Seasonal variation and biogeochemical cycling of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the Yellow Sea and Bohai Sea

The concentrations of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP) and chlorophyll a (Chl-a) as well as the size distribution of particulate DMSP (DMSPp) were determined in the Yellow Sea (YS) and Bohai Sea (BS) during two cruises from 1 to 20 November 2012 and from 21 June to 11 July 2013. Besides, the biological production and consumption rates of DMS and the degradation rates of dissolved DMSP (DMSPd) were also measured. The concentrations of DMS and DMSP showed a significant seasonal variation with higher values in summer, which corresponded well with the seasonal change of Chl-a in the study area. Both DMS and DMSPp concentrations were significantly correlated with Chl-a levels, implying that phytoplankton biomass might play an important role in controlling the distributions of DMS and DMSP in the study area. The size-fractionation research showed that nanophytoplankton (5-20 m) contributed to most of Chl-a and DMSPp in autumn and summer. The average biological production and consumption rates of DMS during summer were 13.35 and 9.80 nmol L-1 d(-1), respectively, which were much higher than those during autumn. The degradation rates of DMSPd during summer ranged from 7.10 to 21.70 nmol L-1 d(-1), with an average of 14.71 nmol L-1 d(-1), which was nearly equal to the average biological production rate (13.35 nmol L-1 d(-1)) of DMS. The average sea-to-air flux of DMS in the YS and BS were 12.06 and 20.81 mol m(-2) d(-1) in autumn and in summer, respectively, which were much higher than the values reported in the ocean.