic Peninsula in the context of climate warming were regulated by marine organisms.（3）Antarctic autumn and winter cruises:Due to limited sampling conditions,little is known about water DOM in the Southern Ocean autumn and winter period.Surface water samples were collected from the Antarctic Peninsula-South Orkney Island-South Georgia Island during five cruises in the austral autumn and winter（2020-48.1 cruise,2020-SOI cruise,2020-48.2 cruise,2020-48.3 cruise,and 2018-FRH cruise）,the DOC concentration,spectra and high-resolution mass spectrometry data of water samples were analyzed and compared.The DOM concentrations and compositions of seawater from the Antarctic Peninsula-South Orkney Island-South Georgia Island show apparently spatial hetergenous.Specifically,for 2020-48.1 cruise,surface water DOM is characterized by the lowest DOC concentration,the lowest BIX andβ/αvalues and the least numer of biolabile compounds,but the highest a₃₅₀,SUVA₂₅₄,HIX and AI_mod values,showing substantial microbial transformation from biolabile protein-like DOM to refractory humic-like DOM.For 2020-SOI cruise,water is characterized by the highest DOC concentration,BIX andβ/αvalues and biolabile compounds proportion,but the lower a₃₅₀,SUVA₂₅₄,HIX and AI_mod values,showing that strong the salps leaching fresh and biolabile microbial DOM is an important contributor to waters,and the spatial distribution trend of DOC concentration is the same as the a₃₅₀ value during the 2020-SOI cruise owing to the salps leaching DOM.Besides,primary productivity may regulate the spatial variation of DOC concentrations in the South Orkney Islands and South Georgia Islands.Similar to water samples in South Shetland Islands among three cruises,this section of water DOM in the Southern Ocean autumn and winter period also shows that microbial activity,salps and chlorophyll are important factors driving the Antarctic autumn and winter marine DOM biogeochemistry.The temporal and spatial variations of marine DOM between summer and autumn-winter are affected by the environmental conditions during the sampling period.In the West Antarctica sea,our study suggests a rapid and great change of DOM biogeochemistry in Antarctic Peninsula in the context of climate warming were regulated by marine organisms.