Transformation of dimethylsulphoniopropionate to dimethyl sulphide during summer in the North Sea with an examination of key processes via a modelling approach

Abstract

Abstract This study describes the routes and rates of transformation of dimethylsulphoniopropionate (DMSP) to dimethyl sulphide (DMS) in a phytoplankton bloom in the northern North Sea (59°N 2°E). A region on the edge of the high reflectance waters that characterised the bloom was labelled with SF6-tracer and tracked for 6 days. Within the framework of this Lagrangian experiment, it was possible to compile a comprehensive budget of the transformation of DMSP to DMS in the surface mixed layer of 15–30 m depth and a subsurface layer that extended to ⩾40 m. As a basis to the synthesis of the DMS(P) biogeochemistry, an attempt was made to accurately constrain DMS emissions from the Lagrangian water mass. Included in the study is a comparison of estimates of the transfer velocity and DMS sea to air flux derived from two well established and two recently introduced parameterisations. The two recent approaches encouragingly gave similar rates of total DMS sea to air flux of 1.60 mg S m−2 over the 6-day experimental period, and this value was intermediate to values derived from the two more routinely used approaches. The DMS sea to air flux was equivalent in terms of sulphur, to 10% of the DMS production and 1.3% of the particulate DMSP (DMSPp) production in the surface layer over the 6 days. Bacterial consumption accounted for the majority of DMS removal in both surface (62–82%) and subsurface layers (average 98%) and DMS concentrations decreased over the 6 days, despite increasing DMS production. Microzooplankton were the main agents of the transformation of DMSPp to the dissolved phase but little DMS appeared to be produced directly by the grazers. Instead, bacteria rapidly turned over the dissolved DMSP (DMSPd) made available by the grazers. An upper limit of 17% of the DMSPd consumed by bacteria in the surface layer was cleaved to DMS over the 6 days. Lower transformation efficiencies of ∼6% were estimated for the subsurface layer. An ecosystem model incorporating DMS(P) biogeochemistry was developed on the basis of the empirical data. Simple sensitivity analyses were employed to demonstrate the importance of both microzooplankton and bacterial metabolism in controlling the yield of DMS from DMSP production and hence, the DMS sea to air flux. Greater yields of DMS are likely to occur when DMSPp is transformed directly to DMS, in this case due to grazing, than when transformed to DMSPd and subsequently cleaved to DMS by bacterial activity. The point of DMS production in the DMSPp to DMS transformation pathway is likely to alter in relation to phytoplankton bloom progression and with season. The complex interaction amongst heterotrophic processes that control DMS concentrations suggests that attempts to predict global DMS flux may need to couple comprehensive ecosystem models of the type used in this study, to general circulation models.