| Dimethylsulfide (DMS) is the most abundant volatile sulfur compound in the surface ocean. Considerable efforts have been devoted to studying the biogeochemistry of DMS in seawater as well as its climatic effects. On one hand, the oxidation products of DMS in the air are the major source of non-sea salt sulfate aerosol (nss-SO42-) and cloud condensation nuclei (CCN) which increase aerosol albedo over the sea and further form a possible negative feedback for global warming. On the other hand, the oxidations of atmospheric DMS could also contribute to the acidity of aerosols and rainfall. The eastern China continental seas are an important part of continental shelf sea area. It is well known that although the continental shelf sea area occupies only a small part of the world ocean, it makes a great contribution to the global oceanic DMS emissions. Therefore, studies on the biogeochemistry of DMS and its precursor-dimethylsulfoniopropionate (DMSP) in representative continental 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 on the environmental and climate changes.In this dissertation, we focus on the spatial and temporal distributions of DMS and DMSP in the eastern China continental seas and to investigate the biological and geophysical factors governing the distributions of these compounds in this study area. Furthermore, we also measured biological production and consumption rates of DMS to obtain more insight into the biological control of DMS and DMSP concentrations in the study area. Chlorophyll a (Chl-α) and particulate DMSP (DMSPp) samples were size-fractionated by passing the sample though different size membrane filters to investigate the relative contributions of different size fractions of phytoplankton to DMSPp. The experiment of diurnal variations was conducted for studying the diurnal control and limiting factors of DMS and DMSP. The sea-to-air fluxes of DMS in the study area were calculated for assessing the contribution of the eastern China continental seas to global oceanic DMS emissions. Another objective of this study is to examine the seasonal variations of atmospheric MSA and nss-SO42- concentrations and to estimate the contributions of the biogenic sulfur to nss-SO42- in aerosols. The main conclusions are drawn as follows:(1)The distributions of DMS and DMSP were determined in the Bohai Sea (BS) and the Yellow Sea (YS) during April 2010, June 2011, September 2010 and December 2009. The surface water concentrations of DMS, DMSPd and DMSPp in the BS and the YS during spring were 1.77 (0.48-4.92),3.98 (0.68-11.32) and 17.89 (2.82-52.33) nmol L-1, respectively; summer concentrations were 6.85 (1.60-12.36), 7.27 (2.28-19.05) and 61.87 (6.28-224.01) nmol L-1, respectively; autumn concentrations were 2.64 (0.78-7.95),4.89 (1.42-11.30) and 26.41 (6.24-137.87) nmol L-1 and winter concentrations were 0.95 (0.07-3.30),1.18 (0.22-3.54) and 5.01 (1.63-12.33) nmol L-1, respectively. DMS and DMSP concentrations in the BS and the YS showed a notable seasonal variation with the highest values in summer and the lowest ones in winter. The concentrations of DMS and DMSP decrease from inshore to offshore, indicating that the BS and the North Yellow Sea (NYS) are greatly influenced by eutrophication from human activities. The spatial distributions of DMS and DMSP were significantly affected by Changjiang River diluted water and South Yellow Sea Cold Water Mass (SYSCWM) in the South Yellow Sea (SYS) and showed the distinct seasonal differences. Depending on the seasonal difference of SYSCWM, high concentrations of DMS and DMSP appeared in the center of the YS in spring and summer, but it was opposite in autumn and winter. The high values of DMS and DMSP were recorded in the south of SYS in summer and autumn, due to the seasonal variations of flow and direction of Changjiang River diluted water. In addition, DMS and DMSP exhibited a consistent diurnal variation in different seasons, with higher levels in the daytime and lower ones in the night, indicating that the production processes of DMS and DMSP may be related to the solar radiation.The distributions of DMS and DMSP were determined in the East China Sea (ECS) during March 2011, June 2010 and November-December 2010. The surface water concentrations of DMS, DMSPd and DMSPp in the ECS during spring were 1.84 (0.79-4.86),2.59 (1.03-7.96) and 12.84 (2.21-30.03) nmol L-1, respectively. Summer concentrations were 2.99 (0.56-5.97),4.74 (2.63-7.26) and 15.92 (6.08-34.78) nmol L-1. Autumn concentrations were 1.72 (0.72-5.95),2.32 (1.24-6.20) and 8.30 (3.27-47.55) nmol L-1.The seasonal variation patterns of DMS and DMSP in the ECS were the same as those in the BS and the YS. The DMS and DMSP concentrations in the ECS also showed a notable seasonal variation with the highest values in summer and the lowest ones in winter. The spatial distributions of DMS and DMSP in the ECS ere obviously influenced by the Changjiang River effluent and the oligotrophic Kuroshio waters. Compared with each other in different seasons, the spatial distributions of DMS and DMSP were nearly decreased from inshore to offshore sites, without being strongly biased by temporal change.(2) In this study, the DMS and DMSPp concentrations were significantly correlated with Chl-a concentritions in the region of algal bloom, such as the SYS during spring, the BS and the NYS during summer and the ECS in autumn. Besides, the significant correlation between DMS, DMSPp and Chl-a was also found in the ECS in summer when the proportion of high DMSP producer was increased. Our results indicated that phytoplankton biomass might play an important role in controlling the distribution of DMS and DMSPp, and the reasonable correlations are to be expected between DMS, DMSPp and Chl-a when the phytoplankton assemblage is dominated by high DMSP producers or monospecific blooms.