Distributions of hydrogen peroxide and superoxide in seawater east of New Zealand

During four seasonally disparate voyages in 2005 and 2006, hydrogen peroxide (H2O2) concentrations were measured in surface seawater east of New Zealand on a transect from 46° 38′ S to 41° 10′ S along the 178° 30′ E meridian. Concentrations of H2O2 along the transect ranged from 8.5 nM to 138.5 nM, and varied seasonally with the highest mean concentrations measured in summer and the lowest in winter. Differences in the intensities of photosynthetically-active radiation between seasons explained much of this seasonal variation in H2O2 concentrations (r2 = 0.997, p < 0.01). These data, as well as results from formation and decay studies, strongly support the present hypothesis that a photochemically-initiated formation mechanism causes seasonal variation in open ocean sea surface H2O2 concentrations east of New Zealand.Steady-state concentrations of superoxide (HO2/O2−) measured in depth-profile samples during a voyage in February 2009 ranged from below the detection limit (218 pM) to a maximum of 32.7 nM. Superoxide maxima were coincident with distinct deep chlorophyll maxima at two sampling sites in the Subtropical Front (STF) east of New Zealand. The data presented in the present study suggest that biologically-mediated production mechanisms can sometimes lead to steady-state concentrations of superoxide in seawater in the nanomolar range. The present study also confirms that solar irradiance alone is capable of explaining much of the natural temporal variation in H2O2 concentrations in surface seawater.

► Seasonal variation in H2O2 concentrations. ► Superoxide associated with subsurface chlorophyll maxima. ► Simultaneous measurements of H2O2 and superoxide.