Spatial and diurnal dynamics of dissolved organic matter (DOM) fluorescence and H2O2 and the photochemical oxygen demand of surface water DOM across the subtropical Atlantic Ocean

Diurnal dynamics of dissolved organic matter (DOM) fluorescence and hydrogen peroxide (H2O2) concentrations were followed in the upper 100 m of the water column at five stations across the subtropical Atlantic Ocean in July and August 1996. The 10% levels of surface solar radiation for the ultraviolet (UV) B range (at 305‐ and 320‐nm wavelengths) were at 16 and 23 m in depth and for the UVA range (at 340‐ and 380‐nm wavelengths) were at 35 and 63 m in depth, respectively. The DOM fluorescence decreased over the course of the day, whereas H2O2 concentrations increased, especially in the diurnally stratified surface water layers extending to 10‐50‐m depth. In situ H2O2 net production varied between 5.5 nmol L−1 h−1 at 5‐m depth and 1 nmol L−1 h−1 at 40‐m depth, resulting in an H2O2 net production of ~38 µmol m−2 d−1 in the upper 50 m of the water column. Photochemical oxygen (O2) demand of water collected at 10‐m depth in the early morning and exposed to surface solar radiation varied between 0.9 and 2.8 µmol O2 L−1 d−1 and was found to be consistently higher (by a 1.3‐8.3‐fold measure) than bacterial respiration (measured in 0.8 µm‐filtered seawater in the dark). UVB radiation was responsible for 0‐30% of the photochemical O2 demand. A simple one‐dimensional physical model was combined with a photochemical/biological model in order to describe the photochemical production of H2O2 at different depth layers over the course of the day and to determine the contribution of physical versus biological processes in terms of the loss of H2O2 from the surface layers in the late afternoon. The model reflects well the observed diurnal H2O2 dynamics. It further provides evidence that mainly biological breakdown determines the loss of H2O2 in the upper 50 m of the water column during the day; however, in the late afternoon, vertical mixing is important in transporting H2O2 from the uppermost 5‐m layer to the 10‐20‐m layers.