(3) The production and consumption rates of DMS were determined in the ECS during November-December 2010 and March 2011 and in the BS and YS during September 2010, March,2011 and June 2011. The production and consumption rates of DMS in the BS and the YS during spring were 12.32 (0.65-34.70) and 7.94 (0.17-27.38) nmol L-1 d-1. The summer rates were 20.78 (2.90-35.86) and 11.42 (1.92-23.34) nmol L-1 d-1; the autumn rates were 9.25 (0.55-28.63) and 5.11 (0.1-16.46) nmol L-1 d-1. On the whole, DMS production and consumption rates in the BS and the YS showed a notable seasonal variation with thew highest values in summer and the lowest ones in autumn. The production and consumption rates in the ECS during spring were 7.15 (2.52-16.63) and 4.75 (0.79-14.66) nmol L-1 d-1; autumn rates were 5.07 (1.49-12.79) and 3.36 (0.36-8.33) nmol L-1 d-1. The seasonal variation in the production and consumption rates of DMS in the ECS was in the following order:spring> autumn.The high production and consumption rates were recorded in the regions affected by human activities or algal bloom, suggesting that the DMS production and concumption rates are closely related to the biological activity of phytoplankton and are significantly influenced by human activity. The production and consumption rates of DMS in spring were higher than those in autumn, indicating that the life activities of microorganism were more vigorous in spring. The correlation analysis indicated that there was a significant seasonal difference in the primary controlling factors for the production and consumption of DMS. In the BS and the YS, the Chl-a and DMS concentrations played significant roles in controlling the DMS production and concumption rates in the region of spring bloom. In the ECS, DMS biological production and consumption rates showed significant negative correlations with the temperature in autumn. These results implied that the biological production and consumption of DMS were influenced by various biological and environmental factors such as in situ temperature, salinity, DMS and DMSPd concentrations, Chl-a level and bacterial abundance, but the degree of influence varied in different seasons.(4) The study of size distributions of Chl-a and DMSPp were conducted in the ECS and the SYS during spring and in the BS and the YS during summer, respectively. Our data showed that larger nanoplankton was the main contributors of DMSPp and Chl-a during the two cruises. It is suggested that larger nanoplankton fraction is mainly composed of diatoms, according to the phytoplankton data obtained in the same cruise. In addition, our data showed that the microplankton fraction significantly contributed to proportions of DMSPp and Chl-a in the estuary area and coastal region such as Changjiang River estuary, while the picoplankton fraction made an important contribution to proportions of DMSPp and Chl-a in the oligotrophic waters, such as the central YS in summer. This result indicates that smaller-sized phytoplankton, particularly pico-phytoplankton, plays a much greater role in oligotrophic waters while microphytoplankton dominance was usually associated with nutrient-rich coastal areas.(5) The sea-to-air fluxes of DMS in the eastern China continental seas varied widely in different seasons, and the mean annual fluxes obtained by the equation of N2000 were 5.59 μmol m-2 d-1 in the BS and the YS and 7.20 μmol m-2 d-1 in the ECS, respectively. According to the corresponding areas of China continental seas, the preliminary DMS emission was estimated to be 2.98×10-2Tg S a-1 in the BS and the YS and 6.48×10-2 Tg S a-1 in the ECS, repectively. Although the eastern China continental seas occupies only 0.13% of the global ocean in area and the ECS occupies only 0.21% of the global ocean in area, the contribution to the total sea-to-air emissions of DMS was estimated to be 0.09%-0.2% in the BS and the YS and 0.20%-0.43% in the ECS, respectively. These results indicate that although the eastern China continental seas occupies only a small part (0.34%) of the world ocean, the emission of DMS from this area has a relative significant contribution to the global oceanic DMS fluxes.(6) The relative contribution of biogenic sulfur to the total nss-SO42- was estimated to be 9.70%,8.90%,4.90% and 1.95% in spring, summer, autumn and winter over the BS and the NYS, respectively. Biogenic contribution to nss-SO42- was 17.60%,8.10%,8.70% and 3.30% during spring, summer, autumn and winter over the SYS, respectively. In the samples collected over the ECS, the contributions accounted for 13.6%,5.3% and 4.3%, respectively. The biogenic sulfur relative contributions to the total nss-SO42- over the eastern China continental seas showed a clear seasonal variation with the highest values in spring and the lowest ones in winter. Especially in the SYS and the ECS, its contribution in spring was significantly higher than that in other seasons. The average annual contribution rates of biogenic sulfur to the total nss-SO42- over the BS and the NYS, the SYS and the ECS were 6.36%,9.43% and 7.73%, respectively, implying that anthropogenic source is the major contribution to sulfur budget in the study area and that the BS and the NYS are significantly affected by human activities, compared with SYS and ECS. This may be relating to the geographical location of the BS and NYS. |
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