MODIS/Aqua May 17, 2003, Guam
18oN
17oN
145oE
146oE
147oE
145oE
146oE
147oE
MODIS/Aqua May 2, 2003,
Equatorial upwelling
1o S
2o S
-2
-1 -1
L (mW cm m sr )
1.00
0.30
Lt Guam bloom
nLw Guam bloom
0.50
0.00
0.20
-0.50
Lt Equatorial upwelling
nLw Equatorial upwelling
-1.00
0.10
-1.50
-2.00
0.00
400
500
600
700
 (nm)
800
900
650
670
690
710
730
750
 (nm)
Fig. 1. MODIS true-color and fluorescence line height (FLH, mW cm-2 m-1 sr-1) imagery showing postvolcano bloom (top) and Equatorial upwelling (middle). The spectra on the bottom show the difference
between the arrow-pointed location and nearby clear-water. The difference is derived from both at-sensor
total radiance (Lt) and normalized water-leaving radiance (nLw). The spectra in the red wavelengths are
zoomed to show details in the bottom-right panel. In both cases, there are clearly peaks at 678-nm,
indicating elevated fluorescence.
Table 1. Characteristics of the Guam bloom and Equatorial upwelling as compared with nearby
clear waters.
Guam bloom nearby clear water
Chlrophyll-a 0.86±0.02
0.03±0.002
FLH
0.014±0.0014 0.0011±0.0004
SST (oC)
28.32±0.12 28.16±0.14
Equatorial upwelling
0.71±0.01
0.027±0.0007
24.57±0.10
nearby clear water
0.19±0.003
0.0076±0.0012
25.24±0.12
Is the post-volcano pattern a phytoplankton bloom or simply suspended non-living particles
(ashes and dusts)?
The standard NASA (SeaDAS) algorithms, which use the blue/green band ratio to estimate
surface chlorophyll-a concentrations (Chl), cannot be used to answer this question because nonliving particles (such as dust deposited in water) can also lead to a decrease in the blue/green
ratio and therefore a false increase in the estimated Chl (Claustre et al., 2002). Spectral shapes
and optical models are the only keys to help answer this question in the absence of in sit
measurements.
The MODIS data were processed using the default SeaDAS5.1 software package. As a
comparison, MODIS data for the well-known equatorial upwelling were also processed.
Differences in the spectral properties and SST between the “blooms” and nearby clear waters for
both cases (post-volcano “bloom” or PV and equatorial upwelling or EU) are shown in Fig. 1
and Table 1.
For the EU case, the 412-nm signal is higher than the 443-nm signal. This is a solid evidence of
phytoplankton bloom. This evidence is lacking in the PV case. Instead, for the PV case there is a
sharp decrease in both 412 and 443 nm, and increase in 531 and 555 nm. In the red part of the
spectra, there is noticeable local peak at 678 nm, a result of fluorescence. Hu et al. (2005)
showed at in CDOM-rich waters MODIS fluorescence line height (FLH) is effective in
delineating phytoplankton blooms. In this oligotrphic ocean, CDOM is expected to be small, but
non-living particles may be high due to volcano deposition. It is known that high concentrations
of non-living particles can result in a false 678-nm signal (Gilerson et al., 2007). However, at
low concentrations, the influence of non-living particles on the 678-nm signal should be small.
Hydrolight simulations showed that for total suspended solids < 5 mg L-1, their influence on the
FLH values are negligible. Additional argument for high concentrations of phytoplankton comes
from the fact that, if only non-living particles exist in the PV patch, the red-part spectra would
not have a noticeable local peak at 678-nm. This is because that of all water constituents
(phytoplankton, CDOM, detritus and other non-living particles), only phytoplankton have an
absorption peak at 670 nm and fluorescence peak at 683 nm. Because nLw  bb/(a+bb),
absorption and fluorescence of phytoplankton between 660 and 690 nm led to the observed local
peak at 678-nm. Other constituents, including water molecules, have monotonous (or smooth)
spectral shapes in this region in both absorption and backscattering, and therefore cannot lead to
the observed peak at 678-nm when their concentrations are low.
Fig. 3. Influence of TSS on
FLH as simulated by
Hydrolight. The influence is
negligible for TSS < 5 mg L-1,
above which FLH is sensitive
to TSS.
FLH (mw sr-1cm-2m-1)
0.200
0.150
Chl=50.0 (mg m -3)
Chl=30.0 (mg m -3)
Chl=3.0 (mg m -3)
0.100
0.050
0.000
0.1
1
10
100
TSS (mg L-1)
In conclusion, the blue-green part of the spectra suggests that the PV patch can be 1) dust
particles; 2) Trichodesimum spp. (blue-green algae); 3) a mixture of the two; 4) a mixture of dust
and other forms of phytoplankton. The red-part of the spectra suggests that 1 is impossible, and
there must be some phytoplankton (Tricho. or others) bloom. Table 1 further suggests that the
PV patch is very likely a mixture of non-living particles and phytoplankton, because while the
blue-green estimated Chl increased from 0.03 to 0.86 (about 30 fold), FLH only increased from
0.0011 to 0.014 (about 12 fold). In contrast, for the EU bloom, both Chl and FLH increased by 34 fold (0.19 to 0.71 and 0.0076 to 0.027, respectively). Therefore, most of the color changes in
the PV pattern are due to non-living particles, while there is still a significant change that is due
to phytoplankton. If we assume that FLH is proportional to chlorophyll-a concentration, the PV
patch should have Chl of 0.36 (12 fold increase from the background of 0.03) instead of 0.86 mg
m-3.
Claustre, H., Morel, A., Hooker, S.B., Babin, M., Antoine, D., Oubelkheir, K., Bricaud, A.,
Leblanc, K., Quéguiner, B. and S. Maritorena (2002). Is desert dust making oligotrophic
waters greener? Geophysical Research Letters, 29, 10, 10.1029/2001GL014056.
Gilerson, A., J. Zhou, S. Hlaing, I. Ioannou, J. Schalles, B. Gross, F. Moshary, S. Ahmed (2007).
Fluorescence component in the reflectance spectra from coastal waters. Dependence on water
composition. Opt. Express. 15:15702-15721.
Hu, C., F. E. Muller-Karger, C. Taylor, K. L. Carder, C. Kelble, E. Johns, and C. Heil (2005).
Red tide detection and tracing using MODIS fluorescence data: A regional example in SW
Florida coastal waters. Remote Sens. Environ., 97:311-321.
Subramaniam, A, E. J. Carpenter, and P. G. Falkowski (1999). Bio-optical properties of the
marine diazotrophic cyanobacteria Trichodesmium spp. II. A reflectance model for remote
sensing. Limnol. Oceanogr. 44:618-627.
Further work can be performed to nail down if it Tricho. or other phytoplankton by comparing
the spectra with any known Tricho. blooms. Can you find the date and location of such blooms
(from literature)? I can then process and compare them.