Abstract
The most recent Visible Infrared Imager Radiometer Suite (VIIRS) is not equipped with a spectral band to detect solar-stimulated phytoplankton fluorescence. The lack of such a band may affect the ability of VIIRS to detect and quantify harmful algal blooms (HABs) in coastal waters rich in colored dissolved organic matter (CDOM) because of the overlap of CDOM and chlorophyll absorption within the blue-green spectrum. A recent HAB dominated by the toxin-producing dinoflagellate Karenia brevis in the northeastern Gulf of Mexico, offshore of Florida's Big Bend region, allowed for comparison of the capacities of VIIRS and Moderate Resolution Imaging Spectroradiometer (MODIS) to detect blooms in CDOM-rich waters. Both VIIRS and MODIS showed general consistency in mapping the CDOM-rich dark water, which measured a maximum area of 8900 km2 by mid-July 2014. However, within the dark water, only MODIS allowed detection of bloom patches—as indicated by high normalized fluorescence line height (nFLH). Field surveys between late July and mid-September confirmed Karenia brevis at bloom abundances up to 20 million cells·L−1 within these patches. The bloom patches were well captured by the MODIS nFLH images, but not by the default chlorophyll a concentration (Chla) images from either MODIS or VIIRS. Spectral analysis showed that VIIRS could not discriminate these high-phytoplankton water patches within the dark water due to its lack of fluorescence band. Such a deficiency may be overcome with new algorithms or future satellite missions such as the U.S. NASA's Pre-Aerosol-Clouds-Ecology mission and the European Space Agency's Sentinel-3 mission.
Highlights
The most recent (2011–present) ocean color satellite sensor, the Visible Infrared Imager RadiometerSuite (VIIRS), is expected to provide a consistent, long-term data record to continue observations from its predecessors, including the Moderate Resolution Imaging Spectroradiometer (MODIS, 1999–present on Terra and 2002–present on Aqua) and the Medium Resolution Imaging Spectrometer (MERIS, 2002–2012)
The enhanced Red-Green-Blue (ERGB) imagery was composed of the Rrs data at 547 (Red), 488 (Green), and 443 nm (Blue), and the dark color was due to the strong absorption at 443 and 488 nm by phytoplankton pigments and/or colored dissolved organic matter (CDOM)
If the dark water was associated with high FLH values, it must be a bloom, and otherwise it must be a CDOM plume [7,24]
Summary
Suite (VIIRS), is expected to provide a consistent, long-term data record to continue observations from its predecessors, including the Moderate Resolution Imaging Spectroradiometer (MODIS, 1999–present on Terra and 2002–present on Aqua) and the Medium Resolution Imaging Spectrometer (MERIS, 2002–2012). Achieving such a goal requires substantial effort in sensor calibration, algorithm development, and data product validation. Recent sensor evaluation in 2014 showed signs of sensor degradation in the near-infrared (NIR) bands and radiometric calibration in other bands (Menghua Wang of NOAA NESDIS, and Bryan Franz of NASA GSFC, personal communications) These artifacts are minimized in data re-processing by both NASA and NOAA, at least to the first order. For a moderately turbid estuary (turbidity generally between 1 and 8 NTU, Tampa Bay, FL, USA), consistency was achieved between
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