Dark production of hydrogen peroxide in the Gulf of Alaska

Dark H 2 O 2 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 H 2 O 2 , both of which are occurring simultaneously (presumably as a result of biological activity) in unfiltered samples. [H 2 O 2 ] vs. time was measured in 24‐h dark incubations of both unaltered samples and the same samples spiked with 100–250 nmol L −1 H 2 O 2 . Data were modeled with zero‐order H 2 O 2 production rates and first‐order H 2 O 2 decay coefficients as fitting parameters, with the assumption that addition of [H 2 O 2 ] to a sample does not change either parameter. H 2 O 2 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 H 2 O 2 production rates in the top 50 m of the water column indicated that dark production is likely to be a significant source of H 2 O 2 . Indeed, many of the unaltered incubations indicated that in situ [H 2 O 2 ] was close to a steady state between dark production and decay, especially in samples from depths of ≥ 10 m.