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Abstract
The apparatus exploited in this work is composed of an optical cable linked to a portable FieldSpec UV/VNIR that records the spectral downwelling radiance in underwater environment, allowing us to calculate the shortwave attenuation coefficient in water. Results for three inland water bodies are presented under different atmospheric conditions (sun zenith angle and wind speed) and water composition (chlorophyll α concentration and turbidity). We show that the spectral downwelling zenith radiance profiles under high sun elevations present a positive slope in the upper layers due to relatively high scattering of direct sunlight compared to attenuation. For deeper layers, attenuation overcomes the scattering of sunlight leading to a constant negative logarithmic slope. For low sun elevations, a negative slope is observed in the entire water column since the scattering of direct sunlight is always lower than attenuation. Whenever a negative logarithmic constant slope is observed, the attenuation coefficient was computed. A relation was observed between attenuation coefficient in the photosynthetically active radiation (PAR) spectral region and water turbidity, for the three water bodies under study.
5. Acknowledgements
This work is financed through an FCT grant SFRH/BD/45577/2008 and through FEDER (Programa Operacional Factores de Competitividade – COMPETE) and National funding through FCT – Fundação para a Ciência e a Tecnologia in the framework of project FCOMP-01-0124-FEDER-009303 (PTDC/CTE-ATM/102142/2008). The authors acknowledge the funding provided by the Évora Geophysics Centre, Portugal, under the contract with FCT (the Portuguese Science and Technology Foundation), PEst-OE/CTE/UI0078/2011. The authors also thank Samuel Bárias for the technical drawing of the device and support in the field campaigns; Flávio Couto for support in the field campaigns; IFREMER (Institut Français de Recherche pour l'Exploitation de la mer) for providing the water turbidity values from Thau lagoon. They also thank the municipal swimming pool complex of Évora for kindly allowing access for underwater measurements.