Light absorption by particulate and dissolved organic matter in coastal and oceanic off the South west coast of Europe: a contribution to MERIS sensor validation

Goela,  Priscila; Cristina, Sónia; Icely, John; Newton, Alice

http://hdl.handle.net/10400.1/2667

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Abstract(s)

The two variable components of light absorption, which are important contributors to the correct adjustments of ocean colour algorithms, are considered and discussed in this study, conducted in the Southwest coast of Portugal; these are particulate absorption – separated into phytoplankton and non-algal particles absorption, and coloured dissolved organic matter absorption, expressed in terms of their coefficients – ap(), aph(), anap() and aCDOM(), respectively. This study is part of a global effort to collect data to validate marine products of MERIS, the ENVISAT ocean colour sensor, as contracted by European Space Agency (ESA). Particulate absorptions were determined with Transmittance-Reflectance approach, using NaClO bleaching to remove phytoplankton contribution to the total particulate matter absorption spectra; YSBPA absorption coefficient was assessed according to MERIS validation protocols. Phytoplankton absorption coefficients were transformed into specific coefficients – a*ph(), normalizing it with respect to chlorophyll a concentrations determined both by HPLC and spectrophotometric methods. Results show that the absorption of light by particulate matter is almost totally dependent on the phytoplankton, with no significant contribution from non-algal particles (mean contribution of 10% to total particulate absorption), both in coastal and oceanic waters, emphasising the dominance of case 1-type waters. Specific chlorophyll absorption coefficients show significant fluctuations between seasons and stations, ranging from 0.007 to 0.09 at 678 nm; however, a good correlation between ap(443) and ap at other MERIS equivalent wavelengths was found. Particulate matter has generally more weight than dissolved matter absorption in the total absorption budget. These results may contribute to improve and correct regional ocean colour algorithms.