No abstract

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 lm) 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 21 d 21 , 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 21 d 21 , with an average of 14.71 nmol L 21 d 21 , which was nearly equal to the average biological production rate (13.35 nmol L 21 d 21 ) of DMS. The average sea-to-air flux of DMS in the YS and BS were 12.06 and 20.81 lmol m 22 d 21 in autumn and in summer, respectively, which were much higher than the values reported in the ocean.

Environmental context. DMSP is one of the most important substrates for marine bacteria and its cycling contributes substantially to fluxes of carbon and sulfur in the ocean. Accurate determination of the concentration of DMSP available to bacteria is essential to quantifying DMSP consumption rates, and this work improves those determinations by identifying non-bioavailable pools of DMSP that have previously gone unrecognised. Improved estimates of DMSP consumption rates will lead to better understanding of its role in ocean food web and biogeochemical dynamics.Abstract. Dissolved dimethylsulfoniopropionate (DMSPd) is an important substrate for marine microbes and a precursor of sulfur gases. We compared DMSPd turnover flux rates in coastal seawater measured with a 35 S-DMSPd tracer to those obtained with the DMSP-uptake inhibitor glycine betaine (GBT). The 35 S-DMSP tracer method yielded DMSPd turnover fluxes (35.7-215 nM day À1 ) that were 1.7 to 152 times higher than those obtained in parallel samples with the GBT inhibitor method (0.34-21.6 nM day À1 ). Tests confirmed that GBT functioned as planned by strongly inhibiting DMSPd degradation and that 35 S-DMSPd gave accurate estimates of DMSPd loss rate constants. This left the initial DMSPd concentrations, determined by small volume drip filtration (SVDF) through Whatman GF/F filters (0.7-mm nominal retention) ([DMSPd] SVDF ), as a potential cause of the discrepancy in rate estimates. Indeed, GF/F filtrate incubations showed that the initial [DMSPd] SVDF overestimated the bioavailable DMSPd concentrations for at least two reasons: (1) a significant fraction (10-37 %) of DMSP passing through GF/F filters was in particles .0.2 mm (likely bacteria) and therefore not dissolved, and (2) a significant pool (0.44-1.0 nM) of operationally dissolved, non-particle DMSP ([DMSPd] ,0.2 mm ), comprising 40-99 % of [DMSPd] SVDF , was refractory to degradation on a time scale of days. The nature of this refractory DMSP is currently unknown. Accounting for DMSP-containing particles and the refractory DMSP pool in GF/F filtrates is necessary to obtain the true bioavailable DMSPd concentrations, which we estimate to be very low (0.006-1.0 nM; mean of 0.41 nM) in the coastal waters examined, and to avoid overestimation of DMSPd turnover fluxes when using the 35 S-DMSP tracer technique.

The horizontal/geographical variations in dissolved dimethylsulfide (DMS), its precursor dimethylsulfoniopropionate (DMSPd and DMSPp), and chlorophyll a (Chl a), as well as the oceanographic parameters influencing the concentrations of dimethylated sulfur compounds, were investigated in the Bering Sea from July to August 2012. Similar to Chl a, the surface DMS and DMSPp levels, as well as DMS(P) production and consumption rates, exhibited a declining gradient from the central basin to the continental shelf, with high‐value areas appearing in the central basin, the slope regions, and Anadyr Strait but a low‐value area occurring on the outer‐middle continental shelf. Considerably high values of DMS and DMSP were measured in the saline Bering Sea Basin Deep Water (>2,000 m) located at the southwest of the Bering Basin because of the release of resuspension in 2,000 m depth and the DMSP production from endogenous benthic bacteria and cyanobacteria population. Chl a was positively correlated with DMSPp and DMS in the surface waters and the upper water of the basin, whereas significant negative correlations were found between DMS and nutrients (dissolved inorganic nitrogen [DIN], phosphorus, and silicate) in the inner shelf of the Bering Sea. DMS microbial consumption was approximately 6.26 times faster than the DMS sea‐air exchange, demonstrating that the major loss of DMS in the surface water occurred through biological consumption relative to evasion into the atmosphere. Average sea‐to‐air DMS fluxes were estimated to be 4.66 μmol/(m2·d), and consequently oceanic biogenic DMS emission had a dominant contribution to the sulfur budget over the observational area.