Spectral reflectance and absorption of a massive red tide off southern California

Spectral reflectance and absorption of a massive Lingulodinium (Gonyaulax) polyedra red tide in March 1995 off southern California are compared to a “baseline” of biooptical measurements from the California Cooperative Oceanic Fisheries Investigations. The red tide was characterized by increased absorption and therefore reduced remote sensing reflectance (Rrs) in the 340–400 nm spectral range. The increased ultraviolet absorption was probably caused by mycosporine-like amino acids in the particulate fraction as well as increased absorption by dissolved organic matter. The chlorophyll a (chl a) specific particulate absorption of the L. polyedra bloom in the visible spectral range remained relatively constant for the chl a range 1–150 mg m−3 indicating accumulation of cells with similar optical characteristics. The difference in the Rrs versus chl a relationship of the red tide and “normal” California Current phytoplankton diminished with increasing wavelength from 340 nm and disappeared at 412 nm. Ratios of Rrs at 340 nm (or 380 nm) and 412 nm (or 443 nm) provided differentiation of the red tide starting at chl a concentration of 1–2 mg m−3. The forthcoming Japanese Global Imager (GLI) satellite sensor has, among others, the 380 nm band. If the signal to noise ratio and atmospheric correction for the 380 nm band are sufficient to retrieve the dynamic range of the water leaving radiance, then it might be possible to differentiate red tides from other phytoplankton blooms with the algorithm described here.