Abstract
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 (aCDOM(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 (SCDOM(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 SCDOM was more constant exhibiting a negative correlation with aCDOM(440). Dissolved organic carbon (DOC) was strongly positively correlated with aCDOM(440) and followed a similar seasonal trend but it demonstrated more variability than either aCDOM or SCDOM 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.
Highlights
1.1 Importance of chromophoric dissolved organic matter (CDOM) in lakesThere are approximately 117 million lakes on Earth greater than 0.002 km2 in surface area collectively covering about 3.7 % of its non-glacial surface (Verpoorter et al, 2014)
There was some seasonal variability in the CDOM concentration measured in Lake Balaton over a year (Fig. 2, Table 1)
High aCDOM(440) values were observed throughout the year at the mouth of the Zala River in the Keszthely basin at ST01 (Table 2), with concentrations increasing from an annual minimum in spring (3.69 m−1 in March) to a peak in August (9.01 m−1) during the warmest and driest period of the year (Anda and Varga, 2010)
Summary
1.1 Importance of CDOM in lakesThere are approximately 117 million lakes on Earth greater than 0.002 km in surface area collectively covering about 3.7 % of its non-glacial surface (Verpoorter et al, 2014). The importance of the role that lakes play as regulators of the carbon cycle and thereby global climate has only recently been recognised (Tranvik et al, 2009) They act as both a sink (sediment storage through flocculation from dissolved to particulate organic carbon) and a source for carbon (degradation and resulting mineralisation to CH4, CO and CO2; Cole et al, 2007; Benoy et al, 2007; Tranvik et al, 2009). E. Aulló-Maestro et al.: Spatio-seasonal variability of chromophoric dissolved organic matter absorption tive ecosystems (Millennium Ecosystem Assessment, 2005; IPCC, 2007), lakes can respond rapidly to external pressures including meteorology, climate and land use change. This has led to the emerging concept of lakes as sentinels of environmental change (Adrian et al, 2009; Williamson et al, 2009a, b; Schindler, 2009)
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