Stuides On Biogeochemistry Of DMS And DMSP In The East China Sea And The Yellow Sea

Dimethylsulfide (DMS) is the dominant volatile biogenic sulfur compound emanating from the ocean, which plays an important role in the global climate change and acid precipitation due to its oxidation products in the atmosphere. Although coastal and shelf regions only occupy a small part of the world ocean, they appear to be responsible for a large part of the oceanic DMS emission. Therefore studies on the biogeochemistry of DMS and its precursor DMSP in representative coastal waters offer a unique opportunity to link atmospheric chemistry and climate to the ecology and evolution of marine community, which will be helpful to accurately estimate the sea-to-air fluxes of DMS on a local and global scale and to predict the influence of oceanic emissions to the environmental and climate changes.In the present dissertation, we choose the East China Sea (ECS) and the Yellow Sea (YS) as the study areas that are affected seriously by human activities. The spatial and temporal variations of distributions of DMS and DMSP, sea-to-air fluxes of DMS and factors influencing them are systematically studied. Another objective of this study is to examine the seasonal variations of atmospheric DMS concentrations and their contributions to non-sea-salt sulfate (nss-SO₄^2-) in aerosols. The main conclusions are drawn as follows:(1) On the ground of gas-stripping chromatographic systems of seawater DMS in our lab, we integrate and develop a series of sampling and analysis methods of sulfur containing compounds including seawater DMS and DMSP, atmospheric DMS and methanesulfonic acid (MSA), non-sea-salt sulfate (nss-SO₄^2-) in aerosol with strict quality assurance and quality control, which lay a solid foundation for the following research work.(2) The distributions of DMS and DMSP and sea-to-air fluxes of DMS are determined in the East China Sea (ECS) and the South Yellow Sea (SYS) during Jun-Jul, 2006, Jan-Feb and Nov, 2007. The surface water concentrations of DMS, DMSPd and DMSPp in summer are 5.64 (1.70-12.24), 8.59 (2.37-14.77) and 18.97 (9.44-36.15) nmol L^-1, respectively. Winter concentrations are 1.78 (1.02-3.51), 3.92 (2.12-6.25) and 7.09 (3.80-13.34) nmol L^-1, and autumn concentrations are 3.38 (1.83-7.26), 5.40 (2.26-10.51) and 9.35 (3.25-26.64) nmol L^-1, respectively. DMS and DMSP concentrations show a notable seasonal variation with highest values in summer and lowest ones in winter, which corresponds well with the seasonal change of Chl-a observed in the study area. The spatial distributions of DMS and DMSP in the ECS and the SYS are obviously influenced by the Yangtze River effluent and the oligotrophic Kuroshio waters. When the distribution patterns are compared with each other in different seasons, they are nearly synoptic and decreased from inshore to offshore sites, without being strongly biased by temporal change. In addition, DMS and DMSP exhibit a consistent diurnal variation in different seasons, with higher levels in day and lower ones at night, indicating that the production processes of DMS and DMSP may be related to the sunshine radiation.Despite the highly variable physical environment of the ECS and the SYS and resultant large ranges in algal biomass, we observe two consistent correlations between DMS, DMSPp and Chl-a concentrations in different seasons, respectively. These results indicate that phytoplankton biomass might play an important role in controlling the distributions of biogenic sulfurs in the study area. The ratios of DMS/Chl-a and DMSPp/Chl-a exhibit obvious seasonal variations, with summer values being 2-fold higher than winter and autumn values, which could be attributed to the seasonal change in phytoplankton community structure. Data observed in the same cruises show that the species and biomass of dinoflagellates (high-DMSP-producers) decrease sharply from summer to autumn and winter, which consolidate the dominant status of diatoms in the phytoplankton community. The lost proportion of dinoflagellates may be responsible for the lower ratios of DMS/Chl-a and DMSPp/Chl-a in winter and autumn. Liss and Merlivat relationship (LM86) and Wanninkhof relationship (W92) are employed to calculate the sea-to-air fluxes of DMS based on the in-situ wind speeds and the measured DMS concentrations in the surface waters. As a result, the higher DMS concentrations in summer and large wind speeds in autumn contribute to the large fluxes of DMS in summer and autumn, respectively. Based on the average fluxes of DMS and the area of the ECS and the SYS, the annual DMS emission is estimated to be 8.47×10^-2 -19.11×10^-2 Tg S a^-1. Although the ECS and the SYS only occupies 0.27% of the total ocean in area, the contribution of ECS and the SYS to the global sea-to-air fluxes of DMS is estimated to be 0.58%, which means that shelf regions contribute significant amount to the total oceanic DMS flux compared to the open sea.