Detection of Karenia brevis harmful algal blooms in the West Florida Shelf using red bands of MERIS imagery

Abstract

Karenia brevis (K. brevis) blooms regularly in the Gulf of Mexico. However, detection from space still remains a challenge using standard bio-optical retrieval algorithms because of the uncertainties of atmospheric correction and spectral interferences arising in optically complex coastal waters from high concentrations of organic and inorganic materials. We propose a simple red band difference (RBD) technique to detect blooms, coupled with a normalized difference technique to distinguish k. brevis blooms specifically, which we label as the K. brevis bloom index (KBBI). K. brevis blooms studied include those from the Medium Resolution Imaging Spectrometer (MERIS) ocean color measurements off the West Florida Shelf (WFS). As is known, K. brevis has low backscatter characteristics, and as a consequence, it is the chlorophyll fluorescence that dominates the red water-leaving radiance signals (over any elastic scatter). The proposed techniques take advantage of this dominance of the fluorescence signal. With the red water leaving reflectance dominated by fluorescence, which is the case for K. brevis, the reflectance spectra has a local minimum around 665 nm (band 7) and the signal at 681 nm (band 8), which falls in the shoulder of the reflectance peak, has higher values than the signal at band 7 due to the chlorophyll fluorescence emission centered around 685 nm.