Photochemically produced H,O, was found to accumulate at micromolar concentrations in intertidal Waddcn Sea areas. Annual amplitudes of solar radiation lead to variations of the intertidal H,O, accumulation with concentrations between 1 and 4 p,mol liter 1 during summer, while winter concentrations were mostly CO.5 pmol liter-l. Diurnal variations of H,O, accumulation over daily low-tide periods in intertidal environments are determined by the incident solar radiation, the concentration of UV-absorbing dissolved organic carbon in the water, as well as the concomitant biological and chemical H,O, degradation within the sediment surface. The efficiency of photosynthetic available radiation (PAR), UVA and UVB photons for photochemical H,O, production was assessed using cut-off filters in the UVB and in different UVA ranges. UVB photons (295-320 nm) displayed an 1 i-fold higher efficiency compared to UVA (335-370 nm) and a 340-fold higher efficiency compared to PAR photons (>400 nm). A 10 % ozone reduction leads to a doubling of UVB surface irradiance at 300 nm, which entails a 30 and 40 % increase of the apparent intertidal H,O, concentrations. Progressive stratospheric ozone depletion via UVB-induced H,O, formation will have yet unpredictable effects on boreal and Antarctic intertidal ecosystems. Hydrogen peroxide (H,O,) is a nonradical active oxygen species, which in aquatic environments predominantly derive
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