8
Chapter 1: Toward an Ecological Geography of the Sea
around the Southern Ocean, except briefly in summer (see Chapter 12). Haptophytes form
a sort of universal background to higher concentrations of the other groups. Taxonomic
mapping of this kind should become routine as experience grows with new sensors, but
I venture to suggest that Alvain et al.’s illustrations are destined to become textbook cases.
A lengthening series of radiometers has already been flown, starting with the Coastal
Zone Colour Scanner (CZCS). This was launched in October 1978 aboard NIMBUS-7 and
remained operational until 1986; because it was a “proof-of-concept” mission, the sensors
were activated only intermittently so that a comprehensive, repetitive coverage of the
oceans was not obtained, and the data were most useful for developing regional, seasonal
climatologies. The CZCS radiometer operated at five spectral bands, and some difficulty
was encountered in correcting for the atmospheric scattering that was responsible for
95–99% of radiance sensed by the instrument (Feldman et al., 1989). Nevertheless, the
images were a revelation to biological oceanographers and were used in many studies,
including the first regional and global estimates of primary production in the ocean:
the regional analyses presented in the first edition of this work were derived from
CZCS output. Subsequent evolution of sensors and their deployment has been rapid,
and an introduction such as this is out of date before going to press; to keep up with
developments, I recommend reading the bulletins issued regularly by the International
Ocean-Color Coordinating Group (IOCCG).
After CZCS failed in 1986, two other sea-surface color missions obtained data for
relatively brief periods before spacecraft failure—the Japanese ADEOS and the CNESFrance POLDER instruments; the IOCCG lists a couple of dozen sensors with ocean color
capabilities, either launched or due to be launched in the next few years by the United
States, France, China, India, Australia, Japan, Korea, and so on. Here, I shall use images
only from SeaWiFS and MODIS, both currently operational.
In 1997, the SeaStar satellite, owned by the GeoEye Corporation, placed the SeaWiFS
sensors in sun-synchronous orbit at a nominal altitude of 705 km, so as to cover a swath
that varied between 1502 and 2801 km, giving a revisit time of 24 hours; at the time of
writing it remains operational. The radiometer is sensitive to eight spectral bands from
the violet (412 nm) through blue-green and green to the near infrared (865 nm). Twelve
binned geophysical parameters are obtained, including the chlorophyll-like pigments
(band 1 for colored dissolved organic material (CDOM) and bands 2–5 at 443 to 555 nm
for chlorophyll-a), the diffuse attenuation coefficient, the ratio of chlorophyll-a to the
diffuse attenuation coefficient at band 3, the epsilon value for the aerosol correction of
bands 6 and 8 (660–785 nm), and the aerosol optical thickness at band 8 (845–885 nm).
For each of these parameter values and for each bin, the mean, median, mode, and
standard deviation may be derived.
SeaWiFS images, like those of CZCS, are available in two formats, Local (1 km resolution) and General Area Coverage (4 km resolution), the former being available for
only a few specific regions where ground stations able to receive data directly from the
satellite have been installed. GAC data, processed at NASA Goddard, are available from
a variety of sources and at a variety of processing levels, from raw irradiance measured
at the satellite (Level 1) to global data integrated over weekly, monthly, seasonal, or
annual periods (Level 3). For NASA, SeaWiFS was a “data-buy” project, and although
the NASA-generated images are freely downloadable on-line, they remain the intellectual
property of GeoEye; their use in a book such as this is restricted to license holders. Most
of the images I shall present, therefore, will be from other sensors.
From the beginning there has been some uncertainty about the validity of radiometer
images of optically complex seawater typical of coastal regions where bed stress keeps
fine particulate material, both organic and inorganic, in suspension. Here, a wide range
of factors influences the optical properties of seawater (Lavender et al., 2005), and such
complexity requires algorithms “soundly based on theoretical considerations” according
Chapter 1: Toward an Ecological Geography of the Sea
around the Southern Ocean, except briefly in summer (see Chapter 12). Haptophytes form
a sort of universal background to higher concentrations of the other groups. Taxonomic
mapping of this kind should become routine as experience grows with new sensors, but
I venture to suggest that Alvain et al.’s illustrations are destined to become textbook cases.
A lengthening series of radiometers has already been flown, starting with the Coastal
Zone Colour Scanner (CZCS). This was launched in October 1978 aboard NIMBUS-7 and
remained operational until 1986; because it was a “proof-of-concept” mission, the sensors
were activated only intermittently so that a comprehensive, repetitive coverage of the
oceans was not obtained, and the data were most useful for developing regional, seasonal
climatologies. The CZCS radiometer operated at five spectral bands, and some difficulty
was encountered in correcting for the atmospheric scattering that was responsible for
95–99% of radiance sensed by the instrument (Feldman et al., 1989). Nevertheless, the
images were a revelation to biological oceanographers and were used in many studies,
including the first regional and global estimates of primary production in the ocean:
the regional analyses presented in the first edition of this work were derived from
CZCS output. Subsequent evolution of sensors and their deployment has been rapid,
and an introduction such as this is out of date before going to press; to keep up with
developments, I recommend reading the bulletins issued regularly by the International
Ocean-Color Coordinating Group (IOCCG).
After CZCS failed in 1986, two other sea-surface color missions obtained data for
relatively brief periods before spacecraft failure—the Japanese ADEOS and the CNESFrance POLDER instruments; the IOCCG lists a couple of dozen sensors with ocean color
capabilities, either launched or due to be launched in the next few years by the United
States, France, China, India, Australia, Japan, Korea, and so on. Here, I shall use images
only from SeaWiFS and MODIS, both currently operational.
In 1997, the SeaStar satellite, owned by the GeoEye Corporation, placed the SeaWiFS
sensors in sun-synchronous orbit at a nominal altitude of 705 km, so as to cover a swath
that varied between 1502 and 2801 km, giving a revisit time of 24 hours; at the time of
writing it remains operational. The radiometer is sensitive to eight spectral bands from
the violet (412 nm) through blue-green and green to the near infrared (865 nm). Twelve
binned geophysical parameters are obtained, including the chlorophyll-like pigments
(band 1 for colored dissolved organic material (CDOM) and bands 2–5 at 443 to 555 nm
for chlorophyll-a), the diffuse attenuation coefficient, the ratio of chlorophyll-a to the
diffuse attenuation coefficient at band 3, the epsilon value for the aerosol correction of
bands 6 and 8 (660–785 nm), and the aerosol optical thickness at band 8 (845–885 nm).
For each of these parameter values and for each bin, the mean, median, mode, and
standard deviation may be derived.
SeaWiFS images, like those of CZCS, are available in two formats, Local (1 km resolution) and General Area Coverage (4 km resolution), the former being available for
only a few specific regions where ground stations able to receive data directly from the
satellite have been installed. GAC data, processed at NASA Goddard, are available from
a variety of sources and at a variety of processing levels, from raw irradiance measured
at the satellite (Level 1) to global data integrated over weekly, monthly, seasonal, or
annual periods (Level 3). For NASA, SeaWiFS was a “data-buy” project, and although
the NASA-generated images are freely downloadable on-line, they remain the intellectual
property of GeoEye; their use in a book such as this is restricted to license holders. Most
of the images I shall present, therefore, will be from other sensors.
From the beginning there has been some uncertainty about the validity of radiometer
images of optically complex seawater typical of coastal regions where bed stress keeps
fine particulate material, both organic and inorganic, in suspension. Here, a wide range
of factors influences the optical properties of seawater (Lavender et al., 2005), and such
complexity requires algorithms “soundly based on theoretical considerations” according