Characteristics of vertical distributions of methane and dimethylsulphoniopropionate in the southern Yap Trench

Methane (CH 4 ) and dimethylsulphoniopropionate (DMSP) are major carbon and sulfur sources for bacterioplankton in the ocean. We investigated the characteristics of CH 4 and DMSP in the southern Yap Trench from sea surface to hadal zone in June 2017. We found that concentrations of CH 4 varied from 1.5 to 4.5 nmol/L with saturation between 94% and 204% in the euphotic layer. Concentrations of dissolved DMSP (DMSPd) ranged from 0.5 to 3.7 nmol/L with higher values in surface water and decreased with depth. Concentrations of particulate DMSP (DMSPp) varied from 0 to 13.6 nmol/L. Concentrations of total DMSP (DMSPt) ranged 2.0–15.2 nmol/L. Their concentrations decreased slightly and reached consistent levels in 200–3 000-m depth due probably to heterotrophic bacterial production in marine aphotic and high-pressure environments. An exception occurred around 4 000-m depth where their concentrations increased considerably and then decreased in deeper water. This previously unrecognized phenomenon sheds light on the elevated concentrations of DMSP in the abyssal layer that might be affected by the Lower Circumpolar Deep Water (LCPW). Concentrations of CH 4 in seawater of the Benthic Boundary Layer of the southern Yap Trench were slightly higher than those in the water column at approximate depth, and concentrations of DMSP in seawater of the Benthic Boundary Layer of the southern Yap Trench were not much higher than those in the water column at the approximate depth, indicating that sediment was a weak source of CH 4 but was not a source of DMSP for seawater in the study area. This study presented clear correlations between CH 4 and DMSP from sea surface to sea bottom, proving that DMSP might be a potential substrate for CH 4 not only in oxic surface seawater but also in deep water.