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J. Gower and S. King
Spectroradiometer (MODIS) and Visible/Infrared Imager Radiometer Suite (VIIRS),
and the European sensor MEdium Resolution Imaging Spectrometer (MERIS) provide global data on ocean and coastal near-surface phytoplankton concentration at
spatial resolutions down to 1 km (300 m for MERIS) over image swaths 2,000 km
across (3,000 km for VIIRS, 1,100 km for MERIS). These have been joined for short
periods by sensors from several other countries. The Indian Ocean Colour Monitor (OCM) has provided some data since launch in 1999 on the Indian Oceansat-1
satellite, and a follow-on instrument (OCM 2) was launched in 2009 on Oceansat-2.
Optical data from these satellites are used primarily to deduce chlorophyll concentrations in near-surface waters. Chlorophyll is a standard indicator of phytoplankton
concentration, which in turn represents the primary-producing biomass at the base
of the marine food chain. Images can be used to examine spatial patterns of highchlorophyll in upwelling zones and blooms, or conversely “ocean deserts” in centres
of mid-ocean gyres, where chlorophyll concentrations are low. The images show seasonal patterns, for example those due to coastal upwelling driven by seasonal winds,
or the spring bloom in extra-tropical waters. On longer time scales, they may also
show how bloom frequency or extent are affected by global warming, changing nutrient input, or the spread of new species in ship’s ballast water. In some cases blooms
are triggered by mixing after intense storms, or by nutrient input from rivers, volcanic eruptions or artificial fertilization. Timing and intensity of patterns of measured
chlorophyll can then be related to fish catch and properties of marine ecosystems.
Chlorophyll concentrations are deduced from the satellite optical data by measuring the combined effects of increased scattering near 550 nm (green light) and
increased absorption near 450 nm (blue light). Increased phytoplankton concentration causes more scattering from cell walls and other structural parts of the organisms,
which increases the amount of green light back-scattered from the ocean. At the same
time, more absorption due to higher concentrations of chlorophyll-a and auxiliary
pigments responsible for energy gathering for photosynthesis reduces the amount
of blue light. This “blue to green” ratio is used to deduce chlorophyll concentrations from observed spectral radiances. The relation is well-determined in “Case 1”
waters, defined as those whose varying optical properties are determined by phytoplankton and not by the presence of other constituents such as suspended sediments
or dissolved organic material, or again by bottom reflection in shallow water.
In addition to deducing chlorophyll content, optical sensors can measure water
reflectance, for example in green light near 550 nm, and use this to deduce the total
amount of scattering material in the water. If this is found to be higher than expected
for the measured amount of phytoplankton, then the water can be flagged as “Case
2” (essentially anything other than Case 1) and the higher scattering can be related
to sediment from rivers, or resuspension from the bottom in shallow water. In some
cases, high back-scatter can be due to blooms of species such as Coccolithophores,
which are considered Case 2 because of the extreme brightness of their blooms.
Concentration of Coloured Dissolved Organic Material (CDOM) can also be deduced
from measured radiances at the shortest wavelengths, typically 412 nm. Waters with
significant concentrations of CDOM are also classed as Case 2.
In order to measure the actual reflectance of water, the water-leaving radiances
have to be computed from those measured at the satellite. This involves computing the
J. Gower and S. King
Spectroradiometer (MODIS) and Visible/Infrared Imager Radiometer Suite (VIIRS),
and the European sensor MEdium Resolution Imaging Spectrometer (MERIS) provide global data on ocean and coastal near-surface phytoplankton concentration at
spatial resolutions down to 1 km (300 m for MERIS) over image swaths 2,000 km
across (3,000 km for VIIRS, 1,100 km for MERIS). These have been joined for short
periods by sensors from several other countries. The Indian Ocean Colour Monitor (OCM) has provided some data since launch in 1999 on the Indian Oceansat-1
satellite, and a follow-on instrument (OCM 2) was launched in 2009 on Oceansat-2.
Optical data from these satellites are used primarily to deduce chlorophyll concentrations in near-surface waters. Chlorophyll is a standard indicator of phytoplankton
concentration, which in turn represents the primary-producing biomass at the base
of the marine food chain. Images can be used to examine spatial patterns of highchlorophyll in upwelling zones and blooms, or conversely “ocean deserts” in centres
of mid-ocean gyres, where chlorophyll concentrations are low. The images show seasonal patterns, for example those due to coastal upwelling driven by seasonal winds,
or the spring bloom in extra-tropical waters. On longer time scales, they may also
show how bloom frequency or extent are affected by global warming, changing nutrient input, or the spread of new species in ship’s ballast water. In some cases blooms
are triggered by mixing after intense storms, or by nutrient input from rivers, volcanic eruptions or artificial fertilization. Timing and intensity of patterns of measured
chlorophyll can then be related to fish catch and properties of marine ecosystems.
Chlorophyll concentrations are deduced from the satellite optical data by measuring the combined effects of increased scattering near 550 nm (green light) and
increased absorption near 450 nm (blue light). Increased phytoplankton concentration causes more scattering from cell walls and other structural parts of the organisms,
which increases the amount of green light back-scattered from the ocean. At the same
time, more absorption due to higher concentrations of chlorophyll-a and auxiliary
pigments responsible for energy gathering for photosynthesis reduces the amount
of blue light. This “blue to green” ratio is used to deduce chlorophyll concentrations from observed spectral radiances. The relation is well-determined in “Case 1”
waters, defined as those whose varying optical properties are determined by phytoplankton and not by the presence of other constituents such as suspended sediments
or dissolved organic material, or again by bottom reflection in shallow water.
In addition to deducing chlorophyll content, optical sensors can measure water
reflectance, for example in green light near 550 nm, and use this to deduce the total
amount of scattering material in the water. If this is found to be higher than expected
for the measured amount of phytoplankton, then the water can be flagged as “Case
2” (essentially anything other than Case 1) and the higher scattering can be related
to sediment from rivers, or resuspension from the bottom in shallow water. In some
cases, high back-scatter can be due to blooms of species such as Coccolithophores,
which are considered Case 2 because of the extreme brightness of their blooms.
Concentration of Coloured Dissolved Organic Material (CDOM) can also be deduced
from measured radiances at the shortest wavelengths, typically 412 nm. Waters with
significant concentrations of CDOM are also classed as Case 2.
In order to measure the actual reflectance of water, the water-leaving radiances
have to be computed from those measured at the satellite. This involves computing the