Figure 2. Comparison of in situ chl a absorption (at a chl a concentration of 1 µg/l) versus total
pigment absorption (with all chlorophylls and carotenoids) based on the specific absorption
coefficients for five pigment classes (Figure 1) and global ocean averages for the proportions of
these five classes (see text).
Chlorophyll distribution may have significant vertical variation, particularly when
stable stratification develops in coastal and estuarine waters (Paerl, 1988) or with the
characteristic deep chlorophyll maximum of oligotrophic ocean waters (Lefevre et al.,
2003). Optical remote sensing above the water surface generally cannot resolve deep
chlorophyll layers and other vertical heterogeneity patterns (Andre, 1992). A further
complication, rarely considered, arises when multiple wavelength algorithms use bands
representing differing optical depths. For example, in a turbid eutrophic water column,
blue and red reflectance signals attenuate much more rapidly (in both the down and
upwelling fields), and the composite upwelling blue and red signals return from
shallower depths than the green signals.
Chlorophyll is a comparatively efficient and effective, albeit indirect, measure of
phytoplankton biomass. There are three primary analytical approaches for laboratory
analyses of chl a concentration in field samples: spectrophotometry, fluorometry, and
high-performance liquid chromatography (American Public Health Association, 1998).
Most analyses begin with filtration of the chlorophyll-containing pigment fraction.
Pigments are then extracted with an organic solvent (acetone, methanol, or less
commonly ethanol or diethyl ether (Rowan, 1989). Freezing, grinding, and other
techniques to physically disrupt algal cells generally improve the extraction pigment
yield. Note that these extraction schemes also bring accessory pigments (except for the
water soluble phycobiliproteins) into solution. Following clarification of the extract
with filtration or centrifugation, the pigment content is assessed using pigment specific
absorption bands in a spectrophotometer, via fluorescence using proper excitation and
emission wavelength settings, or through chromatographic column separation (usually
HPLC) together with in-line optical detection of column fractions. HPLC is now
widely used to simultaneously identify and quantify the entire suite of extractable
31
Optical Remote Sensing Techniques
pigment absorption (with all chlorophylls and carotenoids) based on the specific absorption
coefficients for five pigment classes (Figure 1) and global ocean averages for the proportions of
these five classes (see text).
Chlorophyll distribution may have significant vertical variation, particularly when
stable stratification develops in coastal and estuarine waters (Paerl, 1988) or with the
characteristic deep chlorophyll maximum of oligotrophic ocean waters (Lefevre et al.,
2003). Optical remote sensing above the water surface generally cannot resolve deep
chlorophyll layers and other vertical heterogeneity patterns (Andre, 1992). A further
complication, rarely considered, arises when multiple wavelength algorithms use bands
representing differing optical depths. For example, in a turbid eutrophic water column,
blue and red reflectance signals attenuate much more rapidly (in both the down and
upwelling fields), and the composite upwelling blue and red signals return from
shallower depths than the green signals.
Chlorophyll is a comparatively efficient and effective, albeit indirect, measure of
phytoplankton biomass. There are three primary analytical approaches for laboratory
analyses of chl a concentration in field samples: spectrophotometry, fluorometry, and
high-performance liquid chromatography (American Public Health Association, 1998).
Most analyses begin with filtration of the chlorophyll-containing pigment fraction.
Pigments are then extracted with an organic solvent (acetone, methanol, or less
commonly ethanol or diethyl ether (Rowan, 1989). Freezing, grinding, and other
techniques to physically disrupt algal cells generally improve the extraction pigment
yield. Note that these extraction schemes also bring accessory pigments (except for the
water soluble phycobiliproteins) into solution. Following clarification of the extract
with filtration or centrifugation, the pigment content is assessed using pigment specific
absorption bands in a spectrophotometer, via fluorescence using proper excitation and
emission wavelength settings, or through chromatographic column separation (usually
HPLC) together with in-line optical detection of column fractions. HPLC is now
widely used to simultaneously identify and quantify the entire suite of extractable
31
Optical Remote Sensing Techniques