Temporal and spatial dynamics of inherent and apparent optical properties in a shallow coastal lake (Fogliano Lagoon, Circeo National Park)

Abstract

1 - The spatial and temporal dynamics of apparent and inherent optical properties in the coastal lagoon of Fogliano were measured in three seasonal surveys in 2002. Ultraviolet and visible solar irradiance data from in situ measurements permitted to estimate the related attenuation coefficients. The extinction measurements on filtered (0.22 μm) and unfiltered water samples were also performed. The integrated approach between in situ and laboratory measurements allowed the determination of the role of the suspended and dissolved matter in the attenuation and extinction of ultraviolet and visible radiation within the water column. 2 - The impact of the suspended matter on the lake optical quality was influenced by wind resuspension of particulate matter: the relative role of dissolved matter in the absorption of UV and visible radiation was prevailing at low wind velocity conditions (less than 2.2 ms^-1), while at high wind velocities (3.9 ms^-1), particulate matter resuspension strongly influenced the attenuation and the extinction measurements. It is a surprise that the dissolved matter is not influenced by the resuspension. 3 - The extinction in the analyzed wavelengths of filtered and unfiltered water samples and the in situ irradiance measurements allowed us to define new optical parameters and important correlations with limnological and classical optical measurements. 4 - By sampling at high spatial resolution (18 stations in 4 km²), it was possible to evidence a spatial gradient of the optical and limnological properties, these distributions showed a consistent pattern in all three surveys, and were important for the characterisation of the chromophoric dissolved organic matter that was estimated with the spectral slope of the extinction curve spectra. A relatively higher spectral slope was found in the southern basin respect to the northern, where the maximum values of the attenuation coefficients and limnological parameters were found. 5 - These results suggest different sources of dissolved organic matter and/or a different level of photobleaching.