Studies On The Distribution, Source And Influencing Factors Of Dimethylsulfoxide In The Eastern Continental Shelf Seas Of China

Dimethylsulfoxide (DMSO) is an important component of biogenic organic dimethylated sulfur compounds in seawater, which is the main oxidation and microbial consumption product of dimethylsulfide (DMS). Simultaneously, DMSO is reduced back to DMS by some bacteria and eukaryotic algae under anaerobic and aerobic conditions. As a result, DMSO can indirectly influence the global climate and environment through the interactions with DMS.In the present study, we choose the continental seas of China which are seriously influenced by human activities as the investigation areas, including the East China Sea (ECS), the Bohai Sea (BS) and the Yellow Sea (YS). The spatial and temporal variations of dissolved DMSO (DMSOd) and particulate DMSO (DMSOp) and the environmental factors which were associtated with them were studied. In addition, we investigated the distributions of DMSO in the sea-surface microlayer and sediment pore waters. The conclusions we got were as follows:(1) The distribution of DMSO and its relationship with other organic dimethylated sulfur compounds and various environmental factors were studied in the BS and the YS during April-May,2014 and November,2014. In spring, the concentrations of DMSOd and DMSOp in surface seawater were 16.97 (4.50-44.87) and 14.99 (1.90-47.06) nmol L-1, while they were 11.44 (1.97-30.49) and 11.10 (1.08-29.44) nmol L-1 in autumn, respectively. DMSOd and DMSOp concentrations revealed seasonal variations with higher values in spring than those in autumn, which was the same to chlorophyll a (Chl-a) concentration. The spatial distributions of DMSOp and Chl-a exhibited the similar pattern, with higher concentrations in the northern coastal waters of Shandong Peninsula and the southern waters of Liaodong Peninsula. High concentrations of DMSOd appeared in the regions around the Yalu River and the Yellow River estuary. In addition, the distribution of DMSOd was closely related with DMS and DOC concentrations.The concentrations of DMSOd and DMSOp in the sea-surface microlayer in autumn were 12.81 (1.21-33.43) and 11.46 (1.03-58.22) nmol L-1, which were higher than the values in the surface water. The enrichment factors of DMSOd and DMSOp were 1.36 (0.52-4.12) and 1.40 (0.41-3.36), indicating that DMSOd and DMSOp were enriched to a small extent in the sea-surface microlayer. DMSO and Chl-a concentrations in the sea-surface microlayer correlated well with those in the surface water. In addition, we found a positive correlation between the enrichment factors of DMSOp and Chl-a, indicating that the enrichment of DMSOp was due to the relatively higher phytoplankton biomass in the sea-surface microlayer.The vertical distributions of DMSOd and DMSOp were observed in cold water area of the YS. Generally, DMSOd and DMSOp showed high concentrations in the surface water in spring, corresponding with the high levels of Chl-a. In contrast with the South Yellw Sea (SYS), the stratification was more obvious in the cold water area of North Yellow Sea (NYS). In autumn, the concentrations of DMSOd and DMSOp tended to be more uniform in the whole water column with the disappearance of the Yellow Sea Cold Water Mass.There was a positive correlation between DMSOp and Chl-a concentrations in spring and autumn, suggesting that phytoplankton biomass played an important role in controlling the distribution of DMSOp in the BS and YS. In the study area, ratio of DMSOp/Chl-a was higher in autumn than in spring, revealing a notable seasonal variation, which might be caused by the seasonal difference in phytoplankton community. In addition, we found a positive relationship between DMSOp/Chl-a and salinity, indicating that salinity was closely related with the biosynthesis of DMSOp. A significant relationship was observed between DMSOd and DMS concentrations, along with the correlation between DMSOd and temperature, which demonstrated that DMSOd in the surface water mainly came from the oxidation of DMS and the oxidation process was closely related with the temperature.(2) The distributions of DMSOd and DMSOp in the surface water and the sediment pore waters were studied in the ECS during October-November,2013 and May-June, 2014. The concentrations of DMSOd and DMSOp in the surface water were 10.52 (2.23-28.64) and 8.99 (1.18-20.49) nmol L-1 in autumn, and those in summer were 17.51 (2.31-35.13) and 16.95 (2.22-40.94) nmol L-1,repectively. DMSO showed the same seasonal variation as Chl-a, with higher concentrations in summer than in autumn. The spatial distributions of DMSOd and DMSOp were severely affected by anthropogenic inputs, with high values in coastal regions and low values in the oligotrophic open sea. In addition, the distribution of DMSOd was influenced by DMS and DOC concentrations in seawater.The concentrations of DMSOd in the sediment pore waters were substantially higher than those at the bottom of the water column, implying that the sediment pore waters might be an important source of DMSOd in seawater. This result corresponded well with the observation of high concentrations of DMSOd at the bottom. DMSOd and DMSOp did not show stratification phenomenon due to the intense mixing between deep waters and the upper mixed layer in autumn.DMSOp/Chl-a ratio was obviously higher in summer when compared to that in autumn, which resulted from the higher cell abundance and ratio of algae contributing more to DMSO. In addition, larger nanoplankton (5-20 μm) contributed to the majority of DMSOp and Chl-a in the ECS in autumn. Positive relationships were observed between DMSOd and DMS concentrations and between DMSOd and DMSOp concentrations in summer, reflecting that DMSOd came from DMS oxidation and the release of intracellular DMSOp. DMSOp/Chl-a ratio was also significantly correlated with salinity in summer rather than in autumn, and it highlighted the role of salinity in biosynthesis of DMSOp in summer.