Supplementary materials for photodegradation of added humic substances Gulf of Finland
Photodegradation experiments were conducted using mesocosm samples amended with humic substances (represent additional input of terrestrial dissolved organic matter (DOM)) and nutrients in the Gulf of Finland, Baltic Sea. The following hypotheses were tested: 1) added humic substances can be photochemically degraded rapidly, 2) comparing in situ and laboratory rates will allow us to assess whether photodegradation could be an important process in removing added humic substances, and 3) added nutrients will affect photochemical degradation of added humic substances. Results showed that photochemistry has the potential to make a significant contribution to removing excess terrestrial humic substances added to this coastal system, as laboratory irradiation experiments showed that the CDOM absorption fading rates can be 3 to 5 orders of magnitude higher than in situ rates, when full spectrum solar irradiation was used. However, it was impossible in this study to separate the relative contributions of photochemical processes from other possible in situ processes that can lead to the degradation of added humic substances. The addition of nutrients didn’t lead to any major differences in the optical properties and their change rates in humic+nutrients-amended samples when compared to those in the humic-amended samples, and therefore didn’t seem to compound the effects of added humic substances on DOM optical properties in this experiment. Nutrients-addition seemed to cause the DOC concentration and specific ultraviolet absorbance to fluctuate over time in the humic+nutrients-amended mesocosm, a trend that was not observed in the control or humic-amended mesocosms. This phenomenon may have been caused by DOM addition or removal during any potential phytoplankton changes induced by the nutrients-addition.