Determination of apparent quantum yield spectra of DMS photo‐degradation in an in situ iron‐induced Northeast Pacific Ocean bloom

Abstract

[1] The wavelength dependence of the photochemical removal efficiency for DMS was studied for samples from an iron-induced bloom in the Northeastern Pacific Ocean. In July 2002, a 64 km 2 patch of ocean was iron-fertilized near Ocean Station Papa (50°12 0 N; 144°45 0 W). Only small changes in pseudo-first-order apparent quantum yield (AQY * DMS (l)) were observed outside the iron-patch. However, inside the patch, AQY * DMS (l) decreased considerably over the two weeks following the initial iron injection. A positive strong correlation was found between pseudo-first-order apparent quantum yield determined at 330 nm (AQY * DMS (330 nm)) and NO À 3 concentrations. We propose that NO À 3 -photolysis has a substantial influence on DMS photo-degradation rates in oceanic waters. This finding demonstrates that in addition to control DMS production, marine phytoplankton could indirectly influence the DMS photochemical loss rate via its control on NO À 3 distribution.

Photochemical oxidation of dimethylsulphide to dimethylsulphoxide in estuarine and coastal waters

“…Although DMSO yields for our coastal samples (S > 30) were lower (52e78%, Table 1), they were still significantly higher than previously reported yields for the northern North Sea (5e37%; Hatton, 2002), the equatorial Pacific (14%; Kieber et al, 1996) and the subpolar South Pacific (33e45%; Toole et al, 2004). Our findings clearly indicate that DMS photo-oxidation by pathways other than photosensitised oxidation by 1 O 2 (Bouillon and Miller, 2004;Toole et al, 2004) is of only minor importance for the estuarine and coastal waters studied here. By contrast, contributions to DMS photo-oxidation from nitrate related pathways, extrapolated from the nitrate dependence of DMS photo-oxidation rates, were important in the subpolar South Pacific (35%) and NE Pacific (81%) (Bouillon and Miller, 2004;Toole et al, 2004).…”

“…Our findings clearly indicate that DMS photo-oxidation by pathways other than photosensitised oxidation by 1 O 2 (Bouillon and Miller, 2004;Toole et al, 2004) is of only minor importance for the estuarine and coastal waters studied here. By contrast, contributions to DMS photo-oxidation from nitrate related pathways, extrapolated from the nitrate dependence of DMS photo-oxidation rates, were important in the subpolar South Pacific (35%) and NE Pacific (81%) (Bouillon and Miller, 2004;Toole et al, 2004). To date, DMSO yields (33e45%), a proxy for the 1 O 2 pathway contribution to DMS photo-oxidation (Kieber et al, 1996), and concurrent contributions from nitrate related pathways (35%) have only been reported in Toole et al (2004), for the subpolar South Pacific.…”

“…High turnover rate constants in the Ross Sea were attributed to a combination of high nitrate levels, enhanced CDOM photoreactivity and high daily irradiance during the austral Summer (Toole et al, 2004), while high upper mixed layer-integrated rate constants for the Greenland Sea are due at least in part to shallow, ice melt induced stratification (Galí and Simo, 2010). Given that seawater photoreactivity appears to increase offshore, presumably due to an increasing contribution by nitrate photochemistry and the higher photoreactivity of marine CDOM (Bouillon and Miller, 2004;Toole et al, 2004;Taalba et al, 2013), our low k int DMS can be partly explained by the lower photoreactivity of CDOM-rich, near coastal waters ( Estimates of k int DMS are also sensitive to the choice of water column depth (equation (5)). For our work we chose bathymetric depth (Stubbins et al, 2011) because the water columns in our study area are predominantly well mixed (van Leeuwen et al, 2015).…”

“…However, Brimblecombe and Shooter (1986) did not report DMSO concentrations, and subsequent work reported low DMSO yields, particularly in open ocean waters (Kieber et al, 1996;Hatton, 2002;Toole et al, 2004), that are not consistent with the 1 O 2 pathway. Positive correlations of DMS photo-oxidation rates with nitrate concentrations imply that reactive intermediates deriving from nitrate photolysis are also involved in DMS photodegradation (Bouillon and Miller, 2004;Toole et al, 2004). Their likely contribution, as estimated from relationships between photo-oxidation rate constants and in-situ nitrate concentrations, appears to be rather variable between contrasting open ocean waters of the subpolar South Pacific (35%; Toole et al, 2004) and NE Pacific (81%; Bouillon and Miller, 2004).…”

“…Positive correlations of DMS photo-oxidation rates with nitrate concentrations imply that reactive intermediates deriving from nitrate photolysis are also involved in DMS photodegradation (Bouillon and Miller, 2004;Toole et al, 2004). Their likely contribution, as estimated from relationships between photo-oxidation rate constants and in-situ nitrate concentrations, appears to be rather variable between contrasting open ocean waters of the subpolar South Pacific (35%; Toole et al, 2004) and NE Pacific (81%; Bouillon and Miller, 2004). By implication, these results suggest highly variable contributions from CDOM related pathways of .…”