4.3 Reabsorption 
Figure 5 shows a pronounced difference in curvature for 
detection wavelengths 68.5 nm and 710 nm. Since the devi 
ation from linearity is a measure for reabsorption, Figure 5 
proofs that the 685 nm radiation is much more reabsorbed 
than the 710 nm radiation. The absorption coefficient of 
pure water is greater at 710 nm than at 685 nm [Jer76], thus 
this effect of reabsorption is caused by particular substances 
dissolved in the water, probably by the algae themselves. If 
the absorbers arc spherical, 11 is directly proportional to the 
quantum efficiency for absorption Q a : 
m ~ Qa('l) (2) 
according to [Col85]. Q„ is the ratio of the energy absorbed 
within the sphere to the radiant engergy impinging on its 
geometrical cross-section [Mor81]. Thus the spectral 
dependence of Q a can be obtained by plotting II versus 2. In 
order to get a set of fit parameters a, II, every function 
L1I(N) in the interval 670-730 nm has been fitted according 
to equation (1). Figure 6 shows the curve 11(A). 
According to equation (2), Figure 6 shows the function 
Q„(A) in arbitary units. The solid line is a straight-line-fit. The 
curves are different for the two algae species. They differ 
from 12% at 675 nm to 45% at 700 nm. 
4.4 Emission spectra 
While the parameter H of equation (1) describes the 
absorption spectrum, the parameter a describes the emission 
spectrum. It can be shown [Gege90] that a is nearly pro 
portional to the coefficient of fluorescence, O 0 (2), defined by 
Gordon [Gor79] as the ratio of fluorescence radiation at 
wavelength A to the irradiance impinging on a small volume 
of the particle: 
a(A) ~ <D 0 (A). (3) 
Figure 7 shows this function. 
The emission of light is described by a Lorentzian function 
on the energy scale, thus the transformed a-values have been 
fitted with a Lorentzian function. The results of this fit arc 
listed in the following table. 
Tank 1 
Tank 2 
E 
C 
o 
689.2 
706.7 
694.7 
r (nm) 
13.1 
15.7 
16.2 
f/r 
2.6 
0.9 
1.9 
Table: Parameters of the Lorentz fit of Figure 7. A 0 = wave 
length of fluorescence emission, T = half width at half height, 
f/F = relative intensity. 
As Figure 2 already indicates, the tank 1 phyta emit at two 
wavelengths (689 nm, 707 nm), while the tank 2 phyta emit 
at one only (695 nm). Compared with the 689 nm peak of the 
tank 1 phyta, the 707 nm peak is reduced in intensity by a 
factor 2.9, the 695 nm peak of the tank 2 phyta by a factor 
1.4. I he half width at half height is similar for all peaks. 
4.5 Species dependence of fluorescence intensity 
In section 4.3 the function 11(2) has been derived and in 
section 4.4 the function a(A). Thus the functions 1,11(2,N) arc 
known for the algae species in tanks 1 and 2, and the spe 
cies-dependence of LII can be estimated. The difference in 
LI 1 between the two species Biddulphia sinensis (tank 1) and 
Procentrum micans (tank 2) at same concentrations illus 
trates Figure 8. 
Figure 8: Relative differences (in %) of the fluorescence 
intensity LH between the two tanks. ALH = LHJLH 2 - 1; 
the indices indicate the tank number. 
As Figure 8 shows, the line height depends considerably on 
algae species: differences in LII up to 90% were observed! 
5. Literature 
[Col85] 
[FSA86] 
[Gege90] 
[Gor79] 
[Gow81] 
[Jer76] 
[Kim85] 
[Mor81] 
[Nev77] 
D.J. Collins, I).A. Kiefer, J.B. Soolloo, I.S. 
McDermid: The role of rcabsorption in the spec 
tral distribution of phytoplankton fluorescence 
emission. Deep-Sea Research 32 (1985), 983. 
ESA contract No. RFQ 3-5059/84/NL/MD: The 
Use of Chlorophyll Fluorescence Measurements 
from Space for Separating Constituents of Sea 
Water. GKSS Research Centre Geesthacht 
(1986). 
P. Gegc, II.P. Hofmann: Tank experiments for 
the natural fluorescence of phytoplankton at 
high concentrations. DLR IB Nr. 552-1/90 
(1990). 
II.R.Gordon: Diffuse reflectance of the ocean: 
the theory of its augmentation by chlorophyll a 
fluorescence at 685 nm. Applied Optics 18 
(1979), 1161. 
J.F.R.Gower, G.Borstard: Use of the In Vivo 
Fluorescence Line at 685 nm for Remote Sensing 
Surveys of Surface Chlorophyll a. In: .I.F.R. 
Gower (cd.), Oceanography from Space, New 
York, London (1981), 329. 
N.G.Jerlov: Marine Optics. Elsevier Scientific- 
Publishing Company, Amsterdam - Oxford - 
New York (1976). 
II.II.Kim, II. van der Piepen, V. Amann, 
R.Doerffer: An Evaluation of 685 nm Fluores 
cence Imagery of Coastal Waters. ESA Journal 
9 (1985), 17. 
A.Morel, A.Bricaud: Theoretical results concern 
ing light absorption in a discrete medium, and 
application to specific absorption of phyto 
plankton. Deep-Sea Research 28 (1981), 1375. 
R.A.Neville, J.F.R.Gower: Passive Remote 
Sensing of Phytoplankton via Chlorophyll a 
Fluorescence. J. Geophys. Res. 82 (1977), 3487.