Distribution And Variation Of Organic Carbon And Its Influencing Factors In South Yellow Sea

With the launch of a series of large international projects such as JGOFS and SOLA, marine organic carbon cycling receives considerate attention, in particular, carbon cycling in continental margin sea. The thesis looked at Total Organic Carbon (TOC) concentrations and their seasonal variation throughout the whole South Yellow Sea, based on the four-season investigation from 2006 to 2007. TOC concentrations, distribution and their influencing factors in South Yellow Sea were analyzed. Major conclusions are itemized as following:(1) In Spring, TOC concentration collected in northern part was higher than that in southern part. High value areas were found offshore while TOC concentrations in south Changjiang Estuary and middle open sea were relatively low. Horizontal distribution of TOC had similar patterns in western offshore and southern shallow sea, but in middle sea area, TOC in the surface layer distributed with difference from those in bottom layer. In vertical direction, TOC concentrations did not vary much as the depth, although there seemed to be four typical distribution patterns. The reason of high offshore TOC values could be due to the longshore transportation or biological production, and very likely the low values of TOC in bottom layer were the result of dilution from modified water in Yellow Sea Warm Current.(2) In Summer, TOC distribution had significant difference from South to North. While TOC concentrations in northern area were high and decreased sharply from offshore to open sea, while the TOC in south were relatively low and increased from offshore to open sea. High value areas were found in northern offshore, on the other hand, northern Jiangsu offshore area, northern Changjiang Estuary and central South Yellow Sea had large-scale low values in their bottom layers. In addition, TOC presented significant stratification, as high TOC values were usually in surface layer or the 10 m depth layer while TOC concentrations below thermocline (20m depth) were low.Within western offshore area, TOC distribution pattern in surface layer resembled anticlockwise circulation pattern in Yellow Sea, which indicated that Western Circulation in South Yellow Sea was the major reason for the TOC distribution in offshore surface layer. In addition, the analysis showed that the impact of circulation to TOC concentrations in sea water only had negative relevance to suspended particles, and had little connection to other parameters. Northern Jiangsu offshore currents, Changjiang diluted water and cold water mass from Yellow Sea had dilution effects to TOC concentrations in the area. It was found out that TOC concentrations in Changjiang diluted water decreased as the increase of suspended particles, and although the correlation to Chlorophyll a (Chl-a) was not significant, TOC concentrations increased with consumption of nutrients in sea water. In Yellow Sea cold water mass, TOC had close relation to salinity and Chl-a, primary production from phytoplankton increased the TOC in sea water, while high-salinity water in open sea had dilution effects to TOC concentrations in cold water mass.(3) In Autumn, southern and northern areas had large difference in TOC, i.e., TOC concentrations in north were lower with no variation from offshore to open sea, while the concentrations in south were relatively high. The high value areas were found in southern Changjiang Estuary (Jiangsu), with the highest value of 13.8mg/L, which was one order higher than the average value (1.99mg/L). Vertically, TOC concentrations varied largely among various sections, with four different scenarios. In terms of average values, TOC concentrations decreased accordingly from button layer to 10m depth layer, surface layer, while the 30m depth layer held the smallest value.High TOC areas from southern part seemed to be due to re-suspension from the sedimentation surface, which is consistent to the observation that TOC concentrations from button layer were much higher than other layers. In addition, as the Changjiang diluted water shifted south and the water transparency got increased, biological production contributed a lot to the TOC concentration in the sea area.(4) In Winter, TOC concentrations in north remained low with little difference from offshore to open sea, however, in the south, TOC concentrations were high. High value areas for TOC were found in offshore northern Jiangsu tidal flat while the low value areas were in the central and south side. Since water in the area were homogeneously mixed, TOC concentrations on various water layers presented almost identical patterns. In vertical direction, TOC horizontal pattern from different sections varied largely although the average concentrations of most layers had similar values.(5) Seasonal variation of TOC in South Yellow Sea: TOC concentrations had the highest values in Autumn and the lowest in Winter. In Spring and Summer, northern sea area had higher TOC concentration with the high concentration area in western offshore while the TOC were lower in the south with low concentration area in northern offshore Jiangsu and northern Changjiang Estuary. However, during Autumn and Winter time, northern sea area presented opposite scenarios while the south had the higher TOC. In terms of influencing factors, TOC distribution were strongly affected by mass water movement in all seasons although the effects seemed to more significant in Summer and Winter.