Fraunhofer lines and atmospheric absorption bands interfere with the spectral location of absorption bands of
photosynthetic pigments in plankton. Hyperspectral data were used to address this interference on identifying absorption
bands by applying derivative analysis of radiance spectra. Algal blooms show elevated radiance data even at longer
wavelengths compared to oligotrophic water and may reach radiance values of around 800 W/m<sup>2</sup>/micrometer/sr at a
wavelength of about 0.8 μm. Therefore, the use of a spectral range beyond 0.55 μm is useful to describe bloom
characteristics. In particular, the slope between 0.55 μm to 0.80 μm shows an advantage to depict gradients in plankton
blooms. Radiance spectra in the region from 0.4 to 0.8 μm for oligotrophic water and near coastal water show similar
location of absorption bands when analyzed with derivative analysis but with different amplitudes. For this reason,
radiance spectra were also analyzed without atmospheric correction, and various approaches to interpret radiance data
over plankton blooms were investigated. Cluster analysis and ratio techniques at longer wavelengths were found to assist
in the separation of ocean color gradients and distinguish bio-geochemical provinces in near-coastal waters.
Furthermore, using the slope of spectra from plankton blooms, in connection with scatter diagrams at various
wavelengths, shows that details can be revealed that would not be recognized in single channels at lower wavelength.
Data with 0.4-m spatial resolution acquired ~2 km off the southeast Florida coast using the airborne Portable Hyperspectral Imager for Low-Light Spectroscopy (PHILLS) have been analyzed with the objective of identifying drifting surface macroalgae (Sargassum) through its spectral signature in at-sensor radiance. The observed spectral features of Sargassum include a peak at a wavelength of ~0.570 µm and a photosynthetic 'red edge' between 0.673 and 0.699 µm. Sargassum also exhibits high radiance in the reflected near-infrared but is impacted by the atmospheric absorption bands of water vapor at 0.720 µm and oxygen at 0.756 µm. The spectral signature is clearest and largest in amplitude where the Sargassum occurs as small surface aggregations, or rafts, which tend to lie at the downwind ends of narrow Sargassum windrows. The quantity of floating Sargassum was estimated within a single pixel by linearly mixing a spectrum of Sargassum-free water with varying percentages of a spectrum from a pixel assumed completely filled with floating plants. For our study site about 2.3% of the ocean area is classified as having some Sargassum coverage, with pixels completely filled with Sargassum being rare (only 0.2% of the classified Sargassum pixels) and pixels with the least-resolvable amount of Sargassum (~10% filled) being the most common.
High-resolution spectroscopy using the Portable Hyperspectral Imager for Low-Light Spectroscopy (PHILLS) was applied to the problem of detecting potentially harmful algae blooms in the coastal environment. Data were collected on two aircraft passes, 30-min apart, over the tidally influenced part of the Potomac River. Use of two wavelengths, 0.676 and 0.700 µm, permitted the detection of surface algae accumulations while avoiding the need for atmospheric corrections, which are problematic in Case-2 water. The analysis identified algal accumulations derived from frontal processes, and narrow, linearly coherent streaks, derived from Langmuir circulation. The streaks increased markedly in number between the two passes and formed a two-dimensional pattern across the river, consistent with the advection time of surface material into windrows. The effect of wind on the patches is primarily a local reorganization of the algal material into new streaks. Spectra from within the streaks compared to those from ambient water showed absorption characteristics consistent with the presence of cyanobacteria. This interpretation is reinforced by available in-situ data. This study illustrates the value of high spectral and temporal resolutions in observing the spatial distribution of the algae, in identifying dominant functional groups, and in understanding the response of the algae to physical forcing.