Solar UV‐light penetration and photodegradation of humic substances in peaty lake water

The in situ photodegradation of aquatic humic substances by underwater UV light (300–400 nm) in peaty lake water was studied. The photodegradation of aquatic humic substances (HS) occurred at depths where UV light with wavelengths <320 nm can penetrate. The daily photodegradation rate of aquatic HS under a cloudless summer sky in the northern Netherlands appeared to be of the same order of magnitude as that of the mean daily pelagic photosynthetic fixation of dissolved inorganic C in typically oligotrophic humic lakes. However, the measured UV degradation of aquatic HS on a yearly basis is small compared to the approximate annual primary production of temperate lake phytoplankton (8%) and submerged macrophytes (3%) under fertile conditions. The measured UV‐light‐driven HS degradation implies a higher turnover of organic C in humic lakes than believed so far.