Abstract
The effect of surface solar radiation on the optical properties and the biological availability of dissolved organic carbon (DOC) and its humic and nonhumic component was investigated in the northern Adriatic Sea (44°52'N 13°52'E) and the coastal North Sea (53°00'N 4°45'E). In the northern Adriatic Sea, humic substances of primarily autochthonous origin contributed, on average, 15% to the bulk DOC, whereas in the coastal North Sea the mean contribution of the mainly terrestrially derived humic substances to the bulk DOC was about 43%. DOC‐normalized absorbance and fluorescence were both ∼1.5‐fold higher, and the absorbance ratio 250:365 nm was twofold higher for the humic DOC of the northern Adriatic Sea as compared to the coastal North Sea. Bioassay experiments with indigenous bacterioplankton indicated a ∼5 times higher DOC‐normalized bacterial growth on the humic and nonhumic DOC from the northern Adriatic Sea as compared to the corresponding fractions from the coastal North Sea. Exposure of the different DOC fractions to surface solar radiation for 5–8 h and subsequent inoculation with a natural bacterial community resulted in stimulated bacterial growth on the preexposed humic and nonhumic DOC by 35 and 45%, respectively, as compared to the corresponding dark controls for the northern Adriatic Sea, whereas in the coastal North Sea stimulated bacterial growth (by 50%) was found only for the humic DOC. The increase in the DOC bioavailability upon irradiation was linearly related to a decrease in the absorbance and an increase in the absorbance ratio 250:365 nm. Both DOC‐normalized photochemical and bacterial oxygen demand of surface water were, on average, about tenfold higher in the northern Adriatic Sea as compared to the coastal North Sea.