Dark production of hydrogen peroxide in the Gulf of Alaska

Dark H2O2 production rates were measured in samples collected in the Gulf of Alaska. We used a simple, novel method for determining absolute rates of dark production and decay of H2O2, both of which are occurring simultaneously (presumably as a result of biological activity) in unfiltered samples. [H2O2] vs. time was measured in 24-h dark incubations of both unaltered samples and the same samples spiked with 100-250 nmol L-1 H2O2. Data were modeled with zero-order H2O2 production rates and first-order H2O2 decay coefficients as fitting parameters, with the assumption that addition of [H2O2] to a sample does not change either parameter. H2O2 production rates ranged from < 0.5 nmol L-1 h(-1) to 8 nmol L-1 h(-1), and generally decreased with depth and decreasing chlorophyll. Comparison of dark production with estimates of average photochemical H2O2 production rates in the top 50 m of the water column indicated that dark production is likely to be a significant source of H2O2. Indeed, many of the unaltered incubations indicated that in situ [H2O2] was close to a steady state between dark production and decay, especially in samples from depths of >= 10 m.