Seasonal photoreactivity of dissolved organic matter from lakes with contrasting humic content

We studied seasonal variability in photodegradation of dissolved organic ca rbon (DOC) resulting from artificial ultraviolet-A (UV-A) and UV-B irradiat ion. Water samples were taken approximately monthly from the surface layers of two oligotrophic lakes with contrasting humic content, situated in sout hern Sweden. Lake water was filter-sterilized (0.2 mum) and exposed to arti ficial UV radiation in quartz tubes. Potential DOC photodegradation, measur ed as a photoproduction of dissolved inorganic carbon (DIC) and oxalic, mal onic, formic, and acetic acid in irradiated samples, was observed throughou t the sampling period. In addition, exposure to UV radiation resulted in a decrease in DOC, absorbance, and humic substance fluorescence. The photopro duction of DIC and the low molecular weight (LMW) organic acids varied seas onally, being generally higher in winter and spring (December-May), while D OC appeared to become less photoreactive after the extensive exposure to so lar radiation during summer. Production rates of both DIC and LMW organic a cids were approximately eight times higher in the humic lake despite that t he DOC concentration was only two times higher than in the clearwater lake. This is most probably due to the high input of allochthonous DOC and the r esulting higher absorbance to DOC ratio in the humic system. Furthermore, t he longer hydraulic residence time in the clearwater system could have resu lted in an accumulation of residual DOC, recalcitrant to further photodegra dation.