Spatial and diurnal dynamics of dissolved organic matter (DOM) fluorescence and H 2 O 2 and the photochemical oxygen demand of surface water DOM across the subtropical Atlantic Ocean

Abstract

Diurnal dynamics of dissolved organic matter (DOM) fluorescence and hydrogen peroxide (H 2 O 2 ) 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 H 2 O 2 concentrations increased, especially in the diurnally stratified surface water layers extending to 10‐50‐m depth. In situ H 2 O 2 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 H 2 O 2 net production of ~38 µmol m −2 d −1 in the upper 50 m of the water column. Photochemical oxygen (O 2 ) 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 O 2 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 O 2 demand. A simple one‐dimensional physical model was combined with a photochemical/biological model in order to describe the photochemical production of H 2 O 2 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 H 2 O 2 from the surface layers in the late afternoon. The model reflects well the observed diurnal H 2 O 2 dynamics. It further provides evidence that mainly biological breakdown determines the loss of H 2 O 2 in the upper 50 m of the water column during the day; however, in the late afternoon, vertical mixing is important in transporting H 2 O 2 from the uppermost 5‐m layer to the 10‐20‐m layers.

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