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

• Published in: Limnology and oceanography Vol. 46; no. 3; pp. 632 - 643
• Main Authors: Obernosterer, Ingrid, Ruardij, Piet, Herndl, Gerhard J.
• Format: Journal Article
• Language: English
• Published: 01-05-2001
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2001.46.3.0632

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.