Studies On The Distribution Of DMS And DMSP And Influencing Factors In The Yellow Sea And Bohai Sea, China

Dimethylsulfide (DMS) produced by marine phytoplankton is the dominantvolatile biogenic sulfur compound emanating from the ocean, which is an importantsubstance in the global sulfur cycle. The methanesulfonic acid (MSA) andnon-sea-salt sulfate (nss-SO42-) are important contributors to acid rain, which are themajor oxidation products of DMS in the atmosphere. In addition, the generatednss-SO42-aerosols can increase the number of cloud condensation nuclei (CCN) ormake the original CCN grow larger. As the main precursor of DMS,dimethylsulfoniopropionate (DMSP) is a sulfur metabolite of marine algae. TheDMSP can be cracked into DMS and arylic under the action of DMSP lyase releasedfrom bacteria or some marine algae, which has an important influence on the DMSconcentration distributions.In the present thesis, we chose the Yellow Sea (YS) and Bohai Sea (BS) as thestudy areas which were affected by human activities seriously. The DMS and DMSPconcentration distributions and influencing factors in the YS and BS were studied.Besides, their diel variations and seasonal variations were also analyzed. Furthermore,the size distribution of DMSPp and the sea-to-air flux of DMS were discussed. Inaddition, the DMS biological production and consumption rates and DMSPddegradation rates were preliminary investigated. The main conclusions were drawn asfollows:The concentrations of dimethylsulfide (DMS) and dimethylsulfoniopropionate(DMSP) were measured in situ in the YS and BS during November1-20,2012andJune21-July11,2013to study their concentration distributions and influencingfactors. Besides, their diel variations and seasonal variations were also analyzed.Furthermore, the size distribution of DMSPp and the sea-to-air flux of DMS werediscussed. The concentrations of DMS, DMSPd and DMSPp in the surface water ofthe YS and BS during autumn were (3.92±1.31)(1.15-6.52),(3.63±2.19)(0.57-8.85) and (13.34±4.70)(5.52-28.73) nmol L-1, respectively. And their concentrations insummer were (12.01±6.00)(1.87-27.63),(10.66±3.94)(3.93-18.38) and (25.97±8.31)(11.51-45.64) nmol L-1, respectively. The concentrations of DMS and DMSP in thesurface water of the YS and BS showed a significant seasonal variation with highervalues in summer, which corresponded well of Chl-a observed in the surface water ofthe study area. The horizontal distributions of DMS and DMSP were in line with eachother in autumn and summer, they were gradually decreased from inshore to offshoresites, and their concentrations in the BS were higher than in the YS. Generallyspeaking, the vertical distributions of DMS, DMSPd and DMSPp in autumn andsummer were also similar with each other, their high values occur in the suface ormiddle water. In addition, their diel variations followed the same trend that higherconcentrations in day and lower concentrations at night, indicating that thebiosynthesis of DMSP was closely related with the light.Significant positive correlations were found between DMS, DMSPp and Chl-a,implying that phytoplankton biomass might play an important role in controlling theproduction and distributions of DMS and DMSP. The ratios of DMS/Chl-a presentedobvious seasonal variation, with summer values was1.67times than autumn values,which might be caused by the change of phytoplankton community structure andbiomass in autumn and summer. Moreover, Small phytoplankton (>20μm) and thelarger nanophytoplankton (5-20μm) contributed to the vast majority of Chl-a andDMSPp in the YS and BS.The sea-to-air fluxes of DMS in the YS and BS during autumn and summer were(11.81±10.59)(0.11-43.45) and (20.81±20.37)(0.49-92.30) μmol m-2d-1, respectively.The larger wind speed [(7.23±3.78) m s-1] in autumn and the higher DMSconcentrations [(12.01±6.00) nmol L-1] in summer contributed to the large fluxes ofDMS. According to the average fluxes of DMS in the autumn，summer, spring and thearea of the YS and BS, we estimated the annual DMS emission to the atmosphere tobe9.22(6.30-11.10)×10-2Tg S. Although the area of the YS and BS only accountedfor0.13%of the global ocean, the contribution of the YS and BS emission to theglobal ocean emission was estimated to be0.38%, which demonstrated the fluxes of the YS and BS as a part of shelf regions was very large and the roles of shelf regionsin the global ocean DMS emissions could not be ignored.The biological production and consumption rates of DMS were measured in situin the YS and BS during November1-20,2012and June21-July11,2013. Thebiological production and consumption rates of DMS in the surface water of the YSand BS during autumn were (6.82±2.44)(2.92-11.52) and (3.06±1.57)(0.80-5.52)nmol L-1d-1, respectively. And the biological production and consumption rates ofDMS during summer were (13.35±5.62)(5.28-21.68) and (9.80±4.65)(3.56-16.12)nmol L-1d-1, respectively. As can be seen from the above, the biological DMSproduction and consumption rates presented a significant seasonal variation, whichwere related to the water temperature, salinity, DMS, DMSP and Chl-a concentrations.Their horizontal distributions in the YS and BS during autumn were nearly the sameas during summer, they were gradually decreased from inshore to offshore sites, too.Through the determination of the DMSPd degradation rates in the surface water ofthe YS and BS from June21to July11,2013, the degradation rates of DMSPd rangedfrom7.10to21.70nmol L-1d-1, with an average of (14.71±4.47) nmol L-1d-1. On thewhole, the horizontal distributions of DMSPd degradation rates in summer weregradually decreased from inshore to offshore, and the rates in the BS were higher thanin the YS, which might be connected with the higher DMSPd concentrations andbacterials abundance in the BS. In addition, the average degradation rate of DMSPd[(14.71±4.47) nmol L-1d-1] was nearly equal to the average biological production rateof DMS [(13.35±5.62) nmol L-1d-1], which showed the cracking of DMSPd was themost important source of DMS biological production.