Bio-optical properties of the marine diazotrophic cyanobacteria Trichodesmium spp. II. A reflectance model for remote sensing

The spatial and temporal distribution of Trichodesmium in the world's ocean s is highly variable and can potentially be assessed using satellite imager y. Distinguishing these organisms from other phytoplankton in the upper oce an using remotely sensed information, however, requires an optical model th at uniquely characterizes Trichodesmium. Here, we parameterize a standard r emote-sensing reflectance model using measured values of Trichodesmium's in herent optical properties, namely the spectral dependence of the chlorophyl l-specific optical absorption cross-sections and the spectral dependence of the chlorophyll-specific backscatter cross-sections. Values for the chloro phyll-specific absorption cross-sections are described in the previous pape r. We calculated the spectral chlorophyll-specific backscattering cross-sec tion (b(b)*) from measurements of the chlorophyll-specific volume-scatterin g function and the spectral backscatter coefficients. b(b)* was 0.0027 m(2) (mg chlorophyll a [Chl a])(-1) at 436 nm and 0.002 m(2) (mg Chi a)(-1) at 546 nm; these cross-sections are approximately one order of magnitude highe r than those for "typical" phytoplankton. The optical model revealed that t he combination of high backscatter, absorption, and fluorescence could be u sed to distinguish moderate to high concentrations (>1 mg Chl m(-3)) of Tri chodesmium from other phytoplankton. The model also predicted that surface scum blooms of Trichodesmium would have high reflectance in the near infrar ed. The high reflectance feature of surface Trichodesmium blooms was used i n conjunction with sea truth and data from the advanced very high resolutio n radiometer (AVHRR) to map a 300,000-km(2) Trichodesmium bloom off the Som ali Coast in May 1995. The nitrogen fixed by this bloom was estimated to be 9.4 x 10(8) gN d(-1). These results demonstrate the potential of using rem ote-sensing techniques in the estimation of nitrogen fixation and the contr ibution of nitrogen fixation to global biogeochemical processes.