Photosynthetically Produced Dissolved Organic Carbon Released Of Marine Phytoplankton In China Coastal Seas

Organic compounds synthesized by photosynthesis of phytoplankton, in additionto the part binding to the cell body in the form of the particulate organic matter, still aconsiderable part direct release in the form of photosynthetically produced dissolvedorganic carbon (PDOC). However, most past marine productivity observation wereonly determined on particulate portion of the primary productivity, no simultaneousdetermination on PDOC. Accordingly, most related food web trophic dynamics model,carbon cycle and carbon balance model also ignored this important part of carbonflow. This resulting in primary productivity and carbon sequestration capacityunderestimated in the global ocean.This study first carried out systematic field observation on phytoplankton PDOCin typical areas such as the Yellow Sea, the Yangtze River Estuary-East China Sea,,northern South China Sea, using the improved14C isotope tracer method to obtain thePDOC productivity distribution data as well as the distribution and variationcharacteristics of PER (percentage of extracellular release) ratio in study areas. In thisstudy, in addtion to system analysis on environmental regulation mechanism of thephytoplankton PDOC release in China coastal sea, the relationship of phytoplanktonspecies characteristics and PDOC release, the effects of phytoplankton release onmicrobial food cycle, also discuss the relationship between phytoplankton PDOCrelease and specific ecological processes e.g. phytoplankton seasonal blooms, theseasonal hypoxia in estuaries. The main results are as follows.PER ratio of primary productivity in the euphotic zone ranged from7.6to39.3%,the average PER was22.2%in different China sea. According to this result, carbonsequestration was roughly estimated in China coastal seas. The results showed thattotal carbon sequestration capacity was about9.4×10~8t in China coastal seas includingthe calculation of PDOC release. Ignoring phytoplankton PDOC products, carbonsequestration capacity in China coastal seas was underestimated about2.5×10~8t.Field observation research to spring bloom processes in the Yellow Sea in2009showed that, phytoplankton PDOC productivity and PER ration in the spring bloom were higher than those before the occurrence of algal bloom. While in the observationof two subsurface water bloom processes, PDOC release was significantly higher inthe bloom process that dominant species was smaller size of dinoflagellates than thatwith reasons species as larger particle size diatom. Large amounts of phytoplanktonPDOC release in the bloom periods led to heterotrophic bacterioplankton biomassincreased rapidly in study areas, showing the significance of PDOC productivity tomicrobial food cycle.The study results on the distribution variation of phytoplankton primaryproductivity in the Changjiang Estuary and its adjacent waters, and the relationshipwith the formation and development of hypoxia in the estuary bottom in Jun.-Oct.2006, showed that, the formation of the Yangtze River Estuary Hypoxia Zone insummer was closely related to the variation of phytoplankton productivity. Theformation of hypoxic core area had a direct relationship with strong algal bloomevents happened20days ago in the same area. In high productivity period in summer,phytoplankton PDOC release rate was high, but there was no direct relationshipbetween phytoplankton PDOC release in the euphotic layer and the underlyinghypoxia appear. There was close relationship between Hypoxia Zone movingnorthward in Aug.2006, and a wide range of high temperature and drought weather,Changjiang River Runoff substantially reduced in2006summer.Research results on heterotrophic bacteria distribution and phytoplankton PDOCproductivity distribution in the northern South China Sea in2009summer and wintershowed that, relatively higher bacterial productivity appeared in offshore area thatwith a higher proportion of phytoplankton photosynthetic product release, illustratingphytoplankton PDOC release can more effectively used by heterotrophic bacterialproduction. But the results of bacterial carbon quantity estimation showed that, PDOCproduction can not meet the need of heterotrophic bacterioplankton in northern SouthChina Sea, supplied by other DOC outside of phytoplankton direct release is also veryimportant to bacterial growth.Comprehensive analysis results on the environment control mechanism of PDOCshowed that, phytoplankton PDOC release controlled factors in China coastal seaincluding light, nutrients and phytoplankton size characteristics. The light was aimportant factor controlling the vertical distribution of PDOC production. In verticaldistribution, high PER ratio often occured in surface with the presence of lightinhibition and the euphotic layer bottom with week light. In horizontal distribution, PER in oligotrophic offshore waters dominated with small size phytoplankton weresignificantly higher than that in eutrophic coastal waters with large size phytoplanktondominant.