Hydrologic control and diurnal photobleaching of CDOM in a subarctic lake

Changes in the concentration of chromophoric dissolved organic matter (CDOM ) were determined over short (hours to days) and medium (weeks) time scales during summer in a shallow, polymictic lake in subarctic Quebec, Canada. C DOM fluorescence (F-CDOM) decreased linearly from mid-June until early Augu st, consistent with the zero-order kinetics expected for photodegradation o f organic matter in a regularly mixed lake in which most of the photochemic ally-active radiation is absorbed by CDOM. A major rainfall event then resu lted in a sharp increase in F-CDOM back to levels recorded at the start of the study. These changes showed that CDOM concentrations were controlled by the balance between catchment inputs and subsequent photodegradation proce sses within the lake. Shorter term changes in CDOM concentration were monit ored by measuring the penetration of ultraviolet radiation (UVR) using a Sa tlantic TACCS system moored near the center of the take and set to monitor downwelling 380 nm irradiance at 10 min intervals. The diffuse attenuation coefficient K-d(380) decreased by 12% during the deployment indicating incr eased penetration of UVR and the change paralleled the drop in F-CDOM over the same period. Many of the daily records showed a significant decline in K-d(380) in the near-surface waters over the course of the morning and earl y afternoon indicating diurnal photobleaching of CDOM, and an increase late r in the day consistent with the breakdown of the diurnal thermocline and e ntrainment of deeper waters. The results illustrate the short and medium te rm dynamics of CDOM, the importance of diurnal stratification and mixing fo r photochemical processes, and the variability that aquatic organisms must experience in UV exposure in their natural environment.