lower than the adjacent NIR maximum (8% at 712 nm). The upward spectral shift of
the NIR peak with increased chl a has been widely observed (Gitelson, 1992; Gitelson
et al., 1999) and is explained by an upward shift in the position of minimum combined
absorption by pigment and water with increased chlorophyll concentrations (Figure 6).
The comparative heights of the NIR peak and the chl a red trough are used in several
chlorophyll algorithms for Case 2 waters (Dekker, 1993; Ammenberg, 2000; Gitelson
et al., 2000, Gons et al., 2002; see below).
Cunningham Lake is a eutrophic, clay-rich, turbid reservoir near Omaha within an
agricultural watershed. A late summer example with high chl a and total seston
concentrations of 118.9 µg/l and 106.6 mg/l is shown in the last panel of Figure 7.
Phytoplankton composition was quite diverse, with abundant cynaobacteria and
dinoflagellates as well as diatoms and chlorophytes. Green and NIR maxima occurred
at wavelengths similar to the May Carter Lake example, but with higher reflectance.
Reflectance at the NIR peak exceeded green peak reflectance (4.1% versus 3.7%) in
contrast to green peak dominance in the first Carter Lake example, with an overall
pattern more similar to the August spectrum. The dinoflagellate carotenoid pigment
peridinin has significant activity in the mid-500 nm region and suppresses the green
peak magnitude (Gitelson et al., 1999). Additionally, combined pigment and water
absorption in the green peak region is more affected by increased pigment loads than
the NIR peak position (Figure 6).
6. Tank Mesocosm Studies
6.1 PHYTOPLANKTON DENSITY AND COMPOSITION AND THEIR EFFECTS
ON REFLECTANCE
Experiments with manipulations of OACs in indoor and outdoor tanks (ex.
Krijgsman, 1994; Schalles et al., 1997; Schalles et al., 2001) have provided calibration
data for pigment estimation algorithms, empirical evidence for interactions between
phytoplankton and other constituents, and tests of bio-optical models. The results of
spectral reflectance measurements from a tank mesocosm experiment at the former
National Agricultural Water Quality Laboratory (USDA-Agricultural Research Service)
in Durant, Oklahoma, illustrate the optical interactions of a graded series of
phytoplankton densities with water. In this experiment (Figure 8, from Schalles et al.,
1997), a mixed culture of green algae was grown to bloom state in an 80 m
3 outdoor
tank. The tank depth was 3.2 m. Chlorella sp. accounted for 92% of cell density and
Scenedesmus sp. for an additional 7% after bloom development. The upper 80% of the
culture tank’s water was transferred to a separate tank (dilution tank) to minimize
settled detrital particles and brought to full volume with clear water. In the experiment,
the bloom water was sequentially diluted with clear water from two additional tanks
and volume was balanced by pumping the phytoplankton tank water to a clearwater,
enrichment tank. Complete tank mixing was accomplished in 15-20 minutes with a pair
of pumps drawing water from the bottom center, which was reinjected tangentially at
the tank wall just below surface and at mid depth. For load balance, the enrichment tank
was allowed to overflow to a drain. In this manner, phytoplankton cell densities were
manipulated in a graded manner, achieving a chl a range of 0.4 - 62 µg/l. Initial cell
density in the bloom water tank prior to dilution was 3.2 x 10
5 cells/ml.
A hinge point near 510 nm was observed in the resultant family of spectra (Figure 8).
At wavelengths below 510 nm, reflectance decreased with increasing cell density - direct
41
Optical Remote Sensing Techniques
the NIR peak with increased chl a has been widely observed (Gitelson, 1992; Gitelson
et al., 1999) and is explained by an upward shift in the position of minimum combined
absorption by pigment and water with increased chlorophyll concentrations (Figure 6).
The comparative heights of the NIR peak and the chl a red trough are used in several
chlorophyll algorithms for Case 2 waters (Dekker, 1993; Ammenberg, 2000; Gitelson
et al., 2000, Gons et al., 2002; see below).
Cunningham Lake is a eutrophic, clay-rich, turbid reservoir near Omaha within an
agricultural watershed. A late summer example with high chl a and total seston
concentrations of 118.9 µg/l and 106.6 mg/l is shown in the last panel of Figure 7.
Phytoplankton composition was quite diverse, with abundant cynaobacteria and
dinoflagellates as well as diatoms and chlorophytes. Green and NIR maxima occurred
at wavelengths similar to the May Carter Lake example, but with higher reflectance.
Reflectance at the NIR peak exceeded green peak reflectance (4.1% versus 3.7%) in
contrast to green peak dominance in the first Carter Lake example, with an overall
pattern more similar to the August spectrum. The dinoflagellate carotenoid pigment
peridinin has significant activity in the mid-500 nm region and suppresses the green
peak magnitude (Gitelson et al., 1999). Additionally, combined pigment and water
absorption in the green peak region is more affected by increased pigment loads than
the NIR peak position (Figure 6).
6. Tank Mesocosm Studies
6.1 PHYTOPLANKTON DENSITY AND COMPOSITION AND THEIR EFFECTS
ON REFLECTANCE
Experiments with manipulations of OACs in indoor and outdoor tanks (ex.
Krijgsman, 1994; Schalles et al., 1997; Schalles et al., 2001) have provided calibration
data for pigment estimation algorithms, empirical evidence for interactions between
phytoplankton and other constituents, and tests of bio-optical models. The results of
spectral reflectance measurements from a tank mesocosm experiment at the former
National Agricultural Water Quality Laboratory (USDA-Agricultural Research Service)
in Durant, Oklahoma, illustrate the optical interactions of a graded series of
phytoplankton densities with water. In this experiment (Figure 8, from Schalles et al.,
1997), a mixed culture of green algae was grown to bloom state in an 80 m
3 outdoor
tank. The tank depth was 3.2 m. Chlorella sp. accounted for 92% of cell density and
Scenedesmus sp. for an additional 7% after bloom development. The upper 80% of the
culture tank’s water was transferred to a separate tank (dilution tank) to minimize
settled detrital particles and brought to full volume with clear water. In the experiment,
the bloom water was sequentially diluted with clear water from two additional tanks
and volume was balanced by pumping the phytoplankton tank water to a clearwater,
enrichment tank. Complete tank mixing was accomplished in 15-20 minutes with a pair
of pumps drawing water from the bottom center, which was reinjected tangentially at
the tank wall just below surface and at mid depth. For load balance, the enrichment tank
was allowed to overflow to a drain. In this manner, phytoplankton cell densities were
manipulated in a graded manner, achieving a chl a range of 0.4 - 62 µg/l. Initial cell
density in the bloom water tank prior to dilution was 3.2 x 10
5 cells/ml.
A hinge point near 510 nm was observed in the resultant family of spectra (Figure 8).
At wavelengths below 510 nm, reflectance decreased with increasing cell density - direct
41
Optical Remote Sensing Techniques