Sources and sinks of hydrogen peroxide at Station ALOHA

A. M. E. Gasc, P. J. Morris and D. M. Karl

University of Hawaii, Department of Oceanography, Honolulu, HI 96822

Abstract

Hydrogen peroxide (H₂O₂) dynamics were studied in the water column at station ALOHA (22 45N 158 W). Samples were taken twice a day during 16 cruises from 1994 to 1997 to assess diel and seasonal variability. Measurements were made using the dimerization of (p-hydroxyphenyl) acetic acid by H₂O₂ in the presence of peroxidase and detected by fluorometry. In 2001 the same protocol was used in conjunction with a continuous flow surface pumping and detection system. The average vertical distribution of total peroxide (H₂O₂ plus organic peroxide) showed a "typical" profile with high near uniform concentrations (40 to 70nM) from the surface to about 50m followed by an exponential decrease in concentrations to about 10nM at 150m. In deeper waters, concentrations of < 5nM were observed and dominated by organic peroxides. Although this pattern represents the mean condition of peroxides distribution at station ALOHA, seasonal and higher frequency variations were observed reflecting the different sources and sinks for this compound. Near surface (0-50m) H₂O₂ concentrations ranged between 20nM to 140nM. Seasonal patterns were observed with lower concentrations (20-40nM) in winter and higher concentrations (70-140nM) in summer. The seasonality is mainly due to the high irradiance occurring in summer which sustains a photoproduction of H₂O₂. The effect of wet deposition during episodic rainfalls and a variety of biogenic sources and sinks of H₂O₂ was also documented. Extremely low concentrations (< 10nM) of (H₂O₂) were observed in surface waters on rare occasions, especially during large blooms of the cyanobacterium Trichodesmium. This H₂O₂ scavenging is probably related to high catalase activity in this N₂ fixing microorganism and may be a large sink for H₂O₂ during selected periods. The continuous pumping/detection system documented the dynamic of the diel distribution of H₂O₂ concentrations in the surface waters. Important variations (> 50nM in amplitude) were observed with maximum concentrations (90nM) around 3 pm and minimum (15nM) a few hours after sunset. By contrast, during Trichodesmium blooms we observed only small variations (mean 26 ± 7.5 nM) over diel cycle.