Distributions And Influencing Factors Of Dissolved Carbohydrates In The Coastal Waters Of China

Carbohydrates are the largest identified fraction of dissolved organic carbon in the ocean, and they serve as energy, storage, and structural components in marine organisms, which play important roles in the marine carbon cycle and marine food web cycle. Therefore studies on the biogeochemistry of dissolved carbohydrates in representative coastal waters will be helpful for realizing the characteristics, sources and degradation pathways of the organic matter.In the present dissertation, distributions, influencing factors of dissolved monosaccharides (MCHO) and polysaccharides (PCHO), and their contributions to dissolved organic carbon (DOC) in the coastal waters of China, i.e. the South China Sea, the East China Sea, the Yellow Sea and the Jiaozhou Bay, were systematically studied for the first time. Furthermore, the distributions and enrichment factors of MCHO and PCHO in the surface microlayer water of the Jiaozhou Bay and its adjacent area were detailedly investigated. The main conclusions are drawn as follows:1. The spectrophotometric method has been developed in our lab for the determination of dissolved MCHO and PCHO in seawater based on the references. The detection limit is 2.2μM C and the precision varies from 1.7% to 8.3%.2. The concentrations of MCHO and PCHO were determined in the northeastern South China Sea from March to April 2009. In the surface water, average concentrations of MCHO and PCHO were 7.0 and 9.5μM C, respectively. We found that these values in the study area were much lower than the values obtained from the other coastal and estuarine locations. On the other hand, our data were consistent with those of the Atlantic and Pacific Ocean water, which was attributed to the low phytoplankton biomass and primary production in this area. Higher concentrations of MCHO and PCHO occurred in the Zhujiang delta region, the northwest and northeast regions of the Luzon Island. The ratios of PCHO to MCHO ranged from 0.7 to 2.5 with an average of 1.4, suggesting that the polymeric compounds were much more abundant in total dissolved carbohydrates. Higher concentrations of MCHO and PCHO were observed in the water less than a depth of 150 m for vertical profiles. Higher values were also found in the deeper water (500 m,1000 m, or 2000 m depth). The concentrations of PCHO showed a diurnal variation at an anchor station with the highest value at 15:00 and lowest value at night. The maximum concentration of MCHO appeared at 18:00, which was later than that of PCHO. This observation further supported the hypothesis that MCHO was derived, in part, from the hydrolysis of PCHO.3. The horizontal and vertical distributions of MCHO and PCHO were examined in the East China Sea and the South Yellow Sea in November 2007 and May 2009, respectively. The concentrations of MCHO and PCHO in the surface water ranged from 3.1 to 11.7μM C and from 4.0 to 17.6μM C in autumn, respectively, with average values of 6.2 and 6.7μM C. In contrast, the average values of MCHO and PCHO in spring were 11.5 and 21.6μM C, respectively, which were higher than those in autumn. On average, MCHO and PCHO accounted for 5.1% and 5.5% of DOC in autumn, respectively, while MCHO and PCHO accounted for 6.8% and 13.1% of DOC in spring. This result indicated that dissolved carbohydrates were an important constituent of DOC in the surface seawater of the East China Sea and the South Yellow Sea. The horizontal distributions of MCHO and PCHO were obviously influenced by the Yangtze River effluent and the oligotrophic Kuroshio waters, decreasing from inshore to offshore sites. At transect 03, MCHO, PCHO and phytoplankton biomass showed low values in the Yellow Sea Cold Water Mass. At the southeastern part of transect 05, the low concentrations of NCHO, PCHO and phytoplankton biomass reflected the characteristics of Kuroshio waters.4. Seasonal variations of MCHO and PCHO were investigated during three cruises in the North Yellow Sea from January to October 2007. The concentrations of MCHO and PCHO varied from 4.5 to 19.3μM C and from 0.8 to 27.0μM C in the surface water, respectively, with annual averages of 10.8 and 10.1μM C. On average, MCHO and PCHO accounted for 4.1% and 4.8% of DOC, respectively. The concentrations of MCHO showed an obvious seasonal variation with highest average value in winter and lowest value in autumn. In contrast, the maximum average concentration of PCHO was 