Bio-optical properties of the marine cyanobacteria Trichodesmium spp.


Bio-optical spectral properties were determined on fresh suspensions of Trichodesmium spp. collected in a tropical lagoon and put in seawater tanks (total chlorophyll concentrations range between 0.1 and 3.8 mg m-3). The spectrum of the backscattering coefficient was a hyperbolic function with a slope of 1.2, often showing troughs at 440, 550 and 676 nm, due to absorption peaks of chlorophyll and phycoerythrin. The absorption spectrum computed with a specific beta correction for Trichodesmium, showed a blue to red ratio (B/R) equivalent to the one of a single colony (B/R=2), and also showed the double peak of mycosporine-like amino acids (MAA's, 330 and 360 nm). The CDOM absorption spectrum showed minor MAA peaks when cyanobacterial concentrations were above 1 mg Chl a m-3. The chlorophyll a-specific backscattering and absorption coefficients at 442 nm were respectively 0.0126 m2 (mg.chl a)-1 and 0.027 m2 (mg chl a)-1. Suspensions in tanks exhibited a high backscattering ratio at 660 nm (bb=bbp/bp). The above-water reflectance spectrum clearly showed troughs at the wavelength of the pigment absorption peaks. Datasets of Trichodesmium normalized absorption, backscattering and reflectance spectra will allow its detection with future hyperspectral ocean colour sensors.
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