Influence of photochemistry on the marine biogeochemical cycle of dimethylsulphide in the northern North Sea

Shipboard experiments were conducted in the northern North Sea to assess the rate of removal of dimethylsulphide (DMS) and the rate of production of DMSO due to both UVB and UVA/visible light. Experiments were conducted using 0.2-μm filtered seawater and natural light conditions. The DMS photolysis rate constant was determined to be between 0.03 and 0.07 h−1, and initial photolysis rates were between 1.3 and 2.5 nmol dm−3 d−1. Using these rates, the in situ profiles for downward irradiance, and the DMS concentration in the water column, a photochemical turnover rate constant of between 0.1 and 0.37 d−1 was determined for the upper 20 m of the water column, with a photochemical turnover time of between 2.5 and 9.5 days. DMSO photoproduction rates were up to 1.20 nmol dm−3 d−1. Furthermore, results indicate that under UVA/visible light most of the DMS is photo-oxidised to form DMSO, whereas under UVB radiation DMS may be removed via a second photolysis pathway.