Geoscience Reference
In-Depth Information
Size
Heterotrophes
Fish lavae
2000 µm
Copepods
Autotrophes
200 µm
Ciliates
20 µm
Flagellates
2 µm
Bacteria
0.2 µm
Virus
DOM
0.02 µm
FDOM
Figure 8.5. A simplified example of the lower marine food web with vira as the smallest organisms
and fish lavae as the largest. Grazing is depicted as black arrows and DOM production from each
trophic level is depicted with gray arrows. The production of FDOM has not been documented for all
levels and it seems still unclear whether it is directly released or forms abiotically from compounds
released by organisms.
variable but overall resembled the combined fluorescence of tyrosine and tryptophan
(again blue shifted). The blue-shifted tryptophan signal is due to the fact that the amino
acids where bound in proteins. In addition, some of the degradation products of these
two amino acids were also found to fluorescence with emission maxima between 300 and
450 nm ( Figure 8.6 ). Fluorescence signatures with these characteristics are often found in
natural waters and these results clearly reveal that the decay of bacteria and phytoplankton
or release of cell DOM are a likely source of FDOM in aquatic environments. In support
of these results, Yamashita and Tanoue ( 2003 ) found strong relationships between FDOM
fluorescence centered around 340 and 300 nm with concentrations of tryptophan and tyro-
sine measured using high-performance liquid chromatography (HPLC).
The origins of humic-like fluorescence at longer wavelengths remains to a certain extent
illusive. Correlations between the apparent oxygen utilization (AOU) and humic-like fluor-
escence in the deep Pacific Ocean suggest that FDOM is generated from the oxidation
and remineralization of organic particles in the dark ocean (Hayase and Shinozuka, 1995 ;
Yamashita and Tanoue, 2008 ). Recently this has been found to be a global phenomenon
 
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