(3) Seasonal variations of seawater, atmospheric DMS and aerosol compounds, potentially linked with DMS oxidation, such as MSA and nss-SO₄^2- are examined in the North Yellow Sea (NYS) from Jul, 2006 to Oct, 2007. The concentrations of DMS and DMSP exhibit pronounced seasonal variations, with the highest values in summer and the lowest in winter. As a mean, the surface waters concentrations of DMS, DMSPd and DMSPp in summer are 3.2, 2.5 and 2.9 times higher than those in winter, respectively. The annual averages of DMS, DMSPd and DMSPp in the NYS are 4.05±1.78, 6.94±2.75 and 11.82±5.46 nmol L^-1, respectively. Both DMS and DMSP display a similar distribution pattern in different seasons, decreasing gradually from the shore waters of Liaodong Peninsula and Shandong Peninsula to the open sea. These results reveal the influence of anthropogenic activities on the coastal environment, for example, enhancing the nutrient levels and consequently resulting in high primary production. Two significant correlations are found between DMS, DMSPp and Chl-a concentrations in the surface waters in one separate season, respectively. However, no correlation is found between integrated DMS or DMSPp and Chl-a concentrations at all stations in four seasons, which could be attributed to the different phytoplankton species as well as biomass in different seasons. Not only do different phytoplankton species produce different amounts of Chl-a, they also differ in their ability to form DMSP. The sea-to-air fluxes of DMS in the NYS vary widely in different seasons, and the mean annual fluxes obtained by the arithmetic of LM86 and W92 are 4.92±2.10μmol m^-2 d^-1 and 10.97±4.58μmol m^-2 d^-1, respectively. In connection with the area of the NYS, the preliminary DMS emission from the NYS is estimated to be 8.47×10^-2 -19.11×10^-2 Tg S a^-1. Although the NYS only occupies 0.017% of the global ocean in area, the contribution of the NYS to the total sea-to-air fluxes of DMS is estimated to be 0.029%, which also indicates that the coastal waters is an very important source of atmospheric DMS. Similar to the case of seawater DMS, atmospheric DMS concentrations also show an obvious seasonal variation, with the highest values in summer and the lowest ones in winter. However, no significant correlation appeared between atmospheric DMS concentrations and sea-to-air fluxes of DMS. The concentrations of MSA and nss-SO₄^2- in aerosols also show pronounced seasonal changes, however, their seasonal trends are different, due to the contribution of anthropogenic SO₂ to the nss-SO₄^2-. According to the observed MSA and nss-SO₄^2- concentrations as well as their ratios, the relative biogenic sulfur contribution to the total nss-SO₄^2- are estimated to be 11.0%, 10.4%, 2.0% and 2.8% in spring, summer, autumn and winter, respectively, implying that anthropogenic source is the major contribution to sulfur budget in the study area.(4) The distribution and cycling of DMS and DMSP are studied in the surface microlayer and corresponding subsurface waters of the YS in April, 2006. The average concentrations of DMS, DMSPd and DMSPp are 5.42 (1.78-12.75), 9.22 (2.85-19.73) and 17.50 (4.33-36.09) nmol L^-1 in the subsurface water, and those in the microlayer are 4.92 (1.69-10.66), 17.08 (3.13-38.82) and 22.54 (4.85-47.24) nmol L^-1, respectively. As a whole, no microlayer DMS enrichment is found due to the loss from the thin film during sampling. In contrast, DMSPd and DMSPp appear to be enriched in the microlayer with average enrichment factors of 1.39 and 1.98, respectively. The concentrations of DMS, DMSP and Chl-a in the microlayer are closely correlated with those in the subsurface water, suggesting that the materials in the microlayer could be related to and transported from the underlying water. The ratios of DMS/Chl-a and DMSPp/Chl-a are 4.57 (1.11-9.36), 21.11 (8.02-57.98) mmol g-1 in the microlayer, and 5.92 (1.70-12.90), 18.54 (6.84-41.39) mmol g^-1 in the subsurface water. These relative low ratios reflect the fact that diatoms dominate the phytoplankton array in the YS, which is in good agreement with the phytoplankton data obtained in the same cruise.The biological production rates of DMS in the microlayer and subsurface water are 7.31 (2.41-10.35) and 5.39 (2.96-13.53) nmol L^-1d^-1. By contrast, the microbial consumption rates of DMS are 5.56 (2.59-9.67) and 4.09 (1.89-7.13) nmol L^-1d^-1, respectively. Overall, the production and consumption rates of DMS in the microlayer are mostly higher than those in the subsurface water, demonstrating that the microlayer is biologically active relative to the underlying water. The biological turnover times of DMS in the microlayer and subsurface water are 1.12 (0.73-2.39) d and 1.93 (1.06-3.44) d, respectively, and the sea-to-air turnover time of DMS are 3.53 (0.10-26.28) min and 3.32 (0.08-24.45) d。Thus the above observations lead to a clear conclusion that the major sink of DMS in the microlayer is escape into the atmosphere, which greatly exceeds its bacterial consumption.