Spatio-seasonal variability of chromophoric dissolved organic matter absorption and responses to photobleaching in a large shallow temperate lake

Abstract. The development and validation of remote-sensing-based approaches for the retrieval of chromophoric dissolved organic matter (CDOM) concentrations requires a comprehensive understanding of the sources and magnitude of variability in the optical properties of dissolved material within lakes. In this study, spatial and seasonal variability in concentration and composition of CDOM and the origin of its variation was studied in Lake Balaton (Hungary), a large temperate shallow lake in central Europe. In addition, we investigated the effect of photobleaching on the optical properties of CDOM through in-lake incubation experiments. There was marked variability throughout the year in CDOM absorption in Lake Balaton (a_CDOM(440) = 0. 06–9.01 m⁻¹). The highest values were consistently observed at the mouth of the main inflow (Zala River), which drains humic-rich material from the adjoining Kis-Balaton wetland, but CDOM absorption decreased rapidly towards the east where it was consistently lower and less variable than in the westernmost lake basins. The spectral slope parameter for the interval of 350–500 nm (S_CDOM(350–500)) was more variable with increasing distance from the inflow (observed range 0.0161–0.0181 nm⁻¹ for the mouth of the main inflow and 0.0158–0.0300 nm⁻¹ for waters closer to the outflow). However, spatial variation in S_CDOM was more constant exhibiting a negative correlation with a_CDOM(440). Dissolved organic carbon (DOC) was strongly positively correlated with a_CDOM(440) and followed a similar seasonal trend but it demonstrated more variability than either a_CDOM or S_CDOM with distance through the system. Photobleaching resulting from a 7-day exposure to natural solar UV radiation resulted in a marked decrease in allochthonous CDOM absorption (7.04 to 3.36 m⁻¹, 42 % decrease). Photodegradation also resulted in an increase in the spectral slope coefficient of dissolved material.