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Dissolved Organic Matter and Solar UV Radiation

Solar UV has been shown to degrade dissolved organic carbon (DOC) photolytically, most of the DOC being of terrestrial origin and relatively resistant to other forces (Naganuma et al., 1996). Humic substances are fairly resistant to bacterial degradation, but after photolytic activity the products (e.g., formaldehyde, acetaldehyde, glyoxylate, and pyruvate) are readily taken up by bacterioplankton (Wetzel et al., 1995). Humic substances strongly absorb ultraviolet radiation. Thus, increased breakdown of DOC and subsequent consumption by bacteria increases the UV-B penetration into the water column. Close to the surface, solar UV radiation inhibits bacterioplankton activity and rapidly photolyzes DOC. However, the uptake of the fragments is hampered by the inhibition of the bacterial ectoenzymes. Only when both bacteria and the photolyzed DOC circulate to deeper layers does the uptake rate increase. Another aspect of DOC photolysis is the generation of photosensitizers, which upon absorption of UV radiation produce reactive oxygen species or free radicals. DMS is released from the water at a rate that is closely correlated with the concentration of DOC (Herndl, 1997). DMSP is considered an osmoregulator in phytoplankton.

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