Spatial variability of absorption coefficients over a biogeochemical gradient in a large and optically complex shallow lake

Abstract

In order to improve robustness of remote sensing algorithms for lakes, it is vital to understand the variability of inherent optical properties (IOPs) and their mass-specific representations (SIOPs). In this study, absorption coefficients for particulate and dissolved constituents were measured at 38 stations distributed over a biogeochemical gradient in Lake Balaton, Hungary. There was a large range of phytoplankton absorption (aph(λ)) over blue and red wavelengths (aph(440) = 0.11-4.39 m-1, aph(675) = 0.048-2.52 m-1), while there was less variability in chlorophyll-specific phytoplankton absorption (a∗ph(λ)) in the lake (a∗ph(440) = 0.022 ± 0.0046 m2 mg-1, a∗ph(675) = 0.010 ± 0.0020 m2 mg-1) and adjoining wetland system, Kis-Balaton (a∗ph(440) = 0.017 ± 0.0015 m2 mg-1, a∗ph(675) = 0.0088 ± 0.0017 m2 mg-1). However, in the UV, a∗ph(350) significantly increased with increasing distance from the main inflow (Zala River). This was likely due to variable production of photoprotective pigments (e.g., MAAs) in response to the decreasing gradient of colored dissolved organic matter (CDOM). The slope of CDOM absorption (SCDOM) also increased from west to east due to larger terrestrial CDOM input in the western basins. Absorption by nonalgal particles (aNAP(λ)) was highly influenced by inorganic particulates, as a result of the largely mineral sediments in Balaton. The relative contributions to the absorption budget varied more widely than oceans with a greater contribution from NAP (up to 30%), and wind speed affected the proportion attributed to NAP, phytoplankton, or CDOM. Ultimately, these data provide knowledge of the heterogeneity of (S)IOPs in Lake Balaton, suggesting the full range of variability must be considered for future improvement of analytical algorithms for constituent retrieval in inland waters.