Biodegradation Of Extraneous Dissolved Organic Matter From Kuroshio Intrusion In The Northern South China Sea

Marine dissolved organic matter(DOM)is a major carbon reservoir in the ocean and plays an important role in biogeochemical cycles.Biodegradation of DOM is a critical process governing DOM distribution and transformation,and the extent and rate of such degradation are under the influence of various environmental factors such as microbial community structure and nutrient levels.Kuroshio current,being a western boundary current,intrudes the South China Sea(SCS)when passing by the Luzon Strait and imports high levels of extraneous DOM into the SCS,which remarkably alter the environmental conditions that may have potential influences on the DOM biodegradation process.To investigate the DOM removal process under changing environmental conditions,we estimated the amount of DOM which was removed during the Kuroshio intrusion,using an isopycnal mixing model based on the in-situ data measured from two cruises.A significant amount of DOM removal was found and may be attributed to promoted DOM biodegradation.Besides,high levels of DOM removal were found to coincide with high chlorophyll concentrations,which indicated a potential nutrient replenishment from DOM degradation,supporting a higher level of primary production.How environmental factors affect DOM degradation processes remained unclear.To investigate the mechanisms of DOM biodegradation,we conducted onboard incubation experiments under different environmental conditions.We found that more Kuroshio DOM was degraded by SCS microbes than Kuroshio microbes,and SCS microbes degraded more Kuroshio DOM than SCS DOM.This agreed with our hypothesis that changing environmental conditions stimulated DOM biodegradation.DOM biodegradation was also found to be promoted with an addition of nutrients,the reason of which could be higher N or P demand by microbes inferred from microbial C:N:P ratios.The accommodation of microbial community structure by nutrients level could be another reasonable explanation.The effect of this stimulated DOM biodegradation during Kuroshio intrusion was also studied.Combining field data and incubation experiment results,we calculated a bioavailable DOM flux of 3.58 × 109 mol C d-1 across the Luzon Strait.Upon degradation,0.485-3.007 mmol N m-2 d-1 could be contributed to the surface water in northern SCS.This nutrient contribution is comparable to that resulted from atmosphere deposition and N2 fixation,highlighting the importance of extraneous DOM as a nutrient source in this area.