Succession Of Diatoms And Dinoflagellates In The Changjiang Estuary And Its Impact On The Storage Of Refractory Dissolved Organic Matter In The Ocean

The ocean plays an important role in the sequestration of atmospheric carbon dioxide and mitigation of climate change.Research on the storage and variation of marine organic carbon under multiple stressors is important for the global carbon cycling,proper environmental policy and reasonable climate change interventions.Phytoplankton,the key source of marine organic carbon,plays an essential role in determining the composition and nature of marine organic carbon.In recent years under the climate change and eutrophication,the succession between diatoms and dinoflagellates has been widely taking place in estuaries and offshore waters in the world.The subtle changes in phytoplankton community composition,especially the succession of key species,have implicit but important effects on the composition and stability of the marine dissolved organic carbon.Under this background,the key themes of this study includes:1)the succession of Skeletonema costatum and Prorocentrum donghaiense,the dominant causative species of bloom in the Changjiang estuary and adjacent areas under multiple stressors,2)the potential impact on the accumulation of refractory dissolved organic matter(DOM)under the background of the increasing dinoflagellates.In this study,high performance liquid chromatography was used to detect targeted organic matter molecules such as phytoplankton pigments and amino acids,whereas ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS)was used to analyze non-targeted organic matter molecules.Other basic environmental parameters were also analyzed.In this study,the logical progressive content is carried out in the following three aspects:1)reconstructing of diatoms and dinoflagellates succession in the Changjiang estuary and adjacent areas over the past century based on preserved sedimentary phytoplankton pigments;2)investigating and comparing the composition and early degradation features of diatom-and dinoflagellate-derived DOM,respectively,based on laboratory incubation experiments;3)revealing the marine DOM dynamics off the Changjiang Estuary in terms of its stability and the potential contribution to refractory DOM,with a summer dinoflagellates bloom as a breakthrough.The main findings are:The early diagenetic effects of preserved sedimentary pigments were removed by a organic matter degradation model,in order to ensure that the key diatoms and dinoflagellates succession reconstruction indicator(PER/FUCO ratio)was free from the early diagenetic impact.Based on the PER/FUCO ratio changes in core A5-4,the time window for the succession of diatoms and dinoflagellates(increase of dinoflagellates)was found as the early 1990s.Combined with the findings of the different coupling of the diagnostic pigment of dinoflagellates(PER)in the surface and near bottom waters,relative to other diagnostic pigments,it is proposed that the effect of dinoflagellates blooms on the burial of pigments in sediments is significantly stronger than other phytoplankton blooms including diatoms.Together with the numerical simulation results,we further propose that the changes of PER/FUCO ratio in the sediments should be due to the increase of dinoflagellates blooms,rather than a common all-year round dinoflagellates biomass increase.Further indoor incubations were carried out to clarify changes in the DOM properties due to phytoplankton succession.Both diatom-and dinoflagellate-derived DOM showed high bioavailability during the 90 days incubation.Most of the total hydrolysable dissolved amino acid(THDAA)was degraded and transformed in the first3 days(pre-incubation period),whereas other compounds degraded more slowly and persisted until middle(15-50 day)and end(51-90 day)of incubation stage.The mean values of THDAA carbon yield percent(THDAA-C%),degradation index(DI)values,and protein-like compounds%and the molecular lability index(MLB_L%)of the diatoms-DOM were 3.4%,4.0,19.8%,and 27.2%,respectively,whereas for the dinoflagellates-DOM they were 1.7%,2.7,15.6%,and 20.0%,respectively.Higher values in the diatom-incubations were found than that in the dinoflagellate-incubations,indicating that the DOM released during diatoms degradation was more labile compared to that of dinoflagellates,and hence the DOM released during diatoms degradation was more easily decomposed and transformed.The mineralization fraction of DOC released by the degradation of diatoms and dinoflagellates were 30.3%and21.1%,respectively.The mean value of carboxyl-rich alicyclic molecules percent(CRAM%)in the dinoflagellate-incubation(56.7%)was higher than that in the diatom-incubation(50.6%),indicating that the proportion of refractory DOM released by microbial degradation of dinoflagellates was higher than that of diatoms.Our results of both THDAA and FT-ICR MS showed that the DOM released from the microbial degradation of dinoflagellates had a higher proportion of refractory DOM and was more stable in the incubation system,which was more beneficial to the accumulation of refractory DOC.Based on field surveys conducted in March,July and October 2022 off the Changjiang Estuary,respectively,we were able to reveal the DOM dynamics.On one hand,the FT-ICR MS results showed that DOM in the Changjiang estuary and adjacent area was significantly influenced by the input of terrestrial aromatic compounds in summer,and the terrestrial contribution to refractory molecules of DOM in the study area was significant.On the other hand,the phytoplankton growth was vigorous in summer,corresponding to the labile DOM presence as revealed by amino acids.That is,the THDAA-C%in the surface waters was 2.1%,4.1%,and 1.8%in spring,summer,and autumn,respectively.Terrestrial DOM was the main source of CHOS compounds,while phytoplankton-derived DOM yield a bigger contribution to CHO and CHON compounds.The CRAM%was 59.0%(spring),57.9%(summer),and 61.8%(autumn)in DOM of the surface waters,and it was 58.5%(spring),59.1%(summer),and 60.3%(autumn)in the near bottom waters.After removing the salinity effect,the labile,semi-refractory and refractory fractions were 14.1%,27.5%and 58.4%respectively in the dinoflagellates bloom stations,while in the diatom-dominated comparison stations the three fractions were 15.6%,26.5%and 57.9%respectively,in summer.The significant higher presence of the refractory DOM in dinoflagellate sites relative to diatom sites,was consistent with the findings from the incubations in which CRAM%was 56.7%(dinoflagellates-incubation)and 50.6%(diatoms-incubation).respectively.With further consideration and removal of the terrestrial contribution on the refractory DOM,the difference in CRAM%between the dinoflagellates blooms stations and diatom-dominated comparison stations was 2.6%.Both indoor incubations and field investigations showed that the degradation and release of organic matter from dinoflagellates provided more refractory DOM than diatoms did.Under the background of diatoms and dinoflagellates succession,more dinoflagellates bloom is expected to enhance the stability of the DOM in the Changjiang estuary and adjacent area.Our findings are helpful in clarifying the influence of phytoplankton succession on the stability of DOM pools in estuarine and offshore area,and our work also provides scientific basis for the assessment of carbon storage and marine carbon sequestration potential in estuarine and offshore area.