67 
Reflectance (%) 
3. Measurements 
3.1 Algae development 
All algae cultures showed a similar behaviour in population: 
after a short time of constant population, they increased 
exponentially ('log-phase') until nutrients were exhausted, 
then the population was constant for some time, and finally 
decreased exponentially. The plots of chlorophyll concen 
tration and cell numbers of Figure 1 illustrate this develop 
ment for the algae species Prorocentrum micans over a peri 
od of 14 days. 
1 he growth rate of chlorophyll a is greater than the growth 
rate of cell numbers, thus the chlorophyll concentration per 
cell increased, because of photoadaption. The limiting factor 
for growth was the silicate, which these species need to build 
up their skeleton. Note the great range in chlorophyll con 
centration (4-90 Mg/1), which is greater than in natural pop 
ulations. 
3.2 Reflectance spectra 
Figure 2 shows the reflectance spectra (spectral range 
550-750 nm) of two different algae species at similar chlo 
rophyll concentrations. Because of the relatively high con 
centrations (55-58 Mg/0. the fluorescence peak is well pro 
nounced. These spectra illustrate three main problems which 
have to be taken into consideration for a quantitative inter 
pretation of the fluorescence peak: 
• The intensities (height, integral) are different in spite 
of equal concentrations. 
• The maxima occur at different wavelengths. 
• The reflectance outside the fluorescence peak is different 
(different baselines, see section 4.1). 
These qualitative statements will be quantified in the follow 
ing sections. 
3.3 Red Shift 
The wavelength of the fluorescence maximum in the reflec 
tance spectra is dependent on chlorophyll concentration: the 
maximum shifts towards longer wavelengths when chlorop 
hyll increases. This well-known phenomenon called 'Red 
Shift' is shown in Figure 3. 
The maximum shifts by 0.23 - 0.26 nm when chlorophyll 
increases by 1 Mg/1, hut only up to concentrations of 55-63 
Mg/1. At higher concentrations the maximum is no longer 
shifted; the corresponding wavelength is 695-696 nm. This 
Figure 2: Reflectance spectra of two different algae species at 
similar concentrations N of chlorophyll a. Upper curve: Bid- 
dulphia sinensis, N = 55.1 Mg/1- Lower curve: Prorocentrum 
micans, N = 58.2 Mg/I- 
behaviour is independent of species, thus it is an optical 
effect. The red shift is caused by the reabsorption of the 
fluorescence light (see section 4.3). 
4. Interpretation 
4.1 Baseline 
The baseline ist that mathematical function which describes 
the reflectance curve without any fluorescence. It is not 
possible to measure the baseline, because it is impossible to 
switch off the fluorescence without destroying the algae 
(destroyed algae have a changed baseline). It is also impos 
sible to calculate the baseline exactly, first because the addi 
tional suspended substances generally are unknown, and 
second because the complex biological organisms cannot be 
reduced to a set of parameters which could be treated by 
electrodynamics. 
wavelenght of reflectance maximum (nm) TANK 1 
Figure 3: Wavelength of the reflectance maximum of the 685