Photochemically produced H2O2 was found to accumulate at micromolar concentrations in intertidal Wadden Sea areas. Annual amplitudes of solar radiation lead to variations of the intertidal H2O2 accumulation with concentrations between 1 and 4 μmol liter-1 during summer, while winter concentrations were mostly <0.5 μmol liter-1. Diurnal variations of H2O2 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 H2O2 degradation within the sediment surface. The efficiency of photosynthetic available radiation (PAR), UVA and UVB photons for photochemical H2O2 production was assessed using cut-off filters in the UVB and in different UVA ranges. UVB photons (295-320 nm) displayed an 11-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 H2O2 concentrations. Progressive stratospheric ozone depletion via UVB-induced H2O2 formation will have yet unpredictable effects on boreal and Antarctic intertidal ecosystems.