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Optical variability
associated with a Gymnodinium breve red tide event off northwestern
Florida
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Lohrenz, Steven E.1, Donald G. Redalje1,
and Gary J. Kirkpatrick2 |
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Red tides of the toxic dinoflagellate, Gymnodinium
breve, are commonly observed along the Florida coast.
Our ability to detect and quantify these phenomena has the
potential to be greatly enhanced by observations of in situ and remotely
sensed optical properties. During
October 2000, optical measurements were conducted in the area of a red
tide bloom off the northwest Florida coast as part of the Florida ECOHAB
program. Time-series sampling
of the bloom was carried out over a 48 h period while tracking a surface
drifter (Fig. 1). Cells of G.
breve accounted for the vast majority of particles visible by light
microscopy. Repetitive
profiles with a spectral absorption and attenuation meter (WETLabs, Inc.
ac-9) revealed substantial vertical structure in attenuation and
absorption that was largely attributable to G. breve distributions.
Observed temporal variations in vertical structure were consistent
with vertical migration and apparent wind-induced mixing. Our results illustrate the utility of optical measurements
for characterizing red tide phenomena. |
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1The
University of Southern Mississippi Department of Marine Science, Stennis
Space Center, MS 39529 |
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2Mote
Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, FL
34236 |
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Figure 1. Time-series of beam attenuation at
532 nm measured with a WETLabs, Inc. spectral absorption and attenuation
meter (ac-9). Regions of high attenuation near the surface and
bottom appeart to correspond to dense accumulations of G. breve
cells. |
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Mie
approximation of light scattering by Gymnodinium breve and its
relationship to in situ scattering during a red tide event
Mahoney,
Kevin L.1, Steven E. Lohrenz1, Gary J. Kirkpatrick2,
Gary L. Fahnenstiel3
Scattering
of light by particles in aquatic environments varies in relationship to particle
properties including size distribution, shape and composition (refractive
index). The backscattering of light
strongly influences water-leaving radiance and, subsequently, remote sensing
reflectance. Ideally, information
about particle properties and concentrations can be derived from remotely sensed
signatures. However, the
contribution to backscatter by specific types of particles is difficult to
measure in the field. Backscattering
by particles of known characteristics can be modeled through Mie calculations
for homogeneous spheres. During
October 2000, a cruise was conducted during a red tide event off the northwest
Florida coast as part of the Florida ECOHAB program.
Measurements included spectral absorption and particle size distributions
and concentrations in samples dominated by the red tide organism, Gymnodinium
breve. In addition, in situ
total scattering and absorption were determined using a spectral absorption and
attenuation meter (WETLabs, Inc. ac-9). The
objective of this project was to compare Mie approximations of scattering to
measurements of total in situ scattering, and to assess variability in the
backscattering ratio attributable to G. breve.
This information will be used to evaluate the potential for using remote
sensing to detect red tide events.
1The
University of Southern Mississippi Department of Marine Science, Stennis Space
Center, MS 39529
2Mote
Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, Florida
34236
3NOAA
Great Lakes Environmental Research Laboratory, 1431 Beach St., Muskegon, MI
49441
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ECOHAB Research