Trichome abundance, chlorophyll content and the spectral coefficient for light absorption of Trichodesmium slicks observed in the Southwestern Atlantic

Dense slicks of Trichodesmium were found in the shelf-break region in the Southwestern Atlantic during austral spring and autumn. A total of 14 slicks were sampled, and the absorption coefficients of phytoplankton (aph(λ)) indicated clear spectral features of phycobilin pigments. Although these samples showed low-degradation products and detrital importance, the chemotaxonomy, shape, and magnitude of aph(λ) indicated the importance of co-occurring species in the slicks. In addition to the difficulties of enumerating trichomes in situ, co-occurring species affect the expected chlorophyll-a (Chl-a) to trichome ratio, further complicating the detection of Trichodesmium by ocean colour remote sensing. Our results showed that trichome density could be predicted similarly by Chl-a and by aph(621), especially for trichome densities above 8000 trichomes L−1. The phycocyanin spectral feature is a potential source of quantitative information for the detection of Trichodesmium, but noninvasive techniques for quantifying the abundance of Trichodesmium in natural waters are necessary.