12.8μM C in spring and the minimum value was 8.1μM C in autumn, respectively. Both MCHO and PCHO displayed a similar distribution pattern in different seasons, decreasing gradually from the shore waters of Liaodong and Shandong Peninsula to the open sea. The concentrations of MCHO and PCHO at four stations in autumn decreased gradually from the surface layer to the bottom layer. However, the difference of concentrations between the surface and bottom water was not significant. This trend was consistent with the well-mixing aquatic system between 0 and 30 m in autumn, and the supplement of dissolved carbohydrates from the bottom water by microbial hydrolysis of organic matter.5. The distributions and seasonal variations of MCHO and PCHO in the surface water of the Jiaozhou Bay were investigated during four cruises from August 2007 to April 2008. The concentrations of MCHO and PCHO ranged from 1.7 to 65.9μM C and from 0.3 to 210.2μM C for all samples, respectively, with annual averages of 13.2 and 21.0μM C. On average, MCHO and PCHO accounted for 4.5% and 7.4% of DOC, respectively. This result indicated that dissolved carbohydrates were an important constituent of DOC in the surface seawater of the Jiaozhou Bay. Although the concentrations of MCHO and PCHO varied during difference cruises, higher concentrations occurred in the east and west coast region, decreasing gradually from the coast to the central region. This pattern may be strongly influenced by the industrial and domestic waste discharges, the development of shellfish aquaculture in the western and northeastern eutrophic coast, and the tidal current which moves back and forth in the north-south direction. The concentrations of MCHO and PCHO exhibited a marked seasonal variation, with the highest values in winter and the lowest in spring. As a mean, the concentrations of MCHO and PCHO in winter are 2.2 and 4.4 times higher than those in spring, respectively. The ratios of PCHO to MCHO exhibited two peaks in winter and summer, respectively, as well as the ratios of PCHO to DOC, which fundamentally coincided with those of phytoplankton biomass in the bay. MCHO were biologically labile molecules, which could be taken up directly and metabolized quickly by heterotrophic bacteria. Chemically they would be more stable than PCHO in seawater. Thus, it was reasonable that the ratios of MCHO to DOC varied less than those of PCHO. Negative correlations between concentrations of total dissolved carbohydrates and salinity in summer and spring suggested that riverine input around the Jiaozhou Bay had an important contribution to the concentrations of dissolved carbohydrates in surface seawater.6. The properties and enrichment factors of dissolved MCHO and PCHO were studied in the sea surface microlayer and their related bulk surface water of the Jiaozhou Bay and its adjacent area in July and November,2008, respectively. The concentrations of MCHO and PCHO in the surface microlayer varied in the ranges of 4.7-54.7 and 8.3-48.6μM C in July, respectively, compared to 7.5-42.5 and 11.3-36.6μM C in November. Average concentrations in July were 22.5 and 24.3μM C for MCHO and PCHO, respectively, while those values in November were 15.3 and 20.8μM C. The values were higher in July than in November, presumably due to the remarkably high phytoplankton biomass caused by a green-tide occurred in July 2008 in the bay and its adjacent area. MCHO and PCHO exhibited an extent of enrichment in the microlayer relative to those in the surface water. The average enrichment factors of MCHO and PCHO in the surface microlayer were 1.3 and 1.4 in July, respectively. In contrast, the average enrichment factors of MCHO and PCHO in November were 1.9 and 1.6. Thus, the microlayer enrichment of MCHO and PCHO also displayed seasonal variation, with smaller value in summer, which was attributed to the carbohydrate characteristics and the physical and chemical environment. Our data also showed that the concentrations of MCHO, PCHO, and DOC in the surface microlayer were strongly correlated with those in the surface water, indicating that there was a strong exchange effect between the microlayer and the underlying water. The concentrations of total dissolved carbohydrates in the bulk surface water were closely correlated with salinity during the cruises, suggesting that riverine input and water exchange processes had an important effect on the concentrations of dissolved carbohydrates in surface seawater of the investigated area.