Hydrologic control and diurnal photobleaching of CDOM in a subarctic lake

Abstract

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. CDOM fluorescence (F CDOM ) decreased linearly from mid-June until early August, consistent with the zero-order kinetics expected for photodegradation of organic matter in a regularly mixed lake in which most of the photochemically-active radiation is absorbed by CDOM. A major rainfall event then resulted 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 processes within the lake. Shorter term changes in CDOM concentration were monitored by measuring the penetration of ultraviolet radiation (UVR) using a Satlantic TACCS system moored near the center of the lake and set to monitor down-welling 380 nm irradiance at 10 min intervals. The diffuse attenuation coefficient K d (380) decreased by 12 % during the deployment indicating increased 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 early afternoon indicating diurnal photobleaching of CDOM, and an increase later in the day consistent with the breakdown of the diurnal thermocline and entrainment of deeper waters. The results illustrate the short and medium term dynamics of CDOM, the importance of diurnal stratification and mixing for photochemical processes, and the variability that aquatic organisms must experience in UV exposure in their natural environment.