Response Of Colored Dissolved Organic Matter To The Typhoon And ENSO Events In The Taiwan Strait

Unusual or extreme climate events like typhoon,El Nino Southern Oscillation(ENSO)events happen frequently in the context of global change.Such events undoubtedly have important impacts on the biogeochemical cycle of dissolved organic matter(DOM)in the marginal seas.However,most studies in this field focused on the normal climate conditions,leaving a great blank on how the extreme events will influence the sources-sink issue of coastal DOM cycle.The Taiwan Strait is located in the subtropical region of the Western Pacific,where typhoon and rainstorm events happen frequently.Previous studies also found that the watermass movement and primary productivity of the Taiwan Strait was significantly influenced by ENSO events.Thus,this sea area is an ideal place to study the response of marine DOM dynamics to these extreme events.In this paper,Jiulong Estuary-Taiwan Strait junction area and the Southern Taiwan Strait was selected as the investigation areas.The absorption and fluorescence spectra was used to characterize the DOM properties.The impact of Typhoon Matmo occurred in summer 2014 and 2015-2016 strong ENSO event on CDOM sources,distribution and spectral characteristics of the Jiulong Estuary-Taiwan Strait junction area and the Southern Taiwan Strait were studied,respectively.The main results are as follows:(1)CDOM and FDOM abundance in the upper Jiulong Estuary increased significantly as a response to the heavy precipitation in the watershed triggered by Typhoon Matmo.However,these signals were weakened in the downstream of the estuary as a result of removal process occurred in the river-extuary interface and mixing process by the tidal movement.The estuarine plume signal derived from this event was only limited in the inshore area.(2)Typhoon "Matmo" had significant impacts on hydrological and biological process of the Jiulong Estuary-Taiwan Strait junction area,which will influence the source and composition of CDOM in this area.After Typhoon Matmo,the distribution of temperature and salinity in study area showed a sandwich-like model:high temperature and low salinity in inshore area,low temperature and high salinity in the middle area and high temperature and low salinity in offshore area.As a result,terrestrial input,autochthonous contribution from the algal bloom in upwelling zone and long-distance transport from the Pearl River plume were main sources of CDOM in the study area after Typhoon "Matmo".However,due to the much higher contribution of in situ biological activities before Typhoon "Matmo",the average abundance of surface CDOM in the study area was not increased after the thphoon process.(3)Since the runoff of the Pearl River increased significantly in 2015-2016 under the influence of the 2015-2016 super ENSO event,The contribution of the Pearl river water plume in the Southern Taiwan Strait increased to 4.3%in 2016,compared to the 1.4%in 2010(weak ENSO event)and 2.2%(before the super event).Accordingly,the CDOM abundance was also highest in summer of 2016(0.96 m-1),compared with the 0.69 m-1 in summer of 2010 and 0.90 m-1 in summer of 2014.(4)The summer hydrology in the Southern Taiwan Strait follows a three-end member mixing model.The three end members are the Pearl River plume,the South China Sea Surface Current and the South China Sea Subsurface Current.The contribution of the Pearl River plume to the CDOM pool in the Southern Taiwan Strait increased from 0.93%in 2010 to 32.7%in 2016.Contributions from processes like biological activities and sediment resuspension decreased from 23.9%in 2010 and 25.8%in 2014 to 15.0%in 2016.(5)The Shantou and Dongshan coastal upwelling enhanced while the Taiwan Bank upwelling weakened during 2015-2016 super ENSO event.Chl-a was highest(8.22?g/L)in surface water of Shantou and Dongshan upwelling area in the summer of 2016,much higher than in 2010(1.60 ?g/L)and in 2014(0.23 ?g/L).Thus,the biological contribution to the CDOM pool was also very high in summer,2016.However,the overall contribution of Dongshan upwelling to CDOM pool was lower in 2016(?a280=0.33 m-1)than 2014(?a280= 0.77 m-1).The CDOM abundance in Shantou and Dongshan upwelling area in 2016 was even lower than the average value of the whole study area in the summer of 2016,demonstrated the different response of upwellings in different area of Southern Taiwan Strait to 2015-2016 ENSO event.