The effect of mesoscale iron enrichment on the marine photochemistry of dimethylsulfide in the NE subarctic Pacific

Measurements of underwater light fields and available quantum yield spectra were used to calculate photochemical removal rates of DMS for surface waters of the northeast subarctic Pacific during the SERIES mesoscale iron-fertilization experiment in July 2002. We observed that the UV portion of the solar spectrum was most important in inducing DMS photo-oxidation, and calculated that UV-B accounted for more than 20% and UV-A for more than 68% of the total DMS photo-oxidation near the sea surface. Vertically resolved rates showed that most (>90%) of the DMS photo-oxidation occurs in the upper 15 m of the water column. During the study, calculated rates of DMS photo-oxidation, just below the ocean's surface ranged from 0.34 to 5.9 μmol m−3 d−1. As the study progressed, an initial increase in photo-oxidation rates occurred within the iron-enriched patch and this was followed by a dramatic decrease in rates, whereas little change was observed outside the patch. Changes in DMS concentrations and decreases in the photochemical removal efficiency for DMS were the dominant factors explaining the variation in the DMS photo-oxidation rates. The turnover rate constants for DMS photo-oxidation, calculated for the upper mixed layer (UML) of the water column, (0.03–0.25 d−1) were in the range of those previously published and were at times higher than those calculated for biological consumption of DMS during SERIES. Our results suggest that iron fertilization of an oceanic patch in the northeast Pacific Ocean considerably altered the photochemical removal rates and turnover rate constants of DMS.